Effect of Series Capacitor on Line Protection - A Case Study

Transcription

1 112 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 22 Effect of Series Capacitor on Line Protection - A Case Study Anand Mohan, Vikas Saxena, Mukesh Khanna & V.Thiagarajan Abstract: Series compensation is a time tested reliable technology being used successfully to compensate the reactance of transmission lines. The advantage of use of series compensation on long EHV lines is improvement in Transmission System performance and increase in line loadability. This has been well established and implemented for many years world over. For the first time in India fixed series compensation of 35% has been proposed on a 4kV S/c i.e Kanpur - Ballabhgarh 4kV S/c to enhance the line loadability and system stability. The introduction of a series capacitor into the line affects the line distance protection due to abrupt change in line series impedance (which is inductive) at the series capacitor. The paper discusses in detail the effect of series compensation on distance protection and various Short Circuit and EMTP studies carried out. I. INTRODUCTION Indian Power System, presently with more than 1GW installed capacity, is being operated as four independent regional grids - Northern, Southern, Western and Eastern & North-Eastern Regions. Northern region is one of the largest grid with an installed capacity of about 27 GW, consisting of eight states and a Union Territory. Eastern part of Northern Region has major coal reserves and thermal Power Stations like Singrauli, Rihand, Anpara and Obra have been developed to tap this reserve. Power from these stations is transferred to Western part of the grid, where there is a concentration of loads, through long distance 4 kv lines via Kanpur/Panki axis. For transfer of power beyond Kanpur/Panki towards Western part, presently there are mainly three 4kV AC lines i.e. Kanpur - Ballabgarh (395km), Panki-Muradnagar (4km) and Kanpur Agra (25km) (Figure-1). The loading on Kanpur - Agra line remains generally higher and also due to long length of the other two, it has been proposed to install series compensation on Kanpur-Ballabgarh and Panki-Muradnagar lines. Under Phase-I, series capacitor is proposed to be installed on Kanpur-Ballabgarh and under phase-ii, it is proposed to install on Panki-Muradnagar 4kV S/c line. Application of Series compensation increases the line loadability, But the integration of series capacitor into the line makes the line protection complex due to abrupt change in line series impedance at the series capacitor, which changes the fault current, voltage and apparent impedance measured by the line relays. Moreover, the fault current may change from its normal lagging angle, with respect to voltage, to a leading angle. The presence of series capacitor on a line can cause incorrect operation of the relays on that line and relays on adjacent lines. Anand Mohan, Vikas Saxena, Mukesh Khanna & V.Thiagarajan Engineering Division Power Grid Corporation of India Ltd B-9 Qutub Instt. Area, Katwaria Sarai, New Delhi-11 16, India Figure I- Schematic diagram of Series capacitor Protection This paper discusses in detail the studies carried out to study the effect of series compensation on conventional distance relays and remedial action taken. II. SYSTEM DESCRIPTION A. Network configuration Fig. 1 shows the spatial arrangement of proposed Series Compensation, the series capacitor is proposed to be located at Ballabgarh end of Kanpur-Ballabgarh 4kV S/c line. Ballabgarh, other than Kanpur, is connected to Agra and Jaipur by 4kV Single Circuit line while a 4kV Double Circuit line connects it to Dadri. Similarly Kanpur bus is connected to Panki and Allahabad by two numbers of 4kV lines while it is connected to Singrauli and Agra by 4kV Single Circuit line. Agra is connected to both Kanpur and Ballabgarh by 4kV Single Circuit line. B. Series compensation Theoretically, a transmission system can carry power up to its thermal loading, in the case of Kanpur-Ballabgarh line 4km built with twin Moose conductor it is 87 MVA. But loading on a long line is restricted well below its thermal loading due to angular separation been the two buses, which as per Transmission Planning Criteria should be within 3 degrees. For example, a 4km long 4kV line with per unit voltage at both can be loaded upto only 57MW. Provision of series compensation on the line effectively reduces the line impedance and hence the angular separation between the buses. After detailed analysis 35% series compensation has been proposed on Kanpur-Ballabgarh line. The 35% series compensation consists of banks of 27% and 8%, has been considered at Ballabgarh end of Kanpur-Ballabgarh line. The

