Engineering Fundamentals CBT

Transcription

1 Engineering Fundamentals CBT Printout of CBT Content for Reference Purposes Only Reference CBT: Electrical Engineering Fundamentals V

2 Engineering Fundamentals CBT: Printout of CBT Content for Reference Purposes Only Reference CBT: Electrical Engineering Fundamentals V October 2007 EPRI Project Manager Ken Caraway ELECTRIC POWER RESEARCH INSTITUTE 3420 Hillview Avenue, Palo Alto, California PO Box 10412, Palo Alto, California USA askepri@epri.com

3 DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT EPRI NOTE For further information about EPRI, call the EPRI Customer Assistance Center at or askepri@epri.com. Electric Power Research Institute, EPRI, and TOGETHERSHAPING THE FUTURE OF ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc. Copyright 2009 Electric Power Research Institute, Inc. All rights reserved.

4 Summary PRODUCT DESCRIPTION This document provides a printout of the CBT content for use as a reference document only. Students are encouraged to use the CBT as animations, flash video, and interactive features are intended to enhance their learning experience. NOTE: The CBT should be used to validate information as errors may have been introduced when converting the graphics, equations, etc. Abstract Engineering Fundamentals Electrical Engineering (EF-EE) Version 1.0 web-based training module allows users to access training when desired and review it at their own pace and provides graphics and limited interactive features to enhance learning. Description Engineering Fundamentals Electrical Engineering (EF-EE) covers the basics of electrical engineering topics. The module provides information about basic electrical engineering terms and concepts that help identify electrical-engineering-related design, equipment, and safety considerations as they relate to nuclear power plant employees. EF-EE is intended as orientation training for new engineering support personnel. Platform Requirements The following hardware and software are required: Windows 2000 or Windows XP 1.0 GB RAM, 120 GB Microsoft Internet Explorer 6.0 Windows Media Player 10 Adobe Flash Player 9.0 CD-ROM This personal computer software is designed to run directly from the CD-ROM. EF-EE can also be installed and run on a dedicated server where it can be accessed by multiple users from other computers. Application, Value and Use EF-EE Version 1.0 benefits include the following: Computer-based training module allows individuals to learn at their own pace. Interactive features and graphics are included to illustrate key concepts and enhance training. Review questions are used periodically to check for understanding and reinforce concepts. Keywords Training Electrical engineering Fundamentals Orientation iii

5 ACKNOWLEDGEMENTS EPRI would like to acknowledge the following individuals for their active participation and significant contributions toward the development of this training course: Sam Bailey Ken Caraway Lyn Fraedrich Mat Janus Jeni Kusherman David Richards Stan Short Janet Simmons South Carolina Electric & Gas Co. EPRI Duke Energy Entergy Nuclear Operations, Inc. Handshaw, Inc. Southern California Edison Co. Florida Power & Light Co. Southern Nuclear Operating Co. iv

6 CONTENTS ACKNOWLEDGEMENTS... IV 1 INTRODUCTION ELECTRICAL ENGINEERING FUNDAMENTALS ELECTRICAL EQUIPMENT AC AUXILIARY POWER SYSTEMS DC SYSTEMS AND EQUIPMENT SYSTEM INTERACTIONS AND DESIGN CONSIDERATIONS SAFETY AND OTHER CONSIDERATIONS v

7 1 INTRODUCTION ELECTRICAL ENGINEERING FUNDAMENTALS Introduction Welcome to the Electrical Engineering Fundamentals course. In this course, you will learn about basic electrical engineering (EE) terms and concepts in order to identify EE related design, equipment, and safety considerations, as they relate to your job. If you are new to web-based training or if you experience difficulty viewing this course, please refer to the About This Course Section. You can review this information again at any time by clicking the About button on the toolbar and reviewing the About This Course section. In this lesson, you will learn about the principle EE applications in nuclear power plants. After successfully completing this lesson you will be able to: Describe how a basic electrical circuit works State basic terms and units related to electrical engineering Identify fundamental electrical laws: Ohm s Law and Kirchoff s Laws Describe how a large power system works 1-1

8 Power at Home You are most likely familiar with the electrical systems within your house. The power system provides power for such things as lighting, heating and cooling, fire detection, security, water heating, cooking, and various plug-in devices such as washers and dryers, televisions, DVD players, stereo systems, computers, and vacuum cleaners. Some systems, such as security and fire detection, have battery back-up systems built in to provide service for a reasonable period of time after power is lost. Let s take a look at some applications in a nuclear power plant. 1-2

9 How Does Electrical Engineering Affect Me? As you might expect, the electrical systems in a nuclear power plant are much more complex than those in your house. There are a variety of electrical systems within a nuclear power plant such as: Offsite power system Auxiliary power system Onsite emergency power system DC auxiliary power system Protective relay systems Control systems Instrumentation systems Lighting systems Grounding Lightning protection systems Communication systems Monitoring systems These systems provide power to equipment needed to operate and maintain the plant, to monitor performance of systems and equipment, to handle communication needs, etc. Who is affected by these systems? Almost everyone is impacted by these systems and your work will most likely impact one or more of these systems. It is very important to understand these impacts when performing operations or considering modifications. 1-3

10 Simple Circuit Covering some basic principles will help you better understand the issues and applications being presented in this course. Let s begin by reviewing the basics of a simple circuit. The figure below represents a typical circuit. Click each number to identify the elements of this simple circuit. 1. Voltage source. For example a 120 V outlet or battery. 2. Conductor or conductive path. This is typically wire such as in a drop cord or cable. 3. A control device such as a switch. Current will only flow when the switch is closed, i.e. there is no circuit until a conductive path is established. 4. A load. For example a light bulb. Ohm's Law (V = IR) can be used to determine the parameters of the circuit. Voltage (V), analogous to pressure, determines the amount of electrical insulation required whereas current (I), analogous to flow, determines the conductor size. The R represents the resistance in the circuit. The higher the voltage, typically, the larger the electrical distribution equipment required. System and equipment engineers try to optimize designs by using voltages and currents within normal equipment and cable rating ranges. This relationship is helpful in relating issues that impact power system design. 1-4

11 What Do You Think? Once V & I are known, the amount of power required can be calculated, i.e. Power = voltage X current (P = VI). Many common items are rated in watts (power) and voltage, such as a 250 watt lamp for 120 volt service. In this case, the lamp would draw approximately 2 amps. You can see that current is directly proportional to power when voltage is constant. If a 1000 watt hair dryer were connected to this same source, approximately how much current would it draw? Answer: 8 Amps That's correct. If a 1000 watt dryer were connected to this source, it would draw approximately 8 amps. 1-5

12 Load Example Let s examine an everyday example of loads within an electrical system. Start with the first switch and click the switches in order to see the effects of loads being added to the system. Be sure to notice the effects each load has on V load (voltage available to the load) and V c (voltage drop across the cable) respectively. Use the Reset button to repeat the activity. This example also illustrates Kirchoff's Current and Voltage Laws. Kirchoff s Current Law states the sum of currents at any node (junction) in a circuit equals zero. In other words, the sum of currents flowing into any node must equal the sum of the currents flowing out of the node. A simple circuit with one source and parallel loads illustrates how the current from the source is divided between the loads. Kirchoff s Voltage Law states the sum of the voltages across the different elements in any closed loop equals zero. Note: This was an interactive example that will not work in this word document. 1-6

