Synchronous Condenser : Design, Working, Phasor Diagram & Its Applications

Synchronous condensers are not new but normally used since the 1950s for stabilizing power systems. Synchronous condensers are large machines that turn very freely & can absorb or generate reactive power to stabilize & strengthen a power system. These condensers assist when there are any changes within the load as they enhance network inertia. The kinetic energy which is stored within a synchronous condenser supplies the whole inertia of the power system & is very helpful from a frequency control point of view. This article discusses an overview of the synchronous condenser – working and its applications.

What is a Synchronous Condenser?

An over-excited synchronous motor that runs on no-load is known as a synchronous condenser. This condenser is a DC-excited synchronous machine whose shaft is not connected to any driving equipment. This condenser is also known as a synchronous compensator or synchronous capacitor. This device provides improved stability & voltage regulation by generating  or absorbing continuously adjustable reactive power, improved short-circuit strength, and stability of frequency by supplying synchronous inertia.

Synchronous Condenser
Synchronous Condenser

The main purpose of a synchronous condenser is to use the reactive power control capabilities & the synchronous inertia of the machine. The power system includes an attractive alternative solution to capacitor banks becaus e of the capability to regulate the reactive power amount continuously. These condensers are perfectly suitable to control the voltage on long transmission lines or within networks through a high diffusion of power electronic devices & in networks wherever there is a high danger of ‘islanding’ from the major network.

Synchronous Condenser Design

The synchronous condenser is designed with different components like a stator, rotor, exciter, amor tissuer winding, and frame. A synchronous motor includes a 3-phase stator that is analogous to an induction motor. The unit begins as an induction motor with the amortisseur winding that needs to slip to generate starting torque.

Synchronous Condenser Design
Synchronous Condenser Design

For synchronous motors, the DC is supplied to the field winding of the rotor called an exciter. It is arranged on the shaft of the synchronous motor. A rotor with an equal number of poles like the stator is supplied through a direct current source. The rotor current creates a north-south magnetic pole connection within the rotor pole pairs by allowing the rotor to “lock in step” by the rotary stator flux. The frame is the external part of the machine & is designed with cast iron.

How Does Synchronous Condenser Works?

As synchronous condenser working is similar to the synchronous motor principle. The working principle of this motor is motional EMF which means, a conductor tends to revolve because of the magnetic field effect. Here, there are two ways are used for providing a magnetic field like a 3-phase AC supply & a stable DC power supply to the stator.


The main reason to provide two ways of excitation is that it can revolve at synchronous speed because the motor simply works on the magnetic field interlocking generated because of the stator as well as dc field winding.

The changing of DC field excitation may result in different modes. So synchronous condenser modes of operation are discussed below.

At first by increasing the dc supply, the armature current reduces and shows that the stator is utilizing low current to generate flux, and also the synchronous motor draws less reactive current, so it is called under-excited mode.

Further on increase within dc field excitation, a point comes wherever the armature current is low & the motor works at unity power factor (PF). The requirements of all the field excitation are met by the dc source. So this mode is known as the normal-excited mode.

Further, increase the field current with the dc supply, then flux excessively increases & to offset it, the stator will start supplying reactive power in place of absorbing it. Thus, the synchronous motor draws a leading current.

Synchronous Condenser Vs Capacitor Bank

The difference between a synchronous condenser Vs a capacitor bank includes the following.

Synchronous Condenser

Capacitor Bank

It is a DC-excited synchronous motor, used to improve power factor and power factor correction within power lines by simply connecting it with transmission lines. A capacitor bank is a set of capacitors that are arranged in series
(or) parallel combinations. Capacitor banks are mainly used for power factor correction & reactive power compensation within the power substations.
It is also known as a synchronous compensator or synchronous capacitor. It is also known as a capacitor unit.
Not like a static capacitor bank, the reactive power amount from a synchronous condenser can be adjusted continuously. Reactive power from a static capacitor bank reduces when grid voltage reduces, whereas a synchronous condenser increases reactive power when the voltage decreases.
The synchronous condenser has higher life as compared to the capacitor bank. Capacitor bank lifetime is low.
They give better performance within the high voltage system as compared to the capacitor bank. They give less performance within the high voltage system.
It is more expensive than a capacitor bank. It is economical.

