The Complexities of High-Voltage Power

05/01/2006


Most building electrical services are provided at secondary voltage to the main switchgear by the local electrical utility. However, large facilities, such as multi-building campuses, hospitals or telecommunications and computer data centers, typically employ customer-owned and -maintained primary power distribution and possibly standby or cogeneration systems.

The technical experience and equipment necessary to design, construct, commission and operate such facilities are of a higher order than for secondary power systems.

For example, a typical office building might have a low-voltage power system consisting of service entrance switchboards, panel boards, motor control centers, dry transformers, cables/bus ducts, and finally, generators and automatic transfer switches. The testing associated with start-up and commissioning for such facilities is straightforward:

  • Insulation resistance measurements of low-voltage transformers, power cables, bus ducts and switchgear.

  • Torque testing of switchgear-bolted connections.

  • Setting and testing of the ground-fault protection at the service mains.

  • Setting and testing the overload protection for motor starters.

  • Load bank and functional performance testing of generators and transfer switches.

  • Thermographic inspections of operating switchgear and panels.

However, with a primary power system, especially one that's facility-owned and -maintained, a much greater effort—and responsibility—is required.

In fact, local code enforcement, and the utilities themselves, will often require that an independent electrical testing agency, such as the International Electrical Testing Assn., be employed to test and certify that the installation is safe to energize and operate.

Size matters

The size range of primary power systems covers a broad spectrum. At one end of the continuum is a minimal primary power system with a single primary service to a metal-enclosed switch and fused piece of switchgear serving a single secondary substation. At the opposite end, the system could consist of multiple metal-clad switchgear units with multiple and redundant services and multiple feeders to redundant secondary substations with closed-transition switching between the utility and on-site generation.

The additional equipment for simple metal-enclosed switchgear, over and above that found in secondary services, includes the primary switch and fuse switchgear, primary power cables, high-voltage transformers and building ground systems.

Testing for metal-enclosed switchgear would involve all the same items as those for secondary services, plus the following additional testing for the primary switchgear, cables and transformers:

  • Contact resistance for the closed switch and breaker contacts.

  • Primary power cable DC hypot testing.

  • Ground resistance testing of the earth electrode systems.

  • Turns ratio testing (TTR) for high voltage power transformers.

  • Setting of protective relays or circuit breaker trip settings for secondary switchgear.

  • Thermographic inspections of completed and loaded switchgear.

A significant step up in complexity occurs when metal-clad switchgear is provided on a project. This type of switchgear can include the following equipment: vacuum breakers, current transformers (CT) and potential transformers (PT), metering and instrumentation, programmable-logic controller-based controls and protective relaying and station batteries. This type of equipment is more complex and requires a significant effort.

A typical start-up and commissioning effort would include:

  • Factory witness testing of major capital equipment including transformers, switchgear, generators and such.

  • Point-to-point wire check performed by the technicians after the switchgear control and instrument wiring is completed.

  • Continuity and phasing for polarity-sensitive equipment such as current transformer, potential transformer, meters, relaying and instrumentation wiring.

  • Turns ratio testing for current and potential transformers.

  • A thorough testing of each component in the switchgear, including insulation testing for circuit breakers, switchgear, transformers and cables. Testing should also include contact resistance measurements for breakers and bus connections. Functionally test and operate circuit breakers outside and inside of the switchgear.

  • Testing and calibration of protective relays in accordance with short-circuit and coordination studies.

Modern microprocessor-based relays could require a PC communication interface as host software to program and test the relay. Some multi-function electronic relays, which combine the functions of many relays, could require the setting and verification of hundreds of adjustable settings. The incorrect setting of even one point could impose a risk to mission, equipment and life. PLCs are often employed for switching and controls of substation equipment. This is especially true where load transfer or primary generators are involved. When component testing is completed, function and system integration testing is performed to verify proper operation of the equipment through the various operating modes.

