Effect of Unbalanced Loading On Generator :
The unbalanced loading on a generator can be due to unsymmetrical faults (both internal and external) or due to maloperation of the circuit breaker (resulting in the opening of only one or two phases) or due to open circuiting of a phase or due to failure of one contact of the circuit breaker.
From the theory of symmetrical components, we know that the negative sequence currents are present in all the above abnormal conditions. The negative sequence field produced due to the negative sequence currents in the stator winding will rotate at synchronous speed Ns but in the direction opposite to that of the positive sequence field i.e., opposite to the direction of rotation of the rotor.
So, the relative speed between the negative sequence field and rotor will be, Ns - (-Ns) = 2Ns. Hence, an emf is induced in the rotor conductors whose frequency is double that of the supply frequency.
In the case of a cylindrical rotor-type alternator, the double frequency current flows through the rotor body. Hence, the rotor overheats due to increased eddy current and hysteresis losses. In the case of salient pole type alternator, the double frequency currents flow through the damper winding, and hence they are less affected due to unbalanced loading.
However, if the unbalanced loads persist for a long duration, the rotor stamping and the field winding gets overheated and gets damaged. Hence, the rotor is damaged due to sustained unbalanced currents.
The protection scheme employed against this abnormal condition consists of a negative sequence filter. It detects the presence of a negative sequence component in the stator current and operates the IDMT relay when it exceeds the permissible level. The relay, send the trip signal to the main circuit breaker. Sometimes, it is used to operate an alarm when the negative sequence current exceeds its maximum continuous permissible value.
Unbalanced Loading Protection of Generator :
The protection for unbalanced loading in an alternator using a negative sequence current filter is shown above. There are three current transformers connected in the three phases and a negative sequence filter which consists of resistors and inductors. An overcurrent relay is connected to the secondaries of the CTs through a negative sequence filter.
The negative sequence filter allows the flowing of negative sequence components through the coil of the overcurrent relay. As the rating of the machine is rated to I22 t. The current Is is the negative sequence current. Hence, the overcurrent relay ZL having negative phase sequence protection is used.
The below shows the negative sequence filter explaining its principle. Under balance conditions, the two twin auxiliary windings of current transformers are connected to the line current transformer. The direction of flow of current IR, IY, and IZ are marked in the circuit diagram. Across the auxiliary current transformers Ta and Tb, the impedances Za and Zb are connected. Across the terminals YY, the load impedance ZL is connected.
The current through the current transformers Ta and Tb during the flow of primary current will be equal to (IY - IB) and (IR - IY) respectively. The impedances Za and Zb for a given value of ZL (overcurrent relay) are chosen, such that the potential at points M and N will remain equal.
If O is taken as the midpoint between M and N since the potential held at points M and N is the same, the voltage across OM and ON is the same but the directions will be opposite according to the flow.
During the unbalanced condition, the output produced will be proportional to the negative phase sequence. Hence negative phase sequence relays are used. The output is produced across the terminal YY with voltage E. Now, the relay will start operating and trips the circuit.
