Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1995-09-18
1998-02-10
Wong, Peter S.
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 51, H02J 336
Patent
active
057175802
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for DC transmission between two converters. It also relates to a control arrangement for DC transmission between two converters and to a control device for that purpose.
In order to transmit electrical energy between two AC voltage networks, use is made, inter alia, of so-called DC transmission. In this case, there is connected to an AC voltage network a rectifier which generates a direct current which is fed, in turn, to an inverter. The inverter is connected to the second AC voltage network. In this way, the two AC voltage networks are coupled to one another via a DC connection. Such a DC transmission is disclosed in, for example, WO 92/22 118. The direction of energy flow can also be reversible in this case.
Various control methods are applied for operating such arrangements. An example of this is the so-called marginal current method, which is also disclosed in EP 0 197,352 B1. In this case, the rectifier and inverter each have a current controller whose characteristics are offset from one another by a marginal current. It is also possible in this case for both converters to have a voltage controller. In this way, each converter is assigned a precisely defined dynamic characteristic for the rectifier and inverter, which in each case has a current branch and voltage branch. The point of intersection of the respective dynamic characteristics then yields in operation the working point of the overall arrangement. Depending on the instantaneous operating situation, in such an arrangement only one controller (current or voltage) is ever active. The remaining controllers are not operative.
SUMMARY OF THE INVENTION
The present invention to specifies a method and a control arrangement by means of which the control response is improved in the case of DC transmission. Furthermore, the intention is to specify a control device suitable for this purpose.
The object is achieved according to the present invention by means of a method for DC transmission between converters, in particular for a high-voltage DC transmission system, to which converters there is respectively assigned a control device for forming their respective control angles. Using this method and starting from a common working point, each control device is respectively fed a system deviation relative to the working point in the case of a prescribable current and voltage value, and in which the respective control angle is employed in order to reach the working point, minimizing each system deviation.
Starting from a prescibable current and voltage value belonging to the common working point of the power to be transmitted between the converters, each control device is respectively fed a two-dimensional system deviation from the common working point. The respective control angles of the converters are then calculated in order to reach the common working point by minimizing each two-dimensional system deviation. The minimization of of the two-dimensional system deviation is performed simultaneously for both dimensions of the system deviation.
A substantial improvement in the control response of the DC transmission is achieved by the present invention, the dynamic performance being enhanced. Compared with the past, the DC transmission is substantially more insensitive to harmonic components and instabilities, thereby resulting in an increase in stability during transmission.
The respective system deviations are preferably described as at least two-dimensional variables, in particular as vectorial or complex variables. Current and voltage values are expediently employed as controlled variables. It is advantageous if the values for the working point are derived from a power to be transmitted. All that is then required is to prescribe a value as setpoint selection for the working point and to calculate each system deviation from the instantaneous current and voltage values of the respective converter. These respective embodiments permit the DC transmission to be further improved in a simple way u
REFERENCES:
patent: 4494179 (1985-01-01), Inokuchi et al.
patent: 4598350 (1986-07-01), Sakai et al.
patent: 4649466 (1987-03-01), Rogowsky
Han Y. J.
Siemens Aktiengesellschaft
Wong Peter S.
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