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Page 2. Oct - 09. Objective. Present Scenario: Windfarms connected to existing lines For Economic benefit 2 or 3 collector circuits tapped to a line Now , 2 ended line ? 5 /6 terminal lineIssue Protection Requirement for 5/6 terminal line Aim: To verify the differential scheme in term
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1. Multi-Terminal Differential Relaying as a Practical Solution for Multi-Ended Line Protection for Onshore Wind Farms
Presenter: Andre Smit, Siemens Energy, Inc. USA
Authors: John Ciufo, Babak Jamali , Arnd Struecker , Shyam Musunuri)
2. Page 2 Oct - 09 Objective Present Scenario:
Windfarms connected to existing lines
For Economic benefit 2 or 3 collector circuits tapped to a line
Now , 2 ended line ? 5 /6 terminal line
Issue
Protection Requirement for 5/6 terminal line
Aim:
To verify the differential scheme in terms of
Dependability
Speed
Security
Stability
On shore windfarms are normally located at a considerable distance far away from the main substation utilizing the maximum wind energy. Therefore collector circuit feeders from windfarms are normally tapped into 230kV or 115kV transmission lines. With the increaing use of distribution generation, it is possible to have multiple collector feeder circuits tap into one transmission line. In this condition, a single, two eneded transmission line might be converted to a 5 or 6 ended line.
The protection requirement should be altered with the change in the configuration of the line. There are two protection configurations that can protect a multi-ended line. Distance Protection Configuration and Differential Proteciton Configuration. If distance relays are used to protect a multi-ended line, an operartional time delay needs to be built into the protection scheme in order to overcome the operational selectivity issue. In many applicatios, this time delay might not be tolerable as it may causes equipment damage or may not meet the required stability after fault clearing time. Differential protection configuration, on the other hand, has fast tripping, is sensitive and reliable which results in reduction of the equipment damage.
The aim of the paper was to test the behavior of the differential protection for a multi-ended line application in terms of dependability, speed, Security and Stabilty.On shore windfarms are normally located at a considerable distance far away from the main substation utilizing the maximum wind energy. Therefore collector circuit feeders from windfarms are normally tapped into 230kV or 115kV transmission lines. With the increaing use of distribution generation, it is possible to have multiple collector feeder circuits tap into one transmission line. In this condition, a single, two eneded transmission line might be converted to a 5 or 6 ended line.
The protection requirement should be altered with the change in the configuration of the line. There are two protection configurations that can protect a multi-ended line. Distance Protection Configuration and Differential Proteciton Configuration. If distance relays are used to protect a multi-ended line, an operartional time delay needs to be built into the protection scheme in order to overcome the operational selectivity issue. In many applicatios, this time delay might not be tolerable as it may causes equipment damage or may not meet the required stability after fault clearing time. Differential protection configuration, on the other hand, has fast tripping, is sensitive and reliable which results in reduction of the equipment damage.
The aim of the paper was to test the behavior of the differential protection for a multi-ended line application in terms of dependability, speed, Security and Stabilty.
3. Partners Lets first start with who did what in this study. Application of differential protection on a multi-ended line is Hydro One requirement and they paid for the study. Siemens as one of the manufacturers who provide such a relay, loaned the equipment and Kinectrics did the research, prepared the test plan and performed the tests on the configuration.Lets first start with who did what in this study. Application of differential protection on a multi-ended line is Hydro One requirement and they paid for the study. Siemens as one of the manufacturers who provide such a relay, loaned the equipment and Kinectrics did the research, prepared the test plan and performed the tests on the configuration.
4. Page 4 Oct - 09 Before I get into the testing procedure of the relay, lets talk about the algorithm that is used in this relay which is different from the classical differential algorithm.
In classical differential algorithm, the differential current is the amplitude of the complex summation or phasor summation of all currents in fundamental frequency. The restraint current is scalar summation of the amplitude of all currents again in fundametal frequency. The tripping characteristic is normally a dual sloped line with tripping area on top of the line and restraint area on the bottom.Before I get into the testing procedure of the relay, lets talk about the algorithm that is used in this relay which is different from the classical differential algorithm.
In classical differential algorithm, the differential current is the amplitude of the complex summation or phasor summation of all currents in fundamental frequency. The restraint current is scalar summation of the amplitude of all currents again in fundametal frequency. The tripping characteristic is normally a dual sloped line with tripping area on top of the line and restraint area on the bottom.
5. Page 5 Oct - 09 The tripping algorithm of the line differential protection tested in this work is different from the classical one.
The restraint current of the tested differential protection can be shown in a complex area as a so called Delta I (?I) Phasor. Each relay calculates its own complex current phasor, furthermore a confidence domain is estimated. The error domain includes all errors for of the currents
CT error ( proportional errors from the CT)
Signal distortion errors( error in the estimation of the phasor due to harmonics caused by CT stauration or decaying DC components
Synchronization errors( error caused by asymmetrical transmission times)The tripping algorithm of the line differential protection tested in this work is different from the classical one.
