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2022-07-31
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Electromagnetic ring operation of power system

1 introduction

1.1 the electromagnetic ring of power system refers to the ring electricity formed by transmission lines of different voltage levels through magnetic or electromagnetic coupling of transformers

1.2 in the early days of the advent of a new and higher voltage level in the power system, the network cannot be very strong in structure, but is gradually improved, strengthened and becoming more and more reasonable. In the process of improvement and strengthening, in order to obtain large network transmission power, make rational use of cheap resources, and meet users' requirements for maximum power consumption, most power systems have one or more electromagnetic rings running

1.3 the electromagnetic ring operation of power system usually refers to the operation in which the electrical components of two adjacent voltage levels are connected into a ring. In the case that electrical equipment with more than 2 voltage levels form an electromagnetic ring for operation, this transitional temporary extremely special power system operation mode can be adopted only when there is no problem through checking calculation during the short time when the dispatcher handles the accident or transfers the load

2 necessity of open-loop operation of electromagnetic ring

2.1 in the operation of power system, the operation of electromagnetic ring is often taboo. According to statistics, 187 power angle and voltage stability failure accidents related to electrical structures occurred in China from 1970 to 1990, including 55 accidents related to high and low voltage network structures, accounting for 20.2%. Large scale blackouts related to electromagnetic rings are also common in foreign countries. For example, on july2,1996, the power system in the western United States experienced a large-scale blackout. When the Paci (three circuit 500 kV AC) was disconnected, a large number of power flows flowed to the 345/230 kV network in the East, resulting in damage to voltage stability. The WSCC was separated into five isolated islands, more than 2 million users were cut off, the loss load was 10576 MW, and the longest blackout time was 6.4 H. On August 10 of the same year, accidents of the same nature occurred again in the power system, and the power cut users, load loss and power cut time were far longer than those on July 2. Therefore, people often "talk about the ring color change". As long as possible, the electromagnetic ring operates in an open-loop manner

2.2 open loop operation has many advantages

a. it is stable and easy to control. In case of interference in open-loop network, fault components are often cut off, supplemented by effective accident handling means, the development of the situation can be calmed down. If a fault occurs in the ring, in many cases, power transfer will be caused after the faulty components are removed, so that the power of non faulty components will exceed the limit and cause stability damage

b. convenient power flow control. During open-loop operation, the purpose of power flow adjustment can be achieved by adjusting the active power (or power angle) and reactive power (or power and ensuring the fixed voltage of the equipment) of the power supply at the sending end. When the loop is closed, the power flow is naturally distributed in the loop, and it is difficult to control. Some of the loop components are fully loaded or even overloaded, and some are idle

c. limit short circuit capacity. Open loop operation is an important means to limit short circuit capacity. For closed loop operation, the comprehensive impedance is often small and the short-circuit capacity is relatively large. Buses with large short-circuit capacity are those with more outgoing lines and concentrated power outgoing lines. When these buses fail, they should be replaced with the original components, which is often the culprit for triggering major power system accidents

d. simplify relay protection and safety automatic device. The configuration of relay protection and stability measures of the ring is much more complex than that of the non ring, and the coordination is more difficult. The complication and non cooperation of protection and safety automatic devices are generally the direct or expanded causes of accidents

e. in some loops, because there is no circulation problem in open-loop operation, the transmission loss may be smaller than that in closed-loop operation, which can improve the transmission efficiency

3 possibility of electromagnetic ring operation

3.1 in the actual operation of power system, it does not exclude all electromagnetic ring operation modes. On the contrary, in some cases, electromagnetic ring operation may have more advantages than open-loop operation in terms of making full use of resources, improving transmission reliability and reducing transmission loss for a period of time

3.2 the electromagnetic ring must be operated with caution and adhere to the following principles:

a. in case of an accident, any component (mainly high-voltage component) in the ring is disconnected, even if power transfer occurs, it will not cause stability damage

b. when the electromagnetic ring operates, the relay protection configuration and setting coordination are good, and the implementation of safety and stability measures is simple and reliable

c. after the operation of the electromagnetic ring, the voltage level at the receiving end shall be basically unaffected

d. during the operation of electromagnetic ring, the increase of short-circuit capacity of any bus shall not cause difficulty in switching

e. the transmission loss shall not increase significantly when the electromagnetic ring operates

3.3 if the above five principles are met, the operation of electromagnetic ring is acceptable. However, it must be pointed out that "a wise man will make a mistake if he thinks a thousand times". A large number of accidents are not completely expected by people. Therefore, when the power system adopts the electromagnetic ring operation mode, reliable measures should be taken to prevent the occurrence of accident interlocking, resulting in large power transfer, which may cause stability damage, or cause wrong protection action

4 suggestions on the operation mode of 500/220 kV electromagnetic ring in Hunan power system

4.1 Hunan power system has formed a 500 kV rhombic single ring. The operation of 500 kV and 220 kV electromagnetic rings is shown in Figure 1. The power supply directly connected to the 500 kV network includes Gegang line and Wuqiangxi power plant connected to the central China power system. There is two-way power exchange between 500/220 kV networks of GangShi substation. Under normal circumstances, there is only one-way active power exchange between Loudi and Yuntian 500/220 kV networks, that is, 500 kV networks inject active power into 220 kV networks. 4.2 in this complex electromagnetic ring, there are two main problems in the outdoor experiment: 4.2.1 two "bottlenecks":

a. the "bottleneck" of 220 kV transmission from GangShi substation. The power transmission from GangShi substation to 220 kV network is limited by the transmission capacity of three 220 kV tie lines (Gangtie, gangde and Gangying). The power transmitted by the 3-circuit line and the load level of the whole system, the output of Jiangya, Shimen and Wuqiangxi power plants, the power transmitted by Gezhouba power plant to the south, Mingshan, Panshan, Hucheng and Wushenggong

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