L STUDY OF OPTIMAL LOCATION FOR CAPACITOR INSTALLATION IN A 220/13^33 kV GRID SUBSTATION A dissertation submitted to the Department of Electrical Engineering University of Moratuwa in partial fulfillment of the requirements for the Degree of Master of Science LIBRARY UNIVERSITY OF MORATUWA, SRI LANKA MORATUWA by W.M.S. DEEPIKA WIJESINGHE Supervised by: Dr. } P Karunadasa University of Moratuwa 92425 Department of Electrical Engineering University of Moratuwa, Sri Lanka ,3 foj- 3- C ^ 4 3 j February, 2009 T H 92425 LIBRARY 5 Deceleration The work submitted in this dissertation is the result of my own investigation, except where otherwise stated. It has not already been accepted for any degree, and is also not being concurrently submitted for any other degree. Name of the candidate: W.M.S. Deepika Wijesinghe Date: February 12, 2009 I endorse the declaration by the candidate. 'cy oevp' Name of the Supervisor: Dr. J P Karunadasa Date: 1 Table of Content Item Page Table of Content » Abstract i v Acknowledgement v ' List of Figures V11 List of Tables List of Tables ' x 1. Introduction 1 1.1 Background of the research 1 1.2 Motivation of this study 3 1.3 Objectives 4 2. Literature Review 5 2.1 Applicable Standards 5 2.2 The Capacitor Unit 6 2.3 Fuse technologies 6 2.4 Capacitor Bank Design 10 2.5 Grounded Wye-Connected Banks 10 2.6 Multiple Units in Series Phase to Ground - Double Wye 11 2.7 Ungrounded Wye-Connected Banks 12 2.8 Multiple Units in Series Phase to Neutral - Single Wye 12 2.9 Delta-connected Banks '. 13 2.10 H Configuration 13 3. Methodology 1 4 3.1 Introduction 14 3.2 Software for transient analysis 14 3.3 Development of the Software Model 15 3.4 Different fault conditions used for Analysis 23 4. Results and Analysis 24 4.1 Normal Switching of Capacitor 24 4.2 Fast Switching of capacitor Banks 32 4.3 Switching of 33 kV load to the system 34 4.4 Disconnection of loaded distribution feeder from the system 35 4.5 Capacitor Unit Failure 36 4.6 Unbalance feeder faults in the system - X - - 38 ;; ii V , c w . . . . ' 4.8 Three phase to ground fault at 132 kV feeder 40 4.9 Lightning to 220 kV Feeder 42 5. Conclusion 5.1 Switching of Capacitor Banks 43 5.2 Switching of distribution loads 44 5.3 Capacitor unit failure 44 5.4 Unbalance system faults 44 5.5 Balance system Faults 44 5.6 Lightning to 220 kV Feeder 44 5.7 Summary 45 5.8 Recommendation 45 Annexure 1 - Proposed options for Pannipitiya GSS 48 Annexure 2 - The model Developed for simulation 49 Annexure 3 - 83.3 MVA Transformer data sheets 50 Annexure 4 - 31.5 MVA Transformer data sheets 51 Annexure 5 - 200kVA Transformer data sheet 52 Annexure 6 - Capacitor bank data sheets 53 Annexure 7 - Transmission line data sheets 54 iii Abstract In the island wide transmission network of Ceylon Electricity Board, there are 33kV Breaker Switch Capacitor Banks at twelve locations for improving the efficiency and quality of power. The capacitor bank installed at Pannipitiya Grid Substation, which is the highest capacity installed in a grid substation of the CEB network was failed immediately after connecting to the system. Several studies were conducted to identify reasons of the failure. However the final recommendation is still pending. Placement of capacitor banks in a grid substation is a major factor, influencing the reliability and efficient operation of capacitor banks. Therefore, this study was focused on to determine the preferred location of installing capacitor banks in a 220/132/33 kV grid substation. The two of possible locations are at 33kV tertiary of the power transformers and at the 33 kV load bus. Influences on capacitor banks under different fault conditions were analyzed in this study, while simulating the grid model built using Simulink in MATLAB program. Positive and negative impacts were found in respect of the two identified locations. • Switching stresses on capacitors of the bank is less when capacitors are installed at tertiary of the power transformers compared to the case when capacitors are at the 33 kV load busbar. • For balance or unbalanced feeder faults, preferred location is the 33 kV load busbar, since voltage and current fluctuations are less compared to the other location. iv • In case of lightning strikes at high voltage side, capacitors have less stresses when located at 33 kV load busbar. As such, it is recommended to connect capacitor banks at tertiary of 220/132/33 kV transformers of grid substations in industrial areas such as Export processing Zones, because the capacitor banks are subjected to frequent switching due to high load variations of the industrial load. Installation of capacitor banks at 33 kV load busbars is recommended to the grid substations where there are long power transmission lines with frequent feeder faults because such feeder faults have less influence on capacitor banks when they are located at the 33 kV load busbars. v Acknowledgement I sincerely thank my supervisor, Dr. J.P Karunadasa for his excellent corporation and guidance offered for the successful completion of this study. I extend my sincere thanks to lecturers of postgraduate study course in Electrical Engineering, University of Moratuwa, Sri Lanka, who gave me the theoretical knowledge and encouragement in bringing up this academic work. I specially thank Mrs. Y. M. Samarasinghe, Deputy General Manager (Western Province South), who initially put forth the idea of studying the ideal location for installation of capacitor banks in grid substations. I highly appreciate my friends and colleagues, specially Mr. Kurrupu Dharmasiri for their help offered to collect data and information required to prepare a realistic model for the study. Finally, I thank many individuals for making this research a success, who have not been mentioned here personally. vt List of Figures Figure 2 - Cross section of a Power Capacitor Unit 6 Figure 3 - Capacitor Fuse Technologies 7 Figure 4 - Externally fused type 7 Figure 5 - Internally fused shunt capacitor bank and capacitor unit 8 Figure 6 - Fuseless shunt capacitor bank and series string 9 Figure 7 - Multiple units grounded single Wye 10 Figure 8 - Multiple units grounded double Wye 11 Figure 9 - Multiple units ungrounded single Wye 12 Figure 10 - Multiple units ungrounded double Wye 12 Figure 10 - Two options considered for the Study 15 Figure 12 - Wiring Diagram of Capacitor Banks 19 Figure 13- capacitor model used to study the unit failure effect 19 Figure 14 - Lightning Impulse Model 20 Figure 15 - Lightning Impulse at 220 kV busbar 21 Figure 17- Equivalent model during instant of switching 24 Figure 18 - Current transient during switching of 1st 5Mvar capacitor bank 25 Figure 19 - Voltage transient during switching of 1st 5Mvar capacitor bank 25 Figure 20 - Current transient during switching of 2nd 5 Mvar capacitor bank 26 Figure 21 - Voltage transient during switching of 2nd 5Mvar capacitor bank 27 Figure 22 - Current transient during switching of 3rd 20Mvar capacitor bank 27 Figure 23 - Voltage transient during witching of 3rd 20Mvar capacitor bank 28 Figure 24 - Inrush current at the switching of 3rd step 20 Mvar capacitances 29 Figure 25 - Current transient during switching of 4th 20Mvar capacitor bank 30 Figure 26 - Voltage transient during switching of 4th 20Mvar capacitor bank 31 Figure 27 - Voltage transient during fast switchng of capaciotor banks 31 Figure 28 - Current transient during fast switchng of capaciotor banks 32 Figure 29 - Voltage transient during switched the loaded 33 kV feeder 33 Figure 30 - Current transient during switched the loaded 33kV feeder 33 Figure 31 - Voltage transient during disconnection of loaded 33kV feeder 34 Figure 32 - Current transient during disconnection of loaded 33kV feeder 34 Figure 33 - Voltage transient during capacitor unit failure with grounded case 35 Figure 34 - Current transient during capacitor unit failure in grounded case 35 Figure 35 - Voltage transient during capacitor unit failure with ungrounded case 36 vii Figure 36 - Current transient during capacitor unit failure with ungrounded case 37 Figure 37 - Voltage transient during 33 kV line fault at transformer option 38 Figure 38 - Voltage transient during 33 kV line fault at busbar option 38 Figure 39 - Current transient during 33 kV line fault at transformer option 39 Figure 40 - Current transient during 33 kV line fault at busbarr option 39 Figure 41 - Voltage transient during three phase to ground fault at 33 kV feeder 40 Figure 42 - Current transient during three phase to ground fault at 33 kV feeder 40 Figure 43 - Voltage transient during three phase to ground fault at 132 kV feeder 41 Figure 44 - Current transient during three phase to ground fault at 132 kV feeder 41 Figure 45 - Voltage transient at capacitor during lightning stroke at 220 kV feeder 42 Figure 46 - Transient