AU702779B2

AU702779B2 - Method and apparatus for controlling n converter stations in a multipoint HVDCT network

Info

Publication number: AU702779B2
Application number: AU17657/97A
Authority: AU
Inventors: Franz Karlecik-Maier
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1995-11-30
Filing date: 1996-11-18
Publication date: 1999-03-04
Anticipated expiration: 2016-11-18
Also published as: WO1997020373A2; NO982451D0; NO982451L; DE19544777C1; CA2238970A1; AU1765797A; EP0864192A2; WO1997020373A3

Description

P 3842 1 Description Method and apparatus for controlling n converter stations in a multipoint HVDCT network The invention relates to a method and apparatus for controlling n converter stations in a multipoint HVDCT network.

A control concept for multiterminal high-voltage DC transmission (Conference Proceedings of the East West Energy Bridge International Conference, Warsaw, 24-25.10.1995) is known which comprises a higher-level master regulator and the station's own control functions.

The multiterminal system comprises a total of five bipolar converter stations which are connected by means of two parallel DC overhead lines per pole. The main object of the higher-level control is to coordinate the power and current setpoints for the steady-state operating point. The system is intended to remain stable in the event of failure of one converter station, even if communication between the master regulator and the converter stations is temporarily interrupted. This higher-level control adds up the power setpoints. Should this total not be equal to zero, then the error is split between the various converter stations on the basis of weighting coefficients. The coefficients are freely 25 variable, but their total must be equal to unity. The current setpoints for the individual converter stations are determined from the power setpoints which are determined in this way, in that the power setpoints are divided by the actual DC voltage value of the respective station. The power losses cannot be calculated accurately in advance, as a rule, and have therefore not been taken into account in the determination of the power setpoints, the current setpoints do not produce a total of zero as a rule after division. In a similar manner to the determination of the power setpoints, the current S setpoints are 7*

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P 3842 2 therefore adjusted via a control loop, so that the total current of all the rectifiers and invertors is zero. The weighting factors are set such that the total is unity.

The station control system, as is exists in each converter station, comprises two current control paths and two voltage control paths as well as a minimum current regulator. The instantaneously active control difference is determined via a combination of minimum and maximum failure blocks. In the case of this control concept, only one converter station may determine the voltage, that is to say only one rectifier or one invertor operates in this operating mode. Which converter is best suited for this purpose depends on the specific system configuration. In the case of those invertors which have a current-regulated operating point, two characteristic alternatives are available. On the one hand, in the case of system disturbances with a reduced DC voltage, it is possible to operate with constant voltage regulation at the invertor. The second alternative operates with a current regulator. In this alternative, the current setpoint is reduced in the region of the reduced DC voltage via a VDCOL function :(Voltage Dependent Current Order Limit) The more favourable of the two options must be determined by simulation calculation for a specific system S"configuration. In the case of the proposed multiterminal system, the function which governs the voltage is ensured by a converter station which is operated as a rectifier.

All the other stations are operated with current regulation at the steady-state operating point. The two invertor stations operate with current regulation, using a VDCOL function, in the area of reduced DC voltage.

S~i. n the case of this known control concept for a multiterminal HVDCT, comprising five converter stations, it is not possible to predict how the control of an n-th converter station will appear when n converter stations are intended to operate with one another in P 3842 3 a DC system. In addition, the control concept is of highly complex design and in each case requires telecommunication between the higher-level master regulator and a control system at the station end. Setpoints and actual values are interchanged via this costly telecommunication. Furthermore, the station's own control systems each have a plurality of control modes, an appropriate control mode being selected by means of regulator cut-out.

The control mode cut-out, the higher-level control system and the telecommunication makes the dynamic behaviour of the overall DC system of the multiterminal HVDCT worse, and can also adversely affect the stability of the three-phase systems connected to it.

