METHOD FOR ADJUSTING COMPENSATING INDUCTOR IN DISTRIBUTION NETWORK FOR ELECTRICAL ENERGY

Abstract: The invention relates to a method for adjusting a compensating inductor in a distribution network for electrical energy comprising an adjustable compensating inductor (K) between the star point of a transformer (M) connected to the network, such as a power transformer or a separate earthing transformer, and the earth, the inductance of the compensating inductor then forming a part of a total zero inductance (L) of the distribution network for electrical energy. The method comprises the steps, in which a total earth capacitance (C) of the network is determined by means of a zero voltage transient associated with a connection change or fault situation of the network, when the total zero inductance (L) of the network is known, a total zero inductance value (Lr) of the network is determined, the inductance forming a reactance whose absolute value is the same as the absolute value of the reactance formed by the total earth capacitance of the network at a nominal frequency (fn), and the compensating inductor inductance (Lk) is adjusted so that the total zero inductance of the network equals the value calculated above.

Description: METHOD FOR ADJUSTING COMPENSATING INDUCTOR IN DISTRIBUTION NETWORK FOR ELECTRICAL ENERGY BACKGROUND OF THE INVENTION The invention relates to a method for adjusting a compensating in- ductor in a distribution network for electrical energy comprising an adjustable compensating inductor between the star point of a transformer connected to the network, such as a power transformer or a separate earthing transformer, and the earth, the inductance of the compensating inductor then forming a part of a total zero inductance of the distribution network for electrical energy Compensating inductors are used in distribution and transmission networks for electrical energy to improve the conditions for earth faults to quench automatically. In order for the self-quenching to be at the highest, the current at the earth fault, or a so-called zero current, has to be made as low as possible. The zero current is at the lowest when the network is at so-called resonance i.e. the absolute value of the reactance formed by the zero inductance of the network is the same as the reactance formed by a total earth capacitance of the network. The problem is to adjust the inductor into such a position where the network is at resonance. A conventional method for adjusting the network at resonance is to adjust the inductor to such a position where the zero voltage of a healthy state of the network, or a so-called natural zero voltage, is at the highest. The zero voltage of the healthy state is caused by the asymmetry of capacitances between the phases and the earth. A conventional adjustment method cannot react rapidly to connection state changes, since the adjustment principle per se is slower. Since a plurality of inductor positions, in some of which the network is considerably out of tune, are also gone through, then the conventional adjustment measures may merely weaken the self-quenching conditions of electric arc earth faults. A conventional adjustment method requires asymmetry of the earth capacitances of the network since the adjustment is based on searching for a maximum point of the natural zero voltage. In well-alternating overhead line networks and underground systems the natural asymmetry can remain too small in order for the zero voltage maximum point to be accurately found. The problem has conventionally been solved by connecting to a phase an additional condenser, which make the zero voltage of a healthy state rise. In a more recent solution current is fed into the star point providing as for the adjustment a sufficient zero voltage of the healthy state. In both solutions additional components are needed. A conventional adjustment method has the advantage of being sim- pie. In principle, a controller does not need to calculate anything. An adjustment of only one step in either direction is performed. If the zero voltage rises, a second step is adjusted in the same direction. The process thus continues until the zero voltage drops for the first time. Then, it is noted that the maximum point is bypassed and an adjustment of one step backwards is per- formed and the inductor is left in that position. BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is to provide a method by which a compensating inductor can be adjusted more rapidly and more reliably than before. This is achieved with the method of the invention characterized by comprising the steps determining a total earth capacitance of the network by a zero voltage transient associated with a connection change or fault situation of the network, when a total zero inductance of the network is known, determining a value of a total zero inductance of the network, the inductance forming a reactance whose absolute value is the same as the absolute value of the reactance formed by the total