WO2022167071A1

WO2022167071A1 - A series compensation unit and a method for controlling power in an electric power line

Info

Publication number: WO2022167071A1
Application number: PCT/EP2021/052556
Authority: WO
Inventors: Nicklas Johansson; Haofeng BAI; Kalle ILVES; Jan Svensson; Gunnar INGESTRÖM; Tomas Tengner
Original assignee: Hitachi Energy Switzerland Ag
Priority date: 2021-02-03
Filing date: 2021-02-03
Publication date: 2022-08-11

Abstract

The present invention discloses a method for controlling power in an electric power line (1) by means of controlling a series compensating device (10). The series compensating device (10) comprises a capacitor (17) and a series compensating unit (15). The capacitor and the series compensating unit are connected in series along the electric power line. The series compensating unit comprises a supercapacitor (16) arranged in a full bridge configuration with four thyristor-based switches (11-14). Each thyristor-based switch comprises a thyristor and a reverse-conducting diode. The series compensating unit is configured for selective serial connection of the supercapacitor into the electric power line for charging the super-capacitor, bypassing the supercapacitor, or discharging the supercapacitor. The method comprises obtaining an indication of a desired reactive power, and controlling the series compensating unit for each half-cycle of electric current of the power in the electric power line. The controlling includes: selecting a duration time for the charging of the supercapacitor, selecting a duration time for the discharging of the supercapacitor. The selections are based on the desired reactive power.

Description

A SERIES COMPENSATION UNIT AND A METHOD FOR CONTROLLING POWER IN AN ELECTRIC POWER LINE

TECHNICAL FIELD

The invention relates to control of electric power systems, especially to series compensation of electrical lines in an AC power system.

BACKGROUND

Capacitors, or capacitor banks, are often arranged in series in transmission lines in electric power systems in order to compensate for reactive power. The series capacitor, which may be referred to as a fixed capacitor or uncontrolled capacitor, is arranged in series in the electric power line and may e.g. be connected in parallel with a breaker in order to selectively provide compensation via the fixed capacitor or bypass the fixed capacitor. The time scale of connecting and bypassing the fixed capacitor is much larger than the time scale of the fundamental frequency of the power system that normally would be 50 Hz or 60 Hz.

An example of a control equipment for a series capacitor connected into an electric power line may be found in US 5,801 ,459. However, in the present technical, there is still a need for methods providing an improved control of the reactive power in electric power lines.

SUMMARY

It is therefore an object of the present invention to provide an improved method and series compensating device for controlling power in an electric power line, and particularly for controlling reactive power. A purpose of the present disclosure is also to provide an energy storage system for serial connection into an electric power line, such as a distribution or transmission line, that enables control of active power as well as reactive power in the electric power line. This and other objects are achieved by means of a series compensating device and a method as defined in the appended independent claims. Other embodiments are defined by the dependent claims.

According to a first aspect disclosed herein, a method for controlling power in an electric power line by means of controlling a series compensating device is provided. The series compensating device comprises a capacitor and a series compensating unit. The capacitor and the series compensating unit are connected in series along the electric power line. The series compensating unit comprises a supercapacitor arranged in a full bridge configuration with four thyristor-based switches. Each thyristor-based switch comprises a thyristor and a reverse-conducting diode. The series compensating unit is configured for selective serial connection of the super capacitor into the electric power line for charging the supercapacitor, bypassing the supercapacitor, or discharging the supercapacitor. The method comprises obtaining an indication of a desired reactive power and controlling the series compensating unit for each half-cycle of electric current of the power in the electric power line. The controlling includes selecting a duration time for the charging of the supercapacitor and selecting a duration time for the discharging of the supercapacitor. The selecting of duration time for charging and the selecting of duration time for discharging the supercapacitor are based on the desired reactive power.

According to a second aspect presented herein, a series compensating device for controlling power in an electric power line is provided. The series compensating device comprises a capacitor and a series compensating unit. The capacitor and the series compensating unit are connected in series along the electric power line. The series compensating unit comprises a supercapacitor arranged in a full bridge configuration with four thyristor-based switches. Each thyristor-based switch comprises a thyristor and a reverseconducting diode. The series compensating unit is configured for selective serial connection of the supercapacitor into the electric power line. Further, the series compensating unit comprises a control unit. The control unit is configured to obtain an indication of a desired reactive power and to control the series compensating unit for each half-cycle of the electric current of the power in the electric power line, in order to: charge the supercapacitor, bypass the supercapacitor, or discharge the supercapacitor. The control unit is further configured to select a duration time for the charging of the super-capacitor and a duration time for the discharging of the super-capacitor based on the desired reactive power.

