Synchronous Condensers Transient & First Swing Periods Sudarshan B S Assistant Professor Dept. of EEE RVCE, Bangalore

Introduction Synchronous condensers are designed for shunt reactive power compensation. They are called as active shunt compensators. This is because the reactive power is exchanged with the transmission system using electromagnetic principles. Active compensators are usually shunt connected devices which have the property of tending to maintain a substantially constant voltage at their terminals. Consider the Thevenin equivalent of the two-machine system with synchronous condenser at the mid-point bus. In steady state, the synchronous condenser can be approximated as the generated EMF E0 in series with the synchronous reactance Xd.

Introduction During steady state, the V-I curves of synchronous condensers is as shown by steep lines (solid lines) in this figure. Each line represents a fixed value of field current or equivalent open-circuit voltage E0. The slope of these solid lines is proportional to (Xd + XT). These curves are applicable when the field current is fixed. This may be sometimes due to conditions such as maximum excitation limit used to limit condenser overloading.

Introduction Under transient conditions, the condenser behaves as though its synchronous reactance were reduced to the transient reactance Xd’. Even without changing the field current, the condenser tends to have flatter transient V/I characteristics than the constant field current lines. The slope of the dashed field current lines is proportional to (XT + Xd’). With a fast acting voltage regulator controlling Vm, the machine can be made to operate continuously very near to the dashed transient characteristics.

Synchronous Condenser in Transient Period In the equivalent circuit shown for transient period, the voltage E0’ is a true EMF generated by the air gap flux linkages which tend to remain fixed in the short term (transient period). Because of this, it is impossible for transient voltage changes to deviate very far from the transient V-I characteristics shown. Faster the voltage change, flatter the characteristics will become due to induced subtransient currents in the damper windings. Consider the synchronous condenser transient response (next slide). Let the system initially be at point ‘a’ corresponding to zero current.

Synchronous Condenser in Transient Period

Synchronous Condenser in Transient Period Consider a system disturbance that causes the load line to suddenly change from 1 to 2. If there was no compensator, then the voltage at the compensators would be reduced to E2. With the condenser, the operating point follows trajectory a-b. This trajectory tends to be a little flatter than the transient characteristic because of the subtransient currents induced in the damper winding and its effect dies out in a few cycles required to transition from point ‘a’ to ‘b’. The voltage at point b is higher than E2, thus illustrating the ability of the condenser to instantly supply additional reactive power. At point b, the condenser is operating overexcited, but at less voltage than its original terminal voltage. The voltage regulator will automatically increase the field current to restore the condenser terminal voltage to point c.

Synchronous Condenser in Transient Period If the condenser rating is sufficient, then point ‘c’ will be the final operating point. However, if the point ‘c’ is beyond the condenser capability, the operation at point c would be limited to a minute or more depending on the short-term overload capacity of the condenser. In such a case (short term overload), the voltage regulator set point is to be automatically adjusted to reduce the field current to operating at a new point such as ‘d’. In case of some disturbances, the voltage at the compensator bus may be higher in the first couple of half-cycles. It may even be unbalanced. If only a synchronous condenser is connected, it can reduce the voltage changes very quickly in a period in proportion with its short-circuit armature time constant.

Synchronous Condenser in First Swing & Oscillatory Periods How a synchronous condenser helps reduce voltage swings & oscillations have been explained in chapter-2.

Synchronous Condenser in First Swing & Oscillatory Periods How a synchronous condenser helps reduce voltage swings & oscillations have been explained in chapter-2.

Synchronous Condenser in First Swing & Oscillatory Periods

Synchronous Condenser in First Swing & Oscillatory Periods

Synchronous Condenser in First Swing & Oscillatory Periods