The PSS DG project included replacement of the existing transformer with a new one, installation of new 4000 amp paralleling switchgear, control modifications to all six transfer schemes to allow redundant back up of existing engines, installation of a 1500 KW engine with remote radiator, building addition to house the project, and the removal of a local radiator at (1) diesel generator position with remote radiator installed in replacement. As the existing electrical system was served through (1) utility source, all activities were scheduled as zero tolerance in downtime / interruption of the critical load.

THE DESIGN BUILD FORMULA

PSS Design Build contracts include incremental procedures designed to maximize the clients ROI. For the CRHS DG project, our first step was an audit of the clients electrical utility account, to evaluate contractual alternatives / savings opportunities they might qualify for. This process revealed savings opportunities were available using diesel generators in conjunction with an alternative rate schedule. The next step was a comprehensive Feasibility Study, with an objective of qualifying a formal firm proposal, establishing the project financial pro-forma, and establishing conceptual design. Study submittals were to define risk and reward associated with minor investment / modifications of existing equipment, as compared with a DG scheme capable of carrying the entire hospital campus. 

The PSS Feasibility Study scope of services and deliverables included; 

• Generation of a new “as built” electrical single line. A physical walk down to establish an “as built” facility single line, including the Normal bus / Critical Care / Life Safety branch distribution was conducted over the initial 12 weeks of the project. Field efforts also served to accumulate the necessary information required to model the system within Load Flow, Short Circuit, and Protective Device Coordination studies affiliated with a “paralleling” system design.

• Evaluation of existing load profiles and power quality. A Power Quality Analysis of the system load profile was established to benchmark existing conditions for comparison to revised conditions upon power plant commissioning. Historical loading patterns at the utility service entrance, service provisions for future expansion, and seasonal load flow models were established to ensure compatibility between existing and recommended equipment. A composite assessment of this data finalized an optimum set of equipment recommendations.

• Evaluation of the existing power scheme. An evaluation of the existing generator schemes serving Critical Care / Life Safety branches was conducted. Two emergency generators served a total of (6) transfer switches, but were not configured to allow bus transfer between generators serving life safety / critical care loads. Although a single layer of redundancy met code, CRHS wanted to ascertain the feasibility of reconfiguring each engine to back each other’s transfer switches and critical loads. This was to address concerns that in the event an engine should fail in the absence of utility source, critical care / life safety loads would be unsupported. Due to capacity constraints within the existing distribution system, modifying existing facilities for manual transfer of prioritized load was considered, but not feasible. As approximately half of the hospital normal bus was not backed up by either generator, and as a second redundant source was desired for emergency and life safety loads, a third generator of sufficient capacity to (simultaneously) carry the normal bus and critical loads was recommended.

• Conceptual Design and Equipment Specification. Conceptual design, equipment specifications, and associated project financial pro-forma was provided.

• Utility Contract Evaluation. DG projects include an audit and review process of historical utility contracts, billing statements, and alternative rate structures an account may qualify for. From a composite assessment of the above, the savings estimate associated with our recommendations was qualified. A proposal for a turnkey paralleling generator scheme was then compared with savings estimates to establish a project ROI. A new contract with the utility was brokered, and served to establish funding justification per cumulative savings / billing reductions over the ensuing 60 months. 

ANALYTICAL STUDIES

Load Flow, Short Circuit, Coordination Study, and Power Quality studies were conducted to gain an understanding of the needs of the system to coincide with upgrade recommendations. The Load Flow study determined the hospital loading curves of normal and critical bus systems. Peaking load profiles split at approximately 700 KW (critical bus) / 800 KW (normal bus) respectively. Bracing and equipment interrupt ratings of existing equipment were profiled to compare with the additional fault current contributions the system could see during paralleling sequence. This ensured both existing and pending equipment was adequately rated for the revised system complexion the additional generator would create. 

As the project was to be a Paralleled Power Scheme (see below information) between the new generator and utility, a comprehensive protection scheme was developed to address all conceivable utility / local fault scenarios. A protective device coordination study was employed to ensure selective protection and operational integrity under conditions of duress. Prescribed settings provided coordination between the service entrance utility fuse, paralleling load bus, and downstream branch circuits. 

In order to ensure conformity with IEEE Standard 519 - 1992, harmonic analysis and a power quality profile of the existing system was performed. Harmonic distortion of voltage and current integers was found in compliance at the point of common coupling (PCC) with the servicing utility. The PQ profile revealed circulating ground currents, however, which were remedied prior to project implementation. Ground currents were attributed to faulty wiring practices, as multiple neutral to ground bonding terminations were found within the clients distribution system.

PARALLELED POWER SCHEMES

When two or more electrical sources ( i.e. utility transformer and site generator) simultaneously serve the same load, it is described as a “Paralleled Power Scheme”. “Paralleled” or “Closed Transition” power schemes use multiple sources to accommodate “seamless” load transfer. Upon transfer of the load from utility to generator, the utility is disconnected, but still available, after transition. After the desired run interval, the utility is automatically connected back to the Hospital, load is removed from the generators, generators automatically disconnected, and returned back to “stand-by” mode of operation. The second source, either utility or generator, is always available.