The series compensating device may provide at least one of improved power flow control, improved power oscillation damping, improved fast frequency support/response, improved synthetic inertia provision, and/or providing capacitive and inductive operation range, when compared to a system in which there only is a fixed series connected capacitor. More generally, the method disclosed herein and its associated series compensating device provides a more flexible method for controlling the power, and particularly the reactive power, in an electric line, in that the level of the reactive power can be adjusted and in that it is not limited to the capacitance of the fixed capacitor.

The supercapacitor may be understood as, for example, an energy storage able to handle high current. A supercapacitor may also be referred to as an ultracapacitor. A supercapacitor is a high-capacity capacitor with a capacitance value much higher than other capacitors.

The full bridge configuration may be understood as, for example, a power electronic switch configuration. The series compensating device provides full energy utilization of the supercapacitor, while using a simple and robust full bridge configuration. The full bridge configuration provides a low voltage rating and a low power rating. The series compensating device provides more functionality than a conventional fixed series compensation.

It is to be understood that the present disclosure is not limited to comprising a supercapacitor. For example, the supercapacitor may be exchanged by a battery. A battery may provide an improved active power support over longer time periods, due to the greater energy density of batteries compared to supercapacitor. However, a supercapacitor may provide an improved fast frequency support, due to their greater power density, compared to batteries.

The capacitor may be a fixed capacitor. The capacitor may be a fixed series capacitor. The capacitor may be understood as, for example, a series capacitor. The combination of the series compensating unit and the capacitor may provide improved fast frequency support. The capacitor may provide an improved protection and insulation. The fast frequency support may be more effective when the electric current of the power line is higher since the discharging current of the series compensating device is then higher.

For connecting the capacitor and the series compensating unit in series along the electric power line, the capacitor and the series compensating unit may be arranged in series from a first part to a second part of the electric power line.

The selective serial connection of the supercapacitor includes a selective connection of the supercapacitor or that the series compensating unit is configured to connect the supercapacitor in a number of different modes, and/or that the series compensating unit is configured to connect the supercapacitor in a number of different serial modes. The modes, and/or the serial modes, may comprise a charging mode, a bypassing mode and a discharging mode.

Further, the supercapacitor may be configured to be arranged in a serial connection with regards to the electric power line.

A thyristor has the advantages of being of low cost and providing low loss and high robustness compared to other alternatives. A thyristor-based full bridge configuration may for example be more robust than an insulated-gate bipolar transistor (IGBT). A thyristor-based full bridge may simplify protection of a converter. However, the present disclosure is not limited to using a thyristor. For example, the thyristor may be replaced by, for example, insulated-gate bipolar transistor (IGBT). Further, the thyristor of the thyristor-based switches may comprise a reverse conducting thyristor, ROT. The thyristor of the thyristorbased switches may comprise an integrated gate-commutated thyristor (IGCT).

The method may further comprise obtaining an indication of a desired active power.

The indication of a desired reactive power and/or the indication of a desired active power may be obtained by, for example, a power reference calculator and/or an entity controlling the grid at a higher level. The power reference calculator and/or the higher-level entity may calculate the indication based on a desired function. The desired function may be a function required in order to stabilize a power system to which the electric power line is a part of.

By the term “half-cycle of the electric current” it is meant, for example, a halfcycle of an alternating portion of the electric current, or a time period between two zero crossing of the electrical current, or a time period defined between two following occurrences of the electric current passing zero, and/or a fourth, or a quarter, of the period of the electric current. By the term “period of the electric current”, it is meant, for example, a waveform of the alternating portion of the electric current.

The selecting of duration time for charging and the selecting of duration time for discharging may be further based on the desired active power.

The method may further comprise bypassing the capacitor, the series compensating unit, or the series compensating device.

The method may further comprise selecting, or deriving, a duration time for the bypassing of the supercapacitor within the half-cycle. The duration time may be obtained or derived as a time of the half-cycle minus the duration time for the charging of the supercapacitor and the duration time for the discharging of the supercapacitor.

