Power or voltage oscillation damping in a power transmission system

A method, device and computer program product for providing improved control of power or voltage oscillation damping in a power transmission system. The device includes a magnitude obtaining element configured to obtain an instantaneous magnitude of a signal representing a deviating oscillation in at least one element of the power transmission system, a slope investigating element configured to determine the rate of change of the signal, and a first processing block including an integrating element configured to integrate the instantaneous magnitudes with an integrating factor that is based on the determined rate of change. The first processing block is further configured to form a phase compensation angle based on the integrated instantaneous magnitude for use in a damping control signal generating unit in order to provide power or voltage oscillation damping of the system.

FIELD OF THE INVENTION

The invention relates to the field of power or voltage oscillation damping in electric power transmission systems. The invention more particularly concerns a method, device and computer program product for providing improved control of power or voltage oscillation damping in a power transmission system.

BACKGROUND OF THE INVENTION

Inter-area modes of oscillation are typically characterised by a group of machines in one geographical area of a power transmission system swinging against a group of machines in another geographical area of the system. Inter-area modes of oscillation is for instance described in CN 101202451, U.S. Pat. No. 6,252,753 and EP 1852952.

These oscillations are initiated by e.g. normal changes in the system load or switching events in the system possibly following faults. These oscillations may typically have a frequency of less than a few Hz, for instance in the range of 0.1-0.8 Hz, and are often considered acceptable as long as they decay fast enough. Insufficiently damped oscillations may occur when the operating point of the power system is changed, for example, due to a new distribution of power flows following a connection or disconnection of generators, loads and/or transmission lines. In these cases, an increase in the transmitted power of a few MW may make the difference between stable oscillations and unstable oscillations which have the potential to cause a system collapse or result in loss of synchronism, loss of interconnections and ultimately the inability to supply electric power to customers. Appropriate monitoring and control of the power transmission system can help a network operator to accurately assess power transmission system states and avoid a total blackout by taking appropriate actions such as the connection of specially designed oscillation damping equipment.

The conventional way to perform Power Oscillation Damping (POD) is by adding a modulation signal to the control signal of an actuator which counteracts the power oscillation. Typical actuators which could be controlled to perform such damping include synchronous generators, HVDC and FACTS installations.

There are different ways in which such oscillations can be dampened. One way is through using a POD device that employs lead-lag compensation.

Another way in which power oscillation damping can be performed is through the use of phasor based damping in a so-called phasor POD. A phasor POD is described in U.S. Pat. No. 6,559,561. In a phasor POD an auxiliary signal is provided to a power flow controller or voltage controller for actuators in the power transmission system in order to damp such oscillation. The phasor POD uses a scheme which expresses the active power, voltage or current oscillation in a rotating coordinate system and control action is synthesized in another phasor form to counteract the root oscillation. In this process, the knowledge of the system oscillation frequency, optimal phase difference of control signal with respect to measured signal and appropriate gain is needed a-priori. Thus with this technique an appropriate compensation angle needs to be known a-priori for each operating condition.

However there is a problem associated with using a fixed phase compensation angle in the above-described way. The configuration of a power transmission system may change, for instance because of a line outage following a severe fault. This means that different phase angles may be needed for different conditions. In order to provide efficient damping after a fault it may then be necessary to determine the operating condition after the fault. There is today no existing technique for determining such a post-disturbance operating condition.

There is therefore a need for improvement in this field of technology.

SUMMARY OF THE INVENTION

It is therefore an objective of the invention to enable phasor based power or voltage oscillation damping that can be applied without determining post-disturbance operating conditions. This objective is achieved by a method and a device for providing improved control of power or voltage oscillation damping in a power transmission system and a computer program product for providing improved control of power or voltage oscillation damping in a power transmission system. Further preferred embodiments are evident from the present teachings.

According to a first aspect of the invention, a method is provided for improved control of power or voltage oscillation damping in a power transmission system comprising the steps of: obtaining an instantaneous magnitude of a signal representing a deviating oscillation in at least one element of the power transmission system, determining the rate of change of the signal, integrating the instantaneous magnitude with an integrating factor that is based on the determined rate of change, and forming a phase compensation angle based on the integrated instantaneous magnitude for use in a damping control signal generating unit in order to provide power or voltage oscillation damping of the system.

