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Neutral Grounding Resistors and Wye Wye Transformers

Neutral Grounding Resistors and Wye Wye Transformers

Neutral Grounding Resistors and Wye Wye Transformers

(OP)
At an industrial site we have a 69MVA Delta/Wye transformer stepping down utility voltage from 138kV to 24.94kV. The neutral on the secondary is connected to 50A NGR.  A 24.94kV overhead line distributes power to a fresh water pumping station located about 2km from the sub. At the pump house, there are 3x50kVA pole mounted transformers for 600V step down. The transformers were SUPPOSE to be connected as Delta/Wye with a 5A NGR on the secondary neutral. However, the installation contactor procured and installed Wye/Wye pole mounted transformers without informing until after the fact. The contractor solidly grounded the neutral of the high side WYE and connected the NGR (5A) to the  neutral of the secondary wye. Here is a rough ASCII sketch illustrating the distribution scheme.

138KV-24.94kV delta/wye          24.94kV-600V wye/wye
Xfmr (Secondary)                    Xfmr(Primary)   (SecondarY)
X0    neutrals (not connected)     H0                 XO    
|                                     |                  |
N                                    |                  N    
G (50A)                           |                  G (5A)
R                                    |                  R  
|                                     |                  |
+------------------------------------+------------------+
PLANT GROUNDING SYSTEM

My question is how will this Wye/Wye system behave vs the Delta/Wye system? I did some modeling in PSIM to check the line-ground faults and here are my observations. (For clarity, I will call the 69MVA Delta/Wye transformer as XFMR 1 and the 3x50kVA Wye/Wye transformers as XFMR 2.) The results are as follows:

SCENARIO 1.
Both transformers have ALL their star points solidly ground. (I want to start by studying the faults in a solidly grounded system, the scenario 2 is with resistive grounding.)

*Simulation A.   
A L-G Fault is placed on the secondary side of XFMR 2.
Ground fault current is observed flowing in the  ground cable connected to the secondary neutral of XFMR 1.
Ground fault current is observed flowing in the  ground cable connected to the secondary neutral of XFMR 1.
No fault current is observed flowing in the ground cable connected ton the primary neutral of XFMR 2.

*Simulation B
A L-G Fault is placed on the secondary side of XFMR 1.
No ground fault current is observed flowing on the secondary neutral ground wire of XFMR 2.
No ground fault current is observed flowing on the primary neutral ground connection of XFMR 2.
Ground fault current is observed flowing on the secondary neutral ground wire of XFMR 1.

SCENARIO 2.
XFMR 1 has a 50A NGR connection to its secondary wye neutral point. XFMR 2 has its primary wye neutral point solidly grounded. XFMR 2 has its secondary wye neutral point connected to a 5A NGR.

*Simulation A.   
A L-G Fault is placed on the secondary side of XFMR 2.
5A ground fault current is observed flowing through the 5A NGR connected to XFMR 2's secondary neutral.
No ground fault current is observed flowing on the primary neutral ground connection of XFMR 2.
No ground fault current is observed flowing on the 50A NGR connected to XFMR 1's secondary neutral.  (I amsurprised, but good!)

*Simulation B.   
A L-G Fault is modeled on the secondary side of XFMR 1.
50A Ground fault current is observed flowing through the 50A NGR connected to XFMR 1's secondary neutral.
No ground fault current is observed flowing on the primary neutral ground connection of XFMR 2.
No ground fault current is observed flowing on the 5A NGR connected to XFMR 2's secondary netural point.


I have read the threading regarding a similar situation as described here:
http://www.eng-tips.com/viewthread.cfm?qid=248062&page=7

However in that thread, it seemed that the neutral points of the transformers were connected and thus pybassed the system's NGR. In this application, the neutral points of the wyes are isolated and each secondary is connected to a dedicated NGR. Based on my simulations, the solidly grounded neutral on the primary side did not affect NGR behavior on the 24kV or 600V system. (They worked as intended.) Furthermore, I am not worried about harmonic flow because the load is 2x 575V 3-phase pump motors on full voltage starters. Due to the simplisty of the 3 phase load, there is minimum risk of an imbalance system. Finally, lighting panel exists, but it has an isolated 600V-208/120V XFMR.