The series compensating unit may be arranged in series from a first part to a second part of the electric power line. The series compensating unit and the capacitor (i.e. the series compensation device) may be arranged in series from a first part to a second part of the electric power line.

The series compensating unit may, during each half-cycle, output a reactive power the duration time for charging and the duration time for discharging may be selected such that a total reactive power output of the series compensating device is substantially equal to the desired reactive power.

The series compensating unit may, during each half-cycle, output an active power. The duration time for charging and the duration time for discharging may be selected such that a total active power output of the series compensating device is substantially equal to the desired active power.

The capacitor may output a reactive power. The total reactive power may comprise the reactive power of the series compensating unit and the reactive power of the capacitor.

The control unit may be further configured to obtain an indication of a desired active power. Further, as mentioned above, the control unit may be further configured to obtain an indication of a desired reactive power. The indication of a desired reactive power and/or the indication of a desired active power may be obtained from, for example, a power reference calculator. The control unit may comprise the power reference calculator. The control unit may be communicatively coupled to the power reference calculator. The control unit may be further configured to select the duration time for charging and the duration time for discharging based on the desired active power.

The control unit may be further configured to select the duration time for charging and the duration time for discharging based on the desired reactive power.

The control unit may be further configured to select a duration time for the bypassing of the supercapacitor within the half-cycle.

The series compensating device may further comprise a bypassing arrangement. The bypassing arrangement may comprise a first bypass device configured to bypass the series compensating unit. The bypassing arrangement may comprise a second bypass device configured to bypass the capacitor. The bypassing arrangement may comprise a third bypass device configured to bypass the series compensating device. The control unit may be configured to control the bypassing arrangement. The first, second, and/or the third bypass devices may each comprise a bypass switch and/or at least one surge arrestor.

It is noted that other embodiments using all possible combinations of features recited in the above described embodiments, alternatives or examples, may be envisaged. Thus, the present disclosure also relates to all possible combinations of features mentioned herein.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF DRAWINGS

Exemplifying embodiments will now be described in more detail, with reference to the following appended drawings.

Figure 1 illustrates an electric power line and a fixed capacitor according to prior art;

Figure 2 illustrates an electric power line and a series compensating device comprising a selectively connectable capacitor, the connection of which is controlled by means of a thyristor;

Figures 3A-C illustrate a series compensating device according to a first embodiment;

Figure 4A-H illustrate switching control of the series compensating device during a fundamental time period;

Figure 5 illustrates a method for providing reactive power by means of the series compensating device;

Figure 6 illustrates a fundamental time period of an electric power system and changing between charging, bypassing and discharging during the fundamental time period;

Figure 7 illustrates a series compensating device according to a second embodiment;

Figures 8A-8B illustrate changing between charging, bypassing and discharging during a fundamental time period, wherein figure 8A illustrates operation of the first or the second embodiment of the series compensating device, and wherein figure 8B illustrates operation of the second embodiment of the series compensating device;

Figures 9A-9B illustrate the operating range of the series compensating device of the first embodiment of figures 3A-C;

Figures 10A-10B illustrate the operating range of the series compensating device of the second embodiment of figure 7;

Figures 11 A-11 B illustrate control of series compensation units in case of DC faults in the electric power line. As illustrated in the figures, the sizes of elements and region may be exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of the embodiments. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION

Exemplifying embodiments will now be described more fully hereinafter with reference to the accompanying drawings in which currently preferred embodiments are shown. The invention may, however, 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 fully convey the scope of the invention to the skilled person.

Figure 1 illustrates an electric power line 1 , such as a transmission or distribution line, which electric line 1 affects the electric power as an inductor 9. To counteract the reactive, and inductive, power (-Q) introduced along the electric power line 1 , a fixed capacitor 2 is arranged in the electric power line 1 to provide reactive, and capacitive, power (+Q). The fixed capacitor 2 may be arranged in parallel with a breaker (not shown) in order to selectively connect and disconnect, e.g. bypass, the fixed capacitor in series with the electric line 1 . The solution of figure 1 is a prior art solution, wherein the fixed capacitor 2 is dimensioned to provide approximately the same, but opposite, reactive power, Q, as the electric line 1 consumes.

