By using various modes of operation, engine gen-sets serve emergency needs while offering energy cost-cutting features.
Can you design an alternate power source to power an emergency system and an equipment system at a health facility while reducing electrical demand? The answer is a resounding "Yes." A recent renovation at West Georgia Medical Center in LaGrange, Ga. provides us a good example of such a design.
Early in the conceptual phase of the project, the hospital's representative involved the architect, consulting engineers, general contractor and subcontractors, and power company officials in the planning. The results speak for themselves; a project that fits the owner's needs, built on schedule, and on budget. The hospital's engineer, Richard Hutcheson, may have started a trend in the hospital industry in Georgia. As more hospitals face obsolete, inefficient, and difficult to maintain capital equipment, the West Georgia Medical Center's plant may become the envy of the industry. Lower energy bills, better dependability, and lower maintenance costs are powerful incentives. When the pay back measures a few years followed by decades of savings, the decision may already be made.
Code compliance issues. While renovating its essential electrical systems, the medical center had to meet the requirements of electrical construction and installation criteria of NFPA-70, Art. 517 (Health Care Facilities). It also had to comply with performance, maintenance, and testing criteria noted in NFPA-99 (Standard for Health Care Facilities). Both standards include a requirement for an alternate power source for serving essential electrical systems.
These systems included two separate networks: the emergency system and the equipment system. The former is for circuits essential to life safety and critical patient care (designated life safety branch and critical branch). The latter supplies power to major equipment (such as for areas with key HVAC needs) necessary for major patient care and basic hospital operation.
Humid summer days with temperatures above 95 DegrF are not infrequent in Georgia. And, a hospital suffering an extended power outage could get uncomfortable quickly. High indoor temperatures would cause patient discomfort and hamper infection control in operating rooms. Art. 517-34, (Equipment System Connection to Alternate Power Source) of NFPA-70 requires the control of temperature for heating in patient care areas. This usually means a control system, air handlers, and pumps.
The medical center's new environmental concerns for controlling temperature and humidity for cooling substantially increased its electrical load. This was in the form of large chiller motors and pump motors for the cooling tower. The facility needed an upgrade of its electrical system to handle the add itional loads. The upgrade resulted in additional engine gen-sets.
Two 1500kW gen-sets specified. At the beginning of our design process, we calculated the hospital's existing loads and estimated the future loads. The finding: The medical center needed at least 2000kW of generating capacity. We selected two engine generator manufacturers to secure price estimates and analyzed this pricing. We looked at two gen-sets at 1000kW each and compared that to two gen-sets at 1500kW each. We also looked at a third option: using three 1000kW units. The analysis (based on price per kW prime power and payback in years) showed the two 1500kW gen-set option as the most attractive.
Plans called for selling the existing engine gen-sets for salvage value. The new building (still under construction) had space available for a future (third) gen-set. This would allow for future growth and any possible rate structure changes benefiting the hospital. The manufactures of the gen-sets and intelligent switchgear specified had a good history of providing similar packages. They had to meet the delivery dates, or we would lose the first opportunity for installation during a planned curtailment of service. All the equipment fit together. The building and equipment were in place and ready to go when the time came.
Alternate power source details. The gen-sets each have an 1800-rpm, 12-cylinder diesel engine that powers a generator rated at 1500kW prime power at 480V.
We had the existing six automatic transfer switches (ATSs) (not shown in the figure because of space limitations) modified to meet new conditions. We installed a new ATS (ATS - EECP) for emergency equipment loads.
Personnel can remedy a single failed utility feeder at either the main building switchboards or the central energy plant switchboards by closing (after opening the failed utility breaker) the tie breaker located at the two ends of the two-part bus. Here, the facility may not need the gen-sets.
If both utility mains fail, then the voltage sensing equipment at the ATSs will signal the gen-sets to start. The mode of operation upon loss of utility power at both main building utility breakers (MUA and MUB) or at the central emergency plant breakers (CUC and CUD) follows:
Step 1: The two utility main breakers open, the tie breaker MTI (or CTI) closes, and the emergency main tie GMM (or GMC) closes to allow generator power to go to all the loads connected to the main building switchboard (or the central energy plant). All ATSs lock into emergency mode to provide power from the generator(s).
