Your questions answered: Generator set-based paralleling

This webcast from June 8, 2023, provided a comprehensive overview of a typical paralleling emergency power system and dive into the fundamental key features needed to parallel generator sets. The presenter reviewed critical control functionality for paralleling systems and compared distributed logic architecture with traditional switchgear paralleling. He wrapped up looking into system reliability while reviewing the ability of paralleling and control strategies employed to eliminate potential single points of failure.

Additional questions were answered by Daniel Fields, Technical Advisor, Cummins.

Question: Can you discuss the advantages of true RMS control (FTPM listing vs. NRGU)?

Answer: Genset-based control systems can be either “industrial grade” (UL category FTPM) or (UL category NRGU), which is utility grade. The difference is that NRGU is true RMS sensing and display for all functions. FTPM is susceptible to misoperation due to distortion from nonlinear loads. This is a big deal because gensets are higher-impedance machines than the utility service supply. So power control accuracy and display can be disrupted unless you have NRGU listing.

Question: Can you discuss general utility requirements for paralleling and how Cummins addresses these?

Answer: Protection requirements needed when paralleling to the utility are generally driven by the utility company. Each have their own standards and requirements that have to be tested and verified before putting a system in operation.

Question: What do you recommend for required ground fault protection for generator breakers rated 1000 amps or more? Would you use a solidly grounded neutral configuration with a 3-pole ATS or use a 4-pole ATS?

Answer: The use of 3-pole vs. 4-pole ATS’s would need to be determined by the design/electrical engineer.

Question: How practical is using umbilical (on board) type controls vs. paralleling switchgear? How many gensets can you parallel using umbilical (on board) paralleling?

Answer: Each manufacturer may have different limitations. The main limitation is generally the overall distance between the genset controls. As long as the gensets are close to one another, you can have up to 16 with no issues. If more than that would be required, or the gensets are not right next to one another, you should consult the manufacturer.

Question: Is the Cummins PCC 3300 capable of random access synchronization in an independent island mode EPSS setup?

Answer: With the 3300 control, as long as it is set up properly and the bus sensing is wired properly, it is capable of paralleling to an energized bus using an electrically operated breaker.

Question: What is N+1 and N+2?

Answer: N refers to the number of gensets required to operate the load on the emergency bus. N+1 would be one more genset than needed, N+2 would be two more gensets than needed and so on. This is done so that if there is a genset failure, all emergency loads can still be kept online.

Question: Where can users/engineers go to inquire about list/budget pricing for paralleling switchgear and a ballpark cost to add?

Answer: Your local manufacturer’s sales representative can help you with pricing.

Question: How do you size the generators bus for multiple generators connected on parallel?

Answer: The bus would be sized for total available current. Your local sales representative can help you with this.

Question: What do you do for overall voltage dip when all generators start at the same time?

Answer: I assume you are referring to the genset battery voltage dip when it is cranking. If the battery voltage is dropping below acceptable levels, then the batteries should be serviced and tested.

Question: What are the critical parameters the operator/maintainer would want to monitor to minimize cost of ownership? This includes any parameter that would indicate failure or maintenance need of the genset to increase the reliability.

Answer: There are generally suggested checks and service requirements in the generator set operator manual.

Question: What is the typical time delay to get through the first start arbitration phase of the process?  I’m sure this depends upon the system size.

Answer: Typically, one generator set can be closed to the bus (first start arbitration complete) within 3 to 6 seconds. There are several things that can affect this time such as genset battery condition, the fuel system, etc.

Question: Could you explain the difference between start sequence when utility power is present and when the utility power has failed? I have a client that doesn’t wait for a utility failure, but switches to generator on the approach of a storm front.

Answer: The sequence would be the same for starting except possibly the signal used to start the gensets (remote start or test). The transfer sequence may be different depending on if it is an open or closed transition system. The operator manual should have a sequence of operation section with details for each sequence.

Question: In parallel mode, once one genset is closed to the bus, the loads on the bus may be too large for the first generator to provide for, thus it trips off and the others will do the same if sequentially closed in. What does Cummins recommend here?