Figure 2 illustrates a combination of a “Thyristor Controlled Series Capacitor” (TCSC) 3 and a fixed capacitor 2, arranged in an electric power line 1 . During operation, i.e. power transfer, the electric power line will consume reactive power -QLINE and this reactive power is compensated for by the fixed capacitor 2 providing reactive power +QCAP and the TCSC providing a controllable amount of reactive power ±QTCSC. In the same way as in figure 1 , the combination of TCSC 3 and fixed capacitor 2 can provide reactive power compensating for the reactive, and inductive, power -QLINE from the electric power line 1 . However, the combination of TCSC 3 and fixed capacitor 2 can be used to regulate the reactive power provided, more than merely connecting or disconnecting the fixed capacitor 2. Such regulation of the provided reactive power +QCAP + ±QTCSC, can be used for:

- controlling power flow in the electric line;

- damping power oscillation between two distribution systems; and

- counteract sub-synchronous resonance.

According to some embodiments, Figures 3A-C illustrate a series compensating device 10 arranged in an electric power line 1 . Figures 3A and 3C also illustrate a control unit 18 associated with, and operatively connected to, the series compensating device 10 and configured for controlling the series compensating device 10, especially controlling power exchange between the series compensating device 10 and the electric power line 1 . The series compensating device 10 comprises a series compensating unit, or full bridge unit, 15 in series with a fixed capacitor 17, wherein the series compensating unit 15 is arranged in series with the fixed capacitor 17 between a first part 1A and a second part 1 B of the electric power line 1 , see figure 3C. The series compensating unit 15, see figure 3A, comprises a super-capacitor 16 arranged in a full bridge circuit comprising four thyristor-based switches 11-14, each comprising a thyristor 11A-14A, for selectively connecting the supercapacitor 16 in a positive “+”, or negative direction to the electric power line 1 . As illustrated in figure 3A, each thyristor-based switch 11-14 comprises a reverse conducting diode 11 B- 14B in parallel with each thyristor 11A-14A.

The control unit 18 is configured to control the series compensating unit 15 to selectively remove electric power P from the power line 1 , i.e. charge the supercapacitor 16, and selectively provide electric power P to the power line 1 , i.e. discharge the supercapacitor 16. When connected, the fixed capacitor 17 provides positive reactive power “+Q”, or capacitive reactive power, to the electric power line 1 , whereas the series compensating unit 15 can be controlled to provide negative reactive power “-Q”, or inductive reactive power by means of the supercapacitor 16. Thus, as summarized in figure 3B, the series compensating device 10 can be controlled to provide positive or negative active power “±P” and provide positive or negative reactive power “±Q”.

Figure 3C also illustrates a bypassing arrangement 19A-C for the series compensating device 10, including individual bypasses, or bypass devices, 19A and 19B, for bypassing the series compensating unit 15 and the fixed capacitor 17, respectively, and a bypass device 19C arranged for bypassing the series compensating device 10, i.e. both the series compensating unit 15 and the fixed capacitor 17. The control unit 18 is preferably configured for controlling the bypasses 19A, 19B, 19C of the series compensating device 10, wherein the switching on and off of the bypasses 19A, 19B, 19C should normally be performed at a time scale significantly longer than the time scale of the fundamental time period of the AC power system of the electric power line 1 . In contrast, the control of the series compensating unit, or full-bridge unit, 15 is performed by means of the control unit 18 in a short time scale compared to the fundamental time period of the electric AC power system as will be described in greater detail with reference to figures 4A-4H. The fundamental time period of the AC power system would normally have frequency of 50 Hz or 60 Hz.

Figure 6 illustrates control of the series compensating unit, or full bridge unit 15, during one fundamental time period of the electric AC power system to which the series compensating device 10 is connected. For each half-cycle of the fundamental time period, the control unit 18 is configured for controlling charging, bypassing and discharging of the supercapacitor 16. Each half cycle, such as the first positive half-cycle of figure 6, begins with charging during a charging time period, denoted Tc, continues with a bypassing time period, TBP, and ends with discharging the supercapacitor during a discharging time period TDC. The first positive half-cycle is followed by a second negative half cycle, as illustrated in figure 6, which also comprises a charging time period Tc, a bypassing time period, TBP, and a discharging time period TDC in the same order. Figures 4A-H illustrates the switching of the thyristor-based switches 11-14 of the series compensating unit 15 during one fundamental time period of the electric AC power system, wherein figures 4A-C illustrates switching during the first half-cycle, and figures 4E-G illustrates switching during the second halfcycle.

