Series and Shunt Compensation

1. Need for Compensation

Transmission lines have reactance which causes:

• Voltage drop • Reduced power transfer capability • Poor voltage regulation • Stability problems

Compensation improves voltage profile, stability, and power transfer capability.

2. Series Compensation

Series compensation uses capacitors connected in series with transmission line.

Purpose:
• Reduce line reactance • Increase power transfer capability • Improve stability

Power Transfer Equation

P = (Vs Vr / X) sinδ

If X decreases → P increases

With Series Capacitor

Xeffective = Xline − Xcapacitor

Reduced reactance → Increased power transfer

Degree of Compensation

k = Xc / Xline

Where: Xc = Capacitive reactance Xline = Line reactance Typical value: 30% to 70%

Advantages

• Increases transmission capacity • Improves system stability • Reduces voltage drop • Improves power transfer

Disadvantages

• Risk of sub-synchronous resonance • Protection becomes complex • Costly

3. Shunt Compensation

Shunt compensation uses shunt capacitors or reactors connected parallel to line.

Purpose: • Control voltage • Improve power factor • Reduce reactive power demand

Reactive Power Formula

Qc = V² / Xc

Types of Shunt Compensation

Device	Function
Shunt Capacitor	Supplies reactive power
Shunt Reactor	Absorbs reactive power
SVC	Dynamic voltage control
STATCOM	Advanced reactive power control

4. Effect on Voltage Regulation

Without compensation → Voltage drop high With shunt capacitor → Voltage improves

5. Comparison: Series vs Shunt Compensation

Parameter	Series Compensation	Shunt Compensation
Connection	Series with line	Parallel with line
Function	Reduces reactance	Controls voltage
Improves	Power transfer capability	Voltage profile
Used for	Long transmission lines	Voltage control

6. Important GATE Points

Series capacitor reduces reactance Power transfer increases when reactance decreases Shunt capacitor supplies reactive power Shunt reactor absorbs reactive power Series compensation improves stability Shunt compensation improves voltage profile

7. Power Increase due to Series Compensation

Pnew / Pold = Xold / Xnew

8. Modern Compensation Devices (FACTS)

SVC – Static VAR Compensator STATCOM – Static Synchronous Compensator TCSC – Thyristor Controlled Series Capacitor UPFC – Unified Power Flow Controller

9. Exam Tips (GATE Most Important)

Series capacitor → increases power transfer Shunt capacitor → improves voltage Series compensation → reduces reactance Shunt compensation → controls voltage