Guideline for Parallel Grid Exit Point Connection 28/10/2010

Transcription

1 Guideline for Parallel Grid Exit Point Connection 28/10/2010

2 Guideline for Parallel Grid Exit Point Connection Page 2 of 11 TABLE OF CONTENTS 1 PURPOSE Pupose of the document BACKGROUND TO PARALLEL GXP CONNECTIONS What is a parallel connection? Why do parallel connections occur? Type 1: Parallel Connection near one GXP bus Type 2: Parallel Connection between two GXP s - Network Tie Health and Safety responsibilities How are they of interest to the Grid Owner and System Operator? What does the Electricity Industry Participation Code (the Code ) require? SUPPORTING INFORMATION Switching Device Tie Duration Clearing Multi-phase faults Clearing Earth Faults Substations on a spur with an HV star winding Substations on a spur with an HV delta winding Substations in a mesh with an HV delta winding System Power Angle and Magnitude Reverse Power Flow Fault levels Parallel Paths Assessment Template Code Obligations APPENDIX Relevant Code References - System Operator Relevant Code References - Grid Owner and Distributors Version Date Change 2 12/8/05 Rule added to Appendix 2. Revisions to switching device and tie duration updated with Distributor feedback 3 26/10/05 Revised with Distributor comments 4 28/10/10 Updated for the change from EGRs to the new Code references.

3 Guideline for Parallel Grid Exit Point Connection Page 3 of 11 1 Purpose 1.1 Pupose of the document This document is a technical guideline which highlights the issues to be considered when paralleling Grid Exit Points (GXP s). If not managed appropriately, parallel connection of GXP s can compromise operation of the Grid Owners assets ie:protection systems, and can impact on the ability of the System Operator to maintain a secure Grid While this document it is not to be considered as expert technical advice, it does highlight the key issues that need to be considered at a technical level before a parallel connection is made. This document is to be read in conjunction with the document titled: Process for Assessment of Parallel Grid Exit Point Applications. 2 Background to Parallel GXP Connections 2.1 What is a parallel connection? A parallel connection exists between two points when there is more than one path for electrical power flow between two points. 2.2 Why do parallel connections occur? Distribution networks are connected in parallel with the grid for several reasons, most frequently to enable equipment maintenance without disruption of supply. Parallel connections can also be required, following a contingency in a local network, to facilitate restoration of supply to customers. Distributors can typically make two distinct types of parallel connection: 1. A parallel connection between two radial feeders where both feeders are supplied from the same GXP. 2. A parallel connection between GXP supply buses where the system path between the two buses includes enough impedance to potentially cause voltage differences (angular and/or magnitude) between the two buses. Note: Connection Type 2 has the greatest potential to impact on operation of the power system and grid assets. Assessing the implications of this type of connection is the primary focus of this document Type 1: Parallel Connection near one GXP bus The diagram below shows a parallel connection between two radial feeders (A & B) by bridging the two feeders at Y close to the GXP.

4 Guideline for Parallel Grid Exit Point Connection Page 4 of Type 2: Parallel Connection between two GXP s - Network Tie In this figure, parallel paths are formed when GXP A and GXP B are connected via a local distribution network in addition to the normal grid path. This configuration enables the distributor to change its network configuration e.g. a section of network normally supplied from GXP A can be switched over to receive supply from GXP B. 2.3 Health and Safety responsibilities The Distributors and the Grid Owner are responsible for ensuring the safe operation of their assets, and that assets are sufficiently protected. Before paralleling GXP s the Distributors and the Grid Owner need to assess any risks involved, to ensure they are managed appropriately. The document titled: Process for Assessment of Parallel Grid Exit Point Applications outlines the process that distributors and the Grid Owner can follow to assess these situations where necessary. 2.4 How are they of interest to the Grid Owner and System Operator? Parallel connections can impact on the achievement of the System Operators PPO s, the Grid Owners AOPO s, and the operation of protection systems. The Grid Owner and System Operator, in conjunction with the distributor need to assess the impact of parallel connections to identify, any risks that may be involved, and plan to mitigate these as required. 2.5 What does the Electricity Industry Participation Code (the Code ) require? The Code requirements include: Distributors are obligated to agree the temporary or permanent connection of its assets with the System Operator when those assets become simultaneously connected to the grid at more than one point of connection (Part 8, Schedule 8.3, Technical Code A clause 6). Each asset owner will ensure that it provides protection systems for its assets that are connected to, or form part of, the grid. Each asset owner must also ensure that such protection systems will support the System Operator in planning to comply, and complying, with the principal performance obligations and must be designed, commissioned and maintained, and settings must be applied, to achieve the following performance in a reliable manner (Part 8, Schedule 8.3, Technical Code A clause 4). Refer to the appendix of this document for more Code references