According to a second aspect of the present invention a device for providing improved control of power or voltage oscillation damping in a power transmission system is provided. The power or voltage control device comprises a magnitude obtaining element configured to obtain an instantaneous magnitude of a signal representing a deviating oscillation in at least one element of the power transmission system, a slope investigating element configured to determine the rate of change of the signal, and a first processing block comprising an integrating element configured to integrate the instantaneous magnitudes with an integrating factor that is based on the determined rate of change, said first processing block being further configured to form a phase compensation angle based on the integrated instantaneous magnitude for use in a damping control signal generating unit in order to provide power or voltage oscillation damping of the system.

According to a third aspect of the present invention there is provided a computer program for providing improved control of power or voltage oscillation damping in a power transmission system. The computer program is loadable into an internal memory of a device for power or voltage oscillation damping and comprises computer program code means to make the device, when the program is loaded in the internal memory, obtain an instantaneous magnitude of a signal representing a deviating oscillation in at least one element of the power transmission system, determine the rate of change of the signal, integrate the instantaneous magnitude with an integrating factor that is based on the determined rate of change, and form a phase compensation angle based on the integrated instantaneous magnitude for use in a damping control signal generating unit in order to provide power or voltage oscillation damping of the system.

The invention according to these aspects enables canceling out of oscillations using an adaptively changed phase compensation angle. This can be implemented without prior knowledge of a post-fault operating condition. The invention is furthermore flexible in that any measurement signal that have high observability of the oscillation can be used without any significant modification. This also means that the invention can be used in relation to both local and wide area damping. This is made possible because the adaptive generation of phase angle compensation automatically considers different requirements arising from using different measurement signals. In this way duplicated phasor POD devices for local and wide area power or voltage oscillation damping can be avoided.

In one variation, integrating is performed on the instantaneous magnitude having a first polarity as well as on the instantaneous magnitude having an opposite polarity in parallel with the first polarity, the direction in rate of change of the signal is investigated and switching is made to forming phase compensation angles based on the integrated instantaneous magnitude of the opposite polarity in case the direction in rate of change has not been changed within a first time limit.

In another variation the instantaneous magnitude is multiplied with a proportional factor, and the forming of phase compensation angles is also based on the multiplied instantaneous magnitude.

The deviating oscillation will have a magnitude and it is possible to initially set the proportional factor according to this magnitude.

The rate of change of the signal can also be used for tuning of the proportional factor. The factor can for instance be reduced in case the direction in rate of change has not been changed within a second time limit. The rate of change of the signal can also be used for disabling the generation of a damping control signal in case it has not been changed within a third time limit.

The determining of a phase compensation angle according to the principles of the invention allows the provision of phasor based power or voltage oscillation damping on both locally measured and remotely measured signals.

According to another variation a system operation reflecting signal corresponding to a power property of the at least one system element is obtained and at least one signal representing a deviating oscillation is generated based on the system operation reflecting signal.

This system operation reflecting signal may be multimodal, in which case each mode component can then be extracted from this signal, a signal representing a deviating oscillation can be generated for each mode and phase compensation angles formed for each mode. Each such phase compensation angle may then be supplied to a corresponding damping control signal generating unit provided for the mode. For single actuating devices such as AVR, PSS or FACTS devices in the system, the final damping control signal is obtained by summing all the individual damping control signals from individual damping control units. For multiple actuating devices, if one actuating device is used for damping one particular mode, the corresponding damping control signal from the corresponding damping control signal generation unit is provided to the respective actuating device.

According to another variation the damping signal is formed with an amplitude corresponding to the signal magnitude and a phase corresponding to the phase adjustment angle.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically shows a simplified power transmissions system in which a device for providing improved control of power or voltage oscillation damping according to a first embodiment of the invention is provided. The power transmission system is preferably an AC power transmission system and then operating at a network frequency such as 50 or 60 Hz.

The power transmission system includes a number of geographical areas, which are here two areas A_I and A_II. These areas are indicated through dotted ellipses and are typically provided on great distances from each other, where one may as an example be provided in the south of Finland and another in the south of Norway. A geographical area is here a coherent area. A coherent area is an area where a group of electrical machines, such as synchronous generators, are moving coherently, i.e. they are oscillating together. Such an area may also be considered as an electrical area, because the machines are close to each other in an electrical sense. In these geographical areas there are high-voltage tie lines for connecting geographically separated regions, medium-voltage lines, substations for transforming voltages and switching connections between lines as well as various buses in the local areas. In order to simplify the description of the present invention the first geographical area A_I is here shown only including one first power line12or power transmission bus, one first electrical machine10and one actuating device11(FACTS, HVDC, PSS etc.) and the second geographical area A_II is only shown including one second power line16or power transmission bus and one second electrical machine14. That these geographical areas are a part of the same system is indicated through a dashed line joining the two power lines12and16to each other.

