Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
S2-4: System simulation, parameter determination and relay testing
Testing of distance protection relays A. APOSTOLOV, B. VANDIVER D. THOLOMIER OMICRON Electronics AREVA T&D Automation USA Canada
[email protected]
KEYWORDS Distance protection relays, performance and functional testing
I. INTRODUCTION Distance protection relays have changed dramatically in the last two decades from simple single function distance protection relays into multifunctional IEDs with primary transmission line protection functions based on classical or advanced operating principles. Testing of such devices requires the availability of a set of tools that will simplify the testing process, while at the same time will ensure the required high quality of that testing process. Testing of distance protection relays and IEDs is one of the key requirements to ensure their correct operation under short circuit faults during abnormal system conditions. Since protection technology is becoming more and more complex, with protective relays evolving to multifunctional devices with integrated pre-programmed control logic and additional functions like metering, fault and disturbance recording, programmable scheme logic, etc., ensuring that they are properly configured and implemented requires adequate testing of their functionality. At the same time the operation of the transmission systems close to their stability limit requires significant reduction in the fault clearing times that can be only achieved by using the advanced logic schemes available in modern transmission protection relays. In this case verification of the relay operating times through testing is critical. The relays are also used as a front end to the substation automation system and provide different logging functions for analysis of their operation or different power system events. That is why testing of such devices requires excellent understanding of the available protection and nonprotection functions, as well as the operational logic of the different schemes that require testing. Considering the fact that there is a possibility for simultaneous occurrence of different types of abnormal system conditions, it is clear that it is necessary to be able to test the distance protection relay’s operation under such conditions. The requirements for the testing of all above listed functions are presented in the paper. Different methods for testing are described, with special attention being paid to the testing tools for advanced transmission line protection functions and schemes. II. TESTING OF MULTIFUNCTIONAL DISTANCE PROTECTION DEVICES When we analyze the complexity of modern multifunctional distance protection devices it is clear that their testing requires the use of advanced tools and software that can simulate the different system conditions and status of primary substation equipment and other multifunctional IEDs. The test system should be able to replay COMTRADE files from disturbance recorders or produced from electromagnetic transient analysis programs. It should be able to apply user defined current and
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Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
voltage signals with settable phase angles, as well as execute a sequence of pre-defined pre-fault, fault and post-fault steps.
Fig. 1. Test system block diagram The testing of the different IED elements has to start from the bottom of the functional hierarchy and end with the most complex logic schemes implemented in the device. Protective relays with such schemes operate based on the state of multiple monitored signals such as permissive or blocking signals, breaker status signals, and relay status signals. Time coordination of these signals and synchronization with the pre-fault and fault analog signals is required in order to perform adequate testing of these types of schemes.
Fig. 2. Testing of distance protection IED Figure 2 shows in a simplified way the need for the test device to be able to properly simulate the distance protection environment, as well as to monitor the operation of the relay under the simulated conditions. As can be seen from the figure, the testing should include any visible behavior of the tested distance relay. This requirement is taken into consideration in the following sections of the paper. 1) Testing of the analog signal processing The analog signal processing is the first critical step in the testing of a distance protection relay because if any problems exist at this level, they will be reflected at any other step up the functional hierarchy. The only problem is that the data bus of the IED is usually not directly accessible or visible through the relay communications or user interface. That is why an indirect method is recommended. If we configure the testing software to generate pure sinusoidal waveforms of balanced currents and voltages with their nominal values and no phase shift (zero degrees) between the currents and voltages in the same phases and record the applied waveforms with the tested relay, extracting and analyzing the records will allow us to evaluate if there is any problem with the analog signal processing.
