WO2019086058A1

WO2019086058A1 - The method of connection to limit the value of voltage between the neutral point and ground in an alternating current electric network

Info

Publication number: WO2019086058A1
Application number: PCT/CZ2018/000053
Authority: WO
Inventors: Jaroslava ZAKOVA
Original assignee: Zak, Frantisek
Priority date: 2017-10-30
Filing date: 2018-10-24
Publication date: 2019-05-09
Also published as: CZ2017683A3; CZ307590B6

Abstract

The invention concerns the method and connection to limit the value of voltage between the neutral point (1) of the network and ground (2), namely in networks with directly earthed neutral point (1) or in networks where the limiting impedance (5) is connected between the neutral point (1) of the network and ground (2) to limit the flow of the direct current be through the neutral point (1) of the network. The essence of the invention rests in the fact that with an increased instantaneous value of the voltage UZ between the neutral point (1) of the network and ground (2) above the predetermined reference value UK a protective circuit (3) is connected between the neutral point and ground, an/or in parallel to the limiting impedance (5). Such a protective circuit (3) connects the neutral point (1) of the network and ground (2). Due to this, the voltage UZ between the neutral point (1) of the network and ground (2) will have a value approaching zero.

Description

The Method of Connection to Limit the Value of Voltage Between the Neutral Point And Ground in an Alternating Current Electric Network

Field of the Invention

The invention concerns the field of earthing the neutral point in an alternating current electric network and the issue of preventing the direct current from flowing through a neutral point of the electric network.

Background of the invention

The electric network node that is directly earthed or earthed via a reactor, or - where applicable - a low-Ohm resistor together with the line and other similarly earthed neutral points of the electric network can create a parallel path for direct currents IDC, also known as stray earth currents. If the electric network uses autotransformers, direct current IDC can spread also behind such autotransformers into other electric networks, even at a different voltage level. Direct current IDC has a negative impact on the magnetic circuit of electrical machines (transformers, alternators, and motors) and causes increased losses, warming and noise, and reduces the equipment’s service life. Such direct current IDC flowing through the neutral point of an electric network can be limited by connecting a resistor between the neutral point of the network and ground. The higher the value of resistance, the more limited the flow of direct current IDC through the neutral point of the electric network. In case of a single-phase fault in the network, the value of voltage of the aforementioned resistor will equal that of the phase voltage. For complete elimination of the flow of direct current IDC through the neutral point of the network, it is possible to disconnect the neutral point from ground. This neutral point of the network may be then operated as insulated. However, this cannot always be implemented for operating reasons. With networks that are operated only with phase-to-phase loads, it is possible to include a capacitor between the neutral point of the network and ground. The capacitor will completely eliminate the flow of direct current I_DC via the neutral point of the network and will allow the flow of alternating current IOAC to a certain extent. In such a case, the capacitor will be under permanent voltage with its value depending on the capacitor’s capacitance and the value of alternating current IOAC flowing through the neutral point of the network. In case of a single-phase fault, or also when the network is switched on, the neutral point can have the value of phase-to-ground voltage. Therefore, the capacitor needs to be dimensioned for the phase voltage of the network. Capacitance in the neutral point of the network will considerably limit the value of the single-phase fault current flowing through the neutral point of the network and can also limit the value of current with singlephase load. The installation of a capacitor between the neutral point of the network and ground is preferred especially in networks with only phase-to-phase loads. With series connection of the capacitor and inductance - that is usually given by the equipment comprising the neutral point of the network - series resonance can occur. Due to such series resonance, hazardously high voltage and inductance are generated in the capacitor. Such increased voltage can be the cause of damage to equipment connected into the network. Among the disadvantages of the known solutions described above is the fact that with high voltage networks it is necessary to dimension the limiting impedances for the elimination of the direct current flowing through the neutral point of the network to the values of the phase voltage. In particular with very high voltage and extra high voltage networks the limiting impedance is quite demanding in terms of financing and often difficult to implement for technical reasons.

Summary of the Invention

The essence of the method of the voltage value Uz limitation between the neutral point and ground in an alternating-current electric network rests in the fact that a comparator switch is connected between the neutral point and ground. Then, using the comparator switch, the instantaneous value of the voltage Uz is continuously compared with a predetermined reference value UK; and/or the instantaneous value of the current IOAC flowing between the neutral point and ground is continuously compared with the reference value IK. If the instantaneous value of the voltage Uz > UK, and/or if the instantaneous value of the current IOAC > IK, then the comparator switch switching will connect the neutral point to ground via the comparator switch where such switching is executed within the time interval ti through te, where ti being the time when the instantaneous value of the voltage Uz = UK. and/or the instantaneous value of the current IOAC = IK, and b referring to the time when the amplitude of sinusoidal voltage Uz is attained.

In a beneficial implementation of the method according to the invention, a comparator switch is connected in parallel to the limiting impedance in the alternating-current electric network, in which limiting impedance is connected between the neutral point and ground in to limit the stray DC current IDC flowing through the neutral point, and the value of the voltage Uz between the neutral point and ground is limited after switching the comparator switch.

As an extension of this beneficial implementation of the method intended for an alternating-current electric network with limiting impedance, a single-pole switch connected in parallel to the limiting impedance is then added between the neutral point and ground. The single-pole switch is controlled by a comparator switch and upon switching of the comparator switch the neutral point is directly connected to ground. The single-pole switch is intended for long connection between the neutral point and ground, for example at a persisting fault not to force the comparator switch to keep switching.