13 Large Power System Let s take a look at a large power system to put things in perspective. The electrical engineer tries to optimize current and voltage ratings to minimize the cost of installing and operating the system. It would not be practical or cost effective to transfer energy from a generating station directly to your house. Power Plant: Nuclear fuel is used to produce heat which is used to create steam. The steam is used to spin the turbine. The turbine shaft is connected to the generator rotor. The mechanical energy is converted to electrical energy through the generator. Transformer: The power output from the plant is stepped up in voltage to match the grid voltage. Transformers that are used to provide offsite power to the plant (e.g. when the generator is off-line) step down the voltage. Voltage is stepped down again to serve smaller industrial and commercial customers as well as residential needs. Power Plant Switchyard: High voltage breakers and buses are arranged to allow power flow on the grid while isolating individual generators, transformers or transmission lines as needed. Power from the plant is delivered to the grid via the plant switchyard, and the switchyard serves as the offsite power source when a unit is shutdown. High Voltage Lines: Extra high voltages are typically used for transmission systems which are used for transporting bulk power from large generators to areas with heavy load 1-7

14 concentrations such as cities. These lines are also used to transport power between regions as needed. Transmission lines can be compared to interstate highways which are built to accommodate a high volume of traffic over long distances. Transmission Substation: Analogous to a power plant switchyard but on the load end. Voltage is stepped down to Distribution System voltages. Distribution System: The Distribution System is used to supply power to large industrial and commercial facilities as well as the Retail System. Distribution lines can be compared to state highways and major roads around cities. Retail Substation: Voltage is stepped down again to serve smaller industrial and commercial customers as well as residential needs. Retail Lines: Retail lines are used to supply power to smaller industrial and commercial users as well as residential customers. These lines can be compared to local roads such as the one in your local area or neighborhood, i.e. they're designed to handle the needs of local traffic only. Transformer: Voltages must be maintained within reasonable limits to properly serve customer needs. 1-8

15 Three Phase Power To further optimize costs, 3 phase power is used in these systems. Generators have 3 sets of windings with an output lead connected to each winding. The voltage in each set of windings is represented by a sine wave, which is 120 degrees out of phase with the other two windings. The output of the generator is delivered via 3 conductors, i.e. one per phase. The output power is equal to the product of voltage and current times the square root of 3 (P = V * I * square root of 3). 1-9

16 Overview Now let s take a look at the power systems in and at a nuclear power plant. When a nuclear power plant is in operation, the main generator supplies power to the auxiliary systems in the plant and the net output of the unit is delivered to the grid. Generators connected to the grid supply the various loads connected to the transmission system via the distribution and retail systems. Most homes have 120/240 V single phase power. Most industrial and commercial facilities, as well as most power systems in nuclear power plants, use three phase power typically, 480V, 4160V, and 6900V. 1-10

17 Current Current (analogous to flow) predominantly flows on the surface of the conductor. In order to keep the costs down, large conductors are stranded (because stranded wire has more surface area than the same amount of solid wire) instead of solid like in your home. As currents go up, more surface area is required so more strands are added, making the conductors larger and larger. Circuit breakers are used in power systems for switching and to isolate equipment if the circuit is overloaded or a fault occurs. The sizes of the breakers are affected by both the voltage and current ratings. A breaker in your house (120/240V) would fit on the palm of your hand while a breaker in a power plant would range from the size of a tissue box (480/600 V) to a washing machine (4/6.9 kv) and a breaker in the Switchyard (230/ 500KV) would be bigger than your car. 1-11

18 Voltage Considerations If you recall the previous example with the hair dryer, you will remember the lights dimmed. This indicates the lights and receptacles are on the same circuit where the hair dryer, representing a significant load on the circuit, causes a voltage drop through the wires. Although this is not critical for your hair dryer, voltage drop considerations are a major issue for power systems within a nuclear power plant. When motors are energized, the current demand is typically 600% or more during the period when the motor is accelerating (typically in the range of a few seconds up to several seconds depending on how loaded the motor is or how low the voltage droops). When the current goes up, the voltage drop across the circuit goes up as well, making less voltage available at the load. Since torque varies as the square of the per unit voltage, you can see that margin can be reduced rather rapidly. These changes in voltage caused by motor starts or other loads being switched onto an electrical system are called transients. Voltage 100% 100% 90% 81% 80% 64% 70% 49% Torque 1-12

19 Load Additions Which of the following activities do you think might adversely affect the load in a nuclear power plant? - Trimming a pump impeller - Changing the pitch on a fan blade - Changing gear or pulley ratios to increase speed on a load - Changing the system design to increase the flow of a pump Answer: All of the above That's correct! All of these loads may adversely affect the voltage and current flows in the system. Accordingly, when anyone changes the amount or type of load on a system, the other loads are also impacted. Other examples of load changes include: Adding a new load Installing a larger motor, heater, etc. Changing controls or operation such that loads are starting simultaneously or their start times overlap 1-13

20 Load Example The action may appear simple, but the effects on the system parameters can be significant. For example, you want to increase air flow so you change the speed on a fan. This appears to be a simple task and you may think this action couldn't possibly affect any other systems in a nuclear power plant. However, you would be wrong. Let's examine the cause and effect reaction you created by simply altering the speed of the fan. First, by changing the speed of the fan, you have increased the motor load. This increased motor load requires more current which affects the voltage on this load as well as other loads fed via the same equipment. You can see by this example that electrical systems impacts are synergistic. 1-14

21 Regulatory Requirements We have discussed operation of a nuclear power plant during normal conditions. What happens when a nuclear power plant is not operating under normal conditions? When a nuclear unit trips, power is supplied from the grid to power the loads on the auxiliary power system associated with that unit. Redundant equipment is provided to mitigate a nuclear accident and to maintain the unit in a safe shutdown condition. Each group of this redundant equipment is typically referred to as a train and is classified as nuclear safety related. The Code of Federal Regulations (10CFR50) General Design Criterion (GDC-17) requires two independent and physically separate offsite circuits be available to supply the safety related loads. 1-15

22 Loss of Offsite Power Onsite emergency diesel generators are also provided to supply these loads should offsite power not be available. Either source of power must be capable of supplying the required loads, including starting those that were not previously operating, while maintaining voltage and frequency within required limits. An example of the importance of this concept can be illustrated by the Northeast Blackout that occurred in August There was a blackout that affected the Midwest and up into Canada and into new England. The area in red on the map at right indicates areas that were affected by this blackout. 1-16

23 Apparent, Real, and Reactive Power Real power (watts) relates to the energy used or lost across resistive elements or loads. Reactive power (volt amperes reactive or VARs) relates to the energy stored in or released from a capacitor or an inductor or inductive elements such as the windings of a transformer or motor. Apparent power is the vectoral sum of the true (or real) power and reactive power as shown at right. Power factor is the ratio of real power to apparent power and is always a value greater than or equal to 0 (totally capacitive or inductive load) and less than or equal to 1 (totally resistive load). In other words, power factor = cosine of theta. When a generator is on-line (connected to the grid), it is typically boosting voltage to help support grid operations, i.e. the generator is supplying VARs to compensate for losses on the grid (e.g. through transformers) and those associated with motor loads. Under these conditions (normal operation), the generator is pretty much controlling switchyard (high voltage equipment where the generator is interconnected with the grid) voltage. 1-17