2 INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 72132, DECEMBER 27-29, % bank is proposed to be converted into Thyristor Controlled Series Capacitor in future to improve Dynamic stability. III. CAPACITOR DESCRIPTION BALLABGARH BUS VOLTAGE A Series Capacitor reduces the inductance in a power system leading to higher short circuit currents. It is not feasible to design a series capacitor that has high enough voltage ratings to withstand such high fault currents. Hence to protect the capacitor from damage capacitor banks are bypassed when the current touches the protective level. To protect the Series capacitor banks on Kanpur- Ballabgarh line, a combination of MOV triggered spark gap and Series Capacitor Bypass Circuit Breaker is being provided. With the inception of the fault, MOV bypasses the capacitor carrying the fault current. Energy dissipation capacity of MOV is also limited, therefore triggered spark gap bypasses the MOV. Schematic diagram of the Series capacitor along with protection is shown in Figure-II. The spark gap is triggered within 1msec of MOV conduction, after which bypass circuit breaker will be closed. The effect of series compensation along with its MOV during fault conditions and its effect on line protection need to be studied FAULT CURRENT (NOT TO SCALE) Exhibit-I- Fault Current & Ballabgarh Bus Voltage Exhibit-II- Current thru the capacitor 3 BALLABGARH KANPUR 2 1 8% CAPACITOR BANK 27% CAPACITOR BANK MOV MOV -2 DAMPING CKT SPARK GAP -3-4 Exhibit-III- Current thru the MOV From the above study results, following observations can be made: BYPASS CIRCUIT BREAKER Figure II- Schematic diagram of Series capacitor Protection IV. EMTP SIMULTAION In order to study the behavior of series Capacitor along with its MOV, an EMTP simulation has been carried out. A detailed system upto second level, both from Kanpur and Ballabgarh bus, has been modeled in EMTP. All the bus short circuit has been matched with the expected short circuit level. The powerflow on the lines have been matched with the expected value both in magnitude and direction. The MOV has been modeled in detail as per the V-I characteristics as provided by the manufacturer. A fault was created close to the Capacitor terminal on the line side. The voltage and current monitored are plotted as below. Till the voltage across capacitor builds upto protection level the fault current flows thru the capacitor. (From the Exhibit-II it can be seen that initially the current flows through the series capacitor and increases to the magnitude of about 2kA). When the protective voltage level of MOV is reached, MOV bypasses the capacitor bank and fault current flows thru MOV. (Exhibit-III) Hence the series capacitor remains in service for a definite period and its effect on the relays needs to be studied. The Ballabgarh bus voltage does not reduces to zero even for a fault close to bus. To study the effect of series compensation, till MOV starts conducting a fault was slided along the Ballabgarh-Kanpur line and bus voltage of Ballabgarh bus was monitored, MOV not been modeled. The results are plotted in Exhibit IV

3 NATIONAL POWER SYSTEMS CONFERENCE, NPSC Exhibit-IV- Ballabgarh bus voltage for a sliding fault For a fault at capacitor terminal, the voltage of Ballabgarh bus will rise. And the Voltage varies as the fault slides. The effect of high voltage under fault needs to be studied. Consider a simple system shown in Figure-III, power flows from BUS1 to BUS2 ( Rp>>Xl>>Rl). Under normal condition, the flow on a line is mostly Real power (MW) as the load is predominantly Real/Resistive ( Rp>>jXp) and relay at BUS1 on BUS1-BUS2 line measures a high resistive load ( as Rp >> Xl). Under short circuit condition BUS2 voltage becomes zero and no power will flow thru the load and the entire current will flow thru the low impedance fault to ground. The relay at BUS1 on BUS1-BUS2 will measure the actual line impedance. The line inductance is about 1 times higher than line resistance ( X l >>Rl). Hence the relay will measures a high inductance. BUS1 ( R l +jxl) ( I +ji r x ) ( R p +jx p) BUS2 Figure III- Normal system For a fault at the capacitor line terminal, the voltage at BUS2 does not become zero (rather