Your questions answered: What to consider when designing a paralleling power system

When designing a paralleling power system, electrical engineers need to consider many aspects including transfer switches, grounding and protection

By Hassan Obeid November 3, 2022
Courtesy: CFE Media

This course on Sept. 15, 2022, titled “What to consider when designing a paralleling power system” provided electrical engineers with an overview of power systems and covers the elements to consider for designing generator paralleling systems, automatic transfer switches, withstand/closing ratings, grounding and protection.

Several questions were left open, and they’re answered here by Hassan Obeid, Senior Global Technical Advisor – Energy Management Solutions, Cummins Power Generation

Do you have an example of generator application in a microgrid with solar plus battery energy storage?

Yes, we have a few examples where we have implemented generator sets, battery energy storage system and solar.

Can generators of different kW ratings be paralleled?

Yes, they can. All the generators will take their share of the load based on percentages. The larger the generator, the more load it takes. The generators onboard controller does that on a millisecond timescale.

Can you parallel generators from different manufacture via autonomous paralleling or do you have to use min controller?

Yes, that is possible. It is also possible to add the Cummins paralleling controller on top of other controls to minimize the number of controllers. It is also important to consider the winding pitch of the alternators.

With Onboard Paralleling with generator set mounted breakers using a common collector bus, what is the best practice for generator maintenance? Should a disconnect be provided to isolate the generator from the collector bus? Should both gensets be shut down if work is being performed on the circuit breaker?

Yes, providing a disconnect switch to isolate the generator set for maintenance is one of the items to consider. Please refer to this white paper: “Design considerations for generator set mounted paralleling breakers.”

Can generators parallel with BESS (battery energy storage system)?

Yes, Generator sets can be paralleled with BESS.

Quickly touch on lean burn NG stabilization: How do parallel lean burn gensets react to changing loads/minimum load requirement when operating in an island configuration?

As a rule of thumb, lean burn generator sets should be loaded about 60%. Lean burn generator sets react slower than diesel when to comes to load removal for example. Having said that, the controls are tuned precisely to accommodate for that. Therefore, that is not an issue. The most important thing to consider is if a lean burn is paralleled with a diesel generator, then special adjustments must be made in the generator and engine controls to account for that.

Do you use 4 pole or 3 pole ATSs on 120/208V Y systems

To determine which configuration (3-pole or 4-pole) to select, attention must be paid to the system grounding scheme and ground fault protection requirements. If ground fault detection is required on either source, a 4-pole transfer switch is necessary in most cases. Per NFPA 70: National Electrical Code 230.95 Ground Fault Protection (GFP) of equipment is required at the service disconnect (utility breaker) for systems with all of the following:

  • Solidly grounded wye electrical service.
  • More than 150 volts to ground (277/480 or 347/600 VAC).
  • Over current device rating of 1000 A or more.
  • NEC 700.6 (D) and 701.6 (D) requires Ground Fault Indication (GFI) at the emergency source.

Ground fault sensing depends on being able to sense ground fault current. To accurately sense ground fault current, it must return to its source on a known path relative to ground fault current transformers (CTs). In basic emergency standby systems there are two rules to follow to meet these requirements:

  • There can only be one neutral/ground connection on any neutral bus at one time.
  • Ground fault sensors (the CTs) must be downstream (or on the load side) of the bonding connection.

To meet both rules when connected to either the normal or the emergency source, the neutral must be switched using a 4-pole transfer switch

What does phase angle play in paralleling control?

The synchronizer in the paralleling controller must match the phase angle of the two sources by adjusting the fuel to the engine. Therefore, the onboard paralleling controller matches phase, voltage and frequency.

With an ATS having a WCR-100,000 A and time duration of 0.05 sec, is it possible to adjust the short time protection of the upstream UL 489 breaker to 50,000 A and 0.3 sec time duration since 50,000 A is lower than the WCR of the ATS?

The short time element S in LSI trip units in UL 489 breakers doesn’t allow you to go to that time setting of 0.3 sec. The max will not allow you to go beyond the instantaneous. The instantaneous is always on and cannot be disabled. Only LVPCB (Low Voltage Power Circuit Breakers) UL 1066 allows you to adjust that to go up to 0.5 sec for the short-time S, which in that case you need to specify a short-time WCR for the transfer switch.

If the switchboard is solidly grounded, is the neutral at the generator to be grounded or not?

The neutral from the generator sets should be brought back to the distribution board and there should be one neutral to ground pond at the gear with a CT for ground fault.

How do you coordinate a UL 1558 gear with an ATS?

Specify a short-time WCR of 0.5 seconds.

What is the key point to make the decision of paralleling?

Reliability and redundancy.

What is the manufacturer definition of manual paralleling control?

Manually initiated by the operator, automatically executed by the onboard paralleling control for the maximum safety and reliability

How do you parallel generators from different manufacturers?

It can be done and it has been. We need to think about the winding pitch of the alternators (2/3rd or 5/6th), managing which generator closes its paralleling breaker to the bus and then manage load share (real and reactive) across the units.

Why can’t you install a single HRG on the common neutral bus of paralleled generators? The question applies to LV (480 V) system.

in LV (480 VAC) applications the loads need that neutral from the generators and therefore the neutral is brought back to the distribution board. In the distribution board, the bond between the neutral and ground is established along with CTs around that bond that feed into the ground fault relay.

When do you specify 4-pole ATS?

If ground fault protection is needed then specify a 4-pole transfer switch.

What is a basic formula to calculate the available fault current from a generator?

As a rule of thumb, the fault current is about 8 to 10 times the rated current. This is a very conservative estimate.

Generator Set Available Fault Current =

(kW*1,000)/( √3*V*p.f.*Xd)


kW: kilowatt output rating of the generator set

p.f.: generator set rated power factor

V: generator set rated output voltage

Xd: alternator per unit subtransient reactance based on the generator set rating

What controls are available for loading and unloading for retransfer to grid?

All we need to control is engine fuel and field excitation. For a single generator/single utility Cummins paralleling controller can handle all of that. Base load/peak-shave, extended paralleling and ramping the load on/off the utility. For multiple generator sets with a utility, Cummins also offers it is synchronizer that precisely matches the generator set control to achieve all that was described.


Cummins is a CFE Media content partner.

Author Bio: Hassan Obeid is a global technical advisor for systems and controls — critical protection at Cummins Power Generation, focusing on technical vision, business strategy and solving a wide range of complex problems.