This means that in this simplified example the first geographical area A_I only includes the first machine10, while the second geographical area A_II only includes the second machine14, which thus swing against each other. The actuating device11is in this example a device that can be influenced or controlled for removing such swings. In this first example the actuating device (FACTS, HVDC, PSS etc.)11is therefore an actuator, which is controlled through the use of the device for providing improved control of power or voltage oscillation damping of the present invention.

This first embodiment of the invention is directed towards a phasor based power oscillation damping device, a phasor POD device19, which includes a phasor POD unit20and a compensation angle determining unit26. The phasor POD unit20is here operating on local measurements, i.e. on measurements locally in the first geographical area and performing control also in this area. As will be apparent from other embodiments of the invention, the invention can also be applied on wide area power or voltage oscillation damping.

In order to provide local power or voltage oscillation damping there is here a first measurement unit18connected to the first power line. This measurement unit is in turn connected to an oscillation estimating unit22, which oscillation estimating unit22is connected to a damping control signal generating unit24. The oscillation estimating unit22together with the damping control signal generating unit24do in this first embodiment together make up the phasor POD unit20as described in U.S. Pat. No. 6,559,561, which document is herein incorporated by reference.

Both the oscillation estimating unit22and the damping control signal generating unit24are connected to the compensation angle determining unit26as well as to a gain scheduling unit28. The compensation angle determining unit26is also connected to the gain scheduling unit28, which gain scheduling unit is connected to and controls an actuator.

A device for providing improved control of power or voltage oscillation damping is according to the first embodiment of the invention solely made up of the compensation angle determining unit26. However, it should be realized that a device for providing improved control of power or voltage oscillation damping may according to the principles of the invention be provided through any combination of this compensation angle determining unit26with the other units of the phasor POD device19, i.e. the oscillation estimating unit22and damping control signal generating unit24. It is furthermore possible to also include the gain scheduling unit28in any of these variations of a device for providing improved control of power or voltage oscillation damping according to the invention.

FIG. 2schematically shows a block schematic of the compensation angle determining unit26. The compensation angle determining unit26includes a magnitude obtaining element in the form of a normalizing element32, which receives a signal |ΔP|. There is also a peak detecting element30that also receives this signal |ΔP| and is connected to a tuning terminal of the normalizing element32. The normalizing element32is in turn connected to a first processing block34, to a first integration factor providing element50, and to an inverter element54. Also the first integration factor providing element50is connected to the first processing block. The inverter element54is in turn connected to a second integration factor providing element52and to a second processing block42, where the second integration factor providing element52is connected to the second processing block42. The processing blocks34and42here perform proportional and integrating processing activities and are therefore PI blocks in this first embodiment.

Each PI block34and42includes an integrator element38and44being connected between the corresponding integration factor providing element50and52and a summing element40and48. Each PI block also includes a proportional amplifying element36and46with one end connected to the corresponding summing element40and48. The proportional amplifying element36of the first PI block34is here at the other end connected to the normalizing element32, while the proportional amplifying element46of the second PI block42is connected to the inverter element54. The summing element40of the first PI block34is connected to a first input terminal of a switching element56, while the summing element48of the second PI block42is connected to a second input terminal of the switching element56, which switching element is provided with one output terminal providing a phase angle φC. This output terminal is in this embodiment also a phase compensation angle output.

In the compensation angle determining unit26there is also a slope investigating element58that also receives the signal |ΔP|. The slope investigating element58is connected to an adjustment direction control element62, which adjustment direction control element62is connected to a control terminal of the switching element56for controlling which input terminal is to be connected to the output terminal. The slope investigating element58is also connected to an integration factor tuning element60.

The first embodiment of the invention will now be described in more detail with reference being made toFIGS. 1 and 2together withFIG. 3, which schematically shows a flow chart outlining a number of method steps being performed in a method according to the first embodiment of the invention.

