Resistance Ground System Not working

[rockman7892]

I have attached a schematic for a High Resistance Ground (HRG) system that we cannot seem to get working. The system voltage is 480V.

In the schematic I have referenced where the ground is connected to at the bottom and also where the resistance network connects to the transformer nuetral.

There is a pushbutton labeled "TR" that can be used to test the package however when we push this button we see no current flowing through the network nor does the voltage relay HMR pickup. We have traced through the whole system and checked for coninuity and do not see any problems in that regard.

We took voltage readings from PH1-N at "Measuring Point A" that is referenced and got a reading of 280V. However when we press the TR button to test the system this reading drops to 23V and we see no current flowing in the system as stated above. I then removed the ground connection at the bottom of the drawing and connected the #8 wire shown directly to the bottom of the first resistor HREA. Again I had 280V at the measuring point, however when I pressed the TR pushbutton the reading dropped to 0V.

I cannot firgure out what is going on and why this system isnt working. Am I missing something?

I've had the OEM come into look at this and they are puzzled to and are trying to figure it out but I'd figure I'd see what you guys thought in the mean time.

 

[waross]

With over 300 ohms in the circuit, (HREA 300ohms + HRED ?ohms + HREE ? ohms) the test current would be less than 1 amp. With a 100:5 CT you may not be able to read the current.
You should always read 280V (277V?) at the test points. Check your fuses and connections from the transformer pads to the test points for high resistance. It may be a corroded connection a blown fuse.
Check the resistance of HREE and HRED. Make sure that the taps are set properly. If the resistance is set too low the fuse may have blown.
The 23 volts you see is probably an indication of the capacitive current of the system insulation.

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

 

[stevenal]

With 10 primary turns through the core, the effective ratio is 10:5.

 

[rockman7892]

The 175ohm HRED resistor is jumpered out and the HREE resistor is set at 75ohm. This means that the total series circuit resistance is made up of 375ohm (HREA + HREE). Dividing the 280V I measure by this resistance, I should see aprox .73A. I do not see this current on an amp probe nor does the ammeter budge. Even with little current this voltage relay should pickup.

What should I get if I measure the resistor on the secondary side of resistor HREA to the nuetral measure point. Should I get a voltage reading of 280V minus whatever dropped across the HREA resistor. If this is so, should I get this even know the secondary side of this resistor is connected to ground? I guess the ground is what is throwing me off.

All connections to the transformer look good. The PH1 fuse comes directly of the switchgear bus, the the PH1 nuetral fuse is connected to a wire going to the transformer nuetral. What would be the effect of additional resistance or corroded pads as you mentioned?

 

[waross]

When you see zero volts across the test points, this indicates an open circuit somewhere on the transformer side of the test points.
If you have a 480V wye system, you will have 277V from phase to neutral.
If you do not see 277V (280V) at the test points suposedly connected to a phase and neutral, then there is an open circuit somewhere.
Somewhere there is an open circuit. Probably on the neutral side.
Your 23 volts is an indication of system insulation capacitance to ground. With the ground lifted, the charging current will not have a path through your circuit and you will read zero volts.
Everything points to an open connection between your neutral test point and the transformer neutral.
In test position you should read close to 277 volts across the open circuit when you find it.
Measure the voltage from the transformer neutral and the neutral test point when you test.
Make sure that your transformer neutral pad is a neutral and not a case grounding connection.
In case I have talked too much and confused the isue:

OPEN NEUTRAL!
OPEN NEUTRAL!
OPEN NEUTRAL!


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

 

[burnt2x]

Your switch contacts are no good! Analyze it this way:
When you pushed the TR button, the voltage across the 300 ohm HREA resistor dropped to 23 volts. The current flowing from PH1 through your "faulty" TR switch is 23V/300 ohms = 0.076 amps! You cant read it if your clamp is set high! The voltage drop across your switch is 277-23= 254 volts. that means the contact resistance of your "TR" switch is about 3.3K-ohms!
Hope this helps!

 

[rockman7892]

burnt2X

I think you are forgetting about the 75ohm resistor that is in series with the 300ohm resistor. You are saying that 23V is across the 300ohm resistor but wouldn't the 23v be across the 375ohm total resistance thus making the drop across the 300ohm less than 23V.

Also no matter what the current flowing in this circuit is, shouldn't the 277V difference at the source always remain the same. In othr words if you performed a KVL loop around the circuit the voltage at the measuring points should remain the same no matter what the current is

 

[waross]

Also no matter what the current flowing in this circuit is, shouldn't the 277V difference at the source always remain the same. In othr words if you performed a KVL loop around the circuit the voltage at the measuring points should remain the same no matter what the current is

YES


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

 

[NAZ55]

I am not sure I am understanding the schematic.

