Feeder Outages for Earthing System Current Injection Testing

This article provides a brief overview of Current Injection Testing for earthing system assessment. An earthing system carries fault current back to its source, consisting of electrodes, buried grids, overhead earth wires, cable screens, metallic objects and soil.

Current injection testing simulates single-phase-to-earth faults by injecting low-power, off-frequency current into the 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

For utmost accuracy, the preferred approach is to employ the actual fault circuit. Consider a scenario where a substation operates at 132kV to 33kV, with an overhead earth wire at 132kV. In the event of a 132kV fault at the substation, current is supplied via the 132kV line, with some return current facilitated by inductive coupling through the overhead earth wire. To achieve the highest precision, conducting a test involving current transfer from the source substation to the 132/33kV substation through the out-of-service feeder is recommended.

What if we can’t get an outage?

When faced with the need for temporary injection testing, obtaining valuable data remains possible. To determine the ideal distance for your temporary earth, a rule of thumb often used is ten times the grid size in homogeneous soils. However, in cases of high resistivity on low resistivity soils, shorter distances may suffice, while the opposite may require greater separation. This distance is essential for reducing Mutual Earth Resistance (MER) between the grids. MER can distort the apparent grid resistance, making careful consideration crucial to accurate testing.

The choice of the most suitable testing circuit depends on a combination of factors, as detailed in the provided tables. In urban areas with multiple feeders, a feeder outage can be a practical option, while in rural settings, utilizing a temporary lead is often easier and more reliable and Zero Sequence Earthing offers guidance for these assessments.

Pros and Cons of Feeder Injection Test

Pros

• Any MER (mutual earth resistance) effects are as per the actual fault scenario
• Includes any inductive coupling on cables screens/OHEWs
• Most accurately represents the fault scenario under test

Cons

• Can be difficult to get an outage
• Results may not be applicable to other fault scenarios (especially where inductively coupled paths are included). Multiple test may be required for multiple scenarios, or correction in software required such as CDEGS Hifreq.

Pros and Cons of Temporary Circuit Injection Test 

Pros

• No feeder outage required.
• Set up of the temporary circuit is not dependant on operator, switching and load requirements.
• Lack of inductively coupled paths makes the test conservative, so may be applicable for multiple fault cases.

Cons

• Doesn’t include inductive coupling, so may need to be corrected to the actual fault scenario using software such as CDEGS Hifreq.
• Possibility of having MER if the temporary grid is too close. May need to be corrected in software if unavoidable.
• 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.