HK1165109B - Method of controlling voltage source converter in hvdc system and converter station - Google Patents

Description

Method for controlling a voltage source converter in a HVDC system and converter station

The invention is a divisional application of the invention application of PCT application PCT/EP2007/055875 filed on 14 th 6 th 2007 and 26 th 2008 to China, the national application number is 200780024108.2, and the invention name is 'high-voltage direct current system and method for controlling voltage source converter in high-voltage direct current system'.

Technical Field

The present invention relates to a method of controlling a voltage source converter in a High Voltage Direct Current (HVDC) system and an HVDC system and converter station therefor.

Background

An HVDC system includes first and second converter stations, each containing a Voltage Source Converter (VSC) for transferring power from a first Alternating Current (AC) network to a second AC network.

Voltage Source Converters (VSC) are not only used in High Voltage Direct Current (HVDC) systems, but also as e.g. Static Var Compensators (SVC). In HVDC applications the voltage source converter is connected between a Direct Current (DC) link and an AC network, and in applications as static reactive compensators the voltage source converter is connected between a direct current voltage source and an AC network. In both applications, the voltage source converter must be able to generate an AC voltage with the same frequency as the frequency of the AC network. The reactive and active power flow through the converter is controlled by modulating the amplitude and phase of the AC voltage produced by the voltage source converter with respect to the voltage of the AC network, respectively.

In particular, voltage source converters equipped with series connected transistors (IGBTs) make this type of converter usable at relatively high voltages. Pulse Width Modulation (PWM) is used to control the generated AC voltage, which enables fast control of the voltage.

According to US 6400585, which is incorporated herein by reference, control systems for voltage control of converter stations in HVDC systems are previously known. The purpose of the control system is to keep the voltage of the dc link within safe operating limits also in abnormal voltage conditions.

The known HVDC system comprises a first and a second converter station, each having a voltage source converter connected between the DC link and the AC network on each side of the DC link. The current control system of the converter station has means to control the active power flow between the DC link and the AC network by influencing the phase shift (phase displacement) between the bus voltage in the AC network and the bridge voltage of the voltage source converter. The terms bus voltage and bridge voltage are explained further below. The control system comprises means for generating a phase change command signal in response to an indication of an abnormal voltage condition at the DC link, and means for affecting the phase of the bridge voltage in response to said phase change command signal to ensure that the phase shift between the bridge voltage and the bus voltage results in an active power flow from the DC link to the AC network. A phase-locked loop (PLL) device ensures that the control system of the converter station operates in phase synchronism with the bus voltage of the AC network.

The active power flow in the DC link must be balanced. This means that the active power leaving the link must be equal to the power received by the link. Any difference may cause the DC voltage to increase or decrease dramatically. To achieve this power balance, one of the converter stations controls the DC voltage. Accordingly, the other converter station may control the active power flow of the DC link by controlling the DC current accordingly. Typically, the upstream converter station controls the DC voltage, while the downstream converter station controls the active power flow.

From "An electronic transfer simulation for voltage source based HVDC transmission" of Qahraman et al, CanadianConference on electric and Computer Engineering, Niagara Falls, Ont, Canada, 2-5May 2004, vol.2, pages 1063-: 0-7803-8253-6, separate control of the active and reactive power in a voltage source converter is known, wherein a proportional integral controller acts on the actual reactive power error to generate the magnitude and phase of the voltage reference to the internal PWM controller, respectively.

Restoring power after a large area outage in an AC network or AC power grid can be very difficult. It is necessary to re-grid a plurality of power stations (on-line). Typically, this is done by means of power from the rest of the grid. In the absence of grid power, a so-called black start (black start) is required to lead the grid to operation.

To provide a black start, some power stations are typically fitted with small diesel generators that can be used to start larger generators with capacities of several megawatts, which in turn can be used to start the main power station generator. Power plants using steam turbines require up to 10% of their capacity in service power (station service power) for boiler feedwater pumps, boiler supply air combustion air blowers, and for fuel preparation. However, it is not economical to provide such a large standby capacity at each station, and therefore black start power must be supplied from other stations through the power transmission network.

