WO2012152320A1

WO2012152320A1 - An arrangement and a method for determining a parameter of an alternating voltage grid

Info

Publication number: WO2012152320A1
Application number: PCT/EP2011/057549
Authority: WO
Inventors: Mats Molander; Johan SJÖBERG; Björn Thorvaldsson; Giovanni BECCUTTI
Original assignee: Abb Research Ltd
Priority date: 2011-05-10
Filing date: 2011-05-10
Publication date: 2012-11-15

Abstract

An arrangement configured to determine a parameter indicative of the strength of an alternating voltage grid having a thyristor controlled reactor (4) arranged at a location along a line (2) of the grid and connecting the line to ground, comprises means (21-24) configured to carry out measurements of currents and voltages of said grid including a measurement of the voltage (v_svc) at said location with respect to ground, and to carry out each such measurement together with the other measurements at a plurality of instants of time. The arrangement also comprises an apparatus (20) configured to calculate at least the inductance or the capacitance of the extension of said line between said alternating voltage source and said location by solving a system of equations created by an equation for each said measurement instant of time including said inductance and/or capacitance to be calculated, while utilizing the fact that irregularities are created in said voltage of said location by said switching of said thyristors.

Description

An arrangement and a method for determining a parameter of an alternating voltage grid

TECHN ICAL FI ELD OF THE I NVENTION AN D BACKGROUND ART

The present invention relates to an arrangement configured to determi ne a parameter indicative of the strength of an alternating voltage grid , said grid comprising

• an alternating voltage source,

· a line connecting to said alternating voltage source and configured to transmit electric power provided by said alternating voltage source,

• a thyristor controlled reactor arranged at a location along said line and connecting the line to ground , said thyristor con- trolled reactor comprising a current valve of at least two thy- ristors connected in anti-parallel and a reactor connected in series with the current valve,

• a unit configured to control said thyristors to switch to alternatively conduct for carrying out reactive power compen- sation of the electric power transmitted through said line, and

• a harmonic filter associated with said thyristor controlled reactor and connecting said line to ground by a series connection of at least one inductor and one capacitor for reducing harmonics generated in the electric power transmitted in said line by said switching of said thyristors, as well as a method for determining a parameter indicative of the strength of an alternating voltage grid according to the preamble of the appended independent method claim. Said alternating voltage grid may be of any conceivable type distributing/transmitting electric power from said alternating voltage source through said line to different consumers of electric power by means of converter stations, transformers and/or other equipment. The configuration of the grid may be much more complicated than that of the simplified model defined above. It may for instance include several alternating voltage sources, such as different generators of electric power, and several loads. Said line may carry an alternating voltage having any conceivable number, such as one, of phases, but this number will normally be three. Neither is the invention restricted to any level of the voltage to be carried by said line, although this voltage is mostly above 10 kV and often several hundreds kV.

A said thyristor controlled reactor is arranged for obtaining reactive power compensation of the electric power transmitted through said line by switching the thyristors of said current valve for connecting said line at said location to ground through said reactor when said current valve is conducting , i .e. one of the thyristors are turned on . This type of obtaining reactive power compensation is within this technical field called SVC classic (Static Var Compensation). Thus, the purpose of such SVC, i .e. said control of the thyristor controlled reactor, is to control the voltage of said line at said location with respect to ground . The knowledge of the strength of said alternating voltage grid is often very useful for being able to appropriately control said thy- ristor controlled reactor for obtaining an optimum reactive power compensation . Said grid strength may be expressed as a combination of the inductance, resistance and capacitance of the extension of said line between the alternating voltage source and said location , but it will in most cases be sufficient to determi ne the line inductance or the line capacitance for havi ng a parameter indicative of the strength of said grid . Especially in the case of a cable as said line or a part of the grid line the capacitance may be of interest. It has been realized that it is not possible to determine a parameter indicative of the strength of a said alternating voltage grid , such as the line inductance, in a steady state situation with purely sinusoidal signals by carrying out measurements of currents and voltages on said grid . One remedy would be to inject extra excitations, for instance by changing the reference for said unit controlling said thyristors. However, injection of such excitations would disturb the voltage on said line and it might be difficult to find a level and frequency of excitation that is acceptable both in terms of not disturbing the voltage too much and providing enough excitation for enabling a determination of a said parameter.