5 Guideline for Parallel Grid Exit Point Connection Page 5 of 11 3 Supporting information 3.1 Switching Device In all circumstances suitably rated devices that are safe to use are required. Where large power angles, or voltage magnitude differences occur, the switching device must have the capability to make or break the resulting current flow that will occur. Where two grid exit points are being tied at supply bus and feeder voltage levels, if the switching device is not a circuit breaker then the respective asset owners should ensure that the switching device can safely make or break the expected current. This may require that additional parallel connections are made with suitably rated equipment. The parallel connection must include a switching device which is capable of removing any source of back feed should a fault occur in the Distributors or Grid Owners network while the parallel takes place. If the switching device switches phases one at a time then the asset owner should consider the impact this may have on the operation of sensitive earth fault protection equipment. 3.2 Tie Duration The duration of any tie should be kept to a minimum. The tie duration should be less than 10 minutes. This allows sufficient time to manually operate a circuit breaker or disconnector. This also allows sufficient time to confirm equipment loading and protection systems are correct prior to the tie being broken and load transferred between GXP s. For paralleling situations where protection operation may be uncertain or the tying time is longer than 10 minutes, the Distributors and the Grid Owner are to agree any measures that may be necessary to mitigate the risk. This could be achieved by minimising switching times and/or the use of temporary protection settings. The agreed measures are then noted as conditions in the agreement. Where there are sufficient protection systems in place, the switching time may be extended to a maximum of 30 minutes. 3.3 Clearing Multi-phase faults All multiphase faults on the grid owners HV system must be detected and cleared. HV faults are most likely to be detected and cleared by the Distributors protection at a point close to the tying CB, if discrimination/selectivity principles have been applied. The magnitude of the current may not be large, especially if there are intermediate transformers such as 33/11 kv step down and 11/33 step up transformers included in the current path. Such transformers are generally of high impedance so act as good fault current limiters. However, if the fault current is reduced too much, the grid owners feeder protection cannot be expected to operate. Protection installed in line with current practices, should be time graded away from the grid owners point of supply i.e. the protection should work more quickly, the further you move away from the grid owners site.

6 Guideline for Parallel Grid Exit Point Connection Page 6 of 11 It is likely the protection will become more sensitive the further away the move from the grid owners point of supply. The sensitivity should be such, as to detect any reduced fault level due to step up/down transformers in their normal system configuration and during the periods of tying. Assurances should be sought in advance that adequate protection is installed and set. This sort of information should be recorded in the notes for each tying procedure. A fault on the grid owners HV system will draw current from the Distributor s network in addition to some/all of the grid owners transformer energising current and station load. The relevant Distributor infrastructure probably cannot support this type of load. A tripping in the Distributors system would be expected, rather than trippings being observed in the grid owners equipment for other than normal strong infeeds from the grid to the actual fault. 3.4 Clearing Earth Faults Figure 1: Earth Fault Example Substations on a spur with an HV star winding Refer to Figure 1 above. An earth fault incident at fault position 1 will cause the transmission circuit protection at Bus B to operate in minimum time, leaving the transformer back energised from the Distributors network. In any case, action should be taken to open the transformer LV CB in case the transformer Earth Fault protection is not sensitive enough to operate from the weak infeed or the Distributor s protection does not clear the fault. The CB opening will be prompted by the operation of Bus B transmission circuit protection.