earth capacitance of the network at a nominal frequency, and adjusting the compensating inductor inductance so that the total zero inductance of the network equals the value calculated above. The preferred embodiments of the invention are disclosed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail with reference to the accompanying drawings, in which Figure 1 is a schematic view showing a distribution network for electrical energy quenched by a compensating inductor; Figure 2a is an example showing a damped oscillation of zero volt- age in connection with a connection change; Figure 2b shows a frequency of the damped oscillation according to Figure 2a as a function of time, and Figure 3 is a schematic view showing a connection in principle between a total earth capacitance of the network, a total zero inductance of the network and a total earth resistance of the network. A DETAILED DESCRIPTION OF THE INVENTION Figure 1 is a schematic view showing a distribution network for electrical energy quenched by a compensating inductor K. The compensating inductor K is connected between a star point of a line feeding transformer M and the earth. The schematic view also shows the operating principle of the compensating inductor i.e. the absolute value of a reactance formed by a zero inductance L of the network, from which the reactance of the compensating inductor K forms a part, equals the reactance formed by a total earth capacitance C of the network, in which case the earth fault current moving through an earth fault location is in theory zero. Figure 2a shows a damped oscillation, the frequency of which being f r , caused to a zero voltage of an exemplary substation measured in connection with a connection change of the network i.e. to a voltage being effective over the inductor K in Figure 1. Figure 2b shows said frequency f r as a function of time. As disclosed from Figure 2b the frequency f r has risen over 58 Hz, so the disconnection of a line has resulted in the network becoming overcompen- sated. Always when a connection change takes place in the network, a temporary zero voltage change described in Figure 2b also occurs. The zero voltage U 0 change is caused by the fact that also charging and/or discharging of earth capacitances of the lines are associated with the connection changes. Temporary oscillations are formed in the oscillatory circuits formed by the earth capacitances and network inductances. The result is a temporary asymmetry in phase voltages becoming apparent as a change in the zero voltage. The phenomenon leaves an exponentially attenuating U 0 , as shown in Figure 2a, the frequency of which depends on a degree of tuning of the network. This exponentially attenuating zero voltage transient is summed to the zero voltage of the healthy state of the network caused by the asymmetry of the earth capacitances and is of the same frequency as the network fre- quency f n (50 Hz in Europe). In accordance with the above the invention is based on determining the degree of tuning of the network using the attenuating U 0 . The degree of tuning can be calculated in various ways. One way is to resolve directly the total earth capacitance C of the network using the attenuating zero voltage transient when the total zero inductance of the network is known. The total earth capacitance C of the network, the total zero inductance L of the network and the total earth resistance R of the network can be considered as being coupled in parallel as shown in Figure 3. The following equation can be written for currents moving through the total earth resistance R, the total zero inductance L and the total earth capacitance C: When taking into account that l R = U 0 / R, l L = -(1/L) * J U 0 dt + l L0 and l c = C * dU 0 /dt the following differential equation is obtained: U 0 /R - (1/L) * I U 0 dt + l L0 + C * dUn/dt = 0 Using the above differential equation the total earth capacitance C of the network can be solved, for example, by using the prior art method of the smallest square sum, when the attenuating zero voltage transient U 0 (t), as a function of time t, and the total earth capacitance L of the network are known. However, the most preferable way is to measure the frequency of the attenuating U 0 transient. Said frequency is a resonance frequency f r of the oscillatory circuit formed by the total zero inductance and total earth capacitance of the network. When the total zero inductance L of the network is known, the total earth capacitance C of the network can be calculated: C = 1 / (L* (2 * π * f r ) 2 ) Now when the total earth capacitance C and the resonance frequency f r are known, it is possible to calculate what the total zero inductance of the network has to be in order for the network to be fully tuned. The network is fully tuned when the resonance frequency is the same as a nominal frequency f n of the network. Let L r be the value of the total zero inductance of the network by which the network is tuned. The equation solving L r