General provisions for grounding of electrical installations

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Abstract: The grounding of electrical installations involves two main aspects: one is the functional grounding of the power supply, such as the grounding of the power system, which generally refers to the grounding of the neutral points of generators, power transformers, etc.; this is commonly known as system grounding, or system operating grounding. On the other hand, the exposed conductive parts of electrical equipment are grounded to provide protection; hence it is commonly referred to as protective grounding. Keywords: grounding of electrical installations, grounding for power supply functions, protective grounding. 1.1 Functional grounding and protective grounding The grounding of electrical installations involves two main aspects: one is grounding for power supply functions, such as the grounding of power system neutral points, which typically refers to the grounding of generators, power transformers, etc.; this is generally known as system grounding, or system operating grounding. On the other hand, the exposed conductive parts of electrical equipment are grounded to provide protection; hence it is commonly referred to as protective grounding. The main function of system grounding: – To provide a path for discharging atmospheric or operational overvoltages to the ground, thereby preventing the insulation of electrical equipment from being damaged ； – Provides a ground fault circuit; when a ground fault occurs, it generates a large ground fault current to quickly disconnect the faulty circuit ； – In a system with an ungrounded neutral point, although power supply continuity can be maintained in the event of a ground fault, the voltage across the non-faulty phases relative to ground increases by 1.73 times. The insulation level of the equipment and circuits in such a system is higher than that in a system with a grounded neutral point, which results in increased investment costs ； – In ungrounded neutral systems, a large number of insulation monitoring devices need to be installed. The main function of protective grounding: – To reduce the expected contact voltage ； – Provide power-frequency or high-frequency leakage circuits ； – Provide an installation circuit for the overvoltage protection device ； – Equipotential bonding. Figure 1-1 Functional grounding and protective grounding of electrical installations. Depending on the requirements of the electrical installation, the grounding arrangement can serve both protective and functional purposes, either simultaneously or separately. The requirements for protection purposes should always be given priority. The selection and installation of facilities with a grounding configuration shall meet the following requirements: – The grounding resistance value must comply with the functional and protective requirements of the electrical installations, and it is expected to remain effective over the long term ； – It can withstand ground fault current and leakage current to ground without danger, especially the hazards caused by heat, thermomechanical stress, and electromechanical stress ； – It has sufficient strength or additional mechanical protection to withstand the external influences of its location ； – Measures should be taken to prevent damage to facilities grounded and other metal parts caused by electrocorrosion. 1.2 Grounding Configuration of Substations In a 10kV system, the grounding of the neutral point can be classified as follows: ungrounded neutral point system (including grounding through arc suppression coils), grounded neutral point system with resistance grounding, low-resistance grounding, and high-resistance grounding. 1.2.1 Ungrounded Neutral Point System (1) Characteristics of grounding faults During normal operation of the distribution system, under the action of basically balanced voltages across the three phases, the capacitive currents from each phase to ground, ICL1, ICL2, and ICL3, are equal to one another; these currents lead the phase voltage by 90°, and ICL1 = ICL2 = ICL3 = UΦωC. Moreover, ICL1 + ICL2 + ICL3 = 0, meaning that the neutral point of the system is at the same potential as the ground. In the event of a ground fault in phase L1, ignoring the ground transition resistance and assuming a metallic ground condition, the direction of the capacitive current in each branch of the 10kV system is as shown in the figure below: Figure 1-2 Schematic diagram of a ground fault in a 10kV system. From this schematic diagram, it can be concluded that: a) in all non-faulty branches of the entire system, the capacitive current in the faulty phase is zero, while there is capacitive current in the non-faulty phases ； b) In the faulty branch, the fault phase current equals the sum of the capacitive currents in all branches ； c) The direction of the capacitive current in the faulty branch is from the load to the power supply, while the direction of the capacitive current in the non-faulty branches is from the power supply to the load ； d) The zero-sequence current for fault branch detection is the sum of the capacitive currents of all non-faulty branches ； e) The magnitude of the ground fault current is independent of the location of the ground fault point; it depends only on the transition resistance at that point. In a 10kV system with a ground fault, the relationship between voltage and current vectors is shown in the figure below: Figure 1-3 Vector diagram of a 10kV system ground fault. When a ground