Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-07-24
2003-11-04
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
Reexamination Certificate
active
06643154
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for controlling the extinction angle for a line-commutated converter in dependence on a minimum reference value for the extinction angle, and to a device for carrying out the method.
BACKGROUND ART
An installation for transmission of high-voltage-direct current between two ac voltage networks comprises two converter stations, each one being connected on its ac voltage side to a respective one of the ac voltage net works, and a common dc connection.
Each one of the converter stations comprises a converter, usually at least one converter transformer for connection of the converter to the ac voltage network, and shunt filters for generating reactive power and for filtering harmonics. The converters are normally line-commutated, current-source converters, by which is to be understood that the current commutation between the valves of the converters, which usually are arranged in six-pulse bridges, takes place by means of voltages occurring in the ac voltage network, and that the dc connection, as viewed from the converters, occurs as a stiff current source. A converter valve usually comprises a plurality of mutually series-connected semiconductor elements, capable of being fired, in the form of thyristors.
During normal operation, one of the converters, hereinafter referred to as the rectifier, operates in rectifier operation, and the other, hereinafter referred to as the inverter, operates in inverter operation. Control equipment for the respective converter generates a control signal corresponding to a control angle &agr;, at which firing pulses are supplied to the valves of the converter. For the purpose of minimizing the consumption of reactive power by the converters and reducing the stresses on components included in the converter stations, it is advantageous to control the rectifier with the smallest possible control angle &agr; and to control the inverter with the smallest possible extinction angle &ggr; (margin of commutation) without jeopardizing the controlled operation. The control system of the installation is therefore usually designed such that the inverter is controlled to a maximum dc voltage which is suitable for the operating conditions of the installation, taking into consideration safety margins with respect to commutating errors, voltage variations on the ac voltage network and other deviations from nominal operation which may occur. The rectifier is controlled in current control, the reference value of which is formed in dependence on a current order, which in its turn is formed in dependence on a power order and the actual dc voltage in such a way that the direct current and hence the transferred power remain at a desired value.
Usually, the control equipment for rectifiers and inverters are designed identical, whereby, in the rectifier, a current controller is activated and, in the inverter, control equipment for a control which aims at maintaining the extinction angle at, but not allowing it to fall below, a preselected minimum value is activated.
For a general description of the technique for transsot mission of high-voltage direct current, reference is made to {dot over (A)}ke Ekström: High Power Electronics HVDC and SVC, The Royal Institute of Technology, Stockholm 1990, in particular chapter 4.
The current controller in the inverter is supplied, in addition to the reference value of the current in the dc connection and its actual value, also with a so-called current margin with such a sign that the control equipment of the inverter strives to reduce the direct current controlled by the rectifier. During stationary inverter operation, the output signal from the current controller of the inverter will thereby assume a maximum value limited by a limiting signal and the value of the control angle &agr; ordered by the inverter is determined by the limiting signal.
Between the control angle &agr;, the extinction angle &ggr; and the overlap angle &mgr;, during which commutation between two valves takes place, the known relationship &agr;+&mgr;+&ggr;=180° prevails. It is thus desirable, for the inverter, to determine the control angle such that the extinction angle (the margin of commutation) remains at a predetermined minimum value.
U.S. Pat. No. 4,563,732 describes control equipment where the value of the control angle &agr; ordered by the inverter is formed in dependence on the output signal from an OR circuit. The OR circuit is supplied with the output signal from the current controller of the inverter as well as with a control signal formed in dependence on a continuing predicted value of the extinction angle which would be the result if a commutation were to be started at the calculating instant. The valves of the inverter are thus fired in dependence on the signal which is supplied to the OR circuit earliest.
A control system for determining the above-mentioned predicted value of the extinction angle is further described in an article in a journal, {dot over (A)}ke Ekström and Göte Liss; A refined HVDC Control System. IEEE Transactions on Power Apparatus and Systems, Vol. 59, No. 5/6, June 1970, pages 723-732. The system is based on the fact that a certain voltage-time area is required for carrying out the commutation. The criterion for the predicting control system is that, after completed commutation, the remaining voltage-time area is to exceed a certain prescribed minimum value. In the event that a voltage or current disturbance should occur during a commutation in progress, there is then a possibility of terminating this without any commutating error. A predicting member calculates continuously, by a triangular approximation of the curve shape of the voltage, the total voltage-time area which would remain if the semiconductor elements of the valve were to be fired at the running instant. The calculation is carried out by subtracting, from the continuously calculated triangular voltage-time area, a voltage-time area corresponding to the voltage-time area during the time the commutation process is taking place. This latter voltage-time area is directly proportional to the direct current in the dc transmission. The predicting control system gives a firing signal to the conmuutating valve, that is, the valve which is in turn to take over the current, when the remaining voltage-time area becomes equal to a given reference value, formed in dependence on a given minimum reference value &ggr;
min
for the extinction angle.
In practice, the control equipment also comprises means for controlling that the ac voltage and the minimum control angle lie within the prescribed limits before a firing order is generated.
In {dot over (A)}ke Ekström: High Power Electronics HVDC and SVC, The Royal Institute of Technology, Stockholm 1990, pages 7-14 to 7-16 and FIGS. 7-10, an embodiment of such control equipment is described, where the extinction angel &ggr; occurs explicitly. By means of a predicting member, containing information about the preceding zero crossing of the voltage, there is predicted continuously, in dependence on sensed values of commutating voltage and current, which commutating margin &ggr;
pred
would be obtained if firing were to take place at the moment of prediction. The predicted commutating margin is compared with a reference value &ggr;
order
for the minimum extinction angle and when the predicted value becomes equal to the reference value, a firing order is generated for the commutating valve.
In the same publication, on pages 7-13 and FIGS. 7-9, also an embodiment of such control equipment is shown, based on negative feedback of a sensed value of the extinction angle. A reference value &ggr;
order
for the minimum extinction angle is compared with the sensed value and when this falls below the reference value, a firing order for the commutating valve is generated. To compensate for the delay caused by the feedback of the sensed value of the extinction angle, an addition to the reference value, formed in dependence on sensed commutating voltage and current, is given
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Sterrett Jeffrey
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