Coordinated vector control for a high-voltage DC transmission system is known from DE 44 20 600 Cl. In the case of this coordinated vector control, a setpoint pair is generated for the current and voltage for the converter station which is operated in the "rectified" operating mode, as a function of a power to be transmitted and of a measured actual DC voltage value, and this setpoint pair are compared with a determined actual value pair for the current and voltage. The control errors produced are added up. A signal is generated from this sum signal in such a manner that the total control error tends to zero. A setpoint pair is generated for the current and voltage for the converter station which is operated in the "invertor" operating mode, as a function of the power to be transmitted and a turn-off angle setpoint, and is compared with an actual 30 value pair which is determined. The control errors are subtracted from one another. A control signal is generated from this difference signal in such a manner that the difference between the control errors tends to zero. This coordinated vector control method has setpoint pairs for current and voltage which take account of both the aims of the invertor and the aims of the rectifier.

P 3842 4- The setpoint pair for the invertor is thus determined such that it controls the turn-off angle and at the same time includes the power which is made available by the rectifier. The invertor control characteristic which is produced in this case corresponds to the characteristic of a resistance regulator having a positive gradient. The control characteristic for the rectifier using the vector control method is the tangent to the associated power hyperbola of the nominal power at the weighting point for a setpoint pair. This results in vector control in principle tolerating voltage changes at the invertor, provided the power changes caused lie on the tangent. This design of the two characteristics produces a stable operating point.

The invention is now based on the object of specifying a method and an apparatus for controlling n converter stations in a multipoint HVDCT network.

This object is achieved according to the invention by the features in Claims 1 and 6.

As a result of the fact that each converter station in a multipoint HVDCT network is provided with coordinated vector control, the regulator arrangement for the invertor having added to it a device for determining an additional turn-off angle setpoint, stable operating points can be set in a decentralized manner in each station, despite changes in ehe DC system and/or in the associated three-phase networks. A power control difference which is determined is used for determining the additional turn-off angle setpoint. A constant load 30 flow can be maintained in the DC system by means of this combination of known coordinated vector control and the additional determination of the turn-off angle setpoint as a function of a power control difference. Since this method operates in a decentralized manner, there is no longer any need for a higher-level master regulator or telecommunication, P 3842 5 as a result of which the complexity of this control concept is simplified, and the dynamic behaviour improved, in comparison with the control concept mentioned initially.

Advantageous refinements of the method can be found in the subclaims 2 to 5, and advantageous refinements of the apparatus for carrying out the method according to the invention can be found in the subclaims 7 to In order to explain the invention further, reference will be made to the drawing, which illustrates schematically an advantageous embodiment of an apparatus for carrying out the method according to the invention, and in which: Figure 1 shows a multipoint HVDCT network with n converter stations, Figure 2 shows a diagram of the operating characteristics of a multipoint HVDCT network with three rectifier and invertor stations, Figure 3 shows a block diagram for known coordinated vector control for a lossy DC cable, Figure 4 shows an associated diagram of the operating characteristics, Figure 5 shows a block diagram of an apparatus for carrying out the method according to the invention for one station in the multipoint HVDCT network, and Figure 6 illustrates an associated diagram of the operating characteristics.

Figure 1 shows a multipoint high-voltage DC transmission network 2 with n converter stations 4, r of which are operated as rectifiers, and i as invertors.

Each converter station 4 is provided with its own station control system 6. In addition, each converter station 4 is electrically conductively connected to a three-phase network 12 via a converter transformer 8 with associated P 3842 6 stepping switch control 10. The multipoint HVDCT network 2, also called a general DC system, has any required topology, that is to say the n converter stations 4 are connected to one another as required. The normal voltage operating range of this multipoint network 2 moves between 0.8 and 1.2 pu. The overall rectifier power is intended to be 1 pu, and the overall invertor power is then 1 pu minus losses. Figure 2 illustrates a diagram for the operating characteristics of a multipoint HVDCT network 2, the operating characteristics being illustrated, for clarity, only for six converter stations 4, three of which are operated as rectifiers and three as invertors. In the case of this general DC system 2, the operating points AW1, AW2 and AW3 can be set on all the converter stations 4 in the "invertor" operating mode, these operating points being predetermined by the transformer settings and the coordinated vector control 14 (without any additional device) This means that all these converter stations 4 move [sic] the turn-off angle setpoint yo of, for example, 170 electrical at a predetermined power setpoint Po. The operating points AG1, AG2 and AG3 of the converter sttio~s 4 in the "rectifier" operating mode result from the topology of the DC system 2 (Kirschhoff' s Law, mesh equation and energy conservation law) automatically, which in each case correspond to a predetermined power setpoint Por. Figure 2 in each case uses solid lines to illustrate the power hyperbola of the converter stations 4 which are operated as rectifiers, and dashed lines in each case for the 30 converter station 4 which is operated as an invertor. The resistance characteristics of the converter stations 4 which are operated as invertors are illustrated as straight lines, whose intersections with the associated power hyperbola produce the operating points AW1, AW2 and AW3.