current at capacitor during lightning stroke at 220 kV feeder 42 viu List of Tables Table 1 - Power capacitors installed locations in CEB network 3 Table 2 - 83.33MVA Transformer Data 17 Table 3 -31 .5 MVA Transformer Data 17 Table 4 - 200 kVA Earthing Transformer Data 17 Table 4 - Transmission and Distribution Line Data 18 Table 5- Calculated values for the switching of 5 Mvar capacitor bank 24 Table 6 - Calculated values for the switching of second 5 Mvar capacitor bank 26 Table 7- Steady state currents during switching of 41'1 stage 29 ix Chapter 1 Introduction 1.1 Background of the research Transmission of reactive power should be kept at a minimum level to reduce voltage drops, over voltages, transmission losses and to maximize the flow of active power. In order to minimize the flow of reactive power from the supply source, capacitors can be installed at selected places in the network close to the load. The use of Shunt Capacitor Banks (SCBs) is increasingly popular as they are relatively inexpensive, easy and quick to install and can be deployed virtually anywhere in the network. Load flow calculations will provide information on the total amount of reactive power required and the location of capacitors in the transmission network. As reactive power requirement changes during the daily load cycle, it is necessary to switch the capacitors to match with the load. The size of the individual capacitor step must be limited to minimize voltage fluctuations. This may require large number of switching equipment. Capacitors are installed at medium voltage level to minimize the cost of switching equipment. If the grid substation consists with auto transformers with delta tertiary, it is preferred to connect the capacitors to the tertiary because this will additionally reduce voltage fluctuations that would occur in the system. However, electrical transients appear in the network due to network switching and faults occurrences. Network equipment, including capacitor banks are subjected to high stresses resulting from surge current and voltage.. 1.1.1 Development of the CEB network Prior to 1980s, the Ceylon Electricity Board (CEB)'s transmission system was composed of 132 kV and 66 kV lines, which had been developed in coordination with the growth in demand and development of hydroelectric power projects for 1 delivering power to Colombo and other regions. The major hydropower stations of the Mahaweli and Laxapana Complexes are in central mountains and a number of transmission lines have been constructed towards Colombo area. Initially, 132 kV transmission lines were constructed as the major systems to transmit large power for long distance and 66 kV systems for local power transmission. In 1984 , the first 220 kV transmission line with duplex 9 • Zebra conductors (400 mm ) , Victoria -Kotmale-Biyagama (suburb of Colombo) commenced its operation to transmit the large generated power of the newly constructed two major hydro power stations of Victoria and Kothmale (approximately 410 M W in total) to Colombo. Later, this system was extended from Victoria to Rantambe via Randenigala, and from Biyagama to Kotugoda. Two 132 kV lines, Biyagama-Kelanitissa and Biyagama-Pannipitiya, were constructed with the 220 kV design and have been operated at 132 kV voltage. From Transmission & Substation Development Project 1, the 220 kV network of CEB expanded up to Pannipitiya grid substation. In 1997 NIPPON KOEI CO., Ltd of Japan conducted a master plan study for development of the transmission system of the Ceylon Electricity Board. They identified the requirement of static capacitors and shunt reactors to minimize the voltage variation of the bus voltage regardless of voltage drop or rise in long transmission lines. At that time only three static capacitors of 20 Mvar each have been installed on the 33 kV buses of the Kotugoda, Anuradhapura and Galle Grid Substations. At the Galle Grid Substation, additional static var compensators (SVC), +20 Mvar and -20 Mvar are also in operation for smooth adjustment of 132 kV system voltages. Under the master plan study of CEB, it had identified that low power factor of the Colombo power system as a serious problem to keep the operating voltage of the 132 kV systems. Though gas turbine generators are used in the condenser mode of operation, their available capacity was not enough. The shortage of reactive power was a serious problem at that time. Following to the Power System Analysis, it was proposed to connect 100 Mvar capacitors at Pannipitiya Grid substation by 2000 [1]. Reactive power compensating substations of the CEB network and there capacities are summarized in Table 1. No Location Capacity (MVar) Connection Location 1 Galle (SVC) 20 132 kV network 2 Anuradhapura 20 33 kV load bus 3 Habarana 10 33 kV load bus 4 Kotugoda 50 33 kV load bus 5 Kiribatkumbura 20 33 kV load bus 6 Kurunegala 10 33 kV load bus 7 Matugama 20 33 kV load bus 8 Panadura 20 33 kV load bus 9 Puttalama 20 33 kV load bus 10 Pannipitiya* 100 220/132/33 kV tertiary 11 Athurugiriya* 20 33 kV load bus 12 Thulhiriya* 10 33 kV load bus Table 1 - Power capacitors installed locations in CEB network * Breaker Switch Capacitors installed at Aturugiriya, Pannipitiya and Thulhiriya are not in operation due technical reasons. 1.2 Motivation of this study Under the Transmission & Substation Development Project 1, two number of 83.33 MVA power transformers were installed at Pannipitiya Grid Substation and Pannipitiya-Biyagama transmission line voltage level was improved to 220 kV. In order to improve the voltage profile of transmission network, 100 Mvar capacitor banks were installed at Pannipitiya grid substation under the Transmission & Substation Development Project -2, based on the Transmission Development Plan. 3 However, Breaker Switch Capacitors at Pannipitiya GSS, which were connected to tertiary of the 220/132/33 kV power transformers were failed within the defect liability period. The reasons to failure were under investigating. Therefore, 10 Mvar capacitor banks at Thulhiriya GSS and 20 Mvar banks at Athurugiriya GSS were kept in de-energized state even though no failures observed at those places, due to unresolved issues over this failure. Unavailability of necessary reactive power affects the CEB network in following ways. 1. Network operation is severely constrained in day to day operations due to lack of reactive power. 2. Cost incurred due to increased losses in power transmission. 3. Financial losses: Capital investment cost is around 15% of the total project cost. In Ceylon Electricity Board transmission network, Breaker Switch Capacitors have been connected atto the 33 kV load bus in all the locations, except at Pannipitiya GSS, where the capacitors are connected to the tertiary of 220/132/33 kV transformers. Various problems were uncounted from time to time, even the capacitor banks had been connected to the 33 kV load bus bars. Several technical problems are to be addressed in order to guarantee the availability of installed power capacitors in CEB network. One of the problems is the selection of the location of capacitor banks in a grid substation. Objectives The objective of this study is to identify the most suitable location for the connection of 33kV Breaker Switch Capacitors in a 220/132/33 kV Grid substations. Those possible locations are • Tertiary of 220/132/33 kV power transformer and • 33 kV Load busbar. Chapter 2 Literature Review 2.1 Applicable Standards 2.1.1 IEEE 18-2002 standard [2] Capacitors shall be capable of continuous operation provided that none of the following limitations are exceeded. • 110% of rated r.m.s. voltage 36.30 kV • 120% of rated peak voltage i.e., 1.2 * V2* rated r.m.s. voltage 56.00 kV Including harmonics but excluding transients • 135 % of nominal r.m.s. current based on rated kvar and rated voltage, For 5 Mvar bank 118.09 A For 20 Mvar bank 472.38 A Capacitors shall be capable of withstanding switching transients having crest voltage up to two times. • Reactive power manufacturing tolerance of up to 115% of rated reactive power. 2.1.2 IEC60871 -1: 1997 [3] Under the routing test, capacitor should withstand ac test voltage of 2.15 times rated r.m.s voltage. Long duration power frequency voltages are • 100% of r.m.s. voltage for continuous operation at power frequency • 110% of r.m.s. voltage for 12hours in every 24h • 115% of r.m.s. voltage for 30 minute in every 24h 5 • 120% r.m.s. voltage for 5 minutes • 130% r.m.s. voltage for 1 minute Maximum permissible currents are: • 130 % of r.m.s. current for continuous operation at rated voltage, rated current and rated frequency excluding transients. 