Voltage stability by
compensating reactive power
What this project is about??
• This project will illustrate the basic issues related to voltage instability
by considering the characteristics of transmission system and
afterwards examining how we can improve voltage stability by using
reactive power compensation devices.
• Its about how voltage is being brought back to same(nearby) value by
compensating reactive power
PRINCIPAL CAUSES OF VOLTAGE STABILITY PROBLEMS
• High reactive power consumption at heavy loads
• Generating stations are too far from load centres.
• We have observed this from simulation of a case by increasing the
length of the PI section transmission line.
• Source voltages are too low.
• Poor coordination between various control and protective systems.
Ways of improving voltage stability and control
• Reactive power compensation is often most effective way to improve both
power transfer capability and voltage stability. The control of voltage levels is
accomplished by controlling the production, absorption and flow of reactive
• To control voltage throughout the system we have to use addition devices to
compensate reactive power.
• The device used for this purpose are:
SVC(Static Var compensator)
STATCOM(Static Synchronous Compensator)
• In this project we will be discussing two of them namely
Shunt compensation (by using capacitor banks)
SVC(by using thyristors and PWM blocks)
Shunt Compensation (by using Capacitor)
• The primary purpose of transmission system shunt compensation
near load areas is voltage control and load stabilization. In other
words, shunt capacitors are used to ensure satisfactory voltage levels
during heavy load conditions.
• Shunt capacitors are used in power system for power-factor
correction. The objective of power factor correction is to provide
reactive power close to point where it is being consumed, rather
than supply it from remote sources.
For voltage stability, shunt capacitor banks are very useful on allowing nearby generators to
operate near unity power factor.
The Qc that we got here should be added to the reactive power loss at the transmission line and
Margin of 8-9% of the load reactive power should be added to get the reference voltage value
• We can also calculate the capacitance that will inject the required
power by equations
Advantages & Disadvantages
Much lower cost compared to SVC
Switching speeds can be quite fast when compared to other compensation
With the increase in voltages the capacitor value(in turn the size) increases
which is difficult to design them
Precise and rapid control of voltages are not possible.
• In SVC we are comparing the Input voltage(reference voltage) and the
voltage at the bus by using a summer and the output is given to the
input of PID controller and then it is given to PWM controller (which
can be used as feed back )and then the output are taken as control
pulses to the both thyristors and then the output is given to injection
transformer which will eliminate the unwanted harmonics of the
Advantages and disadvantages
• The main advantage of SVCs over simple capacitor bank
compensation schemes is their near-instantaneous response to
changes in the system voltage
• Dynamic compensation can be achieved by SVC
• However, static VAR compensators are more expensive than