Furthermore, arrangements and methods for determining parameters of an alternating voltage grid of other types than defined in the introduction are known through US 7 346 462 B2 and US 201 1 /0043186 A1 .

SUMMARY OF THE I NVENTION

The object of the present invention is to provide an arrangement and a method for determining a parameter indicative of the strength of an alternating voltage grid being improved in at least some aspect with respect to an arrangement based on the method discussed above. This object is according to the invention , with respect to the arrangement, obtai ned by providing an arrangement according to the introduction , which is further characterized in that the arrangement comprises means configured to carry out measurements of currents and voltages of said grid including a meas- urement of the voltage of the line at said location with respect to ground , i .e. the voltage across said thyristor controlled reactor, and to carry out each such measurement together with the other measurements at a plurality of instants of time, and that the arrangement comprises an apparatus configured to calculate at least the inductance or the capacitance of the combination of the impedance parameters inductance, capacitance and resistance of the extension of said line between said alternating voltage source and said location by solving a system of equations created by an equation for each said measurement instant of time including said impedance parameter(-s) to be calculated while utilizing the fact that irregularities are created in said voltage of said location of the line with respect to ground by said switchi ng of said thyristors.

Thus, the present inventors have realized that the irregularities of the voltage of the line at said location introduced through said switchi ng of the thyristors of the thyristor controlled reactor may be used to set up an equation system that may be solved so as to determine a parameter indicative of the strength of said alternating voltage grid . Accordingly, no excitation disturbing the voltage on said line has to be injected for obtaining the information of the grid strength asked for.

Accordi ng to an embodiment of the invention said means are configured , when carrying out a measurement of said voltage at said location of the line with respect to ground , to measure this voltage at the moment of switching a said thyristor and directly thereafter for obtaining a measurement of a jump of this voltage created by said switching , and that said apparatus is configured to utilize the i nformation of said measurement of said voltage jump i n each equation of said equation system. A value of a said voltage jump will be useful in each equation of such an equation system for enabling a solving thereof for determining said line impedance parameter indicative of the strength of the grid . Accordi ng to another embodiment of the invention said means are configured to measure the voltage across the capacitor of said harmonic filter and the current through said harmonic filter at each said measurement instant of time, and said apparatus is configured to use information about this voltage and current in each equation of said equation system . There will be no problem to measure this voltage and current in the harmonic filter, and these parameters may advantageously be used i n a said equation system by the fact that they must be continuous , si nce the current through the fi lter inductance and the voltage across the filter capacitance have to be continuous.

Accordi ng to another embodi ment of the invention said apparatus is configured to solve a system of eq uations being for each said measurement instant of ti me the fol lowing :

^— L„L_tcrvc + LfL_tcrv_s + L_nL_tcrRfif

L^vr + LfV_L + L tdf

Lf i n which L_n is inductance of said l ine, L_tcr is the inductance of said reactor, L_f is the inductance of said inductor of the harmonic filter, R_f is the resistance of the harmonic filter, i_f is the current through the harmon ic filter, v_c is the voltage across the capacitor of the harmonic filter, Av_svc is said voltage jumps and v_s is the voltage of said alternati ng voltage source , wh ich may be expressed by V_scos(cot + Φ₀), i n which V_s is the ampl itude of this voltage and Φ₀ Β phase shift factor.

Such an equation will have three u nknowns , namely V_s, Φ₀ and „_. These three unknowns may be determined by carryi ng out measurements of said voltage jump, the voltage across said capacitor and the current through the harmon ic filter at three instants of time for creating an equation system of three such equations and solving this system . Accordi ng to another embodi ment of the invention said means are configured to measure said voltage of said line at said location with respect to g round and the cu rrent through said line at said location at at least three said instants of time for creating a system of equations with at least three eq uations, and said ap- paratus is config ured to use a parameter identification method , such as the method of a minimum root mean sq uare, for deter- mining said impedance parameter to be calculated . This constitutes an alternative to the use of explicit expressions for the disconti nuities (said voltage jumps) as in the previous embodiment of the invention and relies just on the fact that the discon- tinuities exist, so that measurements of said voltage of the line at said location with the respect to ground and the currents through said line at said location will be informative enough . It will then be necessary to have a sampling frequency being rather high , such as at least 5 kHz in the case of a frequency of said alternating voltage of 50 or 60 Hz, for reliably "catching" the voltage jumps and by that enabling said calculation .

