Neutral Grounding Resistors (NGR) are used for the low- and high- ohm grounding of the neutral point in medium voltage grids.
In addition to the conductor voltage to the earth, the cold resistance that determines the initial current, the current flow time, the temperature rise during the operating time and the resulting hot resistance, which also determines the current at the end of the operating time, are the parameters for a grounding resistor. The capacity of the resistor is shown by the current-time integral [i2t] = kA2s. The insulation of the resistors is dimensioned for the system voltage. System voltages are 12, 24, 36 and 52 kV. Depending on the climatic conditions, the soiling and the geodetic elevation at the place of installation, higher clearances and creepage distances may have to be considered.
Resistors for indoor installation in electrical operating rooms are manufactured in protection classes IP00 or IP20. For outdoor applications, the minimum protection class required is IP23. Higher protection classes are problematic with a view to the limited ventilation due to thermal stresses acting on the components, insulators and casings.
The maximum temperature pursuant to IEEEStd32-1972 is to be agreed between the manufacturer and the user. Depending on the resistor material used, high temperatures do not represent a problem for the resistor proper. Only for the casing, insulators, converters and cable sealing ends, special measures have to be taken to protect them from inadmissible temperatures. Cast iron resistors are usually dimensioned for final temperatures of 400 to 500°C and resistors made from chromium nickel steel for temperatures up to 650°C.
DESIGN OF NEUTRAL GROUNDING RESISTORS
GINO grounding resistors consist of the resistor banks with resistor elements made from cast iron with or without surface protection or steel grid elements. This list comprises cast iron resistors of the system GINO. Alternatively, also resistors with stainless steel elements can be offered and manufactured.
One or several resistor banks are installed in an insulated module and one or several modules are arranged in one housing. The housings are made from welded sectional steel frame with steel sheet cladding. 3-coat painting systems with 2-component paints protect the surface. Standard color is RAL 7032.