When designing an industrial HV network, a suitable neutral earth arrangement must be selected: the neutral can either be insulated, or it can be connected to earth. The use of an insulated neutral in an HV network has the advantage of ensuring operational continuity since it does not trip on the first fault, however the network capacitance must be such that an earth fault current is not likely to endanger personnel or damage equipment.
On the other hand, an insulated neutral implies the following:An earthed neutral generally implies mandatory tripping on the first fault, however:
The purpose of this study is not to compare the different neutral earth arrangements, but rather, once the neutral earth solution has been adopted, to determine the earthing mode by finding a compromisebetween three often contradictory requirements:
Earthing can be of different types:
This type of earthing is the most efficient in limiting overvoltages; protection selectivity presents no difficulties. However, in the event of an earth fault, the current is not limited, damage and interference occur and there is considerable danger for the personnel during the time the fault persists. This solution is not used for HV distribution.
Tuned reactor (Petersen coil). This solution is sometimes used for public HV networks. It is rarely used for industrial distribution. Protective relays sensitive to the active component of the residual current must be used to obtain selectivity.
This solution can result in serious overvoltages, as demonstrated by Le Verre (the Research and Development division of the E.D.F.). It can be used only where there are low limiting impedances.
This is often the most satisfactory solution. A study is necessary to choose between these two earthing, (through a reactor or through a resistor): accurate determining of these earthing modes depends on the voltage level, the size of the network and the type of receivers. Depending on the earthing mode, a criterion then determines a maximum impedance value corresponding to the overvoltage problem. Next, it is necessary to check its compatibility with the requirements of the network and the receivers.
The study of overvoltages that occur when short-circuits are eliminated from networks with the neutral earthed through a reactor gives the following results:
The neutral-to-earth overvoltage occurring when short-circuits are eliminated is:
In practise, the earth fault current is limited to at most 10 % of the threephase short-circuit current.
The resistance value r is determined in order to obtain a total active power loss: equal to or greater than the capacitive power in the event of a phase-earth fault, i.e.:
When dividing by , this become where:
Determination of the cable capacitance values depends on their design.
The above criterion is used to define the lower limit of the phase to earth fault current. To determine the upper limit, it is necessary to check that the fault current does not cause damage along its path and in particular to the cable shields. The maximum current withstood by the cable shields may be specified by the constructors. As a general rule, the value used is between 500 and 3 000 A for 1 second.
In HV networks, receivers are transformers which have no particular requirements as concerns the neutral earthing in a power supply network. However, industrial HV networks can supply rotating machines with voltages between 3 kV and 15 kV, the earth fault current should not exceed 20 A in order to avoid damage to the steel plating of the machines.