NATIONAL ELECTRICAL ENGINEERING CONSULTANCY

Transcription

1 NATIONAL ELECTRICAL ENGINEERING CONSULTANCY Prepared by: M. Nassereddine

2 Abstract: The existing of the High oltage (H) infrastructure creates a special set of safety circumstances. 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 H 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 (H) infrastructures is growing due to the corresponding growth in industries and population, mishandling H infrastructure can cause damages to properties and may inflict injuries and fatalities. 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. 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 H infrastructure it is imperative to commission the earthing system to certify that the implemented earth grid meets the requirements. 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

3 of the actual voltage transfer to nearby infrastructure such as farm fences, water, gas pipeline, and telecommunications. 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. 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. H substation earth grid commissioning can be divided into two sections: 1. 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

4 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. 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: H transmission main with return path Without Return Path In some cases, H 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. Figure 2 shows the H 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

5 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. Figure 2: Substation Feed without a return path NEEC established the diagram in figure 3 as a guide to assist in determining the injection path and the minimum measurements required to yield the best output results for the CIT. the established diagram can apply to all type of earthing commissioning for high voltage infrastructure. Measuring the grid resistance of the high voltage infrastructure required a reference voltage; the reference voltage path shall be at 90 degree from the injection path if possible to eliminate/reduce the interference between the injection current and the reference voltage route. Also the separation between the voltage reference electrode and the tested earth grid should be large enough to ensure an accurate reading, as a general rule, the voltage reference electrode should be separated by at least 10 times the diagonal distance of the earthing system under test. For a large substation that contains a return path like OHEW, it is hard to establish a new route that represents 10 times the size of the earth gird (the OHEW with the earth grid on each pole form part of the earth grid under test). Under this situation, there are two possible methods for providing the remote earth reference: 1. Use the phase of another feeder as a route for the voltage reference and the earth grid of the substation of this feeder as the electrode reference (this works if this feeder is fed from different substation to the injected feeder) 2. A spare telecommunications circuit, these telecommunication circuit often run over a large distance and have an earth electrode at their pits It is important to insure that the reference voltage electrode is located outside the EPR zone of the injected system. If using distribution transformers earth grid as the reference voltage, it is important to ensure the there is no CMEN for this system otherwise there will be an EPR area around the reference voltage electrode which yield to incorrect measurement. Figure 4 represents the current injection test layout for measuring the earth grid resistance of an H infrastructure.

6 Figure 3: NEEC diagram for CIT procedures

7 Figure 4: Current injection test layout for grid resistance determination Figure 5 represents the shape of the measured reference voltage under different separation distance between the reference electrode and the earth grid under test, the voltage increases as the distance increases until it flatten out, the value of the flatten out can be used to determine the earth grid resistance using equation 1, in this equation, it is vital to use the current that utilize the earth grid as path under the presence of the OHEW, this only can be achieved if split factor measurement was completed. (It is important to note: Under the presence of a return path, I is not the injected current, it is the current that use the earth grid as a path) grid Z Measured flatten area Grid = (1) I grid Safety requirements The measured touch and step voltage shall be assessed against the allowable safety limits, the step and touch voltage can be determined from the two equations 2 and 3, these two equations are calculated using the resistance from a 50 Kg person that is used when assessing the public access area. Equations 4 and 5 calculate step and touch voltage using 70Kg body weight.

8 touch step touch touch Csρs = (2) t Csρs = (3) t Csρs = (4) t Csρs = (5) t Where C s ρ ρs = 2h Cs is the derating factor relating to surface layer thickness and resistivity ρs is the top surface layer t is the primary clearance time s (6) Figure 5: measured voltage against separation distance EPR contour can be determined using the maximum EPR and the measured voltage in figure 8, equation 7 represent the maximum EPR, equation 8 represent the EPR contour. This process can be repeated for different direction to ensure accurate data is obtained to draw the EPR contour of

9 the tested earth grid. Figure 6 represents the EPR contour computation; the EPR is high at the substation and decreases as the distance from the tested grid is getting greater EPR = I grid Z grid max (7) EPR contour = EPR (8) max fig 5 Figure 6: EPR contour against the separation distance III. CONCLUSION This paper shows the important of choosing the right injection path when completing a CIT for H infrastructure, depending on the H feeding arrangement and its route surrounding infrastructures, the CIT route shall be determined to ensure that the test will yield acceptable results. This paper introduce NEEC CIT diagram, it shows the important of using NEEC diagram and how it leads to more accurate results especially when it comes to earth grid resistance measurement.