24 Real Power Example An example might help illustrate this scenario. Joe is getting ready to take a very unique excursion on vacation, so he checks his digital camera. Full battery capacity is 205 minutes in the normal photo mode. Joe turns on the camera and the indicator shows 200 minutes so Joe thinks he has a pretty full charge [unfortunately the camera is in the review mode which requires less power and would therefore indicate a longer discharge time than would be available in the normal photo mode]. Not worried about battery capacity, Joe is liberally taking pictures on his trip and leaves the camera on while riding around. Joe is now faced with the picture perfect moment of a life-time and his battery voltage is too low. The indication Joe saw before he left was not a good indication of the capacity available for his needs. This is very analogous to seeing switchyard voltage with the unit on-line compared with the voltage that might be seen following a unit trip with the grid being used to supply unit loads while accident loads are being started. 1-18

25 Power Systems Branch 1 The Nuclear Regulatory Commission issued Power Systems Branch 1 (PSB-1), which required utilities to install degraded grid voltage relaying to protect against applying loads on the grid if the voltage is inadequate. Unfortunately there is not a meter, as in the case of the camera, that we can check to indicate postaccident voltage. Grid-based analyses must be performed using state estimators and contingency analyzers to predict grid voltage following a unit trip followed by accidental loading. You will learn more about this in the System Interactions and Design Considerations lesson. 1-19

26 Introduction to Electrical Engineering Summary This course is intended to provide a broad overview. The content is primarily focused on power systems; however, it is important that you have an appreciation for other systems and interface issues as well. Suffice it to say that you will need to consult with someone if an electrical circuit or signal is involved before you modify the associated systems, equipment or circuit. In the following lessons, you will learn about specific electrical systems and equipment in a nuclear power plant as well as some of the issues and considerations you need to be familiar with, including interaction issues. Protecting and properly maintaining these systems and equipment is critical to plant operations as well as maintaining nuclear safety. 1-20

27 Conclusion You have now learned about basic electrical engineering terms and concepts and how to identify electrical engineering-related design, equipment, and safety considerations, as they relate to your job. Now that you've completed this lesson, you can: Describe how a basic electrical circuit works State basic terms and units related to electrical engineering Identify fundamental electrical laws: Ohm s Law and Kirchoff s Laws Describe how a large power system works In the next lesson, you will learn about the electrical equipment used in a nuclear power plant. 1-21

28 2 ELECTRICAL EQUIPMENT Introduction Welcome to the Electrical Equipment lesson. In this lesson, you will learn about the types of electrical equipment typically used in nuclear power plants. After successfully completing this lesson you will be able to: Describe the major types of electrical equipment such as: o o o o o o o Generators Transformers Bus and Cable Switchgear Motor Control Centers Circuit Breakers Motors Identify the primary functions of major types of equipment 2-1

29 Generator A generator is used to convert mechanical energy to electrical energy. The main or unit generator rotor is coupled to the turbine shaft, and contains a set of field windings which are energized via a separate DC source - typically a static excitation system. When the turbine shaft is spinning, the energized field windings create an electromagnetic field within the generator. Currents are induced in the stator windings which are arranged to produce a three phase output (the output voltages were animated in the Introduction to Electrical Engineering lesson). The emergency generators operate in a similar manner but their rotors are coupled to the diesel generator shaft. The excitation can be increased or decreased to control the output voltage on the generator and is used to control reactive power (VARs) which is used to help regulate grid voltage. A governor is used to control the speed. Voltage and frequency (measured in hertz or cycles per second) must be adjusted to be approximately the same as the grid before connecting the generator to the grid. This process is called synchronizing. 2-2

30 Isolated phase bus (IPB), non-segregated phase bus, and cable Various types of conductor systems are used in nuclear power plants. IPB is typically used between the main generator and the main step-up and auxiliary transformers. Each phase has a bus enclosure (which looks like a large pipe) which contains a bus conductor mounted on insulators supported from the enclosure. A typical generator output bus is rated at 40,000 or so amperes at 20 to 24 kv. Forced air cooling systems are typically used to cool this bus. Non-segregated phase bus is typically used between auxiliary or start-up transformers and the associated switchgear. Three bus conductors are typically mounted inside of a single enclosure. Cables of various types are used. For example, open conductors suspended from insulators are typically used between the step-up transformers and the switchyard. Insulated medium and low voltage cables are typically used to connect equipment and loads at the switchgear and lower levels. Terminations of medium voltage cables typically require special equipment and procedures. 2-3

31 Circuit Breaker Circuit breakers are used to energize and isolate systems and equipment. Circuit breakers have a continuous current rating (based on load ratings and margin) as well as an interrupting rating. The breaker must be capable of tripping and interrupting the fault current should the conductors be shorted. Switchyard, generator, and switchgear breakers (at right) are provided as an assembly but depend on input from other devices for tripping and control logic. Protective relays (e.g. over-current relays, under-voltage relays, etc.) are connected to potential transformers (PTs) and current transformers (CTs) to monitor associated circuits and initiate tripping if needed. Control inputs are provided as needed to enable remote control. Load center breakers are similar to switchgear breakers; however, they are used at low voltages rather than medium voltage and they contain CTs and protective devices as part of the assembly. These breakers are typically used to supply motor control centers (MCCs) as well as some large low voltage loads. Molded case circuit breakers (MCCBs) are used at low voltage levels (600 volts and below) to feed individual loads. These devices typically contain a thermal magnetic element which provides an inverse time tripping characteristic. 2-4

32 Transformer Transformers are used to convert voltages from one level to another. The system voltages are selected to optimize equipment ratings and costs based on the number and type of equipment involved. When transformer requirements are developed, an impedance value is specified as well as MVA, primary and secondary voltage ratings, etc. The impedance is used in the system to limit fault currents within acceptable ranges as discussed in the section on circuit breakers. The values must be coordinated to assure faults can be adequately isolated should one occur. Transformers may have two sets of windings (e.g. one high voltage rating and one low voltage rating) but may also have multiple sets (e.g. a 6.9 kv and a 4.16 kv set of low voltage windings). 2-5

33 Switchgear Power from the auxiliary and/or start-up transformers is connected to switchgear assemblies via an incoming breaker. Each switchgear assembly contains an internal bus which is used to distribute power to the various loads that can be connected via individual feeder breakers. Separate switchgear assemblies are provided for each train, as well as safety related versus non-safety. You will learn more about these configurations in the lesson on AC Auxiliary Power Systems. Each switchgear assembly contains the circuit breakers, PTs, CTs, protective relays, meters, and local control switches associated with the incoming and feeder circuits connected to the switchgear bus. Each circuit breaker and associated auxiliary equipment is contained in a separate cubicle to provide isolation for maintenance as well as equipment failures associated with an individual compartment. Switchgear is typically used to power medium voltage motors and load center transformers. 2-6

