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Control of Emergency Generator

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  rev. 2008-11-25        

Existing Conditions

The present emergency power system consists of a 1250 kVa diesel powered generator.  The generator feeds power to the entire facility through the 4160 Volt distribution system.  The system goes into operation 2 seconds after the utility power supply fails.  As the generator comes up to the appropriate speed and frequency, the transfer switch engages and the feeds the facility distribution system (and isolates the feed from the utility).

A load shedding control system manufactured by Siemens Westinghouse supervises the generator system.  Once the generator is up to speed and the transfer switch has been engaged, the load shedding system takes control.  The system monitors the generator output current and initially drops all loads.  This is accomplished through the control of 7 feeds to individual buildings in the facility. The system presently has the capacity for three additional control points.  The control system then restores power to the 7 feeds in a step by step process which takes approximately 60 seconds.

Once the initial sequence of operations have elapsed, the supervisory control system will drop the 7 feeds in order beginning at 88% of rated generator kVa up to 105% of rated generator kVa.

Retrofit Conditions

The emergency power system has to be upgraded to be able to provide the necessary standby power in the event of a utility outage.  Ideally the system would require a minimum of manual staff intervention, maintain the telephone systems and not have any adverse effects on resident comfort.

Addition of Generation Capacity

The first option is to add generator capacity to the facility.  A 200 kW unit would provide the additional capacity to provide uninterrupted power during a summer utility outage with a minimum of load shedding.  Budget costs for installing this unit are approximately $70,000.  This report also contains a recommendation for installing a 200 kW co-generation system for the kitchen area.  The measure has a financial payback of over 10 years.  If the HRC opts to install additional generation capacity, utilizing a 200 kW co-generation system may be an option.

Utilization of Proposed Building Automation System

A facility wide Building Automation System (BAS) is part of the energy management recommendations.  The proposed automation system will have direct digital control (DDC) over all of the significant HVAC loads in the facility.  The system could be used to duty cycle loads and prevent the facility electrical demand from exceeding the generator capacity during a utility power outage.  The existing load shedding control can remain in place in order to provide a redundant level of system protection.  In the event of BAS failure the load shedding supervisory control will remain in place to prevent generator overload.

A sequence of operation for power failures could be part of the BAS program.  Rather than shedding buildings totally, the system could enact a duty cycle sequence of operation and shut off selected loads individually throughout the facility.  For example the duty cycle rate could be set at 50%.  This would mean that selected equipment is on for 15 minutes then off for 15 minutes.  Individual loads within a building would offset each other.  For example Building 17 has four rooftop air conditioner units.  They would be sequenced to operate in pairs effectively cutting the connected air conditioning load in the building 50%.   As a result the building is not totally without air conditioning.  Also allowing a 30 minute operation time per hour would in most cases still provide a comfortable environment for the building residents.

The automation system could individually duty cycle air conditioners, pumps, exhaust fans, supply fans, selected lighting circuits (ie street lighting) in order to reduce facility electrical load.  This method could reduce connected load by 663 kW in the summer.  This would limit the peak connected load to 1000 kW.  The winter load could be reduced by 10 kW.  This would limit the peak connected load to 950 kW.

In addition to the BAS duty cycle control, a utility power outage usage protocol could be developed for each building.  The protocol would dictate which lights, appliances and electrical devices should not be used during a utility power outage.  The lighting and miscellaneous electrical demand in the facility totals 639 kW.  It could be reduced by 1/3 during a utility power outage, the facility electrical demand could be reduced by an additional 200 kW.

Employing this method would leave a peak connected load of 800 kW during the summer months and 750 kW during the winter months.  This is well below the existing generator limit of 1,000 kW.

Further Benefits



Application Details


Issues and Concerns

The generator performed flawlessly during the blackout of August 2003.  In fact, it ran for a period of several days after the event due to the possibility of rotating utility blackouts.  The concern is that the peak electrical load of the facility exceeds the generator capacity.  The generator is capable of 1,250 kVa of continuous output.  The utility measured peak load for the facility is approximately 1,400 kVa in the summer and 1,000 kVa in the winter periods.  It must be remembered that these utility measured values are a 15 minute rolling average.  Short term loads are higher.  The equipment inventory indicates that the actual peak loads for the facility are 1,622 kVa in the summer at 1,000 kVa in the winter.

This was evident during the blackout as staff had to manually shut off power to various buildings in order to ensure that resident areas had air conditioning.  This presented problems as the power interruptions impacted telephone and fax functions, food orders, deliveries, resident discomfort and consumed staff resources.






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