0 Introduction Microcomputer protection has been widely used in power systems. In this paper, based on the experience of field operations in recent years, some problems in the application of 220 kV main transformer protection are discussed and corresponding suggestions are made.
1 Configuration scheme for main transformer protection of the computer Before the 220 kV computer main transformer protection of the Hangzhou Electric Power Bureau adopted the configuration scheme of one main and one rear, main and rear, a set of differential protection, a set of backup protection, differential protection and backup protection. Placed in their own separate chassis, back-up protection on each side and separate [1]. The scheme has a clear structure, a simple secondary circuit, and low investment, and is widely used.
With the improvement of the reliability requirements of users for power supply, in order to avoid the forced suspension of the main transformer due to the abnormal protection of the main transformer, the State Power Corporation formulated the “25 Key Requirements for Preventing Major Electric Power Production Accidents†(hereinafter referred to as “Twenty Five countermeasures) pointed out that the microcomputer protection of the 220kV main transformer must be double [2]. At the same time, with the development of technology, many manufacturers have introduced the main backup protection and two sets of completely independent protection schemes. Therefore, in the recent infrastructure and technical upgrading projects, the Hangzhou Electric Power Bureau’s 220kV computer main transformer protection is Dual configuration (except for non-electricity protection). The scheme has a simple layout, a clear circuit, greater independence, and a modest increase in price. It is more advantageous than the configuration of the main one.
2 main transformer protection doubled current loop configuration 220kV microcomputer main transformer protection double, the two sets of protection current loop should be connected to the corresponding switch on the current transformer, when the main change 220kV switch or 110kV switch bypass generation, principle The upper two sets of protective current loops can be switched, but in practical applications, the secondary windings of the bypass switch current transformer are likely to be insufficient (especially for some old substations). At this time, the two sets of protection current loops mainly have the following Kind of connection method:
a. During normal operation, the two sets of protection current loops are connected to the current transformers of the corresponding switches. When the main transformer is bypassed, the first set of protection current loops are switched, and the second set of protection is disabled (three-phase current mutual inductance. Circuit is disconnected and shorted to ground.)
b. In normal operation, the first set of protected current loops is connected to the current transformer of the corresponding switch, and the second set of protected current loops is changed to the corresponding bushing current transformer. When the main transformer is bypassed, the first one is used. The protection current loop is switched, and the second protection does not switch and can continue to operate.
c. In normal operation, the two sets of protection current loops are connected in the same way. When the main transformer is bypassed, the first set of protection current loops is cut to the corresponding bypass switch current transformer, and the second set of protection switches to the corresponding set. Tube current transformer.
Connection a in the normal operation of the differential protection range, the two sets of protection can take into account the main transformer itself, the external lead, the disadvantage is that the bypass generation can only be a single protection operation. The second set of differential protection range in connection b is smaller. If the first set of protection is suspended for some reason, the 220kV side external lead of the transformer is not protected, but both sets of protection can operate when the bypass substitutes. Connection. In the normal operation, the situation is the same as a, and both sets of protections can be operated when the bypass generation is performed. However, the switching operation is the most, which increases the probability of errors due to human factors and is not welcomed by the operators. The author believes that the operation time of bypass bypass generation on one side of the transformer is generally shorter, and the connection method a can also be operated with a single complete protection in the bypass generation, so the connection method a should be adopted. So far, Hangzhou Electric Power Bureau has adopted wiring a in practical applications.
3 main transformer 220kV switch failure protection design 220kV transformer protection 220kV switch due to the protection of the switch and the probability of failure of the switch is very low, while the transformer failure protection in the loop there are many problems, so before the 220kV switch failure protection has not been put into operation . In order to prevent the extension of the accident range and even threaten the safe operation of the system, the “25 countermeasures†require that the failure protection of the switchgear must be put into operation [2]. However, in the practical application of the site, if To invest in 220kV switch failure protection, according to our experience, the following issues should be noted during design:
a. The sensitivity of the 220kV busbar composite voltage blocking element may not meet the requirement when the transformer is in the middle/low-voltage side. Therefore, after the failure is started, the composite differential voltage blocking should be released.
b. In order to ensure the safe and reliable operation of failure protection, it is not appropriate to use the two relays of the same relay or the output of the same circuit to simultaneously complete the function of releasing the busbar compound voltage blocking and starting the busbar failure.
c. Failure protection should be blocked by phase current, zero-sequence current, and negative-sequence current. At the same time, auxiliary criteria for 220kV switch closing position of the main transformer are required.
d. The power protection of the main transformer protection and the non-electrical energy protection output relay are separated, and the non-electrical power protection does not start to malfunction.
e. The failure protection current loop should not be connected to the transformer 220kV bushing current transformer and should be connected to the 220kV switch current transformer to prevent the dead zone from appearing. In the 220kV switch bypass operation of the transformer, the current circuit of the failure protection should be able to switch at the same time. If the secondary winding of the bypass switch current transformer is not enough, it can be considered to share a set of current transformers with the first protection. Windings.
