Z. Kuran Institute of Power Engineering Mory 8, Warszawa (Poland)

Transcription

1 111 Study Committee B5 Colloquium 2005 September Calgary, CANADA Summary TRANSFORMERS DIGITAL DIFFERENTIAL PROTECTION WITH CRITERION VALUES RECORDING FUNCTION Z. Kuran Institute of Power Engineering Mory 8, Warszawa (Poland) A large number of unwanted operations of transformers differential protections was an inspiration to present this study. Differential protections have many advantages and disadvantages. They are selective but also difficult in design, setting and using. Difficulties are connected with the magnetizing currents of main transformers and instrument transformers, and also with the dynamic of short circuits phenomena. The dynamic process of differential protections operation is difficult for mathematical description and therefore we are in favour of recording all the phenomena occurring in the network instead of calculating currents dynamic. All this is performed by the below-mentioned function of criterion values recording which allows to achieve the simplification of setting, using and protection systems testing. Key words: transformer, differential protection, recording, operation reliability 1. Introduction The statistics shows a relatively large number of incorrect operations of transformers differential protections. According to the publication [1], in the Norwegian fault statistics covering the studied period from 1999 to 2000 the unwanted operations constituted as much as 50% of all operations of the transformers differential protections. A similar situation takes place in the Polish power network. Majority of unwanted operations result from reasons associated with: design, installation and technical aspects. Despite introducing the digital protection systems the situation has not improved. The microprocessor technology has made it possible to eliminate the equalizing transformers. However, differential protection systems still make too many difficulties for the personnel responsible for settings. As an effect, unwanted operations caused by incorrect settings still appear. It should be pointed out here that the CTs for transformers differential protections are more and more frequently selected with regard of the need to account for the cost of energy and with disregard of requirements resulting from the differential protection systems needs. Incorrect operations of the differential protections make a lot of stress to their designers. No fault can be repeated and therefore the testing of reasons of incorrect operations is based exclusively on the intuition of the protection systems designers.in case of the analog protection systems the precautions consisted mainly in increasing the setting of the starting current of the protection system, with simultaneous increase of the slope coefficient. Most often it caused reduction of protection sensitivity and extension of its operation time. There was a feeling of inadequate insight into reasons of the mentioned incorrect operations. To improve the situation the digital differential protection systems with the criterion values recording function have been worked out. 2. Characteristics of the transformers differential protections A principle of operation of the differential protection system is very simple. It is based on measurement of the difference in currents flowing into the transformer, with taking into account the transformer ratios and the CT 111-1

2 ratios. The reason of all problems connected with unwanted operations are the magnetizing currents of the transformer and the magnetizing currents of the CTs. The magnetizing current of the transformer, during its energization, may take values considerably exceeding the rated current. The magnetizing currents of the CTs reach the value equal even to a several dozen per cent of the through current. These currents make the main reason of incorrect operations. To make the protection systems resistant to the magnetizing currents, the manufacturers of the protection systems improve their products by making use of the latest achievements in the digital technology. If they are not able to define a value for a given parameter, they create a subsequent element that may be set, thus leaving the problem solution to the operating personnel responsible for settings. In this way the protection system operation manual becomes more and more thick and the protection system settings more and more difficult. The thickest protection system operation manual known to us contains 300 pages. Consequently, the number of incorrect protection system operations caused by the incorrect settings grows. In the presented differential protection system we set out the way to simplify the protection operation handling. According to the statistical data collected in Poland the unwanted operations of the differential protection systems take place during the external faults and the transformer energization. Despite disadvantages such as difficulties in designing, putting into operation, setting and testing as well as such as the occurrence of many unwanted operations, the differential protection systems of transformers are applied more and more frequently, sometimes even for the 250 kva transformers. It is so because the differential protection system promptly and selectively eliminates the internal faults, i.e. faults in the transformer and in the devices that are located in the zone between the transformers responsible for measuring currents inflowing to the transformer. All standard parameters of the differential protection systems, i.e. restraint current conditioned by the percentage of a particular harmonics of the current, are defined for static conditions. The differential protection is to decide on starting operation, or not, within time not longer than 35ms from the moment of fault occurrence. Time constants of the faults constant components decay very often reach value of 200 ms, whereas time constants of the transformer magnetizing inrush current reach even 1000 ms. Thus the protection system operation is influenced by: - the network conditions determining the inrush fault current, amplitude and constant component, - the design of the transformer and its core plate, - the design of the CTs and the type of material it was made of, - the design of schemes responsible for entering information on currents into the protection system, for instance the CTs schemes or current shunts, - the design of blocking during the inrush magnetizing current. Correct calculation of settings should take into account all the above elements, but it is really a difficult task. It requires top class professionals, who are mostly not available from among the equipment service personnel. This is the reason why experience of the personnel is of greater significance than calculations. Even if the differential protection works correctly during subsequent faults and transformer energizations, we are not able to guarantee its correct operation during any future fault events. The network conditions may deteriorate and we will not know what the concerned safety coefficient was. Therefore, it is important that the manufacturer of differential protection systems who knows his product best, should give such method of setting the protection system that would be simple for an average user and would allow to evaluate the safety coefficient. 3. The differential protection system with the criterion values recording function The function of the criterion values recording consists in dynamic recording the amplitude of the restraint and differential currents as well as the amplitude of the second and the fifth harmonics of the differential current, during the actual events. It is important for the recording to be carried out during the internal and the external faults as well as during the transformer energization. The recorded values will be compared in order to identify the faults. We are interested in amplitudes of every particular harmonic of the currents filtrated out in the protection system algorithm. This function is completely different from that performed by disturbance recorders which record the momentary values of currents. The construction of the differential protection system with the criterion values recording function is now referred to as the classical one. It is the protection restrained by the external current of the transformer as well as by the second and the fifth harmonics of the currents.

