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Isolating DC Grounds in Modern Power Plants

This primer provides a few simple steps for troubleshooting ground indications in above-ground DC control systems installed in hydroelectric facilities.

By John R. Hunter

Most hydro project owners have converted from relay logic and/or pneumatic control and protection systems to microprocessor-based systems. However, many have retained a significant portion of the old DC control systems, specifically the devices controlled by discrete output relays in the new systems to allow safe shutdowns of equipment from the stored energy in station batteries. These systems generally provide uninterruptible power to emergency lube oil pumps, auxiliary power system draw-out breakers, and generator and line breakers.

Although the new control systems are less prone to develop DC grounds as a result of the reduced complexity and interconnecting wiring, grounds must be treated with the same importance and using many of the same procedures as before. With infrequent DC ground problems, all project owners run the chance of losing the skill necessary to resolve these issues in a timely manner.

This article addresses troubleshooting of DC ground indications in above-ground DC control systems. Unlike the DC systems in a vehicle where the negative leg of the electrical system is connected through the vehicle chassis, virtually all power plant DC systems have both the positive and negative legs (buses) running throughout the plant. Voltage levels may vary from plant to plant or substation to substation - 48 VDC, 125 VDC or 250 VDC generally - but within an organization there will be similarities in application.

Most of Salt River Project's eight hydropower plants, with a total of 12 turbine-generating units, have a 125 VDC system, and most of the substations and communications networks have 48 VDC systems. The principles discussed here will be the same for any above-ground DC control system.

Steps to isolate grounds

Figure 1 shows a schematic of a plant DC control system with the following major components:

- DC battery bank sized so that power will be provided for a specified time after AC failure;

- Battery charger capable of being fed by an emergency AC generator to extend the time the batteries can provide control power. In addition to alarm and protection functions, the charger will most probably include the ground protection indication, detection and alarm functionality; and

- One or more DC distribution panels to feed common application circuits throughout the plant. In multiple unit power plants, there may be one or more distribution panels per unit.

Figure 2 shows a basic DC system ground detector circuit using indicating lights. This circuit is usually located in the battery charger cabinet. The battery charger may have additional circuitry associated with ground detection to allow alarming, but this circuit more easily displays how a ground detector works. Assuming this to be a 125 VDC circuit, two incandescent lights would be installed in series and the circuit tied to ground. If there were no grounds in the circuit, each light would be at half brilliance. Note in this circuit that a measurement of DC voltage to ground from either phase would yield a reading of about half of the battery voltage, with polarity dependent on which lead was placed where. Also note that the procedures described below assume this connection to the ground plane is intact.

Figure 3 shows the same circuit with a DC ground applied. The upper light is at full brilliance because it has a direct path back to the positive side of the battery through the ground plane. The lower light is out because the voltage is shorted across the light through the two connections to ground. Note in this circuit that a measurement of DC voltage to ground from the positive leg would yield a reading of 0 volts. Measuring from the negative leg to ground would result in a full battery voltage reading.

Note also that this is a solid ground. Often there will be indications of a "partial" ground, or one that floats around a bit, depending on where the ground appears within the control circuit. In those cases, lamp brilliance will be reduced, with one brighter than the other. This is a limitation of the light system.

For an accurate indication of a DC ground, a voltmeter must be used, and indeed, DC voltages to ground should be checked and documented as part of normal maintenance, probably at least once a month. Maintenance of the DC system should also include periodic testing of the ground detector system.

Figure 4 shows a ground on the negative phase downstream of the circuit 4 breaker. Standard operating procedure for isolating grounds is to open each DC breaker in turn until the ground disappears. In this circuit, you can see how this works: opening circuits 1, 2 and 3 will not isolate the grounded circuit. Opening circuit 4 does isolate the ground, and the lights and voltages will return to normal.

Generally, the troubleshooting procedure is to open each breaker in turn, observe the ground lights or voltmeter and reclose that breaker and move on to the next breaker until the circuit is isolated. Then the faulted circuit is further sectionalized with either individual fuses or wiring within the circuit until the ground is located.

This procedure will only work if the ground appears on only one circuit. For example, assume a heavy rain storm in the area causes a DC ground alarm. Furthermore, assume there is a ground on the negative bus in both circuit 2 and circuit 4. Opening breaker 2 will not change the lights/voltages because it still sees the ground in circuit 4. Reclosing 2 breaker and opening 4 also will not change the lights/voltages because there is still a ground in circuit 2.

For multiple grounds, the procedure must change: open each breaker in turn, leaving the individual breakers open until the ground goes away. Then start reclosing each breaker in turn until the ground comes back, at which time you re-open that breaker and continue closing breakers in like manner until you have a complete list of all circuits with grounds on them. Then start sectionalizing the individual grounded circuits.