Answer: The level 1 priority loads should be sized to be capable of being supported by one genset. If this is not the case, the loads should be reviewed in order to move some noncritical loads to a lower priority, which would keep them from transferring to the emergency bus until more gensets have closed.

Question: What conditions are met in the ready to go phase during first arbitration? What is good voltage and frequency?

Answer: Default is generally 90% rated frequency and voltage. For example, on a 480 VAC 60Hz system, ready to load would activate at 432 VAC and 54Hz. This may differ between manufacturers and can generally be adjusted if required.

Question: Which generator leads the frequency control logic when operating stand-alone?

Answer: Each genset control controls its own frequency.

Question: In isochronous mode, what is the frequency reference when operating stand-alone?

Answer: That would be set by the operator, but in North America, that would typically be set to 60Hz. The genset control constantly monitors itself to maintain proper voltage and frequency.

Question: For installations that include large motors, can individual generators be smaller than can supply a single large motor, provided that the total kW is sufficient?

Answer: Yes, when paralleled, the gensets act like one larger genset so large loads can be powered. I would suggest contacting your local sales representative for assistance with your specific application.

Question: Should all paralleled generators be rated the same?

Answer: The kW size of the gensets can be different. However, the smallest genset in the system should be sized to carry all legally required loads.

Question: Is there any advantage in paralleling same size same manufacture generator sets?

Answer: When the gensets are the same size with the same control, it does make the commissioning easier. However, it is possible to parallel different manufacturers’ equipment together. I would suggest contacting your local sales representative for assistance with your specific application.

Question: What type of relays/relaying do you recommend for paralleling?

Answer: Insulated case or power air breakers are typically used in paralleling applications.

Question: Can this work with utilizing site generators to sell back to the utility for peak shaving considerations with natural gas or hydrogen generators?

Answer: This is possible with a single genset, but it depends on the application as well as the site requirements. I would suggest contacting your local sales representative for assistance with your specific application.

Question: At what rate is the first start arbitration token passed?

Answer: The PowerCommand control typically makes decisions at the rate of about 50ms.

Question: All of the paralleling on gensets seems to center on an approved and compatible controller. And also that all gensets have the same controller. Please comment.

Answer: Having the same control on all of the gensets in the system does make it easier. However, it is possible to parallel different manufacturers’ equipment together. I would suggest contacting your local sales representative for assistance with your specific application.

Question: In load management mode, is there a way to tell the system to start a genset before a large motor is started? This would allow one more set to be stopped in load management mode, right?

Answer: This would require an external control such as a system level controller.

Question: Are there limitations on the distance from the generator to the common point of coupling?

Answer: Yes, voltage drop could be an issue if you’re going long distances. I would suggest contacting your local sales representative for assistance with your specific application.

Question: How does the operator accomplish manual paralleling of multiple machines using this distributed control topography?

Answer: The operator would go to each genset control one at a time to start the genset then close its breaker.

Question: None of your diagrams showed ATS’s. Is that accurate?

Answer: ATS’s would be located downstream of the collector bus where the gensets parallel.

Question: If the breakers are set mounted, can the generator be pre-wired at the factory to get voltage sensing of the paralleling bus on the load side of the set mounted breaker, reducing the field wiring?

Answer: This is possible, but it depends on the application as well as the site requirements. I would suggest contacting your local sales representative for assistance with your specific application.

Question: Would there be any advantage to having electrical operated MV breakers on the point of connection and generator output?

Answer: The generator control would only operate one E/O breaker.

Question: How do you exercise paralleled generators with load bank? Gen mounted breakers?

Answer: This can be incorporated into the common point of connection (switchgear) by adding a breaker for a load bank. A remote connection point can also be installed so the system doesn’t need to be LOTO’d to make the connections.

Question: How do you introduce the required temporary source of power for maintenance or repair of the alternate source of power per NEC 700.3(F) if you are paralleling more than one generator? Would that apply to one of the paralleled generator as long as the emergency load is less than one generator’s rating?

Answer: The generators should be sized so that all legally required loads can be supported by one genset.

Question: Can generators from different manufacturers be paralleled using only onboard paralleling controls? Or does this require a paralleling switchboard?