can be written: C = 1/ (L * (2 * π*f r ) 2 ) = (Lr * (2 * π * f n ) 2 ) = > Lr = L * (f/f n ) 2 In order to be able to tune the network, an inductance of the induc- tor and the rest of the zero inductance of the network must be known. There are several ways to find out the inductor inductance. One way is to measure a current I and a voltage U of the inductor during the attenuating zero voltage transient. These measurements now allowing to calculate the inductor inductance L k : L k = U / (2 * π * f r * l) where f r is the measured frequency of the attenuating U 0 , and U and I are the rms values of the voltage and current of the inductor measured at the same time. Another way to measure the inductor inductance is to measure the position of the inductor, for example by potentiometer. The memory of the controller has to include a function indicating the dependence between the adjustment position and the inductance. The function can be linear or nonlinear. It can be stored in the memory of the controller in form of a table or as a mathematical formula. If the actuator used for controlling the inductor is accurate, there is no need to measure the inductor inductance. The memory of the controller then has to include the prevailing value of the inductance, in which case it is possible to calculate what kind of actuator control sequence is needed to alter the inductor inductance so as to obtain a desired inductance. The rest of the zero inductance of the network is formed from the zero inductance of a power transformer or when a separate earthing transformer is used from its zero inductance. These zero inductances can be thought to be coupled in series with the inductor inductance. The zero inductance caused by the voltage transformers connected between the phase and the earth is, in turn, coupled parallel to the inductor. However, the zero induc- tance caused by the voltage transformers exceeds the inductor inductance in general to such an extent that it can be left unnoticed. The zero inductance of a transformer or a separate earthing transformer can be required of the manufacturer or be measured. It is taken into account when the total zero inductance is calculated by calculating it together with the inductor inductance. The controller of the inductor can also measure the part of the inductor inductance of the total zero inductance by adjusting the inductor during the attenuating U 0 to another position, in which case the resonance frequency changes. Measuring the resonance frequencies f r1 and f r2 by the inductor positions L k1 and L k2 the rest of the zero inductance ΔL of the network can be solved as follows: C = 1/ ( (L k1 + ΔL) * (2*π*f r1 ) 2 ) = 1 / (L k2 + ΔL)* (2*π*f r2 ) 2 ) = > ΔL = L k2 * (f r2 /f r1 ) 2 -L k1 ) / ( 1 -( f r2 /f r1 ) 2 ) In order for the rest of the zero inductance of the network to be measured as accurately as possible, the position of the inductor has to be considerably altered. Since the zero inductance of the inductor does not change, it only has to be measured once. The measured value is stored in the memory of the controller. When the rest of the zero inductance of the network is known, the inductor to be adjusted is adjusted to such a position that the network is tuned at the total zero inductance. If the earth capacitances of the network are very asymmetrical, then the natural zero voltage can be harmfully high in the resonance state. In such a case the inductor is finally adjusted in the under- compensating direction to such an extent which falls below the zero voltage of the greatest allowed continuous state set for the controller. The solution of the invention is also able to directly conclude if the control range of the inductor is exceeded, if the network has become heavily overcompensated as a result of disconnecting long wires. If the degree of overcompensation is even at the extreme limit of the control range so high that the self-quenching probability of the electric arc is higher in the corresponding network separated from the earth, then the controller switches the inductor off without trying to adjust it. The controller includes a parameter to be set indicating how much higher the remaining earth fault current of the compensating network may be compared to a corresponding earth fault current of the network separated from the earth so that the inductor is kept on. If said remaining current limit at the extreme limit of the inductor control range is exceeded, then the inductor is switched off. The setting range of said parameter is 1 ... 