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Consequences of P on f and Q on V

Short answer

Real power deficit lowers frequency, risking turbine damage and load shedding; surplus raises frequency. Reactive deficit lowers bus voltage, risking motor overheating and collapse; surplus raises voltage, stressing insulation. Control: P-f via governor and AGC (ACE); Q-V via AVR, shunt compensators, and tap changers.

Explain the consequences of real power on frequency and reactive power on voltage problems in a power system. Also, illustrate the basic control mechanisms in brief to address these are in the real power system.

Consequences

Real power on frequency: if load exceeds generation, the deficit is drawn from the kinetic energy of rotating machines, which slow down, so frequency falls; surplus generation makes frequency rise. Frequency is common to the whole interconnection. Large deviations cause under-frequency load shedding, damage to turbine blades, and slow motors.

Reactive power on voltage: if reactive demand exceeds local supply, Q flows in over the line reactance and causes a drop ΔV≈QX/V\Delta V \approx QX/V, so bus voltage falls; excess Q (light load, line charging) makes it rise. Voltage is a local quantity. Low voltage overheats motors and can lead to voltage collapse; high voltage stresses insulation.

Basic control mechanisms

 P-f loop:  df -> Governor -> Valve -> Turbine -> Pm
                    ^ AGC (integral of ACE)
 Q-V loop:  dV -> AVR -> Exciter -> Field -> Ef -> Q
  • P–f: speed governor (primary, with droop) and Automatic Generation Control acting on ACE=ΔPtie+BΔfACE = \Delta P_{tie} + B\Delta f (secondary) to bring frequency back to 50 Hz.
  • Q–V: generator AVR/excitation control, shunt capacitors and reactors, synchronous condensers, SVC/STATCOM and on-load tap changers.

✓ Answer checked by Claude Sonnet 5.5

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