34 Load center A load center is similar to switchgear except for use in low voltage versus medium voltage applications. The circuit breakers are self contained as noted earlier. A load center is normally collocated with its associated supply transformer such that the transformer output is connected directly to the load center incoming (supply side) bus. Load centers are used to supply power to motor control centers and large individual low voltage motors in some applications. 2-7

35 Motor Control Center (MCC) An MCC is an assembly containing breakers and motor starters mounted in individual compartments (sometimes called buckets) which are used to feed low voltage loads. Molded case circuit breakers (MCCBs) are used as incoming breakers, as feeder breakers, and in starter compartments in conjunction with motor starters which are used to allow remote or automatic control. Motor starters are equipped with thermal overload protection to protect the motors from sustained overload conditions; however, a starter assembly does not provide fault protection. As such, an MCCB, motor circuit protector, or fuses must be used in conjunction with the starter. 2-8

36 Panelboard A panelboard or distribution panel may be used to power various low voltage loads and are typically used for applications that do not require any form of remote control. Lighting panels are good examples where the individual lighting circuits are controlled from local switches as needed, e.g. each room or office is equipped with its own light switch and no other controls are needed. Receptacles would also be fed from panelboards. Panelboards of a similar type are routinely used in homes. Distribution panels are similar but typically have a higher ampere rating and are used to feed larger circuits, e.g. a concentration of machines in a machine shop. 2-9

37 Disconnect Switches Disconnect switches are used to isolate equipment but are typically not designed to be opened or closed while the circuit is energized (i.e. they re not designed to interrupt current). Generator disconnect switches are used to isolate the generator so the step-up and auxiliary transformers can be used to back-feed power from the switchyard to the plant. Disconnect switches are also used in the switchyard to isolate equipment (e.g. circuit breakers) or transmission lines for maintenance. Disconnect switches are also used in low voltage systems to allow equipment to be isolated locally to ensure personnel protection. 2-10

38 Motor A motor converts electrical energy to mechanical energy. A motor is the counterpart to a generator. Power is applied to the stator windings in a motor. The field created by the currents circulating in the stator windings causes currents to flow in the rotor windings. These currents create a force which turns the rotor shaft. Motor type and ratings need to be matched with the load requirements and the power system to assure proper operation under worst case conditions and loading scenarios. More information is provided in the lesson on system interactions. 2-11

39 Electronic Devices Major pieces of equipment or components are discussed above. Electronic devices are often used in control and protective devices to support operation of this equipment. Semiconductors, such as diodes and transistors, have non-linear performance characteristics and are used in many applications in existing power plants. Diodes operate somewhat like check valves such that current only flows in one direction, when the voltage on the input is higher than the output. As such, diodes are often used in battery chargers and rectifiers in a bridge configuration to convert AC to DC. Diodes can also be used in parallel to auctioneer or select the source with the higher voltage. Transistors are used in numerous applications to amplify or perform switching operations. Transistors are often used as building blocks along with resistors, capacitors, diodes, etc., in a variety of equipment such as protective relays, control equipment, timers, power supplies, etc. 2-12

40 Other Electrical Equipment DC equipment will be addressed in the lesson on DC Systems and Equipment. Many other types of electrical equipment are used in power plants to monitor process systems, e.g. pressure switches and transmitters, flow switches and transmitters, level switches, thermocouples, resistance temperature detectors (RTDs), vibration monitors, etc. Meters, strip charts, event recorders, annunciators, alarm panels, indicating lights, etc. are also used in numerous applications, to include the control room. Computer systems are also used to monitor and display information. As you move about the plant where you work and learn more about the systems and equipment, you should become familiar with the variety of components that provide critical functions for your systems and equipment. 2-13

41 Conclusion In this lesson, you learned about the various types of equipment used in electrical systems at nuclear power plants. You should now be able to discuss the major equipment types and their basic functions. Describe the major types of electrical equipment such as: o o o o o o o Generators Transformers Bus and Cable Switchgear Motor Control Centers Circuit Breakers Motors Identify the primary functions of major types of equipment In the next lesson, you will learn about the ac auxiliary power systems used in nuclear power plants. 2-14

42 3 AC AUXILIARY POWER SYSTEMS Introduction Welcome to the AC Auxiliary Power Systems lesson. In the introductory lesson, you got a quick overview of some power system issues in a nuclear power plant. Let s take a closer look at the requirements and the systems that are used to assure nuclear safety. After completing this lesson you will be able to: Differentiate between typical safety related and non-safety related electrical distribution systems Explain the requirement for redundant electrical power trains Describe the various sources and configurations that can be used to power the safety related systems at a nuclear power plant 3-1

43 Nuclear Safety Related Functions Let s first talk about safety functions. Reactor and public safety during normal and accident conditions relies on satisfying certain safety functions. These usually include titles such as Reactivity Management, RCS (Reactor Coolant System) Inventory, Heat Removal, Containment, etc. Maintaining these safety functions ensures that "specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences" and "the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents." The systems and equipment that are required to perform these safety functions are classified as Nuclear Safety Related. Regulations invoke specific requirements on the design of these systems. Let s look at a few of the major requirements. 3-2

44 Redundancy GDC-17 requires designers to provide for "sufficient independence, redundancy and testability to perform safety functions assuming a single failure". That single failure could be a single component or part of the electrical distribution system that affects an entire train of power. Therefore, a robust design that includes at least two complete redundant trains of each type of equipment with multiple power sources is typical. The design provides for separation of trains such that a failure of one train does not cause the other train to fail. The graphic shown right illustrates the two train concept for safety related auxiliary power systems in an NPP. You will notice that the diesel generators, associated 4 kv switchgear and downstream equipment are independent and redundant. These buses and associated loads are redundant such that one train can perform the needed safety functions should the other one be unavailable. Although this is the case, activities associated with this equipment are limited during plant operations to maximize availability of these systems. 3-3

45 What is a Train? This graphic shows the typical assortment of equipment normally associated with one train of the AC auxiliary power system in a nuclear power plant (NPP). The Emergency Diesel Generator, the 4 kv Switchgear, and the downstream equipment are classified nuclear safety related. The equipment upstream of the 4 kv switchgear is associated with the same train; however, it is non-safety related. Voltages and configuration may vary from plant to plant. 3-4

46 Sources of Power To assure high availability of power, multiple sources of power are provided for the plant auxiliary power systems. For safety related (or essential) auxiliary power systems, sources typically include the unit's main generator, offsite power via the switchyard, and redundant emergency diesel generators. Some plants use a normal alignment where their safety related systems are powered from the switchyard, i.e. they are not fed from the main generator so as to avoid a bus transfer following a unit trip. For multi-unit sites, cross feeds from the other unit's auxiliary power system may also be provided. For multi-unit sites, cross feeds from the other unit's auxiliary power system may also be provided. Let s take a look at the source(s) of power typically used based on the status of the plant and availability of power from the different sources. Main Generator Diesel Generator 3-5

47 Normal Operation of a Train When the unit s main generator is on-line, it is typically supplying power to the plant auxiliaries and the net output is delivered to the grid via the step-up transformer(s) to meet customer needs. In this mode of operation, the generator is typically boosting voltage on the grid and as such, is likely providing a higher than normal voltage for the auxiliary loads within the plant. Click the Start button below to view the normal operation of a train. You will notice the source of power is the Main Generator. 3-6

