Low voltage reactive power compensation device sho

2022-06-23
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Low voltage reactive power compensation device should not be switched frequently

Abstract: This paper analyzes the reasons why low voltage capacitor reactive power compensation with contactor as switching switch should remove the small angle device in time and should not be switched frequently, expounds the various hazards of overvoltage and impulse current generated during switching to self-healing capacitor, and puts forward reasonable suggestions

key words: self healing low-voltage shunt capacitor reactive power compensation device switching contactor is applied in low-voltage distribution network. There are a large number of inductive reactive loads that need to be compensated. Otherwise, the network loss will increase and the voltage quality will deteriorate. In order to improve the diversity of power supply, make it more widely used in power quality, reduce line loss and energy saving, and make full use of the capacity of the equipment, the low-voltage capacitor reactive power compensation device with self-healing low-voltage shunt capacitor as the main component and contactor as the switching switch has been widely used. These devices generally divide capacitors into groups. Switch the capacitor according to the change of control physical quantity. However, if the reactive load fluctuates after changing, and the device needs to φ When the control is at a high level, the capacitor is often switched frequently, which may cause harm to the capacitor and early damage. Overvoltage and overcurrent will be generated when the capacitor is put into operation

1.1 the capacitor is put into operation under sinusoidal voltage

the equivalent circuit when the capacitor is put into operation is shown in Figure 1. In Figure 1, R is the circuit medium resistance

as shown in the figure, when the capacitor is at sinusoidal voltage U (T) =umsin( ω t+ φ) When the capacitor is switched on, the differential equation

of the circuit

solves equation (1), and it can be obtained that when the capacitor is switched on, the voltage Uc on the capacitor is:

a is a constant, which is related to the initial state when the capacitor is switched on. The voltage Uc curve when the capacitor is put into operation is shown in Figure 2

u 'and U "in the figure are the steady-state component and transient component of UC respectively. The initial voltage of the capacitor is zero, and the voltage Uc when the capacitor is powered on

in order to control that the capacitor will not generate dangerous overvoltage when it is powered on, it is generally required that the capacitor cannot be powered on under the existing charging voltage, so as not to endanger the safety of the capacitor and other electrical equipment. Therefore, the initial condition when the capacitor is put into operation is

according to equations (3) and (4), the steady-state components of voltage UC and current I are sinusoidal AC, while the transient components are decaying DC. The strength of the transient component is related to the time when the capacitor is put into the circuit ψ+φ= 0 or π, then the transient component is the largest

the curve of voltage Uc at this time is shown in Figure 3

it can be seen from Figure 3 that ucmax can be close to twice the amplitude of voltage Uc at steady state. According to equation (4), the current I is ψ+φ= 0 is:

when the capacitor is just put into operation, that is, when t=0, the initial value of the transient component. Therefore, adding a very large transient component I "(0) to the steady-state component will make the input current far exceed the steady-state value and cause current shock. The curve of current I is shown in Figure 4.

2 harm of frequent overvoltage to self-healing shunt capacitor

2.1 accelerate the aging process of the capacitor's insulating medium

self healing metallized shunt capacitor is made by winding metallized polypropylene film. Its insulating medium polypropylene film, like other types of capacitors, will gradually age. The aging speed is related to the applied voltage, service temperature and other conditions. It is generally believed that the service life is reduced by half when the voltage is increased by 10%. The details can be expressed by the following formula:

where: U0 - rated voltage

- service life under rated voltage

u - actual applied voltage

- service life under overvoltage

α- Constant, generally taken as 7-9

overvoltage generated by capacitor input, although it is instantaneous rather than long-term

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