7 Guideline for Parallel Grid Exit Point Connection Page 7 of 11 Earth fault current flows between the transformer HV star point and earth at Bus A. The transformers earth fault protection would be expected to detect the fault, but will probably be set to a final time of 3 or 4 seconds. Phase current would be expected to increase in the Distributor s network and a tripping to occur close to the tying CB Substations on a spur with an HV delta winding Refer to Figure 1. An earth fault incident at fault position 2 will be detected at Bus C by the transmission circuit protection and normal tripping will occur at Bus C. The Bus D transformer HV winding is a delta winding without an earth reference. Unless there is an intertrip from Bus C to Bus D or there is residual protection installed on Bus D, the fault will remain. This becomes a safety issue. The grid owner needs to be vigilant in situations like this, so that during the period of the tie, if the Bus C transmission circuit protection is seen to operate, the Bus D transformer LV CB is opened without delay. There is a chance that the back-energisation current requirements of the transformer at Bus D will cause the Distributor protection to operate near the tie, but this cannot be guaranteed Substations in a mesh with an HV delta winding An earth fault incident at fault position 3 is less of a concern. There is another HV source of current infeed for faults. Fault current would be delivered by the healthy circuit to allow the fault to be cleared. There may as a consequence be a tripping of the Distributor s assets but this is a risk the Distributor would have to manage. When the Distributor notifies they are going to tie feeders, ensure the bus has sufficient other circuits to develop fault current, or fall back to the fault position 1 scenario. Note, if Bus E faults and all of the incoming transmission circuits trip, the transformer LV CB will need to be opened manually. The bus fault will continue to be fed via the tie. Bus faults are rarer than transmission circuit faults but need to be considered. 3.5 System Power Angle and Magnitude When assessing the option of paralleling two GXP s, the system power angle and magnitude will need to be considered. If the power system angle is large when tying, large active power flows will occur and a tripping is likely to occur in the Distributors network. This is where protection is most sensitive. Large reactive power flows can occur if the magnitudes of voltages are significantly different. The switching device that will be tying the two GXP s needs to be rated to make or break the expected currents that could flow. While it is unusual for a Distributor to want to tie two geographically distant sites, the electrical route between two points can still be great. Normally, as long as the shortest electrical route is healthy, the power system angle should be satisfactory. The Distributor should be satisfied their internal power system angle is within suitable tolerances. In some configuration cases, when a circuit trips it is highly likely the tie will trip also. This could be due to either a strong reactive current flow or some fault contribution being made.

8 Guideline for Parallel Grid Exit Point Connection Page 8 of Reverse Power Flow The connection of two Grid Exit Points must not defeat metering. There is always a risk of a reverse power flow as a consequence of there being a tripping in the core grid during tying. There is also a possibility of reverse power flow where the Distributor has embedded generation in excess of the load. Another situation would be when one substation back feeds another i.e. power is moved across the tie. If site metering is not programmed to be bi-directional, power delivered to the national grid i.e. from embedded generation, will not be recognised. Back feeds into a Grid fault must be cleared by either having backfeed protection installed or as described above. Normal operation of the Grid auto-reclosing may be prevented, where synchronism check detects an uncleared out of synchronism back feed. The loss of auto reclosing affects Grid system security. 3.7 Fault levels The fault levels at both the Grid exit point and the paralleling point must be considered before paralleling can take place. The following three impacts must be considered: The connected equipment must be able to withstand the possible fault current, and break this current if appropriate. The step and touch potential within the substation must be at safe levels and the earth mat design is one way of achieving this. The change in fault level may cause protection discrimination problems and protection operating times may be affected. It is important when considering the increased fault levels at a grid exit point that the public must be protected from injury at all times. 3.8 Parallel Paths When tying two grid exit points a parallel link will be created. If the impedance of this link is low enough it will share current flow with the grid. Larger than normal current flow through the distributors network may cause the protection in the distribution network to operate. 3.9 Assessment Template The following template provides a method for Distributors to identify situations where GXP s can be tied, and at what voltage. This along with any necessary supporting technical information will form the basis for assessing applications to parallel GXP s which is detailed in the document: Process for Assessment of Parallel Grid Exit Point Applications 3.10 Code Obligations Appendices 4.1 & 4.2 provide a summary of the Code references highlighting the various parties responsibilities for managing parallel GXP situations.

9 Guideline for Parallel Grid Exit Point Connection Page 9 of 11 4 Appendix 4.1 Relevant Code References - System Operator This list is provided as a guide. references Code Part/ Clause Part 7 clause 7.2 (1a) Part 8 clause 8.25 (1) Part 8, Schedule 8.3, Technical Code A clause 6 Part 13 clause (1a) Part 8, Schedule 8.3, Technical Code A clause 3 (1) Requirement It is not a complete list of applicable rule Principal Performance Obligations Avoid Cascade failure Act as a reasonable and prudent system operator with the objective of dispatching assets made available in a manner which avoids the cascade failure of assets resulting in the loss of demand and arising from frequency or voltage excursions or supply and demand imbalances Other asset owner performance obligations and technical standards grid owner configuration performance obligation Each grid owner must ensure that the design and configuration of its assets (including its connections to other persons) and associated protection arrangements are consistent with the technical codes and, in the reasonable opinion of the system operator, with maintaining the system operator s ability to comply with the principal performance obligations. In reaching this opinion, the system operator must have regard to the potential impact of the design or configuration of those assets or associated protection arrangements on its compliance with the principal performance obligations and achievement of the dispatch objective. Connection of local networks in parallel with the grid Each distributor must agree with the System Operator any temporary or permanent connection of its assets when those assets become simultaneously connected to the grid at more than one point of connection. Circumstances when revised bids or offers will be submitted Prior to the beginning of the trading period to which the bid applies a purchaser will immediately submit revised bid quantities to the system operator whenever - Bid quantities expected to change by more than 20 MW or 10%. The ability of a purchaser to purchase the quantity bid for a trading period at a grid exit point is expected by that purchaser to change by more than 20 MW or 10% of the quantity bid by that purchaser 3. Requirements for assets information In accordance with rule 4.4 of section III, the following information is required by the system operator to assist it to plan to comply, and to comply, with its principal performance obligations: 3.1 Exchange of information Sufficient information must be exchanged between the system operator and the asset owner to ensure that both fully understand the implications of any changes to the asset capability statement or of any proposed connection of the relevant assets to the grid or to the local network. This information must be exchanged in accordance with a timetable agreed to by both the system operator and the asset owner. 3.2 For planning studies by the system operator Where reasonably requested by the system operator, the asset owner will provide sufficient information to the system operator to demonstrate the compliance of its assets with the asset owner performance obligations and the technical codes.

