DSS/ 007 / 010 Code of Practice for the Protection and Control of HV Circuits

Transcription

1 Version:- 1.1 Date of Issue:- Jan 2011 Page 1 of 30 DSS/ 007 / 010 Code of Practice for the Protection and Control of HV Circuits 1 Purpose This code of practice details the network protection philosophy covering the High Voltage (HV) networks. There is a legal obligation to provide protection to meet the requirements of the Electricity Safety, Quality and Continuity Regulations 2002 and the Electricity at Work Regulations This code of practice is driven by our Quality of Service (QoS) performance, specifically reduction in number of faults, reduction in number of customers per fault and improvements in fault restoration times. The document should be considered in conjunction with IMP/001/912 Code of Practice for the Economic Development of the HV system. This document supersedes the documents detailed in Section Scope This document applies to both rural and urban HV circuits (main line and spur protection) with a nominal operating voltage of between 1kV and 22kV. All other voltages are covered by DSS/007/001 Code of Practice for the Protection of High Voltage Networks (TS1). This code of practice shall be applied to all HV system development including new connections, system reinforcement and asset replacement. It is not intended to apply this code of practice retrospectively. 3 Policy 3.1 Assessment of Relevant Drivers The key internal business priorities relating to the protection and control of HV circuits are: Employee Commitment achieved by developing a safe HV system to ensure that members of the public and employees are not exposed to risks to their health as far as reasonably practicable; Financial Strength contribute towards maximising IIS rewards and minimising operating costs; Customer Service achieved by reducing the impact of fault incidents on customers Operational Excellence achieved by reducing the potential number of customer interruptions and customer minutes lost The external business drivers relating to the protection of HV systems are detailed in the following sections Requirements of the Electricity Safety, Quality and Continuity Regulations The Electricity Safety, Quality and Continuity (ESQC) Regulations 2002 impose a number of obligations on Northern Powergrid, mainly relating to quality of supply and safety. All the requirements of the ESQC Regulations that are applicable to the protection of HV circuits shall be complied with and the Northern Powergrid distribution systems shall be designed to comply with these requirements. Regulation 3 states that generators, distributors and meter operators shall ensure that their equipment is so constructed, installed, protected (both electrically and mechanically), used and maintained as to prevent danger, interference with or interruption of supply, so far as is reasonably practicable. This code of practice details the processes to be followed in order to ensure that the network is designed to limit the number of people that are affected by a fault and hence comply with these requirements. Regulation 6 states that A distributor shall be responsible for the application of such protective devices to his network as will, so far as is reasonably practicable, prevent any current, including any leakage to earth, from flowing in any part of his network for such a period that part of his network can

2 Version:- 1.1 Date of Issue:- Jan 2011 Page 2 of 30 no longer carry that current without danger. This code of practice details the process to be followed in order to ensure that the protection fitted to HV circuits meets these requirements Requirements of the Electricity at Work Regulations 1989 Regulation 5 of the Electricity at Work Regulations 1989 states: No electrical equipment shall be put into use where its strength and capability may be exceeded in such a way as may give rise to danger and places obligations on the business relating to the safety of plant and equipment used on the distribution system. It requires that plant and equipment is designed and operated within the limits of its capability Requirements of Distribution Licences The Distribution Licenses held by Northern Powergrid contain a number of conditions to be complied with which are relevant to system design. In particular, Standard Licence Condition 24 requires the distribution system to be planned and developed to a standard not less than that set out in Engineering Recommendation P2/6 (2006) Security of Supply. This CoP requires that the HV distribution system is designed to at least the standard required by ER P2/6. In addition, Standard Licence Condition 21 refers to the Distribution Code and requires that the licensee must take all steps within its power to ensure that the Distribution Code in force remains a code approved by the authority that complies with the requirement that the Distribution Code must be designed so as to permit the development, maintenance and operation of an efficient, co-ordinated, and economical system for the distribution of electricity. The code of practice requires that the HV distribution system is designed in line with the Distribution Code and therefore in an efficient, coordinated and economical manner as required by the Electricity Act. 3.2 Key Requirements The general objective in developing the protection for the HV network is to ensure the safety of public and staff and to minimise QoS indicators through the management of the number of customer interruptions, the number of customers affected and the restoration time of faults within the constraint of the customers willingness to pay, This code of practice helps ensure that all HV circuits are protected in a manner which: Prevents, as far as reasonably practicable, danger to members of the public and staff; Optimises network security and availability; and Satisfies all other relevant obligations. 3.3 Background Design Policy The objective of this code of practice is to ensure safety and contribute to achieving our strategy to deliver an economic level of reliability in line with the willingness to pay of our customers. This is achieved through the management of the four variables: Number of interruptions; Customers per fault; Restoration time, and Cost. Different design approaches can be used to improve these base measures and this code of practice aims to detail the approaches to be used.