Accordi ng to a further development of the embodiment of the invention last mentioned said apparatus is configured to create a system of equations according to the following equation : v — v — L r — ^d /^■

at in which v₅ = V_s cos(m +§₀ )

and in which L_n is the inductance of said line, i_n is said current through said line, v_svc is the voltage of said line at said location with respect to ground , v_s is the voltage of said alternating voltage source, which may be expressed by V_scos(cot + Φ₀), in which V_s is the amplitude of this voltage and Φ₀ a phase shift factor.

Such an equation system with three unknowns, the line inductance L_n> the voltage amplitude V_s and Φ_0, would not be possible to solve wouldn't there exist any discontinuities, but may here be solved by creating a system of at least three such equations thanks to the presence of this discontinuities (voltage jumps).

Accordi ng to another embodiment of the invention the arrangement is configured to determine a parameter i ndicative of the strength of an alternating voltage grid configured to transmit electric power with a voltage amplitude of at least 10 kV or at least 100 kV, such as 100 kV - 1 200 kV.

The object of the present invention is with respect to the method obtained by providing a method according to the appended independent method claim. The preferred features and advantages of this method and the methods according to embodiments thereof defined in the appended dependent method claims appear clearly from the above discussion of the arrangement ac- cording to the present invention and embodiments thereof.

The invention also relates to an alternating voltage grid comprising an arrangement according to the invention as well as a computer program product associated with a method according to the present invention .

Further advantages as well as advantageous features of the invention will appear from the following description . BRI EF DESCRI PTI ON OF THE DRAWI NG

With reference to the appended drawing , below follows a specific description of embodiments of the invention cited as examples.

In the drawi ng :

Fig 1 is a very schematic view of an alternating voltage grid provided with an arrangement according to the present invention , and

Fig 2 is a graph of the voltage at a location of a thyristor controlled reactor along said line with respect to ground and the current through the thyristor controlled reactor, respectively, versus time. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig 1 illustrates a simplified model of an alternating voltage grid provided with a SVC (Static Var Compensator) in the form of a thyristor controlled reactor. The alternating voltage grid comprises an alternating voltage source 1 , which may be any type of generator of electric power combined with one or more converters and/or transformers. A line 2 connects to the alternating voltage source for transmission of electric power provided by this source. It is illustrated how the extension of the line from the alternating voltage source to a location 3 of a thyristor controlled reactor 4 has a line inductance 5. Thus, it is in this simplified case assumed that the line inductance is the impedance parameter being most important for the grid strength . However, it may also be of interest to determine the capacitance and/or resistance of said line.

A thyristor controlled reactor 4 is arranged at said location 3 along the line 2 and connects the line to ground . The thyristor controlled reactor comprises a current valve 6 of at least two thyristors 7, 8 connected in anti-parallel and a reactor 9 with an inductance L_tcr connected in series with the current valve. A unit 10 is configured to control the thyristors 7, 8 to switch to alternatively conduct or carrying out reactive power compensation of the electric power transmitted through the line 2.

Furthermore, an harmonic filter 1 1 is associated with the thyristor controlled reactor and connects the line 2 to ground by a series connection of at least one inductor 12 with the inductance L_f, one capacitor 13 with the capacitance C_f and one resistor 14 with the resistance R_f. The harmonic filter is arranged to reduce harmonics generated in the electric power transmitted in said line by switching of the thyristors 7, 8. The harmonic filter may have several branches like the one illustrated for removing harmonics of different frequencies. It is pointed out that Fig 1 illustrates the grid with respect to one phase, and that the grid may have three phases with a line conductor, a thyristor controlled reactor and an harmonic filter for each such phase.

Fig 2 illustrates what happens with the voltage v_svc at said location 3 with respect to ground when switching the thyristors 7, 8 for reactive power compensation . It is illustrated how the current i_tcr through the current valve 6 is directed from ground towards the line 2 when the thyristor 7 is conducting (A) and from the line towards ground when the thyristor 8 is conducting (B). The current is 0 when none of the thyristors are turned on (C). It is illustrated how the switching of the thyristors creates a jump J J₂ of the voltage v_svc at said location 3.

