CONTINUITY OF SERVICE-CRITICAL FOR EQUIPMENT, SAFE FOR OPERATORS

When your client is an industrial facility with a coal-fired boiler providing process steam through a utility interconnected extraction steam turbine-generator, the last thing you want is an accidental ground clearing the fault, and the power needed to operate and control your power plant.

This facility received primary utility service at 2,400 VAC, 3 Phase, 3,000 amps, ungrounded delta.  The power plant was being supplied through a heavily loaded 1,000 kVA, 480-volt, ungrounded delta/delta connected dry-type transformer.  As part of a facility upgrade, this existing transformer was replaced with a 1,500/2,250 kVA AA/FA delta/wye connected cast coil transformer.  A high resistance grounding package was chosen, including a ground fault current monitoring scheme.

The primary reason for selecting a high resistance grounding package was the ability to sustain a system ground fault, indicate that a fault occurred, and provide time to locate and clear the fault.  An uncontrolled loss of power in a facility with a boiler and turbine generator poses a real risk of equipment damage, loss of production, and most importantly, puts plant operators and maintenance personnel at extreme risk.  Anyone who has experienced this situation needs no further explanation.

The equipment manufacturer was selected based on their ability to provide a quality product, and their willingness to work with their customers throughout the design, build, install, test, operate and maintain process.  The manufacturer provided the transformer and HRG package, including “Normal” and “Ground Fault” indicating lights, a “Test” button, and a CT and ammeter to monitor the system capacitive coupling current.  This package was modified to include a dual setpoint current meter/relay, and a four-quadrant coordinated manual control (CMC) switch.  The CMC switch included the following: green illuminated windows indicating “Normal” and “Test”, red illuminated windows indicating “Ground Fault” and “Open Ground”, and a center selector switch providing “Ground Fault” and “Open Ground” test positions and push to “Reset” functions.  The relay was tied into the plant annunciator system, indicating “Substation Trouble”.

Using additional primary turns through the CT window, coupled with contact block configurations in the CMC switch, two important testing functions were provided.  The “Ground Fault” test provided the ability to add more “perceived” current through the meter relay, giving a real test of the meter relay to alarm when a current value slightly above the capacitive coupling current was sensed.  The “Open Ground” test shunted current away from the meter relay, simulating a CT or associated resistance/ground wiring failure.  This system worked as designed, alarming for occasional ground faults, and detecting an actual internal CT failure.

No single transformer grounding system is perfect; it can not provide every capability desired in every possible situation, but the HRG system comes close.  I have listed some of the Pro’s and Con’s below.

Pro:

• The ability to operate with a resistive or solid ground until it can be located and cleared.A HRG system typically minimizes the damage to equipment at the location of the fault.
• A pulsing package with current sensing tongs is a good method of locating a fault.
• Finding system grounds after a breaker has tripped can be a frustrating and time consuming effort.  The HRG package gives the benefit of time.

Con:

• An arcing ground could create minor voltage disturbances.
• A second fault on any other phase will trip a breaker or burn the defective component free from the power supply.
• Current flow from the use of a pulsing package could cause sensitive low current components to fail.
• Current flow from the use of a pulsing package can cause wet components can dry and “heal” themselves.  In this case, the problem is not identified or resolved.

The industry can benefit from continued effort to enhance ground detection and fault locating equipment.  Learning from and building on each other’s experiences can help make this happen.  What new grounding product or design is waiting to be marketed?