Step 2: The two gen-sets start upon signal. Both sets automatically and independently start, accelerate to rated frequency, and build up to rated voltage. The First Start sensing unit monitors this process. On finding a gen-set at 90% of rated voltage and frequency, they first disable the other unit from closing to the two-part emergency generator bus on emergency switchboard ES. Then the monitors close the gen-set to the ES bus.
There are priority one loads connected to each of the two sections of the ES bus. The 3000A emergency tie breaker (ETI) stays closed, keeping the two-part bus connected. Step 3: The priority controls prevent overloading of the emergency system's two-part bus by inhibiting operation of selected loads until each of the two gen-sets close to its own section of the two-part bus, or until actuation of a priority override switch.
Step 4: After the first gen-set closes to its side of the emergency bus, the control of the second gen-set switches to the synchronizer of the paralleling control system. This generator then synchronizes with the emergency bus and, at that time, connects to it. When the second generator is providing power to the two-part emergency bus, the priority control unit then allows the system to add further emergency loads to the bus. Each gen-set then assumes its proportional share of the total load on the two-part bus.
If one of the gen-sets fails to start, the system initiates another mode of operation. The overcrank time delay in the gen-set's control unit times out, and the unit shuts down. Also, an audible alarm activates. The priority control system prevents the lowest priority loads from being added to the two-part emergency bus (this being done without manual intervention).
If there's a failure of the synchronizing unit of a gen-set after a preset time delay, an alarm sounds but the unit will continue to attempt to synchronize until signaled to stop by manual intervention.
If there's failure of a gen-set, or if the two-part bus becomes overloaded, various modes of action take place. First, both the alarm lamp and horn activate. If the failure causes a bus overload, a load shed signal initiates load shedding of the emergency system, beginning with the lowest priority loads. If the closed two-part emergency bus does not return to proper frequency within a predetermined time, additional load shed signals generate.
Operation under load demand mode. There's also a load demand mode of operation to conserve fuel and extend the life of the gen-sets. With them running in a load demand mode to reduce energy charges (not emergency mode), the control microprocessor continually monitors the total load on the bus. If one gen-set can safely carry this load on the bus, the controller automatically shuts down one of the sets in an operator-established sequence. This allows the remaining unit on the closed two-part bus to operate closer at the gen-set's rated capacity. This mode reduces fuel consumption, injector fouling, and wear on the system.
Upon sensing the reaching of reserve capacity on the closed two-part bus, or on failure of the operating gen-set, the idle unit automatically starts, parallels, and assumes system load.
For the load demand mode of operation, the system has a locked back-panel-mounted Enable-Disable switch. Currently, this switch is in the OFF position. Should the utility have a need for power, both generators will start and run at prime power rating to allow export of power to the utility lines. However, emergency power requirements are always prioritized.
Return of utility power at both mains. With restoration of power to both utility main breakers (for either the main building or the central energy plant), there is a time delay for retransfer of power at the ATS to ensure the utility source is stable. After the time delay, the emergency generator bus synchronizes and connects to switchboards MSBA, MSBB, MSBC, and MSBD. Separate tie breakers connect the buses of the first two switchboards and those of the last two switchboards. In turn, these buses connect to the closed double bus of the emergency switchboard via the power feeder breakers GMM and GMC.
After a few minutes of power provided by the utility, the ATS "all start signals" are removed from the Master Totalizer Controller. The ATS then transfers the emergency load back to the normal source. The generator paralleling breakers (which connect the generators to either end of the two-part emergency bus) open, and the involved emergency main breaker (GMM or GMC), also opens, leaving the utility source to supply power to all the connected loads.
The gen-sets run unloaded for a cool down period. When the this time delay expires, the units shut down, and the system is again ready for operation.
