SYSTEM

TECHNICAL FIELD

The present invention relates to a power transmission system and a method in a power transmission system.

BACKGROUND

The relative amount of power produced by renewable energy sources such as wind power plants and solar power plants is increasing on a global basis. Renewable energy sources such as wind power plants and solar power plants are variable in the sense that their energy production depends on external conditions such as wind speed and cloudiness. Power production using such renewable energy sources may be predicted based on weather forecasts. Typically, power production using such renewable energy sources is carried out so as to extract the maximum amount of power that can be extracted at a given instant, while power systems connected to such renewable energy sources will have to cope with keeping a balance between energy production and load to maintain a stable frequency in the overall power transmission system. The overall power transmission system may need to be 'reinforced' to handle power transmission from such renewable energy sources and to cope with the relatively large variations over time in power flow from such renewable energy sources. One way to reinforce such a power transmission system is to install one or more High Voltage Direct Current (HVDC) power transmission links as one or more embedded parts of an existing Alternating Current (AC) power transmission system. The AC power transmission system may for example include one or more AC power transmission lines which are interconnecting two (or more) parts, or regions (e.g., geographical regions), of the AC power transmission system. The AC power transmission line(s) may hence form an interface between the parts or regions of the AC power transmission system. The AC power

transmission system may in such a case be reinforced with an HVDC link interconnecting the two parts or regions of the AC power transmission system. The HVDC link may for example be connected in parallel to the one or more AC power transmission lines which are

interconnecting the two parts or regions of the AC power transmission system.

The HVDC link may be provided at both ends with converter stations which are interfacing the respective ones of the two parts or regions of the AC power transmission system at respective Point of Common Couplings (PCCs). If the HVDC link is based on voltage source converter (VSC) technology, then each converter station may have two degrees of freedom with respect to controlling operation of the converter station. The control of each converter may be set up so that a converter station controls either active power injected at the corresponding PCC or Direct Current (DC) voltage, or AC voltage at the corresponding PCC or reactive power injected at the corresponding PCC. For example, one converter station may control DC voltage and AC voltage at the converter station and the other converter station may control active power flow through the converter station and AC voltage at the converter station.

In active power/direct voltage controlling of a converter station, measurements of active power flow and direct voltage at the converter station may be taken as inputs together with set-points, or reference values, therefor for generating a converter valve current order. Similarly, in reactive power/alternating voltage controlling of a converter station, measurements of reactive power and alternating voltage may be taken as inputs together with set-points, or reference values therefor for generating a converter valve current order. The converter valve current orders are used for establishing converter valve voltage references. Either the active power flow through the converter station or the DC voltage of the converter station may be adjusted such that it obtains a value which corresponds to, or comes closer to corresponding to, a reference value of active power or a reference value of DC voltage, respectively (e.g., by way of proportional integral control). Reactive power flow through the converter station or the AC voltage of the converter station may be controlled in a similar manner.

SUMMARY

The control of the converter station as described in the foregoing may aim at a goal of that the actual active or reactive power injected at the corresponding PCC should correspond to the set-point of active power or reactive power. If the power transfer in the AC power transmission system changes, either relatively slowly as weather dependent power production changes, or relatively fast, following sudden disturbances for example due to disturbances (e.g. outage of power system component(s)) or faults (e.g., short circuits) in the AC power transmission system, it may be advantageous if the power transfer on the HVDC link changes in order to support the power transfer on the AC power transmission line(s). However, in the control of the converter station as described in the foregoing, the power transfer on the HVDC link may only change in order to support the power transfer on the AC power transmission line(s) if an operator changes the setpoint(s). Changing of the set-point(s) may however only be possible if the changes in the power transfer in the AC power transmission system are relatively slow. Also, the changing of the setpoint(s) may be relatively time-consuming for the operator since the operator, would have to keep track of the current set-point(s). In the light of the foregoing, the inventors have realized that it would be desirable to be able to support the power transfer on the AC power transmission line(s) at several time scales, from relatively slow changes in the power transfer in the AC power transmission system, for example following changes in weather dependent power production sources, to relatively fast changes in the AC power transmission system, for example following disturbances or faults in the AC power transmission system. Disturbances or faults in the AC power transmission system may cause instabilities therein due to loss of

synchronism or large, poorly damped power oscillations.

In view of the foregoing, a concern of the present invention is to provide a power transmission system comprising at least a first Alternating Current, AC, region and a second AC region interconnected by at least one AC power transmission line, which power transmission system facilitates supporting the power transfer on the AC power transmission line(s) at several time scales in changes in the power transfer in the AC power transmission system.

To address at least one of this concern and other concerns, a power transmission system and a method in a power transmission system in accordance with the independent claims are provided. Preferred embodiments are defined by the dependent claims.

According to a first aspect, there is provided a power transmission system which comprises at least a first AC region and a second AC region interconnected by at least one AC power transmission line. The power transmission system comprises at least one DC link interconnecting the first AC region and the second AC region. The power transmission system comprises at least a first converter station. The first converter station is connected to the DC link at a DC side of the first converter station. The first converter station is connected to the first AC region at an AC side of the first converter station. The first converter station is configured to control flow of power through the first converter station between the DC link and the first AC region. The power transmission system comprises a first phasor measurement entity (PME) configured to measure one or more AC phasors in the first AC region, wherein each AC phasor comprises an AC phase angle. The power transmission system comprises a second PME configured to measure one or more AC phasors in the second AC region, wherein each AC phasor comprises an AC phase angle. The first PME and the second PME employ a common time source such that the AC phasors in the first AC region measured by the first PME and the AC phasors in the second AC region measured by the second PME are synchronized in time. The power transmission system comprises a power flow sensor configured to sense power flow into the first converter station from its AC side. The power transmission system comprises a control unit that is communicatively coupled with the first PME, the second PME, and the power flow sensor, respectively. The control unit is configured to determine a first AC phase angle based on the one or more AC phasors in the first AC region measured by the first PME and a second AC phase angle based on the one or more AC phasors in the second AC region measured by the second PME, wherein the first AC phase angle and the second AC phase angle are synchronized in time (for example, the control unit may be configured to select a first AC phase angle from the one or more AC phasors in the first AC region measured by the first PME and select a second AC phase angle from the one or more AC phasors in the second AC region measured by the second PME such that the first AC phase angle and the second AC phase angle are synchronized in time). The control unit is configured to control flow of power through the first converter station between the DC link and the first AC region by means of generating a control signal (or several control signals) for the first converter station. The control unit is configured to determine the control signal based on a power flow reference value, sensed power flow into the first converter from its AC side, and the first AC phase angle and the second AC phase angle.