48 Generator is Off-line When the unit is off-line, power for the auxiliary power systems is typically supplied from offsite or another onsite unit via the switchyard. When the generator is tripped, power is typically transferred to a start-up transformer. Offsite power is considered the preferred power source for the safety related loads. Should an accident occur; the large safety related loads that were not running will be sequenced on to minimize problems with starting voltages while connecting the loads and to assure core cooling is established as quickly as possible. These loads will typically be sequenced on to minimize the impact of their starting on the auxiliary electrical system voltages. As discussed in the Introduction to Electrical Engineering lesson, measures must be taken per Power Systems Branch [Position] (PSB-1) to prevent applying loads on offsite power if it is vulnerable to degraded grid voltage. 3-7

49 Alternate Offsite Power Source Configurations Some plants have disconnect switches to isolate the generator so power can be back-fed via the step-up and auxiliary transformers as another off-site path. This alignment would usually require manual operation of the disconnect switch so this configuration would not be available immediately after a unit trip. Two generator circuit breakers, step-up and auxiliary transformers are used in some nuclear plants. In these cases, power is back-fed from the grid via the stepup and auxiliary transformers when the generator breakers are opened. In this case, power flows from a different source; however, a power transfer is not required as in the case of the start-up transformers. 3-8

50 Alternate Onsite Power Sources Some nuclear stations have multiple units collocated on the same site. In these cases, cross connects may be provided between the units such that Train A on one unit can be fed from Train A on the adjacent unit. A similar arrangement would be provided for Train B. This arrangement would typically allow the nonsafety switchgear on one unit to be used to supply power to the safety related switchgear on the other unit. 3-9

51 Emergency Diesel Generators (or alternate onsite generators) If the unit is off-line and power is not available via the preferred power source (switchyard is below the minimum allowable switchyard voltage, i.e. the the degraded grid voltage setpoint). The emergency diesel generators are fast started and are typically ready to accept load in 10 to 12 or so seconds and fully loaded in 40 seconds (10 seconds start + 30 seconds to load). Some type of load sequencer is normally used to control the loading such that the required loads are brought on in the order needed and as quickly as possible while maintaining voltage and frequency within acceptable and required limits. 3-10

52 Train Review A safety related train can be powered from which of the following sources? A. Emergency generator B. Main generator at some nuclear power plants C. Offsite power through the switchyard D. Different train on separate unit at multi-unit sites Correct Answer is A. A train can be powered by an emergency generator, offsite power through the switchyard, and the main generator (at some NPPs). 3-11

53 Review Question 2 Which of the following statements is correct regarding the requirement for two trains of safety equipment at a nuclear power plant? A. With multiple trains of safety equipment, the safety functions can still be met in the event of a single failure during an accident. B. Multiple trains allow safer operation with two of any type of equipment out of service. C. Multiple trains provide the ability to equalize equipment run time to enhance reliability during an accident. D. Multiple trains mitigate potential design errors because more equipment is available in an accident. Correct Answer is A. Multiple trains allows for safer operation if one of any type of equipment is out of service. Equalizing run times is a good practice but it is not the reason for multiple safety trains. Engineering uncertainties are accounted for when necessary in design calculations, but multiple trains are not created because of potential errors. 3-12

54 Review Question 3 Two trains of safety equipment, such as safety injection pumps, are provided in case of a single failure. Why are two complete independent electrical distribution power trains included as well? A. If one pump motor short circuits it is likely to affect the entire power train. B. It allows us to cross tie power trains when one power train is undergoing maintenance. C. Electrical distribution equipment could be the single equipment failure that occurs. D. The electrical systems will be more stable with two trains of power. Correct Answer is C. (In other words, one whole power train could fail. The other train and its equipment must be up to the task of meeting all safety functions.) If one pump motor short circuits, it's circuit breaker will trip protecting the rest of the electrical system from the fault. Each power train is independent there is typically no ability to tie them together. Two trains of power will have no effect on stability in this context. 3-13

55 Conclusion You have now learned about trains and their use in a nuclear power plant. Now that you've completed this lesson, you can: Differentiate between typical safety related and non-safety related electrical distribution systems Explain the requirement for redundant electrical power trains Describe the various sources and configurations that can be used to power the safety related systems at a nuclear power plant In the next lesson, you will learn about DC systems and the equipment used with DC systems. 3-14

56 4 DC SYSTEMS AND EQUIPMENT Introduction Welcome to the DC Systems and Equipment lesson. In earlier lessons, you learned about some of the AC systems used in nuclear power plants. You might ask yourself "What happens if AC power is lost?" Although this is very unlikely, it is possible. If AC Power is lost, the DC systems are essential to the Operators in knowing what is going on in the plant and controlling the equipment (the DC system is the last defense in controlling the power plant). DC circuits are widely used for a variety of applications. DC systems are provided to supply loads that are critical for control, monitoring and protection. These must operate should AC power be lost. Some systems also require uninterruptible AC power, or regulated power. Regulated power is conditioned such that the voltage variation is very minimal compared with the variation normally seen on standard service. After completing this lesson you will be able to: List the types of DC Systems typically used in nuclear power plants (NPP) State the purpose of these systems Describe the equipment typically used in DC Systems Identify issues you need to be aware of when considering changes to DC systems Describe precautions when working around DC equipment and circuits 4-1

57 DC Systems The voltage and current in a DC system are relatively constant as opposed to varying and alternating in polarity as in the case of AC systems. Ohm's Law (V = IR) can be used to determine voltage, current or resistance if the other two quantities are known. Power can be calculated as the product of the voltage and current; however, the value of voltage to be used needs to be selected based on the purpose as you will see later in this section. Typical DC system voltage ratings used in NPPs are 125 and 250 VDC. The nominal system voltage determines the approximate number of cells required for the battery, e.g. 125 VDC rating/ 2.06 volts (the approximate open circuit voltage for a typical lead acid battery used in a NPP) = 60 cells. A large battery system in a NPP might be comprised of approximately 60 cells, a few will even have double this number, and each cell is many times larger than an automobile battery, which typically contains 6 cells. 4-2

58 General Information Batteries are widely used in mobile or portable applications. Batteries are also used in many stationary applications to provide services when AC power is lost, such as security systems, fire detection and protection systems, and emergency lighting. Batteries can store energy for specific purposes, such as starting your car. In this example, the energy from the battery is used in starting the car and is recharged by the alternator when the engine is running. Many small batteries are inexpensive and can easily be replaced. Some are for one-time use but others are designed to allow recharging. The large batteries that are used in many applications in NPPs are expensive and are difficult to replace. Below is a large battery application in a nuclear power plant. 4-3

