Ground grid vs ground ring in Unit Substations 1

[rockman7892]

Is there a standard or common practice for installing new grounding systems for new unit substations or electrical rooms that reside within an industrial facilities distribution system. An example would be an elevated electrical room (IPA building) which is supported by a foundation and contains MV & LV switchgear as part of a unit substation with transformers located outside of building.

From what I have seen most electrical room or E-Houses of this nature in an industrial facility typically have a ground ring which surrounds the building with rods at each corner and is also tied into the rebar in the building foundation (UFER Ground).

To me a ground ring designed per IEEE 80 is more reserved for a MV/HV outdoor substation with a large footprint which will be more susceptible to step and touch potentials.

Does anyone have any good guidance on when to install a ground grid per IEEE 80 vs a ground ring as described above?

 

[cuky2000]

The objective of both grounding system is the same to achieve safety to the personnel and allow the protection system to have a good ground reference.

The main difference is while in substation application soil resistivity test and calculation are performed to ensure meet the safety and operation criteria a ground ring is less rigorous without calc and soil resistivity test.

Both systems should be tested to comply with the minimum resistance required. For substation, a common grid resistance acceptance test could vary from 0.5 Ω to 1Ω for HV substation and <5Ω for small distribution substations. Clearing time may vary from less than 0.5 sec (30 cycles) to 1 to 3 seconds respectively.

On the other hand, for switchgear and small MV application under the jurisdiction of the NEC, often is acceptable to be 25Ω or less and traditional grounding calculations and soil resistivity test is not performed. Usually, the protective devices may clear the fault with larger time than in HV substation applications.

 

[7anoter4]

I agree with cuky2000.
However, limiting the grounding resistance to 1 ohm is not already mandatory. The old BS required 1 ohm .The new one-for instance BS 7430/2011- states one has to follow BS EN 50522.[ Annex A(informative) Guidance on typical HV/LV interfaces. A.3 Combined HV/LV earth resistance]
The grounding ring and substation grounding grid are two separate grounding system and are not interchangeable. The grounding ring may be connected with substation grounding grid.
See BS EN 50522:2011 Annex O Global Earthing System
or IEEE 142 /2007 4.2.1 Existing electrodes
Usually the grounding grid is calculated and tested according to IEEE 80/2013 or
BS 7430/2011 or else.
Foundation grounding is not calculated or tested but the design and execution follow the standard details. See:
IEC 60364-5-54 Annex C Erection of concrete-embedded-foundation earth electrodes or [for instance]:

https://www.dehn.mx/sites/default/files/uploads/dehn/pdf/eb/ds162_e_foundation_earth_electrodes.pdf

or Building and Service Entance Grounding and Bonding NECA 331-2009
or else.

 

[cuky2000]

Hi 7anoter4,

Perhaps we need to make a distinction regarding suggested and mandatory requirement (see quote below).

The mandatory compliance usually is specified by the interconnected utility or by manufacturers of sensitive electronic equipment such as hospitals.
Years ago, we have a lot of challenges getting the acceptance test in a substation that not meet the utility requirement of 1 Ohm. This case had a happy ending because we were able convinced the utility engineers that a ground resistance test is not accurate went measured near an energized facility in lieu they accept the grounding calculations.

 

[7anoter4]

Thank you cuky2000.It is very interesting indeed.

 

[cherryg222]

The question posed here is very much a valid one and often poses a dilemma for designers/engineers.

If one reads IEEE 80 carefully, it has some very interesting statements. In the Year 2000 edition, in Sec 15.8 Worst fault type and location, IEEE provides very clear cut statement: "For distribution substations with the transformer grounded only on the distribution side, the maximum grid current Ig usually occurs for a ground fault on the high-side terminals of the transformer. However, if the source of ground fault current on the high side is weak, or if a parallel operation of several transformers results in a strong ground fault current source on the low side, the maximum grid current may occur for a ground fault somewhere on the distribution circuit.For ground faults on the low-side terminals of such a secondary grounded transformer, the transformer's contribution to the fault circulates in the substation grid conductor with negligible leakage current into the earth and, thus, has no effect on the substation GPR, ....."

Thus the requirement for a full fledged grid occurs when you have an Overhead line with source transformer at remote end (solidly grounded), with ground fault occurring on the HV side. This leads to some part of the current flowing in the ground leading to GPR. However, if the power source i.e Transformer is local, as stated above, majority of the L-G current flows in the ground conductors, with no danger to step/touch potential or GPR. Hence, a full fledged grid is not a requirement and a ring serves the purpose. Another factor which is important is the type of grounding. If it is a resistance grounded system, again the L-G current is limited and may not result in any appreciable step/touch potentials. Often a grid is installed instead of a ring with lots of money lost on materials and labor.