Return to utility power at either main. When utility power returns to one of the two main building utility switchboards (MSBA or MSBB and MSBC or MSBC), there is a time delay for retransfer from generator power to utility power to ensure the utility source is stable. At this time the tie breaker connecting the two switchboards is still closed and the two utility breakers (MUA and MUB or CUC and CUD) are still open. After the time delay, the emergency generator bus synchronizes and connects to the live utility source through the live utility breaker (either MUA or MUB for the main building; CUC or CUD for the central energy plant), whichever is energized. The tie breaker connecting the two switchboards remains closed.
During closed transition of power, the load on the generators decreases automatically using generator loading controls. The paralleling breakers (which connect them to the two-part emergency bus) and the emergency main breaker (GMM or GMC) open, leaving the utility to supply power to all the connected loads.
The gen-sets then shut down after a short cool down period. Should the utility source fail during this period, the sequence of operation is as described under loss of utility power failure.
Peak shaving mode. To take advantage of the new utility rate, we developed a special peak shaving mode of operation. For this special mode, we made sure such operation would in no way interfere with the emergency mode of operation per NEC Sec. 517-30(b)(5) and Sec. 700-5(b).
The gen-sets receive a remote signal when placed in the peak shave mode, and both automatically and independently start. The units accelerate to rated frequency and build up to rated voltage. After the two gen-sets connect to both sides of the generator bus, this bus synchronizes and connects to the utility source bus through the emergency main breaker serving the central energy plant (GMC).
By using the VAR power factor controller and the engine governors, the load gradually ramps from the utility source to the gen-sets to a preset utility power import level or a maximum gen-set power level. The system continues to operate in this mode until the peak shave signal is removed or an alarm condition is experienced. Should the system lose utility power, the gen-sets automatically go into an emergency mode of operation.
Emergency mode is prioritized. To manually remove the gen-sets from peak shaving mode, personnel return the generator system test switch to the NORMAL position. With the peak shaving signal removed, the system initiates gradual unloading of the gen-sets to a sufficient load level. The emergency main breaker (GMC) and the gen-set paralleling breakers open. It then goes through a cool down period.
System NO LOAD test mode. With the system test switch placed in the NO LOAD position, both generators start and synchronize to the emergency bus, but both emergency bus breakers (GMM and GMC) stay open. Also, the ATS stays connected to the utility. This action results in no load being connected to the emergency bus. Returning the system test switch to NORMAL causes the gen-sets to open their breakers and go into cool down as described previously.
System ISOLATED test mode (closed transition). The purpose of this mode of operation is to carry out a full test of the emergency system via transferring the critical loads to the generators, isolating these loads from utility power. With the system test switch in the LOAD mode and the transfer mode switch (located in a control console adjacent to emergency switchboard ES) in the CLOSED position, the gen-sets automatically and independently start and accelerate to rated frequency and build up to rated voltage.
After both gen-sets connect to the generator bus, the bus synchronizes and connects to the utility source bus through the emergency main breaker (GMC) serving the central energy plant. Load then gradually ramps from the utility source to the gen-sets. When the utility's load reduces sufficiently, the system signals the utility breaker to open, leaving the generator system supplying power to the central energy plant loads. Returning the test switch to NORMAL causes the system to perform as described above in the system NO LOAD test mode.
Manual operation. Personnel can start the two units, adjust their frequency, and place them in parallel operation. The load transfer circuit breakers are also capable of manual control, when the selector switch is in the MANUAL position. Electrical interlocks ensure the utility and generator system sources can't be manually paralleled.
We coordinated and planned to develop the design and specifications. We had to modify the existing automatic transfer switches for use in the load shedding process, should one generator fail to run. Also, the interrupting capacities of the breakers had to include the fault current available from both sources, since operation is effectively closed-transition. Electronic metering allows the hospital engineer to meter all facets of the system from his desk-mounted computer, as well as at the main switchboards and emergency switchboard locations. In addition, we included some conventional meters on the emergency switchboard for running and testing the gen-sets.