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I have two DG sets which are to be synchronized on to the same AC bus and needless to say, they must share load. I have gathered some parametric data which I felt would decide how well these can work as part of single bus. But I am not sure, and would like to know what the experts feel about these two DG sets getting synced easily.

DG-1

Engine Data:

  • 1200KW
  • 50Hz
  • Power Factor (Cos Phi): 0.7

Generator Data:

  • 3 Phase - 6 Wire
  • X Wye Delta,
  • X Series Parallel
  • 600 Volts
  • 1828 Amps
  • 1714KVA
  • Sub Transient X"D : 0.133 per unit 0.028 Ohms
  • Transient X"D : 0.197 per unit 0.043 Ohms

Excitation Data:

  • 31 Volts, 7.1 Amps
  • Separately Excited

Frame: - 826 Frame - 1500 Rev/Min

DG-2

Engine Data:

  • 1330KW - KW is specified at a higher PF here
  • 50Hz
  • Power Factor (Cos Phi): 0.8

Generator Data:

  • 3 Phase - 6 Wire
  • KH734E Wye
  • P7 Series
  • 600 Volts
  • 1492 Amps - So Amps is much lesser than DG-1
  • 1662.5KVA - So KVA is slightly less than DG-1
  • Sub Transient X"D : 0.101 per unit 0.092 Ohms
  • Transient X"D : 0.581 per unit 0.21 Ohms

Excitation Data:

  • 45 Volts, 3.5 Amps - Excitation power is also much lesser than DG-1
  • Separately Excited

Frame: - Steel Frame - 1500 Rev/Min

Any kind of help shall be highly appreciated.

Best Regards,

Vishal

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  • \$\begingroup\$ Sharing the load is not a generator problem. That a controls problem. What do you know about the controls? Most generators can run parallel, except when rotor wasn't meant for it. (extreme weight reduction for example) \$\endgroup\$
    – Jeroen3
    Jul 2, 2019 at 10:59
  • \$\begingroup\$ @Jeroen3, Thanks. The controls are identical. Excitation transformers have tappings so they can be chosen accordingly. Governors are same, 0 - 200mA for throttle, which is then converted to PWM by the ECU. \$\endgroup\$
    – Vishal
    Jul 2, 2019 at 11:08
  • \$\begingroup\$ Not only the engine has controls. The generator has too, so does the kw sharing (different unit) and the circuit breakers. \$\endgroup\$
    – Jeroen3
    Jul 2, 2019 at 11:24
  • \$\begingroup\$ OK, So I gather from your comment that, if the controls have enough flexibility (Engine-Generator controls) and ACBs, then these can work together, right? No issues are peeping out, right. \$\endgroup\$
    – Vishal
    Jul 2, 2019 at 11:28
  • \$\begingroup\$ The most significant issue peeping out is that you did not share enough information to be able to answer yes or no if you r sets can run load (kw) sharing. \$\endgroup\$
    – Jeroen3
    Jul 2, 2019 at 11:45

1 Answer 1

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Let's give some generic answer to this, because the question lacks the necessary information about the control circuitry to give a dedicated answer for those two DG sets.

Judging from your data both devices are synchronous generators, enabling you to control two parameters: voltage and frequency by changing excitation and speed. For a successful synchronization of two generators (or a generator with a grid) these are the only prerequisites. Synchronization refers to just the moment you connect both DG sets. Successful means not tripping any safety mechanism instantly or damaging one of the DG sets. There is no theoretical limit for synchronizing generators of different power, different current handling capability, very different rotational momentum or other differing factors. In fact you could synchronize an 1 kW DG set with an 1 MW DG set when the four sync conditions (phase sequence, frequency, voltage, phase) are met.

However synchronization is only half the work when you want to make use of more than one generator on a grid. The more difficult task is to enforce a useful sharing of real and reactive power.

Real power sharing can be achieved if

  1. the nominal frequency of at least one DG set can be controlled directly
  2. both DG sets have a governor with a well defined load dependend droop
  3. either both droops are the same over the full load range (e.g. 4%) or if one can be adapted.

Reactive power sharing can similarily be achieved if

  1. the nominal output voltage of at least one DG set can be controlled directly
  2. both DG sets have an excitation regulator with a defined droop dependend on reactive power output/input
  3. either both droops are the same or if one can be adapted.

Without the droops and setpoints for frequency and voltage correctly configured for a power sharing you may face several problems after a seemingly successful synchronization. A non comprehensive list of potential events:

  • One generator running close to its nominal load while the other is nearly idle or even
  • One generator taking up power instead of delivering it (very dangerous)
  • In consequence of these conditions protection circuits of one or even both DG sets tripping
  • One generator picking up the majority of reactive power when connecting inductive or capacitive loads to the grid
  • Both generators exchanging reactive power even with no significant load connected to the grid

The last two effects are even more sneaky than an uneven distribution of real power. Winding losses grow with the square of the reactive power as the generator voltage is fixed, so it is crucial to keep the reactive power under control.

There's one more thing with big generators: They should have a damper winding to prevent oscillations between both sets. Consider the rotor inside the magnetic field as a mass connected with a spring to the outer world (grid). This mass-spring system has a certain resonance frequency which can be excited by external triggers. If more of these mass-spring systems are connected together the oscillation can increase when the resonance frequencies are tuned. This can lead to additional stress on all mechanic parts. Probably the bearings of the diesel are the most affected ones.

Having mentioned the various problems which can arise within a number of already synchronized DG sets it is obvious that the control options (nominal voltage/frequency and voltage/frequency droop) make no sense without the possibility to measure and display real and reactive power for both DG sets in summary and preferably for each phase, too.

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