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SVC for Mitigating 50 Hz Resonance of a Long 400 kV ac Interconnection Dr. A. Hammad * Consultant, NOK Baden, Switzerland S. Boshoff W.C. van der Merwe, C. van Dyk, W. Otto R.P.A. Trans-Africa projects Johannesburg, South Africa Summary A new 400 kV ac intertie between NamPower (Namibia) and Eskom (South Africa) is currently under construction. The 890-km single circuit transmission line is of a novel compact tower design. The existing NamPower system has inherently a low first parallel resonance frequency. This is due to its tree-like configuration transmission network with very long radial lines operated at Extra High Voltages (220 kV and 330 kV) and remote generation. The system also suffers from voltage stability problems. With the introduction of the new 400 kV interconnection to Eskom, the two problems inherent in the NamPower system are accentuated. The large 400 kV line charging capacitance reduces the parallel resonance frequency closer to 50 Hz or even lower and makes the network more voltage sensitive as well. In this case, both phenomena manifest themselves in the form of extremely high and sustained overvoltages. The paper explains the relationship between the 50-Hz resonance and the voltage stability phenomena and presents the approach for solving them. It also describes the criteria used for the development of the configuration and control functions of the new Static VAr Compensator (SVC) at the 400 kV line terminal at Auas. A brief summary of the SVC design features is also given. Results of transient studies using EMTDC simulations and frequency-domain analysis are presented to demonstrate the problems and the effectiveness of the adopted solutions. Keywords: Resonances, Voltage Stability, Temporary Overvoltages, SVC, Controls, Reliability. _______________________________________________________ * NOK, CH-5401 Baden, E-mail: [email protected] U.H.E. Kleyenstüber NamPower Windhoek, Namibia 1. INTRODUCTION The Southern African Development Community (SADC) is committed to the integration of its member countries' energy sectors. Power demands in the SADC are growing with a rate of 4-4.5% annually, with greater growth ahead [1]. In Namibia at present, the power network is built primarily along a 900-km 220 kV double circuit backbone to South Africa in the south as shown in Fig. 1. The bulk generation is supplied from the Ruacana hydro station in the far north through a 520 km 330 kV transmission. Small coal fired generators at Van Eck are used during emergencies to supply the load centre in nearby Windhoek. To cope with the fast growing economy and to provide a secure electric energy to the mining and mineral industry in Namibia, a new 400 kV ac transmission to interconnect the Networks of NamPower and Eskom is planned to be in operation in mid year 2000. The 890-km single circuit 400 kV line is of a novel compact tower design [2]. It interconnects Eskom's Aries substation near Kenhardt in South Africa to the new Auas substation near Windhoek via the existing Kokerboom substation near Keetmanshoop in Namibia as shown in Fig. 1. In the past, due to the long 220 kV and 330 kV lines and the relatively small and sparsely located loads with respect to the generation sources, the NamPower system suffered from voltage stability problems. For this reason, the efficient use of the SVC at Omburu with a dynamic range of +22.5/-37 MVAr supplemented with 60 MVAr of switched reactors and the skilful minute-tominute operation and supervision of the network, through the use of small switched shunt reactors, have ensured that the system narrowly survives. Ruacana 330 kV substations are equipped with breaker-switched shunt reactors. Due to this unique network structure and depending upon the number of generating units connected to the network at Ruacana, the system has a first natural parallel resonance frequency in the range of 55-70 Hz as depicted by curves 1&2 in Fig. 2. 520 km 1000 Omburu 1 800 3 2 SVC 600 Ohm Van Eck Auas 400 4 Hardap 470 km 200 0 Kokerboom 400 kV 0 50 100 150 200 Frequency, Hz 220 kV Fig. 2 System Impedance/Frequency Characteristics 420 km Harib 1000 1 5 Aries 800 2 6 Agganis ESKOM Ohm 600 Fig. 1 Schematic of Main Transmission of NamPower 400 With the introduction of the new 400 kV interconnection, the NamPower system should be strengthened. However, the new line is also very long with large charging capacitance which aggravates the inherent problems in the NamPower system; namely voltage stability and near 50-Hz resonance. 