Semi insulated voltage transformer should not be s

Semi insulated voltage transformers should not be selected for 10kV distribution system

in order to meet the needs of voltage measurement, electric energy measurement and protection in 10 kV distribution network, voltage transformers should be installed to obtain the value of voltage. Voltage transformers can be divided into semi insulated and fully insulated voltage transformers according to the voltage they bear during operation. Semi insulated voltage transformers only bear phase voltage in normal operation, while fully insulated voltage transformers can bear line voltage in operation. With the continuous development of power distribution, the network is expanded, the conductor section is increased, and a large number of cables are used to improve the insulation level, so that the capacitive current of the neutral ungrounded distribution system is increasing, and the faults of voltage transformers are increasing. After analyzing the problems found in the operation practice of applying inorganic sand paddles on both sides of flammable polyurethane, it is believed that the selection of voltage transformers with different voltage levels has different effects on the implementation of anti harmonic measures and the safe operation of equipment, which are mainly reflected in the following aspects

(1) different wiring methods. The high-voltage N pole of semi insulated voltage transformer must be directly grounded for operation. Voltage transformers need to be installed in substations, switching stations, high-voltage user terminals, etc. in the distribution system. Many semi insulated voltage transformers operate in parallel. When the system is slightly asymmetric, it is easy to excite ferromagnetic resonance overvoltage with high amplitude, and the more the number of parallels is, the more likely it is to occur; Fully insulated voltage transformers can operate with direct grounding, indirect grounding (grounding resistance, zero sequence voltage transformer, etc.) and ungrounded V-shaped wiring

(2) different anti harmonic measures. The semi insulated voltage transformer adopts the secondary open delta winding, which is equipped with a special harmonic eliminator, or a parallel bulb, or a parallel resistance anti resonance; In addition to the above measures, the fully insulated voltage transformer can also be connected in series with resistance to eliminate harmonic at the high-voltage neutral point. The fully insulated voltage transformer has good excitation performance due to its normal operation in step-down operation. In order to effectively prevent the ferroresonance over-voltage of voltage transformer, it is necessary to take multiple measures at the same time to play the effect of starting up and preheating the equipment for 1 minute

(3) the voltage borne by single-phase grounding is different. When the semi insulated voltage transformer is single-phase grounded in the system, it needs to withstand the impact of line voltage. Generally, the operation shall not exceed 2 hours, and long-term operation may cause breakdown fault; When the system is single-phase grounded, the fully insulated voltage transformer bears the rated voltage

(4) the effect of safe operation is different. In the late 1980s and early 1990s, more than 10 35 kV substations in our bureau adopted the anti resonance measures of paralleling 500 W bulbs on the secondary open delta winding and connecting zgk-i resistors in series at the neutral point for fully insulated voltage transformers. Over the past 10 years, overvoltage fuse fusing failures have occurred occasionally, and most of them are caused by resistance damage or disconnection. However, several newly-built 110 kV substations in our bureau in recent years use semi insulated voltage transformers, which are in poor operation and have continuous faults. For example, from September 2001 to July 2003, the single-phase, two-phase and three-phase fuses of 110 kV Shimen substation were fused 26 times, and from July 2001 to September 2003, the single-phase, two-phase and three-phase fuses of 110 kV Wuzhen substation were fused 21 times. In addition, the burning of 10 kV No. 2 voltage transformer caused a short circuit in the cabinet (the whole cabinet was burnt out); Overvoltage transformer breakdown also occurred in 110 kV Heshan substation. During this period, we have also taken various forms of harmonic elimination measures, but they have not achieved results. In this regard, at the beginning of 2004, the semi insulated voltage transformers of six 110 kV substations were transformed into fully insulated voltage transformers, and the same harmonic elimination measures as those of 35 kV substations were taken. After the operation test in summer thunderstorm climate, there was no fuse failure

to sum up, in the operation of 10 kV distribution system with ungrounded neutral point, semi insulated voltage transformer is prone to ferroresonance overvoltage, fusing the voltage transformer fuse, burning the voltage transformer, and even causing another system accident, which seriously affects the accuracy of measurement, causes the loss of measurement data, and endangers the correct action of relay protection and automatic devices. It can be seen that semi insulated voltage transformers should not be selected in the 10 kV distribution system, which should provide excellent gas barrier performance and chemical resistance. Selecting fully insulated voltage transformers is conducive to taking various forms of harmonic elimination measures to effectively prevent ferroresonance overvoltage and ensure the safe operation of equipment. Select full insulation