Standards, Procedures and Policies for Grid Connection

Transcription

1 Standards, Procedures and Policies for Grid Connection CONTENTS 1 INTRODUCTION 1 2 EMBEDDED GENERATION OUTLINE OF EMBEDDED GENERATION CHANGES IN EMBEDDED GENERATION ASPECTS OF EMBEDDED GENERATION 1 3 STANDARDS UK GRID CONNECTION STANDARDS OTHER RELEVANT UK / EU STANDARDS G59 PROTECTION GENERATION COVERED BY G SRI LANKA GUIDE INTRODUCTION OUTLINE OF THE GUIDE STUDIES AND INFORMATION REQUIREMENTS INFORMATION REQUIREMENTS INFORMATION PROVIDED BY THE CEB INFORMATION PROVIDED BY GENERATING COMPANY INFORMATION EXCHANGE - INDUCTION GENERATORS INFORMATION EXCHANGE - DRAWINGS STUDIES REVIEW STABILITY FAULT LEVELS DISTRIBUTION SYSTEM PROTECTION VOLTAGE LEVELS EARTHING INTERCONNECTION PROTECTION REQUIREMENTS INTERCONNECTION PROTECTION CASE INTERCONNECTION PROTECTION CASE INTERCONNECTION PROTECTION CASE INTERCONNECTION PROTECTION CASE INTERCONNECTION PROTECTION CASE INTERCONNECTION PROTECTION SELF COMMUTATED STATIC INVERTERS 10 APPENDIX A - SINGLE LINE DRAWINGS 0

2 1 Introduction Some knowledge of regulatory issues and solutions (relating to the grid connection process), are required to facilitate hydro generator connections. Items covered in this section include: 1. A review of Embedded Generation and connection principles and background 2. The requirements for and examples of UK Standards The regulations and guide developed in Sri Lanka will be used as an example. 2 Embedded Generation The four key principles for an embedded generator connection are: 1. The prime Distribution Network Operator (DNO) requirement and responsibility is that the distribution network should not be unacceptably affected by the generator. That is, the quality of supply to customers is maintained. 2. The generating equipment must not be damaged by the distribution system 3. The generator can operate, and export, as intended 4. Safety is maintained for the generator, distribution system and customers Outline of Embedded Generation 1. Embedded generation is used to describe any generation that is connected to an electrical distribution system 2. An electrical distribution system is any part of the electrical network primarily designed for distribution. They usually operate at up to 40 kv 3. The higher voltage systems are usually referred to as the Transmission System 4. Embedded generation is usually relatively small up to 5 or 10 MW. 5. Generation can be very small 1 kw or less Changes in Embedded Generation The regulations and guidance for embedded generator connection were often based on the principle that the embedded generation capacity was disposable: 1. there was a large amount of system main generation capacity 2. embedded generation capacity was relatively small 3. supply security was not dependant upon embedded generation However, in many places, embedded generation capacity is increasing Aspects of Embedded Generation There is no central control over embedded generation the plant can operate under the owner s control or the availability of the resource. System supply security may (or will?) become more dependant upon embedded generation capacity. DNO s may not know the true load on their network only the net load until there is a loss of generation. DNO s may not know the total embedded generation capacity or availability. DR-S10-RevA.doc Section 10 Page 1