In another beneficial implementation of the method according to the presented invention for an alternating-current electric network with directly earthed neutral point the stray DC current IDC flowing through the neutral point, the neutral point is disconnected from ground and a comparator switch is connected between the neutral point and ground, which results in the limitation of the voltage value Uz between the neutral point and ground.

The method intended for an alternating-current electric network with directly earthed neutral point can be completed in a preferred manner by connecting a single-pole switch between the neutral point and ground. The single-pole switch is controlled by the comparator switch and the neutral point is directly connected to ground upon switching the comparator switch. Even in this case, the single-pole switch is intended for long connection between the neutral point and ground, for example not to force the comparator switch to keep switching during a persisting fault. The subject-matter of another beneficial implementation of the method according to the present invention is also the method of control of the moment when the comparator switch is being switched off, namely both in the network with limiting impedance, and in the network with directly earthed neutral point. If the amplitude of the current IOAC flowing between the neutral point and ground is higher than the predetermined reference value IK, at least for one or more consecutive cycles, then the comparator switch remains in the switched-on condition even at the moment when the alternating current IOAC passes through zero value in terms of network frequency. This prevents the occurrence of voltage pulses and EMC interference.

Another beneficial implementation of the method of control of switching the comparator switch according to the presented invention is based on the comparator switch remaining in the switched-on state even at the moment when the alternating compensating current IOAC passes through zero in terms of network frequency.

The subject-matter of the presented disclosure is also the connection for limiting the value of the voltage Uz between the neutral point and ground in an alternating current electric network in particular when the stray DC current be occurs. The method is based on the connection of a comparator switch between the neutral point and ground where the comparator switch comprises of at least one pair of diodes connected in antiparallel, and/or “n” diodes arranged in series and connected in antiparallel, or a combination of at least one of the reference circuits, i.e. voltage or voltage comparator circuit, with a quick-acting switching unit. The voltage comparator circuit compares the instantaneous value of the voltage Uz with the predetermined reference value Uk. The voltage comparator circuit compares the instantaneous value of the current IOAC flowing between the neutral point and ground with the predetermined reference value IK. The quick-acting switching unit, or diodes connected in antiparallel, is used to switch the comparator switch connecting the neutral point and ground. Such switching must be executed at a time within the interval ti to fe. The time ti is defined for the voltage comparator circuit as the time when the instantaneous value of the voltage Uz = UK is attained. For diodes connected in antiparallel the ti is defined as the time when the instantaneous value of the voltage Uz = n.UD is attained, where in product n.UoUD refers to the value of the minimum voltage necessary for opening the diode in forward direction and "n” refers to the number of diodes arranged in series and connected in antiparallel. For the voltage comparator circuit, ti is defined as the time when the instantaneous value of the current IOAC = IK is attained. The time when the amplitude of the sinusoidal voltage Uz is attained is defined as the time ½.

In the aforementioned connection the voltage comparator circuit comprises of a circuit for the continuous comparison of the instantaneous value of the voltage Uz with the predetermined reference value UK, with output“1” for the instantaneous value of the voltage Uz > UK, and with output “0” for the instantaneous value of the voltage Uz < UK. The voltage comparator circuit comprises of a circuit for the comparison of the instantaneous value of the current IOAC with the predetermined reference value Ik, with output“ for the instantaneous value of the current IOAC ³ IK and with output“0” for the instantaneous value of the current IOAC < IK. The quickacting switching unit comprises of a switching element from the following group: triac, diac, a pair of diodes connected in antiparallel, thyristors or GTO thyristors, IGBT switching transistor, etc.

In addition, the connection where a single-pole switch controlled by a comparator switch is connected between the neutral point and ground is preferred.

The method and connection to limit the value of voltage between the neutral point and ground in alternating current electric networks can be used in particular in applications where impedance is connected between the neutral point of the network and ground to limit the flow of the stray DC current IDC through the neutral point concerned. The limiting impedance can be implemented for example by a resistor, capacitor and a combination thereof. For such networks the comparator switch is connected between the neutral point and ground, specifically in parallel to the limiting impedance. In the aforementioned networks it is possible to control the comparator switch by both a voltage and voltage comparator circuit. The voltage comparator circuit performs the control of the comparator switch depending on the value of the current IOAC flowing between the neutral point and ground. If the instantaneous value of the current IOAC exceeds the predetermined reference value IK, the comparator switch switches on. The method and the connection to limit the value of voltage between the neutral point and ground in alternating circuit electric networks can be also used in networks with directly earthed neutral point. In a network with directly earthed neutral point the neutral point of the network is disconnected from ground and a comparator switch with a voltage comparator circuit is connected between the neutral point and ground. However, as stated above, the voltage comparator circuit can also be used in networks where limiting impedance is connected between the neutral point and ground. With the use of the voltage comparator circuit the comparator switch will switch due to an increase of the instantaneous value of the Uz voltage between the neutral point of the network and ground above the preset reference value UK.

After the neutral point of the network is connected to ground via the comparator switch, the IOAC current is earthed. Due to this, the voltage Uz, i.e. the voltage in the neutral point of the network to ground, will have a value approaching zero. The comparator switch must switch on at the time between attaining the reference value UK. or IK, and attaining the amplitude Uz. The switching time of the comparator switch defined in this manner guarantees that switching on will occur before the amplitude of the Uz voltage between the neutral point of the network and ground is attained. This will reduce the Uz voltage. The maximum value of the Uz voltage between the neutral point of the network and ground is then given by the sum of the Uz voltage value at the moment when the reference value UK or IK is attained and the Uz voltage increment over the time of switching-on the comparator switch. If the aforementioned conditions are fulfilled it is possible to dimension the comparator switch, earthing of the neutral point of the network, and/or the limiting impedance at a lower voltage compared to the phase voltage. The value of voltage for dimensioning all the aforementioned equipment depends on the selection of the value of the reference values UK or IK, on the rate of switching of the comparator switch, and/or on the selection of the limiting impedance. For this reason, the application of the presented invention is suitable in particular for high voltage, very high voltage and extra high voltage networks.