Answer: That would depend on the manufacturers and controls used. I would suggest contacting your local sales representative for assistance with your specific application.

Question: Can generators in several locations be paralleled?

Answer: Distance would be a concern. I would suggest contacting your local sales representative for assistance with your specific application.

Question: What are the conditions for winning first start arbitration?

Answer: The genset must be at “ready to load” state, which is generally 90% rated frequency and voltage, the bus must be deenergized and the genset must have the “token” breaker close inhibit removed.

Question: How does this prevent dry stacking?

Answer: Wet stacking can be prevented by getting the engine temperature to the manufacturer’s suggested minimum operating temperature. If this cannot be achieved using building load, then a load bank may be required.

Question: For a given normal load, say 1MW, and a max load of 3MW, would you recommend having fewer gensets with higher ratings or more gensets with lower ratings?

Answer: The smallest genset in the system should be able to support all emergency and legally required loads. For example, if that would require 500KW, then that is the smallest size that should be considered. Beyond that, the main concerns would be how much floor space is available, fuel system constraints, etc.

Question: Can a system level control system be replaced with integrated controls on existing gensets?

Answer: On some systems it can, but this entirely depends on the existing system. I would suggest contacting your local sales representative for assistance with your specific application.

Question: Should the distributed controllers be backed up with UPS?

Answer: The genset controls are powered by the genset starting batteries.

Question: How quickly can gens typically sync to close to a common bus? Cycles or seconds? Is that complicated or lengthened with more than two generators in a paralleled set?

Answer: This generally happens very quickly. The first genset can typically be on the bus (depending on application) withing 3 to 6 seconds. The time it takes for the rest of the gensets to close to the energized bus depends on several factors, such as how far out of sync they are, their adjustments (which determine how aggressively they synchronize), etc. Increasing the number of gensets above two does not complicate this process.

Question: What do you change in the genset when asking it to shed some load? Is that droop set point?

Answer: That would be the load dump set point.

Question: Is system-level control required for “momentary” paralleling with utility (closed transition, 6-cycle overlap) at medium voltage?

Answer: Each manufacturer is different, but utility paralleling with a single genset can be done with an integrated genset paralleling control.

Question: If a decision is made to install a single genset due to cost, but additional sets MAY be added in the future, will modification to the original unit’s controls be required when wanting to later parallel?

Answer: Each manufacturer is different, but if this need is communicated during the ordering process, this can generally be accommodated fairly easily. As long as the correct control is used, changing from a single genset to paralleling would just require a program change to the control.

Question: What are passive close transfer pros/benefits?

Answer: An ATS set up in passive closed transition will wait to transfer until the two sources are synchronized. This allows the transfer to happen without interrupting the load. They typically have an adjustable time delay to revert back to open transition if the two sources do not come into sync within a certain amount of time.

Question: Does integrated PowerCommand control have the capability to control loads such as ATS and circuit breakers? Does it have bus optimization to add and shed loads depending on available gen capacity and load priority?

Answer: There is an integrated load shed level in the PowerCommand control. The control can also operate E/O breakers depending on the application.

Question: For a 480/277V paralleling system, do you need to use 4-pole breakers if the generators neutrals are individually bonded at the generator? The concern is performing maintenance on one generator while other generators are supporting load and neutral current flow.

Answer: The E/O breakers would be 3-pole. Where the neutrals are bonded would be determined by the electrical design engineer.

Question: With the control wiring between the generators and the wiring between the generators’ controllers and the human machine interface (HMI), is this all ethernet-type wiring, or do we also need analog and discrete wiring?

Answer: Manufacturers differ on their requirements. I would suggest contacting your local sales representative for more details.

Question: Is dead bus/dead field synchronization feasible with a Cummins PowerCommand controller, or is a master controller is needed?

Answer: Dead bus paralleling can be done by the PowerCommand control. I would suggest contacting your local sales representative for information for your specific application.

Question: Can we set certain loads to the specific gen set?

Answer:  With distributed logic architecture, the power to all loads is supplied by the emergency bus so the gensets on the bus are providing power. With this system, the only way to have a load supplied by a specific generator would be to only have that particular generator running and the others disabled.

