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.

Click to see In upper diagram capacitor is switched on randomly.Lots of  Harmonics are generated which die down in a few cycles. In lower diagram situation is better by synchronous switching. Capacitor switch gear is switched on  such that voltage is applied to incoming capacitor at zero crossing.

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.

More about synchronous condenser in later issue.
Please do send comments and questions.

Delhi 27 Sept. 2012

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.

1. Reliability is very high.
2. Step-less adjustment of power factor.
3. No generation of harmonics.ow maintenance 
4. Is not affected by harmonics.
5. Expected life almost 25 years.
6. Low maintenance; only periodic bearing greasing is necessary.

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

Untill next time, Cheerio!