3 3 Standards To facilitate the connection of embedded generation Standards are required to define: 1. The connection application and specification process 2. The interconnection protection requirements for the particular situation 3. The types of protection that may be used and their implementation 4. Long term maintenance and re-testing requirements 5. Quality of supply issues 3.1 UK grid connection standards G59 has been in use for many years for generators up to 5 MW. Currently being revised to version G59/2. G83 covers all forms of micro generation up to 16A per phase (very small generators) typically up to 10kW. These standards cover the interconnection protection only. There are many other standards and regulations covering issues such as safety and quality of supply. 3.2 Other relevant UK / EU Standards Distribution General Conditions (DGC) Distribution Planning and Connection Code (DPC) Distribution Operating Code (DOC) Distribution Data Registration Code (DDRC) G5/3 Harmonics and Flicker CENELEC standard EN50160 voltage levels Electricity Safety, Quality and Continuity Regulations (formerly Electricity Supply Regulations) safety and earthing 3.3 G59 protection G59 suggests the types of interconnection protection that may be required for a variety of situations. The actual requirements will usually be determined by the Distribution Network Operator that you are connecting to. Be aware that the statement comply with requirements of G59 does not define the actual interconnection protection needed. DNO s are becoming more conservative and requiring more interconnection protection. Typical G59 protection requirements: Under and over voltage mandatory Under and over frequency mandatory Loss of Mains some type(s) will almost always be required Reverse Power usually required, but required by the generator anyway New G59 protection requirements are being introduced potentially better for generators two stage tripping with longer delay times possible Generation covered by G59 Generation capacities up to 5MW, connected at up to 33 kv. Sometimes used as guidance for generators above 5MW. Requires on site testing with the DNO of the interconnection protection. DR-S10-RevA.doc Section 10 Page 2

4 G59 is supported by Engineering Technical Report ETR113. This document contains a lot of useful technical information. ETR 113 summary recommendations HV Generating Plant LV Generating Plant Small Medium Large No Export < > 250 kva Export kva kva Under / Over Voltage Yes Yes Yes Yes Yes Under / Over frequency Yes Yes Yes Yes Yes Loss of Mains (1) Yes Yes No Yes Yes Overcurrent Yes Yes Yes Yes Yes Earth Fault Yes Yes No Yes Yes Reverse Power Yes (2) Yes (3) No No Yes (3) Directional Overcurrent (3) Yes Yes No No Yes Neutral Displacement (4) Voltage Yes Yes Yes Yes Yes 4 Sri Lanka Guide 4.1 Introduction The situation in Sri Lanka is used as an example of a situation where the design and implementation of appropriate standards has improved the situation for hydro power operators and the grid utility. The Sri Lanka Guide is used as an example of recent document designed to facilitate embedded generation that has been successfully implemented and used. Sri Lanka has a relatively small grid system approximately 2,500 MW total capacity. The grid system is subject to frequency variations, voltage disturbances and load shedding. Embedded generator capacity increasing, primarily hydro in the range ,000 kw. The UK standard G59 had been used as a basis for interconnection protection requirements. Application and implementation process was not clear for all parties leading to delays in some cases. Embedded generators experiencing frequent nuisance tripping. In 2000 the World Bank funded a program through the Ceylon Electricity Board (CEB) to develop standards and procedures designed to suit the Sri Lankan situation. The outcome was the Guide for Grid Interconnection of Embedded Generation, December The work was undertaken by a combination of Sri Lankan and UK consultants working with the CEB and developers in Sri Lanka. The Guide is available from the CEB in 2 parts and will be summarised here. DR-S10-RevA.doc Section 10 Page 3

5 4.2 Outline of the Guide The Guide includes: Introduction to Embedded Generation Context in terms of grid capacity and stability Procedures for Application and Exchange of Information Costs, studies and metering Inter connection Certificate Testing and Acceptance Procedures Fault levels Voltage regulation Earthing Synchronisation Interconnection protection requirements and Islanding Implementation of Protection Surge Protection Forms and explanatory notes including: Information exchange blank forms Form of Interconnection Certificate Notes on NVD Notes on Earthing Typical AVR model Figures and drawings 4.3 Studies and Information Requirements Prior to allowing a new generator connection, the CEB must study the effect of a new generator connection and any particular requirements for that connection. The studies will include: Grid stability and security Fault Level Grid protection Voltage levels Earthing Load flow Grid operation, protection and safety Information requirements The information required by the CEB to undertake these studies is defined in the Guide. These studies will require information from the proposed generating company. The generating company will also require information from the CEB to design suitable protection arrangements and to ensure the proposed designs to suit the particular grid requirements. The Guide defines the information to be provided by the CEB to a potential Generating Company and by the Generating Company to the CEB. The initial information to be provided by the CEB and the generating company are given in a pro forma in Annex 3 of the Guide. This information exchange is to follow the issue of an LOI (Letter of Intent). The Generating Company shall later provide the following information, prior to acceptance testing: 1. the proposed interconnection protection implementation 2. protection test procedures 3. drawings showing the protection arrangements DR-S10-RevA.doc Section 10 Page 4

