Renewable Energy Systems 14

Transcription

1 Renewable Energy Systems 14 Buchla, Kissell, Floyd

2 Chapter Outline The Electric Power Grid 14 Buchla, Kissell, Floyd 14-1 THREE-PHASE AC 14-2 THREE-PHASE TRANSFORMERS 14-3 GRID OVERVIEW 14-4 SMART GRID 14-5 POWER TRANSMISSION 14-6 CONNECTION TO THE GRID

3 14-1 Three-Phase AC Single-phase ac is one alternating voltage that can be supplied with a minimum of two conductors (usually color-coded black and white). Split-phase refers to two ac waves that are out of phase with each other by 180 o. Split phase is supplied with three conductors.

4 14-1 Three-Phase AC Three-phase ac is generated by nearly all electrical utilities. Three-phase consists of three sine waves (L 1, L 2, and L 3 ) that have the same magnitude but are separated by 120 o. In North America, the commonly used colors for wires are black, red, and blue for L 1, L 2, and L 3 respectively. Neutral is white.

5 14-1 Three-Phase AC The major advantage to three-phase ac is for industrial operations, particularly applications with three-phase motors. When three-phase is applied to a motor, a naturally rotating field results that allows high torque without the need for starting windings. On the generating side, transmission systems use threephase voltage because it is more economical to produce and transmit than singlephase. This large wind farm has high voltage three-phase transmission towers, dwarfed by the wind turbines. Source: David Buchla

6 14-1 Three-Phase AC Single-phase is derived from three-phase voltage. A common conversion method from three-phase to singlephase is to use a delta (D) connected transformer. In North America, there is 240 V across each secondary, so the voltage from the center tap (neutral) to the coil ends is 120 V.

7 14-1 Three-Phase AC Power was defined in Chapter 2, but is useful to review. True power (P true ) is the actual power that is dissipated in a circuit; it does useful work and eventually ends up in the form of heat. It is measured in watts. Reactive power (P r ) is the rate at which energy is stored and alternately returned to the source. It is measured in units of volt-amps reactive (VAR). Apparent power (P a ) is a combination of true power and reactive power. It is measured in units of volt-amps (VA). Power factor (P a ) is the ratio of true power to apparent power. That is: Ptrue PF P a

8 14-1 Three-Phase AC The power dissipated in a three phase load such as a threephase motor can be determined from the line current and the line voltage as well as the power factor. The line voltage, V L, is measured between any two phases (excluding neutral) and the line current, I L, is the current in any one line. P I V PF load 3 L L A three-phase motor operates on 240 V line voltage and draws a line current of 12 A. If the power factor is 0.88, what is the true power and the apparent power? Pload 3 I LVL PF 3 10 A 240 V 0.88 P a Ptrue 3.66 kw 4.16 kva PF kw

9 Source: David Buchla 14-2 Three-Phase Transformers Power transformers are generally used to step-up or step down voltage. Single phase transformers have a primary winding and a secondary winding on a common magnetic core. For an ideal transformer (no power loss), a basic relationship is P pri = P sec (ideal transformer) where P pri = power delivered to the primary (W) P sec = power delivered to the load (W) Single-phase transformers can be used in a three phase system by using three transformers: one for each phase. This simplifies handling and replacement issues.

10 14-2 Three-Phase Transformers Three-phase transformers generally have three separate but identical primary and three separate but identical secondary windings that are wound on a common core. Normally, all leads are available externally and both the primary and secondary can be connected in either of two common configurations a delta or a wye. Delta Wye Source: David Buchla

11 14-2 Three-Phase Transformers Three configurations are commonly used: Delta-wye The delta-wye transformer can be configured as either a step-up or step-down transformer. The neutral can be brought out from the secondary side where the three windings are joined, so that a single-phase voltage is also available for lighting or electrical outlets in an office.

12 14-2 Three-Phase Transformers Wye-delta The wye-delta transformer is most commonly used in a step-down transformer application. The wye connection on the high voltage primary side provides a place for a ground to be connected at the wye point.

13 14-2 Three-Phase Transformers Delta-delta The delta-delta transformer is useful when three single-phase transformers are connected as a three-phase transformer. Any one of the three transformers can be replaced individually. It can be used to step-up or step-down voltage.

14 14-2 Three-Phase Transformers The current and voltage for three-phase generators, transformers, and motors are described as either phase currents and voltages or line currents and voltages. Phase current and voltages refer to the coils themselves. Line current or voltage refers to L1, L2, and L3.

15 14-3 Grid Overview The electrical grid is defined as an interconnected network of power stations, transmission lines, and distribution stations that transmit power from suppliers to consumers. Voltage is stepped up the source, transmitted at high voltage, and stepped down for consumers.

