Secondary Voltage of Broken Delta VT for 67N Protection of a Grounded Wye System

[Haroon Baloch]

I'm working on a 4.16kV, 40kA-3sec, 4000A Switchboard. We need Directional Earth Fault (67N) protection on the Generators. The relay manual (Siemens 7SR2..) mentions that the polarizing quantity for the directional protection can be "calculated from the 3 phase voltage inputs or the 3Vo input". We are using a Broken Delta VT configuration to provide the 3V[sub]0[/sub] input. I want to know what should be the secondary voltage of a single phase VT (V[sub]LN[/sub]) so that when connected in Broken delta it will give 120V output during ground fault.

The Broken delta VTs from my previous work experience have connections for 120/Sqrt(3) and 120/3 V, never 120V.

One source I checked mentions that for ungrounded systems 3V[sub]0[/sub] = 3V[sub]LN[/sub] and for grounded systems 3V[sub]0[/sub] = V[sub]LN[/sub]. From this, it seems like my single phase VT secondary should be 120V not 120/3.

Am I right to go for 120V secondary in this case? Should I consider additional connection option for 120/Sqrt(3)? Please let me know and share any references, if possible.

Thanks

 

[waross]

I am assuming that the PTs are rated for line to neutral voltage and are connected line to neutral on the primary side.
With rated voltage applied to the primary of a ?:120V PT the secondary will develop 120 Volts.
The three phase of 120 Volts will sum to zero Volts.
If a primary phase is grounded, that PT output voltage will drop to zero Volts.
The remaining two PTs will act as an open delta and will develop 120 Volts.
Disclaimer:
If the PTs are rated for line to line voltage but then connected line to neutral, then the root-3 factor enters the equation.
I hope that I have answered the right question.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

Agree, seems like 2400/120 PTs would be the correct ratio in your case.
I’m surprised you are using a separate Vo PT input versus having the relay do the 3Vo calculation from the 3 phase PT inputs. Just for my own curiosity, any particular reason?

 

[Haroon Baloch]

@waross You're right. The VTs are rated line-to-neutral and connected in Wye Grounded on the primary side. I went for line to neutral since the system is Wye grounded. I suppose if it was Delta ungrounded then i would have to choose line to line rated VTs, correct me if i'm wrong. Also, let me know if a Ballast Resistor is required in this case. I have attached a link below, can you let me know if their approach is right?

Link

@wcaseyharman Yeah, a 4160/Sqrt(3) // 120V line to neutral rated VT would be right.
Its a project requirement to use a separate Broken Delta winding in the VT. So each single phase VT has one winding for Wye-Wye and one for Wye-Broken Delta. I have seen this practice before too, though I'm not sure why. Apparently, theres some benefit to using dedicated winding for 3V[sub]0[/sub] output.

 

[Kiribanda]

You should have 120V secondary line to line. But your primary should be 5kV line to line and not 5kV/ 1.732.
Therefore, you should install 3 VTs having the turns ratio=5kV/120V=42 where the secondaries
are connected in "BROKEN DELTA" and not open delta.

 

[davidbeach]

In a modern design, the two secondaries would be matched and both connected in wye. One secondary would go to the A relays and the other secondary would go to the B relays. Do that, and 3V0 is just math. Then you would have redundancy against a blown secondary fuse that you don't have now.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[wcaseyharman]

I’ve seen two winding PTs connected like you describe, but it was on an ungrounded bus and the broken delta was connected across a ferro resonance suppression resistor. Seen it also where one set of PTs is open delta and the other set broken delta, but these were for high resistance grounded generators. Can’t think of a reason for a solidly grounded wye system, the math should give the same result as the PT wiring.

 

[Haroon Baloch]

@wcaseyharman My system is also resistively grounded Wye. It's resistively grounded via a Neutral Earthing Resistor. Does it change anything, whether the system is solidly grounded or resistively grounded?

There are conflicting answers here on whether the VT should be line-to-line rated or line-to-linear. From the above replies, It seems to me that the VT should be 120V secondary line-to-neutral rated, since the system is Grounded. Is it correct for solidly grounded systems only? Do resistively grounded wye system require 120V secondary line-to-line?

 

[wcaseyharman]

Yes, assuming it’s not a very small resistance.