fault occurs in phase L1, it is equivalent to applying a voltage U0 = –UL1 to phase L1; the same voltage U0 = –UL1 is also applied to phases L2 and L3. The voltage of the non-faulty phases relative to ground increases by a certain factor, and the angle between them changes from 120° to 60°. As a result, the combined capacitive current increases by that same factor. The ground fault current is three times the value of the single-phase capacitive current, i.e., Id = 3UΦωC. (2) Advantages and disadvantages: a) Ground faults can cause internal overvoltages in the system that reach 3.5 to 4 times the phase voltage, easily leading to the breakdown of the insulation in equipment and circuits. b) Oil-impregnated paper-insulated power cables can handle 20A, polyethylene-insulated power cables 15A, and cross-linked polyethylene-insulated power cables 10A. If a ground fault current causes an arc to form, this arc will not extinguish on its own, resulting in intermittent arcs that generate overvoltage and may lead to inter-phase short circuits or fires ； c) The voltage of the non-faulty neutral rises by a certain factor. The insulation class of the equipment or cables within the system is correspondingly increased; for example, 10kV power cables should use an 8.7/10kV insulation class rather than 6/10kV ； Gapless zinc oxide arresters, by increasing the continuous operating voltage or by adding series protection gaps, etc ； d) In the event of a ground fault, an alarm is issued without shutting off the faulty circuit, ensuring continuous power supply ； e) The ground fault persists for a period of time, and the voltage resulting from such a fault can cause electric shock or trigger a fire, as shown in Figure 1-4 below. 1.2.2 Low-resistance grounding system at the neutral point: Grounding is classified as low-resistance or high-resistance based on the magnitude of the ground fault current. When the ground fault current is greater than or equal to 100A and less than or equal to 1000A, it is a low-resistance grounding method ； When the ground fault current is less than 10A, it is a high-resistance grounding method. For the low-resistance grounding method, the ground fault current is generally set between 300A and 800A; in 10kV systems, the ground resistance for this low-resistance grounding method is chosen to be either 10Ω or 16Ω, depending on the region. Figures 1–4: Voltage transmission of high-voltage ground faults to the low-voltage side. In order to keep the resonance overvoltage factor within 2.5 in the system, the resistive current IR flowing through the neutral point must be greater than or equal to 1.5 times the capacitive current IC of the system. The low-resistance grounding method increases the ground fault current Id. 　　When a ground fault occurs in the system, the ground fault current Id is independent of the location of the fault point; therefore, zero-sequence current quick-break protection cannot be used to ensure selectivity of protection. Instead, zero-sequence current protection with different time delays should be employed to achieve selectivity. Mechanical relay delay time: 0.5s for the output line ； The bus coupler is 1.0s ； The main incoming time is 1.5s to 2.0s. For electronic protectors, the delay time should be set between 0.2s and 0.3s; this range provides a high degree of precision in setting the value. It is recommended to use electronic protectors for zero-sequence current protection. In the case where the neutral point is grounded through a low resistance, the ground fault current Id is high. During the time it takes to disconnect the faulty circuit, a high ground fault voltage Uf is generated. When the low-voltage system uses a TN grounding configuration, the exposed parts that can make contact with the ground share the same grounding electrode as the transformer’s low-voltage neutral point; as a result, the ground fault voltage Uf can be transmitted to the low-voltage side, potentially causing electric shock or fires, as shown in Figure 1-5. In a TT grounding configuration for low-voltage systems, the exposed parts that can make contact with the ground and the transformer’s low-voltage neutral point each have their own separate grounding electrodes. The ground fault voltage Uf can still be transmitted to the low-voltage side, leading to power-frequency overvoltage, as shown in Figure 1-6. According to IEC standards, for general low-voltage electrical equipment, there are requirements regarding power-frequency overvoltages and the time required to disconnect faulty circuits: when the allowable power-frequency overvoltage is U0 + 250V, the time needed to disconnect the faulty circuit must be greater than 5 seconds ； When the allowable power-frequency overvoltage is U0 + 1200V, the time to disconnect the faulty circuit is less than or equal to 5 s. Figure 1-5: The grounding fault voltage from the high-voltage system propagates into the TN system. Figure 1-6: The grounding fault voltage from the high-voltage system causes power-frequency overvoltage in the TT system. When the neutral point is grounded through a low resistance, a grounding fault occurs within the system; the faulty circuit is immediately disconnected, which does not ensure continuity of power supply. 　　Based on the above, in a 10 kV ungrounded system, when a ground fault occurs, the fault voltage amplitude is not high, but it persists for a long time. When a TN system is used for power supply at low voltages, fault voltage is conducted along the PEN or PE conductor, and common equipotential bonding measures are adopted to reduce the expected contact voltage. 　　