WO2013020581A1

WO2013020581A1 - Power grid

Info

Publication number: WO2013020581A1
Application number: PCT/EP2011/063643
Authority: WO
Inventors: Eswar Kumar Chukaluri
Original assignee: Alstom Technology AG
Current assignee: GE Vernova GmbH
Priority date: 2011-08-08
Filing date: 2011-08-08
Publication date: 2013-02-14
Anticipated expiration: 2014-02-08

Abstract

A DC power grid (10) for use in high voltage direct current power transmission comprises a plurality of terminals (12,14,16), each terminal (12,14,16) being configured to import power from or export power to at least one other terminal (12,14,16), and a plurality of control units, each control unit being operably associated with a respective one of the terminals (12,14,16) and being configured to selectively control the respective terminal (12,14,16) to increase its voltage in response to an increase in imported or exported current at the respective terminal (12,14,16) and to decrease its voltage in response to a decrease in imported or exported current at the respective terminal (12,14,16).

Description

POWER GRID

This invention relates to a DC power grid for use in high voltage direct current (HVDC) transmission.

In HVDC power transmission networks, alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance.

Multi-terminal DC transmission and distribution grids are needed to support the emergence of HVDC power transmission. The interconnection between different terminals of a DC power grid to permit power flow within the DC power grid however means that a fault or other abnormal operating condition in one part of a DC power grid could influence power flow within the DC power grid and thereby adversely affect the operation of the remaining parts of the DC power grid.

According to an aspect of the invention, there is provided a DC power grid for use in high voltage direct current (HVDC) power transmission comprising a plurality of terminals, each terminal being configured to import power from or export power to at least one other terminal, and a plurality of control units, each control unit being operably associated with a respective one of the terminals and being configured to selectively control the respective terminal to increase its voltage in response to an increase in imported or exported current at the respective terminal and to decrease its voltage in response to a decrease in imported or exported current at the respective terminal.

For the purposes of this specification, the increase in voltage of a terminal with increasing imported current and the decrease in voltage of a terminal with decreasing imported current when the terminal imports power, and the increase in voltage of a terminal with increasing exported current and the decrease in voltage of a terminal with decreasing exported current when the terminal exports power are referred to as a positive-type droop characteristic.

Controlling each terminal of the DC power grid in accordance with a positive-type droop characteristic has not only been found to allow the DC power grid to respond to transient changes in power flow within the DC power grid, but also has been found to eliminate the risk of power flow reversal between terminals. This improves the reliability of the DC power grid, which reduces the risk of part or whole of the DC power grid going offline unnecessarily, and thereby minimises costs of repair and maintenance of the DC power grid and inconvenience to end users relying on the working of the DC power grid.

The configuration of the plurality of control units in the manner set out above enables local and autonomous control of the voltage at each terminal, which eliminates the need for a global control unit to govern the operation of all of the individual control units. This in turn allows each control unit to respond rapidly to changes in power flow within the DC power grid, since there is no delay resulting from telecommunications between each individual control unit and the global control unit. This also reduces the complexity and costs associated with designing and building the DC power grid, since there is no need for installation of a global control unit and communication cables linking the individual control units and the global control unit.

Furthermore the use of such control units results in a DC power grid that is readily scalable to include a small or large number of terminals without requiring substantial redesign of the DC power grid.

In embodiments of the invention the imported or exported current at the respective terminal increases when one or more terminals are blocked and/or disconnected from the DC power grid in response to a fault occurring, in use, in the DC power grid.

In the event of a fault occurring within the DC power grid, the capability of the DC power grid to respond to transient changes in power flow as set out above allows one or more terminals to be blocked and/or disconnected from the DC power grid to allow isolation and repair of the fault in the DC power grid without interrupting the operation of the remaining terminals of the DC power grid. A terminal may be blocked by, for example, turning off IGBTs in a power electronic converter, while a terminal may be disconnected from the DC power grid using, for example, a DC circuit breaker. Each control unit is preferably configured to selectively control the respective terminal to increase its voltage to achieve power flow equilibrium between the plurality of terminals in response to an increase in imported or exported current at the respective terminal.

The ability of each control unit to carry out local and autonomous control of the voltage at the corresponding terminal allows power flow equilibrium between terminals of the DC power grid to be achieved in a relatively short amount of time, particularly in cases of a fault occurring within the DC power grid or stochastic changes in imported or exported power due to, for example, intermittent power generation such as wind-farm power generation, where time delays due to telecommunications with a global control unit are too long to be admissible. Otherwise, in the event of one or more terminals being blocked and disconnected from the DC power grid, a slow response to the subsequent changes in power flow within the DC power grid may result in unstable and unreliable operation of the remaining unblocked terminals of the DC power grid.

In other embodiments of the invention each control unit may include an over-current detection apparatus configured to detect a change in imported or exported current at the respective terminal. The over- current detection apparatus may be replaced with any other type of equipment that is capable of detecting changes in current.

The structure of the DC power grid may vary depending on a number of factors including, for example, end-user requirements and availability of power sources.

Each terminal preferably includes a converter station or any other apparatus that is capable of importing or exporting power within a DC power grid.

