Discussion on generator overexcitation protection setting

In order to measure the overexcitation of generator, the physical concept of overexcitation multiple is usually adopted. The so-called overexcitation multiple refers to the ratio of the flux density in the core of a diesel generator during overexcitation operation to the flux density in the core at rated operating conditions (rated voltage and rated frequency). Namely: N = B/BN = (U/f)/(UN/fN)

N- overexcitation multiples

B- core flux density during overexcitation operation of generators or transformers

BN- core flux density at rated operating condition of generator or transformer

Actual operating voltage and rated voltage of U, UN- generator or transformer

F, fN- actual operating frequency and rated frequency of generator or transformer

It can be seen from the above formula that the overexcitation multiple is proportional to the voltage and inversely proportional to the frequency.

I. setting of time-limit over-excitation protection

A timed over-excitation protection outlet usually sends a signal or signal and is used to reduce the excitation of a generator. The fixed value that needs to be adjusted has over excitation multiple Ndz1 and action delay.

Ndz1 = Krel/Kr

Ndz1- overexcitation protection action overexcitation multiple with fixed time limit

Krel- reliability coefficient, 1.05~1.1

Kr- return coefficient is 0.96

The action delay of overexcitation protection with fixed time limit should be determined according to the exit mode of protection and the condition of protected equipment. Action when protecting only applies to signal delay the whole for 6 ~ 9 s, over-excitation protection when export applied to signal and reduce excitation, the action time delay setting, as specified in the "escape" the forced excitation generator usually take more than 11 s.

It should be noted that the overexcitation multiple of the time-limit overexcitation protection should not exceed the ratio of the initial saturation magnetic density of the core to the rated magnetic density.

Modern large-scale generators and transformers, whose rated working magnetic density BN=1.7~ 1.8t, and the starting saturated magnetic density BS=1.9~ 2.0t. The ratio is 1.1 to 1.15. Therefore, the overexcitation multiple setting value of the time-limit overexcitation protection should not be greater than 1.15.

Ii. Setting of anti-time over-excitation protection

The action characteristics of generator or transformer reverse time over-excitation protection should be adjusted according to the curve of permissible over-excitation characteristic given by the manufacturer. However, some large generators and transformers made in China do not have the allowed over-excitation characteristic curve, so they cannot match the allowed over-excitation characteristic curve given by the manufacturer.

It is well known that the frequency of the voltage of a generator or transformer operating in parallel network depends on the system frequency. The operation practice shows that there is little possibility of system frequency reduction except for the occurrence of system collapse accidents. Therefore, overexcitation of generators and transformers (especially transformers) is mostly caused by overvoltage.