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MV Service Upgrade assures expansion flexibility

Two 2500kVA transformers had to be temporarily paralleled during the first phase of the power equipment changeover

The installation of new service equipment at the Chromalloy Gas Turbine Corp. plant in Orangeburg, NY required careful attention to detail, engineering know how, and the cooperation of the local utility to complete the changeover within a 12-hr shutdown during a Memorial Day weekend.

To keep pace with the constant growth in business, new service-entrance equipment has been installed in the plant at various periods over the years. The normal peak demand usage at the plant was 5500kVA and, essentially, the existing service-entrance equipment was reaching its limit of capacity. In addition, production forecasting indicated that the plant could expect a 45% increase in electrical consumption over the next five years.

Thus, in early 1989, Chromalloy facilities engineer David Johnson called in his consulting electrical engineer and Vista Electrical Contractors, Inc., West Nyack, NY, that had done electrical installation and maintenance work at the plant over the years. Johnson wanted to establish an orderly plan for increasing the capacity at one of the plant's four medium voltage services, a project that had been under review for about two years.

Power distribution
The plant receives electrical power from the local utility at two separate locations, in accordance with NEC Sec. 230-2, Exception No. 4,the north end and the south end of the building. Two separate services are provided at each location, and in all cases, the incoming utility service of 13.2kV is stepped down to the 480/277V utilization voltage of the plant. Fig. 1 shows the two services at the north end of the building, which were served from utility switchgear via underground conduit runs. A 2500kVA transformer (T1) installed 12 years ago serves the 4000A switchboard No. 1.A total of six 500kVA mat-mounted transformers paralleled together serve switchboard No.2, which was installed in 1965 and is the plant's original service switchboard.

Equipment upgrade plan
The upgrading project called for installing anew 13.2kV medium-voltage service distribution system to refeed the 2500kVA padmounted transformer T1, and to replace the six 500kVA mat-mounted transformers serving switchboard No. 2 with one new 2500kVAtransformer T2. A second new 2500kVA transformer T3 would be installed adjacent to transformer T2 to be used in future expansion. Transformer T3 was to feed a new switchboard, to be called switchboard No 3. The new switchboard No. 3 would be installed in a new electrical equipment room that would be built in the space occupied by the six mat-mounted transformers, which would be removed. Transformers T2 and T3 were purchased in advance by Chromalloy and stored on the plant property, awaiting installation. The largest capacity power transformer allowed by the utility on its lines is a 2500kVA padmounted unit.

However, in the interim, continuous monitoring of switchboard No. 2 indicated that the maximum power demand on the 3000kVAbank of six transformers was reaching its limit. Thus, a decision was made to modify the project events. The new plan called for the six transformer bank to be disconnected and transformers T2 and T3 to be paralleled on their secondary side and connected via a new service switch and a new section of busway to serve switchboard No 2. The paralleling would be temporary, perhaps for less than a year.

Then, after the new electrical equipment room was built, transformer T3 could be removed from its paralleling hookup and permanently connected as was originally intended to the new switchboard.

Plan view of the north service

Fig. 1.
Plan view of the north service shows the two existing service runs to serve switchboards No. 1 and No. 2.

The service changeover included a new run to transformer T1 and two new runs to the paralleled transformers T2 and T3.

Later, transformer T3 will be disconnected from the new 4000A service switch and reconnected to serve a new switchboard to be located in the space occupied by the six 500kVA transformers.

The procedure involving use of a temporary 480V, 3-phase circuit is also shown.

Assuring transformer match
Before finalizing the plan to parallel transformers T2 and T3, a number of approvals had to be granted. First, the serving utility had to give permission for paralleling the two transformers. Then, the electrical inspection agency had to approve the installation. Finally, the manufacturer had to grant assurances that the paralleling scheme was engineeringly sound, since the transformers were not designed and built to be paralleled together. A review of the transformer shipping data indicated that their construction was close enough for paralleling: one transformer had a 6.2% impedance, and the other had a 6.1% impedance. That was close enough for the two transformers to be used in a paralleling scheme.

After the decision was made to parallel the two power transformers and after installation work had begun, an interesting problem came up. Chromalloy's production schedule would allow only one day for Vista to make the service changeover - Sunday of the Memorial Day weekend with the following day a backup date.

Thus, Vista Electric president Bill Maloney began coordinating procedures with the Orange and Rockland (OAR) Utility Co. Working with Ed Blanke, Bob DeGroat and Clint Hesselgrave of the O&R Commercial Operations Dept., Maloney prepared a detailed 12-hr time schedule on all operations for his crew and the utility crew throughout the changeover operation, starting from the 6:00 a.m. deenergizing of the north service, until utility voltage was restored at the new yard pole after 8:00 p.m. that evening.