increases) and hence power will continue to flow through Rp, at the same time as Xc is small compared to Rp there will be reactive current flowing to ground thru Xc. The impedance measured by the relay at BUS1 on BUS1-BUS2 line will depend on the position of the fault. Hence due to installation of series capacitor on Kanpur Ballabgarh 4kV line, the relays on adjacent lines on Ballabgarh bus and on Kanpur Ballabgarh 4kV line which needs to be studied for whether there will be any maloperation for a fault at any location, due to series capacitor is in service. As we are interested when capacitor is conducting, MOV need not be modeled. The study has been carried out on Short Circuit package of Power System Simulator/Engineering (PSS/E), power system software, developed by Power Technologies Inc, USA. Similar studies can be carried out with EMTP, but considering the number of lines and locations on which the faults is to be simulated study has been done on PSS/E A. System Modeling IV. SHORT CIRCUIT STUDIES Detailed system up to 132kV level having about 22GW load has been simulated. The system simulated comprises of 37 nodes, 1 branches and 7 generating plants. All generators have been shown behind a Sub transient Reactance. B. Study Approach For a particular fault, the series capacitor affects the input to the relay only if the location of Series capacitors is between the relay measurement point and the fault point. Hence Relay measurement at Ballabgarh end can be done either at Bus or at capacitor line terminal. In the case of Kanpur-Ballabgarh the measurement shall be at line side. With measurement of Relay inputs at Capacitor line terminal, relay at Ballabgarh end of Kanpur-Ballabgarh will be unaffected by series capacitors for any fault at capacitor terminal or fault along Kanpur-Ballabgarh line and will sense the faults correctly, but will be affected by the series capacitor for faults behind the capacitor terminal like fault on Ballabgarh bus or on any adjacent lines of Ballabgarh. Relay at Kanpur end of Kanpur Ballabgarh line will be unaffected for any fault along Kanpur-Ballabgarh line upto Capacitor terminal. Kanpur end relays will be influenced by the series capacitor for fault just after capacitor, and on adjacent lines terminating at Ballabgarh. Relays on Adjacent lines at Ballabgarh end, i.e. Dadri- Ballabgarh 4kV D/c, Dadri-Agra 4kV S/c etc., shall be affected by the series capacitor for faults beyond capacitor line terminal only on Kanpur-Ballabgarh line. For relays on Adjacent lines at Kanpur end i.e. Kanpur Allahabad 4kV S/c, Kanpur Agra 4kV S/c etc, shall be affected by the series capacitor for fault beyond capacitor, on Ballabgarh bus and on adjacent lines from Ballabgarh. As compensation is only for 35%, for faults beyond Ballabgarh the total impedance from Kanpur to fault shall always be inductive. This will be added to actual line impedance, hence the relay operation on adjacent lines at Kanpur end may be affected only in Zone-2 & 3. Hence effect of series compensation on these lines can be ignored. To summarize, following studies need to be carried out to study the effect of Series capacitor: Response of relay on Kanpur Ballabgarh line for fault behind capacitor, on Ballabgarh bus and on adjacent lines of Ballabgarh Adjacent lines at Ballabgarh end for fault at capacitor terminal and along Kanpur-Ballabgarh line

4 INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 72132, DECEMBER 27-29, C. Short Circuit Studies: Relays on adjacent lines at Ballabgarh: The line in adjacent sections which will get affected due to series compensation on Kanpur-Ballabgarh/ lines which may affect the protection of series compensated Kanpur- Ballabgarh line are: Ballabgarh -Dadri 4kV D/c 53km Ballabgarh-Bhiwadi 4kV S/c- 36 km Ballabgarh-Agra 4kV S/c- 181 km Effect on Dadri-Ballabgarh relays A sliding fault was created along Ballabgarh-Kanpur line and the apparent impedance measured by the relay at Dadri end of Dadri-Ballabgarh line was monitored, the results are plotted in Exhibit-V. For a fault at Ballabgarh bus, the relay at Dadri measures impedance equal to the line impedance,.8 p.u. But for fault at capacitor terminals due to current infeed effect it measures an impedance of.23-j.95 as against expected.1-j.272 p.u. Power flows from Dadri to Ballabgarh hence resistive component is always positive. And as the fault moves towards Kanpur the capacitive impedance measured by the relay reduces and for a fault