B4-209 SCCL A NEW TYPE OF FACTS BASED SHORT-CIRCUIT CURRENT LIMITER FOR APPLICATION IN HIGH-VOLTAGE SYSTEMS

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1 21, rue d'artois, F Paris B4-209 Session 2004 CIGRÉ SCCL A NEW TYPE OF FACTS BASED SHORT-CIRCUIT CURRENT LIMITER FOR APPLICATION IN HIGH-VOLTAGE SYSTEMS V. GOR D. POVH LU YICHUAN E. LERCH, D. RETZMANN*, Southern University of South-East University K. SADEK, G. THUMM California Edison Ljubljana Nanjing SIEMENS AG (United States) (Slovenia) (China) (Germany) 0. ABSTRACT FACTS (Flexible AC Transmission Systems) and HVDC (High Voltage DC Transmission) are powerful devices used to enhance system performance in the evolving power markets. Increasing generation in high load density networks on one hand and on the other hand interconnections between systems increase the short-circuit power substantially. If the short-circuit current rating of the equipment in the system is exceeded, the equipment must be either upgraded or replaced, both of which are very cost- and/or time-intensive procedures. Short-circuit current limitation offers clear benefits in such cases. Current limitation using passive elements, for example reactors, is a well known practice, however it reduces the system stability, increases the risk of voltage collapse and it has impact on the load-flow. By means of innovations in FACTS technology, a new dynamic fault current limiting device, the SCCL (Short-Circuit Current Limiter), is available now. The SCCL is based on developments in series compensation, where the TPSC (Thyristor Protected Series Compensation) has been successfully applied on 3 projects in a 500 kv transmission system in the Southern Californian Grid (USA), at the Vincent substation. Keywords: Market Developments - Bottlenecks in Transmission - FACTS - Series Compensation - Short-Circuit Current Limitation - Power Oscillations - Subsynchronous Resonances 1. INTRODUCTION A global tendency towards an increase in generation capacity can be observed, for example the installed capacity is expected to grow from GW (in the year 2000) up to GW in 2020 worldwide [4]. Globalisation/ Liberalisation Privatisation Deregulation - Privatisation: Opening of the markets, Independent Transmission Companies ITCs, Regional Transmission Organisations RTOs Bottlenecks in Transmission Privatisation Problem of uncontrolled Loop-Flows Overloading & Excess of SCC Levels System Instabilities/ Outages Investments in Power Privatisation Systems System Enhancement & Interconnections: Higher Voltage Levels New Transmission Technologies Renewable Energies Fig. 1: Power Market Developments * Siemens AG, Power Transmission and Distribution, High Voltage, P.O. Box 3220, Erlangen, Germany. dietmar.retzmann@siemens.com