The comparator switch can also comprise of only diodes connected in antiparallel, and/or“n” diodes arranged in series connected in antiparallel. Selection of the value of the UK reference voltage is then given by the product n.UD, i.e. the product of“n" diodes arranged in series connected in antiparallel and the value of the UD voltage that is necessary for opening the diode in forward direction. However, this solution can only be used in networks with no risk of problems due to radio frequency interference that may be generated when switching off the comparator switch whenever the current passes through zero.

Due to radio frequency interference elimination and voltage peaks prevention, the comparator switch is always switched off only after a decrease in the amplitude of the IOAC current flowing between the neutral point and ground under the predetermined reference value IK, namely at least for one or several consecutive cycles. Switching off the comparator Switch can also be implemented by a pre-set delay that will be longer than processes in the network generating a higher value of the IOAC current between the neutral point of the network and ground. This prevents switching off the comparator switch upon every passage of the IOAC current through zero which would otherwise generate voltage pulses or EMC interference.

After switching on the comparator switch of the circuit, the sum of the alternating current IOAC and direct current IDC flows in this circuit. If the value of the direct current IDC exceeds the amplitude of the alternating compensating current IOAC flowing through the neutral point of the network, it is necessary to select the switching elements or connections for the quick-acting switching unit that are also designed for switching off direct currents.

The advantages of the described solution of the presented invention rest in the fact that both the limiting impedance for the elimination of direct current flowing through the neutral point of the network and all other elements of the proposed equipment, can be dimensioned for values lower than the phase voltage of the given network. Therefore, especially in the case of very high and extra high voltage networks, expenses related to the installed equipment can be significantly reduced. The advantage of the method of control of the proposed equipment according to the invention also rests in the possibility to limit the occurrence of radio frequency interference. Explanation of Drawings

In Fig. 1 , an example of the connection according to the invention, including limiting impedance for the elimination of the e direct current flow is shown. In Fig. 2, an example of the connection without limiting impedance is shown. In Fig. 3, a connection diagram of a comparator switch with a voltage comparator circuit is shown, while Fig. 4 shows a connection diagram of a comparator switch with a voltage comparator circuit. In Fig. 5, a connection diagram with a single-pole switch and voltage comparator circuit is shown, while Fig. 6 shows a connection diagram with a single-pole switch and voltage comparator circuit and Fig. 7 shows a connection diagram of a comparator switch with power diodes connected in antiparallel. In Fig. 8, a graph of the instantaneous values of the voltage U_z between the neutral point and ground is shown.

Examples of the Invention Implementations

It shall be understood that the specific cases of the invention implementations described and depicted below are provided for illustration only and do not limit the invention to the cases provided here. People skilled in the art will find or, based on routine experiments, will be able to provide a greater or lesser number of equivalents to the specific implementations of the invention which are described here. Also such equivalents will be included in the scope of the following patent claims.

The examples of the implementations of the invention concern various methods of earthing of the neutral point 1 of the network in an area where the direct current IZDC flows through ground 2. The current IZDC flowing through ground 2 between earthed neutral points 1 of the network, i.e. between the neutral points of the TM and Tr_2 transformers, generates the direct voltage Albc in the ground resistor 2. In principle, both the windings of the transformers and a three-phase line provide a negligible resistance to the flow of direct current. Therefore, the stray DC current be flows through the neutral points 1 of the network and the three-phase line. The current be can cause the oversaturation of the transformer magnetic circuit and the transformer will therefore be operated with oversaturated magnetic circuit, which will result in increased noise and increased losses of the transformer. Oversaturation of the transformer magnetic circuit also causes overheating of its parts and results in an increase in the representation of harmonics in the current, including even harmonics. The compensating alternating current IOAC also flows through the neutral point I of the network.

Fig. 1 shows an example of the invention implementation where the limiting impedance 5 connected between the neutral point 1 of the network and ground 2 is used to eliminate the flow of the stray DC current IDC through the neutral point 1 of the network. The alternating compensating current IOAC then flows through the neutral point 1 of the network causing a voltage loss Uz on the limiting impedance 5. The voltage loss Uz also refers to the voltage between the neutral point 1 of the network and ground 2. In this example, the limiting impedance 5 comprises of a series combination of a capacitor C and a resistor R. The resistor R in the limiting impedance 5 is used to reduce the discharge current of the capacitor C and has only a small value, usually several Ohms. Therefore, the value of the limiting impedance 5 is especially affected by the selection of the capacity of the capacitor C. When designing the value of capacity of the capacitor C, it is necessary to consider the required value of a voltage loss in the capacitor C and the value of the compensating current IOAC flowing through the limiting impedance 5. For example, for a 400-kV network it is possible to ensure that during the regular operating condition of the network the Uz voltage loss on the limiting impedance 5 will equal to the required value, e.g. 250 V, by a suitable selection of the capacitor C. The selection of the Uz voltage value is based on technical and economic conditions for the use of capacitors C and the comparator switch 4. In the given example, the capacitor C voltage dimensioning is at the level of 6 kV regarding transient processes.