59 Applications The following example applications require DC or uninterruptible AC power to meet their mission critical functions. Click each example to learn more. 1. Circuit Breaker Control: Large circuit breakers can be controlled remotely (manually or automatically) to start/stop large pumps, or to energize/de-energize transformers or buses. 2. Protective Relaying Circuits: Protective relaying circuits are powered from DC systems to assure reliable operation, especially when disturbances occur on the AC system they re protecting. 3. Diesel Generator Controls and Auxiliaries: Diesel Generator controls and auxiliaries are fed from DC power to provide starting capabilities when no AC power is available, somewhat analogous to a battery application in your car. 4. Emergency Lighting: This is an example of an emergency lighting unit. These units are self-contained with a voltage sensing circuit such that the lights are turned on, powered from the battery, when normal AC power is lost. 5. Emergency Oil Pumps: These pumps protect large equipment like the turbine generators from incurring shaft damage due to loss of bearing oil following loss of AC power. 6. Nuclear Instrumentation Systems (AC): Nuclear Instrumentation systems (AC) typically require a regulated voltage source [very little fluctuation in voltage] and service is needed following a loss of power. Accordingly, these systems are fed from inverters (opposite of a battery charger) which receive input power from a battery charger or battery and provide a regulated AC output. 4-4

60 7. Plant Process Computers: Plant process computers are used to provide data and information to the plant operators. These computers are typically powered from inverters which are connected to non-safety related batteries. 8. Selected Devices: An example of a selected device is a solenoid valve that must operate after a loss of AC power. 9. DC Motor Operated Valves (MOV) DC motor operated valves are required to operate with precise timing. Engineering maintains voltage and thrust calculations for these valves. 10. Annunciators: Annunciators provide information to the operators so they can know what is going on after AC power is lost. 4-5

61 DC System Overview A DC system is typically comprised of a large battery, a battery charger, and a distribution panel. The charger normally supplies the load on the system while maintaining charge on the battery. The one line diagram to the right represents a typical DC auxiliary power system. For nuclear safety related systems, each train of DC charger(s) will typically be powered from an independent train of AC power. If offsite power is unavailable, these chargers would typically be sequenced onto the diesel generators after they re started. To assure proper operation of the systems and equipment that depend on these DC systems, the batteries are sized to carry the load for a reasonable period of time as might be the case in the unlikely event that all AC power is not available. Where uninterruptible AC power is required, an inverter is used to convert DC power to AC. This is typical for solid state protection systems, nuclear instrumentation systems, etc., in a NPP. Let s take a look at some of the different types of DC systems typically used in NPPs. 4-6

62 Types of DC Systems Depending on the plant type, layout, and number of units, the number and type of DC systems will vary. The following types of systems would typically be found in a NPP. Click each system type to learn more about each one. 1. An emergency lighting system: An emergency lighting system uses emergency lighting units similar to what you would find in commercial applications. These units typically contain a small battery (similar to a small car battery) and a couple of lamps; however, they can be used to power a few remotely mounted lamps as well. 2. A 125 VDC safety related auxiliary control power system: A 125 VDC safety related auxiliary control power system provides power for safety related systems and equipment, e.g. annunciators, controls and protection for the switchgear and load centers that can be fed from the emergency diesel generators, selected solenoid valves, and inverters which are used to power safety related AC loads that require uninterruptible power or voltage within a very narrow band. This system may have 2 or 4 trains per unit to supply four inverters. Some of the inverter fed loads utilize a 2 out of 4 logic scheme. NOTE: Some plants use a 250 VDC safety related system to provide power to safety related motor operated valves. 3. A 125 VDC non-safety auxiliary control power system: A 125 VDC non-safety auxiliary control power system provides power for non-safety systems and equipment, e.g. turbine generator controls, auxiliary power system controls, protection and monitoring; inverters that supply AC power to the plant process computer and various monitoring and control functions requiring regulated or uninterruptible power, etc. This system would typically have 2 trains per unit to match the AC auxiliary power system configuration as discussed in Plant Auxiliary AC Systems. 4. A 125 VDC safety related diesel generator system: A 125 VDC safety related diesel generator system provides power for diesel auxiliaries and controls. The battery is used in starting the diesel generator following a loss of AC power. Typically there is one battery system per diesel generator. 5. A 250 VDC auxiliary power system: A 250 VDC auxiliary power system provides power to emergency oil pumps for the turbine generator and feedwater pump turbine, 250 VDC deadlights, and possibly inverters for specific purposes or other large DC loads. Typically there is one battery system per unit. 4-7

63 Battery Performance A. Temperature B. State of charge C. Type of load Which of the following do you think battery performance depends on? D. Number of cells in parallel Correct Answer is ALL. Just as in the case of batteries used in a flashlight, a portable radio, etc., the functionality depends on the state of charge on the battery, the amount of load connected to the battery, the ambient temperature, and the number of cells. For example, a new battery in a smoke detector will typically function properly well beyond a year whereas ones in a flashlight used extensively might only last several days. 4-8

64 Voltage Considerations The batteries used in large DC systems in NPP applications are typically large lead-acid cells (similar to automobile batteries) with an open circuit voltage of approximately 2.06 volts per cell. Usually 59 or 60 cells are connected in series to form a battery with a nominal 125 volt rating. The charger output voltage is normally set at approximately 133 V (float voltage) to maintain full charge on the battery. Some plants periodically perform an equalize charge on their batteries where voltage is raised to approximately 138 V to assure each cell is adequately charged. If this is done while the charger is connected to the system, all connected equipment must be capable of operating at the higher voltage levels. Open circuit voltage (i.e. with a fully charged battery disconnected from the charger and all loads) is approximately or volts depending on the number of cells used. The voltage under load will be lower depending on the state of charge on the battery and the amount of current required. Some loads like inverters and motors are constant power devices which require more current as the voltage decreases (from the power equation, P = VI). These loads will deplete the battery much more rapidly than resistive loads (e.g. like an incandescent light bulb) where the current will drop proportionately with voltage decay (from Ohm's Law, V = IR). Similar to a typical car battery, these batteries do not perform well in low temperatures. On the other hand, battery life is lost at an accelerated rate when temperatures get much above normal room temperature. The temperature at which a battery is maintained is a critical parameter when determining if the battery can provide sufficient amperage and voltage to meet its design requirements or duty cycle (discussed later). Accordingly, most battery rooms are temperature controlled. 4-9

65 Voltage Considerations for DC and AC Systems From the previous discussion, you can see some interesting parallels between the voltage considerations for DC and AC systems. For example, once the normal source is lost (main generator for AC systems and chargers for DC systems), the voltages on the affected buses are lower even without a change in loading. Both the AC and DC system voltages are impacted by the amount of load. There are also some interesting differences. A generator is typically rated in Volt-Amperes (KVA or MVA) whereas batteries are rated in ampere-hours corresponding to a specific period of time (e.g. a 1 hour discharge, a 3 hour discharge, or an 8 hour discharge period). The current is assumed to be constant during this discharge period. The larger the current, the faster a battery will discharge. If a battery has an 8 hour rating of 800 ampere-hours, it might only be adequate for supplying 250 amperes for 1 hour while maintaining voltage above the minimum required level. The current is assumed to be constant during the discharge period, but the voltage will drop as battery capacity is depleted. Main Generator Battery 4-10

