Question

What is a synchronous condenser, and what are its principal advantages over a switched capacitor bank for power factor correction? Please explain with great detail, thank you!

Answer 1

Static capacitor:The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor. The capacitor (generally known as static capacitor) draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load. For three-phase loads, the capacitors can be connected in delta or star.

Advantages

They have low losses

They require little maintenance as there are no rotating parts

They can be easily installed as they are light and require no foundation

They can work under ordinary atmospheric conditions

Disadvantages

They have short service life ranging from 8 to 10 years

They are easily damaged if the voltage exceeds the rated value

Once the capacitors are damaged, their repair is uneconomical

Synchronous condenser:

A synchronous motor takes a leading current when over-excited and, therefore, behaves as a capacitor. An over-excited synchronous motor running on no load is known as synchronous condenser. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

Advantages

By varying the field excitation, the magnitude of current drawn by the motor can be changed
by any amount. This helps in achieving step less †control of power factor

The motor windings have high thermal stability to short circuit currents

The faults can be removed easily

Disadvantages

There are considerable losses in the motor

The maintenance cost is high

It produces noise

Except in sizes above 500 kVA, the cost is greater than that of static capacitors of the same
rating

As a synchronous motor has no self-starting torque, therefore, an auxiliary equipment has to
be provided for this purpose

Synchronous condenser for Power factor improvement:

All of us are familiar with capacitors used for improvement of power factor. Depending upon present power factor and desired power factor, we calculate the requirement of capacitor kVAR and install them in banks.

An Automatic Power Factor Correction controller is also installed which detects existing power factor and switches ON and OFF banks of capacitors.

These capacitors absorb a lot of harmonics and transients present in the system and develop in the system.as they offer low impedance to harmonics and impulses.

The energy of these harmonics and impulses heats up the capacitor and they may fail prematurely.

Average expected life of a capacitor bank in a system having lots of harmonics and voltage transients is around five years.

An other problem with capacitors is that they generate harmonics, specially at the time of switching on a bank. These harmonics further aggravate situation and may even cause system instability, if system inductance and capacitance form a resonant circuit.

The generation of harmonics is worst if one bank is already On and another bank is switched on randomly. Incoming bank has two sources; supply and existing bank.
The situation is some what redeemed by synchronous switching, which is a technique to switch capacitors such that voltage is at zero crossing at the time of contact.
Still it would be difficult to maintain harmonics below IEEE 519.
All of the above problems are solved with the help of synchronous condenser and a few more advantages are present with synchronous condenser.Synchronous condenser is a synchronous motor running in over excited conditions.Synchronous motor is a three phase motor which has a 3 phase armature winding and a DC field winding.

It also has a starting winding called amortisseur winding. This winding helps motor start from rest as an induction motor and when speed reaches near synchronous speed, excitation is switched on and it pulls rotor in synchronism with armature rotating field.
Synchronous motor's excitation controls the power factor of the motor. At low excitation power factor is lagging. at optimal excitation voltage and currents are in phase and power factor is one.
At excitation more than this armature current leads voltage and power factor becomes leading.

This leading armature current cancels lagging current from other sources and gives very high power factor.
The power factor is steplessly controlled.
Synchronous condenser in construction is very similar to a synchronous motor with out and external shaft for coupling as not load is intended to run.
The field winding is made from thicker wire to continoulsly run on over excitation.
The synchronous condenser offers several advantages over capacitor bank.

Reliability is very high.

Step-less adjustment of power factor.

No generation of harmonics.ow maintenance

Is not affected by harmonics.

Expected life almost 25 years.

Low maintenance; only periodic bearing greasing is necessary.

The only  disadvantage is that it is expensive and therefore mostly used by large power users.