Reactive Power Solutions

Transcription

1 GE Digital Energy Reactive Power Solutions Effects of Series Capacitors on Line Protection Relaying Design and Settings Presented by: Paul Datka, GE Energy Consulting Luis Polanco, GE Energy Consulting

2 GE Guest Speakers Paul Datka GE Energy Consulting, Principal Over 10 years of T&D experience, Series Capacitor equipment design, ratings, and system studies IEEE, CIGRE Member Holds a patent for Series Capacitor Protection equipment invention Luis Polanco GE Energy Consulting, Principal Over 10 years of substation equipment and line protection system design and settings IEEE Member

3 Agenda Why Series Compensation Series Capacitor Operation During Line Faults Effects of Series Capacitors On Line Protection Relaying Mitigation Techniques Live Q & A

4 Why Series Compensation?

5 Why Series Compensation? Series Compensation are primarily applied to Increase long distance power transfer capability Increase dynamic stability Reduce voltage variation Improve voltage profile of the line Improve load division but requires Protection from line faults

6 System Overview Series Capacitor Current Limiting Damping Reactor Varistor Bypass Gap Bypass Switch Series Compensation is an integrated, custom-designed system that consists of power capacitors arranged in series with the HV transmission line. The capacitors are accompanied by a parallel protective system that will prevent damage to the capacitors under power system events (faults).

7 Series Capacitor Operation During Line Faults And The Resulting Equivalent Impedance

8 Equivalent Impedance During Faults This section we will discuss the following types of faults: Low Current Faults High Current Faults Medium Current Faults

9 Equivalent Impedance During Low Current Faults

10 Equivalent Impedance During Low Current Faults Low current faults summarized Current primarily in the Capacitors Very little MOV current Bank voltage less than protective level Impedance ~= the Capacitive Reactance of the Bank Resulting impedance is almost purely the capacitive reactance

11 Equivalent Impedance During High Current Faults

12 Equivalent Impedance During High Current Faults High current faults summarized Current primarily in the MOVs Very little Capacitor current Bank voltage equal to protective level Impedance ~= small Resistance Resulting impedance approximates a small resistance

13 Equivalent Impedance during Medium Current Faults Medium Current Faults

14 Equivalent Impedance During Medium Current Faults Medium current faults summarized: Current is shared in the MOVs and the Capacitors Capacitors conduct during first part of the half cycle, MOV conduct during the second part of the half cycle Bank voltage less than protective level, but becoming more rectangular Impedance = percentage of the Resistive-Capacitive Reactance of the bank Resulting impedance approximates a resistive and capacitive reactance

15 Equivalent Impedance During Faults Resulting Impedance as seen by the relay Series Capacitors affect the impedance as seen by the relay The resulting impedance may fall outside the relay operating characteristic If severe enough may even cause directional discrimination

16 Effects of Series Capacitors On Line Protection Relaying

17 Series Capacitors On Line Protection Relaying Overreach Conventional distance relays operated by comparing an operating quantity with a stabilizing quantity. Stabilizing quantity as seen by the relay is a voltage drop developed by measured current on line model and is affected by system impedance and fault location. Overreach is caused by the relay not including the negative reactance of the capacitor banks in the impedance model of the line Can cause unnecessary sequential line tripping

18 Series Capacitors On Line Protection Relaying Voltage Inversion SC Inserted SC Bypassed Pre-Fault Voltage Source Fault Voltage -jx c V R jx S jx L I f Relay Fault V R = I f x jx L V R = I f x j(x L - X C ) V R Source Relay SC Bank Fault With the SC bypassed, the voltage at the relay point, V R, is equal to the voltage drop on the line and will lead fault current, I f. With the SC inserted, I f will increase due to lower overall impedance and the relay voltage, V R, will be equal to the voltage drop across the combination of the line and SC reactance. If X c > X L, the voltage at the relay point, V R, will lag fault current. In this case the voltage at the relay point is inverted. Voltage inversion can lead to directional discrimination problems at the relay.

19 Series Capacitors On Line Protection Relaying Current Inversion V R SC Bypassed Pre-Fault Voltage Source SC Inserted Fault Voltage -jx c V R jx S jx L I f Relay Fault Source Relay SC Bank Fault With the SC bypassed, fault current normally flows from the power source towards the fault point and fault current will lag source voltage. With the SC inserted, two conditions could exist, either the resultant reactance (X s X c + X L ) is greater than or less zero. When greater than zero, the condition is similar to when the SC is bypassed. When less than zero, the resultant reactance is capacitive and fault current will lead source voltage. This condition is called current inversion. Current inversion can adversely affect distance relays and needs to be examined when the possibility exists.

