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Single Phase Transformer
and Loading Resistor : |
If the system has a Neutral which is available, a single phase distribution transformer can be used in conjunction with a loading resistor, to provide high resistance grounding. This is particularly well suited for grounding of generators, in that it allows the system to operate like an ungrounded system under normal conditions, while still retaining the ability to limit fault currents during a fault. Figure 1 shows a typical schematic.
The primary of the transformer is connected from the system neutral to ground. The loading resistor is connected across the transformer secondary.
When a ground fault occurs downstream of the grounding transformer, ground fault current flows through the fault, back through ground to the grounding transformer.The loading resistor limits the current flow in the secondary winding, which in turn limits the flow of the ground fault current back into the system through the primary of the grounding transformer.
The transformer primary voltage rating should be the same as the system line-to-line voltage. The secondary voltage is normally 240 or 110 volts.
Grounding Transformers :
In High Voltage Systems, where Neutral is not available, as in case of Delta Transformers, the grounding transformers can be used to create a neutral, which in turn is connected to a Neutral Grounding Resistor (NGR). These combinations are known as Artificial Neutrals.
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Figure 1
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Grounding transformers may be either Zigzag or Star Delta Type. The operation of each is similar. They present high impedance to normal 3 phase current, so that under normal conditions, only a small magnetizing current flows in the transformer winding. But, under line-to-ground fault conditions, a low impedance path is provided for the zero-sequence currents. These currents can flow through the fault, back through the neutral of the grounding transformer to the power source. The ground fault current is only limited by the resistance of the ground fault, the NGR, and the small transformer leakage reactance.
The Star Delta grounding transformers have a Star-connected primary and Delta-connected secondary. The three primary line terminals are connected to the 3-phase ungrounded power source. The neutral terminal is connected through a neutral grounding resistor (NGR) to ground. The Delta secondary is not connected to any external circuit.
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