Ground fault detection in Ungrounded System 5

[a10jp]

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.

 

[rbulsara]

Which literature are you reading?

Perhaps you want to read some in davidbeach's faq238-1287 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

 

[slavag]

Hi very shortly.
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

 

[xxjohnh]

In China they commonly install ungrounded MV systems. They have developed a special relays which is able to provide fault location. I think SEL has something similar that has been used as well. Contary to practice in EU and US

 

[slavag]

Hi.
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

 

[a10jp]

Thank you. Just want to add clarify the picture a little more. The system we have is 13.2KV ungrounded system. I did not underatand why we do it this way, but that is what we have. Method of protection was to use 59G to detect, and use 67 to trip. Earlier, rbulsara mentioned ungrounded and HRG system worked the same way as normal ground fault protection and utilize the GF CT for input. When I0 is only a few A, can we still use CTs for such small current? Now I am wondering if device 64 can be used. Also, the system I am looking at use only 67, but not 67N. What is the main difference between 67 and 67N that have been mentioned earlier?

 

[slavag]

67- is directional overcurrent protection
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

 

[odlanor]

There is a very good explanation about this problem in this Sell article:
Advanced Commercial Power System Protection Practices Applied to Naval Medium Voltage Power Systems.

 

[prc]

slavag,The system I had seen for 11 &33 kV ungrounded system was the use of three single pahse PTs or one 5 limbed PT connected to lines with one of the PT secondaries in broken delta to sense voltage during ground fault.How this compares with the practices mentioned by you.

 

[slavag]

Sorry prc.
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

 

[prc]

slavag,We use such a scheme to find out inadvertant earthing of tertiary bus of auto transformers. Single phase Pts will be connected to line with secondary windings of 110/3 V winding will be connected in broken delta.The ends of broken delta will be connected to a voltage sensing relay.Normally voltage across the corners ie ends of broken delta will be zero.In case of a solid ground fault on any line,voltage across the secondary broken delta will rise to 110 V raising alarm or trip.

 

[slavag]

Hi prc.
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

 

[a10jp]

Hi Slava, how are 59N and 59G different?

 

[slavag]

Hi.
It's same-residual overvoltage protection, connected to broken delta of VT secondary..
Best Regards.
Slava

 

[a10jp]

Hi, the system we have used a parameter called neutral displacement which utilizes 3*V0 as the input. The pickup level is set at 0.85 *VT using a definite time curve as part of the overvoltage detection. Our first mistake was to set this all in trip mode. But now we learned you cannot properly trip a circuit (using only V0) until you know where the fault is, which is where 67 is used for directional I0 element. Make sense?

 

[slavag]

Hi.
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

 

[Bronzeado]

Hi folks,

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

 

[odlanor]

We have been experienced with these protections. Never use 67 to trip!It is too much sensitive and you could have many maloperation!
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

 

[davidbeach]

Bronzeado, yes. Arcing ground faults can develop phase-ground voltages well in excess of phase-phase voltage; voltage builds up until something flashes over.

 

[slavag]

In additional to David.
Especialy in case of time betweem restrikes is less than 100ms.