200 0 0 10 20 30 40 50 60 70 80 90 100 Frequency, Hz The paper explains the relationship between the 50-Hz resonance and the voltage stability phenomena and describes the approach for solving them. It also presents the optimisation process used for the development of the configuration and control functions of the new SVC at Auas. Results of transient studies using EMTDC simulations and frequency-domain analysis are presented to demonstrate the problems and the effectiveness of the adopted solution. Fig. 3 System Near 50-Hz Resonance 1. Existing transmission with 4 generators 2. Existing transmission with no generators 3. With 400 kV transmission & 4 generators 4. With 400 kV transmission & no generators 5. During 400 kV line energisation with 4 generators 6. During 400 kV line energisation & no generators 2. NEAR 50-HZ RESONANCE With the addition of the new 400 kV line (AriesKokerboom-Auas) and its 4x100 MVAr shunt terminal reactors, the system first resonance frequency shifts to the range 60-75 Hz (curves 3&4 in Fig. 2). Note also the remarkable reduction of system impedance at 50 Hz due to the new line indicating the strengthening of the system. At Kokerboom, two additional 100 MVAr As shown in Fig. 1, the NamPower network has a treelike configuration with very long radial EHV lines. The total length of the 132 kV and the 220 kV circuits is about 1100 km and 2900 km respectively. Only the 330 kV line is shunt compensated, whereas the 220 kV 2 breaker switched reactors are used for very light load conditions on the 400 kV transmission. The system resonance, however, can shift towards 50 Hz or even lower during system transients such as 400 kV line energisation or recovery after clearing of line faults. Fig. 3 (curves 5&6) shows the network impedance seen at Auas 400 kV bus at the instant of energising the 400 kV line from the Auas side before closing the breaker on the Kokerboom terminal. The condition of a parallel resonance in a circuit is that the system equivalent series reactance Xs equals the inverse of the equivalent shunt susceptance Bs. When this occurs at the fundamental frequency, the net system self admittance defined as: Yo = Go + j (Bs-1/Xs) = Go + j 1/Xo = 1/Zo approaches zero and is limited only by the system loads and generators. At light load conditions and/or no generators connected at Ruacana, the net system conductance (Go) becomes very small and Zo is very large as shown in Fig. 5. To a good extent, this is similar to the load flow Jacobian Q/U condition at voltage instability. Moreover, a small variation in the network configuration can cause an abrupt variation of the system net reactance from very large inductive to vary large capacitive at fundamental frequency as illustrated in Fig. 5 (X2 to X1). Again, this is analogous to the VSF condition at the bifurcation point for voltage instability. Furthermore, when the resonance frequency is below the fundamental frequency (case of X1) the system will be self-excited as well known from the subsynchronous resonance phenomenon [4]. To further elucidate the phenomena of near fundamental frequency parallel resonance in a power system and its implications, analogy to the well-known voltage stability phenomenon is made next. 3. ANALOGY BETWEEN 50-HZ RESONANCE AND VOLTAGE INSTABILITY Voltage stability phenomenon in transmission systems is usually described by the nose-curve relationship between voltage and power transfer as illustrated in Fig. 4. Voltage/reactive power sensitivity factor VSF (U/Q) is one of the simple techniques used to assess the proximity to voltage collapse [3]. In the stable operation region VSF is positive whereas it is negative in the unstable region as depicted in Fig. 4. As the system approaches instability it becomes more sensitive, i.e. VSF takes higher values. At the bifurcation point (A), i.e. transition from stable to unstable conditions VSF changes abruptly from + to -. This is because its inverse (Q/U) passes through zero at A. Therefore, a load flow computation employing NewtonRaphson iterative method diverges at that point since it uses the system Jacobian matrix that contains the factor Q/U which makes it singular. In the unstable region the system behaves in a reverse sense, i.e. switching-in a reactor will cause the voltage to rise. 1000 Zo 800 600 X0 X2 Ohm 400 200 X1 + Y0 0 20 _ -200 30 40 50 60 70 80 Frequency, Hz STABLE Us U/ Q Ur -400 -600 Um A Fig. 5 Near 50-Hz Resonance Phenomenon Um c The impact of the near 50-Hz resonance problem in the NamPower system can best be illustrated by simulating the condition represented by curve 6 in Fig. 3 at Auas substation during the 400 kV line energisation. As shown in Fig. 6, at time t=0.5 the breaker at Auas terminal is closed and it is assumed that the breaker at Kokerboom is synchronised at t=0.7 s. At first, due to the large charging capacitance of the line, the voltage dips before it overshoots. The extreme high overvoltages appearing at Auas with a peak value in excess of 1.7 pu and a sustained TOV of over 1.5 pu is attesting to the severity of the problem. It is clear that as soon as the 50-Hz resonance is triggered, very