16 14-3 Grid Overview The grounding system is used as a safety measure to protect workers and others from shock in the event of a failure. It provides a low-resistance path for current that that can flow to ground and trip a breaker when a failure occurs. The grounding system also serves to protect equipment from damage due to a fault or a lightning strike. The ground system provides a path to earth to dissipate the energy. There are many specific requirements that must be met with any grounding system. 8 feet

17 14-3 Grid Overview Standard electrical outlets in North America for 120 V have three connections: a hot wire, neutral, and ground. The neutral carries return current and the ground should normally not have current. Neutral Ground Hot Notice that neutral is larger than the hot line. Source: David Buchla

18 14-3 Grid Overview GFCI circuits can detect a difference in the hot and neutral current and trip a breaker. One outlet on the circuit will have reset and test buttons. The purpose of detecting a difference in hot and neutral current, is that if these currents differ, the return current has found a different path back to the source, indicating a fault. Neutral Ground Reset Test Hot Source: David Buchla

19 Source: David Buchla 14-4 Smart Grid The smart grid generally refers to technology changes, particularly in communications, for the delivery of electrical power. The advent of highly variable renewable power (solar and wind) and the many new distributed resources have changed how power distribution is managed. In some cases, the resource is very far from the consumer.

20 Source: David Buchla 14-4 Smart Grid Smart Grid Implementation Issues 1. Control of electrical distribution. 2. Sensing and measurement technologies. 3. Improved interfaces. 4. Advanced components. 5. Integrated communications. 6. Cyber security.

21 pedrosala/fotolia 14-5 Power Transmission Expansion of the existing grid to include new systems involves a number of issues. For example many of the best wind sites are in the Midwest and some of the best geothermal sites are in the Nevada desert and Great Basin area, away from population centers. To send power long distances involves transforming it to very high voltage for transmission.

22 Source: David Buchla 14-5 Power Transmission At the production site a transmission substation raises the voltage, which is then stepped down for distribution at a distribution substation. Most substations can be controlled from remote locations. In addition to transformers, substations include switches and relays and a variety of protection equipment. Large, oil-filled or compressed air circuit breakers are in the line to trip in case line conditions exceed preset limits by Pearson Higher Education, Inc.

23 Source: Tom Kissell 14-6 Connecting to the Grid Many renewable energy systems are connected to the grid, which has made distributed power much more prevalent. For privately owned systems, the owner can be reimbursed for excess power using a net meter. A net meter An issue with power companies is to insure that private renewable systems are safe (disconnected from the grid in times of outages) and that they provide grid-quality ac. Grid quality ac has very low noise, no noise, flicker, or significant harmonic content.

24 Selected Key Terms Electrical grid Flicker Ground fault interrupt (GFI) circuit breaker Harmonic An interconnected network of power stations, substations, transmission lines, and distribution systems that transmit power from suppliers to consumers A short-lived variation in the electrical power. A special circuit breaker that can trip if the hot and neutral current differ by more than a few milliamps; this condition can occur when a person is receiving a shock. A frequency that is a whole number multiple of another basic frequency.

25 Selected Key Terms Power quality Smart grid Substation Transmission system A measure of the purity of the ac power including the presence of unwanted harmonics and noise on the power line and the frequency. An electrical distribution technology that uses computer-based remote control and automation to improve the delivery efficiency of electricity and to provide information to the customer for optimal power use. A network of switching equipment, control equipment, and transformers that are used to convert voltage to a different level. The system that moves bulk electricity from the transmission substations located at power plants and renewable energy systems to distribution substations.

26 true/false quiz 1. Three-phase means the same as split-phase.

27 true/false quiz 2. When split phase is applied to a motor, there is a naturally rotating field as a result.

28 true/false quiz 3. Reactive power is the actual power dissipated in a circuit.

29 true/false quiz 4. An ideal transformer does not have any power loss in the transformer.

30 true/false quiz 5. Single-phase transformers cannot be used in a three-phase system.

31 true/false quiz 6. Phase current and voltages refer to the coils themselves in a threephase transformer.

32 true/false quiz 7. The grounding system should provide a low resistance path for current in case of a failure.

33 true/false quiz 8. The smart grid includes a cyber security component.

34 true/false quiz 9. A production substation steps down the voltage for transmission.

35 true/false quiz 10. A net meter allows a private homeowner to be reimbursed for power sent to the grid.

36 true/false quiz Answers: 1.F 2.F 3.F 4.T 5.F 6.T 7.T 8.T 9.F 10. T