A typical black start sequence based on an actual scene is as follows:

battery start a small diesel generator installed in a hydroelectric power station.

The power from the diesel generator is used to operate the hydroelectric power plant.

Supplying power to critical power lines between the hydropower stations and other areas.

The power from the dam is used to start a fire-rated base load (real-fire base load).

The power from the base load plant is used to restart all other plants in the system, including the nuclear plant.

Eventually reapplying power to the main power distribution network and sending to the customer.

Restoring power after a power outage is not a simple process. When the system is vulnerable during the restoration process, small disturbances continue to occur and the grid will experience different conditions from the de-sourced network, through various weak network conditions, to a normal strong AC network. In order to keep the frequency and voltage stable during the recovery process, a recovery plan of the entire cooperative system is required.

"Einflus der Liberalivisiting auf dieVersorgungshiccherheit den Stromnetzen bzw. Regelung serfordernised source d published as Gutachten Glapitsch. pdf from http:// www.versorgungssicherheit.at/downloadie"it is known to perform power restoration in a de-sourced AC network, for example by identifying autonomous areas of the network that are capable of providing power to a local network, followed by gradual connection to other network areas.

When the converter is connected to an isolated network that only generates electricity, such as a wind farm, or only consumes, or a mixture of both, it will be difficult to predict the real and reactive power. It is therefore difficult to determine the desired active power P_refAnd desired reactive power Q_refIt would not be practical to control them.

When the converter is connected to a dead AC power network, i.e. no power supply at all, the known control system described above will not work because there is no AC voltage for synchronization of the PLL, and the current control will not work because the current is naturally determined by the connected load.

The known control system described above is difficult to keep stable when the converter is connected to a very weak AC power network, i.e. the existing short circuit power in the network is approximately equal to or less than the converter rating, because the weak AC network provides a more oscillating AC bus voltage, which results in a PLL and a current controlled oscillation, because both systems use the AC bus voltage as input.

From WO 03/026118a1, a voltage source converter is known which can be used to supply weak or depowered ac voltage networks. How this is done is not discussed here.

Disclosure of Invention

A first object of the present invention is to provide a method and a system for controlling a voltage source converter in an HVDC link, which allows a more stable power supply to a dead AC network. A second object is to find a method for black starting an AC network, wherein the method is based on a control method of a voltage source converter.

A method according to the invention of controlling a voltage source converter in a high voltage direct current system connecting two ac networks, wherein one of the two ac networks is without power supply and the other of the two ac networks is functioning normally, the voltage source converter being connected to the ac network without power supply, the method comprising: operating the voltage source converter as a voltage source generator to generate an alternating voltage having a desired frequency and a desired voltage amplitude, denoted bridge voltage, using direct alternating voltage and frequency control by controlling the frequency and amplitude of the generated alternating voltage, wherein the voltage source converter is powered by a direct voltage of a high voltage direct current system having a nominal value.

Further, the method is applied for black-starting an ac network without power supply, wherein the ac network without power supply is connected to one of at least two ac power stations via a high voltage dc system, and wherein the ac network without power supply is connected to another of the at least two ac power stations via a transmission line, the method comprising: -powering a power line connected to another of the at least two alternating current power stations using the established bridge voltage (UV 1; UV 2); and activating another of the at least two ac power stations.

Furthermore, according to the invention, in a converter station in a hvdc system connecting two ac networks, where one of the two ac networks is without power supply and the other of the two ac networks is operating normally, said converter station comprising a voltage source converter and a control unit, and being connected to said ac networks without power supply, wherein said control unit is adapted to operate said voltage source converter as a voltage source generator to generate an ac voltage having a desired frequency and a desired voltage amplitude, denoted bridge voltage, using direct ac voltage and frequency control by controlling the frequency and amplitude of the generated ac voltage, wherein said voltage source converter is powered by the dc voltage of the hvdc system having nominal values.

