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ENGINEERING AND CONSTRUCTION

E-110 ENGINEERING AND CONSTRUCTION 2012 HydrocarbonProcessing.com

of NGR resistance cannot pick up a failure that occurs later, and some-times the person making the resistance measurement forgets to reconnectthe resistor after the measurement.

Sometimes ground-fault relays are tested by intentionally groundinga phase. If the NGR is open, then the ground fault relay will not oper-ate, and the investigation into the reason should reveal the open NGR.

Testing the ground-fault relays by primary current injection, on the otherhand, will not reveal an open NGR, because the ground-fault relayswill respond to the injected current and appear to operate properly.This would then falsely confirm that ground-fault protection was working.

NGR monitoring. There are several ways to monitor NGRs, some bet-ter than others. A potential transformer and a time-delay voltage relayconnected across the NGR will monitor neutral voltage, but it will notoperate until a ground fault occurs, regardless of the condition of theNGR. This is not continuous NGR monitoring. Another approach is touse voltage measurement and current measurement combined with logiccircuit to the NGR (FIG. 3). If both voltage and current are present, thereis a ground fault on the system, and since there is current flowing throughthe NGR, it cannot be open. If there is voltage but no current in the NGR,

it must be open. Like the previous example, this is not continuous NGRmonitoring because it works only during a ground fault.

Continuous monitoring is the way to go. A far better solution is an auto-matic monitoring device. A continuous NGR monitor detects an openNGR as soon as the failure occurs. It works whenever control poweris applied, whether or not the system is energized and with or withouta ground fault. It also reduces the opportunity for human error duringinspection and maintenance.

At a petroleum facility, engineers performed quarterly insulation testson resistance grounded generators. To perform the test, the engineershad to float the generator, disconnecting the neutral-grounding resis-tor from ground. How could engineers be sure that each generatorwas re-grounded after each test was completed? The solution in this

case was to install NGR monitors that would report the condition ofthe ground to plant control software. Not only would engineers know

for certain that generators had been re-grounded, but that fact wouldbe documented in the control system.

A good way to implement NGR monitoring is to combine an over-voltage measurement, an overcurrent measurement and a resistancemeasurement (FIG. 4). In physical form (FIG. 5), it combines measuredNGR current, transformer or generator neutral voltage and NGR resis-

tance to continuously determine the health of the NGR. The resistancemeasurement is the sum of the resistance from the sensing resistor, to theneutral point, through the NGR to ground, and through ground backto the monitor. Connecting the sensing resistor to a separate lug on theneutral bus assures that the NGR connection to the neutral point is partof the monitored loop.

When there is no ground fault on the system, a measurement ofNGR resistance is enough to confirm NGR continuity. The monitor deter-mines the presence of a ground fault through the voltage and currentmeasurements. Voltage on the neutral and current in the NGR indicatesa ground fault.

When a ground fault is present, a resistance measurement is not suf-ficient to confirm NGR continuity because of the possibility of measuring

continuity through the fault, as mentioned earlier. Because a resistancemeasurement alone is not sufficient to confirm NGR continuity, the moni-tor constantly evaluates resistance, current and voltage measurements.

When neutral voltage is elevated and current is flowing throughthe NGR, the NGR must be continuous. When the neutral voltage iselevated but no current flows through the NGR, the NGR must be open.The ability to detect an open NGR in the presence of a ground faultis particularly important in alarm-only systems where ground faults canremain on the system for long periods.

When a ground fault occurs in a resistance-grounded system, volt-age appears on the system neutral. In the case of a bolted fault, thetransformer or generator neutral rises to line-to-neutral voltage. An NGRmonitor that is directly connected to the system neutral brings a conduc-

tor with line-to-neutral voltage during a ground fault into a low-voltagecontrol cubicle. This is not acceptable in many applications. The sensing

Over-current

Over-voltage

FIG. 3. One way to monitor the status of an NGR is to apply an

overvoltage measurement, an overcurrent measurement and a logiccircuit to it.

Over-current

Over-voltage

FIG. 4. A better solution is the combination of an overvoltage measure-ment, an overcurrent measurement and a resistance measurement.