10 Guideline for Parallel Grid Exit Point Connection Page 10 of Relevant Code References - Grid Owner and Distributors This list is provided as a guide. It is not a complete list of applicable rule references. Code Part/ Clause Part 8 clause 8.22 Part 8 clause 8.24 Part 8 clause 8.25 (1) Part 8, Schedule 8.3, Technical Code A clause 4 Requirement Asset Owner performance obligations and technical standards concerning voltage - Voltage range AOPO s Each asset owner will ensure its assets are capable of being operated within the specified range of voltages Load shedding obligations to support voltage Grid owners and distributors to shed load in accordance with the technical codes or as the system operator reasonably requires Grid Owner configuration performance obligation Each grid owner must ensure that the design and configuration of its assets (including its connections to other persons) and associated protection arrangements are consistent with the technical codes and in the reasonable opinion of the system operator, with maintaining the system operators ability to comply with the PPOs. In reaching this opinion, the system operator will have regard to the potential impact of the design and configuration of those assets or associated protection arrangements on its compliance with the PPOs and achievement of the dispatch objective 4 Requirements for the grid and a grid interface (1) Each asset owner and grid owner must co-operate with the system operator to ensure that protection systems on both sides of a grid interface, which include main protection systems and back up protection systems, are co-ordinated so that a faulted asset is disconnected by the main protection system first and the other assets are not prematurely disconnected. (2) A proposed grid interface, including the settings of any associated protection system, must be agreed between the relevant asset owner and the system operator before being implemented. (3) Each asset owner must ensure that sufficient circuit breakers are provided for its assets so that each of its assets is able to be disconnected totally from the grid whenever a fault occurs within the asset. (4) Each asset owner must ensure that it provides protection systems for its assets that are connected to, or form part of, the grid. Each asset owner must also ensure that as a minimum requirement (a) such protection systems support the system operator in planning to comply, and complying, with the principal performance obligations and are designed, commissioned and maintained, and settings are applied, to achieve the following performance in a reliable manner: (i) disconnect any faulted asset in minimum practical time (taking into account selectivity margins and industry best design practice) and minimum disruption to the operation of the grid or other assets: (ii) be selective when operating, so that the minimum amount of assets are disconnected: (iii) as far as reasonably practicable, preserve power system stability; and (b) it provides duplicated main protection systems for each of its assets at voltages of 220 kv a.c. or above, other than busbars; and (c) it provides, for each of its 220 kv a.c. busbars (i) a single main protection system and a back up protection

11 Guideline for Parallel Grid Exit Point Connection Page 11 of 11 Policy Statement, clause 86 system; or (ii) if the performance of its back up protection system does not meet the requirements of paragraph (a), a duplicated main protection system; and (d) it provides duplicated main protection systems for each of its busbars at voltages above 220 kv a.c; and (e) it designs, tests and maintains its main protection systems at voltages of 220 kv a.c. or above in accordance with the requirements set out in Appendix A; and (f) it provides a circuit breaker failure protection system, that need not be duplicated, for each circuit breaker at voltages of 220 kv a.c. or above. Circuit breaker duplication is not required; and (g) protection system design for a connection of assets to the grid at lower voltages must be similar to existing design practice in adjacent connections of assets to ensure coordination of protection systems 86. Where, pursuant to rule 6 of technical code A of schedule C3 of part C, a distributor requests agreement of the system operator to connection of local networks in parallel with the grid, the system operator will require the distributor to provide the following: Any information reasonably required by the system operator to evaluate the consequences and determine conditions that may apply (and the system operator will list on its website the type of information generally required) Written evidence of the grid owner s agreement to the simultaneous connection of those assets.