Phantom loads

Switchgear PLC-based control systems can also be functionally tested without high-voltage power applied. This is accomplished through the use of simulated voltage and current signals injected through the switchgear instrumentation. This method permits the simulation of various switchgear operating scenarios through the use of phantom-source voltages and load currents to simulate real-time operation of the switchgear controls. The use of phantom loads permits testing of the operation without the risk of the high energy present when the switchgear is energized.

Typically, the phantom loads are provided through the use of multi-phase protective relay test sets that can inject current through the CT secondary circuits and potentially to the PT circuits.

The following example of phantom loads employed for low-power testing of an electrical system at a major hospital and research campus should be helpful. Power source equipment consisted of the following: A new metal-clad switchgear composed of three 1,200-amp, 13.2-kV primary spot network metal-clad switchgear employing six source feeders, two ties and 24 load feeders.

Using phantom loading of the switchgear protection and metering equipment, the commissioning team was able to functionally test and operate all of the switchgear relays, controls and instruments prior to the connection of the high-voltage sources and loads.

During the functional testing, we detected and corrected several wiring and PLC-programming problems in the switchgear. After corrections to the switchgear and a successful functional testing using phantom loading, we were able to transfer the source and load feeders to the new substation and be confident that the system would function as intended during the subsequent systems integration testing that was part of a biannual Joint Commission for the Accreditation of Healthcare Organizations (JCAHO) pull-the-plug test for the hospital.

The use of phantom loading to functionally test multi-function protective relays is of particular importance when the relaying incorporates functions that are phase-angle related. This would include reverse power, reverse VAR, impedance-type relays, etc.

For example, on another major campus project, the installation ran into start-up troubles when the user attempted to transfer the campus' power load to its new power source substation. The new source tripped and dropped the load and no one knew why as the contractor did the standard testing in the initial commissioning process. After investigation on our part, as third-party agent, it turned out that the relay was programmed for an incorrect phase relationship. (See "PQ Analyzer Saves the Day" below.)

Old school and new school

With large facilities owning and monitoring their own high-voltage power systems, commissioning has become a complicated affair that requires a thorough, old-school knowledge of how power system equipment works, and frankly, what test equipment is available to confirm that systems are working correctly. At the same time, it also involves a new-world knowledge of programming of computer-based equipment such as multi-functioned protective relays and PLC equipment.



PQ Analyzer Saves the Day

Using phantom loading to functionally test multi-function protective relays can prove critical, especially when relaying incorporates functions that are phase-angle related, including reverse power, reverse VAR, impedance-type relays, etc. Such functions, however, can prove tricky. For example, one installation where things went sour involved multiple primary service metal-clad switchgear units that served a major campus load. The new substation was placed into service by a contractor who performed all of the typical substation tests including secondary injection testing of the relays and metering equipment. However, some weeks after commissioning, as campus load was being transferred to the new substation, the new power source tripped and dropped the load.

RTKL was called to assist. The substation's main relay protection included a multi-functioned relay that indicated a reverse power trip. The relay had been successfully tested using secondary injection and was working within acceptable ranges. The wiring associated with the relay was retested and found to be as per the shop drawings.

To identify the problem, we employed a graphics power-quality analyzer with a polar display of three-phase, voltage and current and connected the unit in parallel with the tripped relay. We transferred some nonessential load to the new substation and observed the load current, voltage and phase angle on the power-quality analyzer polar display. Again all seemed in order and matched the expected delta-wye shift of 60 degrees, due to the hookup of the potential transformers, between the current and voltage as shown on the polar graphics display of the power monitor. The next step in our investigation included a complete review of the hundreds of various settings in the relay. Of particular importance were entries that related to phase-angle measurements. What we found was that the relay was programmed for an incorrect phase relationship of zero degrees between the current and potential instead of 60 degrees, such as the delta-wye configuration of the potential transformers.

The incorrect phase setting caused the relay to see normal through load, phase-shifted to appear as a partial reverse load. As the new substation load was added, it eventually reached the trip-set point. The relay programming was corrected, and the substation has remained in service without incident.



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