WO2012037957A1

WO2012037957A1 - Dc-dc converter based load flow control in hvdc grids

Info

Publication number: WO2012037957A1
Application number: PCT/EP2010/063772
Authority: WO
Inventors: Praveen Kumar Barupati; Tomas U Jonsson; Subhasish Mukherjee; Sasitharan Subramanian
Original assignee: Abb Technology Ag
Priority date: 2010-09-20
Filing date: 2010-09-20
Publication date: 2012-03-29

Abstract

A DC-DC converter (200) for load flow control in a high voltage direct current (HVDC) grid is provided. The converter comprises four transistor-diode pairs (T1/D1–T4/D4) and an inductor (L), arranged to constitute a combination of two buck and two boost converters. The DC-DC converter may be used to introduce a series direct voltage (U ₀) into a transmission line of an HVDC grid for the purpose of controlling the load flow in the grid. The converter is operable in all four quadrants and is capable of introducing a direct voltage of either polarity in series with the transmission line, resulting in power extraction from the line or power infeed into the line, respectively, for both directions of current (I ₁₃₂) through the line. Further, an HVDC grid terminal and a method of load flow control in an HVDC grid are provided.

Description

DC-DC CONVERTER BASED LOAD FLOW CONTROL IN HVDC GRIDS

Field of the invention

The invention relates in general to high voltage direct current (HVDC) power transmission, and more specifically to DC-DC converter based load flow control in HVDC grids.

Background of the invention HVDC power transmission is becoming increasingly important due to the steadily rising need for bulk power delivery and power grid

interconnections. In particular, using HVDC transmission based on voltage source converters (VSC), power can be transmitted with cables and overhead lines which are embedded or overlapped with high voltage alternating current (AC) transmission networks. An attractive feature of HVDC

transmission is that the direction of power transmission can be changed by changing the direction of current.

An HVDC grid comprises multiple converter stations, also referred to as terminals, which convert an AC power source for transmission over an HVDC transmission line, i.e., cables or overhead lines, or vice versa. Different configurations of HVDC transmission lines are known, such as monopole, symmetric monopole, and bipolar. For instance, a bipolar HVDC transmission line comprises a positive pole line, a negative pole line, and a metallic return line connected to ground. Within the grid, each terminal may be connected to multiple terminals by HVDC transmission lines, resulting in different types of topologies. Such a multiple terminal grid enables efficient load flow control as well as congestion management, and has an improved stability against disturbances.

The known technique for load flow control in HVDC power transmission is to control the DC voltage of the AC-DC converters. In HVDC grids, however, more advanced load flow control techniques are desired. Summary of the invention

It is an object of the present invention to provide a more efficient alternative to the above techniques and prior art.

More specifically, it is an object of the present invention to provide an improved load flow control in high voltage direct current (HVDC) grids.

These and other objects of the present invention are achieved by means of a DC-DC converter having the features defined in independent claim 1 , by means of an HVDC grid terminal having the features defined in independent claim 6, and by means of a method of load flow control defined in independent claim 10. Embodiments of the invention are characterized by the dependent claims.

For the purpose of describing the present invention, the discussion is limited to an HVDC transmission line comprising two poles, e.g., a positive pole and ground. An electronic component, such as a diode, a transistor, an inductor, or a switch, which is described as being connected in series with the transmission line, is to be understood as being connected in series with one of the live pole lines, e.g., the positive pole line. A component which is described as being connected in parallel with the transmission line is to be understood as being connected between the two poles, e.g., between the positive pole line and ground. Embodiments covering other transmission line configurations may easily be envisaged.

According to a first aspect of the invention, a DC-DC converter for load flow control in an HVDC grid is provided. The HVDC grid comprises a first terminal, a second terminal, and a transmission line. The transmission line interconnects the first terminal and the second terminal. The DC-DC converter comprises a first switch, a second switch, a first diode, a second diode, a third diode, a fourth diode, and an inductor. The first switch is connected in series with the transmission line. The second switch is connected in series with the transmission line. The first diode is connected in parallel with the first switch. The second diode is connected in parallel with the second switch. The third diode is connected in parallel with the transmission line. The fourth diode is connected in parallel with the

transmission line. The inductor is connected between the first switch-diode pair and the second switch-diode pair, i.e., in series with the transmission line. The first switch, the third diode, and the inductor constitute a first buck converter. The second switch, the fourth diode, and the inductor constitute a second buck converter.