If a single-phase fault occurs in the network or the value of the compensating current IOAC increases due to a transient process, high Uz voltage can be generated on the limiting impedance 5 that can reach up to the value of the phase voltage of the network in the steady state. For this reason, a protective circuit 3 is connected between the neutral point 1 of the network and ground 2, namely in parallel to the limiting impedance 5. The protective circuit 3 includes a comparator switch 4 and a single-pole switch 6. The comparator switch 4 will switch on based on the increased voltage Uz on the limiting impedance 5 or based on an increased value of the current IOAC and will ensure that the single-pole switch 6 will subsequently switch on. The single-pole switch 6 is intended for long direct connection between the neutral point 1 of the network and ground 2, for example in case of a persistent fault. The voltage Uz between the neutral point of the network and ground 2 is thus approximately zero. Upon the comparator switch 4 switching on the current IOAC will flow especially through the protective circuit 3 with a low value of resistance connected in parallel. Regarding the current, the switching elements of the comparator switch 4 must be dimensioned for the effects of the short-circuit current and the discharge current of the capacitor C.

The comparator switch 4 remains in the switched-on condition even at the moment when the alternating compensating current IOAC passes through zero given the network frequency. Switching off the comparator switch 4 is determined either by the value of the amplitude of the IOAC current flowing between the neutral point 1 and ground 2, or by a predetermined switch-off time. The comparator switch 4 is switched off depending on the amplitude of the IOAC current flowing between the neutral point 1 and ground 2, if the amplitude of the IOAC current flowing between the neutral point 1 and ground 2 is lower than a predetermined reference value IK, namely for at least one or several consecutive cycles. Another possibility to control the switching-off of the comparator switch 4 is given by the expiry of a predetermined switch-on time. The switch-on time is selected to allow transient processes in the network to fade away. This usually concerns a period lasting several seconds. Prior to switching off the comparator switch 4, the single-pole switch 6 is switched off first. Considering the fact that during switching on of the protective circuit 3 even the stray DC current IDC can flow through the neutral point L of the network again, it is necessary that switching on of the protective circuit 3 lasts for only the necessary time.

Fig. 2 shows another example of the invention implementation. This example covers a directly earthed neutral point 1 in an alternating current electric network without any limiting impedance. The description of Fig. 2 is based on the description provided for Fig. 1. The only difference is that for limiting the stray DC current IDC flowing through the neutral point 1* it is necessary to disconnect the neutral point 1 from ground 2 and connect a protective circuit 3 between the neutral point i and ground 2. Another difference of this example is that it is not possible to use the switching on of the comparator switch 4 based on an increased value of the IOAC current.

Both the aforementioned examples do not specify in detail the implementation of the comparator switch 4. The implementation of the comparator switch 4 can comprise of some of the belowmentioned and also other connections.

Fig. 3 shows an example of the comparator switch 4 connection that comprises of a voltage comparator circuit 7 to compare the instantaneous value of the Uz voltage between the neutral point 1 and ground 2 that is designated as uz(t) in the diagram with a predetermined reference value UK. In addition, the comparator switch 4 comprises of a monostable multivibrator 8 with a predetermined time T of the output pulse and a quick-acting switching unit 9 comprising of power switching semiconductor devices (for example thyristors and triacs) that switches on upon the logic level“1” of the signal and switches off upon the zero logic level of the input signal after the current passes through the zero value.

The connection according to Fig. 3 works in a manner that the quick-acting switching unit 9 is in the switched off state during the regular operating mode and does not conduct current. If the instantaneous value of the Uz voltage between the neutral point 1 of the network and ground 2, that is designated as uz(t) in the diagram, increases above the value of the reference voltage UK, the voltage comparator circuit 7 generates a signal at the level of a logic Ί”. This signal is led to the input of the monostable multivibrator 8 that generates a switch-on signal for the quickacting switching unit 9 on its output. The quick-acting switching unit 9 will switch on and bridge over the limiting impedance 5. As a result, the Uz voltage on the limiting impedance 5 decreases to nearly the zero value and the voltage comparator circuit 7 cancels the signal that indicates exceeding the value of the reference voltage UK. On the output of the voltage comparator circuit 7 a logic“0” will occur. This logic“0” will enter the input of the monostable multivibrator 8. However, the monostable multivibrator 8 leaves the output condition in the switched-on state (a logic“1”) until the expiry of the pre-set time T. When the time T expires the output of the monostable multivibrator 8 is set to the logic state“0” and the quick-acting switching unit 9 switches off after the IOAC current passes through zero. If the condition uz(t) > UK is met again, the monostable multivibrator 8 will again respond to the output signal of the voltage comparator circuit 7 and will generate again the switch-on command for the quick-acting switching unit 9. To prevent repetitive switching on of the quick-acting switching unit 9, it is advisable to set the T time longer that the expected duration of non-symmetrical faults in the network.

Fig. 4 shows an example of the comparator switch 4 connection that comprises of a voltage comparator circuit 7 to compare the instantaneous value of the Uz voltage between the neutral point 1 and ground 2 that is designated as uz(t) in the diagram with a predetermined reference value UK. In addition, the comparator switch 4 comprises of an amplitude voltage comparator circuit 12 with the measurement of current 13, a flip-flop 10 with the priority function and a quick-acting switching unit 9 comprised by switching semiconductor devices (for example IGBT switching transistors, thyristors, GTO turn-off thyristors, and triacs).