The control signal may for example comprise or be constituted by a converter valve current order, e.g., in d-axis direction, of one of more converter valves of the first converter station. In the context of the present application, by a converter valve it is meant a building block, or cell, of a converter, which for example may comprise a plurality of solid- state semiconductor devices such as thyristors or insulated gate bipolar transistors (IGBTs) which may be connected together, for instance in series.

The control unit is hence configured to control flow of power through the first converter station between the DC link and the first AC region based on PME measurements in or from the first AC region and from the second AC region, respectively. In particular, the control unit may be configured to control flow of power through the first converter station between the DC link and the first AC region based on AC phase angles resulting from PME measurements from the first AC region and from the second AC region, respectively. For example, the control unit may be configured to determine the control signal based on a difference between a power flow reference value and sensed power flow into the first converter station from its AC side and a difference between the second AC phase angle and the first AC phase angle.

According to one or more embodiments of the present invention, the control unit may be configured to determine the control signal further based on a reference value for the difference between the second AC phase angle and the first AC phase angle, and possibly a difference between the difference between the second AC phase angle and the first AC phase angle and the reference value.

The first PME and/or the second PME may for example be configured to measure AC voltage phasors in the first AC region and the second AC region, respectively, wherein each AC voltage phasor may comprise a AC voltage phase angle.

By controlling flow of power through the first converter station between the DC link and the first AC region based on AC phase angles resulting from PME measurements in or from the first AC region and from the second AC region, respectively, the power transfer on the AC power transmission line(s) may be supported by power transfer on the DC link at several time scales in changes in the power transfer in the overall power transmission system. The power flow PAB in an AC power transmission line between a first AC node (or AC region) A and a second AC node (or AC region) B may be approximated as:

PAB = (V _A VB/XAB) sin(9 _A - Θ _Β ), where PAB is the active power flow from AC node A to AC node B, XAB is the reactance of the power transmission line, VA and ΘΑ are the magnitude of AC voltage and AC phase angle, respectively, in AC node A, and VB and ΘΒ are the magnitude of AC voltage and AC phase angle, respectively, in AC node B. Thus, an increase in the difference in AC phase angle between AC node A and AC node B, ΘΑ - ΘΒ, corresponds to an increase in the active power flow from AC node A to AC node B, and vice versa. Thus, if the AC phase angles ΘΑ and ΘΒ in the respective ones of the AC node A and the AC node B are sensed and there is an increase in ΘΑ - ΘΒ, the active power transferred on a DC link provided between the first AC node A and the second AC node B should be increased, in order to support the power transfer in the AC power transmission line between the first AC node A and the second AC node B and offload the AC power transmission line. The same considerations apply to a case where the first AC node A and the second AC node B would be connected by means of several AC power transmission lines. In that case, XAB can approximately be considered as an equivalent reactance between the first AC node A and the second AC node B. Even though the equivalent reactance between the first AC node A and the second AC node B would not be exactly known, the above-mentioned relation between ΘΑ - ΘΒ and the active power flow from AC node A to AC node B, and vice versa, still holds.

By controlling flow of power through the first converter station between the DC link and the first AC region based on AC phase angles resulting from PME measurements from the first AC region and from the second AC region, respectively, the power transfer between the first AC region and the second AC region may become more effective on, during or after any changes in the power transfer in the power transmission system as weather dependent power production changes. The controlling of flow of power through the first converter station between the DC link and the first AC region could in alternative or in addition be based on measurements of active power transferred on the AC power transmission line(s). However, by controlling flow of power through the first converter station between the DC link and the first AC region based on AC phase angles resulting from PME measurements from the first AC region and from the second AC region, respectively, the power transfer between the first AC region and the second AC region may become more effective on, during or after any relatively fast changes in the power transfer in the power transmission system following sudden disturbances (e.g. outage of power system component(s)) or faults (e.g., short circuits) in the power transmission system. The DC link may for example comprise or be constituted by a HVDC link. The response time of a HVDC link is short in comparison to the electromechanical time constants that will govern the behavior of an AC power system following disturbances such as, for example, short circuits therein.

In the context of the present application, by a phasor measurement entity (PME) it is meant any unit, element, device, system, etc., which is capable of carrying out phasor measurement. The phasor measurement capability may for example be realized by means of a (dedicated) unit specially configured for carrying out phasor measurement, or by means of a unit, element, device, system, etc., which exhibits a phasor measurement capability in addition to other capabilities or functionalities, which unit, element, device, system, etc., for example may be constituted by or included in a protection Intelligent Electronic Device (IED), a transient fault recorder, a control system, etc.

In the context of the present application, by an AC region comprised in the power transmission system it is meant a part or portion of an AC power transmission system or network which includes one or more selected parts or portions of the overall AC power transmission system. The one or more selected parts or portions of the overall AC power transmission system may for example comprise one or more generators, switchgear, and/or any other component(s) of the overall AC power transmission system. The AC region may correspond to a geographical region, but it is not necessary for the AC region to do so.

According to one or more embodiments of the present invention, the AC region may include one or more selected parts or portions of the overall AC power transmission system that is or are 'embedded' in the overall AC power transmission system, or forming an integral part thereof.

According to one or more embodiments of the present invention, the DC link may be considered as being integrated, or embedded, in an AC power transmission system, which, e.g., may comprise a meshed AC power transmission system or network.

The power flow sensor that is configured to sense power flow into the first converter station from its AC side may possibly be a part of the first converter station. Thus, the power flow sensor is not necessarily a component that is separate from the first converter station.