4. Discussion on the disconnection of the current transformer disconnection Compared with the electromagnetic differential protection, the sensitivity of the microcomputer differential protection is greatly improved, but at the same time the problem of disconnection of the current transformer is also more prominent. For microcomputer differential protection, whether the differential protection outlet needs to be blocked after the current transformer is disconnected has existed for a long time in the following two viewpoints:
a. Breakage of current transformer is an abnormal operating state, which will endanger the safe operation of the equipment and should be quickly removed. Therefore, the differential protection is not blocked by the current transformer break, and the differential protection caused by the current transformer break is acceptable.
b. When the main transformer is operating normally, disconnection of the current transformer causes differential protection to remove the transformer. This is an incorrect operation and should be signaled by the operator.
The author believes that the harm to the equipment caused by the current transformer disconnection increases with the increase of the load current, and on the other hand, it increases with the extension of the time. If the main transformer is under heavy load, the current transformer will occur. Disconnection, protection signalling, dispatching by operating personnel, transfer of load after the dispatch permit, and pulling of the transformer, may seriously endanger the safe operation of the equipment. Therefore, the differential protection should not be blocked by the current transformer disconnection.
5 countermeasures in the application of the body protection microcomputer main transformer protection to reduce the volume, increase the speed of action, most of the use of small sealed fast intermediate relay, which has a negative effect on the non-electricity protection (especially used to trip the non-power protection) It may even threaten the reliable operation of the transformer. Because the non-electricity protection directly introduces the switch quantity into the protection device from the transformer body, the cable is long and the capacitor current is large, and the small sealed intermediate relay of the positive power source grounding is easily misplaced. For this reason, non-electricity protection should use intermediate relays with larger starting power, but it does not necessarily require quick action, or the intermediate relay's operating voltage is 50% to 70% of rated voltage. At the same time, shielded cables should be used for cables that incorporate non-electricity protection. The shielding layer should be grounded at both the switchyard and the control room.
In addition to the above problems, the identification of magnetizing inrush current is also a key issue of transformer differential protection, and has been discussed in a large number of literatures [3~5], which will not be repeated here.
6 Conclusion In this paper, based on the operating experience of the Hangzhou Electric Power Bureau, we discussed the solution to several problems in the main transformer microcomputer protection for reference by peers.
1 Configuration scheme for main transformer protection of the computer Before the 220 kV computer main transformer protection of the Hangzhou Electric Power Bureau adopted the configuration scheme of one main and one rear, main and rear, a set of differential protection, a set of backup protection, differential protection and backup protection. Placed in their own separate chassis, back-up protection on each side and separate [1]. The scheme has a clear structure, a simple secondary circuit, and low investment, and is widely used.
With the improvement of the reliability requirements of users for power supply, in order to avoid the forced suspension of the main transformer due to the abnormal protection of the main transformer, the State Power Corporation formulated the “25 Key Requirements for Preventing Major Electric Power Production Accidents†(hereinafter referred to as “Twenty Five countermeasures) pointed out that the microcomputer protection of the 220kV main transformer must be double [2]. At the same time, with the development of technology, many manufacturers have introduced the main backup protection and two sets of completely independent protection schemes. Therefore, in the recent infrastructure and technical upgrading projects, the Hangzhou Electric Power Bureau’s 220kV computer main transformer protection is Dual configuration (except for non-electricity protection). The scheme has a simple layout, a clear circuit, greater independence, and a modest increase in price. It is more advantageous than the configuration of the main one.
2 main transformer protection doubled current loop configuration 220kV microcomputer main transformer protection double, the two sets of protection current loop should be connected to the corresponding switch on the current transformer, when the main change 220kV switch or 110kV switch bypass generation, principle The upper two sets of protective current loops can be switched, but in practical applications, the secondary windings of the bypass switch current transformer are likely to be insufficient (especially for some old substations). At this time, the two sets of protection current loops mainly have the following Kind of connection method:
a. During normal operation, the two sets of protection current loops are connected to the current transformers of the corresponding switches. When the main transformer is bypassed, the first set of protection current loops are switched, and the second set of protection is disabled (three-phase current mutual inductance. Circuit is disconnected and shorted to ground.)
b. In normal operation, the first set of protected current loops is connected to the current transformer of the corresponding switch, and the second set of protected current loops is changed to the corresponding bushing current transformer. When the main transformer is bypassed, the first one is used. The protection current loop is switched, and the second protection does not switch and can continue to operate.
c. In normal operation, the two sets of protection current loops are connected in the same way. When the main transformer is bypassed, the first set of protection current loops is cut to the corresponding bypass switch current transformer, and the second set of protection switches to the corresponding set. Tube current transformer.