To ensure stability between sources, careful consideration of protection scheme design is paramount to ensure operational integrity, stability, and safety. “Relays” are devices used for protection of equipment and operations personnel. When energized by suitable currents, voltages, or both, these devices respond to the magnitudes and relationships of current and voltage to indicate or isolate an abnormal operating condition.

Conway Regional Relaying Scheme
(click on image to enlarge)

The protective relaying design for the CRHS paralleling scheme addressed all conceivable scenarios required to protect both utility and hospital equipment and personnel. The relays employed for this project, and a description of each device's protective function and task, are as follows:


ANSI Device #25: Synchronizing Relay. CRHS is typically operated within an automatically controlled sequence for paralleling breaker closure. Synchronization between the utility bus and generator bus is accomplished by this device. This relay senses the differences in phase angle, voltage magnitude, and frequency of the sources, and initiates corrective signals to adjust the generator frequency and voltage until both systems are in sync. The relay then sends a permissive signal to close the generator breaker or the utility breaker, contingent on whether the generator is closing into the utility, or the utility breaker is closing into the generator. The relay actuates the permissive logic to close the breaker in advance of the generator coming into synchronism with the utility, so that when the breaker is closed, the systems will be exactly in sync. 

This timing is accomplished per a speed matching relay element, which senses the sources and adjusts the governor of the generator with raise or lower signals. This action matches the frequency of the generator with the frequency of the utility bus. The same principle is applied to match the voltage of the two systems; a voltage matching relay element compares the running system and incoming generator voltages and provides raise or lower signals to the generator excitation so that its voltage matches that of the utility bus.


ANSI Device #27: Undervoltage Relay. This relay is used to initiate transfer of the CRHS bus from the utility (the normal source), to any combination of the CRHS generator positions. When the voltage of the normal bus falls below a predetermined value for a predetermined time delay, this relay will initiate start signals to all stand-by generators. A time delay for this device is coordinated with the utility substation breaker / line recloser’s. This provides ride through of momentary faults originating on the utilities medium voltage distribution grid. 


ANSI Device #32: Reverse Power Relay. This style of relay is used to prevent power flow (in watts) of some predetermined level for a predetermined time interval in a direction opposite to normal system operation. This relay is applied between the generator and the generator breaker, to protect the generator from becoming a motor. This can occur when a prime mover loses its input energy (a throttle valve closes), and the generator breaker is not opened. This condition could cause system instability due to reactive var swings, or possibly cause the engine to catch fire / explode. 


ANSI Device #40: Impedance Relay. This relay senses when the generators excitation has been lost. Should a generator lose its field excitation, it will continue to operate as an induction generator, obtaining its excitation from the utility. Loss of excitation would cause the generator rotor to overheat due to the slip-frequency currents induced.


ANSI Device #46: Negative Phase Sequence Relay. Unbalanced loads, unbalanced system faults, open phase conductors, or other unsymmetrical operating conditions result in an unbalance of generator phase voltages. The resultant negative sequence currents induce current in the generator rotor, which cause overheating. This relay is included to provide protection from said conditions both local to the CRHS bus, as well as the utility distribution circuit. It also serves to address the utilities concerns of prohibiting the CRHS generator from paralleling a “dead bus” upon loss of utility source. 


ANSI Device #50/51: Instantaneous / Timed Overcurrent Relays. These two relay characteristics (Instantaneous / time delayed trip) are provided within a single “Overcurrent” relay. This relay is used to trip a respective circuit breaker when current exceeding the desired magnitude flows within the power system. 


ANSI Device #51G: Ground Timed Overcurrent Relay. This style of timed overcurrent relaying is used for protection from both transformer and generator ground faults. The service entrance relays also provide back-up protection to (existing) downstream ground fault relaying. As the utility company does not employ differential (87) scheme relaying for the service entrance transformer, the 51G provides the only local protection against a service entrance load bus ground fault. Said relaying also provides protection for transformer & generator winding / terminal ground faults.


ANSI Device #51V: Backup Overcurrent Relay. This style of timed overcurrent relaying utilizes voltage restraint, which provides an additional level of distinction between no fault conditions (overload) and abnormal fault conditions. The voltage element is constructed in such a way that it applies a torque that opposes the operating torque produced by the current coil. The linear relationship between voltage and current is in turn scrutinized to distinguish between actual fault current and normal system overloads.


ANSI Device #81: Over / Under Frequency Relay. This Relay monitors the status of the utility frequency. It serves to open the utility breaker in the event the system frequency deviates above or below predetermined acceptable levels. 


ANSI Device #87: Differential Relay. This relay provides the means for rapidly detecting internal generator phase-to-phase or phase-to-ground faults.