2.2 The Capacitor Unit Figure 1 - Cross section of a Power Capacitor Unit Figure 1 is the building block of a shunt capacitor bank. The capacitor unit is made up of individual capacitor elements, arranged in parallel/ series connected groups, within a steel enclosure. The internal discharge device is a resistor that reduces the unit residual voltage to 50V or less in 5 min. Capacitor units are available in a variety of voltage ratings (240 V to 24,940V) and sizes (2.5 kvar to about 1000 kvar) [4], 2.3 Fuse technologies Capacitor units are available with Internal or External Fuses or Fuseless as shown in Figure 2. 6 imemal Discharge Devise ^ Bushmg E emerst Hi - X T T T-XI (a) Internally fused unit (b) Externally fused unit (c) Fuseless unit Figure 2 - Capacitor Fuse Technologies The use of fuses for protecting the capacitor units and its location (inside the capacitor unit on each element or outside the unit) is an important subject in the design of Shunt Capacitor Banks. 2.3.1 Externally Fused Externally fused SCBs are configured using one or more series groups of parallel- connected capacitor units per phase as shown in Figure 3. Advantages are • An individual fuse typically protects each capacitor unit. • The capacitor unit can be designed for a relatively high voltage because the external fuse is capable of interrupting a high-voltage fault. • Use of capacitors with the highest possible voltage rating will result in a capacitive bank with the fewest number of series groups. _ 1 S S *•> S S S ^ b :> C JL <_ < > j> > > > rifc . y jy-^T" ''p x ' 3 ± £X3 cara rxi KX2 I A..: Figure 3 - Externally fused type 7 Although the external fuses provide a visual indication of a failure, banks tend to occupy more substation space, are more expensive, have many live parts subject to possible damage by animals, and have higher installation and maintenance costs [4]. 2.3.2 Internally Fused Each capacitor element is fused inside the capacitor unit. Upon a capacitor element failure, the fuse removes the affected element only. The other elements, connected in parallel in the same group, remain in service but with a slightly higher voltage across them. Figure 4 illustrates a typical capacitor bank utilizing internally fused capacitor units. The capacitor units are normally large because a complete unit is not expected to fail [4], JL a; i/i H_ Jl. i^r Tr m <3) tfi Co r r Tr r r as i l/S I «S I Q . 1 T L x d "1 T"" prr • Advantages Designed and coordinated to isolate internal faults at the element level and allow continued operation of the remaining elements of that capacitor unit. Higher reliability. Less space, lower installation and maintenance costs, and fewer live parts Shunt Capacitor Banks without internal Fuses 8 2.3.3 Fuseless shunt capacitor bank To form a bank, capacitor units are connected in series strings between phase and neutral, shown in Figure 5. + 4 " T I L Figure 5 - Fuseless shunt capacitor bank and series string When the capacitor element fails, it welds and the capacitor unit remains in service. The voltage across the failed capacitor element is then shared among all the remaining capacitor element groups in the series. The fuseless design is not usually applied for system voltages less than about 34.5 kV. The reason is that there shall be more than 10 elements in series so that the bank does not have to be removed from service for the failure of one element because the voltage across the remaining elements would increase by a factor of about E (E - 1), where E is the number of elements in the string [4], The discharge energy is small because no capacitor units are connected directly in parallel. 2.3.4 Unfused Shunt Capacitor Banks Contrary to the fuseless configuration, where the units are connected in series, the unfused shunt capacitor bank uses a series/parallel connection of the capacitor units. The unfused approach would normally be used on banks below 34.5 kV, where series strings of capacitor units are not practical, or on higher voltage banks with modest parallel energy. This design does not require as many capacitor units in parallel as an externally fused bank [4], 9 2.4 Capacitor Bank Design 2.4.1 Basics of capacitor bank design and capacitor unit connections. The optimum connection for a SCB depends on the best utilization of the available voltage ratings of capacitor units, fusing, and protective relaying. Virtually all substation banks are connected wye. Distribution capacitor banks, however, may be connected wye or delta. Some banks use an H configuration on each of the phases with a current transformer in the connecting branch to detect the unbalance. 2.5 Grounded Wye-Connected Banks Grounded wye capacitor banks are composed of series and parallel-connected capacitor units per phase and provide a low impedance path to ground. Figure 6 shows typical bank arrangements. Figure 6 - Multiple units grounded single Wye Advantages of the grounded capacitor banks include: • Its lcfw-impedance path to ground provides inherent self-protection for lightning surge currents and give some protection from surge voltages. Banks can be operated without surge arresters taking advantage of the capability of the capacitors to absorb the surge. • Offer a low impedance path for high frequency currents and so they can be used as filters in systems with high harmonic content. However, caution shall be taken to avoid resonance between the SCB and the system. • Reduced transient recovery voltages for circuit breakers and other switching equipment. • Some drawbacks for grounded wye SCB are: • Increased interference on telecom circuits due to harmonic circulation. • Phase series reactors are required to reduce voltages appearing on the CT secondary due to the effect of high frequency, high amplitude currents. 2.6 Multiple Units in Series Phase to Ground - Double Wye When a capacitor bank becomes too large, making the parallel energy of a series group too great (above 4650 kvar) for the capacitor units or fuses, the bank may be split into two wye sections. Figure 7 shows typical bank arrangement for multiple units grounded double wye capacitor bank [4], The characteristics of the grounded double wye are similar to a grounded single wye bank. T Figure 7 - Multiple units grounded double Wye 2.7 Ungrounded Wye-Connected Banks Ungrounded wye banks do not permit • Zero sequence currents, • Third harmonic currents, or • Large capacitor discharge currents during system ground faults to flow. • Over voltages appearing at the CT secondaries are not as high as in the case of grounded banks. 11 However, the neutral should be insulated for full line voltage because it is momentarily at phase potential when the bank is switched or when one capacitor unit fails in a bank configured with a single group of units. Figure 8 shows multiple units ungrounded single wye capacitor bank. For banks above 15kV this may be expensive [4], Multiple Units in Series Phase to Neutral - Single Wye Figure 8 - Multiple units ungrounded single Wye Figure 9 - Multiple units ungrounded double Wye When a capacitor bank becomes too large for the maximum 4650 kvar per group the bank may be split into two wye sections as shown in Figure 9. As for any ungrounded why bank, the neutral instrument transformers should be insulated from ground for full line-to-ground voltage, as should the phase terminals [4], 2.9 Delta-connected Banks Delta-connected banks are generally used only at distributions voltages and are configured with a single series group of capacitors rated at line-to-line voltage. With only one series group of units no overvoltage occurs across the remaining capacitor units from the isolation of a faulted capacitor unit [4], 2.10 H Configuration Some larger banks use an H configuration in each phase with a current transformer connected between the two legs to compare the current down each leg. This arrangement is used on large banks with many capacitor units in parallel [4]- 13 Chapter 3 Methodology 3.1 Introduction Transient disturbances in power systems may damage key equipment, potentially having a great impact on system reliability. These transients may be introduced during normal switching operations, interruption, short circuit faults, lightning strikes, or due to equipment failures. Capacitor banks installed in a grid substation would have great stresses from excessive currents and voltages due to such type of transients. Different fault conditions, which could develop in the power supply network were simulated to determine the most suitable location of capacitor banks to be installed in a grid substation and the simulated results were used for the analysis of voltage and current stress on capacitors and switchgears. 