high UNSTABLE + 0 _ U/ Q Pr Fig. 4 Voltage Stability in a Transmission System 3 dynamic overvoltages appear with large time constant depending on the system load and generation conditions. such a device is to be able to provide smooth VAr absorption in order to continuously track the system during steady state as well as during line energisation, de-energisation, tripping, contingencies and faults. The Auas substation proved to be the optimum location for the device and in order to be most effective it should be connected to the 400 kV bus. 600 400 Four different solutions have been considered as part of a value engineering exercise. The criteria for developing such solutions and for selecting the most appropriate scheme are (in a descending order): kV 200 0 0.4 0.5 0.6 0.7 1. Reliability, availability and maintainability (RAM) 2. Simplicity and Redundancy 3. Flexibility of operation practically during contingencies and degraded modes 4. Cost 5. Technology 0.8 -200 -400 The rating of the device was determined to be 250 MVAr inductive to be able to sufficiently mitigate the near 50-Hz resonance with a good margin and to limit the resulting dynamic overvoltages to an acceptable level under worst conditions. The capacitive range was determined to be 80 MVAr in order to maintain an adequate margin of voltage stability during high power transfers on the 400 kV intertie between NamPower and Eskom similar to the existing margin on the 220 kV interconnection. -600 Fig. 6 Voltage at Auas during energisation of 400 kV line Section to Kokerboom 4. SVC SOLUTION The striking analogy between the near 50-Hz resonance and voltage stability phenomena can in fact make them dual where one produces overvoltage while the other leads to voltage collapse. In fact this points to a common solution method for both problems by means of adequate reactive power compensation. For voltage instability employing capacitive shunt VAr compensation Bc moves the bifurcation point A to a higher value as depicted in Fig. 4 [5 ]. It can be shown that the increase in the voltage (Um) at bifurcation due to insertion of Bc is such that: The developed schemes are: 1. Statcom (Voltage source converters using gate-turnoff thyristors) 2. Single SVC unit in 12-pulse configuration 3. Single SVC unit in 6-pule configuration with 1-phase transformers and built-in redundancies 4. Two SVC units each has a 75% rating and a 3-phase transformer with no built-in redundancies. 2 (Umc/Um) = 1/(1 - Bc.Xs) and Um/Um 0.5 Bc.Xs The detailed optimisation process resulted in choosing the scheme number 3 shown schematically in Fig. 7. Similarly, the system first resonance frequency close to 50 Hz (fo) can be increased by introducing an inductive compensation (-Bc) such that: 5. SVC DESIGN FEATURES The SVC scheme (Fig. 7) consists of four 1-phase transformers including a spare, four TCR identical units (three plus one spare) and two identical damped doubletuned filters. (foc/fo)2 = 1/(1 - Bc/Bs) f/fo 0.5 Bc/Bs Practically, however, the unpredictability of the resonance occurrence presents another problem in that very little preventive solutions could be safely utilised while ensuring the reliable and continuous operation of the NamPower system. This is crucial since only the minimum amount of fixed and switched shunt reactors could be used; otherwise the system voltage would collapse due to its low short circuit capacity. Therefore, the focus for finding a solution turned toward the use of power electronics or FACTS devices. The function of At present, the SVC technology is well established in the power transmission environment with some reasonable reliability and availability figures being achieved on the average. However, classical approaches to the application of SVCs in utility networks are in principle focused on voltage control particularly voltage support against collapse. In the NamPower system, however, the Auas SVC has multiple functions; many are unconventional, in the following priority order: 4 To cope with the sensitivity of the NamPower network to both reactive power and harmonic current injections, multiple TCR units (3x110 MVAr) are used. Each unit consists of direct light triggered thyristor valves and has its individual redundant control and cooling systems. The three TCR units are controlled in a series sequence (Master-Slave Mode) by the SVC redundant Master Controller. The capacitive MVArs required are in the form of 2x40 MVAr filter banks. Each filter is doubletuned to about 3rd/5th harmonics and provides sufficient filtering and damping to achieve a good performance particularly when one filter is out of operation. 