Paralleling Gensets in the Digital Age
Editor's Note: This is the second installment in a two-part series. The first part appeared in the Winter 2004 issue of Pure Power, p. 35. Parallel generator systems offer facilities broad options for ensuring reliable power supplies. But, as discussed in the first part of this article (see Pure Power 12/04, p.
Parallel generator systems offer facilities broad options for ensuring reliable power supplies. But, as discussed in the first part of this article (see Pure Power 12/04, p. 35) difficulty in synchronizing control of multiple generators can make such approaches too expensive for all but the most critical applications. New digital controls, however, can lower installation and maintenance costs.
Bringing Functions Together
The first step in creating an integrated parallel generator design is to combine oversight for all generator functions—speed governing, voltage regulation, genset alarm and monitoring, synchronizing, load sharing and protection—into a single digital controller per generator. This consolidation significantly changes the issues surrounding parallel generation. What was a complex system becomes a simple "plug and play" module. No more hardwiring multiple controllers together. No more difficult calibration processes. No more inherently unstable control loops. No more pulling I/O points back to the master programmable logic controller (PLC), just to secure basic supervisor monitoring capabilities.
This single-controller-per-generator approach significantly enhances system performance, replacing inherently unstable load-sharing methods with stable control loops. Synchronizing processes are greatly enhanced by directly interacting with frequency-control functions. Troubleshooting becomes a simple process of monitoring inputs and outputs using a laptop computer. Repairs that took hours or days are reduced to minutes, using an on-site spare controller and a simple plug-and-play approach. Supervisory control and monitoring are also simplified.
Digital controls also improve reliability. A hardwired maze of different manufacturers' analog and digital controllers is replaced with simplicity. Reducing the component count alone drives reliability through the roof. Furthermore, all control functions are contained on a single CPU, which can be hardened from the effects of environmental degradation, mechanical stresses and electrical interference.
Integrated Paralleling Switch
Step two in this process is to integrate the paralleling switch function into the generator connection box, removing the cost and space of external switchgear. Once a generator is synchronized, the generator controller issues a close command to a paralleling switch connecting the unit to the generator bus. Historically, this switch is a motor-operated breaker located in a large metal cabinet and connected to a bus bar.
With an integrated paralleling system, the paralleling switch is a high cycle-rated contactor specifically designed for switching power circuits, instead of a breaker designed as an overcurrent protective device. The paralleling switch is mounted on, and wired directly to, the generator, resulting in greater system integration. The paralleling switch is then cabled to a common point that is typically a generator distribution panel, replacing the functionality of the generator bus bar inside traditional switchgear.
Various automatic transfer switches (ATS) are fed from the generator distribution panel. For single transfer switch applications, the wiring from the paralleling switches would be terminated directly to the transfer switch generator terminals. The end result: a multiple-unit system at reduced complexity and cost.
Getting Rid of the Wires
Traditional switchgear uses a PLC to coordinate operation of every generator and ATS in the emergency-power system. An integrated paralleling system also needs a system controller for coordinating such functions as starting and stopping the generators, priority loading, load shedding and data collection for supervisory control by building-management systems. However, almost all communication is digital in an integrated scheme, eliminating the hardwiring required in many traditional systems. Also, the system controller does not have to perform relay logic to sequence multiple additional controllers.
Operationally, each ATS monitors utility voltage and signals the system controller upon loss of utility supply. The system controller in turn communicates digitally to the integrated controller on each generator to control unit starting and stopping, along with sequencing for paralleling to the generator bus.
Bringing it All Together
To understand the operating sequence in an integrated application, consider an automatic start sequence initiated by a utility failure. Consider, for example, an emergency power system with two generators and two ATS. The generators are connected to the system controller via a single RS485 data line. A two-wire start line is run from each ATS similar to any single-engine standby solution, except the connection is made to the system controller.
In this illustration, the critical-load ATS is configured to pick up load within ten seconds of an outage. The transfer switches sense utility-power loss and provide a two-wire start signal to the system controller, which, in turn, sends a start command to all the generators in the system. The generators start and accelerate to rated speed. The system controller gives the first generator that reaches rated voltage and frequency permission to close onto the dead generator bus. Upon sensing the energized generator bus, the critical-load ATS will transfer onto generator power.
At this point, with one generator on the bus, the second ATS for equipment load is prevented from transferring onto the generator bus by a priority-loading feature built into the system controller, thus preventing an overload of the first unit onto the generator bus. With the generator bus now energized, the remaining generator must synchronize to this power waveform before it can switch onto the bus for parallel operation. The integrated generator controller manages this process.
As additional generators parallel to the bus, the system controller compares available capacity to expected load. Load is added in order of priority only when sufficient capacity is available. Additionally, if a generator fails to start or fails during operation, load equal to the lost generator capacity remains offline or is removed from the system. Such load shedding can be performed within the ATS or via a shunt-trip circuit breaker within the facility's distribution system.
Call to Action
The benefits of parallel generation are widely accepted in the marketplace; however, implementation has been limited. As mentioned, this is largely due to the constraints posed by traditional solutions, including cost, space, issues of single source responsibility and design complexity.
To meet market price and performance expectations, generator suppliers should migrate to an integrated digital-control philosophy and relocate paralleling-switch function onto the generator. Through these steps, paralleled power solutions can compete cost effectively against single engine/generator price points while maintaining parallel-generation benefits.