The power flow reference value may for example be a reference value for the flow of power through the first converter station between the DC link and the first AC region.

In the context of the present application, by the first AC phase angle and the second AC phase angle being synchronized in time it is meant that they are corresponding or relating to the same, or substantially the same, time instant or period of time.

As mentioned in the foregoing, the control unit may be configured to determine the control signal further based on a reference value for the difference between the second AC phase angle and the first AC phase angle. The control unit may for example be configured to determine the control signal based on a difference between a power flow reference value and sensed power flow into the first converter from its AC side and a difference between the difference between the second AC phase angle and the first AC phase angle and the reference value for the difference between the second AC phase angle and the first AC phase angle.

The control unit may be configured to determine the control signal based on summation of the difference between a power flow reference value and sensed power flow into the first converter from its AC side and a difference between the difference between the second AC phase angle and the first AC phase angle and the reference value for the difference between the second AC phase angle and the first AC phase angle. One of the difference between a power flow reference value and sensed power flow into the first converter from its AC side and the difference between the difference between the second AC phase angle and the first AC phase angle and the reference value for the difference between the second AC phase angle and the first AC phase angle may be multiplied by a power control parameter.

The controlling of (e.g., active) flow of power through the first converter station between the DC link and the first AC region may hence be considered as a form of droop control. For example, at steady state conditions, the following relation may hold: where P _re f is the power flow reference value, Pmeas is sensed power flow into the first converter from its AC side, ΔΘ is the difference between the second AC phase angle and the first AC phase angle, A9 _re f is the reference value for the difference between the second AC phase angle and the first AC phase angle, and D is the power control parameter, which hence be considered as a droop constant. From this relation it follows that if ΔΘ increases, then so will also the power flow into the first converter station from its AC side, Pmeas. The power transfer on the DC link using the droop control may 'behave' similarly to power transfer on an AC power transmission line in the sense that the power transfer on the DC link using the droop control shares the overall power transfer burden between the first AC region and the second AC region as the need for power transfer between the first AC region and the second AC region changes. The extent to which the power transfer on the DC link using the droop control shares the overall power transfer burden may be governed by a droop constant, which may be the power control parameter as mentioned in the foregoing. A relatively small droop constant implies that the DC link will take a relatively large share of the overall power transfer burden between the first AC region and the second AC region, whereas a relatively large droop constant implies that the DC link will take a relatively small share of the overall power transfer burden between the first AC region and the second AC region.

The power control parameter may be a predefined value. Alternatively, the power control parameter may be adaptively determined. For example, the power control parameter may be determined based on the difference between the second AC phase angle and the first AC phase angle. The controlling of (e.g., active) flow of power through the first converter station between the DC link and the first AC region may in case the power control parameter is adaptively determined be considered as a form of adaptive droop control, or nonlinear droop control.

The first PME may be configured to measure a plurality of AC phasors in the first AC region corresponding to different time instants or different periods of time. The second PME may be configured to measure a plurality of AC phasors in the second AC region corresponding to different time instants or different period of time. Thus, the first PME and/or the second PME may repeatedly carry out measurements of AC phasors in the first AC region and in the second AC region, respectively. For example, the first PME and/or the second PME may carry out measurements of AC phasors in the first AC region and in the second AC region, respectively, periodically, at predefined, successive time instants (for example with a selected time interval between successive measurements, such as 20 ms or about 20 ms).

The first PME and the second PME may employ the common time source such that the AC phasors in the first AC region measured by the first PME are synchronized in time with respective ones of the AC phasors in the second AC region measured by the second PME. In alternative or in addition, the first PME and the second PME may employ the common time source such that the AC phasors in the first AC region measured by the first PME and the respective ones of the AC phasors in the second AC region measured by the second PME correspond to the same time instants or periods of time. The AC phasors in the first AC region measured by the first PME may be time-stamped with time instants or periods of time corresponding to when the respective ones of the AC phasors were measured.

Similarly, the AC phasors in the second AC region measured by the second PME may be time-stamped with time instants or periods of time corresponding to when the respective ones of the AC phasors were measured.

For example, the phase angles of the AC phasors measured by the first PMU and the second PMU, respectively, may correspond to the same, or substantially the same, time instants. That is, the first and second PMUs may be configured to generate first and second sets of measured AC phasors including phase angles where corresponding elements of the first and second sets relate to the same or substantially the same time instant when the respective AC phasors were measured.

The power transmission system may comprise a plurality of first PMEs. Each of the first PMEs may be configured to measure one or more AC phasors in the first AC region, wherein each AC phasor comprises an AC phase angle. The power transmission system may comprise a plurality of second PMEs. Each of the second PMEs may be configured to measure one or more AC phasors in the second AC region, wherein each AC phasor comprises an AC phase angle. The first PMEs may be configured to measure one or more AC phasors in the first AC region at different locations, or in different components, in the first AC region. Similarly, the second PMEs may be configured to measure one or more AC phasors in the second AC region at different locations, or in different components, in the second AC region. The first PMEs and the second PMEs may employ the common time source such that the AC phasors in the first AC region measured by the first PMEs and the AC phasors in the second AC region measured by the second PMEs are synchronized in time. The first PMEs and the second PMEs may employ the common time source such that the AC phasors in the first AC region measured by the first PME and the respective ones of the AC phasors in the second AC region measured by the second PME correspond to the same time instants or periods of time. The control unit may be configured to determine the first AC phase angle by averaging the AC phasors in the first AC region measured by the respective ones of the first PMEs, and to determine the second AC phase angle by averaging the AC phasors in the second AC region measured by the respective ones of the second PMEs, the first AC phase angle and the second AC phase angle being synchronized in time. Thus, the first AC phase angle may be determined by averaging AC phasors in the first AC region corresponding to the same time instant or period of time, measured by the respective ones of the first PMEs. And similarly, the second AC phase angle may be determined by averaging AC phasors in the second AC region corresponding to the same time instant or period of time, measured by the respective ones of the second PMEs. The averaging of AC phasors can be carried out in different ways as known in the art.