The alternating voltage grid shown in Fig 1 is provided with an arrangement according to the present invention configured to determine a parameter indicative of the strength of the alternating voltage grid . This arrangement comprises means configured to carry out measurements of currents and voltages of said grid includi ng a measurement of the voltage v_svc of the line at said location 3 with respect to ground and to carry out each such measurement together with the other measurements at a plurality of instants of time. Furthermore, the arrangement comprises an apparatus 20 configured to calculate the line inductance L_n by solving a system of equations created by an equation for each said measurement instant of time including said line inductance while utilising the fact that irregularities in the form of voltage jumps are created in the voltage v_svc by the switching of the thyristors.

Fig 1 illustrates a combination of said measurement means for two different embodiments of an arrangement according to the present invention for determining said line inductance according to two different methods according to the present invention . The first method is based on direct use of said voltage jumps of the voltage v_svc at said location 3. To get expressions for how the discontinuity in the thyristor current influence this voltage v_svc we can use the fact that the current i_f through the filter in- ductance and the voltage v_c over the filter capacitance must be continuous. When one of the thyristors 7, 8 is conducting , we have the relations r ^{d ■}

Vs Vsvc — ⁿ rf£^ⁿ

through which the time derivatives of the currents can be eliminated yielding

With the thyristor not conducting we have

yielding

The d iscontinuity in v_svc at a thyristor switch thus is

^— L„L_tcrvc + LfL_tcrv_s + L_nL_tcrRfif

L^vr + LfV_L + L tdf

Lf

The arrangement comprises means 21 configured to measure said voltage v_svc at the moment of switching a said thyristor and directly thereafter for obtai ning a measurement of a ju mp of this voltage Av_svc. Means 22 , 23 are also arranged for measuring the voltage v_c across the capacitor of the harmonic filter and the current /> through the harmonic filter, respectively.

Furthermore, we may write v_s as V_s cos(⁽S)t+§₀), i n which V_s is the ampl itude of the alternati ng voltage of the alternating voltage source and φ₀ is a phase shift factor. This means that we have for each measurement i nstant of time th ree un knowns, i .e. L_n, V_s and φ₀. By carryi ng out the measurements through the measuri ng means 21 , 22 and 23 at th ree d ifferent i nstants of time and send this i nformation to said apparatus 20 a system of equations for Av_svc may be created and solved for delivering a value of the line inductance L„.

An arrangement and a method accord ing to another embod iment of the invention is based upon estimation usi ng grid measurements only. Th is method is based on measurements of the current in said li ne i_n by a means 24 and of the voltage v_svc by the means 21 and the differential equation d

If we assu me that Vs = Vs cos( + φο )

We have three unknowns in the eq uation V_s cos( }t + φ₀) _ _v namely L_n, V_s and φ₀ (we know ω = 2 nf with f the frequency of said alternating voltage). Just relying on the fact that the discontinuities in the form of the voltage jumps exist, the measurements of v_Svc and i_n will be informative enough for calculating these unknowns. As already said a suitable sampling frequency has to be selected for reliably "catching" the voltage jumps.

The arrangement according to this embodiment of the invention is configured to measure v_svc and i_n at at least three instants of time for creating a system of equations according to (2). The apparatus 20 is configured to solve this equation system and determine L_n, V_s and φ₀ by use of a method for parameter identification , in which a minimum at correctly estimated parameters is sought, such as the method of a minimum root mean square, RLS, LS or Kalman filter. This is possible thanks to the irregularities of v_svc introduced by the switchings of the thyristors.

The i nvention is of course not in any way restricted to the embodiments thereof described above, but many possibilities to modifications thereof would be apparent to a person with ordinary skill in the art without departing from the scope of the in- vention as defined in the appended claims.

It may for i nstance be desired to determine the resistance (R„) of said line, in addition to a determination of the line inductance and/or line capacitance. I n such a case the equation to be used may be d i .

dt " + R n This equation has four unknowns, L_n, V_s, Φ_α and R_n, so that at least four such equations are required for determining these unknowns.