2 The increasing power demand and major environmental constraints (Fig. 1) require advanced solutions for transmission systems: care must be taken to insure, that the transmission system under such dynamic market conditions is not going to produce a bottleneck, but rather be the key for a high return on investment and positive cash-flow. 2. FACTS - A PROVEN TECHNOLOGY FOR POWER SYSTEM ENHANCEMENT Flexible AC Transmission Systems are powerful devices for system performance enhancement in the evolving power markets [1, 2]. FACTS, based on power electronics have been first developed to improve the performance of long distance AC transmission [1, 5]. Later, the technology has been extended to the devices which can also control power flow [2, 12, 15]. Excellent operating experiences are available world-wide and the FACTS technology became mature and reliable. The main idea of FACTS can be explained by the basic equation for transmission in Fig. 2. Power transmitted between two nodes in the systems depends on voltages at both ends of the interconnection, the impedance of the line and the angle difference between both systems. Different FACTS devices can actively influence one ore more of these parameters and control the power flow in the systems, see Fig. 2. In Fig. 3, a summary of the key-issues and the solutions for the evolving power markets is given. P There are 3 typical Situations in Power Systems: U 1, δ 1 U 2, δ 2 G ~ G ~ X Meshed Systems: Load-Flow Problems Weak Systems: Stability Problems Strong Systems: High Fault Currents P = U 1 U 2 X sin (δ ( 1 - δ 2 ) The Solutions: SCCL & B2B, GPFC Load-Flow Control Parallel Compensation Series Compensation Fig. 2: Application of FACTS - The Basic Equation Short-Circuit Current Limiter Grid Power Flow Controller Fig. 3: Extended System Requirements of the Evolving Power Markets Main shunt connected FACTS application is the Static Var Compensator (SVC) with line-commutated thyristor technology, which provides fast voltage control, reactive power control and power oscillation damping features [2, 16]. There are hundreds of these devices in operation world-wide. Since decades, it is a well developed technology and the demand on SVC is increasing further. For long AC lines, series compensation is used for reducing the transmission angle, thus providing stability enhancement. The simplest form of series compensation is the Fixed Series Compensation (FSC). A huge number of these applications are in operation. Thyristor Controlled Series Compensation (TCSC) is used if fast control of the line impedance is required to adjust the load flow or for damping of power oscillations. Special FACTS devices are UPFC (Unified Power Flow Controller, [1, 2]) and GPFC (Grid Power Flow Controller [4-6]). UPFC combines a shunt connected STATCOM with a series connected STATCOM, which can exchange energy via a coupling capacitor. GPFC is a simplified DC back-toback link, which is designed for power and fast voltage control at both terminals [6]. In this way, GPFC is a FACTS back-to-back, which is less complex than the UPFC at lower costs, and it is also suitable for short-circuit current limitation, ref. to Fig. 3 and [3]. FACTS devices consist of power electronic components and conventional equipment which can be combined in different configurations. It is therefore relatively easy to develop new devices to meat extended system requirements of the evolving power markets (Fig. 3). Such recent developments are the TPSC (Thyristor Protected Series Compensation, [13, 14]) and the Short-Circuit Current Limiter (SCCL, [7]), both innovative solutions using high power thyristor technology, which are presented in details in section FAULT CURRENT LIMITATION - STATUS TODAY Increasing generation in high load density networks on one hand and on the other hand interconnections between systems increase the short-circuit power substantially. However, faults in 2

3 electrical power systems are unavoidable. Apart from the damages in the vicinity of the fault - e.g. due to the effects of an electric arc - the fault currents flowing from the sources to the location of the fault lead to high dynamical and thermal stresses on all the equipment being involved. Hence, if the shortcircuit current rating of the equipment in the system is exceeded, the equipment must be either upgraded or replaced, both of which are very cost- and/or time-intensive procedures. Therefore there is a considerable interest in devices which are capable of limiting fault currents. A Fault Current Limiter (FCL) shall limit a fault current passing trough it within the first half cycle [7]. In case of newly planned networks fault current limiters allow the use of equipment with lower ratings which renders possible considerable cost savings. Due to the importance of these issues, a CIGRE Working Group (WG 13.10) was established in 1996 with the task to prepare a specification for fault current limiters. This report [7] was provided in and is available as technical brochure from CIGRE Central Office. Basically, there a two types of fault current limiters, ref. to Fig. 4 a): Fault current limitation (e.g. reactor, superconducting FCL, or the new SCCL ) Fault current interruption (e.g. I S -Limiter, electronic devices) FCL Applications: FCL FCL - Operating Principle: Unlimited Fault Current kv >10xI n SCCL or Reactor 115 kv 115 kv 115 kv I n 3 x I n a) Is-Limiter, Electronic FCL Superconducting Supraleitender FCL b) Fig. 4: Possible Locations of Fault Current Limitation in the System: a) Operating Principle of different Devices b) Application of FCL in the System A major constraint on devices with current interruption, e.g. electronic switches, is that the protection schemes of the neighboring equipment (switchgear, lines, transformers and generators) needs to be modified, depending on the location of FCL. This is due to the fact that an FCL will eliminate a fault current much faster before any protective relay can detect and locate the fault in order to generate a trip signal for the associated breakers. High-Temperature Superconducting Fault Current Limiters (HTS FCL) can be designed as resistive [10, 11] or inductive [9] limiters. An overview on the superconducting devices (mainly high temperature design) and the system requirements is given in [8]. HTS FCL developments are focusing on medium voltage applications, because high voltage applications require tremendous cooling equipment and sophisticated electrical insulation technologies. Fig. 4 b) shows the basic possibilities for an FCL application on different voltage levels in the power system. An FCL can be connected to the generation infeed at the generator voltage level, at different 3