3 Version:- 1.1 Date of Issue:- Jan 2011 Page 3 of Design Guidelines This code of practice shall be read in conjunction with the relevant Engineering Recommendations and other CE Electric UK documents including the following: Code of Practice for the Economic Development of the HV System (IMP/001/912) Code of Practice for the Application of Lightning Protection (IMP/007/011) Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001) Code of Practice for the Setting of Protection and Associated Equipment (TS16/17) (DSS/007/007) Design of HV circuits should be performed using the following design process: Collect circuit information Identify main and spur lines, customer numbers and their distribution Design protection for main line - design source protection (see section 3.4): - position pole mounted autoreclosers (section 3.4.1) Design protection for spurs (see section 3.6): - position auto sectionalising links (section 3.6.1) - design required fuse protection (section 3.6.2) Complete additional circuit protection: - ferreoresonance assessment (section 3.7) - position lightning protection (section 3.8.1) - position fault passage indicators (section 3.8.2) A summary of the process to be followed can be found in Appendix Network Configuration The main purpose of the HV system is to distribute electricity in localised urban and rural areas in an economic, efficient, safe and secure manner, meeting the needs of electricity supply to customers currently and likely to be connected to it. The HV system supplies HV/LV substations, larger demand and generation customers at HV, systems owned and operated by Independent Distribution Network Operators (IDNOs) and is designed in line with IMP/001/912 Code of Practice for the Economic Development of the HV system.

4 Version:- 1.1 Date of Issue:- Jan 2011 Page 4 of 30 Urban HV systems normally comprise underground cables and looped ground mounted HV to LV substations. Rural HV systems normally comprise of a mixture of underground cables, ground mounted HV to LV substations and overhead lines with teed pole mounted HV to LV transformers. The Northern Powergrid HV systems predominantly operate at 20kV and 11kV, with a small proportion of lower voltage (5.25kV to 6.6kV) assets associated with specific customer supplies or small pockets of old industrial areas of urban centres. The majority of 20kV systems serve more sparsely populated rural areas, typically Northumberland whereas the 11kV systems serve more urban areas such as Tyne and Wear, Durham, Cleveland and most parts of Yorkshire. 3.4 Main Circuit Protection Details of the process and requirements for source circuit breaker protection can be found in: Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001) Code of Practice for the Setting of Protection and Associated Equipment (TS16/17) (DSS/007/007) Protection of the main circuit should be considered first. The protection of spurs should be excluded in this section and protection of the main line should be implemented in the following manner Auto-Reclosers Functional Specification The functional specification for auto-reclosers shall be in line with the latest version of NPS/001/009 Technical Specification for 11kV, 20kV and 33kV Pole Mounted Auto-Reclose Circuit Breakers. The protection functional requirements are as follows: 4 trips to lockout with any combination of tripping characteristics. Over Current / Earth Fault (OC/EF) and Sensitive Earth Fault (SEF) sequences to be independent of each other. Selectable Sequence Co-ordination. Individually adjustable Dead Times Adjustable reclaim times Sequence co-ordination. Cold Load Pickup. Magnetising Inrush Restraint. Voltage Measurement on the incoming terminals with provision for an external Voltage Transformer (VT) for measurement of voltage on the load terminals. All auto-reclosers fitted onto main line circuits shall be equipped with remote control Design Rules A summary of the design process can be found in Appendix 1. Source Protection Zone All HV circuits with greater than 1km of overhead line (including spurs) shall be equipped with autoreclose facilities either at the source or at another circuit breaker (see below). The functionality and operation of the primary breaker should then be considered. If the breaker has a multi shot capability, the use of this should be used.

5 Version:- 1.1 Date of Issue:- Jan 2011 Page 5 of 30 Note that consideration should be given to disabling auto-reclose on the primary breaker and installing an auto-recloser at the beginning of the first main overhead line section where the following applies: the first leg from the Primary substation consists entirely of underground cable which feeds more than 50 customers; the primary substation breaker does not have multi shot capability. Primary CBs and Pole Mounted Auto Reclosers (PMARs) being used to provide the source zone autoreclose facilities should be set in accordance with section Further Protection Zones - Location of PMARs In determining possible locations for a PMAR the following should be considered: the current capacity and breaking capacity of the auto-recloser; and staff access requirements. In general the provision of a PMAR on an overhead line distribution circuit will result in a reduction in the number of customers per fault and the fault restoration times on that circuit. The improvement in performance is a function of the location of the PMAR, and is greatest at a particular point in the line. However, it may not be desirable to locate the recloser at this point, for example accessibility may be difficult, and so it is necessary to estimate the benefit of a number of alternative locations and select the most advantageous. The performance benefit is a function of many parameters with wide tolerances, including fault rates, outage times, customer distribution etc. To simplify the criteria for provision of PMARs, guideline rules have been developed to describe the number and location of reclosers that should be installed on a given circuit depending upon its make up and configuration. The objectives include: No customer should experience more than four interruptions of three minutes or longer per annum. No more than 500 customers between remote control points (note that all main-line PMARs are to be fitted with remote control). No more than 2000 customers per circuit. A maximum of three auto-reclose devices should be used in series at 11kV and four at 20kV. No more than 200 customers between switching points. Additional auto-reclose protection zones should be installed where the potential annual gain in customer interruptions (CI) and customer hours lost (CHL) as calculated by approved software meets the following requirements (calculated for DPCR5 IIS incentive rates): (CI saved) + 2*(CHL saved) > 100 Any situation outside of these parameters should be referred to the Design Manager for guidance Application of Settings Selecting protection settings on overhead line circuits is a balance between speed of operation to reduce the amount of energy released at the point of fault and the deliberate insertion of time delays to create grading points and so reduce the number of customers affected by a fault isolating only the part of the network necessary to clear the fault. There is also the need to ensure that spur-line fuses and sectionalisers operate as intended and a delay may be required for this.

