Grounding resistors 4

[qstorm]

What is the difference between a neutral grounding resistor and a neutral grounding reactor. Can one be used instead of the other for grounding substation transformer neutrals?

 

[PWR]

Ask a practical matter, reactance grounding is not used for commercial or industrial power transformers in the US. Whatever you are trying to achieve in system grounding can be achieved by either high resistance, low resistance, or solid grounding. The IEEE "Red" book devotes a paragraph to reactance grounding; it basically states why it is not used.

 

[scottf]

PWR-

You mention commercial or industrial applications...

It is quite common for utilities to use neutral grounding reactors and I would imagine quite a few are for industrial/commercial applications. Certainly resistors are much more common, but reactors do have their applications.

Reactors seem to be used more on generators as resonant grounding systems or on transformers as low-resistance grounding systems.

For resonant ground systems, the reactor is sized to resonate with the system capacitance, thus limiting the fault current to very low magnitude. Depending on the fault type, this can allow generators to remain in operation, through a fault condition. There are stories about generators taking a fault condition for over 1 hour with little to no damage.

For low-resistance grounding systems, the reactors may prove to have some economic advatages over resistors.

 

[PWR]

As a follow up: In the US, system grounding overwhelmingly is either ungrounded(less common today than before 1960), solid grounded (most common on low voltage systems and the standard for utility company practice on distribution systems) or resistance grounded (low or high). If you are not a utility (and the vast majority of their equipment is resistance or solid grounded), and if you are not looking to ground a large generator, a reactance grounded transformer would be a one in a thousand curiosity.

 

[jbartos]

Suggestion: Normally, system grounding calculations are performed within the project engineering phase. The outcome of the calculations determines the type of system grounding. The high-resistance grounding can be applied to certain size of charging or ground fault current. If this current is higher than the high-resistance grounding admits, then medium resistance grounding is applied. The medium resistance grounding can include low resistance grounding; however, it excludes the solidly grounded neutral system grounding. The reactor grounding is used on voltages that are higher, e.g. 25kV, up. The reason for this is that the resistances of the power distribution are negligible and reactances are used. Therefore, to apply a resistor for grounding is somewhat impractical in power distribution where the reactances (or inductances) are prevailing or significant.
D. Beeman "Industrial Power Systems Handbook" includes the low voltage (up to 600VAC) generator grounded via low value reactance with a remark that the grounding reactance should pass ground currents equal to at least 25% of three-phase value.
The Beeman Handbook correctly makes a distinction between the reactance system grounding and ground fault neutralizer that also uses the reactor for the system grounding.
The reactor system grounded generator may have the current for phase-to-ground 5 to 25% of three-phase fault current. It is not used much since it causes excessive overvoltages.
The ground fault neutralizer is used for high voltage overhead lines where faults may be self-healing. It has nearly zero fault currents and the transient voltages are not excessive.

 

[richard1st]

As an engineer in the UK, I thought people in the US worked to IEEE 142 ('The Green Book').

A 1991 version appears at:
standards.ieee.org/colorbooks/sampler/Greenbook.pdf

Am I out of date / showing my age / misunderstood the question?

RAC

 

[busbar]

For ~2-35kV distribution circuits, the two methods are compared in §3.3 of ANSI/IEEE Standard C62.92.4-1991 …Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution

Reactor versus resistor grounding seem to be regional preferences/practices, based to a large degree on operating history versus service continuity and life-cycle costs. Reactor grounding may experience resonant-overvoltage problems not usually associated with resistive grounding.

 

[qstorm]

busbar, jbartos & PWR
Thanks all for your reply to the post and references you've mentioned