at.1 5 For both the relays the minimum impedance measured is greater than the actual line impedance(.8p.u). Effect on Ballabgarh-Bhiwadi relays Exhibit-7 &8 shows the apparent impedance measured by the relay at Bhiwadi and Ballabgarh end relays of Bhiwadi- Ballabgarh line. Power flows from Ballabgarh to Bhiwadi. Hence the Relay at Bhiwadi sees a negative resistance while the relay at Ballabgarh sees a positive resistance. Only for a small percentage of line length the voltage at the bus is low enough that powerflows from Bhiwadi to Ballabgarh Exhibit-VII: Apparent impedance (R-X) measured by Bhiwadi end relay of Bhiwadi -Ballabgarh line Exhibit-V- Impedance seen by Dadri end relay of Dadri - Ballabgarh line for fault on Ballabgarh-Kanpur line about 32% from Ballabgarh end the measured impedance is nearly resistive. But even this resistive impedance is very high due to In-feed current,.9p.u, more than 1% of line impedance. After 32%, the impedance measured by the relay is inductive The apparent impedance measured by Ballabgarh end relay is shown in Exhibit-VI. For a fault at capacitor terminal, the relay sees an apparent impedance of -.24+j.16 and as the fault slides the inductive component reduces but even at the point were it is almost zero the resistive component is -.9, which is about 1% of line impedance Exhibit-VIII Apparent impedance (R-X) measured by Ballabgarh end relay of Bhiwadi -Ballabgarh line For both relays the minimum measured impedance of Bhiwadi end relay is.35p.u, which is more than twice the line impedance(.17 p.u). Effect on Ballabgarh-Agra relays Exhibit-IX &X shows the apparent impedance measured by the relay at Agra end and Ballabgarh end of Agra-Ballabgarh line. The fault current feed for Agra is mainly from Auraiya, Kanpur and Ballabgarh. Under Normal condition power flows from Agra to Ballabgarh hence Resistive component is positive for Agra end Relay. As the fault slides closer to Kanpur, Ballabgarh-Agra- Kanpur lines act as a parallel path to feed the path hence after about 8% distance X becomes negative for Agra end Relay. The minimum impedance measured by the relay is.48 p.u, which is higher than the line impedance of.37 p.u -.15 Exhibit-VI- Impedance seen by Dadri end relay of Dadri - Ballabgarh line for fault on Ballabgarh-Kanpur line

5 116 NATIONAL POWER SYSTEMS CONFERENCE, NPSC 22 Exhibit-IX Apparent impedance (R-X) measured by Agra end relay for fault on Ballabgarh-Kanpur line Exhibit-X Apparent impedance (R-X) measured by Ballabgarh end relay for fault on Ballabgarh-Kanpur line Relay at Ballabgarh End of Kanpur-Ballabgarh Line : A sliding fault was created on Ballabgarh- Dadri 4kV line and Ballabgarh-Agra 4kV line and the apparent impedance measured by Ballabgarh end relay of Kanpur- Ballabgarh line was monitored to study the effect of series capacitor on the relay. The result of studies are plotted below: Exhibit-XI: Apparent impedance (R-X) measured by Ballabgarh end relay of Ballabgarh -Dadri line Exhibit-XII: Apparent impedance (R-X) measured by Ballabgarh end relay of Ballabgarh -Agra line The Zone setting of Kanpur Ballabgarh relay is.656 p.u Under both the cases for a fault on the line close to bus (at gantry) the relay measures an impedance of.29 p.u, the capacitor impedance less than Zone-1 setting It is seen that the measured X component of Relay at Ballabgarh end for fault on Ballabgarh-Dadri line is always less than its Zone-1 setting. Maximum measured value is.287 p.u, which is at the bus. As power flows from Dadri to Ballabgarh R is always measured as a negative Value, but the magnitude is small and Maximum magnitude measured is.188 p.u Similar to Ballabgarh-Dadri line, Ballabgarh-Agra line measures a very small X component, which also becomes negative for a portion of the line. But the negative R component magnitude increases very rapidly and touches up to.13 p.u. Infact, at 1% R is only about -.34 p.u, by the time the fault is at 2% the magnitude of R is about -.68 p.u, greater than the Zone-1 setting of Kanpur-Ballabgarh line. Relay at Kanpur End of Kanpur-Ballabgarh Line : A sliding fault was created on Ballabgarh- Dadri 4kV line and Ballabgarh-Agra 4kV line and the apparent impedance measured by Kanpur end relay of Kanpur- Ballabgarh line was monitored. The result of studies are plotted below : The Zone setting of Kanpur Ballabgarh relay is.656 p.u Exhibit-XIII: Apparent impedance (R-X) measured by Kanpur end relay of Ballabgarh -Agra line For a fault close to Bus the Kanpur end relay measures a fault impedance of.544 i.e the net impedance of line and the capacitor. For any fault on the adjacent lines power shall always flow from Kanpur to Ballabgarh. Hence the impedance measured by the Kanpur end relay Resistance is always positive Exhibit-XIV: Apparent impedance (R-X) measured by Kanpur end relay of Ballabgarh -Dadri line And as the fault slides further the measured impedance increases. The maximum measured impedance is.27+j.77. And upto about 2% distance from the bus