20 Series Capacitors On Line Protection Relaying Transients Most transient problems for relays on series compensated lines occur due to subsynchronous oscillations as they cannot be filtered out unless a very long window is used and the relay is effectively slowed down. These oscillations may increase the operating time of distance protection and cause overreaching of instantaneous distance protection zones.

21 Series Capacitors On Line Protection Relaying Location of Instrument Transformers The selection of the VT and CT locations for line end Series Capacitor banks play an important part in the performance of the protection scheme. Bus Side (CT1 and VT1): Protection is subject to possible voltage and current inversion as well as overreach conditions for line faults. However, the Series Capacitor Bank is in the protection zone and does not require any additional or special protection.

22 Series Capacitors On Line Protection Relaying Location of Instrument Transformers Line Side (CT2 and VT2): Protection is less subject to possible voltage inversion conditions for line faults; however for close in and reverse faults, the negative reactance may affect the protection. The Series Capacitor Banks is not in the protection zone and must be protected separately. Dual Side (CT1 and VT2): Protection is less subject to possible voltage inversion for line faults and the Series Capacitor Bank is in the protection zone, however it is subject to voltage inversion for reverse faults.

23 Series Capacitors On Line Protection Relaying Pilot Protection Schemes on SC Lines Pilot Protection schemes with distance and overcurrent elements - Susceptible to Overreach and Voltage and Current Inversion Pilot Protection schemes using current differential - Susceptible to Current Inversion

24 Mitigation Techniques

25 Mitigation Approaches Protective Zone Reach Reduction Relay Memory Action Adaptive Distance Reach Real Time Digital Simulation (RTDS) and Testing

26 Mitigation Approaches Protective Zone Reach Reduction Reducing the reach of the protective zones can help reduce the affects of overreach in some circumstances. Must be used carefully ensuring line is fully protected. Not fully effective in all cases.

27 Mitigation Approaches Relay Memory Action SC Inserted SC Bypassed Pre-Fault Voltage Source Fault Voltage -jx c V R jx S jx L I f Relay Fault V R V R = I f x jx L V R = I f x j(x L - X C ) Source Relay SC Bank Fault Memory action produces a short duration voltage equivalent to the pre-fault voltage. Can be used to help determine the direction of the fault and aid in proper tripping when voltage inversion is present. Care must be used when setting the duration of the memory action.

28 Mitigation Approaches Adaptive Distance Reach X Original Protective Zone Z Line w/sc Inserted Reach is dynamically reduced based on fault current Reduced Protective Zone Actual Reach for Low Fault Currents R The reach of the relay is dependent on the fault current. Z effective ( I f ) = Z set V/ I f * angle(z set) For high fault currents the reduction is small. For low fault currents the reduction is large. This effectively adapts the reach of the relay to the resulting impedance of the SC bank and therefore properly protects the line for most cases.

29 Mitigation Approaches Real Time Digital Simulation (RTDS) and Testing RTDS TRIP Inputs Current Signals Analog Amplifiers Relay 1 Relay 2 Voltage Signals RTDS Testing is used to test and prove the actual protection settings with the actual relay. An important step as actual relay performance and protection algorithms are difficult to model accurately and in some cases proprietary. Can also be used to test and compare the performance of multiple relay types over a large range of test conditions.

30 Mitigation Approaches Real Time Digital Simulation (RTDS) and Testing RTDS Testing will allow better performance assessments of relay operation. Visualization of the dynamic impedance trajectory can be used/superposed over calculated protective zones to optimize the protection settings based on expectations vs. tested results. RTDS testing is often used to test challenging conditions for the relays.

31 Summary Series Capacitor Banks have protection that operates differently depending on the severity of the fault. Resulting impedance of the banks affects line protection in different ways. There are mitigation approaches available to reduce the severity of the affect. RTDS testing is an important step to ensure the protection system operates correctly and as expected.

32 Look Out For... Further FACTS Educational Webinars For more information go to and select Contact

33 Questions? Opportunity to ask Paul and Luis questions Paul Datka GE Energy Consulting Principal Luis Polanco GE Energy Consulting Principal

34 THANK YOU!