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Ground Potential Rise Analysis between Generating Station and Remote Substation Yoo Jae-seok, Jung In-choul, and Park Hyun-sung Abstract— When making a lot of interconnections between the generating station and substation because of the realistic problem such as too long distance, the remarkable potential differences may occur between them. In that case, the significant potential differences occur between them under fault condition. This paper reviews the differences of potential when occurring fault for the safety of any interconnected cables such as control, communication cables, etc.. Keywords—CDEGS, generating station, grounding system, potential difference, substation I. INTRODUCTION T HE grounding system is very important for the generating station and substation. It is intended to protect personnel in the generating station against dangerous touch and step voltages and also to protect property from damage. For personnel this protection is ensured, in the case of a fault, by either rapid automatic disconnection of the power supply or by limitation of the resulting touch and step voltages to acceptable levels. Ideally, the generating station and substation should be in close proximity to each other. And to achieve an efficiency and equipotential between the two grounding systems, many ground conductors are connected in parallel between them. However in reality, it is sometimes impractical to make interconnections due to long distance, which may give rise to significant voltage differences between the two grounding systems. In that case, any interconnected cable such as control cables, communication cables, etc. is easily influenced under fault conditions. This paper analyzes the grounding potential difference between the generating station and substation located separately under fault conditions. This part should contain the authors’ current affiliations, including current address and e-mail. Yoo Jae-seok is with KEPCO E&C, M-tower, 8 Gumiro, Bundang-gu, Sungnam-si, Gyeonggi-do, Korea 463-870 ([email protected]) Jung In-choul is with KEPCO E&C, M-tower, 8 Gumiro, Bundang-gu, Sungnam-si, Gyeonggi-do, Korea 463-870 ([email protected]) Park Hyun-sung is with KEPCO E&C, M-tower, 8 Gumiro, Bundang-gu, Sungnam-si, Gyeonggi-do, Korea 463-870 ([email protected]) II. DESCRIPTION ON SITE CONDITIONS A. General Description In the Republic of Korea, Shin-Kori Nuclear Power Plant Unit 1 and 2 hereafter PP are apart from 765kV switchyard hereafter SWYD about 2.6 km. Connecting between PP and SWYD grounding system as a grounding conductor, there are two overhead ground wire hereafter OHGW consisting of 120mm2 aluminium-conductor steel-reinforced and one 100 mm2 bare copper conductor embedded in the duct bank. B. Grounding System of PP The area of PP is 515 m 466 m approximately and its grounding system is consistent of 150 mm2 bare copper conductors in 20 m interval below 0.75 m from the ground level. C. Grounding System of SWYD The area of SWYD is 180 m 160 m approximately and its grounding system is consistent of 200 mm2 bare copper conductors in 8 m interval below 0.75 m from the ground level. III. SIMULATION The software CDEGS 1 is used for simulation. For easy modeling, the locations of PP and SWYD are assumed to lie in a straight line. Also, OHGW is modeling to the insulated cables with same size due to reducing the computation time by using MALZ2 module in CDEGS. The basic input data for Case I and Case II is shown in Table I 1 an analyzer program on grounding and lightning system developed by Safe Engineering Services & technologies ltd. 2 MALZ analyzes the frequency domain performance of buried conductor networks and calculates the following quantities: earth and conductor potentials, longitudinal and leakage current distribution in the conductors, electric field and current density in the soil or at the earth surface, as well as magnetic fields in the air. TABLE I. Input Data Soil Resistivity [Ω·m] Fault Current [kA] INPUT DATA FOR SIMULATION Values Remark 100 Uniform Layer 50 Circuit Breaker Rating A. Case I Case I is assumed that a fault current occurs at the transformer area in PP. It may flow through OHGW and grounding conductor in the duct bank, and soil. Fig. 3. GPR of Conductor under Fault Condition <Power Plant> <SWYD> 2.6 km Fig. 1. Modeling of Grounding Systems of SWYD (Left) and PP (Right) with Two OHGW and Grounding Conductor in Duct Bank Above Fig. 1, the grounding systems of SWYD and PP are modeling. Fig. 4. GPR of Segments in Voltage Fault Location In reference to above Fig. 3 and 4, there are about 5443 V potential differences between SWYD and PP under fault condition. B. Case II Case II is assumed that a fault current occurs at SWYD. It may flow through OHGW and grounding conductor in the duct bank, and soil. Fig. 2. Fault Current Location on Grounding System of PP The fault current location is assumed at the transformer area as shown in Fig. 2. <Power Plant> <SWYD> 2.6 km Fig. 6. GPR of Conductor under Fault Condition Fig. 5. Modeling of Grounding Systems of SWYD (Left) and PP(Right) with Two OHGW and Grounding Conductor in Duct Bank Above Fig. 5, the grounding systems of SWYD and PP are modeling. Fault Location Fig. 7. GPR of Segments in Voltage In reference to above Fig. 6 and 7, there are about 5443 V potential differences between SWYD and PP under fault condition Fig. 6. Fault Current Location on Grounding System of SWYD IV. COMPARISON The fault current location is assumed on the center of SWYD. The results from analysis for each case are described in Table II below. TABLE II. COMPARISION WITH COMPUTATION RESULTS Potential Differences [V] Case I Case II 5443 5443 In two cases, the potential differences are exactly same. V. REVIEWS & CONSIDERATIONS When a large potential difference caused under abnormal conditions occurs between two areas, an unintended voltage will be induced at the double end grounded cable shield interconnecting the two areas An equivalent circuit of induced voltage can be assumed that it consists of the capacitance between the grounded shield and conductor in a control cable for a trip command connecting the control room of generating station to the control room of remote substation, the impedance of shield, the impedance of auxiliary relay for the trip circuit breaker and the impedance of ground fault detector. Considering the above results, when any cables interconnected between generating station and substation are designed, the potential differences under fault conditions should be taken into account. For the calculation on the potential differences, it is recommended to use the simulation tool for the accurate analysis. If the analyzed results are not acceptable, it is preferable to enhance the design by adding grounding conductors between the generating station and substation and then analyze again the modeled ground grids under fault condition. REFERENCES [1] [2] Zc △V Zs Zar Vo Zgd △V : Voltage Difference [V] Vo : Voltage at Aux Relay of C.B. [V] Zs : Impedance of Cable Shield [Ω] Zc : Capacitance between Shield and Conductor [Ω] Zar : Impedance of Aux. Relay of C.B. [Ω] Zgf : Impedance of Ground Fault Detector [Ω] Fig. 8. Equivalent Circuit of Induced Voltage If a voltage loaded at auxiliary relay for the operation of trip circuit breaker is larger than a minimum operational voltage, it will be operated accidently. Therefore, the minimum operational voltage of circuit breaker should be considered to prevent a malfunction causing unintended accidents. VI. CONCLUSIONS In situations where the generating station and substation are separated by long distances, multiple interconnections between the two grounding systems are impractical. Due to this, significant voltage differences may occur between them. In general, the generating station and substation are in the proximity and connected together with many conductors for efficiency and equipotential. When making a lot of interconnections between them because of the realistic problem such as too long distance, the remarkable potential differences may occur between them. By using CDEGS software, the grounding systems of generating station and substation are modeled for analysis on the potential differences under fault conditions. It is concluded that in all cases, there might be the potential differences between two grounding systems. They may cause some accidents such as the malfunction of electric devices and insulation breakdown of equipments and others. IEEE Standard for Generating Station Grounding, IEEE Std 665, 1995. IEEE Guide for Safety in AC Substation Grounding, IEEE Std 80, 2000