The power flow sensor may be configured to sense active and/or reactive power flow into the first converter station from its AC side. The control unit may be configured to control flow of active and/or reactive power through the first converter station between the DC link and the first AC region by means of transmitting a control signal to the first converter station. The control unit may be configured to determine the control signal based on an active and/or reactive power flow reference value, sensed active and/or reactive power flow into the first converter from its AC side, and the first AC phase angle and the second AC phase angle. The first AC phase angle and the second AC phase angle may be synchronized in time.

The first PME may be configured to measure one or more AC phasors in one or more locations, or in one or more components, in the first AC region. Similarly, the second PME may for example be configured to measure one or more AC phasors in one or more locations, or in one or more components, in the second AC region. The first PME may for example be configured to measure one or more AC phasors in a first electrical substation in the first AC region. Similarly, the second PME may for example be configured to measure one or more AC phasors in a second electrical substation in the second AC region. The first electrical substation may be different from the second electrical substation. The AC phasors may be measured in the first electrical substation and/or in the second electrical substation for example by means of utilizing one or more voltage transformers (which may in alternative be referred to as measurement transformers) which may be included in the first electrical substation and in the second electrical substation, respectively. For example, the first PME may be communicatively connected with a voltage transformer included in the first electrical substation, and the second PME may be communicatively connected with a voltage transformer included in the second electrical substation, for measuring AC voltage phasors in the first AC region and the second AC region, respectively.

The common time source which may be employed by the first PME and the second PME may for example comprise a Global Positioning System (GPS) based time source.

Each of the first PME and the second PME may be configured to measure one or more AC voltage phasors in the first AC region and the second AC region, respectively. Each AC voltage phasor may comprise an AC voltage phase angle.

According to a second aspect, there is provided a method in a power transmission system. The power transmission system comprises at least a first AC region and a second AC region, the first AC region and the second AC region being interconnected by at least one AC power transmission line. The power transmission system comprises a DC link interconnecting the first AC region and the second AC region. The power transmission system comprises at least a first converter station. The first converter station is connected to the DC link at a DC side of the first converter station, and is connected to the first AC region at an AC side of the first converter station. The first converter station is configured to control flow of power through the first converter station between the DC link and the first AC region.

The method comprises measuring one or more AC phasors in the first AC region, each AC phasor comprising an AC phase angle, and measuring one or more AC phasors in the second AC region, each AC phasor comprising an AC phase angle. The measuring of the one or more AC phasors in the first AC region and the measuring of the one or more AC phasors in the second AC region comprises employing a common time source such that the measured AC phasors in the first AC region and the measured AC phasors in the second AC region are synchronized in time. The method comprises sensing power flow into the first converter station from its AC side. The method comprises determining a first AC phase angle based on the one or more AC phasors in the first AC region and a second AC phase angle based on the one or more AC phasors in the second AC region, wherein the first AC phase angle and the second AC phase angle are synchronized in time. The method comprises controlling flow of power through the first converter station between the DC link and the first AC region by means of generating a control signal for the first converter station. The control signal is determined based on a power flow reference value, sensed power flow into the first converter from its AC side, and the first AC phase angle and the second AC phase angle.

According to a third aspect, there is provided a control unit configured to be used in conjunction with a power transmission system according to the first aspect. The control unit is communicatively coupled with the first PME, the second PME, and the power flow sensor, respectively. The control unit is configured to control flow of power through the first converter station between the DC link and the first AC region by means of generating a control signal for the first converter station. The control unit is configured to determine the control signal based on a power flow reference value, sensed power flow into the first converter from its AC side, and the first AC phase angle and the second AC phase angle, the first AC phase angle and the second AC phase angle being synchronized in time.

The control unit may alternatively be referred to as control and processing circuitry, or a control and processing unit. The control unit may for example include or be constituted by any suitable central processing unit (CPU), microcontroller, digital signal processor (DSP), Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA), etc., or any combination thereof. The control unit may optionally be capable of executing software instructions stored in a computer program product e.g. in the form of a memory. The memory may for example be any combination of read and write memory (RAM) and read only memory (ROM). The memory may comprise persistent storage, which for example can be a magnetic memory, an optical memory, a solid state memory or a remotely mounted memory, or any combination thereof.

According to a fourth aspect, there is provided a computer program product configured to be executed in a control unit according to the third aspect. The control unit is communicatively coupled with the first PME, the second PME, and the power flow sensor, respectively, comprised in a power transmission system according to the first aspect. The computer program product comprises computer-readable means carrying computer program code configured to, when executed in the control unit, cause the control unit to control flow of power through the first converter station between the DC link and the first AC region by means of generating a control signal for the first converter station. The control unit is configured to determine the control signal based on a power flow reference value, sensed power flow into the first converter from its AC side, and the first AC phase angle and the second AC phase angle, the first AC phase angle and the second AC phase angle being synchronized in time.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments. It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the description herein. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described herein. BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present invention will be described below with reference to the accompanying drawings.

Figure 1 is a schematic view of a power transmission system according to an embodiment of the present invention.

Figure 2 is a schematic block diagram illustrating a method according to an embodiment of the present invention.

Figure 3 is a schematic view block diagram illustrating a method in accordance with an embodiment of the present invention.

Figure 4 is a schematic view block diagram illustrating a method in accordance with an embodiment of the present invention.

Figure 5 is a schematic view block diagram illustrating a method in accordance with an embodiment of the present invention.

Figure 6 is a schematic view of a power transmission system according to an embodiment of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate embodiments of the present invention, wherein other parts may be omitted or merely suggested. DETAILED DESCRIPTION

The present invention will now be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments of the present invention set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the present invention to those skilled in the art.

Figure 1 is a schematic view of a power transmission system 100 according to an embodiment of the present invention. The power transmission system 100 comprises a first AC region 10 and a second AC region 20 which are interconnected by two AC power transmission lines 31, 32. It is to be understood that the number of AC power transmission lines 31, 32 illustrated in Figure is according to an example and that the number of AC power transmission lines could be more or less than illustrated in Figure 1. The power transmission system 100 comprises DC link 40, e.g., a HVDC link, that interconnects the first AC region 10 and the second AC region 20. In the following the DC link 40 will be referred to as a HVDC link 40. However, it is understood that the DC link 40 must not necessarily be a HVDC link.