Phase and rotation testing
Prior to the changeover date, the new feeder conduits and conductors, metering enclosure, padmounted switches, transformers T2 and T3,and the 4000A service switch were installed. At the same time, the new feeder conduits for transformer T1 were installed to within 10-ft of the energized transformer. Thus, everything was ready for the work that could be done only with the power system deenergized.

Looking forward to the Sunday service changeover date, Maloney wanted to make sure that when all three transformers were energized, they would be phased properly. That is A-A, B-B, and C-C. In addition, the cable connections on the equipment had to be made so that the phase rotation throughout the power distribution system would remain the same as it was. The utility would not connect the transformers on the primary side(13.2kV) for a testing procedure without imposing an additional power shutdown prior to the changeout day. But the tight time schedule established for the service changeout would not allow for a second, or additional, power shutdown.

Thus, Maloney reviewed some basic electrical theory on transformer turns ratios before figuring out a way to connect a trio of bell transformers in a test hookup that would enable him to energize the system at a reduced voltage level and still take useful test measurements. The concept was first tried out on one of the stored power transformers. When the idea proved out, Vista was ready to use it on the changeover day.

As shown in Fig. 1, a temporary 480V, 3phase, 30A circuit derived from a distribution panel in the plant was brought to the utility yard pole. The temporary circuit was then connected to the medium-voltage conductors at the point where they would normally be connected to the utility lines at the top of the utility yard pole. The temporary 480V circuit was used to energize the metering transclosure, the medium-voltage switchgear compartment, the two paralleled transformers T2 andT3, and the refed transformer T1. In this way, the entire system could be checked out.

Transformer/power system checkout
Fig. 2 shows a temporary test voltage schematic on a hookup at one of the new power transformers. With the 480V temporary circuit connected to the transformer primary, because of the turns ratio, approximately 17V could be measured on the secondary side of the transformers. The 17V on the secondary side was a sufficiently high level to check the phase-to-phase connections: A-A, B-B, C-C, but not sufficiently high to check for phase rotation, because the contractor's phase rotation meter operated only in a 90 to 600V range. The contractor was unable to locate a phase rotation meter that would operate at only 17V.

Schematic of the temporary 480V,
Fig. 2. Schematic of the temporary 480V, 3-phase circuit is shown energizing the new metering transclosure, padmounted switch, feeder run, and power transformer No. 2.

With 480V on the primary of the transformer, voltage between any two terminals on the transformer secondary winding was l 7V.

The three bell transformers, connected with jumpers in a Y-Y arrangement, provided 208V phase-to-phase, thus permitting use of the phase rotation meter.

To develop a suitable voltage level for the phase rotation meter, the three 10VA bell transformers, with a 120V primary and a 10V secondary, came into use. The bell transformers were connected in a 3 phase Y-Y arrangement. The 17V phase-to-phase from the 2500kVA transformer secondary winding then went through each of the bell transformers at about 10V (17V divided by 1.73). On the primary of the bell transformers, the voltage level was 120V phase-to-neutral and 208V phase-to-phase. This voltage level was sufficient for the phase rotation meters to function. Everything worked well, and Maloney was confident that the transformers and feeders were properly connected and that the service equipment could be energized at full voltage. After completion of the paralleling hookup, voltage and amperage recordings were made on both transformers for about one week to confirm that the two transformers were sharing the load.

Refeeding switchboard No. 2
While the outside work was going on, another Vista crew was busy disconnecting and removing most of the existing busway run - the section from the six paralleled transformers up to a wall located close to switchboard No 2. At the same time, the new 4000A busway was installed from the new outdoor service switch (which contains ground-fault protection) and spliced to the existing busway (Fig. 3). Vista had previously made measurements and prepared a sketch showing the best layout for the new busway to connect to the existing busway for refeeding switchboard No2.

The best way to use the existing busway run
Fig. 3. The new 4000A feeder busway extends from the new service entrance bolted pressure switch in a horizontal run and then to a vertical run down the wall where a 9Oº bend is made to join the existing busway.

The entire layout was planned to best utilize the existing busway run.

The entire layout was planned to best utilize the existing busway run.

Preventative maintenance, such as cleaning of the enclosures, inspection of contacts, torquing of busbars, and other adjustments, was also done on switchboards No.1 and No.2 during this downtime period. When the two services were reenergized, additional thermotesting of all connections was also made.

For Vista Electric, Richard Neely was project manager, Michael Sheelan, installation supervisor, and Keith Anser, project foreman.