If it’s high resistively grounded you probably want to go with line to line rated PTs so they don’t see massive overvoltage during ground faults.
It will also make Vo equal Vln when one phase is grounded, which I think means if you want 120V during ground faults your secondary voltage will need to be 69 V.
So for a high resistance-grounded system I think your PTs would need to be 4160-69V.

 

[Haroon Baloch]

@wcaseyharman The resistance of the NER is 24 Ohms. Also, even if I am rating the VT for line-to-line, it should be 4160/120V, since during fault 3V[sub]0[/sub]= 3V[sub]LN[/sub]. So I need the secondary line-to-neutral output (V[sub]LN[/sub]) of each VT to be 120V.

 

[wcaseyharman]

Let’s think about this - 24 ohms will dominate the fault resistance, so I think this would be considered high resistance grounded system.
That means Vo equals Vln as you’ll have full neutral shift during single line to ground fault conditions. Vo then would be 69V. The voltage across the broken part of the delta is 3*Vo. 3*Vo would be 207V, which is 1.73 * 120V.

Thinking this another way, the unfaulted phase voltages will increase to 4160V during the fault. The secondaries would then be 120V on two phases, zero on the third phase. Drawing it out on the board with the phase angles I again get 207V across the broken part of the delta.

Therefore I am pretty sure that if you truly want 120V during a single phase to ground fault, you will need to have a PT ratio of 4160V/69V.

 

[Haroon Baloch]

@wcaseyharman I kinda get what you're saying, but its a bit confusing, so below is my step by step approach. Let me know where I am going wrong.

- The Broken Delta output is 3V[sub]0[/sub].
- V[sub]LN[/sub] is the rated secondary voltage of the single phase VT.
- For a resistively grounded system during ground fault 3V[sub]0[/sub]=3V[sub]LN[/sub].
- Therefore the VT ratio should be 4160 // 120/3.
- During ground fault, 3V[sub]0[/sub]=3V[sub]LN[/sub] = 3*(120/3) = 120V.

 

[Haroon Baloch]

@wcaseyharman You are right, I get it now. I was wrong about the second point V[sub]LN[/sub] is not the rated voltage of the VT. It is the VT secondary voltage under normal(un-faulted) condition. So if we choose 4160 // 120/sqrt(3) = 4160 / 69 VT then under normal operation V[sub]LN[/sub] = 69/sqrt(3) and under fault it will be 3V[sub]LN[/sub] = 3 x 69/sqrt(3) = 120V.

I believe if the system is not designed to run on ground-fault and the VT is rated for overvoltage for a short period then we can use line-to-neutral VT 4160/sqrt(3) // 120/3. It will also give the same Broken Delta output.

Is ballast resistor required even when the system is grounded (with a resistor)? Do you have any reference for calculating the Ballast Resistor value and its application?

Thanks

 

[Haroon Baloch]

@wcaseyharman This is a dumb question but I just want to be sure: If there are two VTs (single phase) 4.16/sqrt(3) // 120/sqrt(3) and 4.16//120 VT, will we get the same output, that is 120 L-L and 120/sqrt(3) L-N for a 4.16kV system voltage, since the ratio of both VT is same? The only difference will be the insulation level (max operating voltage) of the VT?

I want to be sure about this because the VT has another secondary winding for Wye-Wye connection and I want 120V L-L output there.

 

[wcaseyharman]

Yes, that’s correct.
Of course unless the L-G PT saturates at the higher voltage. I would have to brush up on my PT ratings - at some point the iron will saturate, but I am not sure how much overvoltage PTs are capable of before that happens, I’d have to look it up.

A quick google search suggests different overvoltage capabilities of PTs, so I think the performance of the L-G PT under fault conditions will depend on the quality of the PT you end up purchasing. If you go with a 2400V primary I think you’ll need a 2400V/40V PT to get 120V during the ground fault (Vo would equal 40V during a ground fault, 3*40=120)

I don’t think you need the ballast resistor, that’s only for ungrounded systems. The neutral resistance performs that function.

 

[Haroon Baloch]

@wcaseyharman Thanks a lot for your help. I will check with the PT manufacturer for the overvoltage capability. I believe, in this case, its better to go for line-to-line rated PT 4160/120 since then I wont have to worry about PT saturation during fault and it will also give 120V L-L in Wye-Wye when the system is running normal(un-faulted condition).