On the basis of analyzing the configuration of transformer zero-sequence protection, the control of neutral overvoltage and grounding mode of 1 10 kV transformer and the potential safety hazards in the design of transformer zero-sequence protection in 1 10 kV power grid are preliminarily discussed, and the measures to eliminate the gap between neutral rods and improve the coordination of transformer zero-sequence protection are put forward.
Keywords: analysis of transformer grounding mode
1 transformer zero sequence protection configuration
When the neutral point of the transformer has zero-sequence overcurrent, the protection mode of disconnecting the transformer with ungrounded neutral point is called zero-sequence jump. Two main transformers run in parallel, and the neutral point of 1 main transformer is grounded. When a ground fault occurs at point K2, the neutral point zero-sequence overcurrent protection of 1 main transformer acts, tripping the high and low voltage side switches of No.2 main transformer at the first time, isolating the K2 fault point, and 1 main transformer resumes normal operation. If the fault point is at K 1, when the No.2 main transformer trips for the first time, the zero-sequence overcurrent protection trips the transformer for the second time to remove the fault. The obvious disadvantages of zero-sequence jump protection are: ① the probability of selective fault removal is only 50%; (2) There is no selectivity when the bus fails, which will expand the scope of power outage; ③ The time setting of zero-sequence overcurrent protection must be coordinated with the main interphase protection, which is not conducive to the coordination of protection setting; ④ Jumping test can only be carried out when two transformers are shut down at the same time. Bad conditions, secondary wiring is easy to make mistakes.
Characteristics of 2 series wiring and protection configuration
1 10 kV system wiring is mainly radial, with 220 kV substation as the power supply point, radiating to each terminal substation through 1 10 kV line. 1 10kV terminal substation adopts internal bridge wiring or line transformer group wiring, and there is no power supply at the low voltage side.
In the substation with internal bridge, the 100 bus tap changer is not used as the contact element of 103 and 104 lines in normal operation mode. Therefore, there are usually only two operation modes for substations with internal bridge connection: 1 line with two main transformers or two lines with 1 transformers. Under the operation mode of 1 line with two transformers, it is very important that only 1 neutral point of the two main transformers can be grounded, but it must be grounded by the neutral point of the transformer on the side of 1 10kV power supply line. At present, the configuration of transformer zero-sequence protection in substation with internal bridge connection is as follows: neutral zero-sequence current protection trips 100 and 900 bus points within the first time limit; The second time limit of transformer tripping; At the same time, the transformer neutral point is equipped with a rod gap, but there is no gap TA and open delta voltage protection.
In order to save the investment, land occupation and air corridor of 1 100 kV line, the newly built 1 10kV substation mostly adopts line transformer group connection, and 1 line can be "t" connected with two or even three transformers. Transformer zero-sequence protection only has neutral point zero-sequence overcurrent protection, without neutral point gap current protection and 1 transformer. Due to the incomplete configuration of zero sequence protection, the neutral points of all transformers are still grounded in the wiring of multiple T-connected lines-transformer groups. However, the operation of grounding all the neutral points of the transformer has a certain negative impact on the system.
2. 1 When some lines or transformers are overhauled, stopped and the system operation mode changes, the zero-sequence network and zero-sequence impedance value will change greatly, and the magnitude and distribution of zero-sequence currents in each branch will also change greatly. Starting from the coordination of protection setting, it is required to keep the zero-sequence impedance of substation basically unchanged.
2.2 In the process of transformer operation or line reclosing, sometimes neutral grounding transformers running on the same line will produce zero-sequence current with large amplitude, slow attenuation and large DC component caused by inrush current. It is easy to cause unsuccessful power transmission or reclosing.
2.3 All the neutral points of the transformer are grounded, which greatly reduces the zero-sequence impedance of the system, thus causing the short-circuit current of asymmetric grounding fault to increase obviously. Due to the interference of lightning strike and asymmetric grounding fault to secondary equipment, accidents such as misoperation of protection devices and damage of communication equipment still occur from time to time. Therefore, in the effective grounding system, partial transformer neutral grounding should be adopted as far as possible to limit the single-phase grounding short-circuit current and reduce the interference to the communication system.