6 Version:- 1.1 Date of Issue:- Jan 2011 Page 6 of 30 For new overhead lines that are of short or medium length the deliberate delaying of protection to create grading points shall be employed but should be restricted to one grading point. Standard settings will be adopted on short and medium length lines. Where an existing short or medium line is being redeveloped the minimum time delay policy set out in the preceding paragraph shall be implemented. For new lines that fall into the long category the protection shall be graded using both time and current settings. Opportunity shall be taken whenever an existing long line is being refurbished or reconfigured to implement this protection policy. In the case of overcurrent and earth faults, fuses and sectionalisers should, where possible, discriminate with up-stream protection. It is accepted that in some cases 100% discrimination can not be achieved and in most cases discrimination with the SEF settings we normally use will not be possible. General Where the protection in use is currently instantaneous, fault sequencing should not be used however, the facility to employ this in the future should be included to allow for its use should the current drivers on quality of supply change. Where sequencing is currently employed in conjunction with instantaneous settings with the intention of minimising the number of transient interruptions seen by customers, then this should be maintained. Where auto-reclosers are connected in series and delayed protection is enabled then fault sequencing should be used on the recloser with the time-delay settings. Due to the different protection characteristics available on each of the reclosers currently in use and on those proposed for purchase, only units of the same fault interruption type and fitted with configurable relays having suitable characteristics shall be installed in series. Overcurrent Settings These should be set in accordance with DSS/007/007. Earth Fault Settings These should be set in accordance with DSS/007/007. Note that earth fault settings should be a minimum of 80 amps and discriminate with any fuses and sectionalisers (20% margin on actuating current) installed in the zone. Sensitive Earth Fault Settings All HV circuits greater than one span of overhead line shall be equipped with SEF protection. The operational policy for try-in post a confirmed SEF protection operation is currently under review at a national level in conjunction with the HSE and therefore the auto-close arrangements shall continue to be consistent with the operational procedure applied by the control centre for each circuit. Where the control room operational procedure is to wait at least 30 minutes prior to try-in of the circuit then the SEF protection shall be set to inhibit a re-closer sequence so that SEF operation causes a trip and lockout of the circuit breaker. Where the control room operational procedure is to initiate a try-in immediately after an earth fault operation then the SEF protection shall be set to initiate an autoreclose sequence. Where a separate SEF sequence is available it should be selected to 2 trips to lockout. Short and medium line lengths For protection purposes a line will be classified as short if the phase to phase or three phase fault current at the most remote point on the feeder is 800A or greater at 11kV or 1600A or greater at 20kV and of medium length if the fault current at the most remote point on the feeder is 400A or greater at 11kV or 800A or greater at 20kV. Other than the setting for the source Inverse Definite Minimum Time (IDMT) relay which will be calculated, settings at all other relaying points will selected from a standard

7 Version:- 1.1 Date of Issue:- Jan 2011 Page 7 of 30 list approved by the Technical Services Manager and issued in DSS/007/007 The setting of protection and associated equipment. The operating sequence for all source zone auto-reclose protection equipment on the HV system will be 2 instantaneous trips followed by one IDMT trip to lockout with the IDMT set to a 0.7 second clearance time in accordance with DSS/007/007. Where there is one pole mounted circuit breaker in series with the source circuit breaker the pole mounted circuit breaker will be set to 4 instantaneous shots to lock out. Where there is a second PMAR in series with the first, the first auto-reclose circuit shall be set to 4 instantaneous shots to lockout with a 120millisecond delay and sequencing selected for each trip. The second most remote PMAR shall be set at 4 instantaneous trips to lock out. Any further down stream grading points will be achieved through the use of sectionalisers with different pulse settings to trip i.e. grading shall be achieved by trips to lockout. Diagram 1 below illustrates the basic configuration for the maximum number of circuit breakers in series. 3 Stage Auto-Reclose Scheme - Diagram 1 Ground Mount Auto Reclosing Circuit Pole Mounted Auto-Recloser Pole Mounted Auto Recloser Sectionaliser Protection 2 Instantaneous 1 IDMT Protection 4 Definite Time Each 120millisec Delay and sequencing Protection 4 Instantaneous No Delay Protection 3 Instantaneous Opens in third Dead Time SEF 20 Amp 10 Second Delay SEF 16 Amp 7.5 Second Delay SEF 16 Amp 5 Second Delay