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Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
Using any COMTRADE viewer to analyze the waveform record extracted from the relay will immediately show us if there are any deviations from the expected sine waveform, if there is any phase shift or if the amplitude is different from the expected value. COMTRADE viewers are usually readily available as part of the relay software or the testing software itself. They also typically calculate and display the magnitude and phase angles of the currents and voltages, so just by looking at these values and comparing them with the expected nominal values and balanced phase angles it is quite easy to determine if there are problems with the analog signal processing of the tested device. This step does not have to be used every time because it takes some time and it also requires the availability of COMTRADE viewer and communications with the relay in order to extract the recorded waveforms. A much easier way of detecting potential problems in the analog signals processing is the testing of the measurements as described in the next section. 2) Testing of the measurement functions The testing of the measurement functions of the relay is the next step. It can use the same set up as described in the previous section, at least as the initial measurements test condition. The nice thing about this test is that it does not require the use of relay communications, since the relay measurements are normally available through the front panel user interface. The measured phase currents and voltages in this case need to be as close as possible to the nominal balanced values applied to the relay by the test device (within the accuracy range specified by the relay manufacturer). The positive sequence measurements should be within tolerance of the phase values. Since the applied phase currents and voltages are balanced, the measured negative and zero sequence values should be close to zero (again within the expected tolerance range). At the same time the power factor should be close to 1 and the frequency close to the nominal frequency of the applied signals to the relay. Depending on the measurements available in the tested relay, it is quite simple to calculate the nominal balanced values and to compare and see if the measured values are within the expected range and tolerance. If we are interested to check the accuracy of the relay measurements at sub-nominal levels, we can configure the test software to apply 10% or 1% of the nominal values and follow a similar procedure to the one described above. 3) Testing of the main protection functions As discussed earlier, the main protection functions of a distance protection relay are the phase and ground distance elements. The testing of the instantaneous and time delayed elements is different and also should follow a specific order. When testing in a conventional fashion the individual protection elements, it is very important that they are the only enabled protection function (if all protection elements share the same relay output). If the IED has multiple relay outputs and different protection elements are mapped to different outputs, we need to make sure that the test device monitors the correct relay output during the test. For a modern test system, such mappings shouldn’t be necessary. A good fault model will correctly generate a system condition that the relay should distinguish, indicate, and trip correctly for based on the enabled protection element characteristic. If we (based on the measurement functions tests) assume that the relay measures accurately the applied current and voltage signals, the testing of the distance elements should not provide any surprises from the accuracy point of view, but will rather give us an indication of what is the characteristic of the tested zone and expected relay operating time when the apparent impedance seen by the distance element based on the applied currents and voltages is within the operating characteristic. The test system should be configured to apply currents and voltages with magnitudes and phase angles calculated based on the apparent impedance, type of fault and testing method selected. It should measure the time between the start of the test and the sensing of the operation of the relay output when connected to a binary input of the test system. This time should be less than the maximum operating time in the technical specification of the tested relay. It also depends on the time delay setting of the distance zone. 3
Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
Constant voltage and constant current methods may be used for the distance characteristics testing. This is acceptable for electromechanical and solid state relays, as well as for some microprocessor based relays that use distance elements based on the same principles, i.e. the relationship of current and voltage phasors. The operating principles used in the distance relay also need to be taken into consideration in the testing process. While the above described tests are related to checking the distance characteristic of the relay, they may not be suitable for the testing of the relay tripping time. This is especially important for Zone 1. If the relay uses superimposed components for the fault detection, faulted phase selection or directional detection, the ramping of the current or voltage in some of the conventional test methods is not going to be seen as a fault condition and the relay under test is not going to operate as expected. In such cases dynamic testing will be required. We still need to be careful with regard to the understanding of this term. In some cases a state change from pre-fault to fault condition may be sufficient. However, if this is represented as a step change in the fault injection to the distance relay under test, it still may result in an operating time slower than expected due to the fact that the current waveform is not realistic. That is why electromagnetic transient simulation is the best way to generate the signals used for the testing of the distance element. The testing of distance elements with complex characteristics also requires accurate modeling of the distance characteristic as part of the test configuration process. Evaluation of the distance element operation for multiple points on the selected characteristic is typically required. Figure 3 shows some of the configuration for the testing of a distance relay with a complex characteristic. Depending on the tested element the user should be able to configure the type of fault as single-phase-to-ground, phase-to-phase or three-phase and also select the testing method.
