Arrangement and method for balancing power in ac power transmission system

Abstract

To perform power supply and absorption with a system for power system stabilization, efficiently and at low costs.SOLUTION: An arrangement (1) connectable to an alternating current power transmission system (2) comprises a static synchronous compensator device (3) having a converter unit (4) and an energy storage unit (5) connected with the converter unit; a power absorption device (6); and a control device (9) connected with the static synchronous compensator device and with the power absorption device. The control device controls the static synchronous compensator device to exchange power with the alternating current power transmission system, controls the power absorption device in dependence of a charge state of the energy storage unit and a state of the power transmission system, further determines whether or not there is a need to absorb active power from the AC power transmission system in excess of what the static synchronous compensator device is currently absorbing and, in the case of such a need, controls the power absorption device to absorb active power.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Technical Field The present disclosure relates generally to power transmission systems, and more particularly to an apparatus and method for balancing power in a power transmission system with a static synchronous compensator. [Background technology]

[0002] background Power transmission systems, such as alternating current (AC) grids, require stabilization. Historically, the inertia of power generation plants, such as rotating generators in fossil, nuclear, and hydroelectric power plants, has been used as a stabilization method. As the proportion of renewable energy plants, such as solar and wind power plants, increases, the amount of inertia is decreasing. Other methods for stabilizing power transmission systems have been developed. One of these is the use of a static synchronous compensator with a converter and energy storage combination to absorb or supply both reactive and active power from or to the power transmission system. Such static synchronous compensators are commonly referred to as E-STATCOMs or enhanced STATCOMs. Therefore, active power, in addition to reactive power, can be absorbed from and supplied to the AC grid, particularly through energy storage. However, E-STATCOMs have limited energy and power capacity due to the fixed energy storage, which can consist of batteries, supercapacitors, or other types of storage. A simple solution would be to simply add more E-STATCOMS or build them larger, but there are clear practical limitations, both in terms of their actual physical size and the cost of building them, that require some more efficient solution. Summary of the Invention [Means for solving the problem]

[0003] overview In view of the above, the concern of the present disclosure is how to reduce the shortcomings of the prior art.

[0004] To further address this concern, in a first aspect, the present disclosure provides an apparatus connectable to an alternating current (AC) power transmission system. The apparatus includes a static synchronous compensator having a converter unit and an energy storage unit connected to the converter unit, a power absorption device, and a controller connected to the static synchronous compensator and the power absorption device. The controller is configured to control the static synchronous compensator to exchange power with the AC power transmission system and to control the power absorption device according to a state of charge of the energy storage unit and a state of the power transmission system. The controller is configured to determine whether the static synchronous compensator needs to absorb active power from the AC power transmission system in excess of the power currently absorbed, and, if such a need exists, to control the power absorption device to absorb the active power.

[0005] By determining that the STATCOM device needs to absorb more power than is currently possible, real power is dissipated by the power absorption device, improving the balance of the AC power system.

[0006] The power absorption device may include a resistor unit and a current control unit connected in series with the resistor unit, and the control device is configured to control the current control unit to supply current to the resistor unit when controlling the power absorption device to absorb active power.

[0007] The power absorption device may comprise a separate branch for each phase it is configured to be connected to, the current control unit comprising a separate current control module for each branch, and the resistor unit comprising a separate resistor module for each branch, the current control module and resistor module of each branch being connected in series.

[0008] The power absorption device may operate, for example, while the energy storage unit is charging but not absorbing power fast enough, or when the energy storage unit reaches an upper limit, such as being fully charged or charged to a predetermined level less than full charge, and therefore cannot or is not allowed to absorb any more power.

[0009] The power absorption device may be further configured to be connected to the AC power transmission system in parallel with the static synchronous compensator.

[0010] The power absorption device may further be configured to act as a backup device for the static synchronous compensator in terms of power absorption.

[0011] The power absorption device may be configured to be connected to only one or two phases of an AC power transmission system.

[0012] The power absorption device may be configured to be connected to the direct current (DC) side of the static synchronous compensator in parallel with the energy storage unit.

[0013] The current control unit may comprise a semiconductor element such as a thyristor, for example an IGCT or a GTO, an IGBT, an IEGT, or the like.