As mentioned above, the machines10and14in the first and the second geographical areas A_I and A_II swing against each other, which typically takes place after a fault or a disturbance has occurred. This swing is normally a low frequency swing as compared to the operating frequency of the system.

In order to be able to counteract this swinging the measurement unit18provides measurements, typically voltage or current phasor measurements from a system element that is here the first power line12to the oscillation estimating unit22of the phasor POD unit20. These measurements do in this first embodiment make up a system operation reflecting signal P. A system operation reflecting signal is here a signal that reflects a measured power property of at least one system element. It here reflects the power property both in respect of the operating frequency, but also in respect of an oscillator component, i.e. a signal component that causes the swinging. In this first embodiment this system operation reflecting signal is directly made up of measurements, such as voltage measurements made by the measurement unit18. The oscillation estimating unit22then goes on and estimates the oscillatory component ΔP of the system operation reflecting signal, i.e. of these measurements. This oscillatory component ΔP is a deviating oscillation in the system element, i.e. an oscillation that deviates from the system steady state quantities. This component thus has a certain amplitude and frequency. The estimation may for instance be a recursive least-squares estimation (RLS) according to the principles described in U.S. Pat. No. 6,559,561. An amplitude A and phase φ of this oscillatory component is then provided to the damping signal providing unit24, which goes on and generates a damping control signal VPODthat is provided to the gain scheduling unit28. The phase and amplitude may here be pre-set to a default value, which can be any value or values set according to required gain and phase for operating conditions existing before the disturbance. The damping control signal may be generated based on the magnitude of the oscillatory component according to the principles described in U.S. Pat. No. 6,559,561. The oscillation estimating unit22furthermore provides a signal |ΔP| representing the oscillatory component in the form of the absolute value of this component together with a possible average level of this sensed signal to the gain scheduling unit28. It also provides the absolute value of this component to the compensation angle determining unit26.

The oscillatory component ΔP is typically the deviation of power flow or deviation of voltage or deviation of current in the system element from what is expected and typically includes single or multiple damped sinusoidal components. This signal normally appears because of a change of the operating condition of the system. This means that the signal |ΔP| varies between a maximum value and zero, which maximum value is thus detected. As the signal |ΔP| is first emitted from the oscillation estimating unit22, a counter being handled by the adjustment direction control element62is started, step64. The appearance of the signal |ΔP| furthermore causes a selection of an active PI block to be made in the compensation angle determining unit26, for instance by the adjustment direction control element62, step66. This is a selection of which PI block that is to provide phase compensation angles. This selection may with advantage be a default selection and then a default selection of the first PI block34. The selection ensures, through the element62, that the switching element56connects the first PI block34with the adjustment angle output. Typically the compensation angle determining unit26will be activated for ordinary operation within one cycle of the oscillation. The peak detecting element30is provided in the compensation angle determining unit26in order to obtain the instantaneous magnitude |ΔP|por peak of this signal |ΔP|. The first such peak detected is thus here the first peak after one cycle of oscillation. An instantaneous peak magnitude is thus obtained as a peak value of the absolute value of the deviation ΔP. The first detected instantaneous peak magnitude is furthermore provided to the normalization unit32, which uses this instantaneous peak magnitude to set a range between one and zero to which the detected values are being normalized. The instantaneous peak magnitude |ΔP|pof the oscillatory component may be obtained through detecting the peak or RMS (root mean square) of the signal |ΔP| by the peak detecting element30. Thereafter the signal |ΔP| is continued to be obtained or received, step68, and provided to the normalizing element32. The peak value is here thus used to normalize the signal |ΔP| by the normalizing element32, which normalized signal |ΔP| is then provided to the first integration factor providing element50, the first proportional amplifying element36and the inverter element54. The inverter element54inverts the normalized signal |ΔP| and provides this normalized inverted signal −|ΔP| to the second integration factor providing element52and the second proportional amplifying element46of the second PI block42. As this is done the rate of change d|ΔP|/dt of the signal |ΔP| is detected by the slope investigating element58, step70, This element also forms an absolute value |d|ΔP|/dt| of the rate of change or slope. Both these values d|ΔP|/dt and |d|ΔP|/dt I are provided to the integration factor tuning element60, which in turn uses them for influencing the setting of an integrating factor Ki in the two integration factor providing elements50and52. The integration factor providing elements50and52in turn applies this factor Ki in the integrating elements38and44. The integrating elements thus perform integration using the integrating factor Ki. The two versions of the signal |ΔP|, which are a normalized positive instantaneous magnitude and a negative instantaneous magnitude, are thus supplied to the two PI blocks in parallel, which perform a PI activity on the absolute (positive) instantaneous magnitude of the signal |ΔP| and inverted absolute (negative) instantaneous magnitude of the signal |ΔP|, step72.

