Electricity: power supply or regulation systems – For reactive power control – Using impedance
Patent
1995-12-04
1997-09-30
Wong, Peter S.
Electricity: power supply or regulation systems
For reactive power control
Using impedance
323213, 323211, G05F 170
Patent
active
056729570
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention pertains to a method and a device for reducing voltage imbalances in a three-phase network by a static compensator.
Electrical power supply networks are primarily used for transmitting real power. Power which is produced and consumed must always be compensated for so as to prevent frequency changes from occurring. In the same way as the real power balance, the reactive power balance must also always be compensated for so that acceptable voltage conditions prevail in the network. The reactive power is primarily responsible for the voltage level. The real power and reactive power balance in the network must be compensated for at all times so that the voltage and frequency are within predetermined limits.
Power transmission networks are used more and more intensively because of increasing power consumption and limited network extension capabilities. Reactive power flows in the network are the main cause of voltage drops and additional network losses. A compensated reactive power balance in the network can be present, and the influence on the network voltage and network losses can thus be reduced by the objective use of reactive power means, such as capacitors and coils. The different dynamic requirements can be covered by switchable or controllable reactive power elements. However, continuous and dynamic change is in practice possible only through the use of converter circuits. Static compensators using thyristor technology represent the most economical solution at the moment for dynamic reactive power compensation.
The static compensator, also called a Static Var Compensator (SVC), may contain a thyristor-controlled inductor coil, thyristor-controlled capacitors (TSC: Thyristor-Switched Capacitor) and a permanently connected capacitor (FC: Fixed Capacitor). The method of operation and a comparison of two static compensators of different construction can be found in the journal "Energy & Automation X" (1989), No. 1, pages 12 to 17, under the title "A Closer Look at Thyristors in SVC Applications".
The article "BALANCING ASYMMETRIES BY MEANS OF THYRISTOR-CONTROLLED STATIC VAR COMPENSATORS" by B. Klerfors and T. Petersson, printed in Cigre Conference Proceedings, 28.08 to 03.09.1988, pages 1 to 6, proposes two different types of control for a static compensator (SVC) having a thyristor-controlled coil. One type of control is called "open-loop control" and the other "closed-loop control". Unbalanced loads, such as powerful single-phase loads or an electric furnace, lead to voltage fluctuations across the internal impedance of the supplying network, and can interfere with other loads. These unbalanced loads produce current and voltage components of an opposing voltage system, but no neutral system. The components of the opposing system in the network voltage have a negative influence on three-phase motors, as a result of which their rotors are increasingly heated (overload, shortened life). The control of a static compensator has the object of always driving the thyristors of the compensator so that the reactive current load on the network is as low as possible, is as constant as possible and, overall, the network load is as balanced as possible.
In the case of the open-loop control device, phase-related reactive power values are calculated from measured load currents and the determined line to line voltages of the network, from which reactive power values of the compensator are then calculated. These reactive power values are in each case superimposed on an adjustable reactive power value.
These sum signals are subsequently converted into triggering signals for the thyristors of the compensator. This type of control allows unbalanced load currents to be compensated for directly, that is to say the imbalances are reduced to an adjustable value.
The second control device (closed-loop control) in this article is illustrated in greater detail here and is described comprehensively, with reference to FIG. 1. In contrast to the open-loop control device, the line to lin
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Energy & Automation X (1989), No. 1, Gerhard Thumm et al.; A Closer Look at Thyristors in SVC Applications, pp. 12-17.
Cigre Conference (International Conference on Large High Voltage electric Systems), Paris, FR, 28 Sep. - 3 Sep. 1988, pp. 1-6, B. Klerfors et at.: Balancing Asymmetries by Means of Thyristor-Controlled Status VAR Compensators.
IEE Fifth International Conference on AC and DC Power Transmission, 1991, pp. 255-260, C. Welsh: System Variable Evaluation with Digital Signal Processors for SVC Applications.
Riley Shawn
Siemens Aktiengesellschaft
Wong Peter S.
LandOfFree
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