[0014] According to a second aspect thereof, the present disclosure provides a method for controlling an apparatus connected to an alternating current (AC) power transmission system. The apparatus includes a static synchronous compensator having a converter unit and an energy storage unit connected to the converter unit, and a power absorption device. The method includes controlling the static synchronous compensator to exchange power with the AC power transmission system, and controlling a current control unit according to a state of charge of the energy storage unit and a state of the power transmission system, the control including determining whether the static synchronous compensator needs to absorb active power from the AC power transmission system in excess of the power currently absorbed, and, if so, controlling the power absorption device to absorb the active power.

[0015] The determining operation may include at least one of determining that the state of charge (SOC) of the energy storage unit has reached an upper limit, e.g., 100%, i.e., the energy storage unit is fully charged, or a predetermined SOC less than 100%, and detecting that the need for power absorption is increasing faster than the energy storage unit can handle.

[0016] The operation of controlling the static synchronous compensator to exchange power with the AC power transmission system may include controlling the SOC of the energy storage unit between a fully discharged state and a fully charged state when there is no need to exchange power with the AC power transmission system.

[0017] The power absorption device may be connected to only one or two phases of the AC power transmission system and in parallel with the static synchronous compensator, and the operation of controlling the static synchronous compensator to exchange power with the AC power transmission system may include balancing asymmetric currents caused by the power absorption device.

[0018] The method can be used to stabilize the frequency of an AC power transmission system.

[0019] Further scope of applicability of the present disclosure will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present disclosure, are given by way of illustration only, since various changes and modifications within the scope of the present disclosure will become apparent to those skilled in the art from this detailed description.

[0020] Therefore, it is to be understood that this disclosure is not limited to the particular components of the apparatus described, or that the method steps described in such apparatus and methods may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

[0021] BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments will now be described in more detail with reference to the accompanying drawings. [Brief explanation of the drawings]

[0022] [Figure 1] 1 shows a block diagram of an embodiment of an apparatus according to the present disclosure. [Figure 2] 1 shows a block diagram of an embodiment of an apparatus according to the present disclosure. [Figure 3] 1 shows a block diagram of an embodiment of an apparatus according to the present disclosure. [Figure 4] 1 shows a block diagram of an embodiment of an apparatus according to the present disclosure. [Figure 5] 1 illustrates an example of how power is handled by a device. [Figure 6] 1 shows a flowchart of one embodiment of a method according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0023] All figures are schematic, not necessarily to scale, and generally show only those parts necessary to elucidate the embodiments; other parts may be omitted or merely suggested. Like reference numerals refer to like elements throughout the description.

[0024] Detailed Description The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which presently preferred embodiments of the disclosure are shown. However, the present disclosure may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and will fully convey the scope of the present disclosure to those skilled in the art.

[0025] FIG. 1 shows the most general diagram of a device 1 connected to an alternating current (AC) power transmission system 2. The device 1 comprises a static synchronous compensator, or STATCOM, device 3 having a converter unit 4 and an energy storage unit 5 connected to the converter unit 4. The device 1 further comprises a power absorption device 6 having a resistor unit 7 and a current control unit 8 connected in series with the resistor unit 7, and a controller 9 connected to the STATCOM device 3 and the power absorption device 6. Note that the illustrated AC power transmission system 2 is typically a three-phase system, as indicated by the three short parallel lines that diagonally intersect some of the other lines, and the different parts involved typically operate on all three phases. As described below, there are alternative embodiments in which some of them can operate on only one or two phases.

[0026] According to one embodiment of the method, as shown in the flowchart of FIG. 6 , the controller 9 is configured to control the STATCOM device 3 to exchange power with the AC power transmission system 2 (box 61) and to control the power absorption device 6 (box 62) depending on the state of charge of the energy storage unit 5 and the state of the AC power transmission system 2. The control in box 62 includes determining whether there is a need to absorb active power from the AC power transmission system in excess of the power currently absorbed by the static synchronous compensator (box 63). If there is such a need, the method includes controlling the power absorption device 6 to absorb active power (box 64). More specifically, the controller 9 may be configured to determine that there is a need to absorb active power from the AC power transmission system 2 in excess of the power currently absorbed by the STATCOM device 3 and to control the current control unit 8 to supply current to the resistor unit 7. This dissipates active power in the resistor unit 7. As already mentioned above, the power currently absorbed by the STATCOM device 3 may depend on several different factors. For example, the maximum SOC may be preset to a level less than 100% or may be allowed to reach 100%. As another example, the maximum charging rate of the energy storage unit 5 may be less than the required absorption rate, ie, the rate at which the need to absorb active power from the AC power transmission system 2 increases.