When the first half-cycle starts, as illustrated in figure 4A, the reverse conducting diode 11 B of the first thyristor-based switch 11 and the reverse conducting diode 14B of the fourth thyristor-based switch 14, respectively, starts conducting the current through the positive side of the supercapacitor 16 whereby the supercapacitor 16 is charged. The first half-cycle ends with a discharging time period, as illustrated in figure 4C, wherein the second thyristor 12A and the third thyristor 13A conducts the current through the supercapacitor 16 from its negative side. Thus, the control unit 18 is configured to switch the second thyristor 12A and the third thyristor 13A to move from charging to discharging of the supercapacitor 16. Figure 4B illustrates how the control unit 18 controls the series compensating unit 15 to move from the charging state of figure 4A into a bypassing state by switching on the second thyristor 12A, so that the current bypasses the supercapacitor 16 and runs through the second thyristor 12A and the reverse-conducting diode 14B of the fourth thyristor-based switch 14. Figure 4D illustrates an alternative to the bypassing state of figure 4B, wherein the control unit 18 is configured to turn on the third thyristor 13A, instead of the second thyristor 12A as in figure 4B, and bypass the supercapacitor 16 by conducting the current via the third thyristor 13A and the reverse-conducting diode 11 B of the first thyristor-based switch 11. Thus, in this alternative bypassing state, the reverse-conducting diode 11 B of the first thyristor-based switch 11 continues to conduct the current through the bypassing period, whereas in the bypassing shown in figure 4B, it is the reverse-conducting diode 14B of the fourth thyristor-based switch 14 that continues to conduct the current during the bypassing period. When the first half-cycle ends, the first thyristor 11A and the fourth thyristor 14A are “off” and the second thyristor 12A and the third thyristor 13A conduct the current. When the second half-cycle starts, as illustrated in figure 4E, the second thyristor 12A and the third thyristor 13A are turned off by the current running in the opposing direction of the thyristors 12A, 13A. When the second half-cycle starts, the current will instead be conducted through the reverseconducting diode 13B of the third thyristor-based switch 13 and the reverseconducting diode 12B of the second thyristor-based switch 12. The reverseconducting diode 13B of the third thyristor-based switch 13 and the reverseconducting diode 12B of the second thyristor-based switch 12 start the second half-cycle by conducting the current through the positive side of the supercapacitor 16, thereby charging the supercapacitor 16. Similar to the control of the first half-cycle, the control unit 18 is configured to control the switching from the charging state, of figure 4E, to a discharging state of the supercapacitor, as illustrated in figure 4G, via a bypassing state, as illustrated in figure 4F and figure 4H. In the discharging state of the second half-cycle shown in figure 4G, the first thyristor 11A and the fourth thyristor 14A are turned on. The control unit 18 is configured to change from the charging state, wherein the reverse-conducting diode 12B of the second thyristor-based switch 12 and the reverse-conducting diode 13B of the third thyristor-based switch 13B conduct the current, into the discharging state where the first thyristor 11 A and the fourth thyristor 14A conduct the current through the supercapacitor 16 on negative side of the supercapacitor, thereby discharging the supercapacitor 16.

The intermediate bypassing states of figures 4F and 4H are equal alternatives of the order in which the switching of the first thyristor 11 A and the fourth thyristor 14A is performed, similar to the alternatives of the switching order of the second and third thyristors 12A, 13A as illustrated in the bypassing states of figures 4B and 4D.

The control provided by means of the control unit 18 and the selection of the charging time period Tc and the discharging time period TDC will be described with reference to figure 5. The method 100 of providing reactive power to an electric power line 1 starts with obtaining a desired level for the reactive power, such as a reference level QREF. The method 100 may comprise obtaining a desired level for the active power, such as a reference level PREF. The method includes monitoring 115 the voltage level of the supercapacitor (SCAP) 16 and the current of the electric line 1 . The method further includes selecting 120 a charging time period Tc and a discharging time period TDC for the half-cycle so that the reactive power output QOUT equals the reference level QREF. The selection 120 of time periods is based on the monitored current and voltage level. The reactive power output QOUT is the sum of the reactive power “QBRIDGE” of the bridge circuit 15 and the reactive power QCAP of the fixed capacitor 17. If the active power should not be affected, the power charged “Pc“ provided by the supercapacitor 16 should equal the power discharged “PDC“ from the supercapacitor 16.

