Preferential Subject #3 Protection and Control of Series Compensated Networks Special Reporter: Graeme Topham South Africa Study Committee B5 Colloquium 2005 September 14 – 16 Calgary, CANADA

PS-3 Introduction Renewed interest in series compensation:  Increasing need to obtain maximum benefit of existing assets  Economically increase transfer capability of transmission lines  Secure better system stability Challenges in application particularly for protection engineers

PS-3 Introduction Series compensation in modern power systems:  influences power flow  reduces active power losses  connects more robustly, different sub- systems to a stronger integrated network Thyristor controlled series capacitors (TCSCs) can be used to damp system & sub- synchronous oscillations

PS-3 Introduction Introduction of series compensation into existing networks requires extensive studies covering:  expected performance of new systems  the influence of its introduction on the operation of existing protection, control and monitoring systems

PS-3 Introduction Challenges:  Voltage inversion  Current inversion  Low frequency transients  High frequency transients  Sub-synchronous resonance

PS-3 Cigré Working Group B5-10 Formed and commenced work in 2002 Focus is protection, monitoring and control of series compensated networks  protection, control and monitoring of series capacitor banks  protection of series compensated and adjacent power lines 4 regular and 10 corresponding members Target date for report: 2006

PS-3 Cigré Working Group B5-10 International survey questionnaire distributed in December 2004  main objective to obtain better overview of practices currently used in SC networks Questionnaire focuses on:  technical issues  economic aspects  environmental aspects  refurbishment and future trends

PS-3 Cigré Working Group B5-10 Participants urged to respond  utilities who have experience with series compensated networks;  those who are currently investigating the possibility for implementation of series compensation; and,  those who have decided in the past against incorporating SC for various reasons

PS-3 Papers 18 papers received from 15 countries  Paper 301 by WG B5-10 provides overview Main themes covered by remaining papers:  Distance protection  Non-distance protection  Auto-reclosing  FACTS devices  Informational analysis  Shunt capacitor banks

PS-3 Paper 301 Paper 301 gives an overview of:  the intended structure of the WG B5-10 report  the main primary components of Fixed and Thyristor-controlled SC banks;  the basic control philosophies;  the fundamental elements of series capacitor bank protection; and,  the challenges which face the protection of SC and adjacent transmission lines.

PS-3 Paper 301 Question #1: What are the main utility considerations when deciding to series compensate a transmission line and/or what are the challenges faced in the protection, control and monitoring of the capacitor banks as well as of the series compensated and adjacent transmission lines? Question #2: What economic and/or environmental issues deter the implementation of series compensation?

PS-3 Distance protection The papers which had distance protection as the main theme cover a number of different aspects namely:  polarizing voltages, directionality and phase selection;  zero sequence mutual coupling and sub- synchronous frequencies;  weak infeed;  fault locating and calculation of the series capacitor voltage; and,  settings and testing

PS-3 Distance protection: Polarizing voltages, directionality and phase selection Paper 302 (Brazil) describes  study of the effect of different polarizing quantities and their suitability for SC applications  a detailed analysis of relay directionality difficulties imposed by SC Paper 304 (Canada) discusses  possible inadequacy of some protection techniques which function correctly in traditional SC applications but which may perform inadequately in applications such as long lines interconnecting a high capacity wind farm

PS-3 Distance protection: Polarizing voltages, directionality and phase selection Question #3: Are there any experiences of other unique series compensation distance relay applications where special adjustments to settings and/or logic are required to ensure correct protection performance? Question #4: What analysis tools are available to utilities to assist in evaluating distance relay performance on series compensated applications prior to the purchase or choice of a particular relay type?

PS-3 Distance protection: Zero sequence mutual coupling and sub-synchronous frequencies Paper 306 (Germany/USA/Brazil)  presents calculation of sub-synchronous frequencies caused by series capacitors and their influence on measuring algorithms. Paper 316 (UK)  influence of sub-synchronous frequencies on distance measurement as well as addressing the effect of mutual coupling on distance measurement  benefit of introducing zero sequence compensation on parallel transmission lines to improve distance calculation

PS-3 Distance protection: Zero sequence mutual coupling and sub-synchronous frequencies Question #5: Besides utilizing zero sequence current compensation, what other techniques have been successfully applied to distance relays to resolve the impedance measurement inaccuracy on series compensated parallel lines caused by the mutual zero sequence impedance?

PS-3 Distance protection: Weak infeed The weak infeed function was developed to speed up the operation of impedance relays operating in a permissive intertripping mode for cases where there is a lack of, or insufficient, fault current Paper 318 (South Africa)  some experiences of high-risk incorrect operations resulting in overtripping of parallel lines  investigation revealed two areas of possible lack of co- ordination, namely the timing co-ordination due to unforeseen drop-off delays of relays and impedance co-ordination due to a lack of required overlapping of the remote forward zone 2 element by the local reverse blocking element in the worst case scenario (series capacitor does not bypass)

PS-3 Distance protection: Weak infeed Question #6: What are the current experiences with using the weak infeed protection function, particularly in series compensated line protection applications?

PS-3 Distance protection: Fault locating and calculation of the series capacitor voltage Paper 307 (China)  accurate and efficient calculation of the series capacitor voltage is crucial for the reliable operation of a distance relay and for the accurate results of a fault locator applied on series compensated lines  presents a simplified capacitor voltage calculation algorithm which is based on three procedures and requires no iteration  influence of charging current is eliminated with a charging current compensation algorithm based on travelling wave theory

PS-3 Distance protection: Fault locating and calculation of the series capacitor voltage Paper 309 (Japan)  protection and fault location experiences of the SC EHV line in Japan. After analysis of incorrect fault location results, obtained for faults without gap flashover, a two-terminal type fault location scheme was proposed as an effective solution.