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P 3842 7 Figure 3 shows a block diagram for known coordinated vector control 14 for a lossy DC cable 16 in a high-voltage DC transmission system 18, with whose aid two AC networks 20 and 22 are connected to one another.

This HVDCT system 18 comprises two converter stations 4, which are operated as rectifiers and invertors. These two converter stations 4 are connected to one another on the DC side by means of the DC cable 16.

The HVDCT system 18 furthermore comprises measurement sensors, which are not shown in more detail, for detecting current and voltage values Idr, Idi and Udr, Udi, respectively. A control device 24 for driving the active devices or semiconductors in the converter stations 4 is connected upstream of each converter station 4.

Each control device 24 receives a control signal which is produced by a first or second control arrangement (26 or 28 respectively). The first control arrangement 26 essentially comprises a first setpoint generator 30 and a first vector regulator arrangement 32.

This setpoint generator 30 receives as the input signal a power setpoint Por of a predetermined power to be transmitted, and an actual DC voltage value Udr. A setpoint pair lor and Uor are determined for the current and voltage of the converter station 4 from these values SPor and Udr, by means of the setpoint generator 30. The setpoint generator 30 has two characteristic generators 34 and 36. The curve of the first characteristic generator 34 selected for the voltage setpoint Uor exhibits 30 the VDVOC characteristic (Voltage Dependeti Voltage Order Characteristic), a curved response being provided as the characteristic feature at the upper end for the area of steady-state operation. The lower area of the characteristic is designed to be voltage limiting. The characteristic of the second characteristic generator 36 for the current setpoint Ior essentially exhibits a VDCOL characteristic (Voltage Dependent Current Order Limitation), that is to say voltage-dependent P 3842 8 current limiting. The vector regulator arrangement 32 has two comparators 38 and 40, an adder 42 and a control element 44. The setpoint pair Uor, lor which is formed is supplied to this vector regulator arrangement 32 and is compared there by means of the two comparators 38 and with an actual value pair Udr, Idr, which has been determined. The control errors formed for the current and voltage are added up by means of the adder 42. This sum signal is supplied to the control element 44 at whose output the control signal for the control device 24 of the converter station 4, which is operated as a rectifier, is present. The sum of the control errors for the current and voltage is regulated at zero by means of this control signal.

The second control arrangement 28 is analogous to that [sic] of the control arrangement ,26. Further description of the second control arrangement 28 is thus superfluous. There are differences in the number of values supplied to the setpoint generator 46, the characteristics of the two characteristic generators 48 and 50, and a device 52 for determining a power setpoint Poi. As a result of the variance in the input parameters (actual voltage value Udi, actual power value Pdi, power setpoint Poi, turn-off angle setpoint yo, actual turn-off angle value 7, control signal 8) the characteristic, in particular the VDVOC characteristic of the characteristic generator 48 must be capable of being predetermined in terms of its magnitude in the end area and in terms of its gradient. The VDCOL characteristic of the 30 characteristic generator 50 is also adjustable. The essential feature for the second setpoint generator 46 is that a turn-off angle setpoint yo is also predetermined, and must be complied with. The setpoint pair Uoi, loi which is produced is compared by means of two comparators 38 and 40 with an actual value pair Udi, Idi which has been determined. The control errors formed are subtracted from one another by means of the adder 42, since the voltage setpoint Uoi of the setpoint pair Uoi, loi is present at the inverting input of the comparator 38. The *r 4 4C t

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P 3842 8a difference signal is supplied to the downstream control element 44, at whose output the control signal for the control device 24 of the converter station 4, which is operated as an invertor,

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P 3842 9 is present. The difference in the control error for the current and voltage is regulated at zero by means of this control signal.

