Ungrounded systems protected for max safety
Ungrounded systems protected for max safety
(OP)
Assuming we have a power transformer with only 3 output connections, either delta or unearthed star at say 1MVA 400-600V say. What would be the most comprehensive way to ultimately protect the output for safety reasons for
a) a single phase to earth connection , even though it has no return path, but is deemed a no no
b) phase to phase
c) three phase
a) a single phase to earth connection , even though it has no return path, but is deemed a no no
b) phase to phase
c) three phase






RE: Ungrounded systems protected for max safety
But assuming an ungrounded system (there are always better, safer, methods), you can protect against single phase to ground faults by looking at the voltage to ground of all three phase conductors; there are several different approaches here from looking at one phase (undervoltage and it is the faulted phase, overvoltage and another phase is faulted) to looking at all three phases using a three-phase relay. A single overvoltage relay looking at the output of a set of broken delta VTs (not open delta) can detect the presence of a ground fault but not identify the phase.
A bolted phase-to-phase fault will be picked up by the phase elements, no problem. A high impedance phase-ground-phase fault can easily be detected by looking at negative sequence overcurrent.
A three-phase fault doesn't care how the system is grounded, the phase elements will pick it up.
RE: Ungrounded systems protected for max safety
RE: Ungrounded systems protected for max safety
Broken delta VTs: Three VTs, primary connected grounded-wye, secondary connected in delta. Now, open (break) the connection at one corner of the delta and insert a stabilizing resistor and measure the voltage across that resistor with a 59N relay. The voltage is the sum of the three phase voltages, or 3*V0. The presence of 3V0 greater than the set point of the relay is an indication of the presence of a ground fault.
Better yet, use three VTs connected grounded-wye / grounded-wye into a three phase 27/59 relay (solid state or numeric) and actually know which phase has the fault.
None of these techniques will tell you where your fault is though.
A high-impedance grounded system is far safer, and many are provided with a means of alternating the ground fault current between two values, such as 5 and 10 amps to make tracing the ground fault much easier.
Ungrounded is just bad design.
RE: Ungrounded systems protected for max safety
The concern is of course that the tx's can sometimes be mounted well away from the drives.
RE: Ungrounded systems protected for max safety
Have been using them on 500 and 690 V ungrounded systems and they are very reliable. They even compensate for large capacitive currents that often exist in such systems.
Yes David, ungrounded is mostly bad design. But, it has its proponents - and sometimes you even need it. Like in twelve-pulse rectifiers.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ungrounded systems protected for max safety
RE: Ungrounded systems protected for max safety
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Ungrounded systems protected for max safety
The impedance grounded system can have the resistance sized such that the maximum fault current is low enough for continued operation while the first fault is tracked down and corrected while eliminating the devastating overvoltages that can be associated with faults on ungrounded systems.
The transformer/converter pairs are left ungrounded because grounding the transformers would create a fault. Systems left ungrounded so that they can continue to operate during a ground fault are what should be impedance grounded rather than ungrounded.