The power transmission system 100 comprises first converter station 50 and a second converter station 60. Each of the first converter station 50 and the second converter station 60 is configured to convert DC power to AC power, or vice versa. In accordance with the illustrated embodiment of the present invention the DC link 40 is a HVDC link 40, and thus the first converter station 50 and the second converter station 60 may be HVDC converter stations. As illustrated in Figure 1, each of the first converter station 50 and the second converter station 60 may comprise an inverter configured to convert DC power to AC power, and a rectifier configured to convert AC power to DC power. It is to be understood that any one of the first converter station 50 and the second converter station 60 may comprise additional components, elements, or means that may be used in a converter station, or a power transmission system. Such additional components, elements, or means are not shown in Figure 1.

The first converter station 50 is connected to the HVDC link 40 at a DC side of the first converter station 50. The first converter station 50 is connected to the first AC region 10 at an AC side of the first converter station 50, for example via a transformer 43 and a point of common coupling 41 as illustrated in Figure 1. The first converter station 50 is configured to control flow of power through the first converter station 50 between the HVDC link 40 and the first AC region 10.

Likewise, the second converter station 60 may be connected to the HVDC link 40 at a DC side of the second converter station 60. The second converter station 60 may be connected to the second AC region 20 at an AC side of the second converter station 60, for example via a transformer 44 and a point of common coupling 42 as illustrated in Figure 1. The second converter station 60 may be configured to control flow of power through the second converter station 60 between the HVDC link 40 and the second AC region 20.

The operating mode of the first converter station 50 may be (active) power flow control mode, wherein the power flow through the first converter station 50 between the first AC region 10 and the HVDC link 40 is controlled so that the power flow corresponds to, or comes closer to corresponding to, a power flow reference value or range for the first converter station 50. The operating mode of the second converter station 60 may be DC voltage control mode, wherein the second converter station 60 may control the DC voltage of the second converter station 60 such that the DC voltage is equal to a set-point or voltage reference value for the second converter station 60 or that is within a voltage reference range for the second converter station 60, and wherein the first converter station 50 adjusts the DC voltage of the first converter station 50 such that the power flow between the first AC region 10 and the HVDC link 40 corresponds to, or comes closer to corresponding to, a set-point or power flow reference value or range for the first converter station 50. Alternatively, the second converter station 60 may be in (active) power flow control mode and the first converter station 50 may be in voltage control mode.

The power transmission system 100 comprises a first phasor measurement entity (PME) 11 configured to measure one or more AC phasors in the first AC region 10, wherein each AC phasor comprises an AC phase angle. The power transmission system 100 comprises a second PME 21 configured to measure one or more AC phasors in the second AC region 20, wherein each AC phasor comprises an AC phase angle. Each of the first PME 11 and the second PME 21 may for example be configured to measure one or more AC voltage phasors in the first AC region 10 and the second AC region 20, respectively, wherein each AC voltage phasor comprises an AC voltage phase angle.

The first PME 11 and the second PME 21 employ a common time source (e.g., a GPS-based time source) such that the AC phasors in the first AC region 10 measured by the first PME 11 and the AC phasors in the second AC region 20 measured by the second PME 21 are synchronized in time. For example, each of the first PME 11 and the second PME 21 may be provided with an internal clock or clock unit (e.g., a GPS radio clock) which are synchronized with the common time source (e.g., a GPS-based time source). There may possibly be more than one PME in the first AC region 10 and/or more than one PME in the second AC region 20.

The power transmission system 100 comprises a power flow sensor configured to sense power flow into the first converter station 40 from its AC side. In accordance with the embodiment of the present invention illustrated in Figure 1, the power flow sensor is a part of the first converter station 50. Another way to describe this is that the first converter station 50 may be configured so as to be capable of sensing power flow into the first converter station 40 from its AC side. However, the power flow sensor could in alternative be realized as a component that is separate from the first converter station 50. The power flow sensor may for example be configured to sense power flow into the first converter station 40 from its AC side at a location between the transformer 43 and the point of common coupling 41 (e.g., by means of one or more so called measurement transformers).

The power transmission system 100 comprises a control unit 70 that is communicatively coupled with the first PME 11, the second PME 21, and the power flow sensor (or the first converter station 50), respectively.

For example, as illustrated in Figure 1, the control unit 70, the first PME 11, the second PME 21, and the first converter station 50 (or the power flow sensor therein) may each comprise an antenna 71, 12, 22 and 51, respectively. Each or any one of the antennas 71, 12, 22 and 52 may for example comprise a radio frequency (RF) antenna. It is however understood that this according to an example. The communicative coupling of the control unit 70 with the first PME 11, the second PME 21, and the power flow sensor, respectively, may be realized or implemented utilizing one or more wireless and/or non-wireless ('wired') communication means or techniques, for example one or more wireless communication means or techniques such as, for example, RF communication, or free-space optical communication (e.g., based on laser), and/or one or more non-wireless communication means or techniques such as, for example, employing at least one optical waveguide, or optical transmission line (e.g., an optical fiber), and/or at least one electrical conductor (e.g., a cable or wire, e.g., a copper conductor or cable, or copper wire).

The second converter station 60 (or a power flow sensor therein) may comprise an antenna 61, as illustrated in Figure 1. There may possibly be provided a control unit (not shown in Figure 1) associated with the second converter station 60, which control unit may be similar to the control unit 70 and may be communicatively coupled with the second converter station 60 and possibly with the first PME 11 and the second PME and a power flow sensor which may be configured to sense power flow into the second converter station 60 from its AC side. The control unit associated with the second converter station 60 may be configured to control operation of the second converter station 60 similarly to how the control unit 70 is configured to control operation of the first converter station 50 as will be described in the following with further reference to Figure 1 and the other figures.