3.2 Software for transient analysis Dynamic analysis model was to select for the analysis of transient voltages and currents in the network during different switching conditions due to frequency dependency the system. The ATP-EMTP software (Alternative Transients Program - Electromagnetic Transients Program) is mostly used world-wide to compute electromagnetic transients in electrical power systems. PSSE software is available in CEB to analysis the network transient. Currently, the PSSE software is used for the steady state analysis only. Therefore it requires to modify all the data files for dynamic analysis of the system. Due to time limitation, it was difficult to use PSSE for this study. Considering convenience and the complexity of the study model MATLAB simulation software was used for this study. 14 3.2.1 Selection of Model for Study Pannipitiya Grid Substation was selected as the model to study the optimal location for installation of capacitor in a 220/132/33kV Grid Substation. An equivalent model of Pannipitiya GSS was developed to include both the proposed options in the same model as depicted in Figure 9 . The switchgear arrangement of th ePannipitiys gris substation is given in annexure 1. The options studied for connecting shunt capacitors are • Capacitor banks connected to tertiary of 220/132/33kV transformers. • Capacitor banks connected to 33 kV load bus. z § Figure 10 - Two options considered for the Study 3.3 Development of the Software Model The model consist of following equipment, 220 kV Switch Yard 02 Nos.: 220/132/33 kV, 83.33 MVA auto transformers 02 Nos.: 220 kV Line Feeder Bays 01 N o . : 220 kV Bus section bay 15 132 kV Switch Yard 02 Nos: 132/33 kV, 31.5 MVA Power transformers 04 Nos.: 132 kV Line Feeder Bays 01 No.: 132 kV Bus section bay 02 Nos.: 200 kVA earthing transformers 33 kV Switch Yard 04 Nos.: 33 kV Feeder Bays 01 No.: 132 kV Bus section bay 02 Nos.: 50 Mvar Capacitor banks are connected to 33 kV tertiary of each auto transformers or either side of the 33 kV bus section depend on study option. Power System switching transients were initiated by the actions of the circuit breakers and by faults in the power system. The frequency dependent models were selected to represent the various components in the network. 3.3.1 Grounding System 220 kV and 132 kV systems were solidly grounded through power transformers. 33 kV system was grounded via earthing transformers. Capacitor banks were configured as ungrounded Double Wye (STAR) connected capacitor units with a fault limiting reactor connected to each phase. Transformers were not in parallel operation and therefore maximum capacitive reactive power on one 33 kV busbar was 50 Mvar. Power Transformers Transformers were modeled using name plate data and commissioning test data. For transient studies transformer parameters such as percentage impedances, winding resistances, capacitances core losses and hysteresis loss were considered [5], 16 83.3 MVA, 220/132/33 kV Auto Transformer Winding Percentage impedances Percentage resistance Percentage inductance HV-LV 14.18 0.01562 0.04514 HV-TV 16.67 0.01376 0.03826 LV-TV 12.02 0.02343 0.05306 Table 2 - 83.33MVA Transformer Data Selected base values were 83.33 MVA, 220 kV voltages and 220 kV on respective levels. Winding resistances 1U-2U 0.1225 Q 2U-N 0.2654 Q 3U-3V 0.09351 Q Source of supply The fault level at 220kV busbar was taken as 25 kA. 220 kV system was replaced with the thevenin equivalent at 220 kV supply point based on short circuit current value. 31.5 MVA, 132/33 kV Transformer [6] Winding Percentage impedances Percentage Resistance Percentage inductances HV-LV 10.60 0.00055083 0.08922 Table 3 - 3 1 . 5 MVA Transformer Data 200 kVA Earthing Zig Zag Earthing Transformer Winding Zero sequence impedance (%) Percentage Resistance Percentage inductances HV 9.57% 24.67 12550 Table 4 - 200 kVA Earthing Transformer Data 17 Transmission Lines Due to the f requency dependence of transient surges 132 kV t ransmiss ion lines and 33 kV distribution feeders were modeled using Pi -sect ions . Cascaded pi- sections were selected to obtain accurate transients. Actual line parameters were used [7]. Line description Length (km) R/cct (pu) X/cct (pu) Y/cct (pu) P a n n i p i t i y a - Ratmalana 132 kV Transmission Line 6.9 0.00705 0.01588 0.00336 Pannipitiya - Panadura 132 kV Transmission Line 12.3 0 .00629 0.02734 0.00631 33 kV lines 4.7 0.0048 0.01082 0.00229 Table 5 - Transmission and Distribution Line Data Switch Gear - Circuit Breakers Circuit breakers were modeled with introducing small contact resistance, high snubber resistances and capacitances. External control switch was used to control the breaker activities. Loads and Reactors 132 kV and 33 kV loads were modeled using parallel RLC elements. Breaker Switched Capacitors Capacitor bank connection : Ungrounded Split Wye (Figure 11) Number of Series Groups : 2 Number of Parallel Units : 2 Installed Units in Block : 8 All capacitor units are internally fused design type [8]. Capacitor banks are usually modeled as a single lumped element [9]. T o study the system behaviors during capacitor unit failure, capacitor bank was modeled as a separate block with units as shown in Figure 12. 18 % LIBRARY §9 UNIVERSITY OF M O R A T U W A , SRI LANKA m o r a t u w a n phase A p h a s e B -TCT^ •Phase C T T T T T T Current limiting — L reactor 33 kV 0.5 mH TTT /' Each symbol equals 2 capaciotr units in parallel with internal fuse Figure 11 - Wiring Diagram of Capacitor Banks Figure 12- capacitor model used to study the unit failure effect Time steps and Simulation length External controller of the circuit breaker model was used to simulate the switching of capacitors, loads and fault conditions. Time scales in seconds were selected to observe the current and voltage behavior during normal switching and to study the effect due to lightning impulse time scale in micro second range was selected. 3 2 4 2 5 Lightning Impulse Model Lightning strike current wave form was approximated by an expression called Heidler Function. This formula had been developed assuming vertical lightning channel and perfect ground. The analytical expression to represent the channel base current iO(t) is proposed by Heidler [10]. 4% J +i Where 10 = The magnitude of the channel base current t i , T2 = Front and the decay time constant (1.2/50 p.s) n = Exponent having values between 2 to 10 (4.8) 11 = Amplitude correction factor.(0.85) Modeling the lightning stroke Figure 13 - Lightning Impulse Model The induced voltage impulse at 220 kV busbar of the model due to simulated lightning impulse is shown in Figure 14. 2 0 220 KV B U S B A R V O L T A G E - M E A N V A L U E D U R I N G L IGHTNING A T 220 K V S Y S T E M 2 1 . 6 > 01 » 1 o > O.S 0 -0.5 0 1 2 3 4 5 6 7 8 Time (s) x 10-s Figure 14 - Lightning Impulse at 220 kV busbar Assumptions made for modeling Zero sequence impedance of 132kV winding of 220/132/33 kV transformer is approximately equal to 80%-90% of positive sequence impedance of 132 kV to 33kV tertiary winding [11], Zero sequence impedance of 220kV winding of 220/132/33 kV transformer is approximately equal to 85%-95% of positive sequence impedance of 220 kV to 33kV tertiary winding [11], 200 kVA earthing transformer winding inductance is proportional to that of 31.5 MVA transformer. The simulink model of the selected system is shown in annexure 2. 1 1 ; : i — } - ! s i _ I t 1 ! ] : i i t i 21 3.4 Different fault conditions considered for analysis Current and voltage stress in connection with the shunt power capacitors units were studied under the following switching and fault conditions. 1. Normal switching of capacitors 2. Fast switching of capacitors 3. Switching of 33 kV load to the system 4. Disconnection of loaded distribution feeder from the system 5. Failure of a Capacitor Unit 6. Unbalance faults in the system 7. Balance faults in the system 8. Lightning stroke in the 220 kV transmission line 2 2 Chapter 4 Results and Analysis Fault conditions specified in section 0 were simulated and the voltage and current transients at capacitor banks were obtained for each fault. Notations used in the graphs: V33T1- Voltage variation of capacitor bank which was connected to tertiary of power transformer V33B2- Voltage variation of capacitor bank which was connected to 33kV Busbars I33T1- Current variation of capacitor bank which was connected to tertiary of power transformer 133B2- Current variation of capacitor bank which was connected to 33kV Busbar Normal Switching of Capacitor The behavior of capacitor banks were studied while switching capacitor banks at following time intervals. 1st 5 Mvar capacitor bank 1 second 2nd 5 Mvar capacitor bank 2 second 3rd 20 Mvar capacitor bank 3 second 4 th 20 Mvar capacitor bank 4 second The selected switching times for simulation of the study were less than the practical switching times. However these times were selected considering the steady state condition of each switching event. Therefore the effects of previous switching will not influence on the next switching transient. Switching inrush current in a capacitor bank can be explained using natural frequency (Wo) of the circuit, which is a function of L and C of the circuit as shown in Figure 15 and surge impedance (Zo) of the circuit of the circuit. 