According to the invention, the supply of the depowered AC network is achieved by controlling the frequency and the voltage amplitude of the AC voltage generated by the voltage source converter independently of the conditions in the connected AC network such that the voltage source converter operates as a voltage source generator. In contrast, as mentioned above, known converter control systems control the current of the voltage source converter and contribute to the operating state of the AC network. The frequency and voltage control enables to stabilize the voltage and frequency of the generated AC voltage, i.e. to enhance the stability (stiffness) of the weak AC network.

When controlled according to the invention, an HVDC system connecting two AC networks and comprising two voltage source converters is able to provide a black start when either of the two AC networks experiences a power outage.

One of the two converter stations, which is connected to a normally operating power supply AC network, keeps the DC voltage of the HVDC system at a nominal value. According to a second aspect of the invention, the other of the two converter stations connected to the AC network without power supply establishes an AC voltage having a predetermined frequency and amplitude. The established AC voltage is then used to power the transmission lines of an AC network, which is connected to other power stations. With this AC power support from the HVDC converter station, other power stations can be started. By restarting more power plants and connecting more loads, the grid is gradually restored.

An HVDC system comprising a voltage source converter mounted with a control according to the present invention makes the process of recovering power easy and smooth. Unlike a power generating unit which has inertia and involves mechanical power control, the VSC on which the control according to the invention is installed can be made very fast because there is no inertia. Due to the fast control, the VSC acts as a power buffer (slack) that maintains a power balance between generation and consumption, that is, when power generation is greater than power consumption, the converter operates as a rectifier that transfers power from the AC network to the DC side, and when power generation is lower than power consumption, the converter operates as an inverter that transfers power from the DC side to the AC network. In this way, the key issues that traditionally have to be considered during black start and restoration of the grid become less important, which makes restoration of the grid easier.

In an embodiment of the invention, the voltage feedback control is provided with an adaptive voltage droop function, which provides control of the AC voltage generated by the voltage source converter, while providing appropriate reactive power sharing between other reactive power sources in the connected AC network, such as voltage regulation devices. As a result, the adaptive voltage droop function keeps the AC voltage amplitude at the common connection point between the voltage source converter and the AC network stable under different operating conditions from a passive load, through a weak AC system with little power generation, until a strong AC system with all power generation recovered.

In another embodiment of the invention the Phase Locked Loop (PLL) means comprises signal generating means for generating a signal representing the frequency and phase angle of the desired AC voltage to be generated by the voltage source converter in dependence of the frequency command and the desired active power, such that the frequency of the connected AC network is kept almost constant. Accordingly, the signal generating means operates as an adaptive frequency down function. Small variations in frequency are required in order to achieve good load sharing between other power generating units in the connected AC network. The adaptive frequency droop function makes it possible to control the frequency to be almost constant, in addition to the small variations needed to achieve good load sharing between other power generating units.

Further advantageous embodiments of the invention will become apparent from the description of preferred embodiments of the invention and the appended claims.

Simulations of the grid restoration have shown that with the control device according to the invention stable AC voltages and frequencies are obtained under different AC network conditions from a passive load, through a weak AC system with almost no power generation, up to a strong AC system with full restoration of power generation.

Drawings

The invention will be explained in more detail by describing embodiments with reference to the accompanying drawings, which are all schematic and are in the form of a single-line diagram and a block diagram, respectively, wherein:

fig. 1 is a schematic single-line block diagram of a high voltage direct current transmission system known in the prior art;

FIG. 2 is an embodiment of a control device of a voltage source converter of the power transmission system according to the prior art of FIG. 1;

FIG. 3 is a detail of the prior art control device according to FIG. 2;

FIG. 4 is an embodiment of a converter control apparatus according to the present invention;

fig. 5 is an embodiment of an AC voltage control arrangement of the converter control device according to the invention;

fig. 6A is an embodiment of a phase-locked loop device of a converter control apparatus according to an embodiment of the present invention; and

fig. 6B shows a waveform of an output signal of the phase-locked loop device representing a desired voltage frequency and phase angle at the connection point.