The device 52 for determining a power setpoint Poi has a first order delay element 54 with an upper and a lower limit. An actual power value Pdi which is determined and an upper and a lower power limit Pgoi and Pgui are supplied to this device 52. The upper power limit Pgoi is equal to the difference between the power setpoint Por of a power which is to be transmitted from the converter station 4 which is operated as a rectifier and a minimum power loss Pvmin, while in contrast the lower power limit Pgui is equal to the difference between the power setpoint Por and a maximum power loss Pvmax.

Figure 4 illustrates a diagram of the operating characteristics GR and WR for coordinated vector control 14, which is shown in Figure 3, of an HVDCT system 18.

The characteristic GR, which is composed of the sections hl, lm, mn and no, illustrates the rectified characteristic, the section hl illustrating the power hyperbola in the normal operating area, the section lm illustrating maximum current limiting, the section mn illustrating the area of voltage-dependent limiting, and the section no illustrating the minimum current. The characteristic WR illustrates the invertor characteristic. Since the DC cable 16 is lossy, the power hyperbola for the converter station 4 which is operated as an invertor is not consistent with the power hyperbola of the converter station 4 which is operated as a 30 rectifier. Since each point on the characteristic WR is determined by means of a current and voltage setpoint, the invertor characteristic is also resistance control, which represents combined current/voltage control. The :dashed-dotted line corresponds to a turn-off angle setpoint yo.

K P 3842 10 Figure 5 shows a block diagram with an apparatus for carrying out the method according to the invention for controlling n converter stations 4 in a multipoint HVDCT network 2. For reasons of clarity, only one coordinated vector control 14 according to t-le invention is illustrated for one converter station 4 in the multipoint HVDCT network 2. Since the converter system 4 can be operated as a rectifier or as an invertor, the station control system 6 contains a first control arrangement 26 and a second control arrangement 28. Since the control element 44 is present in both control arrangements 26 and 28, it is possible to dispense with one control element 44 in this station control system 6.

To this end, a switch 56 and 58 is in each case connected downstream of the outputs of the adders 42 of both control arrangements 26 and 28, and the outputs of these switches 56 and 58 are linked to the control element 44 by means of an adder The control arrangement 28 has a device 62 added to it for determining an additional turn-off angle setpoint yoadd. This device 62 has a dead band element 64 .ee.

on the input side, and a PI regulator 66 on the output side. Since an additional turn-off angle setpoint yoadd which is determined is intended to be varied only for the normal operating values of the DC voltage of the multipoint network 2, a switch 68 is arranged between the dead band element 64 and the PI regulator 66. This switch 68 is closed as long as the actual DC voltage Udi is greater than a predetermined limit. During a fault which is linked to severe voltage notches, the additional turnoff angle setpoint yoadd can remain unchanged (switch 68 is opened), or can be set to zero. This was done by passing a zero signal SV to the PI regulator 66. The dead band element 64 has a positive power control difference threshold dPo and a negative power control difference threshold dPu. Between these two power control difference thresholds dPo and dPu, the output value of the dead band element 64 remains zero irrespective of the input signal dPo.

P 3842 11 As soon as the value of the input signal dP, namely a power control difference dP which is determined, becomes greater or less than the positive or negative power control difference threshold dPo or dPu, the output value of the dead band element 64 is not zero. This output value is supplied to the PI regulator 66, at whose output an additional turn-off angle setpoint yoadd is present. In order that the turn-off angle yo for the setpoint generator 46 can be varied only within a predetermined range, the PI regulator 66 is provided with a lower limit of zero and an upper limit of maxyoadd. The turn-off angle setpoint yo is composed of a minimum turnoff angle setpoint yomin and the additional turn-off angle setpoint yoadd which is determined, an adder being provided. The limits like dPu and dPo of the dead band of the dead band element 64 are each determined by means of a comparator 72 and 74, a lower power limit Pgui being present at the inverting input of the comparator 72, and a power setpoint Poi being present at the noninverting input. An upper power limit Pgoi is present at the non-inverting input of the comparator 74, and a power setpoint Poi is present at the inverting input. The power control difference threshold dP is determined by means of a further comparator 76, a power setpoint Poi being 25 present at its non-inverting input, and an actual power value Pdi being present at its inverting input.

