Zero Sequence Circulating Current

[tiewrc]

My 11kV Busbar feed by 15/11kV Delta-Start Transformer, Star side Earthed by NER. At the same time another standby diesel Genset connected and neutral point earthed by NER as well. To operate both parallel, what should i set for my Diesel Genset's EarthFault Protection to avoid spurious tripping by 50Hz and 150Hz zero sequence circulating current?

Thanks

 

[rcw retired EE]

Good question. I have had problems estimating circulating zero sequnce currents when no neutral to earth impedance was used. With the two NER units, sero sequence currents will be quite limited. I suggest you set the relays for pickup at 10% of the NER rating, not worrying too much about the impact of circulating currents.

 

[tiewrc]

may i know how to prove this 10% of NER rating will be sufficient?

 

[RRaghunath]

What is the NER rating of generator and Dy trafo. The recommendation I have come across is that you can earth both generator and trafo star points, provided the NER is sized to restrict the current to 100A or lower. This is because th NER value is high enough to limit the circulating current and resultant heating within safe limits. It is more of a thumb rule, I guess.

When it comes to setting the E/F protection pickup, the best would be to actually measure the circulating current and choose a value with a good margin, such as 150% of the measured value or more.

 

[rcw retired EE]

Tiewrc- My 10% setting suggestion is based on:

1. Rule of thumb for setting ground fault overcurrent relays when fault current is limited by a neutral earthing resistor, impedance, or grounding transformer: Set at 10% of the maximum current allowed by the eathing device.
2. The earthing devices limit the maximum fault current for a phase to ground fault. Both the transformer and generator contributions are limited by the earthing device. The driving voltage for ground fault current from both devices is the phase to neutral voltage.
3.Third harmonic voltages in both devices cannot be more than the fundamental voltages. The 3rd harmonic driving voltage is the difference between the the transformer and generator voltages. This difference has to be less than the fundamental phase to neutral voltage unless you have a serious synchronization problem. The difference will usually be a few % at most.
4.Any third harmonic circulating current will have to flow through both resistors and the interconnecting earth and phase wiring which will have some impedance.
5. Since the 3rd harmonic driving voltage is much less and the circuit impedance is at least twice as high, the circulating current should be much less than the designed phase to ground fault current.
6. I have never had a problem with this setting.

But as raghun says, I always measure that circulating current and adjust settings if needed.

 

[sobeys81]

rcwilson,

Interesting comments. I would apprecaite,if you could elaborate a little bit more.

1. Do we have circulating currents even if there is no ground fault ?

2. Could explain this arugument:"The 3rd harmonic driving voltage is the difference between the the transformer and generator voltages. "

I am just having a hard time digesting the above comment.

Thanks

 

[rcw retired EE]

sobeys81- When a 3-Phase, 4 wire generator is operating in parallel with a grounded wye transformer winding the two devices have their neutrals and phase terminals connected together by low impedance wiring or bus. Any voltage difference across that bus will cause current flow.

Look at just one phase. If the transformer phase-neutral voltage is 1.0 per unit and the generator is 1.02 per unit, there is a 0.02 per unit voltage difference between two terminals that are connected together by the low impedance cables or bus.

Current can flow from the generator's phase terminal through the phase cable to the transformer's terminal, down the phase winding to the transformer wye point and back to the generator via the neutral conductor.

If the transformer and generator output sine waves are not perfectly matched, say because the generator has the wrong winding pitch, there will be a voltage difference even if the RMS voltages of the transformer and generator are equal.

On a 3-phase machine the fundamental frequency currents cancel at the neutral, but the third harmonic currents from the three phases add. With the generator and wye transformer windings connected in 3-phase, 4-wire, system, significant currents can flow.

If we remove the neutral point connection but leave the neutrals connected to earth, the current will still flow using the earth path as a return.

Buying the correct generator or inserting neutral earthing resistors is the only way I have found to knock down the circulating currents.

I hope this explanation helps.

 

[sobeys81]

Thanks for the comments rcwilson.

So, let me see if I got it right:
Because there are two sources: generator and transformer are in vicinity of each other and might have different potential at their neutrals, it will lead to circulating currents(Normally in what situation a neutral point of generator or transformer will have voltage?)

Now, 3rd harmonic currents will only flow when you have a either ground fault or high non linear load ?

Suppose there is a non linear load and third harmonic currents from both sources will flow to ground and back to the source.

Now if this system is solidly grounded, there can be more currents because of no resistance available in the path. Agreed.

"If we remove the neutral point connection but leave the neutrals connected to earth, the current will still flow using the earth path as a return"

What does the above statement reflect ?

 

[dpc]

No, the third harmonic voltages are produced in the generator and will flow through the neutral continuously if there is a path. Third harmonic currents are NOT zero sequence currents, but they behave like zero sequence currents.

 

[sobeys81]

dpc, this is new for me. So you mean -ve seq. is not second harmonic also.

When there is an unbalance in voltage or load, generator will develop -ve seq. current which will lead to over heating same as single phasing in voltage.

So how does a relay differentiate between a zero seq. current and third harmonic current.

I woulda appreciate a word.

 

[dpc]

If the relay has harmonic filtering, it can be configured to respond only to fundamental current.

I don't know what "ve seq" means, so I'm not sure what you're referring to. But third harmonic current is definitely not the same as negative sequence current as far as rotor heating is concerned.