Detailed Description

The block diagrams to be described below can be regarded as signal flow diagrams and block diagrams of the control device. The functions performed by the blocks shown in the block diagrams may in applicable parts be implemented by analog and/or digital techniques in hard-wired circuits, but are preferably implemented as programs in a microprocessor. It should be understood that although the illustrated blocks are referred to as components, filters, means, etc., they should be interpreted as means for performing the desired function, especially if their function is performed as software of a microprocessor. Thus, the expression "signal" may also be interpreted as a value generated by a computer program and appearing only as such, depending on the situation. Only a functional description of these blocks is given below, since these functions can be implemented in a manner known per se to the person skilled in the art.

Variables occurring in the control device shown in the figures, in particular variables representing voltage and current, are shown in vector form to illustrate their polyphase behavior. Using upper short barsTo specify the vector unit.

In the various figures, parts that are similar to each other and that appear in more than one figure are given the same reference numerals.

Connecting lines between measured values and blocks and between blocks are sometimes omitted so as not to burden the drawing. However, it should be understood that the individual variables that appear at the inputs of some of the blocks are provided from the block or measurement unit that generated them.

Fig. 1 shows a high voltage direct current transmission system known in the prior art in the form of a schematic single line block diagram. The first converter station STN1 and the second converter station STN2 are connected to each other by a dc-link having two pole conductors W1 and W2, respectively. Typically, the pole conductors are cables, but they may also at least partly comprise overhead wires. Each of the converter stations STN1 and STN2 has a capacitor arrangement C1 and C2, respectively, connected between pole conductors W1 and W2, and each of the converter stations STN1 and STN2 includes a voltage source converter CON1 and CON2, respectively. Each of the converters CON1 and CON2 includes a semiconductor valve having a 2-level or 3-level converter bridge. The semiconductor valve comprises branches of gate on/off semiconductor elements, for example so-called IGBT-type power transistors, and diodes connected in anti-parallel with these elements.

Each converter is connected to a respective three-phase alternating current power network N1 and N2 via a phase inductor PI1 and PI2, respectively. Although not shown in the figures, the converter may be connected to the three-phase network N1 or N2 via transformers, as is well known in the art, in which case the phase inductors PI1 and PI2 may be omitted for some cases. At the connection point between the respective phase inductor PI1 or PI2 and the respective three-phase network N1 or N2, a filter device F1 and F2 are connected as bypass connections, respectively.

The AC voltage of the AC network N1 at the connection point of the filter F1 was designated as UL1 and was measured with the measuring device M1. This voltage UL1 is referred to below as the bus voltage of the ac network N1. The AC voltage established by the converter CON1 is designated as UV1 and is referred to below as the bridge voltage of the converter CON 1. The alternating current at the converter CON1 is designated I1 and is measured with the measuring device M3. Similarly, the AC voltage at the connection point of the filter F2 is designated UL2 and measured with measuring device M4, the alternating current at the converter CON2 is designated I2 and measured with measuring device M6. The AC voltage at the connection point of the filter F2 will be referred to below as the bus voltage of the AC network N2. The AC voltage established by the converter CON2 is designated as UV2 and is referred to below as the bridge voltage of the converter CON 2.

The DC voltage across the capacitor device C1 is designated as Ud1 and the DC voltage across the capacitor device C2 is designated as Ud 2. These voltages are measured with measuring devices M7 and M8, respectively, which are only indicated symbolically.

The first converter station STN1 comprises a control device CTRL1, and the second converter station STN2 comprises a control device CTRL2 of a kind generally similar to control device CTRL1, for generating a chain of on/off commands FP1 and FP2, respectively, for the semiconductor valves of the respective voltage source converters CON1 or CON2 according to a predetermined Pulse Width Modulation (PWM) manner.