The reactive power QOUT outputted for the switching sequence, such as shown in figure 6, that includes starting the half-cycle by charging during a charging time period Tc, bypassing during an intermediate time period “TBP”, and ending the half-cycle during a discharging time period TDC, can be expressed mathematically as follows:

eq. 1

In equation 1 :

- II is the voltage level of the supercapacitor 16;

- 1 is the current through the series compensating unit 15, i.e. the current through the electric power line 1 ;

- co is the angular frequency of the power system;

- QCAP is the reactive power contribution from the fixed capacitor 17. The active power output POUT for a half-cycle with a charging time period, an intermediate bypassing period and a discharging time period in the orders illustrated in figure 6 can be expressed mathematically as follows:

[cos(a> T_c) — cos( a>T_DC)] eq. 2

It should be noted that the active power output POUT will be zero when the charging time Tc is equal to the discharging time TDC, in accordance with equation 2. From equation 1 , it can be noted that the longer the charging time Tc period and the longer the discharging time period TDC are, the larger the inductive reactive power output QBRIDGE of the bridge unit 15 will be. The reactive power output QOUT will be the sum of the inductive (or negative) reactive power output QBRIDGE of the bridge unit 15 and the capacitive (or positive) reactive power QCAP of the fixed capacitor 17. To provide an output reactive power level QOUT in accordance with a reference reactive power level QREF, in combination with a zero contribution to active power POUT, the length of the charging time period Tc and the length of the discharging time period TDC can be selected to be equal (to provide zero active power) and their total length Tc + TDC can be selected to achieve the reference reactive power level QREF; i.e. so that QOUT = QBRIDGE + QCAP = QREF. The operating ranges of the series compensating units 10, 20 will be described later with reference to figures 9A- 10B.

The “ordinary” thyristors 11-14 of the thyristor-based switches in figures 3-4 cannot be turned off when conducting current. Another type of thyristor-based switch is an IGCT (integrated gate-commutated thyristor), which can be turned off when conducting current. The capability of control of an IGCT provides enhanced possibilities for the control of the series compensating device, since an IGCT can be both turned on and turned off by means of a control gate of the IGCT. In a second embodiment of the series compensating device 20, illustrated in figure 7, the thyristor-based switches of the series compensating unit 25 consists of IGCTs 21-24 with reverse-conducting diodes. As in the first embodiment, the thyristor-based switches, in this case IGCTs 21-24, are arranged in a full-bridge configuration for controlling insertion of a supercapacitor 16 in series in the electric power line 1. The control unit 18 is configured to control switching of the IGCTs 21-24. The control unit 18 is configured to control the switching according to a first mode, wherein the series compensating device 20 in each half-cycle includes a sequence of charging, bypassing and discharging, as in the control of thyristors 11-14 illustrated with reference to figure 6, and which provides inductive reactive power, or “-Q”, as illustrated in figure 8A. The control unit 18 is also configured to control the switching according to a second mode, wherein the series compensating device 20 in each half-cycle includes a sequence of discharging, bypassing and charging, i.e. the opposite order, as illustrated in figure 8B and provide capacitive reactive power, or “+Q”.

In addition to being configured to provide output power in accordance with equations 1 and 2, the second embodiment of the series compensating device 20 is configured, in a second mode, to provide reactive power in accordance with equation 3:

^ TJT

QOUT = + — [sin (o)T_c) + sin(o>T_DC)] + QCAP eq. 3

The embodiments of the series compensating device 10, 20 with thyristors 11- 14, and IGCTs 21-24, respectively, are controlled to provide active power POUT in accordance with equation 1 . The series compensating device 10 with thyristors 11-14 is configured to provide reactive power, in this case inductive power, in accordance with equation 2, whereas with IGCTs 21-24, the series compensating device 20 is configured to selectively provide inductive reactive power and capacitive reactive power in accordance with equation 4:

^ TJT

QOUT = ± — [sin (o>T_c) + sin(o>T_DC)] + QCAP eq. 4 Figure 9A illustrates an operating range of the first embodiment of the series compensating unit 10 with thyristors 11-14, which operating range of the reactive power QBRIDGE extends from - QSCAP, MAX to zero, where QSCAP, MAX is the maximum available reactive power of the bridge unit, or series compensating unit, 15. It should be noted that the charging level of the supercapacitor 16 at any given time will limit the maximum available reactive, and active, power output. Figure 10A illustrates the operating range for the series compensating unit, or bridge unit, 25 with IGCTs, which operating range of the reactive power QBRIDGE extends between - QSCAP, MAX and + QSCAP, MAX , where QSCAP, MAX is the maximum available reactive power of the second embodiment of the bridge unit, or series compensating unit, 25.