A buck converter is a DC-DC step-down converter comprising an inductor and two switches, usually a transistor and a diode, which control the inductor. The buck converter alternates between connecting the inductor to a power source, to store energy in the inductor, and discharging the inductor into the load.

According to a second aspect of the invention, an HVDC grid terminal is provided. The terminal comprises means for interconnecting a plurality of transmission lines, and a DC-DC converter. The DC-DC converter is arranged for introducing a series direct voltage into at least one of the plurality of transmission lines.

According to a third aspect of the invention, a method of load flow control in an HVDC grid is provided. The HVDC grid comprises a first terminal, a second terminal, and a transmission line. The transmission line interconnects the first terminal and the second terminal. The method comprises introducing a direct voltage in series with the transmission line.

Even though the invention has been described with respect to an HVDC grid comprising two terminals interconnected by a transmission line, an HVDC grid typically comprises more than two terminals, at least some of the terminals being interconnected over transmission lines with more than one other terminal in the grid. The task of controlling the load flow relates to directing the flow of power, i.e., DC current, from at least one terminal, a power source, to at least one load, through the available terminals and transmission lines of the grid. One of the purposes of load flow control is to avoid excess currents, i.e., currents exceeding the capabilities of a

component of the grid, and congestion by redirecting power to other available paths through the grid. The present invention makes use of an understanding that the load flow in an HVDC grid may be controlled, i.e., the flow of current through the grid may be directed along certain paths through the grid, by introducing a direct voltage, also referred to as DC voltage, in series with at least one transmission line of the grid. The effect of introducing a series direct voltage is to alter the potential difference over the line, i.e., the difference in DC voltage of the terminals at either end of the transmission line. An alternative point of view is that the series direct voltage amounts to introducing a fictive resistance in series with the transmission line. Depending on the sign of the voltage with respect to a current through the transmission line, the resistance may either be positive or negative. For the remainder, the direct voltage is defined as being positive if a positive fictive resistance is introduced into the transmission line, thereby increasing the resistance of the line and, consequently, reducing the current through the line. This amounts to active power extraction from the line, which power is, in consequence, redirected to other transmission lines. A negative direct voltage on the other hand results in a negative fictive resistance, i.e., the resistance of the line is reduced and, consequently, the current through the line is increased. This amounts to feeding power into the line, which power is retrieved from other lines.

Thus, by introducing a series direct voltage into one or several transmission lines of an HVDC grid, the power flow through the grid may be controlled by adjusting the current flow through the individual transmission lines.

A DC-DC converter according to the invention may be used for introducing a positive direct voltage in series with a transmission line, resulting in an extraction of power from the line, for either direction of current through the line. This is advantageous since the direction of current may reverse in an HVDC transmission line if the direction of power flow changes.

According to an embodiment of the invention, the DC-DC converter further comprises a third switch and a fourth switch. The third switch is connected in parallel with the third diode. The fourth switch is connected in parallel with the fourth diode. The third switch, the first diode, and the inductor constitute a first boost converter. The fourth switch, the second diode, and the inductor constitute a second boost converter.

A boost converter is a step-up DC-DC converter comprising an inductor and two switches, usually a transistor and a diode, which control the inductor. The boost converter alternates between charging the inductor, with current from a power source, and discharging the inductor into the load.

The combination of two buck converters and two boost converters is advantageous in that the resulting DC-DC converter may be operated either as a step-down converter or as a step-up converter for either direction of current flow through the transmission line, i.e., it may introduce either a positive or a negative series direct voltage into the line. Such a DC-DC converter may be operated in four different modes and is therefore capable of handling all situations which may arise during load flow control in an HVDC grid, i.e., it may extract power from a transmission line or feed power to the line, irrespective of the direction of current flow through the line.