3 The protection system user has to set: - the transformer s rating and CTs rating, - the starting differential current, - the restraint factor k h for the restraint current I h. The manufacturer s settings (available for service) are as follows: - the restraint factor for the second harmonic current and the limit for the second harmonic restraint, i.e. the limit for k 2h I 2h., - the restraint factor k 5h for the fifth harmonic current and the limit for the fifth harmonic restraint, i.e. the limit for k 5h I 5h., - the limit for blocking sum current I bl, I bl = k h I h + k 2h I 2h + k 5h I 5h The protection starts operating when the differential current Irr exceeds value of the blocking sum current Ibl. Figure 1 shows the computer screen with the recorded criterion values for the internal faults. a) d) b) e) c) Figure 1 The recorded criterion values are shown in five windows: from a) to e). The windows contain: a) The value of the differential current amplitude I rr and the value of the blocking sum current value I bl, b) The value of the restraint current I h, c) The safety coefficient (k s =I bl /I rr ), d) The value of the second harmonic current amplitude I 2h, e) The value of the fifth harmonic current amplitude I 5h. The y-axis of the chart, scaled in amperes, is the currents axis (the currents are recalculated on the secondary side of the CTs of the rated secondary current equal to 5A.). The x-axis is the time axis, and the vertical grid lines on the x-axis are in 20 ms intervals, and the window is 120 ms wide in total. According to the record shown in Figure 1 the differential current Irr reached the maximum value of A. The blocking sum Ibl reached the limit value pre-set to 25 A. The time of fault identification amounted to about 10 ms. At the moment of the fault current occurrence and disappearance the second harmonic current amplitude value is above

4 standard. It is the natural feature of all filters. The amplitude value was increasing and in the final part of the fault amounted to 5.38 A, and this fact proves that the CTs were saturated with the fault constant component. Figure 2 shows the recording of the criterion values for the external fault. The fault current caused a very high saturation of the CTs with the fault current constant component. The differential current reached the value of A which constitutes 71% of the restraint current which reached the value of A. It means that the maximum CT error offset has the value of 71%. The blocking sum Ibl was bounded by the setting limit to 25 A. The second harmonic current reached the value of 7.85 A. The second harmonic current, as a result of saturation of the CTs by the current fault constant component, saved the protection system from the unwanted operation. a) d) b) e) c) Figure 2 Initialization of the criterion values recording starts when the differential current exceeds the setting of the starting current. This condition is always met during all protection system operations as well as during some transformer energizations and external faults. The saturation of the CTs during the external fault also results in the initialization of this recording. Figure 3 shows the transformer energization. The value of the second harmonic current in the inrush magnetizing current was small, namely below 20%. This made the differential current amplitude Irr reach the value close to the blocking sum current Ibl. The safety coefficient Ibl / Irr went down to 1.2. Such value was too low and therefore the setting corrections were entered into the protection system. 4. The operation experiences Since 2000 year over 360 units equipped with the criterion values recording function have been installed in the Polish power industry. And about 90 new units are installed yearly. Until today several hundred recordings has been collected. Users of the protection systems transfer the recordings through electronic mail and in exchange for them they receive the assessment of the correctness of their protection system operation. Operations of the 47 differential protections were recorded. There were no missing operations.