PS-3 Distance protection: Fault locating and calculation of the series capacitor voltage Question #7: What are the main technical difficulties with accurate fault location on series compensated lines and what fault location techniques are currently being successfully employed on series compensated lines?

PS-3 Distance protection: Settings and testing Paper 308 (India)  case study of line relay settings for a 400 kV SC double-circuit line and adjacent lines in India. Each line has 40 % fixed (FSC) and 5 to 15 % variable (TCSC) compensation  paper recommends that instantaneous tripping zones need to be considerably reduced or even disabled at the remote ends of the protected SC lines and on lines adjacent to them  authors point out the necessity for the suitability of existing protection on adjacent lines to be investigated and, if necessary, changed

PS-3 Distance protection: Settings and testing Paper 315 (USA/Sweden)  USA utility’s approach towards testing different line protection relays to be used within the utility’s 500 kV network  special attention is paid to specific project oriented tests which are performed on a complete protection system before installation Paper 306 (Germany/USA/Brazil)  outlines performance verification of protection for SC lines in Brazil using a fully digital real- time network analyzer

PS-3 Distance protection: Settings and testing Question #8: a) What methodologies are used to optimize settings for series compensated line applications?; and, b) What are the requirements of utilities in terms of certification tests for relays and/or protection systems before applying these to series compensated lines?

PS-3 Non-Distance protection Three papers focussed on the application of non- distance protection to series compensated lines Paper 309 (Japan)  indicates that the Japanese utility uses current differential protection as the main protection (with impedance type protection used as back-up)

PS-3 Non-Distance protection Paper 314 (Sweden)  challenge of detecting high resistance earth faults and the determination of a suitable reference quantity for directional measurement  paper identifies the fact that SC of transmission lines can further endanger the correct functioning of directional earth fault protection as the magnitude and phase angle of residual voltage and current is influenced by the series capacitors Paper 317 (USA/France/UK)  presents a directional comparison protection scheme with sub-cycle operating times.

PS-3 Non-Distance protection Question #9: What are utility experiences with non- distance protection applications on series compensated transmission lines? Question #10: What are the advantages, disadvantages, technical challenges and cost implications of using non-distance protection when compared to using distance protection on series compensated line protection applications?

PS-3 Auto-Reclosing Paper 305 (Germany)  a controlled auto-reclosing technique for compensated transmission lines which uses the ‘Prony Method’ to determine the future voltage shape over the circuit-breaker  illustrates that using the conventional zero crossing of the voltage over the circuit-breaker as the switching instant does not produce the smallest transient overvoltages at the end of the transmission line  paper defines a new additional criterion which considers the influence of the resonance circuit of the compensated transmission line.

PS-3 Auto-Reclosing Question #11: a) What are the current experiences with auto- reclosing on series compensated transmission lines?; and, b) What are the technical challenges and proposed solutions?

PS-3 FACTS Devices Two of the submitted papers dealt with the influence of Unified Power Flow Controllers (UPFCs) on protection Paper 310 (Korea)  investigates the impedance variation caused by the series voltage injected by the UPFC according to different UPFC operating conditions under steady- and transient- states in the Korean network  impedance variations seen by the distance protection relay were reviewed under fault conditions to investigate the possibility of relay misoperation and the possible need for relay protection setting modifications

PS-3 FACTS Devices Paper 311 (Poland)  also points out the influence of the series branch of UPFCs on the impedance measurement  paper also considers aspects relating to current differential and phase comparison protection and mentions that adaptive distance protection is a reasonable solution to ensure protection flexibility

PS-3 FACTS Devices Question #12: Have there been any problematic protection issues relating to the use of UPFCs experienced by other utilities and what solutions are available to mitigate against the problems?

PS-3 Informational Analysis Two papers, 312 (Russia/Sweden) and 313 (Russia), deal with the subject of informational analysis as a tool to analyse uncertainty and ambiguity in the identification of fault conditions in electrical systems Question #13: What other novel theoretical tools are available to assist in the analysis and understanding of series compensated networks?

PS-3 Shunt Capacitor Banks Paper 303 (Brazil/Switzerland)  distance protection behaviour under extreme system conditions in the shunt capacitor bank compensated transmission network in Brazil  system was simulated with different software tools and extensive tests were performed on two different model power system simulator tools

PS-3 Shunt Capacitor Banks Paper 303 (Brazil/Switzerland) cont.  based on the investigations and the experience collected during the tests, the paper summarizes how selected power system fault conditions can affect distance protection  authors propose protection strategies, re- adjustment of protection concepts and re- adaptation of protection algorithms to deal with the challenges of extreme network conditions

PS-3 Shunt Capacitor Banks Question #14: What influences of shunt capacitor banks on protection measurement are reported and how are the effects mitigated?

PS-3 Conclusions Evidence of continued valuable work being done to improve the performance of the protection, control and monitoring of series compensated networks New challenges in protecting series compensated and adjacent transmission lines are emerging and innovative techniques to deal with such challenges and also to enhance equipment performance continue to be developed Encouraging to see the high level of co-operation between manufacturers and utilities (evident from the number of joint papers) to solving application problems and also to improve protection equipment

PS-3 Conclusions It is also apparent that a thorough understanding of the subject is advantageous in order to ensure optimal application of the correct equipment for particular applications The importance and value added by international collaboration through international bodies and working groups should not be underestimated.