23 MDMMM* Figure 15- Equivalent model during instant of switching Vs = = Source voltage Vc = = Voltage across the capacitor L = = Equivalent circuit inductance C = = Equivalent circuit capacitance R = Equivalent circuit resistance S'In (r,>nf) i(t: = v(0, *—-—J- W h e r e z c = ^ , a > 0 = — V(0) is the difference between source voltage and the initial voltage of the capacitor at the instant of energization. .1 Switching of first 5 Mvar capacitor bank The calculated surge impedance, natural frequency and steady state current for the switching of first 5 Mvar capacitor bank of the selected two options are as follows. 5 Mvar-1s t Bank switching ZO (ohm) f (Hz) 1(A) Transformer Tertiary 116.52 96 87.477 Busbar 113.44 93 87.477 Table 6- Calculated values for the switching of 5 Mvar capacitor bank Current and voltage wave forms obtained by simulating the model for the switching of first 5 Mvar capacitor bank are illustrated in Figure 15 and Figure 16. The steady state current of both the cases in the above f igures were conf i rmed by the calculated steady state currents values. There are no major d i f ferences except few minor deviations of f requency of oscillation and magni tude of transient current for the two options, during switching of 5 Mvar capacitor bank. First stage - Switching of first 5 Mvar capacitor bank - Switching t ime 1 second t o 33 K V C A P A C I T O R B A N K R M S C U R R E N T F O R T W O O P T I O N S 0.990 1.030 I33T1 I33B2 Figure 16 - Current transient during switching of l st 5Mvar capacitor bank 33 KV C A P A C I T O R B A N K R M S V O L T A G E F O R T W O O P T I O N S 30.00 0.990 1.000 1.010 T I M E (s) 1.020 1.030 V33T1 Figure 17 - Voltage transient during switching of 1st 5Mvar capacitor bank 25 4.1.2 Switching of second 5 Mvar capacitor bank During the switching of the second 5 Mvar capacitor bank also, it was diff icult to observe any differences of the two waveforms corresponds to the two cases of study. Below table shows the calculated values and Figure 18 and Figure 19 indicate the simulated the results obtained from the model. 5 Mvar- 2n d Bank Zo (ohm) f (Hz) 1(A) T/f. Tertiary 82.40 66 174.95 Busbar 80.22 68 174.95 Table 7 - Calculated values for the switching of second 5 Mvar capacitor bank For the second stage switching of second 5 Mvar capacitor bank switching time of 2 seconds was selected. 33 K V C A P A C I T O R B A N K R M S C U R R E N T F O R T W O O P T I O N S T I M E ( s ) I33T1 — I33B2 Figure 18 - Current transient during switching of 2nd 5Mvar capacitor bank 26 T I M E (s) V 3 3 T 1 V 3 3 B 2 Figure 19 - Voltage transient during switching of 2nd 5Mvar capacitor bank 4.1.3 Switching of 3rd 20 Mvar capacitor bank Simulated results of voltage and current transients are shown in Figure 20 and Figure 21 when switching of 3rd capacitor bank with capacity o f 20 Mvar at 3 seconds. For third stage switching of 20 Mvar capacitor bank 3 seconds switching was selected. 33 KV CAPACITOR BANK R M S C U R R E N T FOR TWO OPT IONS 7,000 6,000 _ 5,000 - 4,000 c t 3,000 o 2,000 1,000 0 2.990 3.000 3.010 3.020 3.030 3.040 3.050 3.060 3.070 TIME(S) I33T1 —— I33B2 Figure 20 - Current transient during switching of 3rd 20Mvar capacitor bank 27 T I M E (s) V 3 3 T 1 V 3 3 B 2 Figure 21 - Voltage transient during witching of 3rd 20Mvar capacitor bank At the instant of switching of the 3rd 20 Mvar capacitor bank, it is in series with ] 0 Mvar capacitor bank, which had been switched in Ist and 2nd stage (Paralleling of two of 5 Mvar capacitor banks). Equivalent capacitances when switching on the 3rd step of the capacitor bank is 19.5 |Jf- Therefore the resultant inrush current during 3rd stage 20 Mvar capacitor is higher as shown in Figure 22 due to the series combination of total capacitance of 10 Mvar, source impedance and capacitance of 20 Mvar Capacitor bank. This type of switching is called back to back switching. The energized capacitor bank provides an extremely low source-impedance for the switching capacitor bank leading to extremely high transient currents in both banks. Current limiting reactors of the capacitors should be designed to control such high transient currents. When capacitor is located at 33 kV load busbars, rms value of the capacitor inrush current is very high, compared to the same when the capacitor bank is connected to the 33kV tertiary of the transformer. Obtaining a solution by manual calculation is difficult of this type of circuits which involves large number of R, L and C branches. Numerical methods of solving such a system of differential equations have been developed and EMTP is an example for same. Using the study model developed with M A T L A B simulink, it can observe a higher inrush current during switching of capacitor banks when located at load busbars. Instantaneous current during the switching of the 3 rd step 20 Mvar capacitor bank. 4 Curren Inrush when Switching of 20 Mvar x 10 capacitors located a 33 kV load Busbars. | L 1 IVVVVW-- JWWiAA 2.995 3 3.005 3.01 3 015 3.02 3.025 3 03 3.035 Time (S) Figure 22 - Inrush current at the switching of 3 rd step 20 Mvar capacitances. 4 Switching of 4 th 20 Mvar capacitor bank. Steady state currents derived by manual calculation 20 Mvar- 4 th Bank ZO/ohm f / H z I/A T/f. Tertiary 36.84 29.5 875 Busbar 35.88 30 875 Table 8- Steady state currents during switching of 4 th stage 29 At the instant of switching on the 4th capacitor bank, the equivalent capacitance is 31.5 |jf. Therefore switching inrush current to the capacitor bank is not very high compared with the switching of 3rd step of capacitor bank switching. Simulated results of voltage and current transients are shown in Figure 20 when switching of 4 th capacitor bank with capacity of 20 Mvar at 4_seconds. 33 KV CAPACITOR BANK R M S C U R R E N T FOR TWO OPT IONS 3.950 4.000 4.050 4.100 TIME(s) Figure 23 - Current transient during switching of 4th 20Mvar capacitor bank « o £ 50 0) $ 45 40 35 33 K V C A P A C I T O R B A N K R M S V O L T A G E F O R T W O O P T I O N S 30 3.950 4.000 4.050 4.100 4.150 4 .200 T I M E (s ) V 3 3 T 1 V 3 3 B 2 30 Figure 24 - Voltage transient during switching of 4 th 20Mvar capacitor bank In this case of switching, it takes few cycles to stabilize oscillation of current and voltage. This type of switching transients creates power quality problems on sensitive customer loads. Due to high switching transient current and voltage, switchgears such as circuit breakers and disconnectors connected to the capacitor feeders should be selected to withstand these high transient currents. Fast Switching of capacitor Banks Capacitor banks at the Pannipitiya grid substation can be switched manually. Therefore all four capacitors banks amounting to 50Mvar can be switched very fast. Thus, the behavior of voltages and currents during fast switching were studied using the same simulink. Switching t imes corresponds to four capacitor banks were selected to be 0.03 s, 0.04s, 0.06 s and 0.08 s. Simulation results are illustrated in Figure 25and Figure 26. 33 KV C A P A C I T O R BANK RMS V O L T A G E FOR T W O OPTIONS DURING FAST SWITCHING 0.02 0.04 T I M E ( s ) 0.08 0.10 0.12 — V33T1 V33B2 Figure 25 - Voltage transient during fast switchng of capaciotor banks It can be observed that transient t ime during switching of step 3 was not completed when the forth switching step starts. Therefore the magni tude of transient voltage and transient t ime increased to higher values where capacitor performance could be adversely affected. Undue increase of voltages across the i 31 capacitor units will reduce the life time of the capacitors by dielectric failures. Due to fast switching, quality of the supply power at distribution level also adversely affects. 