2. This setting range is adequate to cover the ratio between the self-quenching limits of the earth fault currents determined in DIN and VDE standards in a network that is compensated and separated from the earth. (VDE 0228, Teil 2) If a resistor is used alongside the inductor to increase the active component of the earth fault current, it is worth switching it on when the zero voltage relay awakes and off when a protective relay functions. The attenuation time constant of the zero voltage can thus be extended, leaving more time for measuring the frequency. The inductor controller of the invention enables to tune the inductor more rapidly than is possible by searching for the conventional natural zero voltage maximum point. The controller of the invention directly calculates each degree of tuning and a new base value for the inductor position. It is not nec- essary to go through a number of inductor positions and to search for a resonance point by testing. If a sufficiently fast inductor control device is used, then the network can be adjusted at resonance immediately after a relay function or another connection change of the rest of the network. This is useful, for example, in a situation where the connection state of the network changes rapidly as a result of relay functions caused by a thunder storm. It is even possible to tune the network for the dead time of a delayed automatic reclosing. With the solution of the invention the inductor can be kept in tune nearly all the time, when the self-quenching probability of the electric arc earth faults is at the highest and a part of the relay functions can be avoided. A conventional adjustment method does not allow rapid reaction to connection state changes because the adjustment principle as such is slower. Since several inductor positions, in some of which the network is considerably out of tune, have to be gone through, the conventional adjustment measures can temporarily weaken the self-quenching chances of the electric arc earth faults. The solution of the invention is also able to directly conclude if the inductor control range is exceeded, if the network has become heavily over- compensated as a result of disconnecting long wires. If the degree of over- compensation is so high even at the extreme limit of the control range that the self-quenching probability of the electric arc is higher in a corresponding network separated from the earth, then the controller switches the inductor off without trying to adjust it. A conventional adjustment method requires asymmetry from the earth capacitances of the network since the adjustment is based on searching for a maximum point of the natural zero voltage. In well-alternating overhead line networks and underground systems the natural asymmetry can remain too small in order for the maximum point of the zero voltage to be accurately found. Conventionally the problem has been solved by connecting to a phase an additional condenser which makes the zero voltage of the healthy state rise. In a more recent solution current is fed into the star point providing a sufficient zero voltage of the healthy state as for the adjustment. Additional components are needed in both solutions. The adjustment method of the invention is based on measuring zero voltage transients associated with network connections and temporary interferences, and does not require a natural zero voltage at all. It is thus possible to do without the accessories required by conventional solutions. The zero voltage transients measured by the inductor controller can also be utilized in monitoring the network. Temporary earth faults frequently occur in the overhead line networks. A degree of tuning v of the network can be calculated on the basis of the attenuating zero voltage frequency occurring after removing the earth faults and a damping ratio d can be calculated using a damping time constant. Using the natural zero voltage a degree of asymmetry k of the network being a vector variable can also be calculated. A change in the degree of tuning of the network in the same inductor position always indicates a change in the total earth capacitance of the network. If no deliberate connection state changes are made, a change in the degree of tuning may signify, for example, a total or partial detachment of a long line from the network as a result of a conductor break. A rise in the degree of attenuation of the network may indicate, for example, an insulator defect. A change in the degree of asymmetry can, in turn, indicate an earth fault of great impedance. Changes in all said parameter values may indicate a defect if pro- tective relay functions taken place or controls performed in the network are not known. It is preferable that the controller of the compensating inductor determines said parameter values from each stored zero voltage transient and informs them and/or the changes exceeding the predetermined amount of them to an operating person. The degree of tuning of the network is determined as follows: v = (l L -l c ) lc By taking into account that l L = U 0 / ( 2 * π * f n * L ) and l c = U (2 * π * f n *C), a formula is obtained for the degree of tuning: v = 1 / (L * C * (2 * π * f n ) 2 ) - 1 , where L is the total zero inductance of the network and C is the total earth capacitance and f n is the nominal frequency of the network. According to the formula presented above C = 1/ (L* (2 * π*f r ) 2 ) where f r is the frequency of the attenuating