By the communicative coupling of the control unit 70 with the first PME 11, the second PME 21, and the power flow sensor, respectively, information, messages, commands and/or data, etc., may be communicated at least from the first PME 11, the second PME 21, and the power flow sensor, respectively, to the control unit 70. Possibly, the communicative coupling of the control unit 70 with the first PME 11, the second PME 21, and the power flow sensor, respectively, may provide not only for unidirectional communication but may provide for bidirectional communication between the control unit 70 and the first PME 11, between the control unit 70 and the second PME 21, and/or between the control unit 70 and the power flow sensor.

The control unit 70 - and also possibly the control unit associated with the second converter station 60 - may be provided with an internal clock or clock unit (e.g., a GPS radio clock) which are synchronized with the common time source (e.g., a GPS-based time source).

By means of the first PME 11 and the second PME 21, time-stamped AC phasors - e.g., AC voltage phasors - may be generated, which may correspond to the same or substantially the same instant in time. Thus, the first PME 11 and the second PME 21 may employ the common time source such that the AC phasors in the first AC region 10 measured by the first PME 11 and the respective ones of the AC phasors in the second AC region 20 measured by the second PME 21 correspond to the same, or substantially the same, time instants. The time-stamped AC voltage phasors generated by the first PME 11 may be denoted Vie" ⁹¹ , and the time-stamped AC voltage phasors generated by the second PME 21 may be denoted Vie" ⁹² , where e is the number whose natural logarithm is equal to one, j is an imaginary number, Vi and θι are the magnitude of AC voltage and AC phase angle, respectively, measured in the first AC region 10, and V ₂ and θ ₂ are the magnitude of AC voltage and AC phase angle, respectively, measured in the second AC region 20. Since the time-stamped AC voltage phasors generated by the first PME 11 Vie" ⁹¹ and the time-stamped AC voltage phasors generated by the second PME 21 Vie" ⁹² have the same global time reference, the AC voltage phasors generated by the first PME 11 Vie" ⁹¹ and the AC voltage phasors generated by the second PME 21 Vie" ⁹² will have a common time reference for the AC phase angles θι and θ ₂ .

Although the control unit 70 is illustrated in Figure 1 as a component separate from the first converter station 50, it is to be understood that the control unit 70 may be a part of the first converter station 50. In case the control unit 70 is separate from the first converter station 50, such as illustrated in Figure 1, the control unit 70 may be communicatively coupled with the first converter station 50 similarly to the communicative coupling of the control unit 70 with the first PME 11, the second PME 21, and the power flow sensor, respectively, as described in the foregoing.

The control unit 70 is configured to determine a first AC phase angle θι based on the one or more AC phasors in the first AC region 10 measured by the first PME 11 and a second AC phase angle θ ₂ based on the one or more AC phasors in the second AC region 20 measured by the second PME 21, wherein the first AC phase angle θι and the second AC phase angle θ ₂ are synchronized in time.

As illustrated in Figure 1, the power transmission system 100 may comprise an additional first PME 13 (or several additional first PMEs) configured to measure one or more AC phasors in the first AC region 10, wherein each AC phasor comprises an AC phase angle. The power transmission system 100 may further comprise an additional second PME 23 (or several additional second PMEs) configured to measure one or more AC phasors in the second AC region 20, wherein each AC phasor comprises an AC phase angle. Each of the first PME 13 and the second PME 23 may for example be configured to measure one or more AC voltage phasors in the first AC region 10 and the second AC region 20, respectively, wherein each AC voltage phasor comprises an AC voltage phase angle. As indicated in Figure 1, the first PMEs 11 and 13 may be configured to measure one or more AC phasors in the first AC region 10 at different locations, or in different components, in the first AC region 10. Similarly, the second PMEs 21 and 23 may be configured to measure one or more AC phasors in the second AC region 20 at different locations, or in different components, in the second AC region 20. The control unit 70 may be configured to determine the first AC phase angle θι by averaging the AC phasors in the first AC region 10 measured by the respective ones of the first PMEs 11 and 13, and to determine the second AC phase angle θ ₂ by averaging the AC phasors in the second AC region 20 measured by the respective ones of the second PMEs 21 and 23, the first AC phase angle θι and the second AC phase angle θ ₂ being synchronized in time. Thus, the first AC phase angle θι may be determined by averaging AC phasors in the first AC region 10 corresponding to the same time instant or period of time, measured by the respective ones of the first PMEs 11 and 13. And similarly, the second AC phase angle θ ₂ may be determined by averaging AC phasors in the second AC region 20 corresponding to the same time instant or period of time, measured by the respective ones of the second PMEs 21 and 23.

The control unit 70 is configured to control flow of power through the first converter station 50 between the HVDC link 40 and the first AC region 10 by means of generating a control signal for the first converter station 50, and possibly transmitting the control signal to the first converter station 50. The control unit 70 is configured to determine the control signal based on a power flow reference value, sensed power flow into the first converter 50 from its AC side, and the first AC phase angle θι and the second AC phase angle θ ₂ .

The control unit 70 may be configured to determine the control signal based on a difference between the second AC phase angle θι and the first AC phase angle θ ₂ . The control unit 70 may be configured to determine a plurality of values for a difference between the second AC phase angle θι and the first AC phase angle θ ₂ based on the AC phasors in the first AC region 10 measured by the respective ones of the first PMEs 11 and 13 and the AC phasors in the second AC region 20 measured by the respective ones of the second PMEs 21 and 23. The control unit 70 may then determine an average value of the plurality of values for a difference between the second AC phase angle θι and the first AC phase angle θ ₂ , based on which average value the control signal may then be determined. For example, the control unit 70 may be configured to determine a first difference between the second AC phase angle θι and the first AC phase angle θ ₂ based on the AC phasors in the first AC region 10 measured by the first PME 11 and the AC phasors in the second AC region 20 measured by the second PME 21, and to determine a second difference between the second AC phase angle θι and the first AC phase angle θ ₂ based on the AC phasors in the first AC region 10 measured by the first PME 13 and the AC phasors in the second AC region 20 measured by the second PME 23. The control unit 70 may then determine an average value of the first difference and the second difference, and based on the determined average value, the control unit 70 may then determine the control signal. Figure 2 is a schematic block diagram illustrating a method 150 according to an embodiment of the present invention, which method 150 for example may be carried out in the power transmission system 100 illustrated in Figure 1. The method 150 illustrated in Figure 2 will be described with reference to the power transmission system 100 illustrated in Figure 1, but it is to be understood that the method 150 may alternatively be carried out in a power transmission system according to another embodiment of the present invention.