8 Version:- 1.1 Date of Issue:- Jan 2011 Page 8 of 30 Long Lines Long lines where the phase to phase or three phase fault current at the most remote point from the source is below 400A at 11kV and below 800A at 20kV will have each protection point individually calculated to ensure that the customers at the end of the lines are adequately protected. Additional protection points may be added to provide further sectionalisation where this is considered economically beneficial. Each additional protection point will require increasing the time delay of the instantaneous settings of the relevant protection by 120milliseconds to ensure grading. The maximum permitted time delay of the instantaneous protection at the source circuit breaker is 360millisec. Where time delays are used in conjunction with instantaneous settings protection sequencing will also be used. Auto-Reclose Sequences The dead time and reclaim time for auto-reclose relays at source protection points with solenoid or motor charged circuit breakers will be set as detailed below: For multishot relays Dead Time Reclaim Time 10.0 seconds 7.5 seconds The dead time and reclaim time for pole mounted auto-reclose relays not at source protection locations will be set as detailed below: For multishot relays Dead Time Reclaim Time 5.0 seconds 10.0 seconds The Use of Magnetising Inrush Settings Magnetising inrush restraint settings will not normally be applied at protection points where instantaneous protection is activated however the use of instantaneous protection settings may cause problems when attempting to restore supplies after a transient fault due to high transformer magnetising inrush currents in some networks. Where problems are identified due to the transformer capacity on a section of network the magnetising inrush restraint feature shall be activated. Network conditions where consideration should be given to applying a magnetising inrush restraint setting are:- PMARs with a 400A setting will have a magnetising inrush restraint setting applied if the installed transformer capacity is above 5MVA. PMARs at positions in the network with lower fault levels and a 200A setting will have a magnetising inrush restraint setting applied if the installed transformer capacity is above 2.5MVA. Notes: Gas-filled Vacuum Recloser (GVR) PMARs do not have facility for selecting a magnetising inrush restraint setting. All MVA values relate to summated transformer capacity not load.

9 Version:- 1.1 Date of Issue:- Jan 2011 Page 9 of Arc Suppression Coils Arc Suppression Coil (ASC) earthing works in conjunction with the existing overhead line protection. The coil will minimise the fault current for all single phase to earth faults whilst it is not bypassed. If the fault current disappears within bypass time no other action is initiated and the supply will not be disconnected, resulting in customers experiencing fewer interruptions and an improved quality of service. After a set period of time, the coil will be bypassed which will allow the protection to deal with interruptions caused by permanent single phase to earth faults and interruptions due to all other types of faults. During the time a single-phase to earth fault is on the system and before the ASC is bypassed most faults will fully displace the system neutral. This will cause a large unbalance in the capacitive leakage to earth between the three phases. Standard earth fault relays (i.e. non-directional) will detect this unbalance as earth fault current. Normal earth fault settings are usually too high to be affected by this unbalanced current however, SEF settings can be in range and care must be taken to set the bypass time within SEF operating time where this occurs Functional Specification The functional specification for the installation of new arc suppression coils shall be as follows: Design Rules An automatically tuned ASC with associated controller. The HV network connected to the primary substation determines the coil size required. The actual coil rating chosen will be 50% greater than the initial requirement, in order to future-proof the site and allow for operational switching. Shunt circuit breaker with auto-opening and auto-closing protection scheme. Standby earth fault switching scheme as required to compensate for the lower earth fault levels with a single Neutral Earthing Resistor (NER). Fitting of disturbance recorders as necessary. Replacement / addition of neutral earthing resistors as necessary. Liquid filled resistors in a poor condition will be replaced with dry type resistors. Protection panel for housing of controller and associated protection scheme Four isolators to allow for maintenance of the ASC equipment and transformer outages Balancing of single phase spur lines as necessary to reduce the standing coil voltage to an acceptable level. SCADA alarm of coil operation SCADA control of by-pass CB. A summary of the design process can be found in Appendix 1. ASCs are currently installed to cover a substantial part of the overhead line network. When any changes are made to feeders out of a site which contains an ASC, then the impact of the changes on the ASC should be considered. ASCs will be installed at selected worst performing primary substations. These investments are driven purely by quality of service performance. Installation is only beneficial at sites where the feeders are made up from a high percentage of overhead line (i.e. cable forms typically no greater than one third of the network). The performance of the feeders out of the sites will be assessed and the decision made to apply ASC earthing to those sites which have the highest number of overhead line faults and the greatest length of normally connected overhead lines. The expected benefits should then also be considered. As the more effective sites are equipped with these installations, the per site benefits can reasonably be expected to decrease. The utilisation of ASC earthing is expected to provide a 10% reduction in permanent faults on overhead lines and to reduce short interruptions by 50% at the selected substations.

