TECHNICAL FIELD

The present invention relates to a device for tapping elec¬ tric power from a high-voltage direct-current (d.c.) trans¬ mission line, which device comprises a first converter with d.c. connections and a.c. connections, the d.c. connections being adapted for connection into a transmission line, potential-separating transmission means, connected to the a.c. connections of the converter, for transfer of a.c. power from the converter to ground potential, a second converter, arranged at ground potential and connected to the transmission means, for transformation of power received from the transmission means into d.c. power, and a third converter connected to the second converter for transformation of power received from the second converter into a.c. power.

BACKGROUND ART, PROBLEMS

Power transmissions with high-voltage direct current (HVDC) operate at high voltages - typically several hundred kv. Arranging a tap of power from an HVDC line is a problem which has been known for a long time. When tapping off large powers, a complete converter station may be arranged in a known manner, since the high power justifies the high cost of the station. It is known to design such a station as a parallel tap, that is, the converter in the station operates at the full line voltage and taps part of the line current . In case of small tapped powers, however, the cost per kw is prohibitively high. It has therefore been proposed to arrange taps of series type, that is, the tapped converter is traversed by the whole line current, whereas the direct voltage drop across the converter is only as large as the

tapped power requires.

A problem with series tappings is to bring down the tapped power from line potential to ground potential in a practical and economical way.

In the Swedish printed patent application with publication number 463 953, a device for series tapping from an HVDC line is described. A line-commutated converter is connected with its d.c. side into the line and is connected with its a.c. side, via a transformer, to a self-commutated conver¬ ter, operating as a rectifier, at ground potential. This converter supplies an inverter, connected to a local power network, via a d.c. intermediate link. Since the potential- separating transformer must be designed for a voltage equal to the line voltage between its two windings, it becomes so expensive that the use of the device in connection with small powers is not economically acceptable.

In Asplund, Hammarsten, Lagerup, Lescale: "Small Series

Tapping in HVDC Systems", Proc III Symposium of Specialists in Electrical Operational and Expansion Planning (III SEPOPE) , May 18-22, 1992, Belo Horizonte, Brazil, a device for series tapping is described in which a plurality of series-connected line-commutated three-phase converters are connected into the HVDC line and each connected to a sepa¬ rate winding of a three-phase transformer arranged at potential. The transformer steps up the alternating voltage from the converters, and the stepped-up three-phase voltage is transmitted to a local network, located at ground poten¬ tial, via three capacitor chains which take up the potential difference between the HVDC line and ground. Both the capa¬ citors and the transformer operate at the frequency of the local network and, therefore, entail large dimensions and a high price.

SUMMARY OF THE INVENTION

The invention aims to provide a device of the kind mentioned in the introductory part of the description, by means of which a tapping can be made in a simple and economically advantageous way also at low tapped powers.

What characterizes a device according to the invention will become clear from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following with reference to the accompanying Figures 1, 2, 3a, 3b and 4, wherein

Figure 1 shows an explanatory diagram of an HVDC transmission provided with a device according to the invention,

Figure 2 shows an example of the design of the converter connected into the HVDC line,

Figure 3a and Figure 3b illustrate two different ways of controlling this converter, and

Figure 4 schematically shows how a control device for the converter may be designed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows an HVDC transmission provided with a device according to the invention. The transmission is of single- pole design and has two converter stations SRI and SRII. On their a.c. sides the stations are connected to the three- phase power networks NI and Nil, between which power is transferred by means of the transmission. The stations are interconnected by means of a d.c. line L, through which a

direct current Id flows. The voltage of the d.c. line rela¬ tive to ground may be of the order of magnitude of 500 kV and the maximum current Id 2 kA.

In the following it is assumed, as an example, that a tapping is desired which may supply a power of a maximum of about 650 kw to a local network LN.

According to the invention, a first converter SRI is connec- ted with its d.c. connections LSla and LSlb into line L and adapted to be traversed by the line current Id. The conver¬ ter is a current-source, self-commutated converter. An example of its configuration and control will be described below with reference to Figures 2-4. The converter is adapted to operate with a frequency which is considerably higher than the ordinary power frequency (50 or 60 Hz), preferably within the range above 3 kHz. From the point of view of acoustic disturbance, it may be advantageous to select a frequency above the audible frequency range, that is, exceeding 15 - 20 kHz. Suitably, however, the frequency is not selected so high that it interferes with a carrier frequency transmission possibly arranged on the line. Another factor which influences the frequency selection are the losses in the converter which increase with the frequency. A frequency within the range 20-30 kHz may be a suitable selection. In the example described here, the converter has an operating frequency of 24 kHz.

The converter is connected in parallel with a gate turn-off thyristor GTO. The thyristor is adapted to be fired very rapidly at overcurrent in the line, and it serves as over- current protection for the converter. An electric switch SW is arranged to bridge the converter when it is not in opera¬ tion and possibly also to relieve the thyristor GTO at an overcurrent.

The converter SRI has the a.c. connections VSla and VSlb. To these a transformer Tl is connected. This transformer (like

the converter SRI) is arranged at the same potential as the line. The transformer is adapted to step up the voltage from the converter SRI to such a high value that the tapped-off power can be transferred to ground potential via capacitors of a moderate size. The output voltage from the converter may, for example, have an R.M.S. value of about 350 V, and the transformer may have such a transformation ratio that its secondary voltage becomes 10 kV. Because of the high operating frequency, the transformer is designed with a ferrite core, and it has proved that the transformer for the same reason may be given very moderate dimensions.