Fig. 3. Distance characteristic test configuration If the results from the testing of the distance characteristics and the operating time are within the expected range, the next step is the testing of the different communications based schemes. III. TESTING OF DISTANCE PROTECTION SCHEMES The testing of distance protection schemes [1] is the final step in the testing of a distance relay and it is based on the assumption that all individual protection elements – distance, overcurrent, directional, faulted phase selection, etc. have already been tested and proven to be operating correctly. An important consideration is the purpose of the test. If the test of a distance scheme is performed as part of a relay acceptance test, the complete test can be performed by the simulation of the analog and binary signals that the relay is going to measure or monitor under the specific test case conditions. However, if the test is part of the commissioning of the protection system of a transmission line before it is put in service, it may be necessary to test the complete protection system, including the communications channel. End-to-end testing using GPS synchronization is the preferred method in this case. 4
Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
The conventional test process requires the programming of the test system to perform prefault, fault and post-fault steps simulating the changing power system conditions to evaluate the performance of the selected transmission line protection scheme logic. There is a need for different options for testing of distance protection logic schemes based on the purpose of the test. Three typical cases are: • Complete evaluation: all logic schemes are selected in a “point-and-click” manner and the test software automatically executes a series of predefined tests, measures the relay’s response, analyses the results and prepares the test report. • Testing of a specific logic scheme: automatically executes all tests required for the selected logic scheme, measures the relay’s response, analyses the results and prepares the test report. • Testing of a specific logic scheme for a specific condition: automatically executes a single test required for the selected logic scheme, measures the relay’s response, analyses the results and prepares the test report. Different control signals are required by the distance protection logic schemes and must be considered in the test definition in order to verify the functionality and the correct settings of such schemes. The simulation of the relay environment is also affected by the location of the fault. A. How are the Tests Performed? The fundamental requirement for advanced testing solutions in today’s utility environment is a combination of efficiency and ease of use. The goal is to achieve maximum results with a minimum effort. That is why, the test configuration, execution efforts and analysis of the results from a series of tests in most cases should be limited to a point-and-click action. The testing of communication aided schemes should be performed in a way that as closely as possible matches’ real life power system conditions. The sequence of steps in a test is different as a function of the requirements for the specific scheme and system condition. For example, if the test is for a Direct Transfer Trip scheme and the test conditions are normal system with a noise triggered Carrier Receive signal, the sequence will include only three steps: • pre-fault with breaker in a closed position, nominal voltage and normal load current • receive of Direct Transfer Trip signal • post-trip condition with breaker opened, nominal voltage(assuming that bus voltage is applied to the relay) and no current If a more complex scheme is tested, the number of steps will increase accordingly. For example if a Permissive Overreaching Scheme is tested, and a fault on an adjacent line with sequential breaker opening is simulated, the test will have to include the following steps: • pre-fault with breaker in a closed position, nominal voltage and normal load current • initial fault condition with current flowing in reverse direction • receive of Permissive Trip signal • current reversal fault condition (simulating the opening of the breaker by the Zone 1 trip of the relay on the adjacent line) • post-fault condition with breaker closed, nominal voltage and normal load current The test device is used to simulate both the analog and the digital signals received by the relay in the field. At the same time its inputs are used to monitor the operation of different relay elements as required by the scheme under test. If the test cases of interest involve current reversal, cross-country faults, reclosing or any other event that includes conditions that change in time, a state sequencing tool can be used to create the simulation. One should be careful to avoid step changes in the currents that do not properly represent the real changes that occur in the electric power system – for example the DC offset.. The preferred method for testing the protection of double circuit lines is using an electromagnetic transient simulation program. The only problem with this suggestion is that usually such programs are not very user-friendly and require very good knowledge of the modeling of the different power system elements that are used for the simulation. The use of pre-defined templates that support the needs for modeling of the different modes of operation and fault conditions on double circuit lines significantly simplify the testing process by limiting the configuration process to entry of the values of the system or test parameters defined in the test template. 5
Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
Fig.4. Double circuit line protection test simulation configuration • • •
The configuration of the test should include: Network model – sources at both ends of the line, the two lines and their mutual coupling Fault model – the types of faults, their location and the time when they occur Breaker model – the operation of all breakers on the double circuit line based on the expected sequence of relay operation
