Earthing of neutral point of power transformer 1

[essamalbanna]

Why does the neutral point of power transformer in MV grid be earthed through NER, but in EHV and HV grids that point be earthed directly?

 

[dpc]

Industrial systems generally use some form of resistance grounding to limit the ground fault current that can flow into expensive equipment such as transformers, generators and large motors. The goal is limit damage to the steel cores in the event of a fault. Since industrial systems are relatively compact and consist mainly of cables, the fault current remains pretty constanst.

For utility systems, the concern is making sure that a phase conductor that falls on the ground, and other types of SLG faults will be detected and the line tripped off line. If the NGR is limiting the maximum fault current to 400A or less, the fault current down a transmission will quickly drop below levels that are reliably detectable. Utilities generally want more fault current, not less on long lines.

 

[prc]

In MV systems, NGR also reduces the risk from arc flashovers consequent to LG faults.

 

[wcaseyharman]

The utility generally refers to its system as “effectively grounded” rather than solidly grounded - reason being effectively grounded is a very specific condition (Xo <= 3*X1 if I recall correctly).
Effectively grounded systems allow a number of things:
-Single phase loads (can run single phase taps)
-Limits the unfaulted phase voltage to 138% of L-L voltage.
-Allow better lighting and surge protection since the maximum voltage is less than L-L
-with better surge protection can potentially reduce cost on power system equipment such as transformers
- Can reduce cost on power transformers by connecting Phase to neutral (single high voltage bushing)
- Allow to clear and coordinate ground faults, and potentially use single pole clearing (leave other two phases in service and only trip the faulted phase). Most faults in the utility are L-G, so this helps with system stability.
-ensures ground faults have sufficient current to trip protective equipment. Since much of the utility is in public right of way this has an important safety element.

It wasn’t always this way - some early 55KV systems ran ungrounded. In my company it appears this ended in the 1960s when the older systems received grounding banks to make them effectively grounded.

There is a good discussion of this in the Westinghouse T&D book.

 

[jghrist]

Your use of the term "earthed" instead of "grounded" makes me think that you are from the IEC world (UK, Europe, SA, etc.) not North America. The MV distribution systems in NA are very different than those in the IEC world. NA utilities have a lot of single phase transformers and laterals that require effective grounding. IEC utilities usually nave only phase-phase or 3-phase loads. Protection schemes are also very different.

 

[Spaceinvader2]

Distribution utilities (generally dealing with MV) can use neutral earthing resistors (NERs) or reactors (NEX) to limit the fault currents of their networks for a number of reasons including:

- Less onerous fault withstand requirements of switchgear, etc.
- Decreased earthing requirements of substations.