This article is intended to provide a brief overview and methodology of the application of Current Injection Testing to earthing system assessment.
For a general description and introduction to earthing, read the Introduction To Electrical Earthing article.
For a guide to applying the most appropriate testing methodology to assess your earthing system read the Application Guide to Earthing System Testing.
For a guide to current injection testing, read our article Earthing Assessment using Current Injection Testing.
An earthing system is any system intended to carry fault current back to the source, and commonly consists of electrodes and buried grid in contact with the soil, and also includes overhead earth wires, cables screens, connected metallic objects, and the soil itself.
Current injection testing is used to simulate a single phase to earth fault by injecting a low power, off frequency current into the earthing system.
Earthing Assessment Aims
Primary aims of current injection testing are to obtain the following, as accurately (with respect to actual fault scenarios) as possible:
- Earthing system impedance and grid resistance
- Earth Potential Rise (EPR)
- Voltage hazards (step, touch and transfer voltages)
- Current distribution measurements
Feeder Outages for Assessment
To be as accurate as possible, ideally the actual fault circuit would be used. For example, let’s assume we have a substation is a 132kV to 33kV, and the 132kV has an overhead earth wire.
A 132kV fault at the substation would be supplied over the 132kV line, with some return current (aided by inductive coupling) over the overhead earth wire. The most accurate test is to pass current from the source substation to the 132/33kV substation, using the out of service feeder.
What if we can’t get an outage?
All is not lost. In many cases, you can still obtain useful and valid data by using a temporary injection circuit. This means installing cables and an earth grid some distance from the substation.
How far away should my temporary earth be?
A common rule of thumb that is often observed is ten times the grid size, in homogenous soils. If the soil resistivity model is high on low, it may not need to be as far away, but if it is low on high, significantly further may be required.
The distance is required to reduce the Mutual Earth Resistance (MER) between the two grids. Imagine if you will, the circuit being so short that the substation and temporary grid were touching – this would be a short circuit, and the substation would appear as though it has a grid resistance of 0 Ω which is of course incorrect. This is an extreme example, but MER can reduce the apparent grid resistance, so it needs to be considered.
To enable a comparison of limitations, the following tables give pros and cons of injection testing with a feeder, and with a temporary circuit.
Pros and Cons of Feeder Injection Test
|Any MER (mutual earth resistance) effects are as per the actual fault scenario||Can be difficult to get an outage|
|Includes any inductive coupling on cables screens/OHEW’s||Results may not be applicable to other fault scenarios (especially where inductively coupled paths are included). Multiple tests may be required for multiple scenarios, or correction in software required such as CDEGS Hifreq.|
|Most accurately represents the fault scenario under test|
Pros and Cons of Temporary Circuit Injection Test
|No feeder outage required.||Doesn’t include inductive coupling, so may need to be corrected to the actual fault scenario using software such as CDEGS Hifreq.|
|Lack of inductively coupled paths makes the test conservative, so may be applicable for multiple fault cases.||Possibility of having MER if the temporary grid is too close. May need to be corrected in software if unavoidable.|
|Set up of the temporary circuit is not dependant on operator, switching and load requirements.||In city environments where the MEN (multiple earth neutral) LV network is bonded to the earth grid, MER may be significant. This is difficult to correct post-test.|
|Very difficult to run the leads in city environments, across roads etc.|
In practise, the most appropriate circuit is chosen based on a balance of the factors outlined in the tables above. In a busy city environment, a feeder outage may the only practical method, and redundant feeders in the city makes it a viable option.
In a rural setting, getting a feeder outage may be impossible, but with plenty of room, testing using a temporary lead is easier and less error prone than the city test.
Zero Sequence Earthing are happy to provide advice and assistance with carrying out earthing system assessment.
Zero Sequence Earthing
(02) 4039 8000 / 0434 190 272