NETWORK

TECHNICAL FIELD

The invention is a protective method and an electric circuit for a three-phase electric power network. The invention relates to a method and an electric circuit enabling a significant reduction of the neutral point voltage of medium-voltage overhead line networks along with the enhancement of efficiency of earth fault protection.

BACKGROUND ART

The normal operation of electric power networks is disturbed most frequently by faults. The fault is a shunt failure of the network caused by a breakdown of the isolation or metallic shunting between phase conductors of the network, or between the phase conductor and the earth or an earthed zero-conductor. Among the elements of electric power transfer, in most cases the fault arises on overhead line networks. Frequent causes of it are lightning stroke, breaking of wire, hunting of the wires and flashover of an isolator.

The earth connection of a phase conductor of not solidly earthed networks is called earth fault. Single-phase earth faults on networks with an isolated neutral point, i.e. not directly earthed neutral point, do not need immediate switching off, because the occurring earth fault current is low. The power supply is maintainable for longer or shorter times in spite of the earth fault, moreover the so-called arching faults which give the major part of the earth faults can be eliminated with compensation without disturbing the operation.

Arching fault arises, if one of the phase conductors is connected to the earth (a point with earth potential) through an electric arc. Practice shows that the electric arc is not able to remain continuous in case of earth fault currents under 5 A, therefore it is the aim to bring down the earth fault current to this range.

The potential of the neutral point is determined by the symmetry of the earth capacity and conductivity of the individual overhead lines for each phase conductor and the kind of the neutral point connection. A generally known and used method in this technical field is the so-called earth fault compensation. The object of the earth fault compensation is to decrease - by establishing a parallel resonance - the current of the earth fault to a value at which the arching fault cannot remain continuous. A generally applied way of realization is the neutral point grounding through an inductance (Petersen coil).

In the case of an earth fault, a voltage arises on the coil connected between the neutral point of the transformer and the earth. The inductive current of the coil flows through the faulty phase conductor, the fault location and the earth, and lags 90° compared to the voltage of the neutral point. The direction of l _c of the network with earth fault, and l _L of the inductance is opposite. Thus, if their values are equal, their sum will be zero, and no current will flow at the fault location. Consequently, the inductance of the compensating coil (arc suppression coil) has to be tuned theoretically so that li_=lc-

The total inductivity of the earth fault compensating apparatus(es) and the total earth capacity of the network constitute a parallel resonant circuit when earth fault occurs. Being compensated to resonance is a compensated state, in which the compensating current li_ is equal to the current l _c to be compensated at the location of the earth fault. This state tuned to resonance, however, has to be avoided because of the possibly occurring harmful overvoltages, and should not be produced and maintained at normal operation. The state tuned to resonance or near to resonance is called in the following normal compensated state.

As the compensated networks are earthed through an arc suppression coil as detailed above, the voltage of their neutral point is determined by the sum of the earth capacitances and conductances. The network can operate safely when the voltage of the neutral point is as low as possible, because the increase of the neutral point voltage always leads to an increased risk of earth fault of one or two phase conductors.

The different earth capacities of the individual phase conductors of medium-voltage overhead line networks cause a so-called zero sequence asymmetry. This causes an increase of the voltage of the neutral point, which is a particularly big problem lately, because the resonance point is regularly crossed by the automatic control of the arc suppression coils, which leads to an unacceptable high increase of the voltage. Therefore, the desired l _L =lc compensation cannot be tuned, but a so-called overcompensation is realized to have a low neutral point voltage.

Overcompensation is a compensating state, in which the compensating current li is higher than the current lc to be compensated at the location of the earth fault.

According to another solution, the neutral point is connected to the earth with a resistor, which, however causes significant loss and it has to be switched off in the case of an earth fault.

An arc suppression coil with adjustable inductivity is disclosed by way of example in patent document HU 149 536. In WO 2008/1 16428 A2 a solution is disclosed, in which in case of detecting an earth fault the sensitivity of the protective current circuit is increased by a resistor connected to the auxiliary coil of the arc suppression coil.

In Fig. 1 a simplified circuit diagram of a known solution of compensation is shown, in which U _f is the phase conductor voltage, C ₀ is the total capacitance of the network, R ₀ is the total resistance of the conductances, L is the actual inductivity of the arc suppression coil, whilst AC ₀ is the excess zero sequence capacitance of the network. At the occurrence of an earth fault the capacitance ACo is replaced by a shortcut and at the location of the earth fault the earth fault current will be proportional to the current flowing through the L and C ₀ parallel resonant circuit. If the resonant circuit is tuned near 50 Hz, this current will be extremely low and therefore the arc will extinguish; this is the process of arc suppression. However, tuning exactly to 50 Hz cannot be maintained, because the circuit is always fed by the zero sequence capacitance AC ₀ and by exact tuning the zero sequence voltage Uo will be high because of the high impedance. To avoid this, the inductivity is always tuned to be lower, i. e. overcompensated, thus the circuit will be slightly inductive. The overcompensation, however, significantly decreases the efficiency of arc suppression. Going through the resonance is unavoidable during the tuning of the arc suppression coil, in this case high neutral point voltage occurs, which increases the risk of the fault. The known compensation, control is slow; therefore it endangers the network for a long period. Accordingly, most of the overhead line networks are provided with an arc suppression (Petersen) coil, because by adjusting around the 50 Hz resonance a significant part of the earth faults is automatically eliminated. This tuning, however, results in the high impedance of the resonant circuit. The high impedance has a favorable effect on the prevention of earth faults, at the same time it is problematic during the normal operation, if the zero sequence asymmetry is large, because it increases the voltage of the neutral point, which, however, increases the number of earth fault occurrences. The problem is contradictory, because the resonant tuning increases the number of the earth faults, on the other side, however, the efficiency of the arc suppression decreases with decreasing number of faults.