According to an embodiment of the invention, the switches are transistors. The switches may, e.g., be bipolar transistors, such as bipolar junction transistors (BJTs) or insulated gate bipolar transistors (IGBTs). As an alternative, the switches may also be field effect transistors (FETs), such as MOSFETs, integrated gate-commutated thyristors (IGCTs), gate turn-off thyristors (GTOs), or forced-corn mutated thyristors.

According to an embodiment of the invention, the first diode, the first switch, and the third diode are arranged in a first unit, whereas the second diode, the second switch, and the fourth diode are arranged in a second unit. The two units may be arranged at separate parts of the HVDC grid, e.g., at two separate terminals. Arranging the components of the converter in two separate units which may be placed separately from each other, the two units being interconnected by a transmission line, is advantageous since the space requirements for housing the respective unit at either location are reduced.

According to an embodiment of the invention, the DC-DC converter further comprises DC line filters. This is advantageous since the current ripple caused by the switches may be smoothened out. According to an embodiment of the invention, a positive direct voltage is introduced for extracting power from the transmission line, and a negative direct voltage is introduced for feeding power to the transmission line.

Introducing a direct voltage of either positive or negative polarity allows for extracting power from the line and feeding power into the line, respectively, for both directions of current flow through the line.

According to an embodiment of the invention, the direct voltage is supplied by a DC-DC converter. The DC-DC converter may, e.g., be a single buck converter. This is sufficient if a only a positive voltage needs to be introduced into the transmission line for a fixed direction of current. As an alternative, the DC-DC converter may be a single boost converter. This is sufficient if a only a negative voltage needs to be introduced into the transmission line for a fixed direction of current. Using a single buck or boost converter is advantageous since only few electric components are required. If a DC-DC converter capable of introducing a positive voltage, and optionally, a negative voltage, for either direction of current through the transmission line is required, the DC-DC converter according to the first aspect of the invention may be used.

Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following. Brief description of the drawings

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of embodiments of the present invention, with reference to the appended drawings, in which:

Fig. 1 shows an HVDC grid, according to an embodiment of the invention. Fig. 2 shows a DC-DC converter, according to an embodiment of the invention.

Fig. 3 illustrates the modes of operation of a DC-DC converter, according to an embodiment of the invention.

Fig. 4 shows a DC-DC converter, according to another embodiment of the invention.

Fig. 5 shows a DC-DC converter, according to a further embodiment of the invention.

Fig. 6 shows a DC-DC converter, according to yet another embodiment of the invention.

Fig. 7 shows an HVDC grid terminal, according to an embodiment of the invention.

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

Detailed description

In Fig. 1 , an HVDC grid according to an embodiment of the invention is illustrated. HVDC grid 100 comprises multiple terminals 1 1 1 -1 14,

terminals 1 1 1 and 1 12 comprising AC-DC converters, and terminals 1 13 and 1 14 comprising DC-AC converters. Each of the terminals 1 1 1 -1 14 is on its AC side connected to an AC transmission line 121 -124, an AC power source 121 -124, or an AC load 121 -124. The DC sides of terminals 1 1 1 -1 14 are interconnected with each other over transmission lines 131 -134. HVDC grid 100 further comprises a DC-DC converter 141 connected in series with transmission line 132 for the purpose of introducing a direct voltage into the line. DC-DC converter 141 may, e.g., be an embodiment of the DC-DC converter according to the first aspect of the invention, such as DC-DC converters 200, 400, 500, or 600, described hereinafter.