[0027] The AC power transmission system 2 may include a high-voltage (HV) transmission line or bus 10, a medium-voltage (MV) transmission line or bus 11, and a three-phase transformer unit 12 interconnecting them. The three-phase transformer unit 12 may be a complete three-phase transformer or three single-phase transformers. The three phases may be connected, for example, as a wye or delta. As shown in FIG. 1 , the STATCOM unit 3 and the power absorption unit 6 may be connected in parallel to the MV transmission line 11. The STATCOM unit 3 has both a converter unit 4 and an energy storage unit 5, and thus can process both reactive and real power. The converter unit 4 may be of any suitable type, such as a voltage source converter (VSC), such as a multi-modular converter (MMC), as will be recognized by those skilled in the art. The converter unit 4 includes a reactance 13 and a main circuit breaker 14 at its AC terminals, or more specifically, one reactance and circuit breaker per phase. Similarly, the power absorption device 6 may include a main circuit breaker 15 at its terminals connected to the MV transmission line 11. The main circuit breakers 14, 15 are optional. The energy storage unit 5 may include a battery module, a supercapacitor module, or a combination thereof. With respect to the power absorption device 6, for each phase, the resistor unit 7 may include one or more resistors, and the current control unit 8 may include one or more elements that can be switched on and off as desired to control the amount of current flowing through the resistor and / or the time distribution between current-on times and current-off times, thereby controlling the amount of power absorbed.

[0028] One embodiment of the device 20 is shown in FIG. 2. Again, those skilled in the art will appreciate that the structure of the power absorption device 6 shown in FIG. 2 applies to each phase, so that in a typical three-phase system there will be three identical structures or branches, one for each phase. One of these branches is shown in FIG. 2. Thus, for each branch, the current control unit 8 comprises a separate current control module 21, and the resistor unit comprises a separate resistor module 22 for each branch, with the current control module 21 and resistor module 22 of each branch connected in series. According to this embodiment, the current control module 21, and thus the current control unit 8, may comprise a thyristor 23, and the resistor module 22 comprises two resistors 24, 25 connected in series to the current control module 21, with one of the resistors 24 being located upstream of the current control module 21 with respect to the flow of current through the branch, and the other resistor 25 being located downstream of the current control module 21.

[0029] The current through the resistors 24, 25 is continuously controllable by setting the firing angle of the thyristor 23. As a result, any current level from zero to the maximum current level can be provided by controlling the firing angle of the thyristor 23. This allows the power absorption device 6 to provide variable additional active power dissipation, which is necessary for the AC power transmission system 2 but is currently unavailable in the STATCOM device 3. In other words, active power absorption can be advantageously extended beyond the capacity of the energy storage unit 5. By controlling the power absorption device 6 together with the STATCOM device 3 through the controller 9, a smooth transition can be achieved between simply charging the energy storage unit 5 and further dissipating power in the resistors 24, 25. Because power dissipation causes a temperature rise in the resistors 24, 25, the allowable duration of power dissipation in the power absorption device 6 is a matter of the power rating of the resistors 24, 25. Therefore, the power rating can be appropriately selected to provide a sufficiently high power dissipation capacity. Additionally, the resistor temperature as a function of the magnitude of the current through the resistor can be estimated to determine the remaining duration for resistors 24, 25. At some point, the current may have to be limited to avoid overheating the resistor.