33 KV CAPACITOR BANK RMS CURRENT FOR TWO OPTIONS DURING FAST SWITCHIN Figure 26 - Current transient during fast switchng of capaciotor banks Due to fast switching of 20 Mvar capacitors high transient current is remained for a considerable duration. Current magnitude is few hundreds of rated rms currents. Due to this type of transient over voltages and current not only capacitors but also other equipments in the capacitor bays subjected to voltage stresses that could result insulation failures and reduced the life time of the equipment. Switching of 33 kV load to the system Switching of 33kV load was simulated for the two cases of study and resulting voltage and current transients when load connected at 0.1s are given in Figure 27 and Figure 28. When the load is switched on, voltage of the capacitors will reduced depending on the capacity of the load and the current flow from the capacitor will increase, if the capacitor banks were located at 33 kV busbar, compared to the case where the capacitor banks were connected to the tertiary of the 220/132/33kV transformer. When the capacitors were at busbars, the capacitors had been more sensitive to the changes in voltage and currents due to sudden connection of load. If capacitor banks were located at tertiary of the auto transformers, capacitor banks would not stress due to switching of load feeders. -a c TO 34.4 34.2 34.0 33.8 33.6 33.4 33.2 33.0 0.09 0.11 TIME (s) 0.13 0.15 0.17 • V33T • V33L2 Figure 27 - Voltage transient during switched the loaded 33 kV feeder c 2! L. 3 o 33 KV CAPACITOR BANK RMS CURRENT FOR TWO OPTIONS WHEN 33 KV LOAD SWTCED TO THE SYSTEM 247 237 227 217 0.09 0.11 0.13 0.15 TIME (s) I33T1 I33B2 Figure 28 - Current transient during switched the loaded 33kV feeder Disconnection of loaded distribution feeder from the system Similarly, for sudden disconnection of 33 kV loads, stresses to the capacitor banks were low if the capacitors were located at the tertiary of the power transformer. 33 With the sudden disconnection of load, voltage of the load busbar will increase. It will directly affect to the capacitors located at the busbars. Simulated results of voltage and current transients are shown in Figure 29 and Figure 30 when load disconnected at 0.1s. 33 KV C A P A C I T O R BANK R M S V O L T A G E FOR T W O OPTIONS 34.4 34.2 34.0 - r ^ 33.8 ' 33.6 33.4 33.2 i 33.0 0.09 0.11 0.13 0.15 0.17 T I M E (s) V 3 3 T V 3 3 L 2 Figure 29 - Voltage transient during disconnection of loaded 33kV feeder 33 KV CAPACITOR BANK RMS CURRENT FOR TWO OPTIONS 219 217 T T 0.09 0.11 0.13 0.15 TIME (s) I33T1 I33B2 Figure 30 - Current transient during disconnection of loaded 33kV feeder 4.5 Capacitor Unit Failure During capacitor unit failure transient voltage and current impact to the capacitor banks were studied for two options. 1. Capacitor case is grounded 2. Capacitor case is isolated from ground g f 3 34 4 . 5 . 1 G r o u n d e d c a s e Simulated results of voltage and current transients are shown in Figure 31 and Figure 32, when a grounded case capacitor unit failed at 1 second. 33 KV CAPACITOR BANK RMS VOLTAGE FOR TWO OPTIONS FOR CAPACITOR UNIT FAILURE-CASE GROUNDED TIME (s) — V33T1 — V 3 3 B 2 Figure 31 - Voltage transient during capacitor unit failure with grounded case 33 KV CAPACITOR BANK RMS CURRENT FOR TWO OPTIONS FOR CAPACITOR UNIT FAILURE-CASE GROUNDED Figure 32 - Current transient during capacitor unit failure in grounded case During a unit failure of the capacitor bank, the grounded cover provides low impedance path to discharge the charges remain in the faulty capacitor. These charges will drain off as a transient current. 35 Total impedance of the fault path was less, when capacitors were located at transformer tertiary compared to the other option, where capacitor banks located at load busbars. Therefore a higher current in transformer tertiary can be observed in Figure 32 during capacitor unit failure of grounded capacitor system compared to the busbar option. 4.5.2 Ungrounded case However, the model prepared for this study contains ungrounded capacitor banks. The ungrounded capacitor banks do not permit to f low fault currents, or large discharge currents during system faults. The equal results were obtained by simulation and results are given in Figure 33 and Figure 34 when an ungrounded case capacitor unit failed at 0.15 second. It is difficult to observe any significant change of voltage or current of the capacitor banks for both options due to failure of a capacitor unit. in -a c CO it) 3 O O > 33 KV CAPACITOR BANK RMS VOLTAGE FOR TWO OPTIONS FOR CAPACITOR UNIT FAIL-COVER UNGROUNDED 34.6 34.4 34.2 34.0 33.8 33.6 33.4 33.2 33.0 0.13 0.14 0.15 TIME(s) 0.16 0.17 -V33T1 - V33B2 Figure 33 - Voltage transient during capacitor unit failure with ungrounded case 33 KV CAPACITOR BANK RMS CURRENT FOR TWO OPTIONS FOR CAPACITOR UNIT FAIL-COVER UNGROUNDED 299 297 S 295 | 293 D o 291 289 287 — - — — 0.13 0.14 0.15 0.16 0.17 0.18 TIME(s) I33T1 I33B2 Figure 34 - Current transient during capacitor unit failure with ungrounded case Unbalance feeder faults in the system Voltage and current behaviors at capacitors for proposed two options were analyzed for the following faults conditions. • Single Phase to Earth Fault at 132kV and 33 kV feeders • Double Phase to Earth Fault at l32kV and 33 kV feeders The behavior of voltage and current for both the options was similar for all the feeder faults described above. Simulated results of voltage and current transients are shown in Figure 39, Figure 40, Figure 37 and Figure 38 when an unbalanced 33kV feeder fault occurred at 23 millisecond time. According to Figure 39 and Figure 40, when capacitor banks were located at the tertiary of the transformers, voltage fluctuations due to unbalanced 33 kV feeder faults were high compared to the busbar option. 37 C a p B a n k V o l t a g e c o n n e c t e d t o t r a n s f o r m e r d u r i n g s i n g l e p h a s e to g r o u n d fau l t at 3 3 k V f e e d e r s « 40.00 •o a 35.00 _ g 30.00 — H 25.00 §> 20.00 o 15.00 10.00 5.00 0.00 0.! V33T V33L2 Figure 39 - Voltage transient during three phase to ground fault at 33 kV feeder 33 KV CAPACITOR BANK RMS CURRENT FOR TWO OPTIONS 1,200.00 1,000.00 = ; 800.00 | 600.00 O 400.00 200.00 0.00 0: Figure 40 - Current transient during three phase to ground fault at 33 kV feeder Three phase to ground fault at 132 kV feeder. Simulated results of voltage and current transients are shown in Figure 41 and Figure 42, when three phase to ground fault at 132 kV feeder occurred at 1 second. With lOOMvar capacitor bank operating in steady state during a three phase ground fault at the end of 132 kV feeder, capacitor discharges into the fault, however does not fully discharge as in the case of 33 kV feeder fault due to the TIME (s) T I M E ( s ) I33T1 — I33B2 impedance of 132/33 kV transformer when the capacitor bank was at the 33 kV load busbar. When the capacitor banks were located at tertiary of the t ransformer, voltage of the capacitor bank would not be reduced to zero due to impedance of 220/132/33 kV transformer. 33 KV CAPACITOR BANK RMS VOLTAGE FOR TWO OPTIONS in 40.00 •a ra 35.00 _ g 30.00 - h 25.00 | 20.00 | 15.00 10.00 5.00 0.00 0.900 1.400 1.900 2.400 2.900 TIME (s) V33T V33L2 Figure 41 - Voltage transient during three phase to ground fault at 132 kV feeder 33 KV CAPACITOR BANK RMS CURRENT FOR TWO OPTIONS 1,200.00 1,000.00 ^ 800.00 | 600.00 O 400.00 200.00 0.00 0.900 1.400 1.900 2.400 2.900 TIME(s) I33T1 I33B2 Figure 42 - Current transient during three phase to ground fault at 132 kV feeder During balance faults at 132 kV system, settling t ime was greater and frequency of oscillation also high when capacitors were located at t ransformer tertiary. 41 4.9 Lightning to 220 kV Feeder Lightning stroke at 220 kV transmission network affected to two options differently as shown in Figure 43 and Figure 44. When capacitors were installed at transformers, total impedances were less compared to the 33 kV load busbar. Therefore high voltage and high current will appeared in capacitors when they were at Transformer tertiary. The affect of lightning stroke to capacitor was much less if they were installed at 33 kV load busbar due to addition of impedances from 31.5 MVA transformer. 33 KV CAPACITOR BANK RMS VOLTAGE FOR TWO OPTIONS DURING LIGHTNING STROKE AT 220 KV FEEDER CO -o 3,000 = 2,500 ^ 2,000 1,500 ~ 1 ,000 S 500 ra o 0 > 0.E+00 5.E-04 1.E-03 2.E-03 2.E-03 3.E-03 TIME (s) —VT1 — VB2 Figure 43 - Voltage transient at capacitor during lightning stroke at 220 kV feeder 33 KV CAPACITOR BANK RMS.CURRENT FOR TWO OPTIONS DURING LIGHTNING STROKE AT 220 KV FEEDER "O o 0.E+00 5.E-04 1.E-03 2.E-03 TIME(S) _ j f l — IB2 Figure 44 - Transient current at capacitor during lightning stroke at 220 kV feeder 42 Chapter 5 Conclusion From the simulation results, optimal location for fixing the power capacitors in a grid can be summarized as below. Switching of Capacitor Banks During energization of capacitors, transient over current and over voltages were high if capacitor banks were located at 33 kV busbar. When consider the switching of capacitor banks, it is preferred to locate the power capacitors at tertiary of the transformers. The reasons were: Capacitor bank switching is one of the most frequent utility operations, potentially occurring multiple times per day and hundreds of time per year throughout the system, depending on the need for system. Utility capacitor bank switching can have negative impacts on power quality, especially for customer power systems. AC and DC drives, along with other electronic equipment, can be very sensitive to transient voltages. Utility capacitor bank switching transients can be magnified at low voltage capacitor locations on customer power systems, causing drives to trip and production and other processes to stop. Since fast switching of capacitor banks can increase the voltage variations which were higher than the specified voltage levels of capacitor banks, could causes dielectric failures in capacitor units. Therefore it is more preferred to switch the capacitor banks in pre decided time intervals to avoid development of unnecessary stress in capacitor units. 