Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
A VSC-HVDC TERMINAL WITHOUT A FULL TRANSFORMER AND WITH A SERIES CAPACITOR
Document Type and Number:
WIPO Patent Application WO/2013/044940
Kind Code:
A1
Abstract:
A multilevel voltage source converter (VSC), having a phase output for direct connection to an AC network via a circuit arrangement, wherein the circuit arrangement includes at least one capacitor and at least one arrester connected in parallel. The circuit arrangement is adapted to limit fault currents. The alternating current network can thereby be connected to phase outputs of each phase leg without any full transformer.

More Like This:
Inventors:
JUHLIN LARS-ERIK (SE)
HARNEFORS LENNART (SE)
SONNATHI CHANDRA MOHAN (IN)
Application Number:
EP2011/066764
Publication Date:
April 04, 2013
Filing Date:
September 27, 2011
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ABB RESEARCH LTD (CH)
JUHLIN LARS-ERIK (SE)
HARNEFORS LENNART (SE)
SONNATHI CHANDRA MOHAN (IN)
International Classes:
H02J3/36; H02M7/483
Domestic Patent References:
WO2011107151A12011-09-09
WO2009149755A12009-12-17
Foreign References:
US5414612A1995-05-09
Other References:
ALLEBROD S ET AL: "New transformerless, scalable Modular Multilevel Converters for HVDC-transmission", POWER ELECTRONICS SPECIALISTS CONFERENCE, 2008. PESC 2008. IEEE, IEEE, PISCATAWAY, NJ, USA, 15 June 2008 (2008-06-15), pages 174 - 179, XP031299971, ISBN: 978-1-4244-1667-7
None
Attorney, Agent or Firm:
AHRENGART, Kenneth (Intellectual Property,Ingenjör Bååths gata 11,Mimer T, Floor E Västerås, SE)
Download PDF:
Claims:
CLAIMS

1. A multilevel Voltage Source Converter, VSC, including at least one converter arm per phase leg and having a phase output for direct connection to an AC network via a circuit arrangement, wherein the circuit arrangement includes at least one series capacitor connected in parallel with at least one arrester.

2. A multilevel VSC according to claim 1 , wherein the circuit arrangement includes a series connection of the at least one capacitor and at least one reactor.

3. A multilevel VSC according to claim 1 or 2, wherein the circuit arrangement includes a parallel connection of the at least one capacitor and at least one closing device.

4. A multilevel VSC according to claim 3, wherein the at least one closing device is adapted to short-circuit the at least one capacitor during a fault condition.

Description:
A VSC-HVDC terminal without a full transformer and with a series capacitor

TECHNICAL FIELD

The present invention relates to the technical field of power conversion. In particular, it concerns a multilevel Voltage Source Converter for conversion between direct voltage and alternating voltage.

BACKGROUND

High voltage direct current (HVDC) transmission lines are advantageously used for transmission of power over long distances. In an alternating current (AC) transmission system, the transmission losses are dependent on both active and reactive power transfers. The losses due to the reactive power transfer, for long transmission lines, will be substantial. However, in an HVDC transmission system, only active power is transferred. The losses in an HVDC transmission line will therefore be lower than the losses in an AC transmission line of the same length. A disadvantage of direct current (DC) transmission compared to AC transmission is that the interruption of a fault current is more difficult. A fault current in an AC system inherently exhibits frequent zero crossings, which facilitate for fast current interruption. In a DC system, no inherent zero crossings occur. In order to break a DC current, a zero crossing of the DC current generally has to be forced upon the system.

A converter for converting alternating voltage into direct voltage and vice versa in a converter station of a high voltage transmission system may be a line-commutated Current Source Converter (CSC) in which switching elements, such as thyristors, are turned off at zero crossing of the AC current in an AC-system connected to the converter. The converter may also be a forced commutated Voltage Source Converter (VSC), in which the switching elements are turn-off devices controlled according to a Pulse Width Modulation (PWM) pattern. Voltage Source Converters are power switches comprising a plurality of semiconductor chips such as e.g. insulated gate bipolar transistor (IGBT) power modules. They are often used in HVDC applications for converting direct current to alternating current and vice-versa or in static var compensators (SVC) for reactive power compensation in power transmission systems. Each converter valve has a number of power semiconductor devices connected in series for being able to together withstand the high voltage to be withstood by such a converter valve in the blocking state thereof.

A multilevel converter which is using a cascade configuration is based on serially connected cells to each alternating current (AC) phase, wherein the cells comprise semiconductor switches. The switches can either insert or bypass the capacitor of the cell, thereby allowing the output voltage to be varied

Three-phase Voltage Source Converter HVDC terminals are normally equipped with a full transformer. The transformer accounts for a significant amount of the total terminal costs and losses. It is a bulky unit, which in some cases presents a logistic problem. There is much to be gained from removing the transformer. This is not trivial, though, as the transformer integrates many desirable features. One important aspect is the leakage inductance, providing surge current limitation.

