high resistance grounding (HRG) Vs solid grounding 1

[MAS2000]

Can anyone help regarding high resistance grounding (HRG) and solidly grounded system?

I am interested to know what are pros/ cons of both systems. In our utility solid grounding is used for the LV distribution systems.

What advantages can we get if HRG system is used instead?

Another clarification required regarding HRG systems is the voltage rise on healthy phases in event of SLG fault? What is extent of this voltage rise?

 

[davidbeach]

It depends. Generally solidly grounded is, in my opinion, better for most applications. HRG has advantages where high continuity of service is required AND there are qualified personnel on site 24/7 to deal with any ground fault when it occurs. For larger generators, the ideal connection is HRG for the generator connected directly to a transformer solidly grounded on the system side.

Voltage rise on the unfaulted phases can reach phase-phase voltage from phase to ground, or 1.73 pu.

 

[electricpete]

I didn't understand the comment about qualified personnel on site 24/7 to deal with ground fault. That applies to ungrounded system where ground fault doens't cause trip. But does not apply to hi-resistance grounding, where ground fault still causes trip.

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(2B)+(2B)' ?

 

[waross]

High resistance grounding does not always cause a trip. I have even seen an HR system that alarmed and intentionally went ungrounded to limit damage in the event of a ground fault. (A long time ago, not recently)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

An HRG system cannot be used where there are Ø-N loads. It can be used on a 480V system where there are only 3Ø motors and delta connected transformers, but cannot be used to serve 277V lighting loads directly. It cannot be used on a 120/208V system with 1Ø 120V loads.

 

[ssantoshr]

Solid grounding system is most preferable on a three phase four wire system. However solid grounding system presents problems with coordination and reliability.
High resistance grounding provides safer, reliable and economic system. HRG is preferred for continuous process industries and limits the ground fault current.
Most common reason for not using HRG is that in a typical power system, one phase or three phase four wire loads are fed. HRG is preferred on three phase three wire systems.
During a ground fault, the steady state L-G voltage of the unfaulted phases would be close to full L-L voltage.

 

[MAS2000]

Many thanks to all who shared their valuable experiences.

Can I ask what systems are in use in US utilities where both single phase and three phase loads are to be supplied (that is distribution systems)? Is it HRG or solidly grounded?

I understand that in past (and still at few places) in US delta systems (3ph-3wire) were in use which were later replaced by corner delta systems (3ph-4wire). Are these systems still exist? if yes what earthing is used for these systems?

In UK only solidly grounded systems are used for low voltage distribution (230/400 volts).

M.A.Sh.
Elect. Engr.

 

[waross]

If single phase loads are supplied it must be be a solidly grounded system.

Corner grounded systems are three wire delta systems. One phase is grounded.

3 ph., 4 wire systems are generally 240 Volt delta systems but at least one side of the delta is center-tapped for 120:240 Volts. Single phase panels are fed with 120/240 Volts.

Three phase four wire panels are fed with the three phases and the fourth wire comes from the center tap. The center tap is grounded.
Two phases are at 120 volts to ground and the third phase is at 208 Volts to ground. This is often called the wild leg, or the high leg. The NEC prohibits connecting single phase loads to the
In a 3 ph. 4 wire panel three phase loads are connected to the three phase bus bars and receive 240 Volts three phase. HRG is mainly found in industrial plants.
Further, there is a code rule to the effect that if grounding a neutral will result in maximum voltages to ground of 150 Volts or less then the neutral MUST be grounded. As far as I know, the high leg is not usable for circuits connected to the neutral and the rule is applied to 120/120/240 4 wire circuits.
But North America is a big place and there may be local exceptions.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[azamfaez77]

By introduce HRG system, it will increase zero sequence impedance. Consequence, the Single Phase Fault will be reduce.

Single phase fault = 3 V(0)/(Z1+Z2+ZO+ZF) where as Z1=Positive sequence impedance, Z2=negative sequence impedance, Z0=zero sequence impedance, V(0)=Prefault voltage.

Benefit by doing this, your transformer will be not overstress when Single Phase fault occured. In transmission and distribution utility, transformer is the most expensive equipment(will cost millions). The utility will alyways want to protect their most expensive equipment.

You may refer to the Ieee141, iee241, ieee 242, iee 399 and power system anaylsis book by Hadi Saadat.

 

[UtilityGy]

After reading replies to these posts, I wonder how come my utility has a 115 kV/27.6 kV system grounding with a 1.5 ohm resistor on the secondary side and then coming out of staton 27.6 kV, an O/H 27.6 kV feeder supplies pole top distribution transformers and I think may be they are single phase 27.6 kV/ 120-240 V or may be three phase I have to confirm.

27.6 kV system is a three phase 4 wire system, I wonder why would they do it and make it a 4 wire and suppply neutral; may be there pole tops primary is 16.7 kV. Just wondering the concept behind this application.

 

[waross]

That's a pretty standard arrangement. It is called a 27.6 system but the transformers will be 15.6 kV and connected phase to neutral. The transformers will often be rated for service on wye systems. In the event of a ground fault on the system, the phase to ground/neutral may rise to the phase to phase voltage on the unfaulted phases. This causes two issues. The transformer may have 1.73 times rated voltage applied to the terminals. This will cause saturation and over current. The fuses will generally protect the transformer. The other issue is internal flash-over. This may not do much damage beyond contaminating the oil, but the cost of flushing a smaller transformer is often more than the cost of replacement. The cost of repairs on a larger transformer will be expensive and the transformer will be out of service for some time.
Some other distribution voltages are;
12,000/6,900 V, 13,200/7,620 V, 13,800/7,960 V, 25000/14,400 V, and 35,000/20,200 V.
The neutral conductor will be grounded at each transformer location and about 4 times per mile.
I am not familiar with the 1.5 Ohm resistor grounding but I imagine it may be to encourage any neutral current to return via the neutral conductor so as to lessen the erosion of the ground electrodes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

The 1.5 ohm resistor may be large enough to limit 1Ø faults to 10 kA (equipment rating of things like cutouts and loadbreak elbows), but still be small enough to qualify the system as effectively grounded. You would have to calculate the rise in the unfaulted phase to see if the system is effectively grounded.

 

[odlanor]

by Transmission Distribution Reference book a system effectivelly grounded means:
X0/X1 ratio should be equal to or less than 3 and R0/X1 equal to or less than 1, at any point in the system.

 

[jghrist]

IEEE Std C62.92, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, defines effectively grounded as a coefficient of grounding not exceeding 80%. The coefficient of grounding is defined as the ration of E[sub]LG[/sub]/E[sub]LL[/sub].

With a 1.5 ohm resistor, the system would not be HRG. HRG is used on low voltage systems, not medium voltage systems. I don't know of any US utilities that provide an HRG secondary service. Generally an HRG system would be provided by the customer.

 

[cuky2000]

The enclosed data are compiled from different sources. The file shows a summary of pros/ cons of grounding system graded in accordance with generalized consensus in the power industry.

Please notice that the solid/effective grounded system has low overvoltage but high fault current. In contrast, the HRG has high overvoltage and low fault current.

For overvoltage associated with the coefficient of grounding, the enclosed link to thread238-237503 may help.

 

[cuky2000]

Here are 3 JPG files related with overvoltage/overcurrent and pros & cons for HRG vs. other grounding systems.

 

[waross]

Hello cuky2000. It's nice to see you active here again. Welcome back.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

Hello Bill,

Thanks for your warm welcome back.

 

[prc]

Cuky2000, Iam not able to open your file. A blank screen is coming!

 

[ScottyUK]

Some discussion of HRG systems from Westinghouse here.



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