U 0 . By disposing these in the for- mula of v it is obtained: v = (f r /f n ) 2 - 1 A time constant τ of the attenuating zero voltage can be determined as follows. An amplitude U 0 of the attenuating zero voltage is calculated as a function of time, for example, using the Fourier transform, which is exponential by mode: U 0 (t) = U 00 *exp (-t/τ) => -t = τ*ln(U 0 (t) / U 00 ) Let us assume that a vertical vector X = ( 0 ... u 0p ( N ) ) includes N U 0 amplitude estimates divided by a first amplitude estimate U 00 , from which a natural logarithm is also taken in the following manner: u 0p (k) = In (U 0 (k) / U 00 ) Let Y = ( 0, t(2) ... t(N) ) be a vertical vector including the corresponding points of time. Then the following equation group can be written from where the time constant τ is solved using the method of the smallest square sum. Y = τ*X => τ = - (Y T* _Y) / (Y J* ) From the time constant τ a degree of attenuation of the network can further be calculated. The degree of attenuation d of the network is determined: d = l R /l c By taking into account that l R = U 0 / R, a formula for the degree of attenuation is obtained: d = 1 / (2*π*f n * C*R ), where R is the total zero resistance of the system. On the other hand the attenuation of the system can also be described by an attenuation constant δ: δ = 1/ ( 2 * R * C ) The attenuation time constant τ and the attenuation constant δ of the attenuating zero voltage exponentially relate to each other as follows: δ = 1 / τ = > τ = 2*R * C The degree of attenuation of the network can thus be calculated from the formula: d = 1 / (π*f n *τ) If the smallest square sum method is used in a previously described manner in analysing the attenuating zero voltage the degree of attenuation d can directly be calculated using a solving parameter x.,: d = 1 / (2*π * f n * C * R ) = - , I ( 2 * π * f n ) If the zero voltage of the continuous state exists in the network a so-called degree of asymmetry k can be calculated as follows. A so-called relative zero voltage of the continuous state U en is obtained from the formula: U_ n = M o I U RS = k / ( v + d 2 ) where U RS is a complex principal volt- age indicator. When L . n and v and d are known the degree of asymmetry can be calculated from the formula: k = U en * V (V + d 2 ) The adjustment principle of the invention requires that the inductor inductance causing a need to either measure the inductor inductance or its position is known. If this is not possible the control of the inductor has to be accurate. Here the requirements exceed the ones in conventional adjustment which does not require that the inductor inductance is known. The presented adjustment method of the inductor also requires a better signal processing capacity and a larger memory capacity than the conventional adjustment method. The required calculation and memory capacity is, however, small compared to the possibilities of modern technology. A method where the inductor is tuned during the attenuating zero voltage can in some cases be adapted as an alternative for the above de- scribed adjustment method. If the zero voltage of the continuous state of the network is small in relation to the attenuating zero voltage transient and the attenuating time constant of the zero voltage transient is long, it is possible to tune the inductor during the transient. Then the frequency of the attenuating zero voltage is at first calculated during some sequences and a new standard value for the inductor position is calculated in the manner described above. Next the inductor is directed to said position and the frequency is measured again. If the frequency already approaches 50 Hz no more new control operations are performed. If the frequency still deviates from 50 Hz, then a new standard value is calculated for the inductor position but the inductor position is only changed by half of what is calculated in order not to exceed the resonance point due to inaccuracy of the control. This is done when several consecutive gradually decreasing control movements are needed and the frequency is measured at appropriate intervals. If the time constant of the zero voltage is sufficiently long and the actua- tor of the controller is adequately fast it is possible to tune the inductor before the interference attenuates. In an overhead line network the zero voltage of the continuous state is generally so high that tuning the inductor during the attenuating zero voltage in the manner described above fails. The reason is that the zero voltage of the continuous state superposes to the zero voltage thus making the measurement of the frequency impossible. In such a situation it is better to store at first the entire zero voltage transient from the beginning until it has attenuated and changed into a mere continuos state zero voltage. A recursive discrete Fourier transform is then performed for the stored zero voltage curve u 0 (1) ... u 0 ( N ) and a complex zero voltage indicator U 0 is calculated: X(k) = X(k- 1) + (2/ N s ) * ( u 0 (k) - u 0 (k-N s ) ) * cos ( - (k)*2*π/ N s ) Y(k) = Y(k- 1) + (2/ N s )*( u 0 (k) - u 0 (k-N s ) )*sin ( - (k)*2*π/ N s ) U 0 (k) = X(k) + i * Y(k) In the above formulas k is an index indicating a point of the stored u 0 curve and N s is the number of samples of the stored u 0 for one network frequency sequence i.e. N s = f s / f n , where f s is a sampling frequency of the con- trailer and f n a nominal network frequency. If the zero voltage transient has already attenuated, then the last zero voltage indicator U 0 (N) represents the zero voltage of the continuous state. On this basis a new zero voltage curve can be formed from which the zero voltage of the continuous state has been removed: U 0 ' (k) = U 0 (k) - U 0 (N) An exponentially attenuating sine curve u 0 ' (I) ... u 0 ( N ), the frequency of which is the resonance frequency of the network, can be formed from a complex indicator vector U 0 ' (k) : u 0 ' (k) = U 0 '(k) * cos (2*π*k / N s + a (k) ), where U 0 '(k) is an absolute value of the complex indicator vector element complex U 0 ' (k) and a (k) is an argument thereof i.e. a phase angle. From the curve u 0 ' (I) ... u 0 ' ( N ) obtained the resonance frequency and a new standard value are calculated using any known frequency estimation method for the compensating inductor position. Using a field bus of a substation measurements of different line protection relays can be utilized in adjusting the inductor. One possibility is to calculate the rest of the zero inductance of the system connected to the inductor in series so that the network can be more accurately controlled in tune. The calculation is based on the fact that each relay calculates the earth capacitance of the line it protects using the attenuating zero voltage and zero current and the frequency as follows. The amplitudes l 0 and U 0 of the zero current and voltage are calculated using the known discrete Fourier transform and the frequency f r of the attenuating zero voltage using any frequency measuring algorithm. From these the earth capacitance C of the line can be calculated: C, = l 0 / ( 2*π*f r * U 0 ) Since the zero current l 0 and zero voltage U 0 continuously decrease and in order for the effect of the measurement errors to be minimized a plurality of C, estimates are calculated during the attenuating zero voltage. The av- erage of these is taken as the final earth capacitance value. The relays send the measured earth capacitance values C to the inductor controller. The controller adds them together and calculates the total zero inductance L using the obtained total earth capacitance C and the attenuating zero voltage frequency f r : L = 1 / ( C* (2*π*f r *) 2 ) The controller then calculates another zero inductance ΔL of the system using the total zero inductance L and the inductor inductance L k in its memory: ΔL = L - L k This method to determine ΔL is an alternative to the method de- scribed above where the inductor is directed to another position during the attenuating zero voltage, and the total zero inductances are measured in both inductor positions. Another way to utilize the field bus of the substation is that the relays inform the controller about the initiated auto-reclosing sequences and about the earth capacitances of the measured disconnected lines, in which case the inductor can be tuned for the dead time of the auto-reclosing and again at the same time as the line is reciosed to the network. Another possibility is that the inductor is not adjusted until after the final connection operation and not at all during the auto-reclosing sequence. Even then it is preferred if the bus informs about the auto-reclosings performed. If no bus is used, it is worth setting a sufficiently long delay for the in- ductor adjustment so that the auto-reclosing sequences manage to pass before the adjustment operation. If the natural zero voltage is small and the controller actuator is fast then adjusting the inductor during the attenuating zero voltage is the most preferable alternative. Then such accurate information on the inductor position is not needed but a frequency measurement is used to search for the resonance point. If the zero voltage of the continuous state is high in relation to the attenuating zero voltage, the zero voltage transient has to be stored until it at- tenuates so that the frequency can be reliably measured. If a field bus is used and the controller actuator is fast it is worth utilizing information transmitted by relays to tune the network as accurately as possible, if the velocity of the actuator allows it. If the actuator is slow the controller should be locked during auto-reclosings. In the method of the invention measurement of the zero voltage is needed in order to tune the inductor. If possible the current and voltage of the inductor should be measured since the accuracy can thus be improved. Measurement of one principal voltage is also needed if the network is to be monitored by measuring the degree of asymmetry.