The capability of the converter station to facilitate power conversion between AC and DC networks or DC and DC networks permits connection of AC networks and other DC networks to the DC power grid.

Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which:

Figure 1 shows a three-terminal DC power grid according to an embodiment of the invention;

Figure 2 illustrates the voltage droop characteristics of the three terminals of the DC power grid of Figure 1 ;

Figure 3 shows the DC power grid of Figure 1, in which a terminal is blocked and disconnected from the DC power grid; and

Figure 4 illustrates the modification of the voltage at a terminal of the DC power grid in response to a change in imported current, in accordance with a "positive-type" droop characteristic.

A DC power grid 10 according to an embodiment of the invention is shown in Figure 1.

The DC power grid 10 comprises a plurality of terminals 12,14,16 and a plurality of control units (not shown) . The plurality of terminals 12,14,16 includes three terminals, each terminal being connected to the other two terminals. A first terminal 12 is configured to be a power-exporting terminal, and therefore has a positive voltage droop characteristic 18. Each of the second and third terminals 14,16 are configured to a power-importing terminal, and therefore has a negative voltage droop characteristic 20,22. Each terminal 12,14,16 includes a converter station capable of carrying out power conversion between AC and DC networks or between DC and DC networks .

In use, the DC power grid 10 is connected to either an AC or DC network at the respective terminal .

The plurality of control units include three local control units, each local control unit being operably associated with a respective one of the first, second and third terminals 12,14,16. Each local control unit is configured to selectively control the respective terminal to increase its voltage in response to an increase in imported or exported current at the respective terminal, i.e. in accordance with a positive-type droop characteristic.

The voltages at the first, second and third terminals 12,14,16 are referred to as VI, V2 and V3 respectively, while the outgoing currents 24,26,28 at the first, second and third terminals 12,14,16 are referred to as II, 12 and 13 respectively.

The currents 30,32,34 flowing from the first terminal 12 to the second terminal 14, from the second terminal 14 to the third terminal 16 and from the first terminal 12 to the third terminal 16 are referred to as 112, 123 and 113 respectively, while the impedances 36,38,40 between the first and second terminals 12,14, the second and third terminals 14,16 and the first and third terminals 12,16 are referred to as Z12, Z23 and Z13 respectively.

The DC power grid 10 is characterised in accordance with a nodal admittance matrix, as shown in Equation ( 1 ) .

[I] = [Y] [V] — (1)

Where [Y] is the admittance matrix,

1 1

711

Z12 Z13

1

712

Z12

1

713

Z13

1 1

722

Z12 Z23

1

723

Z23

1 1

733

Z13 Z23

The voltage droop characteristic of each terminal 12,14,16 is shown in Equation (2) . AV

D — (2)

AI

Where AV is the increase or decrease in a terminal's voltage with a change in the terminal's current ΔΙ .

Equation (2) is re-arranged to define an iterative optimization process to determine the optimum system voltages for the terminals 12,14,16 in order to achieve a required power flow in the DC power grid 10.

Γ -V,

D

I_n I n-l

On the basis of Equation (3), Equation (1) is modified to form Equation (4) .

[ΔΙ] = [Y] [ AV] — (4)

Substituting Equation (3) into Equation

(4) ;

V,

Solving Equation (5) for [V]

3n - results in [V] [A] ^{_ i} [ b ]

Where

1

712 713

712 722 723

D2

1

713 723 - 733

D3

The power exported or imported by the first, second and third terminals 12,14,16 are referred to as PI, P2 and P3 respectively, and are calculated using Equation (6) .

The required level of power to be exported or imported by the first, second and third terminals 12,14,16 are referred to as Pld, P2d and P3d respectively. To meet these required levels of power, the optimum voltage for each terminal 12,14,16 of the DC power grid 10 is determined using an optimization function in the form of Equation (7) in accordance with the conditions set out in Equations (8) to (11) . min f{[V]) = {p\ - pldf + (p2 - p2df + (p3 - p3df --- (7:

[A] [V]= [b]

LRSPl

Initial state of [V] , [VO] LRSP2 --- (9:

LRSP3

Where LRSPl, LRSP2, LRSP2 are load reference set points at the first, second and third terminals 12,14,16 respectively, i.e. the voltage at which there is no power flow in the DC power grid 10. As such, LSRP1 = LRSP2 = LRSP3.

Vsl _ min

[j¾_min ] : ^,2_min ^'11^'

Vs3 min

Where Vsl_max, Vs2_max and Vs3_max are the maximum allowable voltages at the first, second and third terminals 12,14,16 respectively, and Vsl_min, Vs2 min and Vs3 min are the maximum allowable voltages at the first, second and third terminals 12,14,16 respectively.

It is envisaged that in other embodiments, the optimization function may be a different function or a set of functions designed to meet the requirements of the DC power grid 10 such as, for example, grid code compatibility or economic profitability.

A fault or other abnormal operating condition occurring in the DC power grid 10 could adversely affect the operation of the DC power grid 10. For example, a short circuit occurring in the DC power grid 10 causes high fault current to flow at one or more terminals 12,14,16, which could result in damage to these terminals 12,14,16. To minimise or prevent damage to the DC power grid 10, one or more terminals 12,14,16 of the DC power grid 10 must be blocked and/or disconnected from the DC power grid 10 to isolate the fault from the rest of the DC power grid 10 before commencing repair of the fault.