10 Version:- 1.1 Date of Issue:- Jan 2011 Page 10 of Spur Protection Where economical, all spurs, apart from those in the final zone, need to be individually protected at their point of connection to the main line. Major spur 5km or more of overhead line. Major spurs should be protected by auto-reclosers wherever possible provided this does not compromise the main line grading points. Medium spur Spurs between 0.5km and 5km in length. Medium spurs should be protected by an sectionaliser unless prospective fault currents coupled with clearance times are too high for the spur conductors. Minor spur Spur with less than 0.5km of overhead line. The fault risk associated with these is relatively small and it is difficult to justify the costs of retrofitting protection to existing sites. Therefore where there are no existing links, minor spurs should be left solid. Where solid links are currently fitted and in new locations, minor spurs should be protected by fuses unless they are in the final auto-reclose section where an sectionaliser should be used. During an overhead line rebuild the opportunity should be taken to protect all minor spurs with fuses or sectionalisers as appropriate. The following table shows the appropriate protection to be used at the main line connection point. Spur Type Source A/R Zone Middle A/R Zone Final A/R Zone Major Spur A/R A/R 3 shot A/S Medium Spur 2 shot A/S 2 or 3 shot A/S 2 or 3 shot A/S Minor Spur 2 shot A/S or fuse 2 shot A/S or fuse 2 shot A/S *Check fault currents and clearance times are acceptable for the conductor sizes in use. If not suitable for the use of sectionalisers then fuse, or exceptionally, a recloser should be used. Diagrams presenting an overview of the protection to be applied to spurs can be found in Appendix 4. Consideration should then be given to adding further discrimination on major and medium spur lengths Auto-Sectionalising Links Functional Specification The functional specification for auto-sectionalising links shall be in line with the latest version of NPS/001/032 Technical Specification for 11kV, 20kV, Pole Mounted Auto-Sectionalising Links (ASLs). With regard to the protection functional requirements, these are as follows: ASL s are designed to recognise a pre-determined sequence or fault currents and then, during a period when the upstream protective device is open, to disengage and drop-out to the isolating position. All actual sectionalisers deviate to varying degrees from the ideal characteristics required of a sectionaliser. The deviations can take the form of requiring a pre-charge from load current to operate correctly or requiring the fault current to be on for a minimum time. Any such deviations will be determined at the time of tendering to ensure that the devices remain suitable for the locations where they are to be fitted. The ASL s shall have load current ratings available of between 15 and 112 Amps. ASL s shall have a reclaim time of 25 to 30 seconds ASL devices are required to be capable of either two or three shot operation.

11 Version:- 1.1 Date of Issue:- Jan 2011 Page 11 of Design Rules A summary of the design process can be found in Appendix 1 and a guide to calculating the required settings found in Appendix 5. The location of the sectionalisers on an overhead circuit selected for auto-reclosing shall comply with the following guidelines but, when applying them, consideration must be given to local conditions affecting access. Note that additional rules may be introduced from time to time to cover any restrictions as to where the types of sectionalisers being purchased can be used. Normally spurs with more than 5km of overhead line shall be protected by an auto-recloser unless they are in the final main-line protection zone in which case a 3-shot sectionaliser should generally be used Normally spurs between (0.5 km) and 5km shall be protected by an sectionaliser. In new locations, where solid links are currently fitted and during a major rebuild of the main line normally spurs less than 0.5 km long shall be fused however consideration shall be given to the installation of a sectionaliser if the number of connected customers exceeds 50. Note that minor spurs in the final protection zone should be protected by a 2-shot sectionaliser as fuses will not operate with all instantaneous shots on the auto-recloser. Where spurs less than 0.5 km are solidly connected to main lines then these should generally be left solid. However, the opportunity should be taken during an overhead line rebuild of the main line to apply protections to all short spurs. Fuses should not normally be used to protect spurs containing TSG s. Where a spur is located in the first protection zone from the primary, sectionalisers should be limited to a maximum of two counts. Care must be taken to ensure that the maximum fault current and prospective clearance times are compatible with the conductor sizes in use. Particularly in the source protection zone this may limit the use of sectionalisers and fuses will have to be used instead. Care must be taken to ensure that devices are only used within their fault rating e.g. new sectionalisers are limited to 10kA maximum through fault current for one second. Care must be taken to ensure that the manufacturer s recommendations on the discrimination between actuating currents and minimum auto-reclose relay settings are maintained., e.g. the AB Chance 25 amp (load current) rated sectionaliser has an operating current of 1.6 times this, i.e. 40 amps and the recommended minimum recloser setting is 50 amps Details of the fault rating of small overhead conductors are shown in the section on design rules in Appendix 1.