Figure 9B and 10B illustrate the embodiments of the series compensating devices 10, 20, respectively, with a fixed capacitor providing the reactive (capacitive) power level QCAP. The operating range of the first embodiment 10, with thyristors 11-14, extends between reactive power levels of + QCAP - QSCAP, MAX and + QCAP. The operating range of the second embodiment 20, with IGCTs 21-24, extends between reactive power levels between + QCAP - QSCAP, MAX and + QCAP + QSCAP, MAX.

Figures 11 A-11 B illustrate compensating for a DC fault when controlling the series compensating units 15, 25, especially the series compensating unit 15 with thyristors 11-14.

As described with reference to the figures 4A-4H, each half-cycle starts with current running through two reverse-conducting diodes (11 B and 14B; or 12B and 13B, respectively) resulting in the charging of the supercapacitor 16. However, if a DC fault appears in the electric power line 1 , the blocking of the thyristors will be moved and the reverse-conducting diodes e.g. 11 B and 14B will start conducting earlier or later than the start of the half-cycle. This situation is illustrated in figure 11 A wherein a negative DC fault delays the shift from each negative half-cycle to the corresponding following positive half-cycle by a timing deviation A. Since the start of the charging is delayed by A, the charging timeperiod Tc becomes too short.

Figure 11 B illustrates how such DC fault can be handled in accordance with some embodiments. The control of the series compensating device 10, 20 is in figure 11 B adapted to adjust the timing of the switching from the charging state to the bypassing state by the amount A that is equal to the timing deviation caused by the DC fault. In more detail, figure 11 B illustrates how the switching to the bypassing state during the positive half-cycle is delayed a time period A, and the switching from the bypassing state to the discharging state is controlled to be earlier by the same time period A during the positive half-cycle. For the negative half-cycle the control is also adapted to adjust the switching, in the illustrated example of a negative DC fault, by switching from the charging state a time period A earlier, since the charging starts by a time period A too early. The switching from the bypassing state to the discharging state is similarly adjusted to be delayed a time period A since the zero crossing of the current will appear and close the thyristor-based switches, e.g. 11 A, 14A, with a delay corresponding to the timing deviation A caused by the DC fault.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. Further, although features and elements are described above in particular combinations, each features or element can be used alone without the other features and elements or in various combinations with or without other features and elements.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements, and the indefinite article “a” or “an” does not exclude a plurality. 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.

Claims

1 . A method for controlling power in an electric power line (1 ) by means of controlling a series compensating device (10, 20) comprising a capacitor (17) and a series compensating unit (15, 25), wherein the capacitor (17) and the series compensating unit (15, 25) are connected in series along the electric power line (1 ), the series compensating unit (15, 25) comprising a supercapacitor (16) arranged in a full bridge configuration with four thyristorbased switches (11-14, 21-24), each thyristor-based switch (11-14, 21-24) comprising a thyristor and a reverse-conducting diode, wherein the series compensating unit (15, 25) is configured for selective serial connection of the supercapacitor (16) into the electric power line (1 ) for charging (130) the supercapacitor (16), bypassing (140) the supercapacitor (16), or discharging (150) the supercapacitor (16), said method comprising: obtaining (110) an indication of a desired reactive power (QREF, <p), and controlling the series compensating unit (120) for each half-cycle of electric current of the power in the electric power line (1 ), wherein the controlling (120) includes: selecting (123) a duration time (Tc) for the charging of the supercapacitor (16), selecting (125) a duration time (TDC) for the discharging of the supercapacitor (16), and wherein said selecting (123) of duration time (Tc) for charging and said selecting (125) of duration time for discharging (TDC) are based on the desired reactive power (QREF).

2. The method according to claim 1 , wherein the capacitor (17) is a fixed capacitor.

3. The method according to claim 1 or 2, wherein the method further comprises obtaining (112) an indication of a desired active power (PREF, AV).