66 Load Duty Cycles As you might suspect, the loads on a battery in a nuclear power plant is not constant during the required discharge period. The amperage required as a function of time over the period (coping period) that the battery is required to be the sole supply of electrical power is called the Duty Cycle. Electrical engineers (EEs) have to develop load duty cycles that envelop the required loading to make sure adequate battery capacity is available. If you are requesting service for new loads, or are making changes that affect the time when a load would operate or the amount of load, these changes need to be analyzed to determine the impact on voltage, cable and breaker ratings, as well as battery capacity. The capacity of a generator can be demonstrated by putting it in service and verifying full load rating after equipment temperatures stabilize at their operating levels. Batteries need to be tested by applying load as a function of time to verify the voltage is adequate during the discharge period. Discharge test sets have to be used to simulate the duty cycles because it is extremely difficult to operate the various plant systems with DC loads to simulate the design basis conditions. Since batteries deteriorate over time, discharge tests are periodically performed to assure adequate battery capacity (amperage available with voltage staying above the minimum battery voltage), including appropriate margin. 4-11

67 Safety and Operating Conditions During charging of lead-acid batteries, hydrogen is released. Accordingly, air must be circulated through the battery rooms with large stationary batteries to prevent a build-up of a combustible mixture. Since these batteries contain an acidic electrolyte, precautions need to be taken to protect your eyes and clothing. Eye-wash stations are typically provided in close proximity to each battery room. To assure the DC system is always available the DC system is ungrounded, i.e. the first ground on the system does not prevent any equipment from functioning. Ground detection devices are installed to alarm should either leg become grounded. If a ground is detected, it must be cleared in a timely manner to avoid problems, e.g. a ground on the positive leg and one on the negative leg will allow current to flow via an undesired path. If this is a low-resistance path, the fault current could cause one or more circuits to be tripped via the over-current protection. Care should be taken when working on DC circuits to avoid unintentionally grounding a circuit. 4-12

68 Conclusion You have now learned about the importance of DC systems in a NPP. Caution should be exercised around the batteries and DC system equipment as well as the connected loads. Just as in the case of AC systems, adding DC loads or changing the times when loads will operate can adversely affect the system and needs to be evaluated by the appropriate EE. Now that you've completed this lesson, you can: List the types of DC Systems typically used in nuclear power plants (NPP) State the purpose of these systems Describe the equipment typically used in DC Systems Identify issues you need to be aware of when considering changes to DC systems Describe precautions when working around DC equipment and circuits In the next lesson, you will learn about system interactions and design considerations related to the electrical systems at a nuclear power plant. 4-13

69 5 SYSTEM INTERACTIONS AND DESIGN CONSIDERATIONS Introduction Welcome to the Systems Interactions and Design Considerations lesson. After completing this lesson you will be able to: Describe the effects of the NRC Power Systems Branch [Position] 1 (PSB-1) on the design basis on the auxiliary interactions with a power grid Identify the effects of load changes on the auxiliary power system, other loads, and equipment 5-1

70 Introduction Now that you have a good overview of the type power systems and equipment used in a nuclear power plant, let s take a closer look at some of the interaction issues. Whether you are responsible for some of these systems or equipment, or you depend on them to provide services for your systems or equipment, you should note that the safety related power systems provide power for the systems and equipment required to shut down the unit, mitigate an accident, and maintain the plant in a safe shutdown condition. The time it takes to establish (required) flow under accident conditions is critical. In other words, these systems are important for your safety as well as the safety of the public. Changes you make can affect the power system and the impacts may have cascading effects on various systems and equipment. If the impacts are significant, such changes might challenge the available margin on other electrical or mechanical systems or equipment. Accordingly, changes need to be evaluated for synergistic effects. Let s take a look at some examples. 5-2

71 Voltage Considerations As a refresher, reduced voltage can have a significant impact on the capability of equipment to perform properly since the torque delivered by a motor is proportional to the voltage squared (e.g. at 90% voltage, a motor will only deliver 81% of its rated torque). This can present a problem for pumps, especially when being started. Even if the equipment might operate without being damaged, it may not perform its function in the required time based on accident analyses and licensing requirements. The time it takes to establish flow in a worst case accident situation is critical due to the huge amount of energy being released. Rated voltage is 100% rated, but engineers need to address worst case situations. If the motor torque curve intersects the pump torque curve, the starting time will be adversely affected. The difference in torque between the motor and the pump represents the acceleration torque. Motor operated valves used in safety related or critical applications also need to be analyzed to assure proper operation with minimum expected voltages present. 5-3

72 Discovery Question Now that you have seen an impact of reduced voltages, which of the following changes do you think might impact voltage? A. Control logic B. Failure rate C. Additional loads D. Load characteristics Correct Answer is A. Control logic, additional loads, and load characteristics are all changes that may affect voltage. 5-4

73 Load Additions You have seen how voltage considerations affect the operation of the equipment. Now we will examine the effects of load additions and the impacts on the system. As was noted in earlier lessons, the addition of new loads will impact voltages. When a load is added, this affects not only the bus where the loads are connected, but also any upstream buses between the source and the bus where the load is added. In addition, the buses and loads fed from the affected buses will also be impacted. Granted, the significance of a single change might be very minor (maybe even negligible); however, the cumulative effects of load additions might present a challenge when considering the margin available for the worst case bus or worst case load. Accordingly, it is very important that load additions be reviewed by the responsible electrical group and analyzed when appropriate. If voltages on any buses are lower than previously determined or specified, the loads fed from these buses will have to be reevaluated to verify proper operation. 5-5

74 Control Changes Another factor that can affect the operation of equipment is a control change. A change which affects the time when a load will operate can have a significant impact on voltages. Changes can be either intentional (e.g. changing the set-point on a timer) or unintentional (e.g. replacing a relay with a similar one, however, the new one has a wider operating range or tolerance which allows more variation in the operating time). Note that expected operating times as well as variations due to tolerances need to be considered. Where sequential operations are involved, it is also important to evaluate maximum operating times for one step in combination with minimum operating times for adjacent steps and vice versa. Care must be taken to preclude overlapping motor starts unless the affected systems and equipment are analyzed to verify proper operation under these circumstances with minimum expected voltage available. Let s look at an example. Timing is critical. Just as in the case of your personal checking account, you need to make sure your automatic deposit is scheduled prior to bank drafts or electronic transfers. Controls affecting the power system need to assure system parameters are within desired limits before presenting another challenge. 5-6

75 Load Changes Changes may be made to existing loads or equipment which impact the power system and associated voltages. This can happen through changes such as: 1. Installing equipment other than like-for-like, e.g. using a valve actuator of a different type. 2. Changing the speed on the driven equipment, e.g. changing the pulley ratio or the gear ratio. The impact can be substantial as horsepower is a function of the speed cubed. 3. Modifying equipment in such a way that the operating characteristics are different, e.g. trimming the impeller on a pump, or changing the pitch on a set of fan blades. 4. Changing the system configuration, parameters and/or operating procedures such that flow requirements are different. 5-7

76 Mutual Interactions You will recall the synergy graphic from the beginning of the lesson. The same effect can illustrate change. If you make a change which impacts the loading on the power system, you should coordinate a review with the appropriate group to verify the changes are properly evaluated and the results are acceptable. Furthermore, you need to obtain information as to the minimum voltage allowed at the terminals of your load during critical operations so that you can verify your system or equipment will function properly. If minimum voltages on other buses and loads are lower than previously determined or specified, the affected systems and equipment will also need to be re-evaluated (this would typically only be required for significant changes or in cases where the available margin has been eroded due to the accumulated changes over time). Mechanical engineers should also be aware of frequency variations and limits so they can verify acceptable equipment performance consistent with design basis requirements. 5-8