12 Version:- 1.1 Date of Issue:- Jan 2011 Page 12 of Fuses Functional Specification The functional specification for pole mounted expulsion fuses shall be in line with the latest version of NPS/001/004 Technical Specification for 11kV, 20kV and 33kV Pole Mounted Expulsion Fuses and Solid Links Design Rules A summary of the design process can be found in Appendix 1 and a guide to calculating the required settings found in Appendix 5. Fusing shall be used in the first two protection zones only. Normally spurs of less than 0.5 km length that are solidly connected to a main line should be left solid. Spurs of less than 0.5 km in length that are fitted with solid links shall be fused however consideration shall be given to the installation of a sectionaliser if the number of connected customers exceeds 50. Spurs containing TSG s should not be fused unless this is unavoidable because of fault level and conductor size factors.. Any existing fuses protecting ground mounted transformers shall be replaced with pole or ground mounted circuit breakers. Care must be taken to ensure that devices are only used within their fault rating Application of Settings The maximum fuse size should be restricted to 50A for minor spurs to allow for grading to successfully occur. The fuse size should be matched to the size of transformers being protected. 3.7 Ferroresonance Ferroresonance is a term used to describe series resonance in an alternating current circuit consisting of non-linear inductance (the transformer) and capacitance (the HV underground cable). In a distribution system, this condition is most likely to occur when lengths of underground cable are connected in an overhead system. The ferroresonant state can occur when energised and deenergised phases occur in the same section of line leading to the generation of overvoltages. When this occurs, damage to both customers equipment and the distribution system can result. Ferroresonance is also possible during system fault conditions where blown fuses leave the system supplied single phase. Tests indicate that the condition is most likely to occur during de-energising. approximately 5% load is required to dampen ferroresonance. However, only The failure of surge arresters under these conditions is well known. It should be noted that with gapped surge arresters this effect could go unnoticed as the resonant overvoltage would be below the sparkover level of an 11kv surge arrester. However, in the case of metal oxide surge arresters some failures will occur. When a line is refurbished then gapped or porcelain surge arresters should be replaced. The failure mode with a polymeric housed surge arrester of proven short circuit capability is not a violent explosion but nevertheless injury could be caused if personnel are near to the failing arrester.

13 Version:- 1.1 Date of Issue:- Jan 2011 Page 13 of 30 Some conditions when ferroresonance can occur are: During single phase switching operations. Operation of a single-phase over-current protective device. When one or two phases of a three phase circuit become open-circuited (blowing of a fuse, breaking of a line conductor, etc). The increase in the number of incidents of ferroresonance reported is due to modern trends in circuit parameters: Increased use of underground cables in rural systems. More three phase transformers on rural systems. Highly seasonal three phase loads resulting in some transformers operating at very light load. Improved material design of transformers (lower excitation currents). Although measures to protect against ferroresonance should be considered, these measures will normally only be applied where the line is new build or significant alterations have occurred Design Rules A summary of the design process can be found in Appendix 1. Where possible, avoid using HV underground cable in rural situations. Ferroresonance will not occur where the system being energised or de-energised comprises entirely of HV overhead lines. Where HV underground cable exists or is installed, the following rules shall be adopted: Where there are more than 10 transformers in the circuit to be energised (or de-energised), ferroresonance is unlikely to be a problem and no action is required. Where there are less than 4 transformers in the circuit, each transformer should be equipped separately with solid links at the transformer position. These links will be removed before the tee is disconnected and replaced after the tee is re-connected. Where there are between 4 and 10 transformers in the circuit, a 3 phase switching device, for example an isolator, should be installed at the tee off position in addition to any spur protection (sectionalisers). In exceptional circumstances (eg fault level above the rating of sectionalisers) an auto-recloser can be used in this position. In this case this should be set to instantaneous protection with the number of shots to lockout set to mimic a three or two shot sectionaliser as appropriate.

14 Version:- 1.1 Date of Issue:- Jan 2011 Page 14 of Critical Cable Lengths to avoid Ferroresonance Critical cable lengths have been calculated for both the 11kV and 20kV systems. The following tables indicate the maximum aggregate cable length in section without the risk of Ferroresonance: Critical 11 kv Cable Lengths to Avoid Ferroresonance Aggregate transformer capacity Cable Lengths in Metres mm 185 mm 300mm Critical 20 kv Cable Lengths to Avoid Ferroresonance Aggregate transformer capacity Cable Lengths in Metres mm 185 mm 300mm

15 Version:- 1.1 Date of Issue:- Jan 2011 Page 15 of 30 Table for XLPE triplex cable Aggregate transformer capacity 11kV cable lengths in Metres 20kV cable lengths in Metres 95 mm 185 mm 300 mm 95 mm 185 mm Ancillary Circuit Protection Lightning Protection Lightning protection should be applied in line with IMP/007/011 Code of Practice for the Application of Lightning Protection Fault Passage Indicators Functional Specification The functional specification for fault passage indicators shall be in line with the latest version of NPS/001/014 Technical Specification for overhead line Fault Passage Indicators (FPIs). With regard to the protection functional requirements, these are as follows: Initially the FPI s shall be supplied preset as follows:- o o Default current settings 21A Delay 150 ms As an example the Bowden Pathfinders should be preset to: o o o o SEF and Instant 21 A Inrush 98 ms, Instant 148 ms, IDMT 150 ms 3 hour flash time and 60 seconds line trip Type D (transient fault reporting enabled) o The above corresponds to setting codes of The FPI s shall be designed such that earth fault sensitivity levels can be amended from its default value to a user entered value. If fitted with GSM/GPRS communications, this amendment shall be achievable via the remote signalling system. FPI s shall be designed to discriminate between magnetic inrush current and fault current.