5 a) d) Figure 3 Reasons by which the protection operations were initiated: - internal fault in the transformer one case, - internal faults in the protected zone (animals, birds, voltage transformers and lightning protectors) 42 cases, - power supply faults 2 cases of hardware faults; these events initiated the recording of the criterion values which made it possible to eliminate the program error, - incorrect settings 2 cases; the incorrect CT ratios were set by mistake and during the next external fault the protection system activated. The recording of the criterion values made it possible to quickly identify the reason of its operation without the need to examine the transformer. In all cases the protection system operated for less than 35 ms. In case of high current values the time of the differential protection operation was shorter and amounted to 10 ms. The time of fault elimination was extended by the circuit breaker s specific time which oscillated mostly from 40 to 70 ms. The analysis of the collected data confirmed that the application of the second harmonic current for blocking the protection system operation during the transformer energization was a good choice. The correct operation of the applied protection algorithms was also confirmed. Benefits resulting from the analysis of the criterion values recording are as follows: - at the beginning of operating period it was recommended to set the starting differential current to 0.5 of the transformer s rated current, and the restraint factor to 0.5. Following first observations it was recommended to lower the setting of the starting differential current to 0.25 of the transformer s rated current, and the restraint factor to 0.35, - one-time CT ratio incorrect setting was identified. During the internal fault a high value of the differential current was recorded, but the protection operation did not activate, - one-time frequent initialization of the criterion values recording was noted, but the protection operation did not activate. Following the analysis of the recordings it was discovered that in one of the current circuit phase the temporary breaks occurred. A non-tightened clamp was discovered in the current circuit, - during the time of transformer energization the threat of activation of the differential protection operation was observed twice. It was recommended to increase the setting of the second harmonic restraint coefficient, - it was observed that the decrease of the accuracy limit factor (ALF) of the CTs does not pose a threat to the unwanted tripping occurrence but effects the extension of the protection operation time. 5. Conjunctive tests of performance arising from the IEC Protection systems standard. The protection system contains all devices having impact on the operation of protection, such as CTs, current circuits, measuring relays and circuit breakers. Within the conjunctive tests of performance one should check sensitivity, operating time and stability of the protection system. The test should be performed with the actual inrush fault currents that may occur in the location of the protection. The IEC-20 standard does not specify what technical means should be used in performing the tests. Each protection system is tested once. One of methods that can be used to replace the tests of the differential protection is the utilization of the actual differential fault currents recorded in the fault recorders. The recorded currents are used to control the digital testing devices of protection systems which are capable of reconstructing exactly the recorded currents in the protection systems.

6 Such method is effective if: - the recorded currents come from the location of the protection system, - the recorder is of high class and does not cause additional distortions of the current, - you have digital testing devices capable of generating the required quantity of currents (sometimes even twelve) simulating the currents inflowing to the transformer. In the Polish power industry there prevail CTs of the rated secondary current of 5 A. Therefore, testing devices of the protection systems must be equipped with the current amplifiers capable of generating the currents exceeding 100 A very often. That is why such tests are performed occasionally as a presentation of scientific achievements. It is much easier to assess the protective system on the basis of the recorded criterion values from the differential protection. The assessment is limited to checking the value of the safety coefficient Ibl/Irr recorded in the window b). For the external faults and transformer energizations it should not be less than 1.3. For the internal faults this value should be less than 0.7. The only task of the protection system users is to assess values of the concerned coefficients. If the personnel operating the protection systems find that the coefficient does not satisfy one of the requirements, it should send the recordings to the manufacturer of the protection systems or to the service agency. In reply he will receive recommendations concerning correction of the settings of the protection systems. The manufacturer renders this service free of charge, because the collected data makes an excellent material that enables him to improve the design of the differential protections and to optimize their settings. For the several hundred criterion values recordings that have been assessed up to now, the settings were corrected three times. The assessment concerns: - correctness of the protection systems settings, - operation of CTs in faults conditions of a given network, - technical efficiency of the protection system, - influence of electromagnetic interferences on the protection system operation in its location. 6. Routine tests The routine tests should be performed periodically. The objective of the tests is to obtain confirmation that all the protection system elements are technically efficient and also that the protections are correctly set. Usually, such tests are performed in one to five years periods. If the recordings of the criterion values have been obtained in such period, they make a documentary evidence proving the functioning of all elements of the protection essential for its operation. If these are recordings that have not activated circuit breakers operation, only the switch-off circuits remain for testing. 7. Conclusions The differential protection with the criterion values recording function was developed in order to decrease a number of unwanted operations. The objective was achieved by: - simplification of the differential protections criterion settings available for the operating personnel, - shifting the responsibility of setting the most difficult to define protection parameters to the designers of service settings, - utilization of criterion values recordings for optimization of the differential protection settings, - accepting the criterion values recordings as documentary evidences confirming the accomplishment of the differential protection routine tests and the conjunctive type test of the protection systems. 8. References [1] G. H. Kjølle, Y. Aabø, B.T. Hjartsiø, Fault statistics as a basis for designing cost-effective protection and control solutions (CIGRE report ,2002). [2] Z. Kuran, E. Tomczak, K. Woliński, Tests of new differential protection type RRTC during transformer energizations. Automatyka Elektroenergetyczna. 1999, nr 3, p (in polish). [3] K. Woliński, Analysis of the transformers 110 kv/mv differential protections operations in the period on the Białystok S.A. region, Automatyka Elektroenergetyczna. 1997, nr 3 4, p (in polish). [4] S. Skrodzki, Selection of the transformers differential protection type RRTC recordings, Instytut Energetyki, Laboratorium Automatyki i Zabezpieczeń, Warszawa, 2002 (in polish).