3 transformer zero sequence protection problem
In the effective grounding system, the neutral point offset voltage of transformer is limited to a certain level, and the neutral point gap protection will not work. The purpose of gap protection is to prevent the harm of zero sequence voltage rise to transformer insulation in invalid grounding system. Only when a single-phase grounding fault occurs in the system, all the related neutral point directly grounded transformers trip, while the neutral point ungrounded transformer with power supply remains in the fault power grid, and the discharge gap will discharge, thus reducing the voltage to the ground and avoiding harm to the transformer insulation. Gap breakdown will produce wave interception, which is not good for transformer turn-to-turn insulation. Therefore, when the zero-sequence voltage rises due to single-phase grounding fault, we prefer to cut off the transformer through zero-sequence overvoltage protection. On the contrary, the gap current protection has certain contingency, which may cause the gap current protection to fail for various reasons. In this sense, zero-sequence overvoltage protection is more important than gap current protection for transformer neutral point insulation, and it usually forms transformer neutral point insulation protection together with gap current protection. Therefore, it is not perfect to only set gap current protection without zero sequence overvoltage protection, especially when intermittent fault occurs, the discharge current cannot be maintained and the gap current protection will not work.
At present, most of the 1 10kV substations that have been put into operation are only equipped with neutral bar gaps and have no corresponding protection. This configuration has both disadvantages and shortcomings. When the zero-sequence voltage of the power grid rises to near the rated phase voltage, all transformers with ungrounded neutral point feel zero-sequence overvoltage at the same time. If the neutral point gap of terminal transformer with gap overcurrent protection is not pre-discharged, the ungrounded neutral point transformer with power supply will not be able to leave the fault power grid when it cannot discharge continuously. Therefore, for the terminal transformer without power supply on the low voltage side, if complete gap current protection and zero sequence overvoltage protection are not configured, the gap between neutral point bars should be increased or the gap distance should be artificially increased to avoid preemptive discharge.
For the substation with internal bridge connection, it is not the best scheme to jump 900 and 100 bus point in the first time limit of zero sequence current of neutral grounding transformer. When the low-voltage side runs in parallel, a section of bus is lost after tripping the 900 switch, and the low-voltage side switch of the transformer with ungrounded neutral point is still running. At present, in the absence of zero-sequence overvoltage protection, if there is a temporary low-voltage power supply due to 10kV transmission, the ungrounded transformer is in danger of overvoltage. Therefore, it has been installed on the side of 1 10kV.
The first is to ensure that 1 10kV system is an effective grounding system. Preventing misoperation is the most fundamental way to ensure the effective grounding of the neutral point at 1 10kV side of the power supply transformer. If the protection setting allows, the neutral points of two transformers running in parallel on the power supply side can be grounded at the same time.
The transformer with power supply may become an invalid grounding system after losing its grounding neutral point. Therefore, for transformers with power supply or future transformers with power supply, complete neutral point gap protection should be considered in the design stage, including neutral point zero-sequence overcurrent protection, neutral point gap current protection and triangular zero-sequence voltage protection at bus opening.
On the 1 10kV feeder, no matter how many transformers are connected in parallel, when the neutral point of the power supply side is grounded, the neutral point of each terminal transformer can operate without grounding. In actual operation, in order to prevent possible unsafe factors, one of the neutral points can be arranged to be grounded. When selecting the grounding neutral point, it can be considered in the following order: first, select the transformer with power supply on the low voltage side, then consider the transformer without circuit breaker on the high voltage side, and finally select the transformer with the shortest distance from the power supply end for grounding.
Most of the 1 10kV terminal substations that have been put into operation are not equipped with bus TV opening triangle zero-sequence voltage protection and neutral point gap current protection. In order to avoid the pre-emptive discharge of neutral point gap, the original neutral point bar gap should be removed or the gap distance should be artificially increased.
In the 1 10kV substation designed in the future, three-phase voltage transformers should be considered on the high voltage side, and zero-sequence overvoltage protection and transformer neutral point gap current protection should be set. This configuration can provide flexible operation mode and adapt to the changes of power grid structure in the future.
For the substation with internal bridge connection, another ungrounded transformer should be removed within the first time limit of neutral zero sequence current protection of main transformer to avoid expanding the power outage range or possible power frequency overvoltage.
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