4 locations in the high-voltage system (400 kv or higher), or in subtransmission and distribution networks, e.g. on 115 kv or lower. In practice, up to now, for fault current limitation mainly conventional reactors have been applied. The drawback of this solution is that it obviously also influences the system during normal operation, i.e. it results in considerable voltage drops at high load currents [7]. This has impact on voltage quality and load-flow, furthermore, if large induction machines are connected, e.g. in industrial applications or at generator home-loads, there is a strong risk of voltage collapse. Additional measures, such as mechanically or thyristor switched capacitors will be needed for reactive power compensation (voltage drop or voltage collapse) [2, 4, 5]. 4. INNOVATIONS IN FACTS TECHNOLOGY In this section, innovative developments in the area of FACTS technology for series compensation are described and their benefits for short-circuit current limitation are shown FACTS for Series Compensation from FSC to TPSC In series compensation, a capacitor is used to compensate for the lines inductance, thus the line is "virtually" shortened and the transmission angle decreased for system stability improvement, ref. to Fig. 2. However, during transient conditions, the short-circuit currents cause high voltages across the capacitor, which must be limited to specified values. In the past, this limitation was accomplished by a spark gap, by arresters (MOV) or in a combination of both, see Fig. 5. An AC-fault current flowing through a MOV always leads to a high energy dissipation of the MOV. The MOV heats up heavily. Due to an upper temperature limit the MOV must cool down before the next current stress can be absorbed. Cool-down requires a substantial amount of time, time constants of several hours are typical. During this time, the series compensation must be taken out of service (bypass-breaker closed) and consequently the power transfer on the related line needs to be reduced, dependent on the degree of compensation. Gap Protected Sensitive to environmental influences, specific maintenance required MOV Protected Long cool-down time Thyristor Protected Fast cool-down time Fig. 5: Developments in Series Compensation Both the (mechanical) gap function and the MOV can now be replaced by an innovative solution with special high power light-triggered thyristors. These thyristors are designed and tested for a 110 ka peak current capability and they have a very fast cool-down time [13, 14]. Fig. 6 shows the fast cool-down time, which is an outstanding feature of this new development for the TPSC (Thyristor Protected Series Compensation). It can be seen, that the TPSC will be ready for additional contingencies, such as multiple fault conditions, before the end of the auto-reclosure deadtime. For the TPSC, innovative developments in thyristor-technology have been applied: Light-Triggered Thyristors (LTT, now State of the Art for FACTS and HVDC applications) and a special heat-sink to enable a self-cooling of the valves [13]. 4

5 Standard FSC with MOV requires up to 8 hours to cool down 260 C TPSC Valve Temp. 0.6 s after the 1st Fault the Valve is back in Pre-fault Condition 50 C Thyr. Valve Bypass CB Time / s Line Breaker Auto-Reclosure Dead-Time 5 Cycles Fault Clearing Time Fig. 6: Benefits of TPSC Full Availability after Fault Clearing Time Using this new technology, significant cost savings after system faults can be achieved. Fig. 7 shows the principle of the TPSC and the cost savings for each system fault on one of the three lines at the 500 kv TPSC installation at Vincent Substation, USA. In case of faults nearby the substations, all three lines are involved in the fault strategy. Then the savings sum up to US$ per single event. to Substation to Line Benefits of US$* per event on 1 line** due to faster availability of a TPSC e.g. reduction from 1200 MW to 600 MW with FSC/MOV * * 25 US$/MWh x 600 MW x 6 hrs ** US$, if all 3 Lines are involved Long cool-down time of arrester in conventional series capacitor after fault or faults before bank re-insertion Replacement of spark gap and high energy absorption arresters by self-cooled direct-light triggered thyristor (LTT) valves Fast re-insertion of series capacitor due to extremely short cool-down time of LTT valve Fig. 7: TPSC versus conventional FSC Highlights and Cost Savings per Event The TPSC has been successfully put into service in three projects in a 500 kv transmission system at the Vincent substation in the Southern Californian Grid (USA, 1999 and 2000). Fig. 8 shows a photo of a Vincent TPSC and Fig. 9 highlights the benefits of the used LTT technology. Due to its excellent benefit-cost ratio and its fully proven, high availability (robust layout), two new orders have been placed for the TPSC in the same transmission system. In Fig. 10, recordings of a real fault event at the 500 kv system are shown. In the small figure (upper part), the non-affected line current is shown (phase A). In the enlarged part, for one of the affected phases (C) the measured valve current and the (calculated) valve temperature rise is depicted. It can be seen, that the initiation of the thyristor firing is depending on the fault current level: in the first half cycle of the current rise (peak value approx. 5 ka), the preset fault detection level is not yet reached, whereas in the second half-cycle, the thyristor is fired due to the increased current level. The level settings are design parameters with regard to load-flow conditions and TPSC rating. In Fig. 10, the 5