43 5.2 Switching of distribution loads During connecting and disconnecting of 33 kV loads changes in voltage and currents were high at capacitors, when they were connected to the 33 kV load busbars. The reason was that capacitors and loads were in the same busbars and changes in voltage will directly transfer to the capacitors. 5.3 Capacitor unit failure For both options it is difficult to observe significant effect to the capacitor banks due to failure of a capacitor unit. However capacitor units in parallel in the same group may have slightly higher voltage across them. Since the capacitor cover is not grounded a unit failure of capacitor not have much influence for both options. 5.4 Unbalance system faults During 33 kV and 132 kV feeder faults, changes in capacitor bank voltage and current were similar for both options. At transformer locations less damping can be observed. 5.5 Balance system Faults For 33 kV balance faults two options have similar behavior but during 132 kV balance fault less damping and high-oscillations were present when capacitors at transformer tertiary. 5.6 Lightning to 220 kV Feeder Resultant effects due to lightning stroke were high when capacitors located at transformer tertiary. The high voltages and current in capacitor could ultimately lead to the failure to capacitor banks. In practically suitable energy dissipation lightning arrestors were installed to divert these lightning impulses at line end of the grid and at high voltage and low voltage sides of the power transformers. 44 Summary If capacitor banks were installed at tertiary of the power transformers capacitors were open to less stresses during • Switching of capacitor banks frequently If capacitor banks were located at 33 kV busbars the Capacitors have less stresses for • Phase to earth faults in feeders • Lightning strikes at high voltage side. Generally lightning arrestors are installed in the grid substation to suppress the lightning strokes. Lightning arrestor function was not included for this model. Recommendation Capacitor banks at tertiary of 220/132/33 kV transformers are recommended for grid substations at industrial areas where the capacitor banks were switched frequently and distribution line faults are less. Example locations are, • Kotugoda • Pannipitiya • Thulhiriya Capacitor banks at 33 kV busbars are recommended to the grid substations where voltage improvement is required due to long transmission power lines. Since most of these are end grid substations and availability of tertiary transformers also limited. Example locations are, • Anuradhapura • Habarana • Panadura • Puttalama • Kurunegala References [1] Master Plan Study for Development of The Transmission system of The Ceylon Electricity Board, Nippon Koei Co., Ltd, Tokyo, Japan; January 1997 [2] IEC 60871-1, Shunt capacitors for a.c. power systems having a rated voltage above 1000V, Second edition 1997-10 [3] IEEE Std 18™ -2002 d for Shunt Power Capacitors [4] Gustavo Brunello, M.Eng, P.Eng Dr. Bogdan Kasztenny "Shunt Capacitor Bank Fundamentals and Protection 1",2003 Conference for Protective Relay Engineers - Texas A&M University,April 8-10, 2003, College Station [5] Transformer Test Reports, Serial No. 01FT240101, Mitsubishi Electric Corporaion, Japan, August 2001. [6] Transformer Test Reports, Serial No. 0605201/01, Siemens Transformer S.P.A, September, 2007. [7] CEB, Long Term Transmission Plan, 1998 and 2006 [8] Internally Fused Capacitor Bank, Name Plate, COOPER Power Systems, McGraw Edision Power CAPACITORS, Greenwood, SC 29646, USA. [9] Modeling Guidelines for switching Transients, Switching Transient Task Force, IEEE Modeig and Analysis of System Transient Working Group. [10] IEE Transaction on Electromagnetic Compatilibity, Vol 40, No. 04, November 1998. 46 Martin Heathcote,"J & P Transformer Book", Edition 13, PP 33-34, Newnes 2007. Electrical Transients in Power Sysems, Allan Greenwood, John Wiely & Sons Inc, 1971. 47 Annexure 1 - Proposed options for Pannipitiya GSS BREAKER SWITCH CAPACITOR INSTALLATION LOCATIONS TWO OPTIONS Annexure 2 - The model Developed for simulation o o V v/ n u n m r F H - c co o 4. " A . « s •J ^ CL U U U U f i m n m ) a r ^ 1 r J J L S L JL f l - f i J p m p i 49 n nnexure 3 - 83.3 MVA Transformer data sheets LAI M 1 v r W I T H O N - L O A D T A P - C H A N G E R 5 0 H z I E C - S 0 Q 7 6 1 1 9 9 3 ) | T Y P E S R S - M R M S 1 N G L E P H A S E C O N T I N U O U S R A T I N G S H E L L F O R M 1 1 C O O L ! N G O N A N / G N A F H V L V TV 5 6 3 3 2 / 8 3 3 3 3 K V A 2 2 0 0 0 0 / < T V 4 5 5 / 6 S 6 A 5 8 3 3 3 / 6 3 3 3 3 K V A 1 3 2 0 0 0 / / T V 7 6 5 / 1 0 9 3 A 2 0 0 0 C / 2 0 0 0 0 K V A 3 3 0 0 0 V 6 0 S / S O S A " E M P E R A T U R E R I S E O I L 5 0 K W I N D I N G 5 5 K I N S ' J L A H V L I N E L V L I N E N E U T R A L T V T I O N L E V E L 9 5 0 K V 6 5 0 K V 6 5 0 K V 1 7 0 K V ! M P E D A N C E H V - L V 6 3 3 2 3 K V A H V - T V 2 0 0 0 0 K V A L V - T V 2 0 0 0 0 K V A V O L T / f /8 •/. /b.bl v. n. o •/• O I L T R A N S F O R M E R T A P C H A N G E R M A S S C O R E 4 T A N K 4 O I L T O T A L W I N D I N G S F I T T I N G S L L KS K 9 K9 K 9 U N T A N K I N G M A S S S H O R T - C I R C U I T C U R R E N T H V 6 5 5 0 A L V 9 7 9 8 A T V 9 1 1 3 A T i M E - F A C T O H 3 S E C O N D S [ S E R I A L N U M B E R 0 I f T 24-01 Q~]~ R Y P E O F I N S U L A T I N G O I L I E C - 6 0 2 9 S Y E A R O F M A N U F A C T U R E 1001 D E S I G N N U M B E R H E 1 1 1 7 7 P U R C H A S E R ' S S E R I A L N U M B E R 1 U O 2 ' J O 3 U c FOR W I N O I N G TEHP. 1 N O [ C A T O f i c FOR W I N D I N G TEMP. IND ICATOR C COS W I N D I N G TEMP. I N D I C A T O R O N O 2 V Q 1 U O n 3 V 3 LJ 2U S U B T R A C T I V E P O L A R I T Y T E R M I N A L A R R A N G E M E N T W . I POSITION CONNECTION ON AN ONAF TAP SELECTOR REVERSING SWITCH 1S1J0C SiS 951 1 I I ^9600 675 965 2 I47J00 EIS 979 3 u s , - c o 696 994 ' 4 143000 707 1009 5 5 I V w o a c o 7 1B 1025 6 6 1286C0 729 1041 7 K 136400 741 1058 i 2 134700 753 1076 9 3 132000 765 1093 10 4 179800 "76 111? 11 s 127600 792 1131 12 6 126400 so t 1151 13 7 VOLT AMP POSITION CONNECTION ON AN ONAF TAP SELECTOR 37950 527 527 1 1 37 400 535 535 2 2 36850 543 | 543 3 3 3S300 551 551 4 4 35750 559 559 5 5 TV 35200 568 568 6 6 34650 577 577 7 34100 587 587 » 32550 596 596 s 33000 606 BOB | 10 10 32450 616 S16 U 11 31900 627 627 12 12 31350 639 536 13 13 o L b v JflPflfl o C h a r a c t e r i s t i c l i s t Serial No. 01FR240101 1. No-load loss at 100% of rated voltage Guaranteed value 48/3 kW (Tole. + 0%) Measured value 15.57 kW 2. No-load current At 100X of rated voltage 0.15 X (Primary) approx.l.OA 3. Impedance voltage at 83.333MVA , rated frequency, rated voltage and 75'C. HV winding to LV winding LV Tap pos. Tap Voltage Impedance 1 220.0/V~3 151.8AT3 10.00 I (Tol. + 15%/-15%) 10 220.0/V~3 132.0/7~3 13 220.0/V3 125.4/V~3 14.00 X (Tol. + 10X/-10X) 16.50 X (Tol. + 15X/-15X) 0.09 0.59 10.00 X 14.18 X 16.38 X H winding to TV winding TV Tap pos. Tap Voltage 1 220.0/V3 37.95 10 220.0/7~3 33.00 13 220.0/V3 31.35 Impedance 76.00 X (Tol. + 15X/-15X) 75.00 X (Tol. + 15X/-15X) 75.00 X (Tol. r 15X/-15X) --220. LV winding to TV winding IV Tap pos. TV Tap pos. Tap Voltage 1 1 151.8/V3 37.95 10 10 132.0/V~3 33.00 13 13 125.4/V3 31.35 4. Load loss at rated frequency, rated voltage and 75°C. At 83.333 MVA on HV winding to LV winding. 420/3 kW At 10.000 MVA on LV winding to TV winding. 50/3 kW 60 .00 56.00 Impedance (Tol. + 15X/-15X) (Tol. + 15X/-15X) 56.00 X (Tol. + 15X/-15X) (Tole. + OX) (Tole. +' OX) 69.17 X' 69.46 X 69.75 X 52.62 X 50.08 X 49.58 % 137.62 kW 16.19 kW 5. Efficiency at 75°C, rated voltage and frequency on HV and LV windings. Base on 83.333 MVA P.P.:1.00 At 100 X of 83.333 MVA At 75 % of 83.333 MVA At 50 X of 83.333 MVA At 25 X of 83.333 MVA 6. Regulation at 75'C, rated voltage and frequency on HV and LV windings. At 83.333 MVA P.P. 1.0 99.82 X 99.85 X 99.88 X 99.88 X 1.18 X 7. Temperature rise test T O P OIL T E M P . R I S E 50 K W I N D I N G S E R I E S 55 K C O M M O N 55 K TV 55 K ONAN 40.5 42.0 42.7 49.8 ONAF 32.0 42.9 45.9 44.6 CUSTOMER SEYLON ELECCTRICITY BOARD, SRI LANKA ORDER No. 07-FT24.01 TRANSFORMER CAPACITY 83.333 MVA A MITSUBISHI ELECTRIC CORPORATION, JAPAN C h a r a c t e r i s t i c l i s t • Serial No. 01FR240101 7. Acoustic sound level 75 dB(A) 67.9 dB(A) 8. Power consumption for cooling equipment All fans operated 21.00/3 kW (Tole. + OX) 3!22 kW 07-FT24.01 A MITSUBISHI ELECTRIC CORPORATION, JAPAN CUSTOMER SEYLON ELECCTRICITY BOARD, SRI LANKA T R A N S F O R M E R T E S T I * ate of test: 6th Aug. 2001 • R E P O R T Tested by[ Arita Serial No 01FT240101 »asurement of winding resistance Hi nding Tap position Calculate Terminal ( 0 ) Oil temp. °C at 75 °C ( O ) Series - 1U - 2U 0.1047 30.0 0.1225 Common 1 2U - N 0.2361 30.0 0.2762 2 2U - N 0.2346 30.0 0.2744 3 2U - N 0.2331 30.0 0.2727 4 2U - N 0.2317 30.0 0.2710 5 2 LI - N 0.2303 30.0 0.2694 6 2U - N 0.2289 30.0 0.2678 7 2U - N 0.2269 30.0 0.2654 ~ 8 2U - N 0.2287 30.0 0.2675 9 2U - N 0.2300 30.0 0.2691 10 _2U_r N 0.2316 30.0 0.2709 11 2 U—N 0.2330 . 