6 4.3.2 Information provided by the CEB This information shall include the planned (or prospective) fault levels expected by the CEB in 10 years from the time of connection application. Maximum fault levels (for equipment selection and earthing design) Network design symmetrical fault level (ka or MVA) Peak asymmetrical fault level at half cycle (ka) 3-phase symmetrical fault level at half cycle (MVA or ka) 1-phase to earth fault level (ka) (neglecting earth system resistances) X/R ratio for 1-phase to earth fault (neglecting earth system resistances) X/R ratio for 3 phase symmetrical fault Minimum fault levels (for protection design): 3-phase steady state symmetrical fault level (MVA or ka) X/R ratio for 3 phase symmetrical fault 1-phase to earth fault level (ka) (Neglecting earth system resistances) X/R ratio for 1-phase to earth fault (neglecting earth system resistances) Information provided by Generating Company The CEB may require more detailed information on particular sites or types of generating or protection systems. This information shall be provided by the Generating Company Synchronous Generators with a capacity above 500 kw Site Name Location... Site Reference Number.... Generating Company Name. Contact Point of Supply (location). Maximum export capacity Maximum import capacity... Power factor operating range.. Generator (for each synchronous generator): Terminal voltage (kv).. Machine rating (MVA). Stator resistance (pu) tolerance %. Sub-transient reactance (pu). tolerance %. Transient reactance (pu). tolerance %. Synchronous reactance (pu). tolerance %. Sub-transient time constant (ms)... tolerance(ms).. Transient time constant (ms). tolerance (ms). Transformer (for each generator transformer); Rating (MVA).. Reactance (pu). tolerance %. Resistance (pu).. tolerance %. Voltage Ratio Vector group. DR-S10-RevA.doc Section 10 Page 5

7 Cable or Line between the Generator and Point of Common Coupling where this cabling distance exceeds 50 metres Voltage (V).. Reactance (Ohm) Resistance (Ohm) Where a total generating capacity is less than 500 kw there is a reduced requirement for information from the Generating Company. This information requirement is listed on page A3:4 The CEB may require more detailed information on particular sites or types of generating or protection systems Information exchange - Induction generators Where Induction, or Asynchronous, generators are proposed the same information as required for synchronous generators should be provided where relevant to the induction type generator. Additional information on the particular induction generator proposed may be required. This would be particularly relevant to inrush current limitation method(s) and the method of synchronization Information exchange - Drawings The Generating Company will provide a single line drawing of the installation showing all major components, switching and protection. This drawing shall indicate the Point of Supply and the Point of Common Coupling if different. Typical single line drawings are attached in appendix A. 4.4 Studies review Following is a review of the typical studies that will be required by the network operator. These studies are required to enable the network operator to asses the possibility of allowing a new generator connection and the appropriate interconnection protection requirements Stability The affect on local stability of new embedded generation capacity should be analysed when the capacity of the new plant exceeds 5MW, or when the total capacity on a single distribution line exceeds 5MW. For small generators, typically less than 1MW, the requirement for stability information from the Generating Company may be waived. The Generating Company shall provide a model of the AVR of the proposed generators where the capacity exceeds 5MW Fault Levels The cumulative effect of the embedded generator(s) on the design fault level for the distribution system shall be assessed by the Network Operator. A study should be undertaken when the cumulative fault level reaches 90% of the rating of the associated switchgear, or the design fault level. The Network Operator may require more detailed information from the generator than that specified in the standard forms for information exchange. DR-S10-RevA.doc Section 10 Page 6