The operation of the DC power grid 10 in the event of an occurrence of a fault in the DC power grid 10 is described with reference to Figure 3.

In the event of a fault occurring in the DC power grid 10, the second terminal 14 is blocked and/or disconnected from the DC power grid 10 to isolate the fault from the rest of the DC power grid 10. This results in the third terminal 16 experiencing an increase in imported current. In response to the increase in imported current, the local control unit operably associated with the third terminal 16 controls the third terminal 16 to effect an increase 42 in voltage in accordance with the positive-type droop characteristic, as shown in Figure 4. This thereby increases the voltage at the third terminal 16 and thereby reduces the voltage difference between the first and third terminals 12,16. As such, this results in new stable operating voltages at the first and third terminals 12,16 and a reduction in power transferred between the first and third terminals 12,16. This thereby establishes power flow equilibrium between the first and third terminals 12,16. Consequently stable and reliable operation of the first and third terminals 12,16 is achieved during the period in which the second terminal 14 is kept offline.

For the purposes of this specification, both the decrease in voltage of a terminal with increasing imported current and the increase in voltage of a terminal with decreasing imported current when the terminal imports power, and the decrease in voltage of a terminal with increasing exported current and the increase in voltage of a terminal with decreasing exported current when the terminal exports power are referred to as a negative-type droop characteristic.

In a DC power grid where the control of all terminals are associated with a negative-type droop characteristic, there is a risk of power reversal between terminals during changes in power flow within the DC power grid. For example, when increasing the amount of power exported by a power-exporting terminal, the voltage of the power-exporting terminal is reduced in accordance with the negative-type droop characteristic. Such reduction of the voltage of the power-exporting terminal may result in the new voltage being lower than the voltage of a power-importing terminal, which causes the direction of power flow between the two terminals to be reversed. To avoid such power reversals, the negative-type droop characteristic of the terminals may be moved along its voltage axis. Such modification of the control of each terminal is however complicated to design, which not only decreases the reliability of the DC power grid but also adds cost to the design and installation of the DC power grid.

In contrast, the aforementioned control of the voltage of each terminal of the DC power grid in accordance with a positive-type droop characteristic not only eliminates the risk of power reversal between terminals during changes in power flow within the DC power grid, but is also relatively straightforward to design and implement, since there is no requirement to move the positive-type droop characteristic along its voltage axis.

This therefore results in a reliable DC power grid 10 and thereby reduces the risk of the entire DC power grid 10 going offline unnecessarily, which would otherwise result in additional costs of repair and maintenance of the DC power grid 10, and inconvenience to end users relying on the working of the DC power grid 10.

The provision of a local control unit operably associated with the respective terminal 12,14,16 enables the voltage at each terminal 12,14,16 to be locally and autonomously modified in response to a change in incoming or outgoing current without having to communicate the fault to a global control unit and wait for instructions. This in turn allows the voltage at each terminal 12,14,16 to be rapidly modified in response to transient changes in power flow within the DC power grid 10, since there is no delay resulting from telecommunications between each individual terminal and the global control unit. This also reduces the complexity and costs associated with designing and building the DC power grid 10 since there is no need for installation of a global control unit that acts to modify the voltage of each terminal in response to transient changes in power flow within the DC power grid, and communication cables linking the individual terminals and the global control unit.

In other embodiments it is envisaged that the DC power grid include a plurality of terminals and a plurality of control units, the number of terminals and control units being dependent on the requirements and capabilities of power applications associated with the DC power grid. This is because the structure of the DC power grid in Figure 1 is readily scalable to include a small or large number of terminals without requiring substantial redesign of the DC power grid.

Claims

1. A DC power grid (10) for use in high voltage direct current power transmission comprising a plurality of terminals (12, 14, 16), each terminal (12, 14, 16) being configured to import power from or export power to at least one other terminal (12, 14, 16), and a plurality of control units, each control unit being operably associated with a respective one of the terminals (12, 14, 16) and being configured to selectively control the respective terminal to increase its voltage in response to an increase in imported or exported current at the respective terminal (12, 14, 16) and to decrease its voltage in response to a decrease in imported or exported current at the respective terminal (12, 14, 16) .

2. A DC power grid (10) according to any preceding claim wherein the imported or exported current at the respective terminal increases when one or more terminals (12, 14, 16) are blocked and/or disconnected from the DC power grid in response to a fault occurring, in use, in the DC power grid.

3. A DC power grid (10) according to any preceding claim wherein each control unit is configured to selectively control the respective terminal (12, 14, 16) to increase its voltage to achieve power flow equilibrium between the plurality of terminals in response to an increase in imported or exported current at the respective terminal.

4. A DC power grid (10) according to any preceding claim wherein each control unit includes an over-current detection apparatus configured to detect a change in imported or exported current at the respective terminal (12, 14, 16) .

5. A DC power grid (10) according to preceding claim wherein each terminal includes converter station.