VOLTAGE CONTROL

Introduction

• A power system engineer must ensure that the power demand is satisfied and the voltage at each load bus is between a specified limit ( 0.95 ≤ V ≤ 1.05)

Factors affecting voltage profile :-

• System load – high voltage at low load - low voltage at high load• Impedance of the transmission line

– High impedance, high voltage drop– Low impedance, low voltage drop

Long transmission line – high impedance and hence caused high voltage drop.

• High reactive load caused low voltage condition.

Control of Voltage Profile

• Control by generator (Reactive Power Dispatch)– By rescheduling the Q generated by the PV bus. This

will increase the voltage at the PQ buses and hence reduces the transmission losses. Thus reducing the total active requirement and therefore reduces the total production.

– However the reactive power generation limits at the PV buses and the voltage limits at the PQ buses must be observed.

• Control by Var generators :-1. Static capacitors (capacitor banks)

2. Synchronous capacitors / condensor

3. Static Var Compensators (SVC)– These compensation techniques would

directly inject reactive power at the load buses and thus improve the voltage at each load bus. This technique would also improve the power factor at the load buses.

• Controlling the on-load tap changing transformer (OLTC) – vary turn ratio– Controlling the tap setting of an OLTC could

improve the receiving end voltage of the OLTC.

Benefits obtained from optimal reactive power planning

• More uniform voltage profile.

• Reduce the MVA loading.

• Ability to avoid over commitment of reactive sources.

• Ability to maintain a stable voltage stability condition.

• Reduce the transmission losses.

FACTS (Flexible AC Transmission System) Devices

• The main control actions in a power sytem such as changing the transformer tap setting, switching current or governing turbine steam pressure are currently achieved through the mechanical devices – which imposed limit on the speed at which the control action can be made.

• a power electronic based system that provide control of AC transmission system parameters to enhance controllability power transfer capability

• FACTS devices which are based on solid state control are capable of control action at far higher speed.

• 3 parameters that control transmission line power flow :-1. Line impedance

2. Voltage magnitude

3. Voltage angle

• FACTS devices can control these 3 parameters

Examples of FACTS Devices

• Thyristor controlled series compensator (TCSC)– Able to controls the transmission line

impedance.– Able to control power within the network– Able to damp the local a inter-area oscillation

and resonance.

• Static on load tap changing (OLTC)– Widely used to control the voltage difference

between the sending and receiving end voltages of a transformer i.e maintaining a constant voltage at the receiving

• Static Var Compensator (SVC) – It is a paralled combination of capacitor and inductor to provide a fast variable source or sink of reactive power.

• Static Compensator (Statcom) – it is a voltage source converter which is capable to compensate (source or sink) for reactive power.

• Unified Power Flow Controller (UPFC) – it is a thyristor controlled series compensator and phase shifter (change the angle), which is capable of controlling the active and reactive power as well as voltage angle.