6 valve temperature raise is only about ten degrees (K), which shows the efficiency of the thyristor selfcooling: there is still a very large margin for more temperature rise up to 260 degrees in case of higher currents stresses, ref. to Fig. 6. The safest Valve Technology LTT: Technical & Economical Advantages 80 % less Electronic Components Less Electric Wiring & Fiber Optic Cables Reduced Spare Parts Requirements Wafer-integrated Over-voltage Protection Thyristor Valve with Direct-Light Triggering 100 mm Thyristors with integrated Break-over Protection Maximum Reliability & Availability - Benefits of LTT Fig. 8: Site-View of one of the three Vincent TPSCs (500 kv System) Fig. 9: Benefits of LTT Technology 5000 Line Curr. Ph A1 Valv.Curr. Ph A Line Current Phase A (Healthy Phase) Amps Line Current Phase C Line Line Breaker breaker open time / msec Bypass Bypass switch Switch close closed , , , I [A] Fault Detection Level 30 Tvj [ C] Valve Current Phase C Valve Temperature Rise Phase C t [sec] Fig. 10: TPSC Vincent On-Site Recordings Line Fault in Phase BC, June 18, 2002 (ref. to Text) Based on these successful project experiences, the use of TPSC for short-circuit current limitation has been initiated, because this highly reliable technology can fulfill the challenging requirements for an utmost reliability and availability, which is a must be for all electronic FCL solutions [7]. 4.2 SCCL - Application of TPSC as Short-Circuit Current Limiter By combining the TPSC with an external reactor, whose design is determined by the allowed shortcircuit current level, this device can also be used very effectively as short-circuit current limiter (SCCL). This new device operates with zero impedance in steady-state conditions, and in case of a short-circuit it is switched within a few ms to the limiting-reactor impedance. Fig. 11 shows the basic 0 6

7 function, the operating principle and a 3-D view of the SCCL. In comparison with the TPSC site view in Fig. 8, it can be seen, that the TPSC is just complemented by an additional reactor for the current limitation. A detailed schematic of the SCCL equipment is given in the single-line diagram Fig. 12. Thyristor Valve Housing Capacitor Bank to Bus 1 BYPASS Breaker Communication SCCL Reactor TPSC + Reactor to Bus 2 Impedance X Fast Increase of Coupling Impedance AC AC Zero Ohm for best Load Flow t Bus 1 Bus 2 Fig. 11: SCCL - Short-Circuit Current Limitation with TPSC (ref. to Text) Valve Monitoring & Supervision SCCL Current CT Platform Fault Current CT Capacitor Overload CT Capacitor Unbalance CT Fig. 12: Components of the SCCL The main bypass breaker and the current limiting reactor LO (Fig. 12) are mounted nearby the platform, ref. to Fig. 11. All other equipment is mounted on the platform itself. As an option, an additional bypass bus coupler can be provided (not shown in the 3-D view). The reactors L1 and L3 are designed for discharging the capacitor either by thyristor or breaker. Fault detection is done by the SCCL current CT, the other functions which marked in Fig. 12 are needed for the SCCL internal protection. The bybass-breaker is operated only in cases of very high fault currents. Applications of SCCL in typical systems configurations, which are expanded by new generation infeeds, are depicted in Fig. 13. Optional, also B2B/GPFC can be used for fault current limitation. 7

