NATIONAL ELECTRICAL ENGINEERING …

1 NATIONAL ELECTRICAL ENGINEERING consultancy Design Management Construction 4 54/60 Links RD St Marys, NSW, 2760 ACN: 132586675 ABN: 86132586675 Current Injection Test (CIT) Procedures for High Voltage Infrastructure NATIONAL ELECTRICAL ENGINEERING consultancy Current Injection Test (CIT) Procedures for High Voltage Infrastructure Prepared by: M. Nassereddine NATIONAL ELECTRICAL ENGINEERING consultancy Design Management Construction 4 54/60 Links RD St Marys, NSW, 2760 ACN: 132586675 ABN: 86132586675 Current Injection Test (CIT) Procedures for High Voltage Infrastructure Abstract: The existing of the High Voltage (HV) infrastructure creates a special set of safety circumstances.

2 Earthing system is one of the main elements to mitigate any unsafe conditions. Commissioning the earth grid certify that the implemented system fulfill to the pertinent necessities. This paper endeavor to present vital information on how to perform the earth grid commissioning of an HV infrastructure; it minutiae the minimum necessities to guarantee the test will symbolize the actual fault case that the design was based on. A flow chart diagram was established and presented in this paper to allow for the determination of the most suitable injection route. I. INTRODUCTION The demand on High voltage (HV) infrastructures is growing due to the corresponding growth in industries and population, mishandling HV infrastructure can cause damages to properties and may inflict injuries and fatalities.

3 High voltage infrastructure necessitates earthing design to warrant the safety and the acquiescence of the system to the confined standards and regulations. Earthing system presents a safe working environment for workers and people passing by during a fault or malfunction of the power system. The assumption that any grounded object can be safely touched is not always correct. A serious hazard may result during a ground fault from the transfer of potential between the ground grid area and grounded object locations. Adequate earthing system provide a safe pass for the fault current and warrant all the ground voltages potential are within the acceptable limits.

4 During the design stage of the earth grid, the designer gather all the possible information from soil resistivity, services in the area and the layout of the proposed substation. This information may not be very accurate which will not symbolize the real case. For example, it is hard to institute an exact soil resistivity structure to represent the actual one of the area; a number of designers follow the single layer structure approach, some others follow the two layers soil structure approach. The output of these two approaches provides a slightly different earthing system data. Before commissioning the HV infrastructure it is imperative to commission the earthing system to certify that the implemented earth grid meets the requirements.

5 Commissioning the earthing system is to simulate an actual fault using low frequency injection set; the commissioning of the earthing system can be divided into: 1. Earth grid resistance measurement 2. Step & Touch voltage measurement 3. EPR contour measurement 4. Transfer voltage measurement The current injection test facilitates inclusive testing of earthing systems, the test can certify many vital parameters such as touch, step and transferred voltage, it determines the actual voltage contour and the actual split factor under the presence of the return path. The CIT allows the test NATIONAL ELECTRICAL ENGINEERING consultancy Design Management Construction 4 54/60 Links RD St Marys, NSW, 2760 ACN: 132586675 ABN: 86132586675 Current Injection Test (CIT) Procedures for High Voltage Infrastructure of the actual voltage transfer to nearby infrastructure such as farm fences, water, gas pipeline, and telecommunications.

6 Also the test permits measuring the actual AC interference on any metallic structure running parallel the feeder This paper endeavour at presenting a general overview of the earthing system commissioning, it discuss the minimum requirement to complete the commissioning which will correspond to an actual fault situation. II. THEORETICAL STUDY The commissioning team of the earthing system could be from different company to the one that completed the design, therefore, the first step is gathering the information, which will assist to understand the extend of the test and what shall be tested. It is important to make sure all the following information are available: Site layout Grid layout Maximum fault current Clearance time Back up time Location of the feeding station Near-by utility Communication network in the area Feeding characteristics Any special considerations Surrounding infrastructure If is vital to evaluate the gathered information with the one from the earthing design report.

7 It is important to choose an injection set that will prevent any interference from occurrence especially the interference from another transmission mains or substations. Every AC ELECTRICAL power run on a nominated frequency range, in the USA 60 Hz is used and in Australia 50 Hz is used. Each frequency has its own harmonic series, for example, the 60 Hz frequency has 120, 180, 240, 300 Hz as harmonic frequencies, for the 50 Hz the harmonics are 100, 150, 200, 250, 300 Hz. The injection set frequency should be chosen to ensure no overlap on the harmonic frequency between the 50Hz and the injection frequency. HV substation earth grid commissioning can be divided into two sections: 1.

8 Substation supplied with return path (cable screen/OHEW) 2. Substation supplied without return path Return Path Under the existing of the return path, and in order to achieve a real fault scenario, it is important to inject using the in-feed to the substation to allow for the mutual impedance between the faulted NATIONAL ELECTRICAL ENGINEERING consultancy Design Management Construction 4 54/60 Links RD St Marys, NSW, 2760 ACN: 132586675 ABN: 86132586675 Current Injection Test (CIT) Procedures for High Voltage Infrastructure phase and the return path, figure 1 shows a tested circuit with an OHEW as a return path, if the injection test occur in any other way, it will not yield to an accurate output.

9 The current injection test shall provide the followings as the minimum outputs: 1. Earth grid resistance value 2. Split factor (under the existing of a return path) 3. Actual Touch voltage 4. Actual Step voltage 5. Actual EPR contour 6. Actual transfer voltage to near by infrastructure In order to achieve the actual value of Step, Touch and EPR, it is important to have the actual current that use the return path and the actual current that use the earth grid of the substation, this only possible if the test allow for the mutual impedance to be present during the test. Figure 1 shows the mutual impedance under the presence of the return path Figure 1: HV transmission main with return path Without Return Path In some cases, HV substation is supplied with and over head feeder without the existence of an over head earth wire, or it could be an underground feeder, with the cable screen not bonded both end.

10 Figure 2 shows the HV substation supplied with an OH feeder without any return path. Under this condition, it is possible to set up an injection path as long it maintains the required separation between the tested earth grid and the injection electrode. Two methods to commission the earth grid system without a return path: 1. Use the feeder phase as an injection path 2. set up an injection lead with temporary injection electrode NATIONAL ELECTRICAL ENGINEERING consultancy Design Management Construction 4 54/60 Links RD St Marys, NSW, 2760 ACN: 132586675 ABN: 86132586675 Current Injection Test (CIT) Procedures for High Voltage Infrastructure If the injection uses the feeder phase as the injection path, it will allow for an AC interference measurement check if there is any metallic object running parallel to the feeder such as pipe line or metallic fence.