Why do we use ground wire
Where and why do we use earthing transformers (zig-zag, earthed star-delta)
Purpose of the grounding transformer
Grounding transformers are sometimes used in distribution systems. A grounding transformer provides one Source for the zero current . Sometimes they are used to convert a 3-wire ungrounded circuit to a 4-wire, grounded circuit.
Why do we use earthing transformers - zigzag and earthed star-delta connections (Photo: swedishneutral.se)
Figure 1 (see below) shows the two most common grounding transformers. The zigzag connection is the most common grounding transformer.
Figure 1 - Grounding transformer connections (zig-zag and grounded star-delta)
Figure 2 shows how a ground bank delivers power to a ground fault. Grounding transformers, used as the sole ground source for a distribution circuit, should be in service whenever the three-phase power source is in service.
If the grounding transformer is lost, a line to ground will cause high phase voltages in the healthy phases, and Load imbalances can also cause neutral displacements and overvoltages .
Figure 2 - A grounding transformer providing a ground fault
A grounding transformer must have both the Unbalanced load in the circuit as well as the operating voltage during phase-to-earth faults.
If the circuit has minimal unbalance, we can drastically reduce the transformer's rating. It only needs to be designed for short-term (but large) errors, usually a 10 second or 1 minute is used. We can also select the impedance of the grounding transformer to accommodate ground fault currents to limit .
Each leg of a grounding transformer carries a third of the neutral current and has a line-to-neutral voltage. In an earthed star-delta transformer, the total nominal power including all three phases is the residual current multiplied by the earth voltage:
S = V LG × I N
A zig-zag transformer is more efficient than a grounded star-delta transformer. Each has a zigzag winding one by a factor of 3 lesser value as the ladder -Earth voltage so the bank can be rated lower:
S = V LG × I N / 3
ANSI / IEEE Std. 32-1972 requires a continuous rating of 3% for a 10 second session (meaning the short term rating is 33 times the continuous rating).
A 1-minute bank has a continuous current of 7% . For a 12.47 kV system delivering a ground fault current of 6000 A, a zigzag require a nominal power of 24.9 MVA . We're going to size the bank so that it can handle the 24.9 MVA for 10 seconds, which equates to a continuous rated power of 0.75 MVA, so that bank continuous 180 A neutral current can handle.
For both the zigzag and the grounded star-delta, the zero sequence impedance is equal to the impedance between a primary winding of the transformer and its secondary winding.
Another application of grounding transformers is in cases of telephone interference due to the flow of electricity in the Grounding. By bringing a ground bank closer to the source of the neutral current, the ground bank shifts some of the current from the neutral to the phase conductors to lower the neutral current that interferes with the telecommunication wires.
Earthing transformers are also used there where utility lines require a ground source in unusual conditions .
One such application is for a combo feeder that serves secondary network loads and other non-network line-to-earth connected loads. If the network transformers are connected to a grounded star point, the network will feed the circuit back during a ground fault.
If this happens while the main line breaker is open, the single-phase load will experience an overvoltage in the healthy phases as the circuit is fed back by the line loads as a non-earthed system. A grounding bank installed on the feeder prevents overvoltage in feedback conditions. Another similar application is found when distributed generators are used .
A grounded star-delta transformer is often used as a connection transformer specified to prevent overvoltages, when the generator drives an island that is separated from the supply source.
Even if a ground bank is not the only ground source, it must be sized to support the voltage imbalance. The zero-sequence current drawn from a bank is the zero-sequence voltage divided by the zero-sequence impedance:
I. 0 = V 0 / Z 0
A severe voltage imbalance can occur when a phase voltage is opened upstream (usually by a blown fuse or a tripped single-phase recloser). In this case, the zero sequence voltage is equal to the line-to-zero voltage. The ground bank will try to keep the voltage high on the open phase and to supply the entire load of that phase, which could severely overload the transformer.
Reference // TA Short Electrical Power Distribution Devices and Systems (Get Hardcopy from Amazon)
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