Connection a in the normal operation of the differential protection range, the two sets of protection can take into account the main transformer itself, the external lead, the disadvantage is that the bypass generation can only be a single protection operation. The second set of differential protection range in connection b is smaller. If the first set of protection is suspended for some reason, the 220kV side external lead of the transformer is not protected, but both sets of protection can operate when the bypass substitutes. Connection. In the normal operation, the situation is the same as a, and both sets of protections can be operated when the bypass generation is performed. However, the switching operation is the most, which increases the probability of errors due to human factors and is not welcomed by the operators. The author believes that the operation time of bypass bypass generation on one side of the transformer is generally shorter, and the connection method a can also be operated with a single complete protection in the bypass generation, so the connection method a should be adopted. So far, Hangzhou Electric Power Bureau has adopted wiring a in practical applications.
3 main transformer 220kV switch failure protection design 220kV transformer protection 220kV switch due to the protection of the switch and the probability of failure of the switch is very low, while the transformer failure protection in the loop there are many problems, so before the 220kV switch failure protection has not been put into operation . In order to prevent the extension of the accident range and even threaten the safe operation of the system, the “25 countermeasures†require that the failure protection of the switchgear must be put into operation [2]. However, in the practical application of the site, if To invest in 220kV switch failure protection, according to our experience, the following issues should be noted during design:
a. The sensitivity of the 220kV busbar composite voltage blocking element may not meet the requirement when the transformer is in the middle/low-voltage side. Therefore, after the failure is started, the composite differential voltage blocking should be released.
b. In order to ensure the safe and reliable operation of failure protection, it is not appropriate to use the two relays of the same relay or the output of the same circuit to simultaneously complete the function of releasing the busbar compound voltage blocking and starting the busbar failure.
c. Failure protection should be blocked by phase current, zero-sequence current, and negative-sequence current. At the same time, auxiliary criteria for 220kV switch closing position of the main transformer are required.
d. The power protection of the main transformer protection and the non-electrical energy protection output relay are separated, and the non-electrical power protection does not start to malfunction.
e. The failure protection current loop should not be connected to the transformer 220kV bushing current transformer and should be connected to the 220kV switch current transformer to prevent the dead zone from appearing. In the 220kV switch bypass operation of the transformer, the current circuit of the failure protection should be able to switch at the same time. If the secondary winding of the bypass switch current transformer is not enough, it can be considered to share a set of current transformers with the first protection. Windings.
4. Discussion on the disconnection of the current transformer disconnection Compared with the electromagnetic differential protection, the sensitivity of the microcomputer differential protection is greatly improved, but at the same time the problem of disconnection of the current transformer is also more prominent. For microcomputer differential protection, whether the differential protection outlet needs to be blocked after the current transformer is disconnected has existed for a long time in the following two viewpoints:
a. Breakage of current transformer is an abnormal operating state, which will endanger the safe operation of the equipment and should be quickly removed. Therefore, the differential protection is not blocked by the current transformer break, and the differential protection caused by the current transformer break is acceptable.
b. When the main transformer is operating normally, disconnection of the current transformer causes differential protection to remove the transformer. This is an incorrect operation and should be signaled by the operator.
The author believes that the harm to the equipment caused by the current transformer disconnection increases with the increase of the load current, and on the other hand, it increases with the extension of the time. If the main transformer is under heavy load, the current transformer will occur. Disconnection, protection signalling, dispatching by operating personnel, transfer of load after the dispatch permit, and pulling of the transformer, may seriously endanger the safe operation of the equipment. Therefore, the differential protection should not be blocked by the current transformer disconnection.
5 countermeasures in the application of the body protection microcomputer main transformer protection to reduce the volume, increase the speed of action, most of the use of small sealed fast intermediate relay, which has a negative effect on the non-electricity protection (especially used to trip the non-power protection) It may even threaten the reliable operation of the transformer. Because the non-electricity protection directly introduces the switch quantity into the protection device from the transformer body, the cable is long and the capacitor current is large, and the small sealed intermediate relay of the positive power source grounding is easily misplaced. For this reason, non-electricity protection should use intermediate relays with larger starting power, but it does not necessarily require quick action, or the intermediate relay's operating voltage is 50% to 70% of rated voltage. At the same time, shielded cables should be used for cables that incorporate non-electricity protection. The shielding layer should be grounded at both the switchyard and the control room.
In addition to the above problems, the identification of magnetizing inrush current is also a key issue of transformer differential protection, and has been discussed in a large number of literatures [3~5], which will not be repeated here.
6 Conclusion In this paper, based on the operating experience of the Hangzhou Electric Power Bureau, we discussed the solution to several problems in the main transformer microcomputer protection for reference by peers.
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