Neutral earthing in an industrial HV network

Earth protection mode

        The neutral earthing impedance affects the required method of protection against phase to earth faults. As a general rule, the higher the fault currents, the easier they are to detect; the lower they are, the harder they are to detect .
        

• either by a core balance transformer around the three phase conductors and which directly detects the sum of their currents (zero if no earth fault) ;
• or by three current transformers the secondary windings of which are connected to form a neutral conductor through which the sum of the three currents flows.

        The core balance transformer solution is the most accurate, however it can only be installed on cables, not on busbars or overhead lines. The 3 current transformers solution is often used, in particular when these 3 transformers are already required for another application. But the measurement obtained is degraded by the inaccuracies of all three transformers, in particular in the event of transient overcurrents when the transformers become saturated.

Earth protection setting

        This must be adjusted according to the measurement accuracy. It must ensure maximum protection and authorize selectivity.
        If the measurement is carried out using the sum of the secondary currents of the three transformers, it will be degraded by the dispersion of the transformers. In particular, a residual current is measured if there is no earth fault when the transformers become saturated.
        Saturation is caused by excessive amplitude of the phase current, but more specifically by the DC component induced in a short-circuit or unbalanced inrush current.
         Note that, in transient conditions, the DC component can induce saturation of the transformers even though the peak value of the transient current is around 10 times less than the saturation value for a steady-state balanced current. An earth protection device supplied by 3 transformers must therefore include a time delay in order to avoid spurious triggering resulting from transients. The current setting must not be lower than 6 % of the transformer rating at best, or 15 or 20 % of the transformer ratings in the most unfavourable cases.
         Moreover, if an earth fault occurs in a star winding near a neutral point, the maximum fault current is only a small part of the maximum fault current which is limited by the neutral earth impedance. For this reason, the current setting is usually 20% of the maximum current limited by the neutral earth in order to protect 80% of the windings. But, as the calculation shows, in the event of a fault, a residual capacitive current flows through the sound parts of the network. So, to prevent the protective device of a sound line from tripping spuriously, the threshold must be set to 30% higher than the capacitive current flowing through this sound line when a phase to earth shortcircuit affects the network.
         Moreover, we must take into account the possible presence of voltage harmonics likely to produce currents which increase as the order of the harmonics increases. Note that 3rd harmonics and multiples of 3 exist even in steady-state conditions. Finally, the neutral earth impedance characteristics must be coordinated with the protective devices so that this impedance itself is not degraded by the fault current before it is eliminated.

Note:

that this concerns circuit protection and not personnel protection.

Conclusion:

when the earthed neutral solution is selected for a medium voltage network, it is advisable to use earthing through a resistor rather than other systems.

Calculating r and I_L

        This resistance r and the maximum current I_L=U/3·r are determined taking the following requirements into account:

• the current I_L must be greater (or equal) to twice the network capacitive current in the event of an earth fault I_L >2·L_Cin order to limit overvoltages;
• the current IL must be less than the maximum overcurrent that can be withstood by the cable shields, normally between 500 and 3 000 A, depending on the cable cross section;
• in a network containing HV motors, it is preferable to respect the relation:5A< I_L< 20A but, if this is incompatible with the first condition, I_L can be up to 50 A;
• in order to ensure correct protection of receivers, the Ir threshold settings should not exceed 0,2·I_L i.e.I_r< 0,2·I_L ;
• in order to ensure selectivity with regard to sound connections, the relation I_r>1,3·I_C , must be respected, where I_C is the capacitive current of the line when a phase to earth fault occurs;
• if the earth current is measured by 3 transformers of rating In , then Ir must be >0,06·I_n ;
• термическая устойчивость резистора r должна учитывать ток IL, протекающий в течение максимально необходимого времени отключения короткого замыкания (1 до 1.5 с.), или, наоборот, замыкание на землю должно быть достаточно быстро определено для предотвращения повреждения резистора.the thermal withstand of resistor r must allow for current IL to flow during the maximum fault clearing time required (1 to 1.5 s) or, conversely, the earth fault must be cleared sufficiently rapidly to avoid damaging the resistor.

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