With reference to Fig. 1 , the voltages and currents used to elucidate the invention are defined. ϋ_ΛΛ2 and denote the voltages at the DC sides of terminals 1 12 and 1 14, respectively, and /-132 denotes the current through transmission line 132. Current / ₃₂ is determined according to Ohm's law by the potential difference between terminals 1 12 and 1 14, i.e., ϋ_{Λ Λ2}-ϋ_{Λ Λ Δ}„ divided by the total resistance /¾otai of line 132 and terminals 1 12 and 1 14. Current A|₃₂ is defined as being positive, as is illustrated by an arrow in Fig. 1 , if current is flowing from terminal 1 12 to terminal 1 14, i.e., if l/ ₂ is larger

For the purpose of controlling the flow of power in grid 100, a direct voltage U₀ may be introduced in series with at least one of the transmission lines 131 -134. As an example, a series direct voltage U₀ is introduced into transmission line 132, by means of DC-DC converter 141 , for controlling current A|₃₂ through transmission line 132. U₀ is defined as being positive if the potential difference between terminals 1 12 and 1 14 is reduced, thereby reducing current A|₃₂ which is now dictated by U^₂-U^₄-U₀ divided by F?_totai- In other words, introducing a positive direct voltage U₀ results in a reduced potential difference between terminals 1 12 and 1 14, which is equivalent to a fictive increase of F?_totai-

For instance, if the power flow through transmission line 132 is to be reduced, a positive voltage U₀ may be introduced, resulting in a fictive increase of the resistance of transmission line 132. In other words, current / ₃₂ through the line is reduced and power is actively removed from the line. If, on the other hand, the power flow through transmission line 132 is to be increased, a negative voltage U₀ may be introduced by means of DC-DC converter 141 . This results in a fictive decrease of the resistance of transmission line 132. In other words, current /i₃₂ through the line is increased and power is fed into the line.

In Fig. 2, a DC-DC converter according to an embodiment of the invention is illustrated. DC-DC converter 200 comprises four transistor-diode pairs T1 /D1 -T4/D4 and an inductor L. The components are arranged such that a combination of two buck converters and two boost converters is accomplished. DC-DC converter 200 is operable in all four quadrants, or modes. In other words, it may provide both positive and negative voltages U₀ for both directions of currents, i.e., A|₃₂ being positive or negative. With reference to Fig. 3, the four modes of operation of DC-DC converter 200 are illustrated for the different quadrants. For each quadrant, i.e., depending on the direction of current /-i₃₂ through transmission line 132 and the desired polarity of voltage U₀, i.e., whether power should be extracted from the line or fed into the line, one of the four buck or boost converters comprised in DC-DC converter 200 is activated. This is achieved by controlling the state of each transistor T1 -T4 such that the transistor is in its on-state (denoted as "1 " in Fig. 3), in its off-state (denoted as "0" in Fig. 3), or in a state of duty ratio between 0 and 100%. The latter state may be achieved by supplying periodic pulses to the control inputs of transistors T1 -T4, such that the transistors periodically switch between their respective on- and off- states, and by adjusting the pulse width such that the desired duty ratio is achieved. This technique is referred to as pulse width modulation (PWM). The term duty ratio describes the portion of time a transistor is in its on-state, as compared to a period of time. Thus, a duty ratio of 0% corresponds to the off- state, whereas a duty ratio of 100% corresponds to the on-state.

It should further be noted that, in all transistors-diode pairs T1 /D1 - T4/D4, the transistor and the diode are connected in parallel with each other, albeit with different polarities, as is illustrated in Fig. 2.

Thus, returning to the different modes of operation illustrated in Fig. 3, in the first quadrant Q1 only T1 is operated in its PWM state whereas T2-T4 are switched off. In that way the first buck converter, which is made up by T1 , D3, and L, is active and may be used for introducing a positive voltage U₀ into transmission line 132 through which current is flowing from terminal 1 12 to terminal 1 14, i.e., A|₃₂ being positive. In the second quadrant Q2, T1 is in its on-state, bypassing D1 and allowing current to flow from terminal 1 12 to terminal 1 14, whereas T4 is operated in PWM, and T2 and T3 are switched off. This activates the second boost converter allowing to introduce a negative voltage U₀. In the third quadrant Q3, T2 is operated in PWM while T1 , T3, and T4, are switched off. In that way only the second buck converter is activated, being operable for current flowing from terminal 1 14 to terminal 1 12, i.e., / ₃₂ being negative. Finally, in the fourth quadrant Q4, T2 is in its on-state, bypassing D2 and allowing current to flow from terminal 1 14 to terminal 1 12, whereas T3 is operated in PWM, and T1 and T4 are in their off-states. This activates the first boost converter only.