At block 101, one or more AC phasors in the first AC region 10 are measured (e.g., by the first PME 11), each AC phasor comprising an AC phase angle.

At block 102, one or more AC phasors in the second AC region 20 are measured (e.g., by the second PME 21), each AC phasor comprising an AC phase angle. The measuring of the one or more AC phasors in the first AC region 10 and the measuring of the one or more AC phasors in the second AC region 20 may comprise employing a common time source (e.g., a GPS-based time source) such that the measured AC phasors in the first AC region 10 and the measured AC phasors in the second AC region 20 are synchronized in time.

At block 103, power flow into the first converter station 50 from its AC side is sensed. As described in the foregoing with reference to Figure 1, a power flow sensor may be comprised in the first converter station 50, and hence be a part of the first converter station 50. However, the power flow sensor could in alternative be realized as a component that is separate from the first converter station 50. As described in the foregoing, the power flow that is sensed may be active power flow and/or reactive power flow.

At block 104, flow of power through the first converter station 50 between the HVDC link 40 and the first AC region 10 is controlled by means of generating a control signal I _or der for the first converter station 50. The control signal I _or der may for example comprise or be constituted by a converter valve current order, e.g., in d-axis direction (or in q- axis direction), of one of more converter valves of the first converter station 50. The control signal er for controlling flow of power (reactive and/or active) through the first converter station 50 between the HVDC link 40 and the first AC region 10 is generated based on AC phasor measurements from the first AC region 10, obtained at block 101, and AC phasor measurements from the second AC region 20, obtained at block 102. The control signal er may be generated based on a difference between a power flow reference value and sensed power flow into the first converter station 50 from its AC side, obtained at block 103, and a difference between a first AC phase angle θι, resulting from AC phasor measurements from the first AC region 10, obtained at block 101, and a second AC phase angle θ ₂ , resulting from AC phasor measurements from the second AC region 20, obtained at block 102. The first AC phase angle θι is determined based on the one or more AC phasors in the first AC region 10 obtained at block 101, and the second AC phase angle θ ₂ is determined based on the one or more AC phasors in the second AC region 20 obtained at block 102, wherein the first AC phase angle θι and the second AC phase angle θ ₂ are synchronized in time. Possibly, the first AC phase angle θι may be determined by averaging the AC phase angles of a plurality of AC phasors in the first AC region 10 obtained at block 101, and the second AC phase angle θ ₂ may be determined by averaging the AC phase angles of a plurality of AC phasors in the second AC region 20 obtained at block 102. The plurality of AC phasors in the first AC region 10 obtained at block 101 may have been measured at different locations or in different components in the first AC region 10 at the same time instant or during a same time period, and the plurality of AC phasors in the second AC region 20 obtained at block 102 may have been measured at different locations or in different components in the first AC region 20 at the same time instant or during a same time period. According to one or more embodiments of the present invention, time-stamped AC voltage phasors generated by the first PME 11, Vie" ⁹¹ , are obtained at block 101, and time-stamped AC voltage phasors generated by the second PME 21, Vie" ⁹² , are obtained at block 102. The time-stamped AC voltage phasors Vie" ⁹¹ and the time-stamped AC voltage phasors Vie" ⁹² are time-aligned at block 105 such that AC voltage phasors corresponding to the same, or substantially the same, time instants may subsequently be employed at block 106. At block 106, an AC phase angle difference ΔΘ = θι - θ ₂ is determined, wherein, as indicated in the foregoing, the AC phase angles θι and θ ₂ may correspond to the same, or substantially the same, time instants (or periods of time). Also at block 106, the time delays ΔΤ between the time instants when the time-stamped AC voltage phasors and V ₂ e> ⁹² were measured (e.g., by the first PME 11 and the second PME 21, respectively) and the time instants when the respective AC voltage phasors Vie" ⁹¹ and V ₂ e> ⁹² were received at block 106 (e.g., the time instants when the respective AC voltage phasors Vie" ⁹¹ and V ₂ e> ⁹² were received by the control unit 70) may be determined. The determination of the time delays may utilize the time provided by a local clock or clock unit at block 107 (e.g., a clock or clock unit comprised in the control unit 70), which possibly may be synchronized with the common time source (e.g., a GPS-based time source) using information provided by the common time source at block 108.

The AC phase angle difference ΔΘ = θι - θ ₂ determined at block 106 is used in determining the control signal I _or der at block 109, as will be further described in the following with reference to Figure 3.

The control signal er may be determined at block 109 provided that the time delay ΔΤ determined at block 106 does not exceed a threshold value, which is checked at block 110.

If the time delay ΔΤ determined at block 106 does exceed the threshold value, which is checked at block 110, a control signal Iorder, fallback may be determined at block 111 instead of the control signal I _or der determined at block 109. The control signal I _or der, fallback determined at block 111 could for example be determined by means of sensed power flow into the first converter station 50 from its AC side together with set-points, or reference values, therefor as inputs in active power/DC voltage control or reactive power/ AC voltage for the first converter station 50 such as described in the foregoing background section. Thus, the control signal Iorder, fallback may, similarly to the control signal Iorder, comprise or be constituted by a converter valve current order, e.g., in d-axis direction, of one of more converter valves of the first converter station 50, but may be determined in a way different from the control signal Iorder .

Figure 3 is a schematic view block diagram illustrating a method in accordance with an embodiment of the present invention. The method, or algorithm, illustrated in Figure 3 is for determining the control signal I _or der at block 109 in the method illustrated in Figure 2. At block 201 , a difference between a power flow reference value P _re f and sensed power flow, Pmeas, into the first converter station 50 from its AC side, obtained at block 103 in the method illustrated in Figure 2, may be determined.