[0030] The need for further consumption of active power can be caused, for example, by the energy storage unit 5 being fully charged (i.e., SOC = 100%), or by reaching a predetermined maximum allowable SOC below 100% and therefore unable to absorb any more power, or by the need for the energy storage unit 5 to rise faster than its maximum charge rate can handle. The reason the energy storage unit 5 is fully charged can be due to the need to absorb power lasting too long. This scenario is illustrated in FIG. 5, which shows that the energy storage unit 5 has a state of charge (SOC) of approximately 50%, or about half of its total capacity, at the start of the illustrated period. This SOC is also idle. Then, a need arises to absorb power from the AC power transmission system 2, and charging of the energy storage unit 5 begins. However, as indicated by the top dashed line, the need for power absorption increases faster than the energy storage unit 5 alone can satisfy it, i.e., the required absorption rate is higher than the maximum charge rate the energy storage unit 5 can achieve. Therefore, power balance in the AC power transmission system is not fully achieved. This is detected by the controller 9, which then controls the power absorption device 6 to support the STACOM device 3 by starting to consume power in the resistor unit 7. As the difference between need and the capacity of the energy storage unit 5 increases over time, the current control unit 8 adapts the firing angle of the thyristor 23 to increase the current through the resistors 24, 25.

[0031] Additionally, the power absorption device 6 may be used as a backup in case the STATCOM device 3 is out of service. In that case, the device 1 can only absorb power and not generate power, but is still available in this degraded mode of operation.

[0032] For completeness, it should be noted that the controller 9 is, of course, also configured to determine that power needs to be supplied to the AC power transmission system 2. The controller 9 then controls the STATCOM device 3 to supply that power, which may include discharging the energy storage unit 5.

[0033] It may also be possible to use a power absorption device 6 to discharge the energy storage unit 5 instead of the external discharge resistor often used on the DC side of the STATCOM device 3. A smaller backup discharge resistor may still be needed if the converter unit 4 is unavailable. This is useful for controlling the SOC of the energy storage unit 5 between 0% and 100% when there is no need to exchange power with the AC power transmission system 2. At this time, the SOC of the energy storage unit 5 may be referred to as an idle charge state and may be controlled to approximately 50% to have a good margin for both charging and discharging, depending on the needs of the AC power transmission system 2.

[0034] Furthermore, an advantage of the power absorption device 6 is that the idle SOC can be set above 50% in case of excess demand, depending on the power absorption capacity of the power absorption device 6. This increases the ability to supply active power to the AC power transmission system 2.

[0035] There may be some reactive power consumed by the power absorption device 6. This reactive power consumption may be compensated for by the converter unit 4.

[0036] According to one embodiment of the device 30, as shown in FIG. 3 , the power absorption device 6 has two branches 31, 32. The first branch is connected to the first and second phases of the three-phase MV power transmission line 11, and the second branch 32 is connected to the second and third phases. In this embodiment, the number of current control modules, resistor modules, etc. is advantageously reduced compared to a full three-phase configuration. Meanwhile, operating the power absorption device 6 with two branches creates an imbalance. The controller 9 is configured to compensate for this imbalance via the STATCOM device 3. Even in the above-described embodiment, configuring the controller 9 to compensate for phase imbalance with three full branches offers advantages. If one or two phases are out of order, the controller 9 can better handle this situation, which can be considered a degraded operating mode of the device 1. This provides increased energy availability to the AC power transmission system 2.

[0037] According to one embodiment of the device 40, as shown in FIG. 4, the power absorption device 41 is directly connected to the STATCOM device 3. More specifically, the power absorption device 41 is connected to the direct current (DC) side of the STATCOM device 42 in parallel with the energy storage unit 43. Because the power absorption device 41 is connected to the DC side of the STATCOM device 43, it is appropriate to use IGBTs / IGCTs or similar elements instead of thyristors for the current control unit of the power absorption device 41. The overall system benefits are the same regardless of where the power absorption device 41 is located, and the resistor unit of the power absorption device 41 is expected to have the same energy rating in both cases. Note that when the power absorption device 41 is connected to the DC side, active power must be transmitted via the converter unit of the STATCOM device 44, compared to the above embodiment in which the power absorption device 41 is connected to the MV transmission line 11 in parallel with the STATCOM device 3.

[0038] While the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive.

[0039] Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements, or in various combinations with or without the other features and elements.