Question: Is the control architecture by Cummins proprietary, or is this approach common to other manufacturers so that specifications/designs may be developed for open/competitive bidding of systems from multiple manufacturers?

Answer: The controls used by Cummins are proprietary. Other manufacturers have similar systems, but each do differ.

Question: Should the electrical-operated circuit breaker be located inside the SWGR in lieu of in the generator enclosure?

Answer: This differs depending on the application and layout of the site. I would suggest contacting your local sales representative for assistance with your specific application.

Question: What is the condition for winning first start arbitration?

Answer: The genset must be at “ready to load” state, which is generally 90% rated frequency and voltage, the bus must be deenergized and the genset must have the “token” breaker close inhibit removed.

Question: With distributed paralleling, can you mix gen manufactures?

Answer: It is possible, but the solutions vary depending on the equipment. I would suggest contacting your local sales representative for more information for your specific application.

Question: Beside the electrically operated generator circuit breaker, does Cummins also use a contact switch for the power output from the generator?

Answer: The E/O breaker is the means of connecting the genset output to the bus. Codes may require additional means of disconnect.

Question: Can a “new” paralleling switchgear use a generator-side breaker/PT to monitor a remote “old” paralleling switchgear to import the capacity.  How is this normally done? Alternately, does it make sense to have a “normal” breaker, etc., at the “new” gear, with new control/instrumentation on each legacy generator? What would be needed in between the gear? Is this something you have seen before?  What was the chosen solution?

Answer: When upgrading or adding new equipment to existing gear, the solutions can vary greatly. I would suggest contacting your local sales representative for assistance to determine the best path for your particular application.

Question: If there is a switchboard-based paralleling breaker, do you need a genset breaker?

Answer: You would have to refer to your local codes and standards.

Question: What is the maximum medium voltage generator ? 15kV or 34 kV?

Answer: 15kV.

Question: For speed control, you always talked about ECM. Is generator set-based paralleling feasible with electro-mechanical control generator sets (speed control made by genset controller only)?

Answer: Yes, different manufacturers may use different methods of speed controls for their engines.

Question: Does the integrated PowerCommand controller provide generator differential (87) protection?

Answer: Differential protection is typically provided by an external relay.

Question: Can you discuss a possible interface/connection to a plant DCS?

Answer: If you are referring to communications, some manufacturers have built-in Modbus communication in the genset control. I would suggest contacting the manufacturer to see what options are available.

Question: Can generator set-based paralleling be used to parallel generators of different sizes and different fuels (e.g., diesel and natural gas)?

Answer: Yes, it can.

Question: Why are 4-pole changeover switches recommended rather than 3-pole changeover switches?

Answer: The use of 3-pole vs. 4-pole transfer switches is decided by the design/electrical engineer.

Question: When we have 18 generators to work as a bank, does that arrangement need to be in temperature-controlled generator rooms, rather than having a pad-mounted generator in separate enclosures?

Answer: Either setup would work, but there could be other factors that need to be considered such as the distance between gensets. I would suggest contacting your local sales representative.

Question: In a fail to synchronize situation, can an operator manually synchronize from any unit controller, or does it have to be at the unit that is not synchronizing?

Answer: With a distributed logic architecture design, the synchronizer would be part of the genset control so the operator would need to go to the specific genset having issues.

Question: Is it the load shedding made by the BMS system or by the ATS controls?

Answer: A single load shed level can be set up from the genset control. If more than one load shed level is required, you would need a load management device such as a system level control. The signal is sent to the ATS, which causes the ATS to go to the neutral position removing power to those loads.

Question: For multiple genset designs, can a portion of the gensets be connected to supply local load and redundant genets be connected to the grid for utility time of day loading?

Answer: This can be done, however, more details would be needed. I would suggest contacting your local sales representative.

Question: Can a PowerCommand controller support ring bus with multiple ties and utilities breakers design? Does it support swing gen bus design where each paralleling generator has two sets of paralleling breakers?

Answer: The 3300 control can support these configurations, however, a system level control would be required.

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