16 Version:- 1.1 Date of Issue:- Jan 2011 Page 16 of Design Rules A summary of the design process can be found in Appendix 1. Locations for fitting pole mounted FPIs: As near as possible to manual switching points As near as possible to both ends of cable sections Major spurs that are not protected with an auto-recloser as near as possible to the point of connection with the main line. To ensure that no more than 500 customers are between monitored points To ensure that there is no more than 3km of main line without an FPI Pole mounted FPIs should not be installed on poles: With underground cables With transformers With double circuit lines At tee-off positions Closer than 300m to kV lines Closer than 150m to 132kV lines Closer than 100m to 66kV lines Closer than 50m to 33kV lines Closer than 100m to 25kV overhead conductors All pole-mounted FPIs used on main lines shall be capable of indicating back to Network Management System (NMS) that they have operated. 3.9 Urban Circuits Urban circuits are generally all underground (up to 1 km of overhead line is permitted) and will be equipped with standard overcurrent and earth fault protection in accordance with the Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001). Settings should be as the Code of Practice for the Setting of Protection and Associated Equipment (TS16/17) (DSS/007/007). Circuits with in excess of one span of overhead line will be equipped with SEF protection. Auto-reclose is not normally enabled. Where practical, all overhead spurs on urban circuits shall be fused. On circuits normally supplying in excess of 2,000 customers consideration should be given to installing intermediate protection when this can be easily achieved, e.g. when replacing switchgear at a distribution substation and there is space available for a feeder CB. The economics of doing this should be checked against the interruption performance benefit that can be obtained using approved software tools.

17 Version:- 1.1 Date of Issue:- Jan 2011 Page 17 of Assumptions It is anticipated that all calculations regarding design of protection for HV circuits will be carried out using GROND. A set of standard assumptions for use in these calculations can be found in Appendix 2 of this document. Several aspects of the policy may be subject to subsequent review. This includes section 3.7 on Ferroresonance and the parts of section relating to SEF protection. Improved interruption performance has been evaluated using the IIS incentive rates for the DPCR5 period (2010/11 to 2014/15). This policy will be reviewed should there be a significant change in these values after DPCR Implementation and Monitoring The main accountabilities for implementation and monitoring of this policy lie with: Designation Design Manager Protection and Technical Services Manager Policy Manager Policy Production Manager Responsibility The manager appropriate to the part of the network where the policy is being applied, who is accountable for the implementation of this policy. Will ensure responsible persons are appointed to implement this policy The manager appropriate to the part of the business where the policy is being applied, who is accountable for the implementation of this policy. Will ensure responsible persons are appointed to implement this policy The manager who is accountable for the derivation of this policy Responsible for monitoring compliance of all business divisions with this policy 3.12 Planned Policy Review This policy shall be proposed for review on a biennial basis or at any time when external or internal influences drive a change in policy e.g. a change in legislation, or learning points from the initial implementation stage. The following responsibilities shall apply to policy control and review: Designation Publication Manager Policy Production Manager Responsibility Responsible for issuing a quarterly report to the Policy Production Manager (or representative) detailing policies scheduled for biennial review within the next six months Responsible for assessing the continued applicability of this company policy and for amending this document and communicating any changes in policy.

18 Version:- 1.1 Date of Issue:- Jan 2011 Page 18 of Superseded Documentation This document supersedes the following documents, all copies of which should be destroyed. Ref Version Title DD.554 Dec 2000 Code of Practice for the protection of HV rural lines DSS/007/010 Draft (Sept 1999) The protection of 11kV overhead lines

19 Version:- 1.1 Date of Issue:- Jan 2011 Page 19 of 30 4 References 4.1 External Documentation Reference Title Version and date EAW Regulations The Electricity at Work Regulations 1989 Statutory Instrument 1989 No 635 ESQC Regulations The Electricity Safety, Quality and Continuity Regulations 2002 As amended at the date of issue of this policy. Statutory Instrument 2002 No Internal documentation Reference Title Version and date IMP/001/912 Code of Practice for the Economic Development of V1.0; Jan 2009 the HV System DSS/007/001 The Protection of High Voltage Networks (TS1) V2.0; Mar 2000 DSS/007/007 The Setting of Protection and Associated Equipment V2.0; Jun 2001 (TS16/17) DSS/007/009 Standard for the Application of Tele Auto-Reclose to V1.0; Mar 2001 the 66kV and 33kV Systems IMP/007/011 Code of Practice for the Application of Lightning V1.0, Aug 2006 Protection NPS/001/009 Technical Specification for 11kV, 20kV and 33kV V2.0, Feb 2008 Pole Mounted Auto-reclose Circuit Breakers NPS/001/004 Technical Specification for 11kV, 20kV and 33kV V2.0, Dec 2007 Pole Mounted Expulsion Fuses and Solid Links NPS/001/014 Technical Specification for Overhead Line Fault V2.0, Jan 2008 Passage Indicators NPS/001/032 Technical Specification for 11kV, 20kV Pole Mounted V1.0 Dec 2010 Auto Sectionalising Links 05/61 An Investment Appraisal for Arc Suppression Coil V3.0, Mar 2005 Earthing An Investment Appraisal for Under Performing HV V5.0, Mar 2008 Feeders An Investment Appraisal for the Protection of 11kV Overhead Lines V5.0, Dec 2008