Claims: CLAIMS 1. A method for adjusting a compensating inductor in a distribution network for electrical energy comprising an adjustable compensating inductor (K) between the star point of a transformer (M) connected to the network, such as a power transformer or a separate earthing transformer, and the earth, the inductance (L k ) of the compensating inductor (K) then forming a part of a total zero inductance (L) of the distribution network for electrical energy, characterized by the method comprising the steps of determining a total earth capacitance (C) of the network by a zero voltage transient associated with a connection change or fault situation of the network, when the total zero inductance (L) of the network is known, determining a value (L r ) of a total zero inductance of the network, the inductance forming a reactance whose absolute value is the same as the absolute value of the reactance formed by the total earth capacitance of the network at a nominal frequency (f n ), and adjusting the compensating inductor inductance (L k ) so that the total zero inductance of the network equals the value calculated above. 2. A method as claimed in claim 1, characterized by the step of determining a zero voltage transient frequency (f r ) associated with the connection change or fault situation of the network, by which frequency the total earth capacitance (C) is determined. 3. A method as claimed in claim ^characterized in that the adjustment of the compensating inductor inductance (L k ) comprises determining an initial value of the compensating inductor inductance based on meas- uring a voltage (U) and a current (I) of the inductor. 4. A method as claimed in claim ^characterized in that the adjustment of the compensating inductor inductance (L k ) comprises determining an initial value of the compensating inductor inductance based on measuring an adjustment position of the inductor, for example by potentiometer, and determining an inductance value corresponding to said adjustment position using a mathematical function or table describing the dependence between the inductance and adjustment position. 5. A method as claimed in claim ^characterized in that the adjustment of the compensating inductor inductance (L k ) comprises retrieving the prevailing value of said inductance from memory where it is stored, calculating a control sequence based on this initial value and a desired new value and feeding said control sequence to an actuator used to control the inductor adjustment position. 6. A method as claimed in any one of the preceding claims 1-5, characterized in that the adjustment of the compensating inductor in- ductance (L k ) comprises determining the total zero inductance (L) of the network by summing a rest of the zero inductance (ΔL) of the network to the compensating inductor inductance (L k ). 7. A method as claimed in claim 6, characterized in that the value of the rest of the zero inductance (ΔL) of the network is determined by retrieving it from memory where it is stored. 8. A method as claimed in claim 7, characterized in that the rest of the zero inductance (ΔL) of the network is determined based on resonance frequencies (f r1 , f r2 ) measured by two different inductance values (L k1 , L^) of the compensating inductor during the zero voltage transient and the obtained value is stored in memory. 9. A method as claimed in any one of the preceding claims 2-8, characterized in that the inductor is tuned by consecutive adjustment measures during the zero voltage transient by adjusting the transient frequency to equal the nominal frequency (f n ) of the network. 10. A method as claimed in any one of the preceding claims 2-8, characterized in that the method further comprises the steps of comparing an adjustment reference value (L k ) of the compensating inductor calculated for the inductance to be adjusted with the control range of the compensating inductor inductance and if said control range is exceeded by a predetermined amount the compensating inductor is disconnected from the network. 11. A method as claimed in any one of the preceding claims 2-8, characterized in that the zero voltage transient is stored in memory of the apparatus, the zero voltage of a continuous state is eliminated therefrom and the frequency of the zero voltage thus obtained is determined, whereby a more accurate estimate is obtained to the frequency of the attenuating zero voltage transient and the inductor can be tuned more accurately. 12. A method as claimed in any one of the preceding claims 1-7 or 9-11, characterized in that protective relays of the lines send utilizing a field bus the earth capacitance values of different lines measured during the zero voltage transient to the compensating inductor controller determining from said values the total earth capacitance (C) of the network and the rest of the zero inductance (ΔL) of the network. 13. A method as claimed in any one of the preceding claims 1-12, characterized in that the protective relays of the lines send information on the initiated auto-reclosing sequences to the compensating inductor controller, which tunes the network when the velocity of the controller actuator allows it for a dead time of the auto-reclosing and again when the line is re- closed to the network or awaits until the end of the auto-reclosing sequence before performing any adjustment measures of the inductor. 14. A method as claimed in any one of the preceding claims 1-12, characterized in that the compensating inductor controller also monitors the network by determining a degree of tuning (v) of the network, a degree of attenuation (d) of the network and a degree of asymmetry (k) of the network using the zero voltage transient and informs the values of these parameters and/or the changes exceeding the predetermined amount thereof to the operating person after each zero voltage transient.

Inventors: KUISTI HARRI, KUISTI, HARRI
Applicants: ABB TRANSMIT OY, KUISTI HARRI, KUISTI, HARRI
Application year: 1998
Main classification: H02H9
IPC codes: H02H9