The converter stations STN1 and STN2 may operate in four different ways, DC voltage control, active power control, AC voltage control or reactive power control. Typically, one of the converter stations, e.g. the first converter station STN1, is operated in a DC voltage control mode for voltage control of the DC link, while the second converter station STN2 is operated in an active power control or AC voltage control or reactive power control mode. The operating mode may be set manually by an operator or may be set automatically by a sequence control system, not shown, in certain situations.

Fig. 2 shows an embodiment of a prior art control device representing the control device CTRL1 and the control device CTRL2, wherein the references 1 and 2 are omitted for simplicity.

The control device CTRL comprises a DC voltage controller 21, an AC voltage controller 22, selector means SW1 and SW2, a converter current control system IREG, a pulse width modulation unit 23, and a switching logic unit 24.

The actual value of the measured DC voltage Ud at the respective capacitor device (C1 or C2) and its voltage reference value UdR are supplied to the difference component 25, the output of which is supplied to the DC voltage controller 21.

The actual value of the respective bus voltage UL measured and its voltage reference value ULR are supplied to a difference component 26, the output of which is supplied to the AC voltage controller 22.

The output signal of the DC voltage controller 21 and the reference value Pref for the active power flow through the converter are provided to the first selector means SW 1. The first selector device SW1 outputs a signal pR, which is an output signal of the DC voltage controller 21 or a reference value Pref, according to the mode signal MD 1.

The output signal of the AC voltage controller 22 and the reference value Qref for the reactive power flow through the converter are provided to a second selector means SW 2. The second selector means SW2 outputs a signal qR, which is the output signal of the AC voltage controller 22 or the reference value Qref, according to the mode signal MD 2.

The AC and DC voltage controllers 21 and 22 have, for example, a proportional-integral characteristic. The reference values Pref and Qref may be formed in a conventional manner as outputs from controllers (not shown) for the active and reactive power flows, respectively.

The output signals pR and qR of the first and second selector devices SW1 and SW2 are supplied to the converter current control system IREG. The current control system IREG provides an internal AC current control feedback loop based on outputs based on switching devices SW1 and SW2Providing current reference vector generation voltage reference vector formed by output signals pR and qR and phase reference synchronization signalA voltage reference template of the form. The voltage reference vectorA voltage reference representing the bridge voltage UV1 or UV2 of the respective converter CON1 or CON 2. By designating the phases of the three-phase ac network as a, b and c, the superscript abc of the vector refers to the three-phase voltage of the converter, and the vector therefore has componentsAnd

the converter current control system IREG is also provided with the actual value I of the alternating current at the converter and the nominal value f0 of the frequency of the AC network N1 or N2, which is typically 50 or 60 Hz.

Reference voltage vectorProvided to the pulse width modulation unit 23, determines the times ta, tb and tc at which the valves in the phases a, b and c of the converter CON1 or CON2 commutate, from which the switching logic unit 24 generates the on/off command chains FPa, FPb and FPc, which are provided to the semiconductor valves.

Preferably, the converter current control system IREG is implemented as software running on a microprocessor and executed as a sampling control system.

For practical reasons, i.e. to facilitate the calculations, the converter current control system IREG operates in a conventional manner, wherein the voltages and currents of the three-phase unit, i.e. the ac network, are converted to and expressed in a rotating two-phase dq reference plane via conversion to a stationary two-phase α β reference plane. The three-phase units of the ac network are thus converted into dc quantities that can be processed with per se known control system techniques.

By designating the phases of a three-phase alternating current network as a, b and c, the three-phase system is called abc-system. In the text and figures that follow, superscripts are used where appropriate (e.g., as) To represent a reference plane.

Fig. 3 shows the basic structure of a converter current control system IREG according to the prior art. The current control system is implemented as a sampling control system having a sampling period time Ts.