What is PH1?

Shouldn't high resistance ground resistor connect between neutral and ground?

If the neutral is connected to the ground through HRG then another scenario that Bill didn't mention would be the disconnect between ground and HRG, which would cause the test points between the neutral and the ground side of HRG to be at the same potential (Thevenin), thus giving you 0.

 

[burnt2x]

First off: your grounding system is originally designed for high resistance grounding of 1A. The test contact connects your phase winding to ground through HREA, a 300-ohm resistor. By closing contact "TR", you're basically pulling down the phase potential almost to ground; limited by the 300-ohm resistor VD. The normal current on HREA branch should be = 277/300 = 0.923A. However, the return path to the source is through series resistors HREB-HREC (Pulser branch) and HREE-HRED (HMR MR HSP branch).

Second: Normally, the current through the HREB-HREC (Pulser)branch will be intermittent (depending on whether the pulser contact PR close or open).
If the HRED is set to Zero ohms, the HREE-HRED total resistance (trafo neutral grounding resitance) will now be 75 ohms only.

Your case: To analyze properly, forget about the case where the PR contact is closed (HREB-HREC circuit out). The current through the HREA circuit will be the same current through the trafo neutral or HRED-HREE circuit = 23/300 = 0.076A (23V as you measured at Point A). That raises the trafo neutral from the normal zero potential to 23V, above ground potential.

The current on the neutral CT secondary in your present case is very small = 23V/300 X (100/[5 X 10]) = 0.0038A!

Another thing is, if your HMR MR HSP relay is set to activate at =0.0.923 X 75 ohms = 6.9 volts, and you have bad contact at your TR, as what I have analyzed, the VD on HREE resistor (which your HMR MR HSP relay gets input voltage from will now be = 0.076 X 75ohms = 5.7 volts - your relay will not activate.

I strongly recommend checking the contacts or connections on your TR switch! Crystal?

 

[waross]

I suggest that any analysis of the grounding circuit is moot and confusing until it is determined why you are not reading 277 volts between Line and neutral on the transformer.
Measuring point "A" is shown as from line to neutral on the transformer. If the voltage here drops to 23 volts That is a voltage drop on the transformer of about 92%. If the transformer has 5% regulation, that is a current of about 18 times full load current. We either have a grounding circuit that is imposing a load of 1800% of full load or a high resistance path in the circuit from the transformer terminals to measuring point "A".
Check the conductors and fuses supplying the grounding circuit. Once the high resistance in the supply conductors is fixed the grounding circuit will probably work properly.
Beware of bad adjustments made in error to the grounding circuit during troubleshooting efforts. I would recheck all resistor tap settings before energizing after the problem is located and repaired.

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

 

[rockman7892]

Waross

I think I now see what you are saying about an open nuetral or a high resistance connection to nuetral. I have drawn a schematic (attached) to help explain my understanding.

The attached drawing shows an example circuit that would be similar to the one I had in order to help explain why the voltage is dropping.

In figure I there is a high resistance connection to ground shown represented by the 10k resistor. With the switch open in this cirucit I would still read 280V at the measuring points because with the switch open I'm seeing two differnet potentials and no current is flowing.

In figure II with the switch closed there is now current flowing. If I measure voltage at my measuring points it appears as 0V because now with the switch closed these two points are connected to the same potential. The reason that they are connected to the same potential is because the high resistance connection is causing little current to flow and therefore there is little to no voltage dropped across the 10ohm resistor. With very little voltage dropped across the 10ohm resistor the measuring points then appear to be the same potential. However in order to satisfy a KVL for this circuit, the current must cause all of the voltage to drop across the 10k resistor. Therefore if I was measuring voltage between the actual source outputs I would still see 180V but since I'm not I'm seeing the results I'm seeing.

Am I understanding this correctly now?

 

[rockman7892]

I'm sorry I posted the wrong schematic in the last post.

Here is the correct one.

 

[waross]

That looks right. One other thing, there may be an open connection rather than a high resistance. The system capacitive charging current would give you 280 volts at the test point.
I would check the neutral side fuse and fuse holder first. After that check all connections back to the bus connection.
If the following suggestions are against plant safety policy do not follow them, but if it is possible hold the test button so that you read 23 volts at the test points. Leave one volt meter probe connected to the neutral side of the test points and use the other volt meter probe to check the circuit. It will probably show about 257 volts to ground but that is not important. With one probe one the neutral test point, use the other probe to check both sdes of the neutral fuse holder. Then work back to the neutral bus. You should read zero volts or close to it until you pass the bad connection or high resistance area. Then the voltage will jump to 280 volts. Be careful. Almost all of your circuit may be at over 250 volts to ground. There may not be enough energy to cause arcs or burns but there will be enough energy to cause death by electrocution.

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