The biggest issue is generally the continuous rating of the grounding resistor, since most grounding resistors have only a 10 second rating. The generator rating or heating is not generally an issue with third harmonic currents.

 

[tiewrc]

Raghun- The NER for Transf rated at 10ohm /667A and NER for Genset rated at 10ohm/635A.

If the unblance occur or with connection of non linear load, may i know how the NER able to limit the zero sequence current? If it able to limit the zero seq current, what will happen to the load and is there any changes on flow of zero seq current path? It should be used to limit E/F current right?

 

[rcw retired EE]

Sobeys81 - I guess I confused you. As dpc said, the third harmonic voltages (180Hz if in US) are generated in the generator. They are superimposed on the 60 hZ waveform. The generator output is not a pure sine wave.

Assume the instantaneous generator votlage is Vgen = V1 sin(wt)+ k3 sin(3wt) + higher harmonics.

The transformer output is closer to a pure sine wave: Vtx = V2 sin(wt)

We have a circuit with two voltage sources, Vtx & Vgen, interconnected by the phase conductors and neutral conductors. The driving voltage across the circuit is (Vtx -Vgen) = k3 sin(3wt). For B and C phase the same 3rd harmonic voltage is present since sin(3wt +120) = sin(3wt)=sin(3wt - 120). (The 180hz sine wave is the same whether shited 120 degrees plus or minus).

The third harmonic cuurents in each phase have no phase shift and are all the same frequency. When combined, they add instead of combining like "normal" phase currents.

Third harmonic currents flow in each phase, circulating from the generator terminal, through the phase wires, down the transformer winding to the neutral and back to the generator neutral. The neutral wires carry all three third harmonic currents that add instead of cancelling.

If there are no neutral wires, but the transformer and generator neutrals are both grounded, the currents travel in the grounding system.

The currents are always present, fault or no fault.

As DPC said, the curents will probably not overheat the generator or the transformer, but they can create problems with ground fault sensing. If a grounding resistor is inserted in the generator's neutral to earth connection, it adds impedance to the circuit, reducing, but not eliminating the circulating currents. The resistor's continuous current rating needs to match the total circulating current.

One side note if that transformer is wye-wye, neutral currents will be induced on the primary side and can trigger the high voltage ground fault proteciton. Even if the HV is 115 kV with a 20 mile transmission line. I know.

Solutions are to buy a generator with a better waveform, increase the size and impedance of the groudning device to limit the current, use a delta transformer winding, or do not parallel the generator and transformer.

Back to your origianl quesition - Set it at 10% of the resistor rating and mesasure the 180 hZ current swhen you get on line.

 

[RRaghunath]

tiewrc,

"If the unblance occur or with connection of non linear load, may i know how the NER able to limit the zero sequence current?"

NER is in the neutral circuit and the zero sequence and triplen harmonic currents (third and its multiples) are supposed to flow through the neutral circuit. This is because all the three vectors will be in phase and hence addup arithmetically in case of zero sequence and triplen harmonic currents.
The triplen harmonics could be due to non-linear loads and zero sequence currents appear only when there is an earth fault in the system downstream.
Higher impedance in the circuit (by way of higher NER ohmic value) is obviously is going to restrict the magnitude of current.

"If it able to limit the zero seq current, what will happen to the load and is there any changes on flow of zero seq current path?"

The load currents are do not flow in the neutral circuit and hence the NER will not have any effect on the load currents which are balanced three phase currents (with the three vectors at 120degrees apart).

Trust the above is helpful.

 

[tiewrc]

Hi rcwilson, raghun, sobeys81, dpc: Thanks!

My standby diesel genset rated at 11kV, 2.5MVA gounded by 10ohm NER (Earth Fault setting 20% of CTR=150/1). Can i say when my genset parallel with 11kV, 12MVA incomer source gounded by 10ohm NER, considering genset loaded at 2MW ~131.2A. Then assume unbalance of 10%(What is the std value??)=131.21 X 0.1 =13.1A.

So 20% of 150A =30A for E/F pickup

So will this proven my Earth fault relay will not caused spurios tripping? Should i consider loading from incomer as well?

 

[RRaghunath]

If there are no non-linear loads at 11kV level, I think setting as suggested by you is ok. If there are any non-linear loads, suggest measure the current in the neutral and then adjust the pickup setting as required.

 

[jimgineer]

Three things:

Does someone mind explaining/expanding upon what the NER is?

Also, I have been told that one of the drivers of problems when it comes to circulating currents (and a reason that 4 pole ATS's are required in many >1000A applications) is due to having two separate earthing points, and connecting nuetrals directly in this case would cause two different potentials, induce circulating currents in the nuetral and ground conductors. This would trip ground fault in many cases, when its required for over 1000A services.

Can someone weigh in on this?

 

[davidbeach]

NER=Neutral Earthing Resistor.

 

[rcw retired EE]

Jimgineer- Your description of circulating currents is correct. A 4-pole ATS is one way of preventing problems.

In several applications the Emergency Diesel Generator (EDG) is designed to operate in parallel with the utility service. An ATS does not allow parallel operation.

Circulating currents can cause ground fault trips.

Placing a neutral earthing resistor (NER) in the DEG's neutral to earth connection knocks down the circulating current to acceptable levels. It also reduces the ground fault current available from the DEG.

Only use this solution if there are no phase to neutral loads.

Ground fault protection settings can get tricky due to the limited fault current through the NER.