Figure 6 illustrates a diagram of the operating characteristics GR and WR of the control concept proposed in Figure 5. In comparison with the diagram according to 30 Figure 4, a new operating point NP has been determined, which is located on the same power hyperbola of the invertor characteristic. This new operating point NP is set in a decentralized manner at a converter station 4, since the voltage in the DC voltage system 2 has been reduced. The load flow remains unchanged irrespective of this voltage change.

V P 3842 12 This control concept according to the invention for multiterminal high-voltage DC transmission has a simple structure, has the same structure for all the converter stations 4, is a decentralized control concept as a result of which there is no need for costly telecommunication, and has an improved dynamic behaviour, since there is no regulator cut-out and no higher-level master regulator. In addition, this concept can contribute better to stabilization of the DC voltage system 2 and the three-phase systems 12.

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Claims

1. Method for controlling n converter stations (4) in a multipoint HVDCT network a setpoint pair (Uor, Ior) being generated for the current and voltage of the converter station for each converter station which is operated in the "rectified" operating mode, as a function of a power (Por) which is in each case to be transmitted, and of a measured actual DC voltage value (Udr) control errors for the current and voltage being determined as a function of a determined actual value pair (Udr, Idr) and of this setpoint pair (Uor, Tor) produced, these control errors being added up and a control signal being generated, as a result of which this total turns to zero, a setpoint pair (Uoi, Ioi) being generated for the current and voltage of the converter station for each converter station which is operated in the "invertor" operating mode, as a function of a power and turn-off angle setpoint (Poi, yo) which is in each case formed, control errors for the current and voltage being determined as a function of a determined actual value pair (Udi, Idi) and of this setpoint pair (Uoi, loi) produced, these control errors are subtracted from one another and a control signal is generated, as a result of which this difference turns to zero, 30 the power setpoint (Poi) for a converter station (4) which is operated in the "invertor" operating mode being determined from its determined actual power value (Pdi) and an upper and a lower power limit ,.•ooo (Pgoi, Pgui) which are formed, 0 the turn-off angle setpoint (yo) for a converter station which is operated in the "invertor" operating mode being determined as the sum of a minimum turn-off angle setpoint (yomin) P 3842 14 and of an additional turn-off angle setpoint (yoadd) which is produced and this additional turn-off angle setpoint (yoadd) being proportional to a power control difference (dP) which is determined from the power setpoint and actual value (Poi, Pdi), as soon as a negative or positive power control difference threshold (dPu, dPo) is respectively undershot or exceeded.

2. Method according to Claim 1, the additional turn- off angle setpoint (yoadd) being variable only during normal operation.

3. Method according to Claim 1, the additional turn- off angle setpoint (yoadd) being set to zero when not in normal operation.

4. Method according to one of the abovementioned Claims 1 to 3, the negative and positive power control difference threshold (dPu, dPo) being variable. Method according to Claim 4, the negative and positive power control difference threshold (dPu, dPo) being variable as a function of a power setpoint (Por) of a power to be transmitted and of a maximum and minimum power loss (Pvmax, Pvmin).

