power system control

21 April 2023

DelftUniversity ofTechnology

Electrical Power System Essentials

ET2105 Electrical Power System EssentialsProf. Lou van der Sluis

Power System Control

25. Power System Control | 18

Electrical Power System Essentials ET2105

Introduction (1)

• The customer expects electrical power at constant frequency and voltage

• The system load changes continuously

• Electricity can not be stored in large quantities

• Maintaining the balance between generation and consumption



Introduction (2)

• The active power balance is controlled by generators

• Another method is load shedding

• The reactive power balance is controlled by generators and static components

• The synchronous generator plays an important role



Increase of Active Power Consumption

• Kinetic energy in the rotating parts of the generator and turbine are 200 MJ

• Suddenly a 10 MW load is connected. What happens to the frequency?• In 1 second the load consumes 10 MJ more active power P• The frequency will drop to 48.7 Hz

f1 = 50 Hz (3000 RPM)



10 kV system:P = 2 MW; cos φ = 0.9Xgen = 3 Ω

This results in:I = 128 A ∠-26°Q = 969 kvar

• Suddenly the load consumes Q = 1.5 Mvar and P remains at 2 MW. What happens?• The power factor drops to 0.8• The current becomes I = 146 ∠-37°• The terminal voltage drops with 100 V

Increase of Reactive Power Consumption (1)



• P remains 2 MW• Q increases from 969 kvar 1.5 Mvar• I increases from 128 A ∠-26° 147 A ∠-37°• V drops with 100 V





• Conclusion:• The voltage depends on the reactive power• The angle of transmission depends on the active power



Some Important Conclusions

• The frequency is a common parameter throughout the system

• The voltage is controlled locally

• The control mechanism for P (rotor angle) and Q (voltage amplitude) operate more or less separately



The Primary Control• Speed governor control of a generating unit:

• Speed governor characteristics:



The Secondary Control or Load Frequency Control

Power exchange between three control areas

a) The original (scheduled) situationb) Incremental generation after losing 400 MW of generation in

control area B



Voltage Control and Reactive Power (1)

Automatic voltage control




Tap-changing transformer




Capacitor banks

Courtesy of TenneT TSO B.V.



Reactive Power Injection

Current through a Thyristor Controlled Reactor

Static Var Compensator (SVC)



Controlling Active Power FlowsThe Phase Shifter

Phasor diagram of the phase shifter



Controlling Reactive Power Flows (1)A transmission line with a series capacitor



Controlling Reactive Power Flows (2)Thyristor-controlled series capacitor

TCSC reactance as a function of the Thyristor firing angle

power system control

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