At block 202, a difference between the AC phase angle difference ΔΘ = θι - θ ₂ , determined at block 106 in the method illustrated in Figure 2, and a reference value A9 _re f for the AC phase angle difference may be determined.

The difference between the power flow reference value P _re f and the sensed power flow Pmeas may be multiplied by a power control parameter D. The power control parameter may be a predefined, or constant, value. According to one or more embodiments of the present invention, the power control parameter D may however be adaptive. For example, the power control parameter D may according to one or more embodiments of the present invention be determined based on ΔΘ. According to one or more embodiments of the present invention, the power control parameter D may for example be determined based on a difference between ΔΘ and A9 _re f.

At block 204, a summation of the output from block 202 and 203 is

determined, and the output from block 204 may be fed to a proportional integral controller block 205. The output from proportional integral controller block 205 may be the control signal I _or der. In alternative or in addition to a proportional integral controller, another or other types of controllers may be employed at block 205.

At steady state, when there is a zero input to the proportional integral controller block 205,

should hold. Put differently,

It follows that if ΔΘ increases, then so will also the power flow into the first converter station 40 from its AC side, Pmeas.

Figure 4 is a schematic view block diagram illustrating a method in accordance with an embodiment of the present invention. The method, or algorithm, illustrated in Figure

4 is for determining the control signal er at block 109 in the method illustrated in Figure 2.

The method, or algorithm, illustrated in Figure 4 is similar to the method illustrated in Figure

3, but differs from the method illustrated in Figure 3 in that a power control parameter K is multiplied with the output from block 202, and not with the output from block 201 , such as in the method illustrated in Figure 3 where the output from block 201 is multiplied with the power control parameter D. At steady state, when there is a zero input to the proportional integral controller block 205,

Pref - Pmea _S + (A9 - 0 should hold, with K = 1/D.

The method illustrated in Figure 2 may according to one or more embodiment be preceded by the followings steps. The first converter station 50 may initially be operated in accordance with active power/DC voltage controlling of first converter station 50 such as described in the foregoing background section. The desired active power transfer on the HVDC link 40 may be established by changing a power flow reference value for the first converter station 50, and controlling the power flow through the first converter station 50 between the first AC region 10 and the HVDC link 40 is so that the power flow corresponds to, or comes closer to corresponding to, the power flow reference value for the first converter station 50. Once the desired active power transfer on the HVDC link 40 has been established, ABref may be set to the current ΔΘ. Subsequently, the method illustrated in Figure 2 may be carried out.

Figure 5 is a schematic view block diagram illustrating a method in accordance with an embodiment of the present invention. The method, or algorithm, illustrated in Figure 5 is for determining the control signal I _or der at block 109 in the method illustrated in Figure 2. The method, or algorithm, illustrated in Figure 5 is similar to the method illustrated in Figure 4, but differs from the method illustrated in Figure 4 in that at block 201 , a difference between a reactive power flow reference value Q _re f and sensed reactive power flow, Qmeas, in the first converter station 50 from its AC side, which may be obtained at block 103 in the method illustrated in Figure 2, may be determined. As described in the foregoing, the power flow that is sensed may be active power flow and/or reactive power flow. At steady state, when there is a zero input to the proportional integral controller block 205,

Qref - Qmeas + (ΔΘ - 0 should hold. Possibly, instead of the output from the block 202 being multiplied with the power control parameter K, the output from the block 201 could be multiplied with a power control parameter D, similar to in the method illustrated in Figure 3. By considering the reactive power flow in the method or algorithm such as described above, an increase in the total power transfer via the DC link and the AC power transmission line(s) may be achieved for example during an AC system fault (e.g., in one or both of the first AC region and the second AC region) and if the DC link is experiencing a high load (e.g., more than 80% of the rated power transfer). During such circumstances it may not be possible to increase active power transmission on the DC link due to current limitations of the first converter station(s). By injecting reactive power to the AC system, e.g., the first AC region, the AC voltage level at the first converter station may be increased, which may facilitate or allow for an increase in the total power transfer via the DC link and the AC power

transmission line(s).

The power transmission system 100 may possibly comprise more than one

HVDC link (or generally, more than one DC link), wherein each HVDC link interconnects the first AC region 10 and the second AC region 20. Figure 6 is a schematic view of a power transmission system 100 according to an embodiment of the present invention which illustrates this case. The power transmission system 100 illustrated in Figure 6 is similar to the power transmission system 100 illustrated in Figure 1, and the same reference numerals in

Figures 1 and 6 indicate the same or similar components, having the same or similar function. The power transmission system 100 illustrated in Figure 6 differs from the power

transmission system 100 illustrated in Figure 1 in that the power transmission system 100 illustrated in Figure 6 comprises an additional HVDC link 40. The two HVDC links 40 may be substantially parallel. The two HVDC links 40 must not necessarily be connected to the same locations in the respective ones of the first AC region 10 and second AC region 20. For each of the two HVDC links 40, the control flow of power through the first converter station 50 between the HVDC link 40 and the first AC region 10 may be controlled in accordance with a method according to an embodiment of the present invention, for example such as described in the foregoing with reference to Figures 2 to 5. The power control parameter D which may be used in the control flow of power through the first converter station 50 between the HVDC link 40 and the first AC region 10 for the respective ones of the two HVDC links 40 may be the same or different.

In conclusion, it is disclosed a power transmission system comprising at least a first AC region and a second AC region interconnected by at least one AC power transmission line, a DC link interconnecting the first AC region and the second AC region, and at least a first converter station configured to control flow of power therethrough between the DC link and the first AC region. A control unit is configured to control flow of power through the first converter station between the DC link and the first AC region by generating a control signal based on PME measurements in or from the first AC region and in or from the second AC region, respectively, in particular based on a comparison or difference between a second AC phase angle, determined based on one or more AC phasors measured in the second AC region, and a first AC phase angle, determined based on one or more AC phasors measured in the first AC region.

While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.