6. Apparatus for carrying out the method for controlling n converter stations in a multipoint 25 HVDCT network according to Claim 1, a control arrangement which has a setpoint generator (30) with a downstream vector regulator arrangement (32) being provided for each converter station which is operated in the "rectified" operating mode, S an actual voltage value (Udr) and a power setpoint (Por) of a power to be transmitted being supplied to the setpoint generator and an actual value pair (Udr, Idr) determined for the current and voltage being supplied to the vector regulator arrangement (32) I P 3842 15 a regulator arrangement (28) being provided for each converter station which is operated in the "invertor" operating mode and having a device (52) for determining a power setpoint (Poi), a device (62) for determining an additional turn-off angle setpoint (yoadd), a setpoint generator (46) and a vector regulator arrangement (32) an actual power value (Pdi) which is determined and an upper and a lower power limit (Pgoi, Pgui) being supplied to the device (52) for determining a power setpoint (Poi), a power setpoint and actual value (Poi, Pdi), a turn-off angle setpoint and actual value (yo, an actual voltage value (Udi) and a control signal being supplied to the setpoint 5 generator an actual value pair (Udi, Idi) which is determined for the current and voltage being supplied to the vector regulator arrangement (32) and a power actual value and setpoint (Pdi, Poi), an upper and a lower power limit (dPo, dPu) and a minimum turn-off angle value (yomin) being supplied to the device (62) for determining an additional turn-off angle setpoint (yoadd)

7. Apparatus according to Claim 6, it being possible to connect or disconnect the control arrangements (26, 28) for each converter station as a function of its operating mode.

8. Apparatus according to Claim 6, the device (52) for determining a power setpoint (Poi) having a 1st order delay element (54) with an upper and a lower limit (Pgoi, Pgui).

9. Apparatus according to Claim 6, the device (62) for determining an additional turn-off angle setpoint (yoadd) having a dead band element (64) and a PI regulator a power control difference threshold (dP) which is determined being present at one input of the deaad band element (64) and the additional turn-off angle setpoint (yoadd) being present at the output of the P 3842 16 PI regulator (66) Apparatus according to Claim 6, each setpoint generator (30, 46) of the two control arrangements (26, 28) of each converter station having two characteristic generators (34, 36; 48, 50) for the setpoint pair (Uor, Ior; Uoi, loi) for current and voltage.

11. Apparatus according to Claim 6, each vector regulator arrangement (32) of the two control arrangements (26, 28) of each converter station (4) having two comparators (38, 40), an adder (42) and a control element one output of a comparator (38, in each case being linked to the adder whose output is connected to the input of the control element (44).

12. Apparatus according to Claim 9, a switch (68), which is closed during normal operation of each converter station being provided between the dead band element (64) and the PI regulator (66) of the device (62) for determining an additional turn-off angle setpoint 2 0 (yoadd)

13. Apparatus according to Claim 9, the PI regulator (66) of the device (62) for determining an additional turn-off angle setpoint (yoadd) having a set input at which a zero signal (SV) is present when each converter station is not in normal operation.

14. Apparatus according to Claim 9, each limit input of the dead band element (64) of the device (62) for determining an additional turn-off angle setpoint (yoadd) being linked to one output of a comparator (72, 74), at each of whose inputs an actual power value (Pdi) and an upper and lower power limit (Pgoi, Pgui) are present. I t I -17- Apparatus according to claim 6, a microprocessor being provided as the control arrangements (26,28) of each converter station

16. A method for controlling n converter stations in a multipoint HVDCT network substantially as herein described with reference to Figs. 5 and 6.

17. Apparatus for controlling n converter stations in a multipoint IIVDCT network substantially as herein described with reference to Figs. 5 and 6. DATED this Fifth Day of May 1998 Siemens Aktiengcsellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON 9. o S. o o o•* e ti I IAI> P 3842 18 Abstract Method and apparatus for controlling n converter stations in a multipoint HVDCT network The invention relates to a method and an apparatus for controlling n converter stations in a multipoint HVDCT network each station control system generating a control signal by means of coordinated vector control. According to the invention, the turn-off angle setpoint (yo) of a converter station which is operated in the "invertor" operating mode is determined as the sum of a minimum turn-off angle setpoint (yomin) and an additional turn-off angle setpoint (7oadd) which is produced, the additional turn-off angle setpoint (yoadd) being proportional to a power control difference 15 value (dP) which is determined, as soon as a negative or positive power control difference threshold (dPu, dPo) is respectively undershot or exceeded. A control concept is thus obtained for multiterminal high-voltage DC transmission which is structured in a simple manner, and 20 is of decentralized design, as a result of which it is possible to dispense with a higher-level master regulator and costly telecommunication. FIG t e. T^ l