They are widely used in synchronizing generators and offering multiplexing solutions.

ADVANTAGES OF SYNCHRONIZATION

When a single load is more than the capacity of feeding Genset either during running or starting.
When load has to be transferred from one Genset to other without interruption.

Pre conditions to be satisfied before two sets can be paralleled (synchronized):-

Voltage – of both the DG sets should be same. Instrument used to ensure this is dual voltmeter.
Frequency (RPM) – of both the sets should be same. Instrument used is dual frequency meter.
Phase – of both the voltages should be same. Instrument used to ensure this is phase angle meter.

Other methods used to ensure above three conditions are:

Three lamp method for manual synchronization.
Check synchronization relay for manual synchronization.
Auto synchronization relay for automatic synchronization.

Facilities required on engine to make it suitable for paralleling:-
1. It should be possible to change engine RPM by means of push buttons e.g. raise & lower. If engine is with electronic governor then motorized potentiometer with auxiliary input for remote speed change. Engine speed change is required to adjust frequency before paralleling.
2. Engine RPM should drop by 4% from zero to full load (Droop setting), for engines with mechanical governor.

Facilities required on engine to make it suitable for paralleling:-

It should be possible to change Alternator voltage by means of push buttons e.g. raise & lower. Alternator should be with electronic AVR with auxiliary input for remote voltage change & capable of accepting 5A droop CT input. Motorised potentiometer for adjusting voltage. Voltage change is required to adjust voltage before paralleling and for adjusting KVAr sharing after paralleling.
Droop CT to ensure 4% voltage droop from No load to full load. Droop is required for stability of both the DG sets against full load fluctuation.

Getting Two Generating sets into same phase:-

Genset 1 & Genset 2 are connected to the phase angle meter.
Genset 1 is switched ON and allowed to run.
The Genset 2 is switched ON and the frequency is increased a bit so that the Phase angle as shown by the meter begins to shift towards Zero.

Parallel Operation with the Mains Grid:-
AC Generator ===== Paralleled ====== Grid

Why?

Supplement the power they receive and
To improve the load power factor
To utilize excess generating capacity by exporting it to the grid.

Note:
* While paralleling with the grid an Import – Export meter is necessary.
* For generator protection (df/dt) relay is a must besides the reverse power relay.

Paralleling of Generators sets: ( Electrical Explanation)

Various makes of controllers used for paralleling:-
COMAP ; WOODWARD ; GAC ; DEEPSEA ; DEIF etc.

Why paralleling is sometimes not economical in present scenario? :-

For critical loads like computers & communication equipment 'ON LINE' UPS is perfect solution as this does not interrupt supply even for a microsecond & gives total solution against, high voltage & surges.
Continuous loads which are more than capacity of single DG set needs Auto Load sharing to ensure proper load sharing against thermal drift which is inherent. Without this facility one DG slowly gets loaded more and finally trips.
Power cable economy also is another limiting factor.
Expertise required for maintaining the system.

Advantages of Paralleling the Gen sets -
1. Increased reliability
a. Enhanced reliability is assured for the most critical loads in the system
b. Backup power remains available in the event of one unit’s failure

2. Flexibility
Large single engine gensets are inefficient when not operated near or at full load. The flexibility of paralleling allows multiple configuration options for various types of loads.

a. The multiple unit configurations permits operation levels at partial load to be handled only by the generators needed, while the others remain offline.
b. Limited operation is possible based upon kilowatt demand levels.
c. The flexibility to combine two, three, four, or five gensets of the same or different kilowatt outputs (400, 500 or 600 kW) to more precisely match load requirements from 800 to 2400 kilowatts
d. Paralleling allows commonality of eqpt with a lower cost structure and ease of adding additional units.

3. Redundancy by design
Greater power availability and redundancy as units back up each other, which also provides coverage during maintenance.

4. Reduced maintenance costs
Serviceable by diesel technicians, unlike larger single engine units requiring more specialized service.

5. Less expensive parts
Replacement parts less expensive and more commonly available than for larger single engine units.

6. Ease of expansion
Ease of expansion is another reason for paralleling. If electrical demand is expected to grow substantially over time, the initial investment can be reduced by installing one smaller genset, then adding additional units in parallel as the load increases. Power capacity often can be added with minimal disruption.

7. Economy
a. Costs are lower for the breakers and other system components associated with smaller sets.
b. Paralleling also permits closer matching of the power produced to the actual loads. For ex: it may be possible to operate a single genset when loads are light. When loads increase, other gensets in the paralleled system can be added. This saves fuel and wear & tear on the gensets since they run only when needed. This type of operation, known as load demand mode, is often used in prime power situations or during long power outages.

Salient Features of Sync Panels:

433V, Upto 1010KVA DG’s Synchronization Panels
Auto / Manual Operation, for Manual override
Equipped with a manual over-ride control
Stable operation even for sudden variations in the load
Auto load Sharing / Transfer / Shedding features
Default Automatic Mains Failure (AMF) Feature is inherent.