400kV Auas Substation 400kV/15 kV SVC Transformer (spare) 15 kV Aux. supply TCR1 TCR2 TCR3 TCR4 The spare TCR unit fulfils the high reliability requirement of the SVC that is considered an integral part of the 400 kV transmission system. Also in case one TCR unit fails no filter bank would unnecessarily be switched off thus avoiding causing a double contingency. Filter 1 Filter 2 (spare) Fig. 7 Schematic of Auas SVC Scheme - Control the near 50-Hz resonance Figures 8 and 9 depict the Auas 400 kV bus and the TCR currents (at 15 kV) during the 400 kV line energisation with the SVC using a classical voltage controller. With the use of a special supplementary overvoltage open-loop controller it is expected that the SVC performance would be more superior. - Limit temporary overvoltages during 400-kV line energisation, system contingencies and post fault conditions - Avoid voltage collapse and provide sufficient voltage stability margin - Damp power oscillations on the 400 kV interconnection to Eskom. 600 Moreover, the weak NamPower network conditions add various constraints such as low order harmonic resonances and harmonic current injection limitations. 400 The fact that the entire NamPower system depends on the successful, reliable and continuous operation of the Auas SVC presents special high demands on the design, quality, functionality and layout of the individual components and sub-systems as well as the SVC scheme as a whole. Furthermore, the Auas substation is relatively remote from technical support and is planned to be unmanned. Any outage time would, therefore, impact severely on the entire system. kV 200 0 0.4 0.5 0.6 0.7 0.8 -200 -400 The following are some highlights on some salient aspects of the SVC design: Time, s -600 5.1 Single-Phase Transformer Units It is well known from other SVC schemes that their availability figures are much higher when 1-phase transformers are used compared to 3-phase units since the repair and transportation times for transformers are very long. Fig. 8 Voltage at Auas during 400 kV line energisation with SVC 5.3 Maintainability Even though it is acknowledged that reliability of SVC devices are continuously improving, the maintainability of the Auas SVC received special attention, based on the experience gained in the operation and maintenance of SVCs in South Africa [6]. Because of this, emphasis has been placed on accessibility of maintenance personnel to the Auas SVC scheme while it is energised. The secondary voltage of 15 kV is chosen as an optimum value for both thyristor valves and bulbar designs. 5.2 TCR and Filter Units 5 The intent is to achieve maximum availability even during equipment outages, by allowing on-line maintenance, dual redundant equipment, monitoring and By analogy to voltage stability phenomenon, a solution can be achieved by variable reactive power absorption. The -250/80 MVAr SVC scheme at Auas 400 kV substation is developed through value engineering criteria. Special features are implemented in the SVC design and layout to economically meet the high levels of performance, reliability, availability and maintainability requirements. 8 TCR1 6 4 kA 2 References 0 0.4 0.6 0.8 [1] H. Henyon, "Integrating Southern Independent Energy, Jan. 1999. -2 -4 [2] H. White, F. Ritky, "Unique Suspension System Conquers Rugged Terrain", Transmission and Distribution World, Aug. 1997. -6 Time, s -8 6 [3] A. Hammad, W. Kuhn, "A Computation Algorithm for Assessing Voltage Stability at AC/DC Interconnections", IEEE Trans. on Power systems, Feb. 1986, pp 209-216. TCR2 4 kA 2 0 0.4 0.5 0.6 0.7 [4] A. Hammad, M. El-Sadek, "Application of a Thyristor Controlled VAr Compensator for Damping Sub-synchronous Oscillations in Power systems", IEEE Trans. on Power apparatus & Systems, Jan. 1984, pp 198-212. 0.8 -2 -4 Time, s -6 [5] A. Hammad," Comparing the Voltage Control Capabilities of Present and Future VAr Compensating Techniques in Transmission Systems", IEEE Trans. on Power delivery, Jan. 1996, pp. 475-484. 6 TCR3 4 kA 2 0 0.4 Africa", 0.5 0.6 0.7 0.8 [6] T. Magg, S. Boshoff, C. Hitchin, H. Mohabir, A. Hammad, "Review of Performance and Operational Experience of the Natal SVC's, Eskom, South Africa", Proc. CIGRE SC14 Int. Colloquium on HVDC & FACTS, Sept. 1997, paper 4.2. -2 -4 Time , s -6 Fig. 9 SVC TCR 15-kV currents during 400 kV line energisation controls wherever necessary and have safe access to all equipment while the SVC is in operation. It is also expected that thyristor valves employing stateof-the-art direct light triggered thyristors would be used to achieve the required high levels of RAM as well as to cope with the stringent controls for fast limiting the extremely high system transient overvoltages. 5. CONCLUSIONS The new 890 km 400 kV ac intertie between Namibia and South Africa is essential to the economic development and prosperity of the region. Due to the unique structure of the NamPower network, the new transmission line can cause a system near 50Hz parallel resonance. Such a phenomenon leads to extremely high sustained overvoltages. 6