In other words, by controlling transistors T1 -T4 as described with reference to Fig. 3, all four modes of operation of DC-DC converter 200 may be realized. Controlling T1 -T4 accordingly may, e.g., be achieved by supplying the control inputs of transistors T1 -T4 with control signals generated by a control unit (not shown in Fig. 2). For instance, if bipolar transistors are used, the control inputs are the bases of T1 -T4. As an alternative, if field effect transistors (FETs) are used, the control inputs are the gates of T1 -T4.

With reference to Fig. 4, another embodiment of the DC-DC converter according to the invention is described. DC-DC converter 400 comprises only two transistor-diode pairs T1 /D1 and T2/D2, whereas diodes D3 and D4 do not have transistors connected in parallel. DC-DC converter 400 may only be operated in two of the four modes described with reference to Fig. 3. More specifically, converter 400 is a combination of two buck converters only and is therefore operable in the first Q1 and the third quadrant Q3 alone. Thus, it can only be used for introducing a positive voltage U₀ into transmission line 132, for both directions of current / ₃₂. In other words, DC-DC

converter 400 may only be used for extracting power from transmission line 132.

A further embodiment of the DC-DC converter according to the invention is described with reference to Fig. 5. DC-DC converter 500 is similar to converter 400, described with reference to Fig. 4, in that it can only operate in two quadrants. However, converter 500 comprises two units 501 and 502, the first unit 501 comprising a transistor-diode pair T1 /D1 , a diode D3, and an inductor L1 , the second unit 502 comprising a transistor-diode pair S2/D2, a diode D4, and an inductor L2. Preferably, the sum of the inductance of inductors L1 and L2 is of the same size as the inductance of inductor L. As an alternative to inductors L1 and L2 shown in Fig. 5, a single inductor a either unit may be used, i.e., either L1 or L2.The two units 501 and 502 may be arranged at different parts of an HVDC grid, e.g., at either side of a

transmission line 503. For instance, referring to Fig. 1 , unit 501 may be arranged at terminal 1 12 whereas unit 502 may be arranged at terminal 1 14, the units being interconnected by transmission line 132. Even though DC-DC converter 500 has been described as being similar to converter 400, being operable in two quadrants only, one may easily envisage an embodiment of DC-DC converter 200 comprising two units, which units are arranged at separate parts of an HVDC grid, e.g., at either end of a transmission line.

With reference to Fig. 6, yet another embodiment of the DC-DC converter according to the invention is described. DC-DC converter 600 is similar to DC-DC converter 400 but is further arranged with DC line

filters C1 /L3 and C2/L4 for reducing the current ripple caused by the switching of the transistors. Even though DC-DC converter 600 has been described as being similar to converter 400, being operable in two quadrants only, one may easily envisage an embodiment of DC-DC converter 200 comprising DC line filters.

An embodiment of the HVDC grid terminal is described with reference to Fig. 7. Terminal 700 comprises an AC-DC converter 701 for converting power from an AC source 702 to DC power, or vice versa, a DC bus 703 for distributing power to HVDC transmission lines 704-706, and a DC-DC converter 707. DC-DC converter 707 may be used for introducing a series direct voltage into transmission line 706. Even though terminal 700 has been illustrated as being arranged for distributing power from one AC power source 702 to three HVDC transmission lines 704-706, an embodiment of the invention for other power distribution system configurations may easily be envisaged.