In that same rain storm, you can continue operation with multiple grounds as long as they all appear on the same DC bus (same polarity). Once you develop a ground on the other bus, however, you will start tripping DC circuit breakers.

Once the ground is isolated to the individual circuit breaker, any fuses would be pulled to isolate it further. Figure 5 shows an example circuit. With the relay contacts as shown, pulling FU-1N would clear the ground. You then sectionalize the circuit at TB2, termination by termination, until you have the faulted circuit identified. The actual procedure used is up to the individual. Some will start with TB2-1, then 2, then 3, etc. Others will start halfway through the circuit at TB2-3, etc.

Figure 5 demonstrates another interesting situation you may run across with regard to the location of the ground. In this case, if the contact is open (coil de-energized), the system would indicate a negative ground. When the contact closes, it would transition to a positive ground.

You may also encounter situations where you suspect a conductor is grounded somewhere due to poor insulation integrity. The proper procedure is to un-terminate the suspect conductor at both ends and check it with an ohmmeter to ground. Another important piece of equipment that will prove invaluable in locating faulty insulation in wiring, especially for an intermittent ground, is a meggar. You can energize an isolated conductor up to the insulation rating to confirm that the wiring is good along the entire length.

The procedures above need to be performed with operational considerations, personnel and equipment safety in mind. Although continuity of operations will probably be possible when opening a small number of breakers on your DC panel (emergency lighting, etc.), many of them will require shutting units down to allow the DC breaker operation. The procedure to isolate DC grounds must be performed with the following in mind:

- Do not open DC breakers to generator, transformer or line protection circuits with the equipment in service;

- Do not open DC breakers that provide power to the trip circuits of generator, transformer, line or auxiliary breakers with the breakers in the closed position;

- If you are left with the possibility of a ground in the battery bank, battery charger, light circuit or any of the interconnecting wiring, one thing to consider in whichever procedure you use next is that all of your protection systems assume the stored energy in the battery bank will be there when needed. If you open the battery disconnect and lose AC to the charger, you will have no DC power anywhere until the battery disconnect is closed or AC is restored to the charger. That can be very dangerous to system security. You are most probably looking at a plant outage for this, including the transmission system. However, redundant battery banks can greatly reduce the need for equipment outages to find grounds.

This being said, you will probably find someone in your plant who has done what is described above in the past, thinking that the chance of a fault on the system at the same time the DC control breaker is opened is small enough to justify the risk. The problem is that the probability is not zero and the potential damage to the equipment could be catastrophic. It is simply not worth the risk.

Our operating environments are different, along with our O&M philosophies. If you are running a peaking plant, most probably you have a few hours each day or two that can be used to complete this kind of plant troubleshooting. If on the other hand you are running a base loaded plant, shutting down a unit for this maintenance becomes more problematic. There are also plants that have redundant circuits for the more critical systems, perhaps allowing removal of one system from service for troubleshooting while the redundant system keeps the unit in service. Plant procedures for DC ground troubleshooting and isolation should already be developed and a part of your plant O&M training programs.

While we are on the subject of DC ground identification and isolation, we should spend some time discussing DC grounds in another of our plant systems: generator excitation systems. Again, these are above-ground systems providing DC voltage to the generator rotor and are separate from the DC control system except for any DC used for field flashing. Troubleshooting these is similar in that a structured approach is necessary. Because most excitation grounds should result in a unit trip, the procedures and options are a bit easier. For a generator exciter or rotor ground, you may wish to consider the following:

- More often than not, you will find the problem in the brush/collector ring area, especially on vertically stacked machines. With the unit shut down, ensure the exciter is de-energized, including any rotor ground detection power sources provided on newer exciters. Remove the brushes from the collector ring and meggar from either of the collector rings to the rotor metal. (Do not meggar from a collector ring to station ground because the voltage will be applied through a bearing.) If a ground is indicated, thorough cleaning of the brush/collector ring area will most probably clear the ground.

- If the ground is on the rotor after cleaning as described above, you may find it on top of the rotor where the bus bars go from the center of the rotor out to the pole pieces. Cleaning any brush dust off the top of the rotor, including any insulators, may be successful.

- If the above two do not work and you still have a rotor ground, you need to disconnect and check the cabling running down through the rotor, and separate the pole pieces from the bus bars at the rotor perimeter to determine if the ground is in a pole piece.

- If you determine from the first step above that the ground is not on the rotor, you will need to separate the cables from the exciter out to the brush rigging and meggar them. If it is not on the cabling, it is probably in the exciter or a bad ground detection system.

The procedure is similar for almost all excitation systems, whether rotating or static, on vertical or horizontal units.

As in all procedures in plants, personnel and equipment safety must be the first consideration. Plant procedures for DC ground isolation should already be developed and a part of your plant O&M training programs.

John Hunter is operations and maintenance supervisor, instrumentation controls and electrical, for the hydro generation division at Salt River Project.

This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.

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