In theory, it is possible to limit the amplitude of the fault current by replacing the transformer with a sufficiently large series reactor. However, such a large series reactor will give unfavourable steady state characteristics of the converter. SUMMARY

It is an object of the present invention to provide an improved alternative to the above techniques and prior art. More specifically, it is an object of the present invention to provide a circuit arrangement connected to each of the phase legs of a multilevel Voltage Source Converter for limiting the fault current without impacting the steady state properties. To achieve these and other objects a multilevel VSC according to the independent claim is provided.

The invention is based on the insight that the main function of a full transformer in HVDC applications using Voltage Source Converters is to adapt a voltage between the alternating voltage network and the converter and that it is not an integral part of the converter as in line commuted HVDC transmissions. Since the converter voltages are increasing, there is no great purpose of using a transformer for adapting voltages between an alternating voltage network and a direct voltage network as these voltages are fairly similar. However, removing the full transformer will provide disadvantageous steady state qualities and high fault currents. Therefore, a circuit arrangement is connected to each of the phase legs of a multilevel VSC, wherein the circuit

arrangement is adapted to limit fault currents without impacting the steady state properties. The alternating current network can thereby be connected to the phase output of each phase leg without any full transformer.

According to an aspect of the invention, a multilevel VSC is provided as defined in claim 1. In accordance with an embodiment of the invention, a multilevel VSC includes at least one converter arm per phase leg. The multilevel VSC has a phase output for direct connection to an AC network via a circuit arrangement, wherein the circuit arrangement is connected in series between the phase output of each of the phase legs and the corresponding phase output of the AC network. The circuit arrangement includes at least one capacitor connected in parallel with at least one arrester. By combining the at least one capacitor with the at least one arrester the fault current can be limited and lower impedance in normal cases and higher impedance in cases of faults is provided. The present solution thereby enables the VSC to function without using a full transformer. In accordance with another embodiment of the invention, the circuit arrangement further includes a series connection of the at least one capacitor and at least one reactor.

In accordance with another embodiment of the invention, the circuit arrangement includes a parallel connection of the at least one capacitor and at least one closing device. The at least one closing device is adapted to short-circuit the at least one capacitor during a fault condition.

The primary advantage with the present solution is saved costs as the costs for said circuit arrangement is considerable lower than for a transformer.

To include at least one capacitor in a circuit arrangement can be very advantageous, if the capacitor is chosen correctly. It will give lower impedance in normal cases and higher impedance in cases of faults. Low impedance results in reduced losses in normal cases. High impedance limits the current, which is desirable in cases of faults.

DRAWINGS

Further characteristics and advantages of the present invention will emerge more clearly to a person skilled in the art from the following non-limited detailed description when considered in connection with the attached drawings, wherein:

Figure 1 schematically illustrates a phase leg of a VSC according to the prior art.

Figure 2 shows a schematic illustration of a phase leg of a multilevel VSC according to an embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic illustration of a phase leg of a VSC (10) according to the prior art. A control device (1 1 ) is adapted to control the switching cells by controlling the semiconductor devices thereof. Figure 1 also shows that a transformer (12) is

connected to the VSC. With reference to figure 2, an illustration of a multilevel VSC (20) according to an embodiment of the present invention is shown. The multilevel VSC (20) includes at least one converter arm per phase leg and has a phase output (21 ) for direct connection to an AC network via a circuit arrangement (22), wherein the circuit arrangement (22) is connected in series between the phase output of each of the phase legs and the corresponding phase output (21 ) of the AC network. The circuit arrangement includes at least one capacitor (23) connected in parallel with at least one arrester (24). Preferably, there are a plurality of arresters connected in parallel. In those cases when the alternating voltage network and the direct voltage network have similar voltages there is no need for a transformer and it can be removed without having to change the voltage. However, one of the reasons of using a transformer is its ability to isolate so that a DC component will not reach the AC network. Therefore, one of the main functions of the at least one capacitor (23) is to physically isolate to prevent a DC component from reaching the AC network. The at least one capacitor (23) is chosen in such a way that the total impedance is reduced. When a fault occurs, the at least one arrester (24) steps in which will lead to increased impedance. By including the at least one capacitor (23) in the circuit arrangement (22) it will result in lower impedance in the normal cases and higher impedance in the cases of faults, as compared to a circuit without a capacitor.

The function is that when in steady state condition the equivalent impedance of the combined circuit is:

where the total reactance should be positive.

During fault conditions with high fault currents, the voltage of the at least one capacitor (23) will rise to the protective level of the at least one arrester (24) and the fault current will be limited by the inductance only. The effect can be further increased by shortening the at least one capacitor (23) by at least one very fast closing device (25). The same principle as used for series capacitors in transmission lines. The at least one closing device (25) is adapted to short-circuit the at least one capacitor (23) during a fault condition.