4. The method according to claim 3, wherein said selecting (123) of duration time (Tc) for charging and said selecting (125) of duration time for discharging (TDC) are further based on the desired active power (PREF, AV).

5. The method according to any one of the preceding claims, wherein the method further comprises: bypassing the capacitor (17), the series compensating unit (15, 25), or the series compensating device (10, 20).

6. The method according to any one of the preceding claims, wherein the method further comprises selecting (124) a duration time (TBP) for the bypassing of the supercapacitor within said half-cycle.

7. The method according to any one of the preceding claims, wherein the series compensating unit (15, 25) is arranged in series from a first part (1A) to a second part (1 B) of the electric power line (1 ).

8. The method according to any one of the preceding claims, wherein the series compensating unit (15, 25), during each half-cycle, outputs a reactive power (QBRIDGE); and wherein said duration time (Tc) for charging and said duration time for discharging (TDC) are selected such that a total reactive power (QOUT) output of the series compensating device (10, 20) is substantially equal to the desired reactive power (QREF).

9. The method according to claim 9, wherein the capacitor (17) outputs a reactive power (QCAP); and wherein the total reactive power (QOUT) comprises the reactive power (QBRIDGE) of the series compensating unit (15, 25) and the reactive power (QCAP) of the capacitor (17).

10. A series compensating device (10, 20) for controlling power in an electric power line (1), comprising: a capacitor (17); and a series compensating unit (15, 25), wherein the capacitor (17) and the series compensating unit (15, 25) are connected in series along the electric power line (1 ), the series compensating unit (15, 25) comprising a supercapacitor (16) arranged in a full bridge configuration with four thyristor-based switches (11-14, 21-24), each thyristorbased switch (11-14, 21-24) comprising: a thyristor; and a reverse-conducting diode, wherein the series compensating unit (15, 25) is configured for selective serial connection of the super-capacitor (16) into the electric power line (1 ); and a control unit (18) configured to: obtain an indication of a desired reactive power (QREF, <p); and control the series compensating unit (15, 25) for each half-cycle of the electric current of the power in the electric power line (1 ), in order to: charge (130) the supercapacitor (16); bypass (140) the supercapacitor (16); or discharge (150) the supercapacitor (16); and wherein the control unit (18) is further configured to: select a duration time (Tc) for the charging of the supercapacitor (16); and select a duration time (TDC) for the discharging of the supercapacitor (16); wherein the control unit (18) is further configured to select said duration time (Tc) for charging and said duration time for discharging (TDC) based on the desired reactive power (QREF).

11 . The series compensating device according to claim 10, wherein the capacitor (17) is a fixed capacitor.

12. The series compensating device according to claim 10 or 11 , wherein the control unit is further configured to obtain an indication of a desired active power (PREF, AV).

13. The series compensating device according to claim 12, wherein the control unit (18) is further configured to select said duration time (Tc) for charging and said duration time for discharging (TDC) based on the desired active power (PREF, AV).

14. The series compensating device according to any one of claims 10 to 13, wherein the control unit (18) is further configured to select a duration time (TBP) for the bypassing of the supercapacitor within said half-cycle.

15. The series compensating device accord to any one of claims 10 to 14, further comprising a bypassing arrangement comprising: a first bypass device (19A) configured to bypass the series compensating unit (15, 25); a second bypass device (19B) configured to bypass the capacitor (17); and a third bypass device (19C) configured to bypass the series compensating device (10, 20); and wherein the control unit (18) is configured to control the bypassing arrangement.

16. The series compensating device according to any one of the preceding claims, wherein the series compensating unit (15, 25) is arranged in series from a first part (1 A) to a second part (1 B) of the electric power line (1 ).

17. The series compensating device according to any one of the preceding claims, wherein the series compensating unit (15, 25), during each half-cycle, outputs a reactive power (QBRIDGE); and wherein the control unit is further configured to: select said duration time (Tc) for charging and said duration time for discharging (TDC) such that a total reactive power (QOUT) output of the series compensating device (10, 20) is substantially equal to the desired reactive power (QREF).

18. The series compensating device according to claim 17, wherein the capacitor (17) outputs a reactive power (QCAP); and wherein the total reactive power (QOUT) comprises the reactive power (QBRIDGE) of the series compensating unit (15, 25) and the reactive power (QCAP) of the capacitor (17).