20 Version:- 1.1 Date of Issue:- Jan 2011 Page 20 of 30 5 Definitions Term ASC ASL CoP EF FPI GVR HSE HV IDMT Interruption NER NMS OC PMAR QoS SEF Short Interruption TSG VT Definition Arc Suppression Coil Automatic Sectionalising Link Code of Practice Earth Fault Fault Passage Indicator Gas-filled Vacuum Recloser Health and Safety Executive High Voltage (between 1,000 and 22,000 Volts) Inverse Definite Minimum Time A supply interruption with a duration in excess of three minutes Neutral Earthing Resistor Network Management System Over Current Pole-mounted auto-recloser Quality of Service; referring to interruptions performance in terms of CI and CML Sensitive Earth Fault A supply interruption with a duration of less than three minutes Triggered Spark Gap Voltage Transformer

21 Version:- 1.1 Date of Issue:- Jan 2011 Page 21 of 30 6 Authority for issue 6.1 Author I sign to confirm that I have completed and checked this document and I am satisfied with its content and submit it for approval and authorisation. Claire Thomas Asset Management Engineer Claire Thomas 16/12/ Policy Sponsor I sign to confirm that I am satisfied with all aspects of the content and preparation of this document and submit it for approval and authorisation. Andrew Ellam Policy & Risk Manager Andrew Ellam 16/12/ Technical Assurance I sign to confirm that I am satisfied with all aspects of the content and preparation of this document and submit it for approval and authorisation. Sign Sign Date Date Glen Hodges Jim Paine Programmes & Strategic Design Leader Protection & Technical Services Manager Sign Date Glen Hodges 17/12/2010 Jim Paine 16/12/ CDS Assurance I sign to confirm that this document has been assured for issue on to the DBD system. Sign Date Sean Johnson CDS Administrator Sean Johnson 16/12/ Approval Approval is given for the content of this document. Sign Date Mark Nicholson Head of System Strategy Mark Nicholson 16/12/ Authorisation Authorisation is granted for publication of this document. Sign Date Mark Drye Director of Asset Management Mark Drye 22/12/2010

22 Version:- 1.1 Date of Issue:- Jan 2011 Page 22 of 30 Appendix 1 Summary of Design Process 1. Collect circuit information and identify main and spur lines, customer numbers and distribution. 2. Design main line protection: 2.1. Source Circuit Breaker If protected circuit has less than 1 km of overhead, no auto-reclose and settings as per TS1; else Source CB does not have auto-reclose facilities. Fit PMAR as close as possible to the start of the first section of overhead line; else Source CB should be set for auto-reclose unless there is a section of underground cable greater than 500m in length or supplying grater than 50 customers prior to the first section of overhead line. If so then consider placing the auto-recloser at the first section of overhead line rather than at the primary Position additional auto-reclosers on the circuit in line with the following guidelines using the approved software modelling tool to assist in achieving the optimal solution: No customer should experience more than four faults per annum Aim for no more than 500 customers between remote control points Aim for no more than 2000 customers per circuit Aim for a maximum of 200 customers between switching points In general, a maximum of three auto-reclose devices should be used in series at 11kV and four at 20kV 2.3. Consider the impact of the changes on any arc suppression coil fitted at the primary substation. 3. Design spur protection 3.1. Define the type of spur (major, medium or minor) and refer to table in section 3.6 for detailed protection requirements 3.2. If spur is longer than 0.5 km then protect using a sectionaliser as long as the fault rating at the point of installation is below the rating of the conductor. Small conductor within the first protection zone may need to be protected by a fuse where fault ratings are exceeded. See appendix 5 for the detailed rules on sectionaliser locations and calculation of sizes If spur supplies more than 50 customers then consider protecting using an sectionaliser 3.4. If spur contains any triggered spark gaps then avoid using fuses unless fault level and conductor size considerations force the use of fuses If spur is located in the final protection zone then check the benefits of fitting a sectionaliser. Where there are a low number of customers in the final protection zone then it will be unlikely that spur line protection can be justified Otherwise protect with a fuse. 4. Consider protection of transformers. 5. Ensure circuit is protected against ferroresonance (where new build or a significant alteration only) 6. Design lightning protection in line with IMP/007/011 and re-check position against Position fault passage indicators as required.

23 Version:- 1.1 Date of Issue:- Jan 2011 Page 23 of 30 Appendix 2 Summary of Modelling Assumptions for use in calculations Fault rates to be applied when using GROND application for CI & CML performance figures. All studies should be carried out using the Light, Medium & Heavy fault performance figures for both Underground cable and Overhead Lines as follows: Light Medium Heavy Underground Overhead All transient fault rates per km are set at zero. Value per CI and CHL to be set as follows: Type Value ( ) CI 5 CHL 10 Restoration times to be set as follows: Switching Time (Mins) Manual 80 Manual/EFI 80 Tele 10 Auto 2 Recloser 1 Repair times to be set as follows: Type Time (Mins) Underground 480 Overhead 480

24 Version:- 1.1 Date of Issue:- Jan 2011 Page 24 of 30