The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, even though embodiments of the DC-DC converter have been described as comprising single transistors or diodes, an assembly of transistors or diodes, respectively, is typically used in high voltage

applications. Further, other semiconductor switches than the transistors described hereinbefore may be used in a DC-DC converter according to an embodiment of the invention, such as gate turn-off thyristors (GTO). It will also be appreciated that embodiments of the second and third aspect of the invention merely using a single buck or boost converter, being operable in only one of the modes described with reference to Fig. 3, may be envisaged. Finally, embodiments of the DC-DC converter according to the invention may be used for applications other than load flow control in HVDC grids.

In conclusion, a DC-DC converter for load flow control in an HVDC grid is provided. The converter comprises four transistor-diode pairs and an inductor, arranged to constitute a combination of two buck and two boost converters. The DC-DC converter may be used to introduce a series direct voltage into a transmission line of an HVDC grid for the purpose of controlling the load flow in the grid. The converter is operable in all four quadrants and is capable of introducing a direct voltage of either polarity in series with the transmission line, resulting in power extraction from the line or power infeed into the line, respectively, for both directions of current through the line.

Further, an HVDC grid terminal and a method of load flow control in an HVDC grid are provided.

Claims

1 . A DC-DC converter (141 , 200, 400, 500, 600, 707) for load flow control in a high voltage direct current, HVDC, grid (100) comprising:

a first terminal (1 12, 700),

a second terminal (1 14, 700), and

a transmission line (132, 706) interconnecting the first terminal and the second terminal,

the DC-DC converter comprising:

a first switch (T1 ) connected in series with the transmission line, a second switch (T2) connected in series with the transmission line, a first diode (D1 ) connected in parallel with the first switch,

a second diode (D2) connected in parallel with the second switch, a third diode (D3) connected in parallel with the transmission line, a fourth diode (D4) connected in parallel with the transmission line, and an inductor (L) connected in series with the transmission line, the inductor being connected between the first switch-diode pair and the second switch-diode pair,

wherein the first switch, the third diode, and the inductor constitute a first buck converter, and the second switch, the fourth diode, and the inductor constitute a second buck converter.

2. The DC-DC converter (141 , 200, 707) according to claim 1 , further comprising:

a third switch (T3) connected in parallel with the third diode, and a fourth switch (T4) connected in parallel with the fourth diode, wherein the third switch, the first diode, and the inductor constitute a first boost converter, and the fourth switch, the second diode, and the inductor constitute a second boost converter.

3. The DC-DC converter (141 , 200, 400, 500, 600, 707) according to claim 1 or 2, wherein the switches are transistors.

4. The DC-DC converter (141 , 500, 707) according to claim 1 , wherein the first diode, the first switch, and the third diode are arranged in a first unit (501 ), and the second diode, the second switch, and the fourth diode are arranged in a second unit (502).

5. The DC-DC converter (141 , 600, 707) according to claim 1 , further comprising DC line filters (C1 , L3, C2, L4).

6. A high voltage direct current, HVDC, grid terminal (700) comprising: means (703) for interconnecting a plurality of transmission lines (704-

706), and

a DC-DC converter (707) being arranged for introducing a series direct voltage into at least one (706) of the plurality of transmission lines.

7. The terminal according to claim 6, wherein the DC-DC converter is a buck converter.

8. The terminal according to claim 6, wherein the DC-DC converter is a boost converter.

9. The terminal according to claim 6, wherein the DC-DC converter is the converter according to any one of the claims 1-5.

10. A method of load flow control in a high voltage direct current, HVDC, grid (100) comprising:

a first terminal (1 12, 700),

a second terminal (1 14, 700), and

the method comprising introducing a direct voltage ( Uo) in series with the transmission line.

1 1 . The method according to claim 10, wherein a positive direct voltage is introduced for extracting power from the transmission line, and a negative direct voltage is introduced for feeding power to the transmission line.

12. The method according to claim 10, wherein the direct voltage is supplied by a DC-DC converter (141 , 707).

13. The method according to claim 12, wherein the DC-DC converter is a buck converter.

14. The method according to claim 12, wherein the DC-DC converter is a boost converter.

15. The method according to claim 12, wherein the DC-DC converter is the DC-DC converter according to any one of the claims 1 to 5.