In the first embodiment this means that the instantaneous magnitude and inverted instantaneous magnitude is multiplied with a proportional factor Kp in elements36and46and integrated with the integrating factor Ki in the integrating elements38and44. In this first embodiment the proportional factor Kp is stationary or fixed, while the integrating factor is variable, i.e. it is varied based on the rate of change d|ΔP|/dt or slope of the signal. Typically a high value, i.e. a steep slope leads to a high factor being used while a low value or a small slope leads to a low factor being used. This means that the instantaneous magnitude is integrated with an integrating factor Ki that is based on the determined rate of change. The proportionally controlled and integrated instantaneous magnitude and inverted instantaneous magnitude are then in each PI block combined in a corresponding summing element40and48for forming two candidate compensation angles, where the result of the combination made based on the positive instantaneous magnitude is originally provided as a phase compensation angle φCvia the switching element56output and the combination based on the negative instantaneous magnitude for is initially not used. Thus a compensation angle φCis formed based on the instantaneous magnitude of the signal |ΔP| using the active PI block, which is here the first PI block, step74.

The compensation angle φCis then provided to the damping control signal generating unit24of the phasor POD unit20, which goes on and uses this phase compensation angle φCin the forming of the damping control signal VPOD. This damping control signal VPODis then supplied to the scheduling unit28as is the average value Pavg, and the absolute value of the deviating oscillation |ΔP|. The compensation angle determining unit26here also supplies the peak value |ΔP|pof the oscillation and the slope values d|ΔP|/dt to this scheduling unit28. This data is then used together with knowledge of the structure of the actuator used for generating a specific actuator control signal used to control the actuator10. The damping control signal VPODis then with advantage a modulation signal, which can be added to a control signal generated by gain scheduling unit28for controlling the actuator11. Phase adjustment angles are thereafter continued to be determined in the same way for later detected values of the signal |ΔP|.

In this way the oscillation is canceled out with the help of an adaptively changed phase compensation angle. Through the use of a phase compensation angle generated in this way the magnitude of the absolute oscillation will then be reduced to zero. This means that as the absolute oscillation magnitude starts to decrease, the output of the first PI block slowly saturates to a phase compensation angle value that provides appropriate damping to the system oscillation after the change of the operating condition. This is furthermore done without any prior knowledge of the post-fault operating condition.

In this way the oscillation is dampened out adaptively. There is no need to know any pre-conditions regarding the system after a fault in the system. It is furthermore flexible in that it can adapt to any situation. The invention does not need any linear model of the system for obtaining the phase compensation angle. This means that any measurement signal that has high observability of the mode of oscillation can be used without any significant modification. This also means that the phasor POD device can be used for both local and wide area damping. This is made possible because the adaptive generation of phase angle compensation automatically considers different requirements arising from using different measurement signals. In this way duplicated phasor POD devices for local and wide area power or voltage oscillation damping can therefore be avoided.

FIG. 4shows two diagrams where adaptive power oscillation damping is compared with power oscillation damping with a fixed phase angle of 25 degrees after the occurrence of a system fault. Here the upper diagram shows an angle difference α over time. This angle difference α is here the phase difference between the two power transmission buses fromFIG. 1over time, where the difference angle for power oscillation damping using the adaptive scheme of the invention is shown with a dark dashed curve, while the difference angle for power oscillation damping with a fixed phase angle is shown with a lighter solid curve. The lower diagram inFIG. 4shows the normalized susceptance of an actuating device, which actuating device in this example is in the form of a Static Var Compensator. As in the upper diagram control using a fixed phase angle is represented by a solid light curve and control using the adaptive scheme of the invention is represented by a dashed darker curve. As can be seen in the diagrams the power oscillations damping with an adaptively determined compensation angle provides wide-area control that stabilizes a power system following a fault, while the control using a fixed compensation angle leads to an unstable system.

