Consulting-Specifying Engineer - January 2009 - (Page 43) be designated as the first unit to shut down as the emergency loads are restored to normal power supply source. The emergency generator control system can monitor the remaining emergency load and generator capacity and control the unloading of the generator units for a smooth shutdown sequence. Source transfer scheme For emergency load equipment vital to the protection of life and safety, the automatic restoration of electrical power within a prescribed period is required. For example, hospitals are required to have life safety and critical branch loads restored to operation in a maximum of 10 seconds. Noncritical emergency loads and essential equipment loads such as HVAC and elevators, which can withstand momentary power outages, typically are set up for either delayed automatic or manual connection to an alternate power source. An outage to this equipment can exceed 10 seconds. An automatic transfer scheme reduces load restoration time and removes the human factor from a high-pressure situation; however, the emergency power system tends to be more complex and difficult to troubleshoot when it does not operate properly. Manual operation provides the most flexibility, and the system is less complex. For manual operation, the emergency response is totally dependant on the operator, which means it may not be as reliable or responsive for the facility function requirements. Methods for transferring emergency loads between the normal power supply source and the emergency power supply sources include: in-phase transition, delayed (or opened) transition, and closed transition. In-phase transition is when the automatic transfer switch, equipped with an in-phase monitor, determines when to transfer the load from one source to another. The switch contacts operate in a “break-before-make” sequence. For delayed or opened transition, the programmed or set delayed transition transfer switch completely disconnects the load from both sources for an adjustable period of time to allow regenerative voltage to decay to a safe level prior to connections to the new source. By allowing motor fields to decay, nuisance tripping breakers and load damage System sequence of operation The following is an example of a sequence of operation for a typical emergency power system. 1. Normal utility power supply source fails. 2. The automatic transfer switch (ATS) or normal power supply switchgear relay (loss of voltage/frequency sensing) sends a start signal to the generator control system. 3. Generators start and parallel to bus. Normal operating voltage and frequency are achieved. 4. ATS senses the availability of an alternate power source or receives a permissive transfer signal from the emergency generator control system to transfer to the alternate source side. Emergency loads are picked up sequentially by the emergency generator’s bus in accordance with load priority setting. 5. The generator control system optimizes the control of the generators, shutting off and starting generators as needed. 6. The generator control system also should have the capability to control load shed operation, sending signals to load breakers and/or ATS devices removing loads from the emergency system in accordance with load priority setting to prevent generator overloading. This requires fast control processing as well as direct control of load serving breakers. 7. Normal utility power supply source returns. 8. ATS transfers back to normal power supply source. The transfer can be either an open transition (momentary power interruption) or a momentary closed transition (no power interruption), depending on the emergency load requirements and the function and feature of the ATS installed. 9. Generators cool down and shut down. are prevented. Delayed transition transfer is recommended by the National Electrical Manufacturers Assn. (NEMA) MG-1. A closed transition transfer is required in applications where loads are sensitive to momentary power interruptions. The switch contacts operate in a “make-before-break” sequence. This allows the seamless transfer of critical loads from one source to another by paralleling the two sources momentarily. The “make-before-break” operation is useful during testing of the engine generator under load and where a predetermined transfer to the generator is desired. Source paralleling duration is typically limited to less than 100 milliseconds (6 cycles). The closed transition operating mode requires that both power sources be synchronized in voltage, frequency, and phase angle within prescribed limits. Protective relaying is required to prevent paralleling of power sources out of synchronization. Protective relaying typically is not supplied with the standard transfer switch. The generator used with a closed transition transfer switch must be equipped with an isochronous governor. System paralleling An emergency power system with paralleling capability has tremendous advantages. The parallel capability allows for closed transition transfer, provides the ability to load test the emergency generator system without using load banks, and provides electrical peak shaving or demand management capability. To operate in parallel, the system voltage, frequency, and phase angle must be within prescribed limits. In other words, the generator units must be at the same speed and frequency, operate with the same frequency directional rotation, and produce almost exact sine waves. When paralleling sources, fault current contributions from parallel sources and protective relay protection requirements should be considered in the system design. Wagner is an associate chief electrical engineer and project manager at Stanley Consultants. With 20 years of experience, he spends much of his time developing and designing electrical systems for large power plant projects. Consulting-Specifying Engineer • JANUARY 2009 43
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