77 Voltage Margin Since we re dealing with a generating station, many people think voltage margin would never be an issue. This thought is further entrenched when people look at grid voltage, battery voltage, etc., during normal plant operations. However, voltage margin is the difference between the minimum voltage expected during an event compared to the voltage required for the equipment to function properly. Let s look at an example. A 125 VDC system is normally operated at 133 V to maintain the charge on the battery. Let s assume the minimum acceptable battery voltage is 108 V to assure proper operation of the connected equipment. Is the voltage margin 25 V ( ) or 17 V ( )? Neither one; as the system voltage and voltage during normal operation don t address the minimum voltage expected during an accident. If the charger is not available and the battery is supplying the design basis loads, let s assume the minimum expected voltage would be 110 V. In this case, the voltage margin on the system is 2 V ( ). 5-9

78 Degraded Grid Voltage (DGV) Margin Earlier in this course you heard about Power Systems Branch (PSB) 1 and the subject of DGV. The graph to the right illustrates this concept in more detail and shows the available margin. The blue line indicates the change in voltage that might occur when a safety injection actuation (SIA) signal is generated with a DGV condition. Once the voltage drops low enough to cause the Degraded Voltage (DV) relays to drop-out, the timers will start timing and will continue to do so until voltage exceeds the DV relay reset. The margin is the difference between the voltage achieved prior to or coincident with the DV Relay Timeout compared to the DV Relay Reset value. Although a straight line is used in this illustration for simplicity, voltage is very dynamic during this period as large loads are being started to mitigate the accident. Computer models or simulations are used to analyze voltages during such conditions to determine minimum acceptable grid voltages. 5-10

79 Summary Hopefully you have gained an appreciation for the interactions and the cascading effects that occur when load changes are made on the power systems in a nuclear power plant. If margin for load growth has been included in the analyses (on the bus associated with your load addition or change), your changes would have no impact if they are bounded by the margin. If this is not the case, analyses would need to be done to determine the impact on minimum expected voltages. If the amount of load addition is significant, the voltages on other buses may also be impacted. If lower voltages are determined, the affected loads would need to be reevaluated as well. When considering significant changes, options should be considered and discussed with the appropriate groups to minimize impacts while assuring objectives are met. 5-11

80 Conclusion You have now learned about the importance of system interactions in a NPP. Caution should be exercised when making decisions of load changes considering the effects this may have on other systems in a nuclear power plant. Now that you've completed this lesson, you can: Describe the effects of the NRC Power Systems Branch [Position] 1 (PSB-1) design basis on the auxiliary interactions with a power grid Identify the effects of load changes on the auxiliary power system, other loads, and equipment In the next lesson, you will learn about safety and other considerations related to the electrical systems at a nuclear power plant. 5-12

81 6 SAFETY AND OTHER CONSIDERATIONS Introduction Welcome to the Electrical Safety and Other Considerations lesson. Now that you have learned about electrical systems and equipment as well as some interaction issues, let s talk about electrical safety and other considerations that you need to be aware of. After completing this lesson you will be able to: Describe electrical safety precautions Describe electromagnetic and radio frequency interference Describe environmental qualification issues Describe seismic mounting issues Describe fire protection and general considerations 6-1

82 Electrical Safety When working around electrical equipment, precautions must be taken to avoid contact with energized equipment. As you probably know, you can not usually tell from looking at an electrical component or conductor whether it is energized or not. Accordingly, you should maintain a safe distance from electrical equipment consistent with your plant practices and procedures unless you have verified it is deenergized and properly isolated. Remember: You can not usually determine if equipment is energized by looking Electricity is NOT forgiving Electricity travels at the speed of light so you can not see it coming or get out of the way If you get too close, you can get shocked (clearance distances increase with voltage) If you re in proximity, you may be vulnerable to flash hazards Procedures are used in nuclear power plants to assure circuits and equipment are deenergized, tagged out, and grounded before personnel work on any of the connected equipment. Even if a circuit has been isolated, it may still have a residual charge that can be harmful. You should also note that different sources may be provided to some equipment, especially electrical distribution equipment (e.g. a normal and alternate source, a low voltage circuit for space heaters, and one or more DC sources for control and protective features may be common in switchgear assemblies). 6-2

83 Electromagnetic and Radio Frequency Interference Electromagnetic interference (EMI) and radio frequency interference (RFI) can cause some equipment/devices to misoperate or malfunction. Some equipment such as welding machines generate electromagnetic noise or signals. Other devices such as walkie talkies, cell phones, etc., transmit signals at radio frequencies. If these signals are generated in close proximity to sensitive devices, the installed devices may experience a false operation or malfunction. EMI/RFI warning or caution signs are typically posted on cabinets or around equipment which contain EMI/RFI sensitive equipment. If you re involved in activities which require welders or portable equipment to operate in these areas, you need to coordinate with the proper groups to assure these activities are acceptable or scheduled at such time that vulnerable equipment malfunction can be tolerated. 6-3

84 Environmental Qualification (EQ) Issues Safety related equipment must be capable of operating after it has aged, when subjected to postulated accident conditions including radiation and/or the effects of high-energy line breaks. Electrical equipment is more easily damaged by such conditions than Mechanical equipment. Thus, safety related electrical equipment (commonly referred to as Class 1E) located in an area where the accident environment (harsh environment) is different from the norm must be tested, environmentally qualified and properly installed to ensure it will operate in its post accident environment. EQ procedures typically specify sealing materials and procedures to be used when installing and maintaining EQ equipment. 6-4

85 Seismic Mounting All Safety related equipment (both mechanical and electrical) must be capable of operating following an earthquake consistent with the plant s licensing basis. During the initial plant design as well as during subsequent modifications, equipment selection and mounting were evaluated to assure seismic requirements were met. If you re adding new devices, replacing equipment other than like for like, or you re relocating equipment/devices; a review needs to be performed to verify seismic requirements are satisfied. If the modification involves a new or different device (a different mass, configuration or center of gravity) mounted on a panel, the design of the panel has to be reevaluated to assure no adverse impacts on the other devices mounted there. Seismic Monitoring Equipment 6-5

86 Seismic Shaker Test Tables This is a short video of a seismic qualification test. (NOT INCLUDED IN WORD DOCUMENT) Seismic shaker test tables are used to test and qualify equipment based on the mounting configuration and the seismic response spectra for the location where the equipment will be installed. Industry qualification testing is often done to envelope the needs of various utilities. Although this might be an unusual configuration, it demonstrates the robustness of a test and the need to evaluate proper equipment mounting to avoid problems. 6-6

87 Fire Protection Nuclear plants are designed to be safely shutdown should a fire occur. These requirements are known as Appendix R requirements. When making changes to the plant, you need to be aware of changes that could adversely impact these design requirements such as: Replacing a circuit breaker or fuse with a different type/rating, or changing the trip setting on a breaker can affect the Fire Protection Safe Shutdown Analysis. Cable ratings are based on installation criteria and assumptions about temperatures and heat dissipation. Adding fire retardant blankets or closing off normal ventilation paths may degrade cables. 6-7