Question

a. Mention the two basic control channels of synchronous generators

b. Which of the two has a much faster response?

c. Explain why are the two control loops are non-interacting?

d. Write down four voltage control equipment used in power system

Answer 1

Ans.a. Control channels of synchronous generators are:
 The Load Frequency Control, also known as generation control or P–f control, deals with the
control of loading of the generating units for the system at normal frequency.
 Q–V CONTROL In this control, the terminal voltage of the generator is sensed and converted into
a proportionate DC signal and then compared to DC reference voltage. The error in between a
DC signal and a DC reference voltage, i.e., Δ |V | is taken as an input to the Q–V controller. A
control output ΔQ is applied to the exciter.
Ans.b. In general, the Q–V loop is much faster than the P–f loop due to the mechanical inertial constants
in the P–f loop.
Ans.c. The active power P is mainly dependent on the internal angle δ and is independent of the bus
voltage magnitude |V|. The bus voltage is dependent on machine excitation and hence on reactive
power Q and is independent of the machine angle δ. Change in the machine angle δ is caused by a
momentary change in the generator speed and hence the frequency. Therefore, the load frequency and
excitation voltage controls are non-interactive for small changes and can be modeled and analyzed
independently.
Ans.d. Four voltage control equipment used in power system are:
 Series capacitors: Capacitors are installed in series with transmission lines in order to reduce
voltage drop. The series capacitors compensate the reactance voltage drop in the line by
reducing net reactance.
 Tap-changing transformers: A tap-changing transformer is a static device having a number of
tap settings on its secondary side for obtaining different secondary voltages. The basic function
of this device is to change the transformation ratio, whereby the voltage in the secondary circuit
is varied making possible voltage control at all voltage levels at any load. The supply may not be
interrupted when tap changing is done with and without load.
 Synchronous condensers: A synchronous condenser (synchronous phase modifier) is a
synchronous motor running without mechanical load. It is connected in parallel with the load at
the receiving end of the line. Depending upon its excitation, it either generates or absorbs the
reactive power. It takes leading current when its field is overexcited, i.e., above normal speed
and takes lagging current when it is under-excited. Thus, the current drawn by a synchronous
phase modifier can be varied from lagging to leading by varying its excitation. It is a very
convenient device to keep the receiving-end voltage constant under any condition of load. It
also improves the p.f. and the output can vary smoothly.
 Booster transformers :The booster transformer performs the function of boosting the voltage. It
can be installed at a sub-station or at any intermediate point of line.