[0040] 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 claims, the words "comprising" or "comprising" do not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. An apparatus (1) connectable to an alternating current (AC) power transmission system (2), the apparatus (1) comprising: a static synchronous compensator (3) having a converter unit (4) and an energy storage unit (5) connected to the converter unit; a power absorption device (6); and a control device (9) connected to the static synchronous compensator and the power absorption device; the control device is configured to control the static synchronous compensator to exchange power with the AC power transmission system and to control the power absorption device according to a state of charge of the energy storage unit and a state of the power transmission system; The control device is configured to determine whether there is a need to absorb active power from the AC power transmission system in excess of the power currently absorbed by the static synchronous compensator, and if there is such a need, to control the power absorption device to absorb the active power.

2. 2. The device according to claim 1, wherein the power absorption device (6) comprises a resistor unit (7) and a current control unit (8) connected in series with the resistor unit, and the control device (9) is configured to control the current control unit to supply current to the resistor unit when controlling the power absorption device (6) to absorb the active power.

3. 3. The arrangement according to claim 2, wherein the power absorption device (6) comprises a separate branch for each phase to which it is configured to be connected, the current control unit (8) comprises a separate current control module (21) for each branch, and the resistor unit (7) comprises a separate resistor module (22) for each branch, the current control module and the resistor module of each branch being connected in series.

4. 10. The apparatus of claim 9, wherein the need to absorb active power from the AC power transmission system (2) in excess of the power currently absorbed by the static synchronous compensator (6) results from at least one of the state of charge of the energy storage unit (5) reaching an upper limit and the need for power absorption rising faster than the energy storage unit (5) can handle.

5. 10. The arrangement according to any one of the preceding claims, wherein the power absorption device (6) is configured to be connected to the AC power transmission system (2) in parallel with the static synchronous compensator (3).

6. 10. The device according to any one of the preceding claims, wherein the power absorption device (6) is configured to act as a backup device for the static synchronous compensator (3) in terms of power absorption.

7. 10. The arrangement according to any one of the preceding claims, wherein the power absorption device (6) is configured to be connected to only one or two phases of the AC power transmission system (2).

8. The device according to any one of claims 1 to 4, wherein the power absorption device (41) is connected in parallel with the energy storage unit (43) on the DC side of the static synchronous compensator (42).

9. 10. The device according to any one of the preceding claims, wherein the current control unit (8) comprises a semiconductor element.

10. A method for controlling a device (1) connected to an alternating current (AC) power transmission system (2), the device (1) comprising a static synchronous compensator (3) having a converter unit (4) and an energy storage unit (5) connected to the converter unit, and a power absorption device (6), the method comprising: controlling the static synchronous compensator to exchange power with the AC power transmission system; and controlling the power absorption device according to a state of charge of the energy storage unit and a state of the power transmission system, the method comprising: determining whether there is a need to absorb active power from the AC power transmission system in excess of the power currently absorbed by the static synchronous compensator; and, if there is such a need, controlling the power absorption device to absorb the active power.

11. 11. The method of claim 10, wherein the power absorption device (6) includes a resistor unit (7) and a current control unit (8) connected in series with the resistor unit, the device (1) further includes a control device (9) connected to the static synchronous compensation device (3) and the power absorption device, and controlling the power absorption device to absorb the active power includes controlling, by the control device, the current control unit to supply current to the resistor unit.

12. 12. The method according to claim 10 or 11, wherein the determining comprises at least one of determining that the state of charge of the energy storage unit (5) has reached an upper limit and detecting that the need for power absorption is increasing faster than the energy storage unit can handle.

13. 13. The method of claim 10, wherein the controlling the static synchronous compensator to exchange power with the AC power transmission system comprises controlling a charge state of the energy storage unit between a fully discharged state and a fully charged state when there is no need to exchange power with the AC power transmission system.

14. 14. The method of claim 10, wherein the power absorption device (6) is connected to only one or two phases of the AC power transmission system (2) and is connected in parallel with the static synchronous compensator (3), and wherein controlling the static synchronous compensator to exchange power with the AC power transmission system comprises balancing asymmetric currents caused by the power absorption device.

15. The method of any one of claims 10 to 14, wherein the method is used to stabilize the frequency of the AC power transmission system.

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