30.0 0.2726 12 2U - N 0.2344 30.0 0.2742 13 2U - N 0.2359 30.0 0.2760 jlertiary 1 3U - 3V 0.08691 30.0 0.1017 2 • 3U - 3V 0.08577 30.0 0.1003 3 3U - 3 V 0.08456 30.0 0.09892 . .. 4 3U - 3V 0.08347 30.0 0.09764 5 3U - 3V 0.08225 30 ;o 0.09622 6 3U - 3V 0.08107 30.0 0.09484 7 3U - 3V 0.07994 30.0 0.09351 8 3U - 3V 0.07872 30.0 0.09209 9 3U - 3V 0.07763 30.0 0.09081 10 3U - 3V 0.07641 30.0 0.08939 11 3U - 3V 0,07529 30.0 0.08808 12 3U - 3V 0.07403 30.0 . 0.08660 13 3U - 3V 0.07292 30.0 0.08530 Customer CEYLON ELECTRICITY BOARD,SRI LANKA Order 07-FT24.01 Capaci ty 83.333 MVA M I T S U B I S H I E L E C T R I C C O R P O R A T I O N , J A P A N i i l E M E N S 7 Annexure 4 - 31.5 MVA Transformer data sheets D a t e 2 1 * 2 4 / 0 9 / 0 7 „ fens T r a n s f o r t r j ^ S . p . A . Test ing Secvi* C u s t o m e r Ser ia l n u m b e r n° : 0 6 0 5 2 0 1 / 0 1 I R a t e d P o w e r [ k V A ] : 2 4 0 0 0 O N A N / 3 1 5 0 0 O N A F [Ra ted V o l t a g e H V [V] : 1 3 2 0 0 0 + 7 - 1 0 x 1 , 5 % i i R a t e d V o l t a g e M V [V] : 3 3 0 0 0 - S u m m a r y of guaran teed and measu red values M e a s u r e m e n t o f n o - l o a d l o s s a n d c u r r e n t . ONAF: 31,5 MVA; r G U A R A N T E E D T O L E R A N C E S M E A S U R E D Loss W V n 18000 + 0 % 17188 Current % V n 0 ,20% + 3 0 % 0 ,086% Current % 0,9 V n 0 ,15% + 3 0 % 0 ,067% Current % 1,1 V n 0 ,40% + 3 0 % 0 ,243% Current % 1,2 V n 5 ,00% + 3 0 % 1 ,291% M e a s u r e m e n t o f s h o r t - c i r c u i t i m p e d a n c e a n d l o a d l o s s . O L T C O f f - L T C P o s i t i o n P o s i t i o n G U A R A N T E E D T O L E R A N C E S M E A S U R E D ONAN: 24 MVA; i Loss W 87070 + 0 % . 84026 8 Impedance 75°C % 7,62 . ± 7 , 5 % 7,71 No-Load + Load Losses W 105070 + 0 % 101213 • Loss W _ + 0 % 83771 ! f ' 1 • I m p e d a n c e 7 5 ° C % 8,08 ± 7 , 5 % 8,21 ; No-Load + Load Losses W - + 0 - % 100959 Loss W _ + 0 ' % " ~ — 87297 1 8 Impedance 75°C % • 7,16 ± 7 , 5 % 7,17 P * V No-Load + Load Losses W - + 0 % 104484 ONAF: 31,5 MVA; Loss w 150000 + 0 % 144747 8 Impedance 75°C % 10,00 ± 7 , 5 % 10 ,12 No-Load + Load Losses w 168000 + 0 % 1 6 1 9 3 5 Loss w _ + 0 % 1 4 4 3 0 9 1 I m p e d a n c e 7 5 ° C % 10,60 ± 7 , 5 % 10 ,77 No-Load + Load Losses w - + 0 % 161496 Loss w + 0 % 150382 18 Impedance 75°C % 9,40 ± 7 , 5 % 9,41 No-Load + Load Losses w - + 0 % 167570 J GUARANTEED TOLERANCES MEASURED Loss W V n 18000 + 0% 17188 Current % V n 0 ,20% + 30 % 0,086% Current % 0,9 V n 0 ,15% + 30 % 0,067% Current % 1,1 V n 0 ,40% + 30 % 0,243% Current % 1,2 V n 5 ,00% + 30 % 1,291% M e a s u r e m e n t o f s h o r t - c i r c u i t i m p e d a n c e a n d l o a d l o s s . O L T C O f f - L T C P o s i t i o n P o s i t i o n GUARANTEED TOLERANCES MEASURED ONAN: 24 MVA; k Loss W 87070 + 0 % 84026 8 Impedance 75°C % 7,62 . ± 7 , 5 % 7,71 No-Load + Load Losses W 105070 + 0 % 101213 Loss W _ + 0% 83771 f ' 1 Impedance 75°C % 8,08 ± 7 , 5 % 8,21 » No-Load + Load Losses W - + 0 - % 100959 Loss W _ ' + 0'% ~ — 87297 1 8 Impedance 75°C % . 7,16 ± 7 , 5 % 7,17 fi » V No-Load + Load Losses W - + 0 % 104484 ONAF: 31,5 MVA; Loss W 150000 + 0 % 1 4 4 7 4 7 8 Impedance 75°C % 10,00 ± 7 , 5 % 10,12 C; ( No-Load + Load Losses W 168000 + 0 % 1 6 1 9 3 5 > f E Loss W _ + 0 % 1 4 4 3 0 9 1 Impedance 75°C % 10,60 ± 7 , 5 % 10 ,77 No-Load + Load Losses W - + 0 % 161496 Loss W + 0 % 150382 K 1 8 Impedance 75"C % 9,40 ± 7 , 5 % 9,41 No-Load + Load Losses W - + 0 % 167570 Date 21 + 24/09/07 lens Transfo Testing Se: S.p.A. Customer i J I E M E N S ~ ~ ~ 7 Annexure 4 - 31.5 MVA Transformer data sheets f e r i a l numbe r Rated Power ,Rated Vo l tage HV |Rated Vo l tage MV • S u m m a r y of guaran teed and measu red values M e a s u r e m e n t o f n o - l o a d l o s s a n d c u r r e n t . ONAF: 31,5 MVA; n° : 0605201/01 • i [kVA] : 24000 ONAN / 31500 ONAF j I [V] : 132000 + 7 - 1 0 x 1 , 5 % i 1 [V] : 33000 - I J Siemens Transformers S.p.A., Zona Ind.le Nord - Settore C 38014 Spini di Gardolo, Trghto - Italy Page 1 / 34 Customer Order Serial number Rated Power Rated Vol tage Rated Vol tage Vector group Frequency Type of cooling Plant location HV MV n° n° [kVA] [V] [V] [Hz] S IEMENS A G OESTERREICH fo r Ceylon Electicity Board 9501411529 dated 01/02/2007 0605201/01 24000 ONAN / 31500 ONAF 132000 + 7 - 1 0 x 1 , 5 % 33000 - Y N d 1 50 O N A N / O N A F PSDTP - LOTA - SRI LANKA L I S T O F T E S T S P E R F O R M E D A C C O R D I N G T O S T A N D A R D : IEC 60076. R O U T I N E T E S T S Check of guaranted values. \ Page 2 Measurement of voltage ratio and check of phase displacement. Pages 3 * 4 L i g h tn i ng i m p u l s e tes t . HV side. Pages 5 -r 12 Separate-source voltage withstand test. Pages 13 T 14 Core insulation tests. Page 15 Measurement of insulation resistance. Page 16 Induced overvoltage withstand tests. Page 17 -r 20 Measurement of no-load loss and cur ren t Pages 21 23 Measurement of winding resistance. Page 24 Measurement of short-circuit impedance and load loss. Pages 25 -r 31 Test on on- load tap changer. Page 32 Dielectric strength test for oil. Page 33 Dimensional,Auxl iary,Paint ing. Page 34 Date 21 + 24/09/07 Siemens Tn feforme Testi g Servii Siemens Transformers S.p.A. Customer A Sede sociale e Direzione: Zona Ind.le Nord - Settore C 3 8 0 1 4 - Spini di Gardolo (TN) Italy Tel. +39 0 4 6 1 957111 Fax +39 0461 9 9 3 4 1 7 siemenstrafo.it@siemens.com ! Capitale sociale: Euro 2 .028 .000 i.v.; Iscrizione Registro Imprese Trento, Codice fiscale e partita I.V.A.: 0 0 8 1 4 4 5 0 2 2 7 ; R.E.A. Trento N. 9 8 4 1 2 Serial n u m b e r Rated P o w e r Rated V o l t a g e Rated V o l t a g e 0 6 0 5 2 0 1 / 0 2 2 4 0 0 0 O N A N / 3 1 5 0 0 1 3 2 0 0 0 + 7 - 1 0 x 1 , 5 % 3 3 0 0 0 O N A F Wind ing cold res i s tance m e a s u r e m e n t HV Wind ing P H A S E S : 1V-1W MV Winding 0 , 1 1 6 0 3 0 , 1 1 6 0 4 0 , 1 1 6 0 6 OIL TEMPERATURE T o p of Radiators : 26,4 Bot tom of Radiators : 24,8 Top of Transformers tank : 26,8 Average value of HV winding (RT 1V-iw) Average value of MV winding (R-, 2v-2w) 1 8 / 0 9 / 0 7 C u s t o m e r Annexure 5 - 200kVA Transformer data sheet TRANSFORMER ROUTINE TEST REPORT ABB -Ltd. CUSTOMER: SIEMENS A G OESTERREICH - GERMANY S e r l N o : 1LVN2070637 J o b o r d e r : 200001266 2 O r d e r N o : 1002139 D a t e : B/11/2007 Rating (kVA) : 200 Work No : 1002139(10_61) High vol tage (kV) : 33 Current of HV side (A): 3.49 Frequency(Hz) : 50 Vector group : Z N Tempera ture ( ° C ): 30 Type of oil : . NYTRO GEMINIX S teps o f tes t : 1- lnsulat ion res is tances f M O ) : 2-Wincfing res is tance measu remen t : HV-Earth: 6 5 0 0 Phases | HV s i d e ( n ) A _ B : I 6.5900 B _ C : j 6 .5600 C _ A : | 6.6000 ?-N9-ta»5 4 « Total 132kV route length UG 13.0 Total 132kVcct length UG 13.0, t=~ 732 kV transmission lines OH $ingk Circuit New Laxapana-Canyon 132 54 Lynx 10 Ukuwela-Bowatenna 132 54 Lynx 30 Rantembe-B adulla 1 132 75 Lynx 37 Rantcmbc-Badulla 2 132 75 Lynx 33 Badulla-Inginiyagala 132 54 Oriole 79.9 Inginiyagala - Ampara 132 Lynx 25 Habarana-Valachchena , 132 75 Lynx 99.7 Total 132kV 1 cct route length 314.6 Total 132kV 1 cct circuit length 314.6 Kotmale-Kiribatkumbura 132 54 Lynx 22.5 0.02299 Kiribatkumbura-Ukuwela 132 54 Lynx 29.9 0.03055 Ukuwela - Habarana 132 54 Lynx 82.3 0.08408 Habarana - Anuradhapura 132 54 Lynx 48.9 0.04996 3 Polpitiya-Kotmale 132 54 Lynx 29.5 0.03014 Biyagama-Sapugaskanda PS 132 75 Zebra 2.1 0.00092 Kelanitissa-Kolonnawa 132 75 Invar 2.2 0.00119 Kolonnawa-Pannipitiya 132 54 Lynx 12.9 0.01318 Kotugoda-Bolawatta(T) , ;3 i' 132 75 Zebra 22.0 0.00960 Bolawatta(T)-Madampe(T) 132 54 Lynx 22.6 0.02309 Balangoda-Ratnapura • , 1 3 2 . .75 Zebra 40.0 0.01745 Madampe(T)-Puttalam • ; 132 ; 54 Lynx 61.4 0.06272 Note: The line parameters are given in p.u. values w.r.t Z ^ = Vbase2/ M V A ^ (MVAblse=100, Long Term Transmission Development Studies 2004- 2013 Table 3.1 Existing transmission lines/UG cables cont. Long Term Transmission bivelopment Studies 2004- 2013 : '' " : "•••.'.' c ^ : ^ ,: Pat>* Af,^.—-