8 4.4.3 Distribution System Protection The effect on the distribution system protection ratings and settings shall be studied if any of the following apply: 1. the proposed generating site maximum short circuit current is greater than 20% of the distribution system short circuit current 2. the cumulative short circuit current from all embedded generators on a distribution line will exceed 30% of the distribution system short circuit current 3. there will be a net export of power from the distribution system to the 132 kv transmission system Voltage Levels The nominal voltage at the Point of Supply (POS) shall be stated by the Network Operator in the LOI (Letter of Intent). The voltage rise at the POS must be within operational limits. A two stage approach shall be made to studies: 1. Exclude load connections 2. Include load connections The stage 2 study is required when the stage 1 study indicates a potential problem. Voltage rise at remote locations is often the main limiting factor for the connection of embedded generation capacity Earthing The Guide provides information on acceptable earthing practices and earthing requirements for a variety of situations. An Annex on earthing is included to provide background information on earthing. The Generating Company shall provide information about the proposed earthing arrangement to the CEB. It is the responsibility of the Generating Company to provide adequate earthing at a generating site. The interconnection of generating site and CEB earth systems should be considered for each site situation with reference to the Guide. 4.5 Interconnection protection Requirements The type and size of generator connection has been classified into five cases and the protection requirements defined for each case.the factors that define which case applies are: 1. Generation site capacity 2. Generator type 3. Ratio of minimum captive load on the line and maximum generation capacity 4. Generation capacity and interconnection protection of other generators on the same distribution line and sub station The interconnection protection requirements specified for each case are the minimum requirements. In some situations there may be a requirement for further interconnection protection. If the CEB were to consider that additional protection is required in a particular situation the reasons and requirements shall be clearly defined. The table below summarises the interconnection protection requirements for each case. DR-S10-RevA.doc Section 10 Page 7

9 Case 1 Case 2 Case 3 Case 4 Case 5 Generator type All All See case 3 descript ion Minimum captive load Maximum cumulative export capacity Max site export capacity Under and over voltage protection Under and over frequency protection All See case 5 description L L L L <0.5 x L < 5 MW <0.8 x L >0.8 x L < 5 MW < 5 MW > 5 Mw * >0.8 x L Self commutated static inverters Vector shift protection * ROCOF protection * True ROCOF * protection NVD protection *(1) Intertripping * * * * Other requirements * * * * * = mandatory * = possible additional requirement or alternative A reminder that the interconnection protection specified does not cover all the protection requirements for a generating site. Additional protection will be required. This is the responsibility of the generating company Interconnection protection case 1 All types of Generator. The maximum cumulative export capacity is less than half the minimum distribution line (or captive) load the maximum capacity is less than 5MW. Protection Required: 1. Under and over voltage 2. Under and over frequency 3. Optional (at the discretion of the CEB): 4. Three phase vector shift, subject to generator preference DR-S10-RevA.doc Section 10 Page 8

10 4.5.2 Interconnection protection case 2 All types of generator. The maximum cumulative export capacity is less than 0.8 times the minimum captive load, and the maximum capacity is less than 5MW. Protection Required: 1. Under and over voltage 2. Under and over frequency 3. 3 phase vector shift 4. Optional: 5. True RoCoF may be used as well as vector shift Interconnection protection case 3 All types of generator except mains excited generators defined in Case 5. The maximum cumulative generation capacity is greater than 0.8 times the minimum captive load, such that load/generator balance is possible, and the maximum capacity is less than 5 MW. Protection Required: 1. Under and over voltage 2. Under and over frequency 3. 3 phase vector shift, or true ROCOF 4. NVD 5. Dead line check Alternative Protection As a replacement for the combination of Vector shift and NVD any one of the following may be used: 1. Inter tripping 2. Fault thrower 3. Reverse VAR, where applicable NVD is not required when the maximum export capacity is less than 1MW if the cumulative export capacity on a line is less than 0.8 times the minimum captive load Interconnection protection case 4 All types of generator. The maximum capacity of an Embedded Generation site is greater than 5 MW. It is preferred that the Embedded Generator is connected directly to the primary bus rather than teed into an HV distribution feeder. Protection Required: 1. Under and over voltage 2. Under and over frequency 3. Inter tripping from primary bus intake 4. Parallel earthing or NVD protection Interconnection protection case 5. If the Embedded Generator is teed into a distribution feeder, the following is also required: 6. Inter tripping from the feeder breaker or 7. Fault throwing or 8. Reverse VAR protection where applicable. DR-S10-RevA.doc Section 10 Page 9