Phasor Diagram

The synchronous condenser phasor diagram is shown below. Whenever a synchronous motor is normally over-excited then it takes the leading power factor current. If this motor is on no load condition, where load angle ‘δ’ is extremely small & also it is over excited like Eb > V then the PF angle will increase nearly to 90 degrees. So, this motor runs with approximately ‘0’leading PF condition that is shown in the following phasor diagram.

Phasor Diagram
Phasor Diagram synchronous motor

This characteristic is related to a typical capacitor that uses a leading PF current. Thus over excited motor working on no load condition is known as a synchronous condenser. This is the main property because which motor is utilized as a power improvement device or phase advanced.

Advantages and Disadvantages

The advantages of a synchronous condenser include the following.

  • It can boost system inertia.
  • Short-term overload capacity can be increased.
  • Low-voltage ride-through.
  • Quick response
  • Extra short-circuit strength.
  • There are no harmonics.
  • The reactive power is continuously adjusted.
  • It is maintenance-free.
  • A high amount of security can be maintained.
  • It has a high lifetime.
  • The faults can be easily removed.
  • The magnitude of current drawn through the motor can be easily changed by changing the field excitation with any amount. So this helps in attaining step-less power factor control.
  • The thermal stability of motor windings is high for short-circuit currents.

The disadvantages of a synchronous condenser include the following.

  • It is expensive.
  • It generates noise.
  • There are huge losses within the motor.
  • It occupies more space.
  • It requires continuous cooling.
  • Field current continuously needs to be checked.
  • It has no self-starting torque so; auxiliary equipment has to be provided.


The uses or applications of synchronous condensers include the following.

  • The typical applications mainly include HVDC, Wind  or  Solar, Grid Support & Regulation.
  • These are used at both transmission & distribution voltage levels to enhance stability & maintain voltages in preferred limits in changing load conditions & contingency situations.
  • These condensers are used in electric power systems for voltage control on long transmission lines, particularly for transmission lines with a fairly high inductive reactance to resistance ratio.
  • It is utilized in power lines for enhancing power factor (P.F) and PF correction by simply connecting it with transmission lines.
  • These condensers are utilized in hybrid energy systems.
  • These condensers behave like a variable capacitor or variable inductor, used within power transmission systems for controlling line voltage.

Why is it called a Synchronous Condenser?

When a synchronous motor at no load condition is overexcited then it performs like a capacitor because it begins using a leading current at no load. Thus, a synchronous motor that is overexcited at no load is known as a synchronous condenser. It is simply connected to the load in parallel for improving the power factor.

Where is Synchronous Condenser Used?

It is used within power transmission systems for regulating line voltage, in HVDC, wind/solar, grid support, regulation, power factor correction, and VAR compensator.

Is Synchronous Motor Self Induced?

A synchronous motor is not a self-starting motor because of the inertia of the rotor. So, it cannot follow the revolution of the magnetic field of the stator immediately. When the rotor attains the synchronous speed, then field winding is excited & the motor will pull into synchronization.

What are the Advantages of Installing a Synchronous Condenser in an Electrical System?

A synchronous condenser is very helpful at both transmission & distribution voltage levels to enhance stability & also to maintain voltages in desired limits in changing load conditions as well as contingency situations.

Why Synchronous Machine Is Synchronous Condenser?

A synchronous machine runs with no load will lead the current. So synchronous motor runs without a load that is overexcited are known as a synchronous condenser.

Thus, this is an overview of a synchronous condenser which is used mainly used in power factor (PF) correction to enhance the PF from lagging to leading. Since this condenser works like a variable capacitor or a variable inductor, then it is used to control line voltage within power transmission systems. Here is a question for you, what is a synchronous motor?