How To Determine The Size Of The Neutral Grounding Resistor?

The size of neutral grounding resistance (resistor) must be determined according to the system voltage level of the system, the sensitivity of the protective device, current limitation requirements of the fault and system parameters. Its core principles are balancing flow limitation and protection reliability. Specific identification methods are as follows:
1.Determination of resistance range Based on system voltage level
Low pressure system (e.g. 400V):
Grounding resistance is usually between a few hundred and a few thousand ohms. For example, a small industrial control circuit can be set to around 1 kΩ. Due to low voltage, high resistance and low fault current, the protective device cannot be detected. Too much resistance leads to too much fault currents, which damages equipment.
Medium pressure system (e.g., 6-35 kV):
Grounding resistance is typically in several thousand ohms to tens of thousands of ohms. For example, the grounding resistance of a 10 kV system can be between 10 and 20 km. Medium-voltage systems require high resistance to limit fault currents, prevent equipment damage and ensure reliable operation of protective devices.
High-pressure systems (e.g. 110 kV and above):
Grounding resistance is high, usually in tens to hundreds of thousands of ohms. For example, the grounding resistance of a 220kV system is 50-100omega. The stability of the system and the safety of the equipment will be seriously threatened due to the excessive voltage levels and ground fault currents.
2.Resistance regulation based on the Sensitivity of protective device
High-Sensitivity Protective Devices:
If the device detects a fault current of milliamps,such as microcomputer-based protective devices, the grounding resistance should be set higher to tens of thousands of ohms. In this case, the resistance value must meet the protective device's minimum detection current requirements.
Low-Sensitivity Protective Devices:
If the protective device requires a large fault current to operate, the grounding resistance should be reduced accordingly to ensure that the fault current is within the protective device's operating range.
3.Calculation of resistivity based on Fault Current Limiting Requirements
Limiting Fault Current to a safe range:
Grounding resistance must limit the fault current of the single-phase grounding to a reasonable range (e.g., a few amperes to a few a few a few dozen amperes) to prevent overload and damage to equipment.
4.Determination of resistivity based on Comprehensive System Parameters
Single-Power Supply System:
Resistance is relatively simple, mainly considering the limitation of fault current and the cooperation of protective device. For example, a small single-source power plant can set a ground resistance of several hundred to several several thousand ohms, depending on the insulation level of the equipment and the sensitivity of protective device.
Multi-source power supply systems:
The interaction between different power sources, such as ground fault current distribution, must be considered. Improper resistance settings can lead to failures or malfunctions of protective devices, even multiple failures. In this case, the resistance value must be determined by complex calculation and simulation to ensure safe and stable operation of the system.
V. Resistance Value References reference reference Typical Application Scenarios
10kV Systems:
Grounding resistance is usually between 10 and 20 Om, and the fault current is controlled within several hundred amperes to meet the operation requirements of the protective device.
Generator Neutral Grounding:
A high-resistance resistor connected by a single phase grounded transformer. The calculation of resistance must take into account the capacitance current of the system and the sensitivity of protective device. For example, the midpoint grounding resistance of a generator is calculated at 1,357.35 omega. The voltage side resistance of transformer conversion is 0.214 omega. Cable distribution network:
In cable-based distribution networks, neutralizingpoint resistance is usually low (7.2 Omega-14.4 Omega, for example) to ensure that the fault current is high enough to meet the sensitivity requirements of the protective device.