High-voltage electrical emerges as area of increasing concern among GT users

Until relatively recently, the technical programs at user-group meetings focused almost exclusively on gas-turbine problems and solutions. But as issues were resolved and items crossed off punch lists, the reliability and availability of the basic engine improved to the point where other plant equipment was at least as likely to cause an outage—more so in some cases.

New topics appeared on meeting agendas. CTOTF™ may have been the first user group to expand beyond the engine; its willingness to add sessions to cover the total-plant information needs of supervisory and management personnel made this possible. Under Former Chairman Bob Kirn’s leadership, the program expanded to include environmental, NERC/FERC, and high-voltage (HV) equipment roundtables, etc. Some others evidently believed the idea a good one and followed suit.

At its last spring meeting (April 2015), CTOTF invited NAES Corp’s Chief Engineer, Bill Lovejoy, PE, to provide guidelines on the inspection and maintenance of isophase bus (IPB). (Access details in CTOTF’s Presentations Library.) Neglect of this component at some plants has contributed to avoidable outages.

A month later, Gary Whitehead of Electrical Builders Inc, a service firm operating in the HV space, spoke about the causes and prevention of bus failures at the 7F Users Group meeting in Denver.

Then, at the 2015 meeting of the 7EA Users Group in Santa Fe last November, Bruce Hack of Crown Electric Engineering & Manufacturing LLC, which designs, fabricates, installs, and provides field support for HV bus reviewed the various equipment options available to owner/operators for both new and retrofit applications.

Given user interest, and the success of the start-up Generator Users Group, which held its first meeting the week before the 7EA conference, perhaps it’s time to launch an HV Users Group—to share information on all the equipment between the generator and the transmission line. Recall that when the industry was regulated, this equipment generally was maintained by the utility’s electrical department. Deregulation has transferred responsibility to generating-plant personnel in most cases and they typically do not have deep technical backgrounds in HV work.

Hack began his 7EA presentation saying that in the electrical industry there are few products as rugged and reliable as bus duct, and among the bus-duct options, there is no product with the integrity and long lifetime of isophase bus. He then put up on the screen a typical system drawing to get everyone in the room on the same page. In Fig 1, note that electrical energy produced by the generator is moved via IPB to transformers that boost voltage for export to the grid and via segregated-phase bus for in-plant use.

HV elec Figs 1-5

Hack next reviewed the three basic types of bus construction described in the ANSI/IEEE Standard for Metal-Enclosed Bus (C37.23-2003):

      • Isolated phase. In this arrangement, the conductor for each phase is enclosed by an individual metal housing separated from the adjacent conductor housing by an air space (Fig 2). The bus may be self-cooled or forced-cooled.

      • Segregated-phase bus. Conductors for each phase are housed in a common metal enclosure but separated by metal barriers between the phases (Fig 3).

      • Non-segregated phase bus. All phase conductors are in a common metal enclosure without barriers between the phases. There are two configurations: rectangular (most common, Fig 4) and circular (Fig 5).

IPB is specified for applications with highest voltage and ampere requirements. One leading manufacturer designs its IPB for voltages between about 15 and 38 kV and ampere ratings to more than 40 kA. The same supplier offers segregated- and non-segregated-phase bus for 600-V to 38-kV applications at ampere ratings up to 8 kA. Another manufacturer of segregated-phase bus offers ANSI ratings up to 12 kA for 600-V class bus and up to 6 kA for 15-kV class bus.

Reinforcing his belief that isophase bus is the “top of the line” but perhaps not economically justifiable at amperages below about 8 kA, Hack’s goal was to make users aware of circular non-seg bus as an alternative to the conventional rectangular bus configuration illustrated in Fig 4. He said the circular design, pioneered by Westinghouse Electric Corp in the 1970s, offers most of IPB’s advantages while avoiding issues often associated with rectangular non-seg bus.

