2010 PotM Testing Teleprotection Schemes | Simulation | Electrical Impedance

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  © OMICRON  electronics GmbH 2010   Testing Teleprotection Schemes by Transient Network Simulation Boris Bastigkeit OMICRON electronics GmbH boris.bastigkeit@omicron.at Introduction  Absolute selectivity and short tripping times of the line protection in meshed networks can be achieved by using line differential protection - or distance protection with teleprotection schemes. As a result of the ex-change of information between the protection devices at the line ends, fault tripping is also quicker in the range of the last 10-20% of the line, where instantaneous tripping is not performed in conventional distance pro-tection. Testing of such distributed protection systems with communication features is performed according to the state of the art using GPS synchronized end-to-end testing. The example of testing distance-teleprotection systems is used in this article to demonstrate how the simulation of faults on the protected line by means of a network model can substantially simplify testing. Hence, not only the correct parameter setting and function of the protection device are ensured but also the plausibili-ty of the calculated parameter settings is verified. Until now, using a transient network model for testing has rather not been a topic in the field of commissioning and routine testing. Typical users of network models are universities, specialized research institutes, engineering departments of protection equipment manufacturers, protection laboratories of major power utility companies, etc. Network models are reputed to be expert tools that are complex and difficult to handle. This is often due to the fact that the available network models are very powerful and allow modelling of very complex networks. However, if we consider that a test of protection devices by transient network simulation a most realistic test, there is a growing wish for a simple network model which is also suitable for commissioning and routine testing. The main advantage of such a method is that it offers a very high degree of reliability that the protection device fulfils its designated main function and that the parameter settings have also been calculated correctly. If one knows the protection concept and how the pro-tection equipment should respond to certain faults, testing is possible independently of the protection de-vice type used. Recently network model software has become available that enables easy network simulation also in substa-tions. When one wants to open up to the idea of using a network model in a substation, it is important to avoid being discouraged by the concept of transient network simulation . An easy-to-use tool may even simplify the testing task by using a network model. The following application report describes the practical implications of this. Description of the distance-teleprotection system  A field test was carried out to verify the usability of a transient network model for the purpose of routine or commissioning testing of a distance teleprotection scheme. The protected line and the protection concept are described below. Line data Line type: Overhead line Grounding: Compensated network Nominal voltage: 110kV X (primary): 7.175 ohms R (primary): 3.44 ohms CT/VT data Side A: Voltage transformer: : 110000/100 Current transformer: : 600/1 CT starpoint towards busbar Side B: Voltage transformer: :110000/100 Current transformer: :600/1 CT starpoint towards busbar  © OMICRON  electronics GmbH 2010   Protection Concept/ Test Concept Distance protection devices with communication channels for teleprotection are used. Teleprotection method: A release signal is sent to the opposite end when a fault is detected in zone 1 or in the extended zone. The extended zones are released for instantaneous tripping when the opposite end sends the release signal. If there is no pick-up, a received release signal is returned to the other end (echo scheme). Fig. 1: Grading of distance protection zone 1 and extended zone / fault locations The following test cases (1 – 5) are typically carried out. They describe the function of the teleprotection scheme in detail and are the basis for designing a test plan. (1) Fault at the middle of the line: Both ends feed into the fault. Teleprotection communication channel active. The protections in A and B detect the fault as a zone 1 fault and both perform instantaneous tripping. (2) Double-end feeding, teleprotection communication channel active. Fault near busbar B. The protection in A detects the fault as a zone 2 fault and issues a Send PSIG . Upon the PSIG receipt, the extended zone is released in A and instantaneous tripping is performed. (2a) Single-end feeding, teleprotection communication channel active. Fault near busbar B. The protection in A detects the fault as a zone 2 fault and issues a Send PSIG . Since protection does not pick up in B because of single-end feeding, B returns an echo signal upon the PSIG re-ceipt. Upon the PSIG receipt, the extended zone is released in A and instantaneous tripping is performed. (2b) Double-end feeding, teleprotection communication channel failed. Fault near busbar B. The protection in A detects the fault as a zone 2 fault. The protection in B detects the fault as a zone 1 fault and performs instan-taneous tripping. Due to a PSIG failure, both ends have to process the zone timers. (3) Fault beyond busbar B. The protection in A detects the fault as a zone 2 fault and issues a Send PSIG . The protection in B detects the fault in reverse direction, trips with the time for the reverse zone, and must not issue a Send PSIG . Since the protection in A does not receive a PSIG, tripping is performed in zone 2. (4) Double-end feeding, teleprotection communication channel active. Fault near busbar A. The protection in B detects the fault as a zone 2 fault and issues a Send PSIG . The protection in A detects the fault as a zone 1 fault, initiates instantaneous tripping and issues a Send PSIG . Upon the PSIG receipt, the extended zone is released in B and instantaneous tripping is performed. (4a) Single-end feeding, teleprotection communication channel active. Fault near busbar A. The protection in B detects the fault as a zone 2 fault and issues a Send PSIG . Since protection does not pick up in A because of single-end feeding, A returns an echo signal upon the PSIG receipt. Upon the PSIG receipt, the extended zone is released in B and instantaneous tripping is performed. (4b) Double-end feeding, teleprotection communication channel failed. Fault near busbar A. The protection in B detects the fault as a zone 2 fault. The protection in A detects the fault as a zone 1 fault and performs instan-taneous tripping. Due to a PSIG failure, both ends have to process the zone timers. (5) Fault beyond busbar A. The protection in B detects the fault as a zone 2 fault and issues a Send PSIG . The protection in A detects the fault in reverse direction, trips with the time for the reverse zone, and must not issue a Send PSIG . Since the protection in B does not receive a PSIG, tripping is performed in zone 2.  © OMICRON  electronics GmbH 2010   Testing with a transient network model:  A new easy to use transient simulation test tool can be used to establish a field compatible test plan. The field test is carried out in the course of a cyclic routine test.  After conventional end-to-end testing is carried out, the new method - using the transient network model - is applied. The test plan is prepared with an appropriate tool for automatic testing. The following describes how a test plan is created using the new method. Selecting the network type The first action in the transient network model is the selection of the network type to be used for the test. The appropriate network configuration that is selected is single line . This is one line with double-end feeding. When the circuit breakers are opened this model also simulates a line with single end feeding. Both cases are required for the test. Fig. 2: Transient network model - Selecting the single line configuration Setting the sources  As the next step the sources of the network model are configured. The described transient network model offers the advantage that the pre-assignment of the individual parameters of the sources has already been done for typical cases and it is - especially for testing distance protection - usually not necessary to make any changes to the default settings. The only adjustment recommended for the given example is to set the zero sequence impedance of the sources to the maximum impedance since the case discussed is a compensated network which does not permit a high fault current in case of a ground fault and causes a voltage offset. However, since in the case described only 2-phase faults are simulated the zero sequence impedance of the sources is not relevant. If it is desirable that a load current flows in the prefault condition, one can achieve this, for example, by defining the phase angles of the two sources slightly out of phase. The amount of the fault currents for 2-phase and 3-phase faults is con-trolled simply by the value of the positive sequence impedances of the sources. Fig. 3: Transient network model - Configuring the sources Specifying the line data Entering the line impedances correctly in the network model is the most important step for testing distance protection devices. The network model supports both entering the line data as primary values or as second-ary values. In this example secondary data are used, therefore the primary line data have to be converted to secondary data using the voltage (V) and current (I) transformer ratios. rV rI   Xprim X   = sec   rV rI   Rprim R  = sec   Xsec ... Secondary Reactance Xprim ... Primary Reactance Rsec ... Secondary Resistance Rprim ... Primary Resistance Secondary line data (positive sequence): X1 (sec): 3.914 Ω  R1 (sec): 1.876 Ω  Z1 (sec): 4.34 Ω  Phi Z1: 64°  © OMICRON  electronics GmbH 2010   The data for the zero sequence impedance of the line are not available. In compensated networks, this is only relevant in case of a cross country fault. However, this is not the focus of the test. Testing ground faults is more important in networks with other starpoint han-dling. If the zero sequence impedance should not be known in such a case, it can be measured. [5] For the purposes of the trial, the following arbitrary assumptions have been made: Z0(sec): 16 Ω  Phi Z0: 85° Fig. 4: Transient network model - Entering the data of the line to be protected Test sequence In accordance with the protection concept as described at the beginning of this article, the various test steps (for example, one transient network model module per test step) are now generated in the test plan by copying the transient network model test module defined so far. The following items are defined for each test step: -  Fault location / line affected -  Fault type -  Prefault duration -  Fault duration -  Postfault duration Fig. 5: Transient network model - Configuring the fault quantities The fault inception angle is not relevant in the example discussed. The fault resistance (arc resistance) is de-fined as metallic fault (0 Ω ). To provide automatic assessment of the test steps, it is possible to define which response is to be expected from the protection (for example, instantaneous tripping, send release). This can be done first for the relay at the  A end. Later, these settings have to be adjusted accor-dingly in the test plan copied for the B end for cases where the protections at the both ends are expected to react differently. Fig.6: Transient network model - Definition of the measurement conditions for the automatic assessment of the test steps In each transient network model module inserted, GPS is defined as the trigger for the start of the test. This allows the generation of time-synchronized test signals at both ends of the line. State of the art test equipment allows to synchronize test signals via GPS with an accuracy in the range of a few microseconds. Test plan for end B Once one has defined all the test steps and the corres-ponding measurement conditions, one can produce the test plan for the B end by creating a copy of the test plan for the A end. As already mentioned it is necessary to adjust the measurement conditions in the test plan for the end B for the test cases (3) and (5).
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