Claims

CLAI MS

1 . An arrangement configured to determine a parameter indica- tive of the strength of an alternating voltage grid , said grid comprising

• an alternating voltage source ( 1 ),

• a line (2) connecting to said alternating voltage source and configured to transmit electric power provided by said alter- nating voltage source,

• a thyristor controlled reactor (4) arranged at a location (3) along said line and connecting the line to ground , said thyristor controlled reactor comprising a current valve (6) of at least two thyristors (7, 8) connected in anti-parallel and a re- actor (9) connected in series with the current valve,

• a unit (10) configured to control said thyristors to switch to alternatively conduct for carrying out reactive power compensation of the electric power transmitted through said line (2), and

· a harmonic filter (1 1 ) associated with said thyristor controlled reactor and connecting said line to ground by a series connection of at least one inductor (1 2) and one capacitor (1 3) for reducing harmonics generated in the electric power transmitted in said line by said switching of said thyristors, characterized in that the arrangement comprises means (21 -24) configured to carry out measurements of currents and voltages of said grid including a measurement of the voltage (v_svc) of the line at said location with respect to ground , i .e. the voltage across said thyristor controlled reactor (4), and to carry out each such measurement together with the other measurements at a plurality of instants of time, and that the arrangement comprises an apparatus (20) configured to calculate at least the inductance (L_n ) or the capacitance (C„) of the combination of the impedance parameters inductance, capacitance and resistance of the extension of said line between said alternating voltage source and said location by solving a system of equations created by an equation for each said measurement instant of time incl uding said impedance parameter(-s) to be calculated while utilizing the fact that irregularities are created in said voltage of said location of the line with respect to ground by said switching of said thy- ristors.

2. An arrangement according to claim 1 , characterized i n that said means (21 ) are configured , when carrying out a measurement of said voltage at said location of the line with respect to ground , to measure this voltage (v_svc) at the moment of switching a said thyristor (7, 8) and directly thereafter for obtaining a measurement of a jump of this voltage created by said switching , and that said apparatus (20) is configured to utilize the information of said measurement of said voltage jump in each equation of said equation system.

3. An arrangement according to claim 1 or 2 , characterized in that said means (22, 23) are configured to measure the voltage across the capacitor (13) of said harmonic filter and the current through said harmonic filter at each said measurement instant of time, and that said apparatus (20) is configured to use information about this voltage and current in each equation of said equation system.

4. An arrangement according to claims 2 and 3, characterized in that said apparatus (20) is configured to solve a system of equations bei ng for each said measurement instant of time the following :

^— L„L_tcrvc + LfL_tcrv_s + L_nL_tcrRfif

L^vr + LfV_L + L tdf

Lf in which L_n is the i nductance of said line, L_tcr is the inductance of said reactor (9), L_f is the inductance of said inductor (1 2) of the harmonic filter, R_f is the resistance (14) of the harmonic fil- ter, i_f is the current through the harmonic filter, v_c is the voltage across the capacitor (13) of the harmonic filter, Av_svc is said voltage jumps and v_s is the voltage of said alternating voltage source, which may be expressed by V_scos(cot + Φ₀), in which V_s is the amplitude of this voltage and Φ₀ a phase shift factor.

5. An arrangement according to claim 4 , characterized i n that said means (21 -23) are configured to carry out measurements of said voltage jumps, the voltage across said capacitor and said current through the harmonic filter at at least three said instants of time for creating an equation system of at least three said equations for determining said line inductance (L_n) .

6. An arrangement according to claim 1 , characterized i n that said means (21 , 24) are configured to measure said voltage of said line (2) at said location (3) with respect to ground and the current through said line at said location at at least three said instants of time for creating a system of equations with at least three equations, and that said apparatus (20) is configured to use a parameter identification method , such as the method of a minimum root mean square, for determining said impedance parameters) to be calculated .

7. An arrangement according to claim 6, characterized i n that said apparatus (20) is configured to create a system of equations according to the following equation : v —v — L — ι

at in which v₅ = V_s cos(m +§₀ )

8. An arrangement according to any of the preceding claims, characterized in that it is configured to determine a parameter indicative of the strength of an alternating voltage grid config- ured to transmit electric power with a voltage amplitude of at least 10 kV or at least 100 kV, such as 100 kV - 1 200 kV.

