Problems and solutions in the application of current limiting reactors

1 Introduction

In the power system, the main function of the current limiting reactor is to limit the short-circuit current of the system by using its inductance characteristics when a short-circuit fault occurs in the power system, reduce the impact of the short-circuit current on the system, and reduce the rated breaking capacity of the circuit breaker selection, saving investment costs, and increasing the residual pressure of the system. However, after using the current limiting reactor, there will be a large loss of electric energy. When there is a large fluctuation in the system, such as starting a large-capacity motor, a large voltage drop will affect the normal operation of other equipment, making it difficult to regulate the voltage of the generator and affecting the stability of the system. At the same time, it will have a great impact on the surrounding power supply equipment, buildings and communication facilities, and even cause equipment abnormalities. How to eliminate the impact of the reactor has become a very important issue faced in the normal operation of the power system.

2 Problem Statement

Hengtong Chemical Thermal Power Plant currently has three 60MW generators and three 15MW generators, with a total installed capacity of 225,000 KW and an annual power generation of 1.27 billion kWh. The power supply for the plant is 6KV, which is directly supplied by the generator outlet. In order to limit the short-circuit current, a current-limiting reactor is connected in series to supply power to two sections of 6KV busbars. The 6KV system adopts a single busbar segmented wiring. The main wiring of the plant power system (taking 4# generator-transformer group as an example) is shown in Figure 1.

Figure 1

During operation, the following problems were found

2.1 When starting a large-capacity motor such as a water pump, the 6kv bus voltage drops to about 80%, affecting the normal use of other motors and users. There was an accident in which the reactor backup protection tripped when two water pumps were started at the same time, which seriously affected the safe and stable operation of the system.

2.2 When the reactor operates at about 70% of the rated current, the reactor coil heats up severely and the winding overheats and changes color.

2.3 The reactor is connected in series for a long time in the system, which consumes huge reactive power and active power, resulting in uneconomical operation.

2.4 When the system is subject to a large disturbance, the voltage fluctuates greatly, making it difficult to regulate the generator voltage and affecting system stability

3 Solutions to the problem

3.1 Solution

In order to solve this problem, after technical consultation and investigation, it was found that the influence of the reactor can be eliminated by using a domestic large-capacity high-speed switching device (FSR device for short). The specific solution is to connect the FSR device in parallel at both ends of the reactor, as shown in Figure 2.

Figure 2

During normal operation, the high-speed switch device (FSR) short-circuits the reactor. Since the FSR impedance is about 0.1mΩ (of which the DC resistance is about 20μΩ), and the reactor impedance is generally between 0.15 and 0.9Ω (i.e., 150 mΩ to 900 mΩ), the reactor is short-circuited by the FSR and does not function during normal operation. The current flows through the high-speed switch device. When a short-circuit fault occurs on the factory's 6KV busbar or below, the high-speed switch quickly blows and puts the reactor into the main circuit to limit the short-circuit current, while the short-circuit fault is still interrupted by the circuit breaker at the fault location.

3.2 Working principle of FSR device

The device is mainly composed of bridge body FS, fuse FU, nonlinear resistor FR and measurement and control unit, referred to as FSR. The impedance ratio of FS to FU is 1:2 000. Therefore, the working current flows through FS during normal operation. When a fault short circuit occurs in the system, after receiving the disconnection command from the measurement and control unit, FS will explode and disconnect within 0.15 ms, and the current will be transferred to FU. After FS is disconnected, all short-circuit currents are transferred to FU, causing FU to fuse within 0.5 ms and generate sufficient arc voltage. The arc pressure generated when FU is disconnected makes it conductive, absorbs the arc energy generated after FU is disconnected and the energy injected by the power supply, so that FU can extinguish the arc smoothly, and limits the overvoltage at the time of disconnection to the allowable range of 2.5 times the phase voltage. Detect the current and the current change rate. When the current amplitude and the current change rate exceed the set value at the same time, it is judged that a short circuit has occurred, and three identical independent measurement and control components are used to make a judgment in a "three-out-of-two" action mode, and send a disconnection signal to FS. As shown in Figure 3

Figure 3

Bridge body FS: Since the resistance of FS is uΩ level, and the resistance of fuse FU is mΩ level, the working current flows through the bridge body under normal circumstances. When a short circuit occurs, after receiving the disconnection command from the measurement and control unit, it will explode and disconnect within 0.15ms, and the current will be transferred to the fuse FU.

Fuse FU: After FS is disconnected, all short-circuit current is transferred to the fuse, which blows within 0.5ms and generates sufficient arc voltage.