The phase compensation is here angularly performed with a positive phase compensation angle, i.e. for instantaneous magnitudes having a first polarity, a positive polarity. However, it is possible that the phase compensation angle should have an opposite sign, i.e. be a negative phase compensation angle based on instantaneous magnitudes having the opposite polarity.

In order to handle this situation, the slope investigating element58continuously provides slope detection values to the adjustment direction control element62. These values can be positive, negative or zero. Originally the signal |ΔP| will have a slope that is positive. If phase adjustment is made in the correct angular direction, this slope will then decrease, become zero and eventually turn negative in a given time period. However if it does not do this within the given time period, the angular direction in which adjustment was made was wrong and the opposite direction should have been used. The first embodiment of the present invention addresses this situation through the adjustment direction control element62receiving the rate of change measurements d|ΔP|/dt from the slope investigating element60and investigating the sign of these rates of change or slope. If the sign changes, step76, the adjustment direction control element62resets the counter, step78, and then continues and obtains magnitude values, step68. However, if the sign is not changed, step76, the adjustment direction control unit60continues and compares the time of the counter with a first time threshold T1, step80. If this threshold T1is not exceeded, generation of the phase compensation angle continues as before, step68, while if it is exceeded, step80, the adjustment direction control element60actuates the switching element56so that now the phase compensation angle φCis provided by the second PI block42. In this way the adjustment direction control element60changes the active PI block, step82, and ensures that the compensation angle φCis formed based on the inverted instantaneous magnitude instead. Thereafter the forming of phase compensation angles is continued based on the inverted instantaneous magnitude.

In this way it is ensured that the oscillation is cancelled out quickly even though the wrong direction is initially selected.

The adaptively changed compensation angle φCafter the occurrence of a fault being provided according to the first embodiment of the invention is schematically shown inFIG. 5. From this figure it can be seen that the phase compensation is initially performed in a positive direction, which is found to be wrong after about 5 seconds after the fault, which time is thus an exemplifying first threshold. Therefore the phase compensation is thereafter performed in the negative direction. From the curve it can be seen that a stable phase compensation angle of about −77 degrees is obtained after about 12 seconds.

It should here be mentioned that there are a number of variations that can be made in relation to this first embodiment of the invention. It is possible that the second PI block is used first and the first PI block is used if the second PI block provided a phase compensation angle in the wrong direction. It is furthermore possible that only one direction is investigated, i.e. that one PI block is removed. In this case there is also no need for the adjustment direction control element and switching element. It is also possible to omit the proportional leg of the PI blocks, i.e. to only use integrating activity. Also normalization may be omitted. It is possible to provide a low pass filter before the slope investigating element58in order to filter out high frequency elements before slope detection is performed.

The negative direction may thus not be investigated. It is here possible that instead of this or in addition, that if the magnitude grows then the proportional factor is influenced such that it is decreased. For this reason the compensation angle determining unit may also include a proportional factor tuning element that changes the proportional factor. This change may be performed based on the rate of change or slope of the absolute oscillation magnitude, i.e. the rate of change of the signal. It is also possible to completely disable the damping control signal. Thus the timing of the counter may be compared with a second threshold and if this is exceeded the proportional factor is decreased. The timing of the counter may also be compared with a third threshold and if this is exceeded, the damping operation is disabled. In this latter case the compensation angle determining unit may include an operation aborting element25that would send a disable signal to the damping signal generating unit24of the phasor POD unit20, which as a response would no longer generate the damping signal V.sub.POD. This third threshold may with advantage be the same as the first threshold. It is furthermore possible that the compensation angle determining unit receives the magnitude of the signal .DELTA.P. In this case it is possible that proportional factor is initially set according to this magnitude. Finally it should be mentioned that the start of operation of the compensation angle determining unit is not limited to the first peak after one cycle of oscillation.

The phasor POD device shown in the first embodiment is a phasor POD device operating locally, i.e. operating on locally measured values. However, it is possible also to use the phasor POD device as a wide area phasor POD device, i.e., for remotely measured values. The phasor POD device may thus be used in other situations than for local damping. It may be used for wide area damping. This means that it may receive measurements from other geographical areas than a local area. It is then possible to use the POD assisting unit described in the first embodiment of the invention. However, it is also possible to use a compensation angle determining unit according to a second embodiment of the invention, which furthermore considers also the time delay of such measurements. A phasor POD device according to the second embodiment of the invention will now be described with reference being made toFIGS. 6 and 7, whereFIG. 6shows four geographical areas of a power transmissions system together with a phasor POD device and gain scheduling unit andFIG. 7shows a block schematic of a compensation angle determining unit according to the second embodiment.