The restraint current of the tested differential protection can be shown in a complex area as a so called Delta I (?I) Phasor. Each relay calculates its own complex current phasor, furthermore a confidence domain is estimated. The error domain includes all errors for of the currents
CT error ( proportional errors from the CT)
Signal distortion errors( error in the estimation of the phasor due to harmonics caused by CT stauration or decaying DC components
Synchronization errors( error caused by asymmetrical transmission times)
6. Page 6 Oct - 09 The figure shows the computation of the differential current in a complex plane of one relay for one phase.
The relay calculates its ?I-Phasor ( blue phasor with the light blue confidence domain). Then it receives a matching ?I-Phasor (red phasor with the orange confidence domain) from the remote relay. The two phasors are added and the result is the differential phasor(green). Both the confidence domains are added (light green, orange dotted circle). At the end the bias threshold is added to the resultant ?I-Phasor and results in the pink circle (?IRest). If the origin of the complex plane is part of the last pink circle the relay is stable. If the origin is not part of the pink circle the relay trips. This is done for all phases separately and as a result the tripping decision is phase seggregated. To be fast in one hand and sensitive on the other hand, the tested relay has two differential algorithms which are independent and running in parallel. The other algorithm operates on charges time integral of the current and will not be explained in here.
The new algorithm has several advantages like:
phase segregated tripping (no summation transformer)
different CT Ratios allowed (no matching transformer)
the algorithm handles each error separatelyThe figure shows the computation of the differential current in a complex plane of one relay for one phase.
The relay calculates its ?I-Phasor ( blue phasor with the light blue confidence domain). Then it receives a matching ?I-Phasor (red phasor with the orange confidence domain) from the remote relay. The two phasors are added and the result is the differential phasor(green). Both the confidence domains are added (light green, orange dotted circle). At the end the bias threshold is added to the resultant ?I-Phasor and results in the pink circle (?IRest). If the origin of the complex plane is part of the last pink circle the relay is stable. If the origin is not part of the pink circle the relay trips. This is done for all phases separately and as a result the tripping decision is phase seggregated. To be fast in one hand and sensitive on the other hand, the tested relay has two differential algorithms which are independent and running in parallel. The other algorithm operates on charges time integral of the current and will not be explained in here.
The new algorithm has several advantages like:
phase segregated tripping (no summation transformer)
different CT Ratios allowed (no matching transformer)
the algorithm handles each error separately
7. Page 7 Oct - 09 Test model This slide illustrates the power system which was modeled on RTDS. Before I get into the detail of modeling, I need to give a brief description of RTDS for people in the room who are not familiar with this device. RTDS stands for Real Digital Simulator which consists of software and hardware parts. The software part is more or less like the EMTP or PSCAD which allows you to model a section of a power system. The hardware part consists of several parallel processors which allows you to run the simulation in real time. The current and voltage signals of each bus are digitally available which can be converted to analog waveforms. Now the waveforms can be injected to the relays after proper amplification. By using this kind of simulation, the relay is placed in an actual power system configuration and behavior of the relay can be tested the same as if the relay is installed in an station control room.
This picture shows three racks of RTDS hardware, the voltage and current amplifiers and the five line differential relays for testing a five ended line.
Now, lets get back to the power system modeling process. As you see a double ended line with infinite sources (S1 and S2) at each end with sources impedances of ZS1 and ZS2 are modeled as a backbone. These sources are connected with a 120km line which is broken into four 30km lines, each modeled as PI sections. The wind turbine is tapped into this 230kV line through a transformer. There were a lot of debates around modelling of the wind turbine. We believe that a synchronous generator is a sufficient model for our purpose. To make this a bit more clear, I should note that there are three kinds of wind generators: permanent magnet generators, synchronous generators and asynchronous generators. Nowadays technology is leaning toward partially decoupled and asynchronous generators which both are acting like synchronous machines. In this project we were not concerned about effect of wind turbines on each other so an aggregated wind generator model is used which is common in power system stability studies. In order to test a five ended differential protection configuration with this three ended power system model, R4 and R5 relays were wired back to back. This means that the same current passing through R3 will pass thorough R4 and R5 relays but R4 and R5 currents cancel each other. This way we need just one set of current amplifiers for the three relays.
Now behavior of the protection configuration can be assessed by introducing a fault at any point of the power system model.This slide illustrates the power system which was modeled on RTDS. Before I get into the detail of modeling, I need to give a brief description of RTDS for people in the room who are not familiar with this device. RTDS stands for Real Digital Simulator which consists of software and hardware parts. The software part is more or less like the EMTP or PSCAD which allows you to model a section of a power system. The hardware part consists of several parallel processors which allows you to run the simulation in real time. The current and voltage signals of each bus are digitally available which can be converted to analog waveforms. Now the waveforms can be injected to the relays after proper amplification. By using this kind of simulation, the relay is placed in an actual power system configuration and behavior of the relay can be tested the same as if the relay is installed in an station control room.
This picture shows three racks of RTDS hardware, the voltage and current amplifiers and the five line differential relays for testing a five ended line.
