Ground fault detection in Ungrounded System
Ground fault detection in Ungrounded System
(OP)
I am trying to understand the method to detect ground fault if you have an ungrounded system. In a few literatures I read that you will use relay 59G to sense gorunded overvoltage. Because the ground fault current in this setup is very small, they said that you will require a sensitive device like directional overcurrent relay like device 67 in order to trip the circuit. First, is this a common strategy for these type of system? Is you have high resistance grounded circuit, would the situation be different? When using HRG system, the benefit is the grounded voltage cna be stablized, as compared to unground sytsem, so, then, is that an issue with using device 67 which uses the grounded voltage (in ungrounded system) to determine a trip setting, knowing the setting range will be sensitive. My apology if the question is confusing, as I am confused.






RE: Ground fault detection in Ungrounded System
Perhaps you want to read some in davidbeach's FAQ238-1287: What are good references for a Power Engineer? mentioned under Power System Protection.
Very briefly:
59G is most common for ungrounded and HRG systems. Since there is no current to speak of you depend on sensing the voltage distortion during a fault.
67 is just a directional device and has nothing to do with sensitivity. What you are referring to is perhaps the polarizing voltage. 67 do need a polarizing input (a reference for direction) either in form of a current or a voltage. The current sensing part still requires a CT input.
Sensitive ground fault detection is something different which can be applied to HRG system where current is very small but it still works the same a normal ground fault protection and needs an input from a GF CT. or residual connection.
Rafiq Bulsara
http://www.srengineersct.com
RE: Ground fault detection in Ungrounded System
59G is goodest ground fault detection for whole type of grounding, but is protection per bus, per tansfromaer.
For ungrounded, HGR and compenstaed grunded systems used 67N function based on classical wattmetric or varmetrec methods.
You can use 51N too, but you need calculated possible capasitive current.
I recommend to you read great documents on the issue, from SEL, some registration, but isn't important.
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
Few contries are still used ungrounded MV systems.
In few places ground fault protection used for signal only, not for trip.
I don't want continieu with lot of problems of this networks, such of transient overvoltages, repetitive restrikes with less than 100ms, etc..etc... Big problem is cross-country faults. Up today used method of only two CT-s in such systems for avoid part of cross-countries faults and tripped two CB's instead one.
Protection methods for such systems are:
1. Varmetric method : U0xI0xsinPfi(0) or I0xsinPhi(0) and U0 as starter.
2. Method are base on high harmonics in the strikes.
3. Injection method.
Lot of companies have a good products with years of expirience: ABB, VAMP, Siemens.
Siemens, for exapmle, used letters 67Ns, VAMP CAP/RES, ABB cos/sin Pfi, for protections in ungrounded and compenesated systems.
Don't forget, you have ground fault in ungrounded systems too, it's capacitive current, for 20kV cable 500sq.mm is about 4-5A per 1km.
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
67N- is directional earth/ground protection
67Ns- is directional sensetive earth/ground protection.
See here possible some porblems
67N is directional, base on the I0, U0 and angle between them. Isn't exactly good for ungrounded system.
Siemens use 67Ns naming.
SEL, if I remeber right, used 32W or 32V naming.
Size of toroid..is depend. I hev expirence with ungrounded system only once. 6.9kV.
We install toroid 100/1A ( now burnt2x saied something
connect 1A to 0.2A input of relay and put minimum setting 1%. That means 0.2A primary.
It's work w/o any problem.
Always Q, why 100/1A, this size have a goodest performence for angle and amplitude errors for the small currents.
Hope that help.
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
Advanced Commercial Power System Protection Practices Applied to Naval Medium Voltage Power Systems.
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
I don't know about this method.
Are you have some scheme of such connection? Could you please attach it and we'll try understand, what is it.
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
OK, now is clear.
It's classical connection of broken delta for tertiary AT winding ( delta)protection. We used it too, btw, tertiary AT(T) winding in our area is not used.
It's simple 59N function, not directional, as I was posted
"59G ( 59N) is goodest ground fault detection for whole type of grounding, but is protection per bus, per transformer".
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
It's same-residual overvoltage protection, connected to broken delta of VT secondary..
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
Yep it's possible too.
BTW, neutral displacment and residual voltage and zero seq. voltage are same. Neutral displacment is more Siemens's naming.