9. A method for determining a parameter indicative of the strength of an alternating voltage grid , said grid comprising · an alternating voltage source (1 ),

• a line (2) connecting to said alternating voltage source and configured to transmit electric power provided by said alternating voltage source,

• a thyristor controlled reactor (4) arranged at a location (3) along said line and connecting the line to ground , said thyristor controlled reactor comprising a current valve (6) of at least two thyristors (7, 8) connected in anti-parallel and a reactor (9) connected in series with the current valve,

• a harmonic filter (1 1 ) associated with said thyristor controlled reactor and connecting said line to ground by a series con- nection of at least one inductor (1 2) and one capacitor (13) for reducing harmonics generated in the electric power transmitted in said line by said switching of said thyristors, characterized by the steps of

• measuri ng currents and voltages of said grid including the voltage (v_svc) of the line at said location with respect to ground , i .e. the voltage across said thyristor controlled reactor (4), in which each such measurement is carried out together with other such measurements at a plurality of instants of time, and

· calculating at least the inductance (L_n ) or the capacitance (C„) of the combination of the impedance parameters inductance, capacitance and resistance of the extension of said line between said alternati ng voltage source and said location by solving a system of equations created by an equation for each said measurement instant of time including said impedance parameter(-s) while utilizing the fact that irregularities are created i n said voltage of said location of the line with respect to ground by said switching of said thyris- tors.

10. A method according to claim 9, characterized i n that said step of measuring comprises, when carrying out a measurement of said voltage of said location (3) of the line (2) with respect to ground , measuring of this voltage (v_svc) at the moment of switchi ng a said thyristor (7, 8) and directly thereafter for ob- taining a measurement of a jump of this voltage created by said switchi ng , and that i n said calculating step the information of said measurement of said voltage jump is utilized in each equation of said equation system.

1 1 . A method according to claim 9 or 10, characterized in that said measuring step comprises measuring of the voltage across the capacitor (1 3) of said harmonic filter and the current through said harmonic filter at each said measurement instant of time, and that said calculating step comprises use of information about this voltage and current in each equation of said equation system.

12. A method according to claims 1 0 and 1 1 , characterized i n that said calculating step comprises solving of a system of equations being for each said measurement instant of time the following :

Av_svc = L_nL_tcrvc + LfL_tcrv_s + L_nL_tcrRfif

„ + L_f in which L_n is the inductance of said line, L_tcr is the inductance of said reactor (9), L_f is the inductance of said inductor (1 2) of the harmonic filter, R_f is the resistance (14) of the harmonic filter, i_f is the current through the harmonic filter, v_c is the voltage across the capacitor of the harmonic filter, Av_svc is said voltage jumps and v_s is the voltage of said alternating voltage source, which may be expressed by V_scos(cot + Φ₀), in which V_s is the amplitude of this voltage and Φ₀ a phase shift factor.

13. A method according to claim 12 , characterized in that said measuri ng step comprises measuring of said voltage jumps, the voltage across said capacitor and said current through the harmonic filter at at least three said instants of time, and that said calculating step comprises creating of an equation system of at least three said equations for determining said line inductance {L_n).

14. A method according to claim 9, characterized i n that said measuri ng step comprises measuring of said voltage of said line (2) at said location (3) with respect to ground and the current through said line at said location at at least three said instants of time, and that said calculating step comprises creating of a system of equations with at least three equations and the use of a parameter identification method , such as the method of a minimum root mean square, for determining said impedance parameters) to be calculated from said system of equations.

15. A method according to claim 14 , characterized in that said calculating step comprises creation of a system of equations ac- cording to the following equation : v — v — L r — ^d /^■

at in which v₅ = V_s cos(m +§₀ ) and in which L_n is the inductance of said line, i_n is said current through said line, v_svc is the voltage of said line at said location with respect to ground , v_s is the voltage of said alternating voltage source, which may be expressed by V_scos(cot + Φ₀), in which V_s is the amplitude of this voltage and Φ₀ a phase shift factor.

16. An alternating voltage grid comprising an arrangement according to any of claims 1 -8 for determining a parameter indicative of the strength of said grid .

1 7. A computer program product storable on a computer usable medium containing instructions for a processor to control execution of the steps of the method according to any of claims 9- 15.

18. A computer program product according to claim 1 7 provided at least partially through a network, such as the I nternet.