Unbalanced Loading Protection of Generator:
Unbalanced Loading Protection of Generator – Whatever be the cause of the unbalance it is obvious that the negative phase sequence current will result in heating of the stator. In case of high-speed turbo-generators, the continuous current which can be carried is usually between 10 and 15% of the positive sequence continuous rating.
The negative sequence heating follows a normal resistance law and hence is proportional to the square of the current. The heating time constant of the machine is largely a function of the cooling system employed.
This is expressed by a rating equation:
where
I2 – is the negative sequence current expressed on a per unit basis of continuous maximum rating (CMR),
t – is the current duration in seconds, and
K – is a constant which for turbo type machines will usually have a value between 3 and 20.
The problem of protecting against this condition lies in obtaining a relay characteristic which will accurately match this heating characteristic. The usual arrangement is an inverse with definite minimum time delay relay connected to a network which segregates the negative sequence current from the positive and zero sequence current Fig. (6.18).
The relay has a long operating time and has a range of settings to allow its characteristic to be accurately matched to those of the machine.