And not 67 element, 67N element ( if it Siemens relays 67Ns function). You need take in account biggedt problem of such system, cross country faults.
For reduce percentage of such faults, used only two CT's and toroidal CT on all 3-ph. 0.85 seems a big value. Are you use broken delta VT connection or internal calculated?
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
I wonder if the phase-to-earth voltage in an isolated (ungrounded) system can reach values more than phase-to-phase voltage.
Best regards,
Herivelto Bronzeado
RE: Ground fault detection in Ungrounded System
It is used in power plant with multiple generator paralleled at same busbar and connected one step up transformer. Generating Voltage system is high impedance grounding by each neutral of generator(5A).
Each generator has 59N and 67NS very sensitive connected at each CT summation close to busbar.
There are 2 trips:
67NS + 59N => trip 52G
59N with delay => trip 52G
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
Especialy in case of time betweem restrikes is less than 100ms.
RE: Ground fault detection in Ungrounded System
Do you have literature on this phenomenon (neutral instability)?
Regards,
HeBron
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
A star for you!
Regards,
HeBron
RE: Ground fault detection in Ungrounded System
RE: Ground fault detection in Ungrounded System
it is almost the same.To decipher the numerous numbers and letters that protection experts use,refer to IEEE std C37.2-2008 Device Function Numbers,acronyms and contact designation.Equivalent IEC is 61850-7-4.
"The suffix N is preferred when the device is connected in the residual of a polyphase circuit, is connected across a broken delta, or is internally derived from the polyphase current or voltage quantities. The suffix G is preferred where the measured quantity is in the path to ground, or, in the case of ground fault detectors, is the current flowing to ground. See Figure C.2 in Annex C for examples."
Similarly "Device number 59—overvoltage relay
A device that operates when its input voltage exceeds a predetermined value." etc,etc
Let me share some "stories" on the subject.
In mid 70's, when one of the earliest 400 KV transformer banks were commisioned in India-it was a 250 MVA bank with 11 kV tertiary formed by power cables,one corner of the delta was earthed.After a couple of monsoon rains,one of the cable developed ground fault creating a L-L fault on tertiary causing tertiary winding failure.
To overcome this, we removed one corner earthing and put three single phase PTs (with primary neutral grounded) at tertiary terminals with 110/3 V secondary winding connected in broken delta to 59N relay.So in case of a LG fault on any line,59N relay will pick up 110 V residual voltage to give alarm.
After some years,during mid 80's I had to face a case where 20-50 v was coming across the broken delta without any apparant earthing on tertiary bus anywhere. Cliet was not ready to accept the transformer suspecting some defect.
Then I found out that American GE and Westinghouse engineers had experienced the same problem in ungrounded MV systems during mid 40's and they explained the reason for this neutral displacement as the resonance between the inductance and capacitance of the tertiary circuit.Solution for this is to connect surge absorbers (capacitors) to tertiary terminals (which we did) or connect a resistor in the closed delta of the PT.Those old AIEE papers saved my skin at that time.
RE: Ground fault detection in Ungrounded System
I, too, obserevd that all the feeder circuit in our system has a surge protection/arrestor device on EVERY feeder circuit, I believe it conforms to your statement before.
Also, after readin all the discussion above and tech papers in the referneces, at this point, I wanted to ask my original question: is the method that we have discussed so far a common method used for ungrounded system, with applications between 59N and 67N? Are there other methods available?
RE: Ground fault detection in Ungrounded System
Thanks prc.
a10jp.
Answer-yes. I would like say again, please check what type of 67N you will be use. I strongly recommned use I0xsin(phi) operation criteria. Is good and work w/o problem. Maybe, here, we have guys from Finland, them have a good expirience with such systems.
59N you can use for alarm and/or back up trip.
67N/67Ns/34Q for detect, what feeder is faulty.
BTW, in newer realys, you don't need additional 59N relay for start 67N function, you have parameter U0 in relay.
That menas, criteria will be I0xsin(phi) and U0.
Best Regards.
Slava
RE: Ground fault detection in Ungrounded System
I suggest the following papers which you can find in the respective sites.
1) A review of system grounding methods- Gerald Johnson
www.basler.com
2)Ground fault protection on ungrounded systems-Application Guide.www.i-gard.com