In a 10kV system with low-resistance grounding, the fault voltage during a ground fault is of high amplitude, although it lasts for only a short time. When a TN system is used for power supply at low voltage, the following measures should be taken: two sets of grounding electrodes shall be installed in the substation ； Total equipotential bonding measures are adopted ； When supplying power outside the overall equipotential bonding area, a local TT system is used. When low-voltage systems are powered by a TT system, the grounding resistance of the exposed conductive parts in the substation shall not exceed 1 Ω, or the total length of the high-voltage and low-voltage cables with an earthed metal sheath shall be more than 1 km. 1.3 Extra-Low Voltage 　Being powered by an Extra-Low Voltage is one of the measures to prevent electric shock. The IEC classifies very low voltages into three categories, as summarized below: 1.3.1 SELV (Self-sufficient ELV). Figure 1-7 shows a SELV circuit diagram. A SELV circuit is insulated from ground; when the PE wire is subjected to a fault voltage Uf or in the event of a ground fault, the voltage across the exposed conductive parts of the electrical equipment with respect to ground remains zero. No additional auxiliary measures are required to prevent electric shock. 1.3.2 PELV (Protective ELV) In a PELV circuit, one of the conductors is connected to ground, while the exposed conductive parts of the electrical equipment are not grounded, as shown in the figure below: Figure 1-8(a) PELV circuit diagram. When the PE wire has a fault voltage Uf, the voltage of the exposed conductive parts of the electrical equipment with respect to ground is zero. When both the PE conductor is under fault voltage Uf and a grounding fault occurs in the electrical equipment, the voltage of the exposed conductive parts of the equipment with respect to ground is the vector sum of Uf and UELV. It is essential to place the electrical equipment within the effective range of equipotential bonding in order to prevent electric shock hazards to people caused by those exposed conductive parts. In PELV circuits, the exposed conductive parts of electrical equipment are grounded, as shown in the figure below: Figure 1-8(b) PELV circuit diagram. The exposed conductive parts of the electrical equipment are connected to ground; when the PE wire is under fault voltage Uf, the voltage of these exposed conductive parts with respect to ground is also Uf. When both the PE conductor is under fault voltage Uf and a grounding fault occurs in the electrical equipment, the voltage of the exposed conductive parts of the equipment with respect to ground is the vector sum of Uf and UELV/2. It is essential to place the electrical equipment within the effective range of equipotential bonding and to use protective devices to cut off the power supply, in order to prevent electric shock hazards to people caused by those exposed conductive parts. 1.3.3 FELV (Functional ELV) When a very low voltage is used for functional reasons, and all the requirements of SELV or PELV cannot be met, or when SELV or PELV is not necessary, a protection mechanism that ensures protection against both direct and indirect contact is employed. This combination of methods is referred to as FELV. If the insulation of the electrical equipment in a FELV circuit cannot withstand the test voltage required for the primary circuit, then the insulation of the non-conductive parts of the equipment that are accessible must be strengthened during installation, so that it can withstand a test voltage of 1500 V RMS for a duration of 1 minute. Connect the exposed conductive parts of the electrical equipment in the FELV circuit to the protective conductor of the primary circuit ； At this time, the live conductors in the FELV circuit may still be connected to the protective conductors of that primary circuit; electrical equipment must be installed within the range where equipotential bonding is effective, and protective electrical devices must cut off the power supply, in order to prevent electric shock hazards to persons caused by the exposed conductive parts of the electrical equipment. For electrical isolation protection, the exposed conductive parts of the equipment in the FELV circuit are connected to the ungrounded equipotential bonding conductor of the primary circuit. Figure 1–9 FELV circuit diagram. When the insulation of the electrical equipment in the FELV circuit cannot withstand the test voltage required for the primary circuit, the insulation of the accessible non-conductive parts of the equipment must be strengthened during installation so that it can withstand a test voltage of 1500 V RMS alternating current for a duration of 1 minute. 　　Connect the exposed conductive parts of the electrical equipment in the FELV circuit to the protective conductor of the primary circuit ； At this time, the live conductors in the FELV circuit may still be connected to the protective conductors of that primary circuit; electrical equipment must be installed within the range where equipotential bonding is effective, and protective electrical devices must cut off the power supply, in order to prevent electric shock hazards to persons caused by the exposed conductive parts of the electrical equipment. 　　For electrical isolation protection, the exposed conductive parts of the equipment in the FELV circuit are connected to the ungrounded equipotential bonding conductor of the primary circuit.