In the system inFIG. 6there are four geographical areas A_I, A_II, A_II and A_IV, that may each swing against one of the other geographical areas. Each area is provided with a measurement unit18,84,86and88. It should here be realized that there may be more measurement units in each geographical area. Measurement units are furthermore normally connected to power lines and buses. A measurement unit may here be a Phasor Measurement Unit (PMU). A PMU provides time-stamped local information about the system, in particular currents and voltage phasors. A plurality of phasor measurements collected throughout the network by PMUs and processed centrally can therefore provide a snapshot of the overall electrical state of the power transmission system. Such PMUs are normally also equipped with GPS synchronized clocks and will send phasors, such as positive sequence phasors, at equidistant points in time, e.g. every 20 ms. These phasors are thus time stamped with high accuracy, and the time stamp may represent the point in time when the phasor was measured in the system. The phasors could be time stamped, i.e. receive time indicators, using Assisted GPS (A-GPS). In order to perform such time stamping each measurement unit18,84,86and88is therefore provided with an antenna for communicating with a GPS satellite.

The phasors are thus obtained at distant geographical locations and time stamped by the measurement units, normally using a GPS clock and sent via communication channels, which are potentially several thousand kilometers in length, to a phasor aligning unit90.

The measurement units18,84,86and88are thus all connected to a phasor aligning unit90, which may be a Phasor Data Concentrator (PDC). This phasor aligning unit90thus receives the above-described phasors and synchronizes them, i.e. packages the phasors with the same time stamp.

The phasor aligning unit90listens to measurement units that are sending time stamped phasors on a regular basis (e.g. every 20 ms). The phasor aligning unit90aligns the phasors according to the time stamp, expecting one phasor from each measurement unit per time slot, and forwards all phasors when these corresponding to a given time slot are available.

The phasor aligning unit90is furthermore connected to a combining unit91, which combines phasors from at least two areas in order to provide a system operations reflecting signal P(t). In this embodiment the system operation reflecting signal is thus a combined signal reflecting a measured power property, like voltage, of more than one system element, here two. It here reflects the power property both in respect of the operating frequency and the oscillatory component This combing unit91is then connected to a phasor POD unit, which phasor POD unit20is connected to a compensation angle determining unit26and a gain scheduling unit28in the same way as inFIG. 1. The gain scheduling unit28is furthermore connected to an actuator92for performing damping in the system. This could be a local actuator, in the same geographical area where the phasor POD is located or in another geographical area. Also the compensation angle determining unit26is connected to the gain scheduling unit28in the same way as inFIG. 1. There is one difference with the compensation angle determining unit26though and that is that it is provided with an antenna. The device for providing improved control of power or voltage oscillation damping of the present invention may here include any combination of the compensation angle determining unit with oscillation estimating unit, damping control signal generating unit, gain scheduling unit, combining unit and phasor aligning unit.

As can be seen inFIG. 7, the compensation angle determining unit26according to the second embodiment is in most parts the same as in the first embodiment. However it is provided with a latency compensating element94connected to the output of the switching element56. This latency compensating element94is provided with said antenna for communicating with a GPS satellite, for instance using A-GPS, in order to obtain an accurate time. It also receives the time stamps associated with samples of the deviating oscillation or rather the time stamps associated with the measurement values on which these samples are based.

The phasor POD unit20does in this embodiment determine a signal representing the deviating oscillation in the form of the absolute value of this oscillation based on phasors from two areas that swing against each other from measurement units having the same time stamps, which is done based on the combined signal P(t). The combined signal may here be a difference signal based on the difference between phasors from two areas having the same time stamp. A signal |ΔPs| may then be generated by the phasor POD unit20based on such a combined signal. The signal |ΔPs|, the frequency f of the swing and the value tsof the time stamps of the corresponding samples are provided to the compensation angle determining unit26, where the latency compensating unit94receives such time stamp values tsand the swing frequency f. The latency determining unit94thus receives the time stamp or time value associated with the deviating oscillation at system elements for a specific instantaneous magnitude of the signal |ΔPs|, i.e. for the instantaneous magnitude that is determined based on the measurements having these time stamps or time values. The compensation angle determining unit26according to this second embodiment determines a phase adjustment angle φcin the same way as in the first embodiment, which angle is provided at the output of the switching element56. However this angle is further adjusted in order to obtain a correct phase adjustment value that also considers the latency of the measurements. More particularly the latency compensating unit94based on this received time value tsand an own current time tcdetermines a time delay Tdaccording to:
Td=tc−ts.