Based on my analysis, I conclude that the Wye/WYE system will work. However, this is my first experience dealing with wye/wye transformers and I would like to have some input for others who have worked with this configuration.

Cheers,
Majesus

RE: Neutral Grounding Resistors and Wye Wye Transformers

majesus,
Grounded WYE/Grounded WYE will act similar to a DELTA/Grounded WYE connection and therefore will not contribute to a ground fault on the 69 kV side from 600V side.That means the Utility ground fault protection relays will see NO CHANGE in their ground fault currents and no need to revise their settings.(This will happen if the transformer is WYE-grounded/DELTA)But since the connection is WYE-Grounded/ WYE-Grounded,when there is a ground fault on the 600 V side,the zero seq currents will flow back to the 69 kV network.As a result the Utility ground fault relays "MIGHT" trip unnecessarily.Also if there are any 3 rd harmonics generating from 600 V side,they will flow back to 69 kV Utility side.That is why always Utility prefers to have DELTA/WYE-Grounded for their customer transformers.Other than the above I cannot think of any other negative issues.Any comments from others please.

RE: Neutral Grounding Resistors and Wye Wye Transformers

Normally, a floating neutral on a wye primary will result in voltage unbalances and phase shifts in the secondary voltages.
But the question may be how much unbalance and how much phase shift.
A simple resistor based solution is not accurate because saturation and phantom deltas both tend to limit the unbalances. In your case a three transformer bank will not have a phantom delta. It is an effect seen with three phase transformer with three legged cores
What may happen in your case?
For NGR resistors the voltages will be the line-neutral voltages.
24.94 kV = 14.4 kV line to neutral.
50 amps at 14.4 kV = 288 Ohms.
0.6 kV = 0.347 kV line to neutral
5 amps at 0.347 kV = 69.4 Ohms.
A ground fault on the 0.6 kV system will be limited to 5 amps phase current by the NGR.
5 Amps of neutral current on the 0.6 kV system will result in 0.12 Amps on the 14.4 kV system.
0.12 Amps through the 50 amp, 288 Ohm NGR will result in a voltage drop of 34.56 Volts. This is on the 14.4 kV line to neutral voltage.
This is about 0.24%. This won't cause much unbalance or phase shift.
BUT the analysis is not accurate because it does not include the resistance of the ground electrodes.
Possible safety issues are high touch and step potentials resulting from high ground electrode resistance. Ground electrode resistance may rise to unexpectedly high values in hot dry weather, particularly if harmonic currents on the ground/neutral are drying out the soil surrounding the ground electrode.

It is extremely poor practice to rely on a ground connection to serve as a neutral conductor. This may affect your ground fault protection in you use low settings.
Suggestions:
At the very least have the contractor install (at his expense) a neutral conductor back to the 138/24.94 kV transformer. This conductor should be installed to comply with code requirements for a neutral conductor.
You will still be faced with the decision as to where to connect the neutral at your 24.94 kV switchgear.
Connecting at the XO of the 24.94 kV system will not benefit from the NGR and will have high fault current levels.
Connecting to the ground side of the NGR is a code violation.
Suggestion #2.
Have the contractor install three small 14.4kV transformers in parallel with the 50 KVA transformers. The secondary voltage is not important as long as all three are the same. If the secondary is connected in delta the primary wye point will form an artificial neutral for the 14.4/0.6 kV transformer bank.
5 Amps at 347 Volts = 1.735 KVA. So three 2 KVA transformers with 14.4 kV primaries will be adequate to form an artificial neutral for a transformer bank with ground fault current limited to 5 Amps.
Suggestion #3.
Have the contractor replace the 14.4/0.6 kV transformers with 24.94/0.6 kV transformers as specified.
I personally don't much like the neutral conductor solution. It involves fault current or code issues.
 

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

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