Types of failures and protection of static capacitor banks (BSC)

Purpose of Static Capacitor Banks (BSC)

Static capacitor banks (BSC) are used for the following purposes: reactive power compensation in the network, regulation of the voltage level in the buses, equalization of the voltage waveform in the control circuits with thyristor regulation.

The transfer of reactive power through a power line results in a voltage drop, especially noticeable in overhead power lines with high reactive resistance. In addition, the additional current flowing through the line results in increased power losses. If active power is to be transmitted in exactly the amount required by the user, then reactive power can be generated at the point of consumption. Capacitor banks are used for this purpose.

Asynchronous motors have the greatest consumption of reactive power. Therefore, when technical specifications are issued to a user who has a significant proportion of induction motors in the load, cosφ is usually suggested to be 0.95.At the same time, the losses of active power in the network and the voltage drop on the power lines are reduced. In some cases, the problem can be solved using synchronous motors. A simpler and cheaper way to obtain such a result is the use of BSC.

At minimum system loads, a situation may arise where the capacitor bank creates excess reactive power. In this case, redundant reactive power is returned to the power source while the line is again charged with additional reactive current, which increases the active power loss. Bus voltage rises and can be dangerous to equipment. That is why it is very important to be able to adjust the capacitance of the capacitor bank.

In the simplest case, at minimum load modes, you can turn off BSC — jump regulation. Sometimes this is not enough and the battery consists of several BSCs, each of which can be turned on or off separately — step regulation. Finally, there are modulating control systems, for example: a reactor is connected in parallel to the battery, the current in which is smoothly regulated by a thyristor circuit. In all cases, a special automatic control of the BSC is used for this purpose.

Types of capacitor block damage

The main type of failure of capacitor banks—capacitor failure—results in a two-phase short circuit. Under operating conditions, abnormal modes associated with overloading of capacitors with higher harmonic current components and voltage increase are also possible.

Widely used thyristor load control schemes are based on the fact that the thyristors are opened by the control circuit at a certain moment of the period, and the smaller part of the period they are open, the less effective current flowing through the load. In this case, higher current harmonics appear in the composition of the load current and the corresponding voltage harmonics at the power source.

BSCs contribute to reducing the level of harmonics in the voltage, because their resistance decreases with increasing frequency and, therefore, the value of the current consumed by the battery increases. This leads to a smoothing of the voltage waveform. In this case, there is a danger of overloading the capacitors with currents of higher harmonics and special overload protection is required.

Capacitor bank turn-on current

When voltage is applied to the battery, an inrush current occurs, depending on the capacity of the battery and the resistance of the network.

Let's determine, for example, the inrush current of a battery with a capacity of 4.9 MVAr, taking the short-circuit power of the 10 kV busbars to which the battery is connected-150 MV ∙ A: rated current of the battery: Inom = 4.9 / (√ 3 * 11) = 0.257 kA; peak value of inrush current for selection of relay protection: Iincl. = √2 * 0.257 * √ (150 / 4.9) = 2 kA.

Selection of a switch for switching a capacitor bank

The operation of the circuit breaker when tripping the capacitor bank is often decisive in the selection of a circuit breaker.The choice of switch is determined by the manner in which the arc is re-ignited in the switch when a double voltage may occur between the switch contacts — the capacitor charge voltage on one side and the mains voltage in anti-phase on the other side. The tripping current of the breaker is obtained by multiplying the tripping current by the surge factor of the gearbox. If a switch with the same voltage as BSK is used, the CP factor is 2.5. Often a 35 kV surge switch is used to switch a 6-10 kV battery. In this case, the CP coefficient is 1.25.

Thus, the re-ignition current is:

When a switch is selected, its current rating (peak value) must be equal to or greater than the re-ignition breaking current rating. The rated breaking current depends on the type of circuit breaker and is equal to: IOf.calc = IPZ for air, vacuum and SF6 circuit breakers; I Off = IPZ / 0.3 for oil switches.

For example, we will check the switch parameters for the inrush currents calculated earlier when using a 10 kV oil circuit breaker with a breaking current of 20 kA in rms or 28.3 kA in amplitude (VMP-10-630 -20).

a) One battery 4.9 mvar. Ignition current: IPZ = 2.5 * 2 = 5kA Estimated shutdown current: I Calculated = 5 / 0.3 = 17kA.

A 10kV oil circuit breaker can be used. With an increase in the short-circuit power of the 10 kV busbars, also in the presence of two batteries, the calculated tripping current may exceed the allowable one.In this case, as well as to increase reliability in BSC circuits, high-speed switches are used, for example, vacuum switches, in which the speed of contact separation when turning off is greater than the speed of the recovery voltage.

It should be noted that the same requirements must be met by the incoming and sectional switch, which can also supply the switched-off voltage to the switched-on capacitor bank.