Fig.5. Double circuit line protection test simulation with current reversal Using these three models the user can easily configure a transient simulation corresponding to any of the cases described earlier in the paper. As soon as the configuration process is complete, the network simulator calculates the current and voltage signals corresponding to the pre-fault and fault conditions selected, also taking into consideration the operations of the breakers defined by the user. Fig.5 shows the waveforms for a fault followed by an opening of a breaker that leads to current reversal. 6
Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
The user also can define which groups of signals are required, that can be used for end to end testing as well. The test can be executed right away and the performance of the tested relay can be evaluated based on its operating time. B. Test Results Analysis The results from each test performed are automatically analyzed by the test software. The analysis is based on an expert system comparing the operating time of a combination of monitored protection elements that have picked-up during the test. The operating time of the monitored protection elements is defined based on the protective relay manufacturer’s technical specifications. The results are displayed in a graphical format in the user interface and in detail in an automatically generated test report. The test system is used to simulate both the analog and the digital control signals received by the relay in the field. At the same time its inputs are used to monitor the operation of different relay elements as required by the scheme under test. IV. END-TO-END TESTING The next step in the Distance Protection Scheme testing is the extension of the same test cases described above into the full operational protection system test. This is commonly referred to as Endto-End Testing or System Testing. These tests depend on two critical factors, first the coordination of the test cases for both ends of the line to be tested must be the same and a mirror of the fault being simulated. In other words, both ends must see the same Zone 1 fault so the scheme logic will respond accordingly. If the local terminal “sees” a Zone 1 fault at 30% of line length, then the remote end must “see” it at 70% of the line length. But care has to be taken to make sure the test cases are structured properly to provide the correct prefault conditions and prefault to fault transition to correctly simulate the system condition of the test case. A simple step change transition can only test the gross performance of the protection scheme; most modern distance relays utilize complex algorithms to detect real system events. The step change may invoke additional logic that would conflict with the scheme logic being tested, resulting in an undesired or unexpected response. Second, playback synchronization accuracy is very important. A synchronization error of 1ms at 60 Hz produces a 21.6° error between two referenced vectors. For successful end-to-end testing, synchronization has to be better than 10µs, which produces a phase error of 0.216°. The best test equipment in the world today achieves a synchronization accuracy of 1µs, which is more than adequate to test even traveling wave relays (a 0.0216° phase error). Today, GPS systems can provide this level of timing to the test set; (Figure 5) one must ensure that the output of the test set is as accurate and more importantly consistently repeatable. In addition to the positive test cases where a response from the distance relay is expected, in end-to-End testing it is even more important to have a suitable set of negative test cases where the distance relay or communication system is verified it does not miss-operate. This requires an in depth knowledge of the entire protection scheme and understanding of when it should not trip. Many utilities are moving to system testing as their normal maintenance test procedure for many reasons. A few of these are: • Understanding and verification of the entire protection system is critical to utility operations • A digital relay’s reliability, once functionally verified, is seldom enhanced with additional element testing except in the case of settings or firmware changes • System testing reveals more in depth data and pinpoints problems faster with proper test cases • Costs for system testing can actually be less, with better trained personnel, as compared to conventional routine testing • System testing requires development of good testing habits resulting in more consistent testing
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Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)
Fig. 6. Testing of the Communication based Power System Protection Schemes using GPS satellite system for the test synchronization V. CONCLUSIONS Testing of multifunctional distance protection relays requires complete understanding of their functionality, operating principles of the individual elements and different distance protection schemes. Software and hardware tools to appropriately simulate the test conditions and the permissive, blocking or other status signals received from the relay under test are essential for successful logic scheme performance evaluation. The testing should follow the functional hierarchy of the distance protection relay. It should start with testing of the analog signal processing and measurements, followed by individual protection or other elements and finish with the distance protection logic schemes. Testing of complex distance characteristics using different methods for single-phase-toground, phase-to-phase or three phase faults is an important step in the overall testing process of distance relays. The operating principles and algorithms implemented in the relay should be taken into consideration when selecting the fault simulation method. Testing for internal and external faults, as well as faults on a parallel circuit (if an application on a double circuit line is tested) should be included in the test cases. Automatic rules based expert system relay performance analysis and test report generation significantly improves the efficiency and further simplifies the overall testing process. V. REFERENCES Automated Testing of Communications Based Schemes in Transmission Line Protection Relays / A. Apostolov, B. Vandiver. Power Industry Computer Applications PICA 2001, Sydney, Australia, May 2001
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