Neutral Earthing without transient overvoltages 4

[makuku]

Could somebody please assist me with the following problem:

I work for a utility and I have a client who needs a modification as follows:

To install a neutral earthing resistors on 2 x 25MVA, YNyn0, XHL = 9.7%132/33kV Transformers with a 33kV fault level of 219.57MVA, 3.84kA.

Apparently to avoid transient overvoltages, the neutral current must be limited to just above the capacitive discharge current. How do I obtain the value of the capacitive discharge current and the correct resistor value?

 

[jbartos]

Comment: 33kV is very high level for high-resistance system grounding. The medium-resistance system groundig may also be unsuitable because of the voltage level of 33kV. Then, the Ground Fault Neutralizer (Resonant Grounding) is used for above 15kV transmission lines.
References:
1. IEEE Std 142-1991 Green Book
2. AIEE Committee Report "Application of Ground Fault Neutralizers," Electrical Engineering, Volume 72, July 1953, p. 606
etc.
Contact a Ground Fault Neutralizer manufacturer for tech support
Visit

http://www.selinc.com/techpprs/6124.pdf

http://www.selinc.com/techpprs/6123.pdf

etc. for more info

 

[Kiribanda]

Hi makuku,

I think you are trying to introduce a high resistance grounding system which depends on the total capacitive charging current of the 33 kV network. Based on that I too cannot justify a high resistance grounding system for a 33 kV system as jbartos noted in his post.

This is my explanation.

In a high resistance grounding system the value of the NGR is normally sized for three times the total capacitive charging current on the 33 kV network. The total charging current of all the 33 kV transmission lines add up to say 10 Amps (very rarely even with 33 kV cables) then the NGR is about 30 Amps cont. rating!!

Then,
The Power that NGR will handle till the fault is cleared = 33000 x 30 / 1.732 kVA.
= 57 kVA

The power dissipation at the fault point till the fault is cleared = 57 kVA

This seems to be too high to handle at 33 kV level. That is why High resistance grounding systems are effective and justifiable up to 5 kV the most.

Therefore ground fault neutralizer is another solution as jbartos pointed out.


Regards!

Kiribanda

 

[busbar]

Added grounding impedance may conflict with 33kV-secondary insulation-grading during ground faults from neutral shift. Its effect on surge protection should also be reviewed.

ANSI/IEEE C62.92.1-2000 …Application Of Neutral Grounding In Electrical Utility Systems TOC at

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.1-2000_desc.html

ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution TOC at

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.4-1991_desc.html

ANSI/IEEE Std C62.92.5-1992 [l]…Application of Neutral Grounding in Electrical Utility Systems, Part V—Transmission Systems and Subtransmission Systems[/i] TOC at

http://standards.ieee.org/reading/ieee/std_public/description/surge/C62.92.5-1992_desc.html

ANSI/IEEE Std 32-1972 …Requirements, Terminology, and Test Procedure for Neutral Grounding Devices TOC at

http://standards.ieee.org/reading/ieee/std_public/description/surge/32-1972_desc.html

 

[rsherry]

It is common practice in the UK to have Resistance earthing on these 132/33kV transformers. Traditionally the fault current has been limited to 1 p.u. (i.e. transformer rated winding current) but this makes for expensive resistors so more modern designs restrict to lower currents if the utility in question allows. Typical resistors for the size of transformers you suggest would be about 43 ohms (each) on the traditional approach (rated current 437Amps).
I think the 'ground fault neutralisers' mentioned are what we in the UK know as Petersen coils (resonant earths) - for most UK utilities these are no longer acceptable on safety grounds at 33kV and are being removed by the utilities that have them.

 

[digitrex]

There is something I need to understand more. Overvoltages experienced in the past is seemed to be related to ungrounded system.

Can you share with us whether the existing YNyn transformer already has a neutral earthing(and is it at 132kV or 33kV side)?

If there is no neutral grounding at all at both HV & LV side and you need to install 2 neutral groundings, at both HV and LV side. One neutral grounding at either side is equivalent to grounding at all.

I don't think you need a resistor grounding. Direct(solid) grounding of both HV & LV neutrals are OK.

 

[digitrex]

Sorry, I need to re-phrase the 2nd last line:

One neutral grounding at either side is equivalent to no grounding at all.

 

[jbartos]

Suggestion to makuku (Electrical) Mar 13, 2004 marked ///\\How do I obtain the value of the capacitive discharge current and the correct resistor value?
///Capacitances of cables, transformers, motors, buses, etc. with respect to ground must be obtained. Then, the capacitive charge current can be obtained. Some margins have to be added. The single phase distribution transformer for line voltage is sized. Then, the resistor size and its power rating are calculated. These calculations are usually considered proprietary since the results and the setting can make a big difference in the system ground fault protection, alarms, outages, etc. The calculation can become involved depending on the complexity of the power distribution downstream of the transformer secondary or generator output.\\\

 

[makuku]

Hi Mengho

Thank you Mengho for your enquiries.

Maybe I should give an overview of this problem.

Our utility supplies power to a mine through 4 tranformers, one set is 132/6.6kV 20MVA YNd11 and another set is 132/33kV 25MVA YNyn0.The 33kV transformers are solidly earthed on the HV and LV. Within the mine network, the two supplies have absolutely no interaction (by transformation).

Now whenever the mine has faults on the 33kV network (mostly overhead lines prone to earthfaults), the 6.6kV voltage dips to about 0.5p.u. and most of their sensitive process loads dropoff. The 6.6kV loads are the most critical. The 6.6kV has NERs.

Apparently the Mine did a study that revealed that introducing a resistance on the LV neutral of the 33kV will lessen the severity of the voltage dips on the 6.6kV.

Now the literature that I have been reading recommends that "the resistor must be sized that groundfault limit is greater than the system's total capacitance-to-ground charging current, in order to avoid transient overvoltages.

Does this make sense anyone?

Busbar recommends some standards, unfortunately I can't get these online due to some access constraints.

Thank you all

 

[busbar]

One [not-so-much-ANSI] online reference is

http://www.schneiderelectric.com/en/pdf/ect62.pdf

No doubt Siemens and ABB have similar descriptive material.

Although on the North American continent, high-resistance grounding in distribution above 5-8kV is not popular, the “resistor… sized that groundfault limit is greater than the system's total capacitance-to-ground charging current” described compares to the general description of high-resistance grounding in ANSI parlance.

In Cahier Technique 62, the term “network capacitive current” seems equivalent to North American “system capacitive-charging or capacitive zero-sequence current”. Depending upon regional engineering customs, resistance grounding at 33- and 132kV may be simply not possible where a routine practice of transformer-insulation grading in YNyn0 banks has been specified.

 

[digitrex]

How about doing this in simpler ways:

1. Operate the 33kV switchboard with bus-coupler(and also for the further downstreams' as well) open such that the 2 YNyn transformers will not be parralleled. When there is fault on 33kV system, the fault currents will be reduced. Then the effect of voltage dip at 6.6kV network will be lessened.

2. Apply faster protections(not sure if it is already so) in the 33kV networks to isolate the fault as fast possible.

3. Try to make the process loads in 6.6kV system less sensitive to voltage dips, eg,

3a. apply stored energy device to hold the motor contactor coil for a few hundred millisecond during voltage dip

3b. apply restart relay on the motor starter for voltage recovery within 3seconds.