Nonlinear resistor FR: The arc voltage generated when the fuse blows makes it conductive, absorbing the magnetic energy in the inductor and the energy injected by the power supply, so that the fuse can extinguish the arc smoothly, and at the same time limit the overvoltage during breaking to within 2.5 times the rated phase voltage.

Measurement and control unit: detects current and current change rate. When the current amplitude and current change rate exceed the set value at the same time, it is judged that a short circuit has occurred. Three identical independently working CPU components are used to make a "two out of three" voting judgment and issue a disconnection command to the bridge body.

3.3 Characteristics of FSR device

3.3.1 Large current carrying capacity: At present, the rated current of domestic vacuum circuit breakers can only reach 4kA, while the maximum rated current of FSR devices can reach 12kA, which can fully meet the needs of the current power system.

3.3.2 Fast breaking speed: The short-circuit current is cut off within 1ms, decays to zero within 3ms, and the fault is completely cut off. Compared with the traditional circuit breaker relay protection method, the short-circuit fault cutting speed is increased by more than 20 times.

3.3.3 No harmful overvoltage during the breaking process: Zinc oxide has good nonlinear characteristics, which can limit the breaking overvoltage to within 2.5 times the rated phase voltage.

3.3.4 The breaking capacity can be large enough: currently it can reach 240kA, which is sufficient to meet the needs of cutting off short-circuit current under normal circumstances.

3.3.5 Higher sensitivity: The current change rate increases significantly during a fault. The device introduces the current change rate as a criterion, which has higher sensitivity.

3.4 FSR device operating current setting principle

3.4.1 The current limiting reactor was originally designed at the 6kV busbar inlet to reduce the three-phase short-circuit current of the 6kV system. When a three-phase short-circuit fault occurs, the reactor should be reliably put into operation, which requires the FSR device to be reliably disconnected at the initial stage of the short-circuit current rise. Therefore, the FSR action value should be 90% of the three-phase short-circuit current value.

3.4.2 When other loads operate normally, they should avoid the maximum no-load closing current of the distribution transformer and take a reliability factor of 1.3 times.

3.4.3 When other loads are operating normally, they should avoid the starting current of the largest motor and also take a reliability factor of 1.3 times.

Based on the above three situations, the maximum current is selected as the operating current of the FSR device.

3.5 Application Practice

After the No. 4 unit of our plant adopted the parallel operation scheme of reactor and FSR device, the influence of reactor was solved.

3.5.1 During normal operation, the reactor is short-circuited and no current flows through it, which solves the problem of severe heat generation during operation. When the system is subject to a large disturbance, the voltage fluctuates greatly, and the generator voltage regulation is also very convenient.

3.5.2 Eliminates the voltage drop caused by the reactor and improves the voltage quality.

3.5.3 When starting a large-capacity motor such as a water pump, the 6kv bus voltage drop is only about 5℅, ensuring the safe and stable operation of the system.

3.5.4 The power loss caused by the reactor is eliminated, the energy saving effect is significant, and the annual electricity bill is saved by 124,000 yuan.

By referring to the rated capacity, power loss and other parameters of the reactor in the reactor manual and the actual operation of the 4# unit in our plant, the annual power loss of the reactor is calculated as follows: A=3×⊿A×T=3×β2× (⊿P+⊿Q)×T

(⊿P=PK=12.196KW ⊿Q=0.02Se β=I /Ie Ie =2000A operating current I=1240A)

A=3×⊿A×T=3×β2×(⊿P+0.02Se)×T=3×0.622× (12.196+0.02×924×24×365=10102.032×30.676=309889.9336

Calculated at 0.4 yuan per kWh, the annual loss of the reactor is:

0.4×A =0.4×30.9889=123,955 yuan/year

4 Conclusion

In summary, the use of high-speed switching devices in parallel with reactors is an economical and practical current limiting solution. It not only solves the economical selection of high-voltage circuit breakers, but also fundamentally avoids the problems of voltage drop, power loss and leakage magnetic field caused by the series connection of reactors in normal operation, improves the power factor and power quality of the factory power system, is conducive to the safe and energy-saving operation of the system, and is an economical operation mode worthy of promotion.

References:

[1] Wang Chuan, Li Yanjun. Large-capacity fast-breaking device for large synchronous generator outlets and plant transformer branches [J] 2002, (1-2), 92-98

[2] Fang Daqian’s Electrical Calculation Manual (Revised Edition) Shandong Science and Technology Press, June 1993

[3] Li Guizhong, Modern Power Engineer Technical Manual, Tianjin University Press, 1994.12