The connection according to Fig. 4 works in a manner that the quick-acting switching unit 9 is in the switched-off condition during the regular operating mode and does not conduct current. If the instantaneous value of the Uz voltage between the neutral point i of the network and ground 2, that is designated as uz(t) in the diagram, increases above the value of the reference voltage UK, the voltage comparator circuit 7 generates a signal at the level of a logic“1”. This signal is led to the input of the flip- flop 10 that generates a switching-on signal for the quick-acting switching unit 9 on its output. The quick-acting switching unit 9 will switch on and bridge over the limiting impedance 5. As a result, the Uz voltage decreases to nearly the zero value on the limiting impedance 5 and the voltage comparator circuit 7 cancels the signal that indicates exceeding the value of the reference voltage Uk. On the input of the flip-flop 10. the level of the logic signal is“0”. The output“Q” of the flip-flop 10 remains at the level of a logic Ί” and the quick-acting switching unit 9 remains switched on. In addition, the IOAC current flows through the quick-acting switching unit 4. The amplitude voltage comparator circuit 12 analyses the amplitude of the measured current 13 that flows between the neutral point 1 of the network and ground 2. If the amplitude of the current IOAC is lower than the pre-set reference value of the current IK, a signal with the logic level Ί” is generated on the output of the amplitude voltage comparator circuit 12. This signal enters the input of the flip- flop 10. If a logic Ί” is on the input of the flop-flop 10 and if a logic“0” is on another input of the same flip-flop, the logic level“0” is set on the output of the flip-flop 10, which results in switching off the quick-acting switching unit 9. The current IOAC will flow only through the limiting impedance 5 again.

Fig. 5 shows an example of the connection of a protective circuit 3 with a single-pole switch 6 and a comparator switch 4. The comparator switch 4 comprises of a voltage comparator circuit 7 to compare the instantaneous value of the Uz voltage between the neutral point 1. and ground 2 that is designated as uz(t) in the diagram with a predetermined reference value UK. In addition, the comparator switch 4 comprises of an amplitude voltage comparator circuit 12 with the measurement of current 13, a flip-flop 10 with the priority function and a quick-acting switching unit 9 comprised of switching semiconductor devices (for example IGBT switching transistors, thyristors, GTO turn-off thyristors, and triacs).

The connection according to Fig. 5 works in a manner that the quick-acting switching unit 9 is in the switched-off condition during the regular operating mode and does not conduct current. If the instantaneous value of the Uz voltage between the neutral point 1 of the network and ground 2, that is designated as uz(t) in the diagram, increases above the value of the reference voltage UK, the voltage comparator circuit 7 generates a signal at the level of a logic“1 This signal is led to the input of the flip- flop 10 and then through the break-type contact of a single-pole switch 6 where it enters the input of the monostable multivibrator 8. The flip-flop 10 generates on the output a switch-on signal for the quick-acting switching unit 9. In addition, the monostable multivibrator 8 generates a switch command for the single-pole switch 6. After the quick-acting switching unit 9 switches on, the limiting impedance 5 is bridged over. As a result, the Uz voltage on the limiting impedance 5 decreases to nearly the zero value and the voltage comparator circuit 7 cancels the signal that indicates exceeding the value of the reference voltage UK. On the input of the flip-flop 10. the level of the logic signal is “0”. The output of the flip-flop 10 remains at the level of a logic Ί” and the quick-acting switching unit 9 remains switched on. In addition, the IOAC current flows through the quick-acting switching unit 4. The monostable multivibrator 8 will have a logic Ί” on its output for the time T. This ensures that the single-pole switch 6 will be switched on even in case of a lost signal from the voltage comparator circuit 7. Switching on of the single-pole switch 6 can last tens of ms. After the single-pole switch 6 is switched on, the auxiliary break- type contacts of the single-pole switch 6 will break. In addition, the auxiliary switching contact of the single-pole switch 6 will engage. The current IOAC can flow through the protective circuit 3 connected in parallel to the limiting impedance 5.

If the amplitude of the current IOAC decreases under the pre-set reference value IK, the amplitude voltage comparator circuit 12 generates a signal indicating a decrease of the current IOAC. AS the single-pole switch 6 is switched on, its auxiliary break-type contact is in the broken state and the signal from the amplitude voltage comparator circuit 12 will not get to the input of the flip-flop 10. This eliminates the switching off the quick-acting switching unit 9 before the single-pole switch 6 is switched off. This signal is also led via the auxiliary switching contact of the single-pole switch 6 to its switching-off circuit. After the single-pole switch 6 is switched off, the auxiliary break- type contact of the single-pole switch 6 switches on and the signal indicating a decrease of the current IOAC enters also the input of the flip-flop 10. On the output of the flip-flop 1^ a logic“0” will occur. This will also switch off the quick-acting switching unit 9. The current IOAC will again flow only through the limiting impedance 5.

The control of the quick-acting switching unit 9 can be completed with the option for its switching off with the single-pole switch 6 engaged. If the single-pole switch 6 is not able to switch on/off high-value currents, the quick-acting switching unit 9 must be always switched on after any change in the single-pole switch 6 condition.

Fig. 6 shows an example of the connection of a protective circuit 3 with a single-pole switch 6 and a comparator switch 4. The comparator switch 4 comprises of a voltage comparator circuit 1 to compare the instantaneous value of the IOAC current between the neutral point 1 and ground 2 that is designated as i(t) in the diagram with a predetermined reference value IK. In addition, the comparator switch 4 comprises of an amplitude voltage comparator circuit 12 with the measurement of current 13, a flip-flop 10 with the priority function and a quick-acting switching unit 9 comprised of switching semiconductor devices (for example IGBT switching transistors, thyristors, GTO turn-off thyristors, and triacs).