11 Generators larger than 5 MW will be encouraged to obtain more secure connections. For large generators remote from the primary bus, adequate security may only be achieved by double circuit connection to the primary bus Interconnection protection case 5 Mains excited asynchronous or induction generator with local power factor correction less than the reactive power demand, or a line commutated inverter. The CEB network/circuit capacitance is not sufficient to self excite the generator. The maximum cumulative connected generation export capacity is greater than 0.8 times the minimum captive load. No synchronous generation or self-excited generation are connected. Protection Required: 1. Under and over voltage 2. Under and over frequency 3. 3-phase vector shift The total generation connected to a primary substation using the vector shift method for loss of mains protection shall not exceed 20MW Interconnection protection Self commutated static inverters Some wind turbines and photovoltaic system inverters are examples of this type of generator. The general requirements are covered with synchronous machines in cases 1-4. However inverters commonly include proprietary protection methods, including ROCOF. It is the responsibility of the Generating Company to demonstrate that the protection meets the acceptable levels of dependability and reliability. David Roberts MorbenHydro November 2008 DR-S10-RevA.doc Section 10 Page 10

12 Appendix A - Single Line drawings NOTE This is a typical single line drawing showing example connection and protection equipment. It does not represent any particular case or situation. Any protection shown here is for illustration and advisory purposes only SYNCH Required for stand alone and grid operation Circuit breakers and contactors should be shown with rating and protection types Point of Supply CB1 Transformer capacity kva and nominal ratio 2 OC I EI NVD Generator 1 Generator 2 as per generator 1 (if applicable) SB EF OV & UV 3 OF & UF 1 EF CB2 SYNCH CB3 Local loads Approximate capacity and use 3 OC I LOM State Generator rating kva and voltage NAND SB EF Earthing transfromer may or may not be required for stand alone parallel opeation of generators For clearance of faults inside the generator, obtain advice from the manufacturer Trip to all generator circuit breakers Figure 3 - Typical Single Line Drawing

13 CB1 Point of Supply for HV connection This breaker may be replaced by fuses for LV connected generators NOTE This drawing shows the required interconnection protection only. Additional generator and earthing protection will be required OV & UV 3 OF & UF 1 CB2 ( optional) Point of Supply for LV connection The interconnection protection m ay operate any breaker to isolate the generator from the grid. For m ultiple generators, the interconnection protection m ay be shared by all generators, or there may be a protection set for each generator Generator 1 SYNCH CB3 Generator 2 as per generator 1 (if applicable) Local loads Figure 4 - Interconnection Protection Arrangem ent for Case 1 DR-S10-RevA.doc Section 10 Page 1

14 This breaker may be replaced by fuses for LV connected generators CB1 Point of Supply for HV connection Point of Supply for LV connection NOTE This drawing shows the required interconnection protection only. A dditional generator and earthing protection will be required OV & UV 3 OF & UF 1 VS 3 ph CB2 (optional) T he Interconnection P rotection m ay operate any breaker to isolate the generator from the grid. For m ultiple generators, the Interconnection P rotection m ay be shared by all generators, or there m ay be a protection set for each generator Generator 1 Generator 2 as per SYNCH CB3 generator 1 (if applicable) Local loads Figure 5 - Interconnection Protection Arrangem ent for Case 2 DR-S10-RevA.doc Section 10 Page 2

15 Inter trip from grid substation and / or distribution breaker CB1 Point of Supply for HV connection NOTE This drawing shows the required interconnection protection only. Additional generator and earthing protection will be required NVD 5 limb VT NVD shall operate CB2 or CB3. CB1 operation is optional OV & UV 3 OF & Reverse VAR UF 1 (optional) CB2 (optional) The Interconnection Protection m ay operate any breaker to isolate the generator from the grid. For m ultiple generators, the Interconnection Protection m ay be shared by all generators, or there may be a protection set for each generator CB3 SYNCH Generator 2 as per generator 1 (if applicable) Local loads Generator 1 Figure 7 - Interconnection Protection Arrangem ent for Case 4 DR-S10-RevA.doc Section 10 Page 3