How to choose the zero-sequence voltage and zero-sequence current protection device for capacitor bank

When selecting the zero-sequence voltage protection device, the protection setting value can be calculated according to the following formula: Udx=(Unδ➗Klny)✖((ε-1)➗(nz))

Where

Unδ is the phase voltage of the capacitor bank, volt; Kl is the sensitivity coefficient, generally 1.1~1.2; ny is the transformer ratio of the voltage transformer; ε is the ratio of the allowed long-term operating voltage of a single capacitor to the normal operating voltage; n is the number of series groups of capacitors per phase.

Since neither the external voltage nor the three-phase capacity will be absolutely balanced, the capacitor will have a certain neutral point displacement voltage during normal operation. In order to prevent the overvoltage multiple on the phase with a smaller capacitance from being too large, the capacitance value of the three-phase capacitor should be adjusted so that the capacitance difference is less than 55%.

Generally, a current transformer is connected to each phase of a single triangular capacitor bank, and the three secondary windings of the current transformer are connected in parallel and connected to the current relay, which can form a zero-sequence current protection device. When a capacitor in the capacitor bank is broken down, due to the change in the capacitance of the faulty capacitor, the current through the faulty phase capacitor bank increases, and the sum of the three-phase currents will not be equal to zero. A zero-sequence current passes through the current relay, causing the protection device to operate, thereby disconnecting the capacitor bank.

When selecting the zero-sequence current protection device, the protection setting value can be calculated according to the following formula: Idx=ΔIg➗(Klnl)={In(λ➗(1-λ))}➗Klnl

Where

ΔIg is the increment of the fault phase current when 50~70% of the series components inside a capacitor are broken down, amp; nl is the converter ratio of the current transformer; λ is the breakdown coefficient, λ = m➗n (m is the number of breakdown components and the number of components inside the capacitor); Kl is the sensitivity coefficient, generally 1.25~1.5.

In order to prevent the unbalanced current ln during normal operation from causing the protection device to malfunction, the normal unbalanced current Itp (A) should be adjusted during installation:

Itp≤Idnl➗Kk

Where Kk is the reliability coefficient, generally 1.5~2.0.

To prevent misoperation during input, the zero-sequence current protection device should have a time limit of 0.5 seconds.