8 Existing 500 kv 500 kv Existing SCCL Expansion 115 kv 115 kv Expansion Location(s) of SCCL depends on Source Impedances I 1 SCCL I 1 Bus 1 I 1 V S, I S I 1+2 a) Bus 1 Bus 2 b) Fig. 13: System Configurations for SCCL Application a) SCCL as Bus-Coupler b) Ring Network Configuration I 2 Expansion b) B2B / GPFC SCCL Expansion 10 ka Bus 2 U 2 I 2 40 ka b) I ka Bus 1 U 1 I 1 40 ka 10 ka Bus 2 I 2 40 ka I Byp c) a) I ka Fig. 14: Simulated Fault Events for a Set-up in Fig. 13 a). The Current Magnitudes are Initial Symmetrical Short-Circuit Currents (I k ) a) Without SCCL b) With SCCL c) View on the Thyristor Current (Capacitor Discharge plus Fault Current) In Fig. 14, for an application of SCCL as bus-coupler, simulation results for a reduction of the fault current from 80 ka to 50 ka are shown. This fault current reduction would lead to significant costsavings for the substation upgrade, because a rating of the existing equipment with 63 ka would be sufficient in the case of system expansion, e.g. by connection of new power plants, hence an expensive uprating on 80 ka could be avoided if SCCL is used. It must to be stated, that the SCCL application is not limited to high-voltage systems only, it can also be applied with same benefits to medium voltage levels, e.g. for the connection of generator homeloads. 8

9 5. TPSC AND SCCL - DYNAMIC CONTROL ADD-ON FUNCTIONS 5.1 Power Oscillation Damping In case that power systems are expanded or interconnected, in addition to the task of current limitation, there is often a need for damping of power oscillations. This would not be possible with conventional reactors and other FCL devices or with fixed series compensation elements. However, with the TPSC, this feature is available. Using the thyristor for control, an additional value for the user can easily be created by means of software add-on functions. In the same way, this feature can also be applied for the SCCL. Fig. 15 gives an example of this innovative solution, applied in a typical, interconnected AC power system. With POD Control: Fast & effective Damping No POD Control: System close to Instability Fig. 15: First Add-On Control Function Power Oscillation Damping In the upper traces of Fig. 15, the TPSC (or SCCL, respectively) is operated in a bang-bang control mode, thus the oscillations are damped very efficiently, whereas in the case without active damping (lower traces), there is a large risk for instability of the coupled parts of the system. 5.2 SSR Mitigation For systems interconnections, in many cases Fixed Series Compensation is used. However a significant disadvantage of FSC is the possibility of exciting subsynchronous resonances (SSR) in neighboring network generators under certain conditions: SSR is a specific and disastrous phenomenon with large (thermal) generation units, which often are constructed with long shaft configurations. These shafts can oscillate at low order frequencies, and such phenomena can lead to shaft destruction [17]. This circumstance is excited when there is a matching electrical frequency value, which may occur in a neighboring series compensated line. A typical example: a turbine generator shaft is resonant at 25 Hz, the system frequency is 60 Hz, then, a resonance of the series capacitor and the line plus source impedance at 35 Hz has to be avoided. However, as system conditions change, this often cannot be prevented. As a consequence, the degree of series compensation needs to be modified (reduced) immediately, when SSR is detected. However, in practice, SSR detection is a rather sophisticated task, and a reduction of the compensation leads to a decrease in transmission stability, so the power transfer must be reduced (less return on investments). Now, for TPSC and SCCL, a control add-on function can be provided, with automatic SSR detection and mitigation to avoid such severe transmission constraints. Fig. 16 shows the simulation results of a power system interconnection with a series compensated line. In the simulation, a TPSC has been used, which can either be blocked or be operated in a controlled mode (like TCSC). In Fig. 16 a), the TPSC is fully blocked (= conventional FSC), and the 9

10 subsynchronous oscillation quickly rises above the critical torque level of the generator under investigation, see the first trace in the figure. Typical Dangerous Level - High Probability of Shaft Damage TPSC blocked: SSR exceeds critical Level a) 1 ph Autoreclosure TPSC active: SSR Mitigation 1 ph Fault Time for Remedial Actions: Permanent TCSC Operation or System Adaptation b) Fig. 16: Second Control Add-On Function of TPSC/SCCL SSR Mitigation a) TPSC blocked b) TPSC active In Fig. 16 b), the TPSC control add-on function is enabled, hence the SSR oscillation remains below the critical and dangerous torque level. This leaves time for remedial actions, ref. to the figure. 6. CONCLUSIONS New developments for Thyristor Protected Series Compensation have been successfully applied in a 500 kv transmission system in the United States. Using the TPSC, in combination with an additional 10