The connection according to Fig. 6 works in a manner that the quick-acting switching unit 9 and the single-pole switch 6 are in the switched-off condition during the regular operating mode and do not conduct current. The value of the current IOAC is handed over from the current sensor 13 for current measurement to the voltage comparator circuit H and to the amplitude voltage comparator circuit 12. If the instantaneous value of the current i(t), flowing between the neutral point 1 of the network and ground 2, exceeds the predetermined reference value of current IK, i.e. i(t) > IK, the voltage comparator circuit H generates a signal at the level of a logic“ on its output. This signal is led to the input of the flip-flop 10 and then through the break- type contact of a single-pole switch 6 to the input of the monostable multivibrator 8. The flip-flop 10 generates a switch-on signal for the quick-acting switching unit 9 on its output. In addition, the monostable multivibrator 8 generates a switching command for the single-pole switch 6. After the quick-acting switching unit 9 switches on, the limiting impedance 5 is bridged over. This will cause a decrease of the Uz voltage on the limiting impedance 5 to nearly the zero value. On the output of the flip-flop 1£, the level of the logic signal“1” will remain until the following condition is fulfilled i(t) > IK. When the current IOAC passes through zero in terms of network frequency, the aforementioned condition will be cancelled on a short-time basis. However, the output of the flip-flop 10 remains at the level of a logic“1" regardless of changes of its input. The quick-acting switching unit 9 remains engaged. The monostable multivibrator 8 will have a logic“ on its output for the pre-set time T. This ensures that the single-pole switch 6 will be switched on regardless of changes in the output of the voltage comparator circuit 11. Switching on of the single-pole switch 6 can last tens of ms. After the single-pole switch 6 is switched on, the auxiliary break-type contacts of the single-pole switch 6 will break. In addition, the auxiliary switching contact of the single-pole switch 6 will engage. The current IOAC can flow through the protective circuit 3 connected in parallel to the limiting impedance 5.

If the amplitude of the current IOAC decreases under the pre-set reference value IK, the amplitude voltage comparator circuit 12 generates a signal indicating a decrease of the current IOAC. As the single-pole switch 6 is switched on, its auxiliary break-type contact is in the broken condition and the signal from the amplitude voltage comparator circuit 12 will not get to the input of the flip-flop 10. This eliminates the quick-acting switching unit 9 switching off before the single-pole switch is switched off. This signal is also led via the auxiliary switching contact of the singlepole switch 6 to its switch-off circuit. After the single-pole switch 6 is switched off, the auxiliary break-type contact of the single-pole switch 6 switches on and the signal indicating the decrease of the current IOAC enters also the input of the flip-flop 10. On the output of the flip-flop 10 a logic“0” will occur. This will also switch off the quickacting switching unit 9. The current IOAC will again flow only through the limiting impedance 5.

As in the case of the example provided in Fig. 5 the control of the quick-acting switching unit 9 can be completed with its switching off with the engaged single-pole switch 0. If the single-pole switch 6 is not able to switch on/off high-value currents, the quick-acting switching unit 9 must be always switched on after any change in the condition of the single-pole switch 6.

Based on the previous examples, it is possible to demonstrate the benefits of the invention compared to the known solution in the following summary.

In case of the standard solution where the protective circuit 3 is switched on by a power switch instead of the quick-acting switching unit 9, the rate of switching that would need to be considered is 30 ms up to 70 ms. Over this time the Uz voltage will be repetitively reaching the phase voltage of the network. For this reason, the entire implementation of the earthing of the neutral point 1 of the network and the elements of the protective circuit 3 would have to be dimensioned for the value of the phase voltage of the network, including the limiting impedance 5 where applicable.

On the other hand, if fast switching on of the protective circuit 3 is used within the time from reaching the reference value, i.e. UK or IK until the amplitude of the voltage Uz according to the described invention, the value of the Uz voltage will be limited. Limitation of the Uz voltage depends on the rate of the protective circuit 3 switching. In a specific case it is possible to use the quick-acting switching unit 9 that will switch on the protective circuit 3 for example within 100 ns after reaching the pre- set reference value for the Uz voltage. If at the same time the amplitude of the voltage Uz is not reached, the Increase of the voltage Uz over the defined time will not exceed 1 % of the value of the network phase voltage. The maximum increase of the voltage Uz occurs during a single-phase fault. For example, for the single-phase fault current IOAC in the 400-kV network, the use of the fast switching on of the protective circuit 3 described above allows the maximum value of the Uz voltage increase to be reached from several to up to thousands of volts. Based on the provided values, it is obvious that through the fast rate of switching on the protective circuit 3* it is possible to achieve a considerably lower value of the voltage Uz compared to the phase voltage value. Therefore, it is possible, if this solution is applied, to dimension the entire implementation of the neutral point A earthing for the voltage from hundreds up to thousands of volts.

In a network with directly earthed neutral point 1* it is possible that the single-phase short-circuit current flows through the neutral point of the network during a singlephase fault. Duration of the short-circuit current depends on the pre-set time characteristics of the respective protecting device in the network or in the neutral point A of the network. To ensure the reliable functioning of the protective circuit 3_j it is necessary to dimension the protective circuit 3, comparator switch 4 and singlepole switch 6 for short-circuit currents, including the short-circuit current duration.

If the amplitude of the compensating current IOAC is lower that the value of the direct current IDC flowing through the neutral point A of the network during the regular operating condition, the result current given by the sum of IOAC and IDC will not change polarity or pass through zero. This means that the comparator switch 4 must be connected in a manner allowing its elements to switch off also this result direct current. Common diodes and thyristors do not allow direct current switching off. This needs to be considered in particular in case of use of diodes 14, 15 connected in antiparallel. The protective circuit 3 switching off must be ensured in the same way as the direct current switching off by semiconductor devices.