6 INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 72132, DECEMBER 27-29, the impedance is less than Zone-1 setting. And as the fault approaches Dadri Bus the Fault impedance reduces again. For fault on Ballabgarh Agra line initially the X measured by the Kanpur end relay increases rapidly. For fault at 1% distance from the bus the magnitude of X is about.84p.u. Since Agra-Ballabgarh is a long line (18km), as the fault slides from Ballabgarh the voltage of Ballabgarh bus increase and power flows from Kanpur to Ballabgarh. Hence measured R value increase heavily, maximum magnitude is about.14 p.u. V. ATP SIMULATION: To study the Relay operation with series capacitor and to see if there is any relay malfunction by transients due to MOV conduction, capacitor etc an ATP simulation was carried out. The ASCII values obtained were converted into COMTRADE format and then fed to distance relays of type that are existing on concerned lines, through an automatic relay test kit. Study results are as follows: For a fault on Kanpur-Ballabgarh line the relays on adjacent lines at Ballabgarh did not maloperate. The Kanpur end Relay tripped in Zone-1 for fault, on Ballabgarh-Dadri Line, at Ballabgarh end, at 1% and Dadri end. Similarly the relay tripped in Zone-1 for fault, on Ballabgarh-Agra Line, at Ballabgarh end. The relay did not operate for fault at 1%, 5% and Agra of Ballabgarh-Agra Line The Ballabgarh end Relay tripped in Zone-1 for fault, on Ballabgarh-Dadri Line, at Ballabgarh end, at 1%, 5% and Dadri end. Similarly the relay tripped in Zone-1 for fault, on Ballabgarh-Agra Line, at Ballabgarh end and 1% from Ballabgarh. The relay did not operate for fault at 5% and Agra end of Ballabgarh-Agra Line The test results match with the results obtained through Short circuit studies for conventional distance relays. The same tests were also simulated on new generation numerical distance relays of different makes compatible with series capacitor. It was observed that these relays are able to give correct directional discrimination. VI. REMEDIAL MEASURES The conventional Ballabgarh end relay of Kanpur Ballabgarh end line has lost its sense of direction and trips for behind the bus fault. The Relay has been replaced with new generation numerical distance relays compatible with series capacitors. To overcome the over reach of Kanpur end relay for fault beyond the capacitor following zone settings were made. Z1=.8*(1-k)*ZL Z2=1.2*ZL Where k is the degree of compensation ZL is the uncompensated line impedance Permissible Over-Reach scheme (POR) has been adopted. VII. CONCLUSION This paper presented in detail application of Short Circuit studies carried out to examine the effect of installation of series capacitor on Kanpur-Ballabgarh 4kV line on line protection on principle as well as adjacent lines. The study results indicate requirement for replacement of conventional distance relays at Ballabgarh end with numerical relays compatible with series capacitor and change in Zone setting of Kanpur end relay and adoption of permissible Overreach Scheme. While Short circuit studies are helpful in identifying faults and distance relay locations which are of concern, it is necessary to make ATP simulation test on actual relays to ascertain performance and adequacy of protection scheme provided. VIII. ACKNOWLEDGEMENT The authors wish to thank POWERGRID for granting permission to present the paper. Views expressed in this paper are of the authors and need not necessarily be of the management of POWERGRID. IX. REFERENCES [1] Application guide on protection of complex transmission network: working group of CIGRE-34 [2] Series compensation of Power system P.M.Anderson and F.G Farmer [3] Limits to conventional distance relaying in series compensated transmission lines-case study by V.K.Prashar, Vikas Saxena, P.Jayachandran & V.Thiagarajan : Integrated Protection, Control and Communication Experience, Benefits and Trends, CBIP, 21