Now, lets get back to the power system modeling process. As you see a double ended line with infinite sources (S1 and S2) at each end with sources impedances of ZS1 and ZS2 are modeled as a backbone. These sources are connected with a 120km line which is broken into four 30km lines, each modeled as PI sections. The wind turbine is tapped into this 230kV line through a transformer. There were a lot of debates around modelling of the wind turbine. We believe that a synchronous generator is a sufficient model for our purpose. To make this a bit more clear, I should note that there are three kinds of wind generators: permanent magnet generators, synchronous generators and asynchronous generators. Nowadays technology is leaning toward partially decoupled and asynchronous generators which both are acting like synchronous machines. In this project we were not concerned about effect of wind turbines on each other so an aggregated wind generator model is used which is common in power system stability studies. In order to test a five ended differential protection configuration with this three ended power system model, R4 and R5 relays were wired back to back. This means that the same current passing through R3 will pass thorough R4 and R5 relays but R4 and R5 currents cancel each other. This way we need just one set of current amplifiers for the three relays.
Now behavior of the protection configuration can be assessed by introducing a fault at any point of the power system model.
8. Page 8 Oct - 09 Verification Tests Balanced and unbalanced external faults at low/ high fault currents
CT saturation issue
Security on wind unit power swing
Loss of communication channels Several scenarios were tested on the system.
In order to test the security of the protection configuration we tested the relay for out-of-zone (external) balanced and unbalanced faults. Some of the differential relays with classical differential characteristic have difficulties in securing the cases with severe CT saturation so we include this into the test plan. Also we wanted to check the security of the protection configuration if the wind units start swinging. Complete loss of a communication channel was of an interest to us since in some cases, this creates communication errors which resulted in mal-operation of the relay.
In the dependability section, a balanced and unbalanced in-zone (internal) fault were introduced to the system. Almost all of the differential relays behave as expected for high fault current but are slower for low fault currents. Both low and high internal fault currents were tested in this case. In some protection algorithms, the relay blocks the differential function for a fixed time after an external fault. This type of algorithm can not detect an evolving fault which is an external fault evolving into an internal fault. So we included the evolving fault condition into the test plan. And at the end, behavior of the relay with complete loss of one communication channel was tested with in-zone faults.Several scenarios were tested on the system.
In order to test the security of the protection configuration we tested the relay for out-of-zone (external) balanced and unbalanced faults. Some of the differential relays with classical differential characteristic have difficulties in securing the cases with severe CT saturation so we include this into the test plan. Also we wanted to check the security of the protection configuration if the wind units start swinging. Complete loss of a communication channel was of an interest to us since in some cases, this creates communication errors which resulted in mal-operation of the relay.
In the dependability section, a balanced and unbalanced in-zone (internal) fault were introduced to the system. Almost all of the differential relays behave as expected for high fault current but are slower for low fault currents. Both low and high internal fault currents were tested in this case. In some protection algorithms, the relay blocks the differential function for a fixed time after an external fault. This type of algorithm can not detect an evolving fault which is an external fault evolving into an internal fault. So we included the evolving fault condition into the test plan. And at the end, behavior of the relay with complete loss of one communication channel was tested with in-zone faults.
9. Page 9 Oct - 09 Test Results Overall behavior of the relay was acceptable with very fast operation (less than one cycle for most of the cases) of the relay for internal faults and no mal-operation for external faults with even sever CT saturation.
The first picture in this slide shows current waveforms of one of the relays for an internal unbalanced fault. As you see the direction of the fault current is changed on the faulted phase which makes it harder for the relay to respond. The relay tripping time for this case was less than a cycle.
The second picture shows current waveform of the relay in case of a balanced external fault with CT saturation. Normally, the phase with the highest DC component has the highest amount of saturation and in this case it looks like that the red phase waveform is more distorted and the other two phases. Again the relay reacted as expected with no mal-operation.Overall behavior of the relay was acceptable with very fast operation (less than one cycle for most of the cases) of the relay for internal faults and no mal-operation for external faults with even sever CT saturation.
The first picture in this slide shows current waveforms of one of the relays for an internal unbalanced fault. As you see the direction of the fault current is changed on the faulted phase which makes it harder for the relay to respond. The relay tripping time for this case was less than a cycle.
The second picture shows current waveform of the relay in case of a balanced external fault with CT saturation. Normally, the phase with the highest DC component has the highest amount of saturation and in this case it looks like that the red phase waveform is more distorted and the other two phases. Again the relay reacted as expected with no mal-operation.
10. Page 10 Oct - 09 Conclusions Differential protection scheme is:
a reliable solution for multi-ended transmission lines
fast and secure
No distance limitation over SDH / SONET
We conclude that the differential protection is a reliable solution for multi-ended transmission line protection. It is fast and secure. I also should mention that with present day reliable communication technologies, it is now possible to extend the differential protection scheme for use on the transmission systems. Modern line differential relays can communicate over direct fiber optics to about 170km and distance is no longer a limitation.We conclude that the differential protection is a reliable solution for multi-ended transmission line protection. It is fast and secure. I also should mention that with present day reliable communication technologies, it is now possible to extend the differential protection scheme for use on the transmission systems. Modern line differential relays can communicate over direct fiber optics to about 170km and distance is no longer a limitation.