Fig. 7 shows the comparator switch 4 comprising of only the quick-acting switching unit 9. This quick-acting switching unit 9 comprises of power diodes 14 connected in antiparallel or“n” power diodes arranged in series and connected in antiparallel 15, where the minimum value of natural number“n” equals 1.

The application of this connection is preferred only in a limited number of cases. It can be used especially where there is a sufficiently low voltage Uz and the direct current voltage AUDC between the neutral point 1. and ground 2. For the function of this circuit, the value of the voltage Uo, which is the catalogue value of the minimum voltage necessary for the opening of a specifically used diode 14 in forward direction, is of key importance. If the amplitude of the voltage Uz is lower than n.U_D, the diodes 15 will be closed during regular operating condition and will conduct current. Upon increasing the instantaneous value of the voltage Uz above the value n.UD, the diodes 15 will automatically close in reverse direction. Only the UD voltage will be present on individual diodes 14 in reverse direction, i.e. the voltage of the 14 diode in forward direction. After the current passes through zero, the opened diodes 15 will automatically close due to a change in the current direction. If an increase in the instantaneous value of the voltage Uz above the value n.UD occurs within the next half-cycle, the diodes connected in antiparaliel 15 that were in the previous half-cycle in reverse direction will open. This comparator switch 4 does not need any control for its activity. For the use of this connection, a suitable selection of the limiting impedance 5 can affect the value of the voltage Uz. By increasing the number of“n” diodes arranged in series connected in antiparallel 15, the required level of the voltage UK = n. UD can be increased. For example, the voltage UD for the silicone diodes 14 is approximately 0.6 V up to 0.7 V. If the voltage Uz is higher than the voltage UD under regular operating conditions, it is necessary to use a relevant number of diodes 15 arranged in series connected in antiparallel. The value of the voltage Uz between the neutral point 1 of the network and ground 2 will then attain the value corresponding to the product of“n” diodes 15 arranged in series and connected in antiparallel and the value of the voltage UD.

The advantage of the simplicity of this connection can be utilized in cases where - under regular operating conditions - a low compensating current IOAC flows through its neutral point 1 that generates a loss of the voltage Uz of several up to tens of volts in the limiting impedance 5. In addition, it must apply that the amplitude of the alternating current IOAC will be higher than the value of the direct current IDC. In Fig. 8 a graph of the instantaneous values of the U_z voltage on the limiting impedance 5, or in other words of the voltage between the neutral point 1 of the network and ground 2, are shown:

Curve 1 : a graphic representation of the instantaneous value of the sinusoidal voltage Uz in the limiting impedance 5 without the protective circuit 3 switching, Curve 2: the instantaneous value of the voltage Uz in the limiting impedance 5, upon switching on the protective circuit 3 every time after exceeding the reference value UK by the instantaneous value of the voltage Uz and switching off the protective circuit 3 every time the current passes through the zero value, Curve 3: the instantaneous value of the voltage Uz on the limiting impedance 5, upon switching on the protective circuit 3 upon exceeding the reference value UK by the instantaneous value of the voltage Uz only after a decrease in the amplitude of the current IOAC under the predetermined reference limit of the current IK.

Power semiconductor devices switch off when the current passes through zero. If the zero current flows through the connected protective circuit 3, the voltage Uz will also be zero. After the protective circuit 3 is disconnected, it is switched on again upon an increase of the voltage Uz above its reference value UK. This will repeat every half-cycle of the flowing alternating current IOAC. This will generate high frequency voltage peaks that will spread along the network and emit an interference electromagnetic field (Fig. 8). Thus, the protective circuit 3 becomes a source of radio frequency interference during the flow of the IOAC current through the neutral point 1 of the network. To eliminate the process described above, the comparator switch 4 is equipped with the control ensuring its permanent switching-on until a decrease in the amplitude of the current IOAC flowing through the neutral point 1 of the network is below the level of IK. For high outputs and voltages, it is advisable to complete the comparator switch 4 with a single-pole switch 6 connected in parallel that will switch on after the comparator switch 4 is switched on and will connect the neutral point 1 of the network directly to ground 2. When the current IOAC decreases to the regular operating value, the comparator switch 4 and the single-pole switch 6 are always automatically switched off. Permanent switching on of at least one of the 4 and 6 switches during an increased flow of the compensating current IOAC through the neutral point 1 of the network limits the generation of radio frequency interference.

Industrial Applicability

The method and the connection to limit the value of voltage between the neutral point of the network and ground can be used especially in networks with directly earthed neutral point or with neutral point earthed via impedance that is connected between the neutral point and ground and prevents the direct current IDC flowing through the neutral point of the network. The application of the present invention is suitable in particular for high voltage, very high voltage and extra high voltage networks.

Overview of the Positions and Abbreviations Used in the Drawings

1 neutral point

2 ground

3 protective circuit PO

4 comparator switch KSO

5 limiting impedance Z

6 single-pole switch S1

7 voltage comparator circuit UKO

8 monostable multivibrator MKO

9 quick-acting switching unit RSJ

10 flip-flop RS

11 voltage comparator circuit IKO

12 amplitude voltage comparator circuit AIKO

13 measurement of the IOAC current between the neutral point and ground

14 diodes connected in antiparallel

15 “n” diodes arranged in series connected in antiparallel

Tr_1, Tr_2 ... three-phase transformers with Y-connection of windings

IOAC alternating current flowing between the neutral point of the network and ground

IDC direct current flowing between the neutral point of the network and ground IZDC earthed direct current

Uz instantaneous voltage between the neutral point of the network and ground AUDC direct current voltage between the neutral points of the network Tr_1 and

T r_2 generated by the flow of the direct current IZDC to ground

R, C resistor, capacitor - elements that the limiting impedance can be comprised of