This time delay is then used for determining a time delay compensation factor φdaccording to:
φd=2*π*f*Td

This leads to the obtaining of an adjusted phase compensation angle
φa′=φc+φd

As can be seen the latency determining unit94thus determines a time delay compensation factor φdbased on the time value ts, the current time tcand the frequency f of the oscillation and adjusts the phase compensation angle with this time delay compensation factor. In this way it is also possible to take account of the time delay of the measurement signals, which is of importance when these are taken far from the phasor POD device. This speeds up the damping process in wide area power or voltage oscillation damping situations.

According to a variation of the second embodiment it is also possible that the system operation reflecting signal is a signal reflecting a property in only one element, a remote element far from the phasor POD device.

As can be seen inFIG. 7, it is possible that several areas may swing against each other. This swinging can also take place simultaneously. It is therefore possible that a combining unit91provides one system operation reflecting signal that is made up of several such swings. A deviating oscillation may thus be multimodal. The system operation reflecting signal is thus a combined signal reflecting a measured power property, like voltage, of more than one system element, here two. It here reflects the power property both in respect of the operating frequency and more than one oscillatory component. Such a combined signal may here be provided through generating a number of difference phasors each being provided as the difference between the phasors of two geographical areas and then summing these difference phasors for obtaining a combined signal. As an alternative it is possible that this system operating reflecting signal is made up of measurements from only one system element, which system element thus experiences swinging between several geographical areas.

In order to handle such a combined or multimodal signal P there may be provided a phasor POD device19as outlined inFIG. 8. Here there is a signal extracting unit96, which splits the combined signal according to the estimated frequencies, i.e. it extracts each mode component from the combined signal P. In this example it does this through splitting the combined signal P into a first, second, . . . and n-th signal P1, P2and Pn, where each such signal is provided to a corresponding phasor POD unit20,98and102. To each of these phasor PODs units there is connected a corresponding compensation angle determining unit26,100and104providing a phase adjustment angle φc1, φc2and φcnto the corresponding phasor POD unit20,98and102. The phasor POD units then each provide a corresponding damping control signal VPOD1, VPOD2and VPOD3. It is here possible that these damping control signals are combined and provided to a common gain scheduling unit connected to one actuator. It is also possible that each such damping control signal is provided to a corresponding gain scheduling unit, which controls an actuator. The gain scheduling unit selected is generally dependent on which areas that swing against each other. An actuator in an area involved in several such swings can therefore be controlled by a damping control signal cancelling these swings. For multiple actuating devices, if one actuator is devoted for damping one particular mode then corresponding VPODsignal can be directly fed from individual phasor PODs to the corresponding actuating devices.

It should here be realized that time delay compensation may be applied also in this variation of the invention.

There are a number of further variations that are possible to make of the present invention. The oscillations estimating unit and damping signal generating units need not be provided together in the same entity, but they may be separated. The system elements from which measurements are being made are with advantage power lines. However, it should be realized that also other types of system elements can be envisaged, such as converters and transformers.

The device according to the invention, i.e. the compensation angle determining unit either alone or in any of the previously described combinations, may with advantage be provided in the form of a controller having processor together with an internal memory including computer program code, which when being operated on by the processor performs the above mentioned functionality of the units included in the device. The program code can also be provided on a data carrier, which performs this functionality when being loaded into such a memory. It will therefore be apparent to the skilled man that the device for providing improved control of power or voltage oscillation damping of the present invention may be hardwired or implemented as a computer program.

The device for providing improved control of power or voltage oscillation damping may be provided via a wide-area monitoring and control platform.

In a further embodiment, the device for providing improved control of power or voltage oscillation damping of the present invention may be run on a FACTS device, specifically the low level power electronics control platform for the FACTS device, or alternatively on a fast acting device such as an AVR or a direct load modulator.

The present invention is therefore only to be limited by the following claims.