The components described so far lie at the same potential as the line L. To provide the necessary potential separation between line potential and ground potential, two capacitors Cl and C2 are each arranged in one of the two conductors from the secondary winding of the transformer Tl. The vol¬ tage and the operating frequency may be selected so high that, in case of moderate tapped powers, capacitors with small capacitances may be used. It has proved that capacitor voltage transformers of standard type may often be used, which, since an existing standard component may be used, provides a simple and economically favourable solution. In typical cases, such a capacitor has a capacitance of the order of magnitude of 10 nF.

A transformer T2 is adapted for stepping down the voltage and designed, in principle, in the same way as the transfor¬ mer Tl. The transformers need not, however, have the same ratio, but the ratio of transformer T2 may be selected to obtain a suitable alternating voltage to the subsequent rectifier SR2 and hence a suitable direct voltage for the inverter SR3.

The transformers are so dimensioned that their leakage reactances together with the capacitors Cl and C2 form a series-resonance circuit with a resonance frequency equal to the selected operating frequency. In this way, the voltage

drop may be greatly reduced when transferring between the line and ground.

The secondary voltage from the transformer T2 is supplied to a rectifier SR2 , which preferably consists of an uncontroll¬ ed single-phase diode rectifier. Its output direct voltage feeds a direct voltage intermediate link with a capacitor C3.

A third three-phase converter SR3 is connected with its d.c. terminals LS3a and LS3b to the intermediate link and with its a.c. terminals VS3 to the local network LN. This conver¬ ter may be a controllable inverter of any arbitrary known kind. In those cases where the local network has its own alternating voltage sources, the inverter may be a line- commutated converter, but otherwise a self-commutated inver¬ ter is selected.

Figure 2 shows an example of how the first converter SRl in Figure 1 may be designed. It consists of a single-phase bridge with a semiconductor valve in each branch, which valve may be turned on and off by means of control signals. The valves are designated TRl - TR4 and may preferably consist of so-called IGBT transistors (IGBT = Insulated Gate Bipolar Transistor) . Such transistors may be obtained for such high currents and voltages that moderate tapped powers can be handled with one single transistor or only few transistors in each bridge branch, for example a small number of parallel-connected transistors. The valve bridge is connected, with its d.c. terminals LSla and LSlb, into the line L, and the a.c. terminals of the bridge constitute the a.c. terminals VSla and VSlb of the converter. Between the a.c. terminals, a capacitor C4 is connected, across which the alternating voltage of the converter is generated.

A coil X, a so-called line trap, is arranged in series with the bridge and prevents the bridge from disturbing a high- frequency carrier frequency transmission arranged on the

line L. A capacitor C5 bridges the bridge and allows carrier frequency signals to pass by.

Figure 3a illustrates a method of controlling the bridge shown in Figure 2. The figure .shows, plotted against time t, the currents II - 14 in the four branches as well as the capacitor current Ic and the capacitor voltage Uc- A cycle of the generated voltage comprises the time between the times tl and t5. The following shows which valves are con- ducting during the different intervals during the cycle:

The capacitor current consists of pulses with the pulse length tp and with alternately opposite polarities. The current pulses charge and discharge the capacitor C4, whereby an a.c. voltage Uc is obtained across the capacitor, which voltage constitutes the output voltage of the conver¬ ter. The amplitude of the currents through the valves and the capacitor becomes equal to the line current Id.

By control of the pulse length tp, the amplitude of the capacitor voltage may be maintained at a desired constant value independently of variations in the line current and in the tapped load. Figure 4 schematically shows how this can be achieved. The capacitor voltage Uc is sensed and a measurement quantity U'c is formed, which is a measure of the capacitor voltage, for example its peak value or recti¬ fied mean value. This measurement quantity is compared in a difference generator DB with a reference value U _r and the difference dU is supplied to a control device SD, which generates control pulses spl - sp4 to the valves for control thereof between non-conducting and conducting states and vice versa in the manner stated above.

An alternative control method is shown in Figure 3b. The valves which are conducting during each interval are as follows:

The capacitor current and the capacitor voltage have the same form as in Figure 3a, and the pulse width tp of the capacitor current pulses may be used in the same way for controlling the capacitor voltage. The difference between this control method and that shown in Figure 3a is that, according to Figure 3a, only two valves are conducting during the intervals when the capacitor current is zero, whereas according to Figure 3b all four valves are conduc¬ ting during these intervals, during which the current through each valve is equal to half the line current Id.

The device according to the invention shown above is only an example, and many other embodiments are feasible within the scope of the invention.

Thus, the converter SRI connected into the line may consist of some other type of converter than that described above and/or with some other type of controllable semiconductor valves. In principle, any self-commutated current-source converter may be used, for example a converter known per se of the type in which an approximately sinusoidal alternating voltage is generated through pulse-width modulation. In the above example, the converter is of single-phase design, which provides the simplest embodiment, but, alternatively, it may be of three-phase design (in which case three coupling capacitors instead of two must be arranged, and in which case the rectifying converter SR2 must be of three- phase design) .

Likewise, the two converters SR2 and SR3 arranged at ground potential may be designed differently from what is described above. Thus, for example, the converter SR2 may be designed as a controllable converter with control means for keeping the intermediate link voltage constant .

As mentioned above, the use of capacitor voltage transform¬ ers of standard type affords great economical and practical advantages. Alternatively, however, especially in the case of large tapped powers, conventional power capacitors may be used.

The control system for the device according to the invention may be designed in other ways than what has been described above. Thus, for example, the controlled quantity may alter¬ natively consist of the direct voltage in the intermediate link between the converters SR2 and SR3 in Figure 1.

In those cases where the power in question can be trans- mitted via capacitors of a reasonable magnitude without step-up of the voltage, the transformers Tl and T2 may be omitted.