For simplicity, all variables are shown in vector form, but it should be understood that the components of each vector are signal processed in a manner known per se. Since the current control system is similar for both control devices CTRL1 and CTRL2, the reference numerals on the various variables described below have been omitted with reference numerals 1 and 2 for simplicity.

The converter current control system IREG according to fig. 3 comprises a current command calculation unit 41, a current controller 42, a first transformation means 43, a second transformation means 44, a first Phase Locked Loop (PLL) means 46 and a first calculation unit 48.

The converter current control system IREG receives the generated signals pR and qR as explained above with reference to fig. 2. The signals pR and qR are supplied to a current command calculation unit 41, from which the current command calculation unit 41 calculates and outputs a reference value of the alternating current at the converter. The reference values are used in dq reference planes respectivelyAndis shown as a current reference vector in the figureShown. The calculation is performed according to a relationship known per se.

Wherein the voltage UL^dAnd UL^qRepresenting the d and q components, respectively, of the bus voltage UL measured in the AC network and transformed to the dq reference plane.

The current reference value may be limited in accordance with a specified operating condition of the power transmission system before further processingAnd

it should be noted that in the dq reference plane rotating synchronously with the bus voltage UL described above with respect to the PLL, the q-component UL of the bus voltage UL^qBecomes zero. Then, following expressions (1a) and (1b), the current reference valueD component ofBecoming a reference value, q-component, of active powerBecomes the reference value of reactive power.

Measuring an actual value I of an alternating current at a converter in an AC network and converting it to a dq reference plane as an actual current vector

Providing a current reference vector to a current controller 42Actual current vectorAnd the average value of the bus voltage UL transformed to the dq reference planeThe current controller 42 outputs a designation according to thisIs the voltage reference vector of the bridge voltage of the converter in the dq reference plane.

Will exchangeVoltage reference vectorIs supplied to a first transformation component 43 for transforming the vector to the α β reference plane. The output of the first transformation section 43 is supplied to the second transformation section 44, and the supplied vector is transformed to the abc reference plane as a vectorThis vector is the bridge voltage reference vector of the converter, which has as components the voltage reference values of the respective three phases of the alternating current system.

Reference vector of bridge voltageIs supplied to the pulse width modulation unit 23 described above with reference to fig. 2.

The first transformation component 43 transforms in a manner known per se using the transformation angle ξ ═ ω t, the rotation frequency ω of the AC network and the time t

The transformation angle signal, designated xi in the figure, is generated in a conventional manner by a Phase Locked Loop (PLL) component 46 from the nominal value f0 of the frequency of the AC network and the phase of the bus voltage transformed to the α β reference plane, and then supplied to the first transformation component 43.

The signal ξ can be regarded as a phase reference synchronization signal, in the following simply referred to as synchronization signal or phase angle signal. The aim is to synchronize the rotating dq reference plane with the bus voltage abc system, which represents an electrical angle that increases linearly with time, the rate of which is proportional to the actual frequency of the AC network. At least in a steady state condition, the synchronization signal ξ is locked in phase with the phase of the bus voltage UL of the AC network. The rotating dq reference plane is then also locked and kept in synchronization with the three-phase abc system, in particular with the bus voltage UL. Under these conditions, the q-component UL of the bus voltage UL^qAlso becomes zero.

Fig. 4 shows an embodiment of the converter control device according to the invention. In contrast to the known embodiment of such an apparatus described with reference to fig. 2 and 3, the control of the active power, the DC voltage, the reactive power and the AC voltage by means of the AC current is replaced by AC voltage control and a new phase locked loop device. The voltage generation means 51 generate the converter reference voltage by using the outputs from the AC voltage control means UACREG and the phase locked loop means PLL _ INI.e. the reference value of the bridge voltage of the converter in the dq reference plane.

Converter reference voltage to be generatedAnd the output from the phase-locked loop means PLL _ IN, including the signals of the desired frequency f _ ord and the desired phase angle ξ _ ord of the bridge voltage UV and the sampling period Ts _ ord, are supplied together to the PWM component 52. The PWM section 52 may be implemented in the manner shown in fig. 2 and 3, or may be implemented in other ways depending on the modulation method selected, as will be readily understood by those skilled in the art.

