Your questions answered: Specifying generator set paralleling controls

In this additional Q&A, learn how to parallel generators for emergency power systems, along with their fundamental control features

By Hassan Obeid February 25, 2022
Specifying generator set paralleling controls. Courtesy: Cummins

This course provides a comprehensive overview of paralleling generators for emergency power systems and dives into the fundamental control features needed to parallel generator sets together and with the electrical grid. Traditional switchgear paralleling is reviewed and compared with the integrated paralleling controls that use distributed logic architecture to help design engineers specify a reliable paralleling system.

During the Feb. 17 course, not all questions were answered by Hassan Obeid, Global Technical Advisor – Critical Protection, Cummins Power Generation. He answers several additional questions here.


Does Cummins offer integrated paralleling?

Yes. In 1995 Cummins Power Generation introduced the first fully integrated PowerCommand digital controls across its entire range of commercial generator sets. These microprocessor-based controls are the industry’s only total integrated platform which is capable of integrating engine, alternator control and protection functionality into a common system.

Some of the integrated control responsibilities are:

  • First Start Arbitration
  • Synchronizing (phase, voltage. frequency)
  • Voltage Regulation
  • Load Sharing (kW, kVAR)
  • Generator Set Metering
  • Generator Set Protection
  • Load Demand

Do you have an example of a good spec that you can forward to us?

Yes, The MasterSpec has all the paralleling, transfer switch and generator sets specifications.

Is it possible for the DMC to make a Main-Tie-Main act as a closed-transition ATS for downstream feeds via synchronizers on the mains and the tie breaker?

Yes, it is possible for the DMC control to do so. However, utilities will not allow for that.

Does integrated paralleling switchgear allow manual syncing of gen sets without having to be right at the generator i.e., in the control room versus the engine room?

Yes, a system HMI can be added in a remote location to manually initiate the automatic paralleling.

Can the paralleling switchgear have an HMI to display data from each engine generator i.e., kw, kVAR, volts, amps, frequency?

Yes, a system HMI and/or remote generator set HMI can be added to the switchgear to view generator set data.

What are the advantages of non-droop (isochronous) systems?

With isochronous load share, the output frequency and voltage of the paralleled generator sets stay constant regardless of the load. Isochronous load share is the preferred method since the output voltage and frequency are constant whether the load is 0% or 100%.

With droop, frequency and voltage vary as the load varies. As the load on the generator sets increases the output voltage and frequency dips.

Based on your description of manual paralleling, you are not doing manual paralleling, so when the electronics dies, which it will you will not be able to parallel. What about having sync lights or sync scope, manually adjusting the voltage and manually closing the breaker when the units are in sync? The integrated system will eventually fail and with no true manual paralleling available you are out of business.

Using sync lights and sync scopes don’t solve this issue at all. An electronic engine control module and a voltage regulator are still needed. Performing manual paralleling with sync lights and sync scopes creates a false sense of reliability not to mention safety concerns for operators.

What typically causes reverse kW or kVAR, what is the resolution?

Typically, a generator set can absorb a certain amount of reverse kW and kVARs without causing damage. The cause of reverse kVARs is loss of excitation due to diode failure for example or if closing out of sync and one unit is at a higher potential than the other one. Reverse kW is caused if the relative phase angle and or frequency of a generator set is higher than the other. Cummins integrated control have built protection to protect against reverse kW and kVARs.

What is the cost benefit for integrated parallel controls?

Traditional paralleling requires more real estate, adds more complexity for wiring, programming and maintaining. It is component based and could add over $100,000 of unnecessary cost to a project compared with the integrated paralleling control.

With the integrated paralleling, the design is consistent, design and wiring are reduced, no switchgear paralleling controls are needed and it is easier to learn operate.

When we have outdoor utility power coming into the indoor ATS and outdoor standby generator coming into the indoor ATS, should we place a surge protection device at the ATS or at the load connected to the ATS?

The surge protection should be placed in the transfer switch to protect the load the transfer switch control. Cummins just released its latest transfer switches and the engineers can select from a few different SPD options.

What is done to ensure the system remains redundant? Are the controls managed by a master controller, or are the controls done in a loop configuration?

The integrated paralleling controls are autonomous and don’t require interaction with the master level control. All of the paralleling functions are included on each generator set. This is what was is meant by distributed logic architecture. No paralleling master is needed and therefore single point of failure is eliminated.

If the Point of Common Connection has overcurrent protection, is protection still required at the alternator terminals?

The Cummins integrated controls have a built-in utility-grade overcurrent protection, AmpSentry.

AmpSentry protection is designed to eliminate nuisance generator set failures that occur with molded-case breaker protection to provide a higher standard of protection for the alternator. It guards the electrical integrity of the alternator against thermal damage.

How do you parallel different kW gensets ratings?

The Cummins integrated controls are capable of paralleling and load sharing different size generator sets. The controls accounts for the size of the generator sets and the load (kW and kVAR) is shared equally across the paralleled units based on equal percentages. No action is needed from the operator.

Specifying generator set paralleling controls

Specifying generator set paralleling controls. Courtesy: Cummins

What is the advantage of 2/3 pitch with harmonics?

The 2/3 pitch windings suppress the triple harmonics current.

Just to be sure, if two gensets, one of 500 kW and one of 250 kW, are connected in parallel on a load of 600 kW, the load on each generator will be 400 kW and 200 kW?

That is correct. The total load is 600kW and the total capacity is 750kW. 600kW/750kW = 0.8 and therefore each generator set will be loaded by 80% of its capacity. The 500kW takes 400kW of the load and the 250kW generator set takes 200kW.

The load is equally distributed across the paralleled units and they are both 80% loaded.

Don’t we need a master controller if generators from different manufacturers are being paralleled?

Not, necessarily. The Cummins integrated control (PowerCommand) can be retrofitted on existing units which will make the units act and behave as if they are the same from a controls standpoint.

Do generators and transfer switches have to be automatic?

No, the transfer switch can be nonautomatic. And the generator set can be placed in a nonautomatic configuration. The application needs must be considered to guarantee that nonautomatic functionality meets the needs of the application.

So, you can only parallel the same size generators?

No, the generator sets can be of different sizes.

With medium voltage a HRG is typically included. Does this create issues?

No, adding a high-resistance ground doesn’t create issues.

What happens if communications between generators is lost?

If the generator sets are paralleled and the load share lines are lost between, for example generator set No. 1 and the rest of the generator sets, the other generators on the line will continue running and generator set No. 1 will open its circuit breaker and shut down. The load share lines can be run in a loop for redundancy.

Hassan Obeid
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.