Hence a demand had arisen for a solution, in which the voltage of the neutral point can be kept near zero, and the mentioned effective compensation is also ensured.

DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a method and an apparatus for a three- phase electric network ensuring effective compensation, and at the same time keeping the voltage of the neutral point near zero. It is another object of the invention to provide a simple, fast and safe protective method and apparatus, which is exempt from the disadvantages of the prior art solutions.

The object according to the invention has been achieved by the protective method according to claim 1 and the protective apparatus according to claim 7. Preferred embodiments of the invention are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary preferred embodiments of the invention will be described hereunder with reference to drawings, where

Fig. 1 is a schematic circuit diagram demonstrating the compensation, and

Fig. 2 is a scheme of a protective apparatus according to the invention.

MODES OF CARRYING OUT THE INVENTION

As shown in Fig. 2, an exemplary protective apparatus according to the invention is for a three-phase electric network 10, and comprises an inductive device 13 suitable for realizing an overcompensated state and being connected between a neutral point 11 and the earth. The apparatus further comprises a sensor device 16 for detecting an occurrence of an earth fault current on the inductive device 13, and a switching device 18 being controlled on the basis of the signal of the sensor device 16, and an additional inductance 19, which can be switched off by the switching device 18 in case of an occurrence of an earth fault current to increase the inductivity of the inductive device 13, and therewith establishing a normal compensated state. The switching device 18 is preferably an electric switch.

The inductive device 13 is preferably realized as an arc suppression coil 14 provided with an auxiliary coil 15. The sensor device 16 is preferably a voltage signal transmitter being in inductive connection with the arc suppression coil 14, and the additional inductance 19 is an additional coil suitable for controllable connection with the additional coil 15 of the arc suppression coil 14.

The apparatus further comprises a control unit 17 for receiving the signal of the voltage signal transmitter, and in the case of an earth fault, disconnecting the additional inductance 19 from the auxiliary coil 15 by means of the switching device 18. At a switched off state of the additional inductance 19, the inductivity of the inductive device 13 is normal, i.e. chosen to be compensated to resonance or near to resonance state, so that effective arc suppression effect can be reached. When the earth fault disappears, the additional inductance 19 is connected back to the auxiliary coil 15 by means of switching device 18, and so the overcompensated state will be restored with the decreased total inductivity. The restored overcompensation ensures that the voltage of the neutral point remain in a state near to zero voltage. The switching action is preferably carried out within a half period in such a way, that the switching is realized at a zero crossing. Therefore, the switching actions can be limited to such time points, when no current flows in the auxiliary coil.

In the course of the protective method according to the invention an overcompensated state is realized by earthing the neutral point 11 through the inductive device 13. In the case of the occurrence of an earth fault, the overcompensated state is terminated and a normal compensated state is realized in such a way, that the additional inductance 19 is disconnected from the auxiliary coil 15, and when the earth fault disappears, the overcompensated state is restored by connecting back the additional inductance 19.

In case of the occurrence of the earth fault the overcompensated state is terminated within a half period, and the termination and/or restoration of the overcompensated state is carried out at a zero crossing.

An overcompensated state characterized by a residual current of 10 to 20 A and a normal compensated state characterized by a residual current of less than 5 A is realized by the method and apparatus according to the invention. The residual current (current at the fault location) is the effective value of earth fault current occurring in the compensated network.

The low value of the neutral point voltage can be achieved by the fast control of the arc suppression (Petersen) coil 14 in such manner, that the additional shunt inductance 19 is connected to the auxiliary coil 15 of the coil tuned preferably exactly to resonance. Thus the zero sequence impedance will be rather inductive and low, which keeps the voltage of the neutral point 11 at a low value. The arc extinction capability of the Petersen coil strongly decreases in the overcompensated state, therefore in the case of an earth fault the shunt inductance is disconnected within 10 ms. Thus, the fault ends with the increased extinction capability and the voltage of the neutral point 11 reaches a low value again with the additional inductance 19 connected back.

It can be seen that the method and apparatus is extremely suitable to reach the proposed objects, since on the top of keeping the voltage of the neutral point at a low value, also the effectiveness of the arc suppression coil is maintained.

The invention, of course, is not limited to the above detailed preferred embodiments, but further modifications, variations and further developments are possible within the scope defined by the claims.