Fig. 5 shows an embodiment of the AC voltage control unit UACREG shown in fig. 4. The reactive power Q measured at the connecting bus to the AC network N1 or N2 in fig. 1 is supplied to the selection component 62 via the filtering unit 61. The selection part 62 selects a preset constant according to the value of the input signal and generates a signal Δ Uref proportional to the selected constant and the input signal. A predetermined constant, known as the slope, forces automatic reactive load sharing between the converter station and other voltage regulating devices. The predetermined constant is typically a value from 0.01 to 0.1. If the input signal is large, a larger value may be selected in order to avoid overcurrent and obtain good AC voltage control. The output from the selection part 62 and the preset AC reference voltage ULR are supplied to the first addition part 63. The first adding part 63 outputs the actual AC voltage reference and supplies it to the second adding part 64. The other input signal to the second adding component 64 is the AC voltage amplitude UL measured at the connecting bus provided via the filtering unit 57. The output of the second adding component 64 is supplied to a regulator 65, which regulator 65 may be of the PI type, i.e. comprising a proportional part and an integral part. The regulator 65 generates a voltage correction section. The correction portion is added to a predetermined AC reference voltage ULR to form a voltage magnitude command UMR.

Fig. 6A shows an embodiment of the phase locked loop component PLL _ IN according to fig. 4. The phase angle generation unit 71 generates a phase angle ξ _ ord of the desired bridge voltage UV, the waveform of which is shown in fig. 6B, from the frequency f _ ord and the sampling period Ts _ ord of the desired bridge voltage UV. The desired frequency, which is also the actual frequency of the connected AC network, is the sum of the pre-set reference frequency fo and the frequency correction part obtained from the third adding means 75. The selection unit 77 selects a predetermined constant according to the value of the input signal, and generates a signal proportional to the selected constant and the input signal. A preset constant known as droop forces automatic active load sharing between the converter station and other power generating units. The predetermined constant is typically a value from 0.1 to 1.0. If the input signal is large, a larger value may be selected in order to avoid overcurrent and obtain good frequency control. The input signal of the selection component 77 is the measured active power P provided via the filtering unit 72. The sampling period Ts _ ord is determined by

Ts_ord＝(2·p·f)^-1

Where p is a value pre-selected according to the switching frequency.

As mentioned above, in the known control system, the output signal ξ of the PLL represents the synchronism between the PLL and the measured bus voltage UL. In other words, signal ξ represents the phase angle as well as the frequency of the AC network voltage. As a result of this synchronization, the dq reference plane also rotates synchronously with the bus voltage UL, which causes the q-component UL of the bus voltage UL to rotate^qBecomes zero, d component UL of bus voltage UL^dEqual to the magnitude of the bus voltage. It should be noted that this synchronicity is generalAnd the feedback control is performed. In the present invention, instead of using feedback control, the q-component UL of the bus voltage is controlled by^qSet to zero and d-component UL of the bus voltage^dSet to the voltage magnitude command UMR, i.e.

Forced synchronization between the signal ξ _ ord of PLL _ IN and the bus voltage is achieved IN the voltage generation component 51 (fig. 4).

Further, the converter reference voltage in the dq reference plane is obtained as follows

WhereinIs the predicted voltage drop across the inductance byWith measured current including filteringIs obtained from the dq component of (1).

Converter reference voltage in control methods and systems according to the inventionCorresponding to the voltage reference vector in the prior art with reference to fig. 3Similarly, signals ξ _ ord, f _ ord and Ts _ ord according to the invention correspond respectively to signals ξ, f and Ts in the prior art. Once these signals are generated, generation of switching commands for controlling the valve may be effected.