Uz(t) instantaneous value of voltage between the neutral point of the network and ground

i(t) instantaneous value of the measured current flowing between the neutral point of the network and ground

UK reference value of voltage

IK reference value of current MKO monostable multivibrator

AZD diodes connected in antiparallel

n.AZD “n" diodes arranged in series connected in antiparailel

Claims

PATENT CLAIMS

1. The method of limiting the voltage Uz between the neutral point (1) and ground (2) in an alternating current electric network, characterized in that a comparator switch (4) is connected between the neutral point (1) and ground (2), then - using the comparator switch (4) - the instantaneous value of the voltage Uz is continuously being compared to the reference value UK, and/or the instantaneous value of the current IOAC flowing between the neutral point (1 ) and ground (2) is being continuously compared with the predetermined reference value IK, and if the instantaneous value of the voltage Uz ³ UK, and/or if the instantaneous value of the current IOAC > IK, then the switching on of the comparator switch (4) connects the neutral point {1) with ground (2) via the comparator switch (4), where the switching on is carried out within the time interval ti to h, where ti refers to the time when the instantaneous value of the voltage Uz = UK is attained, and/or to the time when the instantaneous value of the current IOAC = IK is attained, and te is the time when the sinusoidal voltage Uz is attained.

2. The method according to claim 1 , characterized in that the alternating current electric network in which the limiting impedance (5) is connected between the neutral point (1) and ground (2) in particular to limit the stray DC current IDC flowing through the neutral point (1 ) includes a comparator switch (4) connected in parallel to the limiting impedance (5) and the value of the voltage Uz between the neutral point (1 ) and ground (2) is limited after the comparator switch (4) switching on.

3. The method according to claim 1 , characterized in that the stray DC current IDC flowing through the neutral point (1 ) is limited by disconnecting the neutral point (1) from ground (2) and by connecting a comparator switch (4) between the neutral point (1) and ground (2) in an alternating current electric network with directly earthed neutral point (1) where the value of the voltage Uz between the neutral point (1 ) and ground (2) is limited after the comparator switch (4) switching on.

4. The method according to claim 2, characterized in that a single-pole switch (6) is also connected in parallel to the limiting impedance (5) between the neutral point (1) and ground (2). The single-pole switch is controlled by the comparator switch (4) and the neutral point (1) is directly connected to ground (2) after the comparator switch (4) switching on.

5. The method according to claim 3, characterized in that a single-pole switch (6) controlled by the comparator switch (4) is connected between the neutral point

(1 ) and ground (2) and the neutral point (1 ) is directly connected to ground (2) upon the comparator switch (4) switching on.

6. The method according to any of claims 1 through 5, characterized in that if the amplitude of the current IOAC flowing between the neutral point (1) and ground

(2) is higher than the predetermined reference value IK for at least one or more consecutive cycles, then the comparator switch (4) remains in the switched-on condition even at the moment when the alternating compensating current IOAC passes through zero in terms of network frequency.

7. The method according to any of claims 1 through 5, characterized in that the comparator switch (4) remains in the switched-on condition during a pre-set period of time even at the moment when the alternating compensating current IOAC passes through zero in terms of network frequency.

8. The connection to limit the value of the voltage U_z between the neutral point (1 ) and ground (2) in an alternating current electric network, in particular in case of the stray DC current e occurrence, using the method according to claim 1 , characterized in that a comparator switch (4) is connected between the neutral point (1 ) and ground (2), where the comparator switch comprises of at least one pair of diodes (14, 15) connected in antiparallel, or a combination of at least one of the reference circuits (7, 11) with a quick-acting switching unit (9), where the voltage comparator circuit (7) is designed to compare the instantaneous values of the voltage Uz with a predetermined reference value Uk, the voltage comparator circuit (11) is designed to compare the instantaneous values of the current IOAC flowing between the neutral point (1) and ground (2) with a predetermined value IK, where the quick-acting switching unit (9) or the diodes (14, 15) connected in antiparallel, are designed to switch on the comparator switch (4) connecting the neutral point (1) and ground (2) at the time falling within the time interval ti to fe, where ti is the time when the instantaneous value of the voltage Uz = UK is attained for the voltage comparator circuit (7), the time ti is the time when the instantaneous value of the voltage Uz = O.UD is attained for the diodes (14, 15) connected in antiparallel, where in the product n.Uo UD refers to the value of the minimum voltage necessary for opening the diode (14) in forward direction and“n” is the number of diodes (15) arranged in series and connected in antiparallel, and for the voltage comparator circuit (11) ti is the time when the instantaneous value of the current IOAC = IK is attained, where t2 is the time when the amplitude of the sinusoidal voltage Uz is attained.

9. The connection according to claim 8, characterized in that the voltage comparator circuit (7) comprises of the circuit of continuous comparison of the instantaneous value of the voltage Uz with the predetermined reference value UK, with output“ for the instantaneous value Uz > UK and with output "0” for the instantaneous value of voltage Uz < UK, where the voltage comparator circuit (11) comprises of the circuit for the comparison of the instantaneous value of the current IOAC with the predetermined reference value Ik with output“ for the instantaneous value of the current IOAC > IK and with output “0” for the instantaneous value of the current IOAC < IK, and that the quick-acting switching unit (9) comprises of a switching element from the following group: triac, diac, a pair of diodes connected in antiparallel, thyristors or GTO thyristors, and switching transistor (IGBT).

10. The connection according to claims 8 or 9, characterized in that a single-pole switch (6) is also connected between the neutral point (1) and ground (2) where the single-pole switch is controlled by a comparator switch (4).