11 reactor, a new FACTS element for system expansion, the SCCL, is available now. It offers significant benefits for short-circuit current limitation, in high-voltage and also in medium-voltage systems. By means of control add-on functions, both TPSC and SCCL offer additional advantages for power system enhancement. Further investigations have shown that with both TPSC and SCCL, an active damping of SSR phenomena is feasible up to resonance frequencies of about 30 Hz (electrically). 7. ACKNOWLEDGEMENTS The authors would like to thank the convenor of Cigré WG A3-10 and the colleagues of Hydro- Québec, Canada, for their valuable contributions and discussions during the developments of the new SCCL. 8. REFERENCES [1] N. G. Hingorani, Flexible AC Transmission, [IEEE Spectrum, pp , April 1993] [2] FACTS Overview, [IEEE and Cigré, Catalog Nr. 95 TP 108] [3] Economic Assessment of HVDC Links, [CIGRE Brochure Nr.186 (Final Report of WG 14-20)] [4] Han Yingduo, Wang Zhonghong, D. Povh, X. Lei, D. Retzmann, Role of HVDC and FACTS in future Power Systems, [Cigré Symposium, April 2003, Shanghai, China] [5] V. Sitnikov, W. Breuer, D. Povh, D. Retzmann, M. Weinhold, Benefits of FACTS for large Power Systems, [Cigré Conference, Sept. 2003, St.-Petersburg, Russia] [6] M. Mohaddes, D.P. Brandt, M.M. Rashwan, K. Sadek, Application of the Grid Power Flow Controller in a Back-to-Back Configuration, [CIGRE Report B4-307, Session 2004] [7] Fault Current Limiters in Electrical Medium and High Voltage Systems, [CIGRE Brochure Nr. 239 (Final Report of WG A3-10)] [8] S. Fischer, H. Schmitt, R.R. Volkmar, Y. Brissette, System Requirements and Tests of Superconducting Fault Current Limiter, [CIGRE Report , Session 2000] [9] J. R. Cave, R. Nadi, Y. Brisette, D.W.A. Willen, W. Zhu, Inductive Fault Current Limiter Developments, [2nd European Conference on Applied Superconductivity, 3-6. July 1995, Edinburgh, UK] [10] G. Ries, H.W. Neumüller, H.P. Kramer, B. Gromoll, W. Schmidt, S. Fischer, Development of Resistive HTSC Fault Current Limiters, [2nd European Conference on Applied Superconductivity, 3-6. July 1995, Edinburgh, UK] [11] R. Witzmann, W. Schmidt; R. Volkmar: Resistive HTSL-Strombegrenzer. Energietechnik für die Zukunft, [Internationaler ETG-Kongress 2001, Nürnberg. ETG Fachberichte Band 85, VDE-Verlag, 2001] [12] R.K. Johnson; D.R. Torgerson; K. Renz; G. Thumm; S. Weiss: Thyristor Control Gives Flexibility in Series Compensated Transmission, [Power Technology International, 1993] [13] L. Kirschner, L., J. Bohn, K. Sadek, Thyristor protected Series Capacitor - Part 1: Design Aspects, [IEEE - T&D Conference 2002, Sao Paulo, Brazil] [14] A. Kumar, G. Kuhn, V. Gor, K. Braun, Thyristor protected Series Capacitor - Part 2: Control and Protection Concepts, [IEEE - T&D Conference 2002, Sao Paulo, Brazil] [15] D. Retzmann, K. Bergmann, M. Claus, I. Baran, P. Forsyth, T. Maguire, G. Kuhn, A. Kumar, X. Lei, Advanced Fully digital TCSC Real-Time Simulation, [IPST 01, June 24-28, 2001, Rio de Janeiro, Brazil] [16] S. Geeves, K. Bergmann, D. Retzmann, R. Witzmann, Improvement of System Stability by the HARKER Static Var Compensators/UK - Verification of System Performance by Digital and Real-Time Simulation, [ICPST 94, Oct. 1994, Beijing, China] [17] M. C. Hall, D. A. Hodges, Experience with 500 kv Subsynchronous Resonance and resulting Turbine Generator Shaft Damage at Mohave Generating Station, [IEEE Publication 76 CHI066-0-PWR, pp ] 11