In an embodiment of the invention, the voltage source converter may be installed with control modes known in the art, referred to as first control mode, i.e. active power/DC voltage and reactive power/AC voltage control by AC current control, and with control modes according to the invention, referred to as second control mode, i.e. direct AC voltage and frequency control. A software or hardware switch is added to select the desired control mode.

In HVDC power transmission or back-to-back systems, only one of the voltage source converters may be equipped with a control according to the invention. In this case, the other converters control the DC voltage of the HVDC system using a control known in the art.

Claims (11)

1. A method of controlling a voltage source converter in a high voltage dc system connecting two ac networks, wherein one of the two ac networks has no power supply and the other of the two ac networks is operating normally, the voltage source converter being connected to the ac network having no power supply, the method comprising: operating the voltage source converter as a voltage source generator to generate a voltage having a desired frequency (f _ ord) and a desired voltage amplitudeIs represented as a bridge voltage (UV 1; UV2), is achieved using direct ac voltage and frequency control by controlling the frequency and amplitude of the generated ac voltage, wherein the voltage source converter is supplied by the dc voltage of a high voltage dc system having a nominal value.

2. Method according to claim 1, wherein in a converter station connected to an ac network without power supply a forced synchronization between a desired phase angle (ξ _ ord) of the bridge voltage and the ac voltage of the connected ac network, expressed as bus voltage (UL1, UL2), is achieved.

3. The method of claim 2, wherein forced synchronization is by setting a q-component of bus voltage to zero and a d-component of bus voltage to a voltage magnitude command (UMR) such that a reference value of the bridge voltage on a dq reference plane becomes the reference valueAnd the realization is that,

whereinIs a reference value for the bridge voltage on a dq reference plane, UMR is a voltage magnitude command,is the predicted voltage drop.

4. The method according to any of claims 1-3, wherein the voltage magnitude of the bridge voltage is controlled by a voltage feedback control (UACREG) comprising an adaptive voltage droop function (62) acting on the reactive power (Q) measured at the connection point with the alternating current network (N1; N2) without power supply.

5. The method of claim 4, wherein the adaptive voltage droop function (62) increases the alternating voltage reference signal (ULR) as the reactive power (Q) increases.

6. A method according to any of claims 1 to 3, wherein the frequency is controlled by a phase locked loop (PLL _ IN) comprising an adaptive frequency droop function (77), the adaptive frequency droop function (77) acting on the active power (P) measured at the connection point to the AC network (N1; N2) without power supply.

7. The method according to claim 6, wherein the adaptive frequency droop function (77) increases the frequency reference signal (f0) with increasing active power (P).

8. Method according to any of claims 1-3, wherein the method is applied only after detecting that the connected AC network (N1; N2) is without power supply.

9. A method according to any one of claims 1 to 3, wherein the method is applied to black start an ac network without power supply, wherein the ac network without power supply is connected to one of at least two ac power stations via a high voltage dc system, and wherein the ac network without power supply is connected to another of the at least two ac power stations via a transmission line, the method comprising:

-powering a power line connected to another of the at least two alternating current power stations using the established bridge voltage (UV 1; UV 2); and

starting another of said at least two ac power stations.

10. The method of claim 9, wherein an ac network without power supply is connected to more than two ac power stations and at least one load, wherein the ac network is restored by starting the remaining ac power stations one by one after the start of another of the at least two ac power stations, and thereafter connecting the at least one load.

11. Converter station in a hvdc system connecting two ac networks, where one of the two ac networks is without power supply and the other of the two ac networks is functioning normally, said converter station comprising a voltage source converter and a control unit, connected to said ac networks without power supply, wherein said control unit is adapted to operate said voltage source converter as a voltage source generator to generate ac power having a desired frequency (f _ ord) and a desired voltage amplitudeIs represented as a bridge voltage (UV 1; UV2), is achieved using direct ac voltage and frequency control by controlling the frequency and amplitude of the generated ac voltage, wherein the voltage source converter is supplied by the dc voltage of a high voltage dc system having a nominal value.