To set the stage, Hack asked how many attendees have had issues with non-seg bus. A few hands went up. Next question: Has anyone had issues with IPB? No hands visible. Point made.

HV elec Figs 6

TheHV elec Figs 7n, to illustrate some of the problems identified with conventional non-seg bus, the speaker put up a couple of ugly photos (Fig 6) and offered the following reasons for why such damage might occur:

      • Neglect was at the top of the list. Many plant employees had not been made aware of inspection and maintenance requirements of bus duct.

      • Design was a mitigating factor. As a rule, Hack said, bus duct is supplied in sections of about 8 to 12 ft in length. Reasons: Copper and aluminum conductors typically are offered commercially in this size range and manufacturers have invested in facilities and tooling compatible with it.

With such short sections, each of which is joined to the next by an array of splice plates and bolts that require torqueing, micro-ohm resistance measurements, and insulation with tape or boots, a nominal run of say 100 ft might have as many as 15 or so joints for each of the three phases. All of these joints, plus the associated flexible braid connections to the capital equipment (Fig 7), demands inspection and maintenance at regular intervals.

      • Cycling impacts. Conductors historically have been insulated with sleeving or heat shrink, both performing well if not mechanically stressed. But stress is what some designs caused by bracing conductors using a method known as “choking” the bus. Specifically, the need to brace the bus to accommodate the specified fault forces, required that heavy strips of GPO-3 (red insulating fiber material) or Micarta (for older bus) be bolted to the tops and bottoms of the conductors and then to bracketing angle supports on the inside wall of the bus duct.

Over time, rubbing caused by thermal expansion and contraction (a particular concern in plants powered by gas turbines and starting daily), wears away conductor sleeving and ultimately can fail the insulation at each location so affected. Hack also mentioned that GPO-3 is hygrocopic. If this material gets wet and dirty, a tracking path to ground is created.

      • Bus-duct enclosure section joints create opportunities for water to enter the bus and puddle, possibly saturating insulation. In cold climes, slow-building icicles develop and eventually could flash over.

The circular non-seg bus described by Hack avoids the issues identified above by design. It is of welded all-aluminum construction, the same as the industry standard, isophase bus, and makes it virtually impervious to contaminants. It also promotes rigidity, allowing transport and installation in sections 40 to 50 ft long (Fig 8). The inherent rigidity also minimizes structural support requirements (Fig 9).

Conductors, supported by high-strength porcelain, are air-insulated. The only regular maintenance required—annually or less frequently—is to wipe down the insulators. Both the conductor and enclosure use effective shunt (Fig 10) and expansion (Fig 11) assemblies similar to those found on IPB.

HV elec Figs 8-11

Hack conceded that welding of the conductor and enclosure during installation does require a higher skill level than bolting. However, he said, the total man-hours for joining bus sections are comparable.

Options available for non-seg rectangular geometries are available for the circular non-seg offering—including space heaters and connections to all types of capital equipment (steam and gas turbines, transformers, etc). Plus, bus taps for auxiliary connections and high-amperage disconnect switches for isolation and grounding of the connected transformers when maintenance is required.

Economics. Hack closed with the suggestion that users would likely find circular non-seg bus more attractive financially over its lifecycle than the alternatives in the range of 3.5 to 7.5 kA (up to 110-kV BIL)—especially so if the plant is located in a difficult environment and/or must perform at a high level of reliability.

Example. If the condition of the non-seg bus serving your gas turbine suggests replacement would be a good idea to improve plant reliability, here’s how to determine if a circular non-seg retrofit is worth considering:

Assume the engine is a 501D5A, which has a nominal ISO rating of 121 MW. Dividing by the power factor (0.85 in this case) translates to 142 MVA. For the line voltage of 15 kV, phase current is 142, divided by 15, divided by the square root of 3 = 5.5 kA. This is in the middle of the “sweet spot” for circular non-seg bus suggesting consideration be given to this alternative.

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