Paralleling of DGs with dissimilar ratings

This is an extract of Web Chat conversations on paralleling of DGs with dissimilar rating – some valuable tips available. Read in sequence fully. The names and organisations have been changed to impersonal tags, but the chat conversations have been edited for better readability alone.

Person A (Org A) 17 Feb 01 22:55

I need to know if it is possible to run a 180KVA DG set in parallel with a 500KVA DG set (Synchronising). If yes, what are the precautions to be taken for safe operations?

Person B (Org B) 18 Feb 01 21:55

If properly engineered and designed, one may run the above DG sets in parallel. They have to be properly:

  1. System grounded (system ungrounded is possible but to be avoided, if possible)
  2. Individually protected
  3. The downstream power distribution must withstand their total short-circuit fault currents and short-circuit MVAs.
  4. Designed for the same frequency
  5. Avoid a short intermittent on-off duty of one of them to keep system stable.
  6. Loaded according to their permissible loading, kVA vs time curves.

Person C (Org B) 19 Feb 01 9:15

In addition to the general factors mentioned by Person B, you need the following specifics:

  1. The governors must be set up for parallel running. This usually means that they must be set for the same percentage droop, so that they can share load in proportion to their ratings. Other control options are available, depending on the type of governor in use.
  2. The voltage regulators must also be set up to share reactive load in proportion to rating. This will usually also mean load droop control, or cross-current compensation between the two AVRs.
  3. Of course, you will need synchronizing instrumentation & controls to parallel the units in the first place. This can be automatic or manual & can be simple or complex depending on your requirements.

Person A (Org A) 19 Feb 01 14:23

Thanks to Person A and Person B for their helpful hints. The system where I am going to implement this is already running 3 sets of 500 KVA in parallel. The neutral (star point) of each alternator is connected to ground through an isolating contactor, only one of which is on at a time (the rest being floating). The latest set of 180 KVA is being added to take care of peak
load requirements.

The paralleling instruments already in place are : Synchroscope (rotating LED type), Check Synchronising Relay (SKE11 Electro-mechanical) and also the dark lamp method!

The other protections are Reverse Power Relay (CCUM21) and Earth Fault Relay (CAG14) for each DG set. Each Alternator is protected by motorised air circuit breakers.

I was wondering if you know any reference book on this subject of synchronising of DG sets.

Person C (Org B) 19 Feb 01 15:10

As far as references are concerned, I suggest that you try the appropriate manufacturer websites. Basler and Woodward governor would be good starting points.

Person B (Org B) 25 Feb 01 16:26


    1. I had to keep my answers in general to avoid any unrelated specifics such as the power management, loading percentages, etc.
    2. The original posting calls for safe operation of the additional 180 KVA DG. The power management strategies are considered on safe side.
    3. More info on DG units and their controls is available over manufacturers such as
  •   Type Governor under Product / Service that will return Governors: Diesel Engines and 2 companies in addition to the mentioned ones in the previous posting.
  •  Type Generator Product / Service that will return Generator Sets: Diesel Electric 181 Companies for good selections on more info

4.   References:

  • Bergen A. R. “Power System Analysis,” Prentice-Hall, Inc., 1986 Page 390 Section 12.5
  • Special Case: Two Generator Units describes in more detail “power control of two generating units,” including frequency droop characteristics

Person A (Org A) 27 Feb 01 10:56

Even as we were corresponding thru Eng-Tips,I was carrying out the installation work (cabling, control wiring etc.)at the site. Yesterday, I commissioned the set (180 KVA, 415V, 3 phase) by synchronizing it with the existing 3 x 500 KVA sets. I am glad to report that the set ran for over 4 hours in a synchronised state without ever tripping. As the load sharing is being done manually, I tried to load the 180 KVA set but could raise it to only to 90 KW. At this loading, it was drawing a current of 200 Amps. This ampere load remained fairly constant even as I decreased the KW load even to 20kW ! I am still scratching my head over that !

I have asked for the governor to be calibrated afresh ( mainly to give me time to think of a viable load sharing scheme ! ) Well, thanks to Person A & Person B for their interest and without whose prompt replies, I would be still groping in the dark.

Person C (Org B) 27 Feb 01 12:35

Glad to hear that you were successful in commissioning the set. A couple of thoughts regarding your results:

– At 90 kW, 200A, 415V you were operating at 144 kVA and a power factor of 0.63; reducing the real power to 20 kW, you kept at 200A with a power factor of 0.14. How is the control mode of AVR set up?
– You may want to try reducing the excitation to increase the power factor up to the rated value (0.80? 0.85?) for this loading. How did the bus voltage respond when the unit came on-line? It should have increased noticeably, so that reducing the unit excitation would bring it back towards nominal.
– Assuming that you are not connected to the grid, you may have to back load off of one of the other running sets to get the 180 kVA unit to pick up more load – it all depends on the droop settings. What control mode is the governor set up for? You may also want to verify that the prime mover isn’t limiting the output due to some mechanical problem.

Person B (Org B) 27 Feb 01 13:31


  1. Power flow simulation by suitable software performing power flow analysis/management could help.
  2. 180 kVA generator is supplying power to the parallel generators. Please, notice that various regulators, controllers, governors, etc. have their boundary conditions and operating regions. If the current setup does not fit the limiting conditions, there may be a need for their customizations.

Person B (Org B) 5 Mar 01 18:42

Suggestion – Visit:


For more info, and help, contact:

Selco USA, Inc. for Load Sharer T4300 for application diagram at 770-455-9110 (Atlanta, GA, USA) or email

That is the end of the extract. Hope that this chat extract was of assistance to you, or at least opened your thought process on the synchronising and paralleling of DG sets.


10 thoughts on “Paralleling of DGs with dissimilar ratings

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