Electricity: power supply or regulation systems – For reactive power control – Using converter
Reexamination Certificate
2000-02-28
2002-02-19
Vu, Bao Q. (Department: 2838)
Electricity: power supply or regulation systems
For reactive power control
Using converter
C363S034000
Reexamination Certificate
active
06348778
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method and an apparatus for improving the voltage quality of a secondary power supply unit through the use of a compensation device which has a pulse-controlled power converter with at least one capacitive storage device, a matching filter, a closed-loop and open-loop control device and an incoming feeder device, the compensation device being coupled serially to the power system through the use of a transformer.
Such a compensation device is known from an article entitled “Netzqualität im Griff [Tackling Power System Quality]” in the journal “EV Report-Information des Bereichs Energieübertragung und -verteilung [Electronic Processing Report—Information on Power Transmission and Distribution]”, from the firm Siemens, pages 16 to 18, 1996, Order No. E50001-U700-R964. That compensation device, which is also referred to as SIPCON S, is connected directly into the load flow. Through the use of that compensation device, an additional voltage is added to the power system voltage and the supply voltage of a load is thus kept constant (secondary power supply unit). The energy which is fed-in in that device is taken from the voltage link to which power is continuously fed from the power system through the use of a diode rectifier as an incoming feeder device. An energy accumulator may also be provided as an incoming feeder device. Through the use of that compensation device, it is also possible to eliminate asymmetrical voltage dips or increases (1 or 2 pole faults) in the power system. It is necessary to make the incoming feeder device capable of feedback in order to compensate voltage increases. In addition, voltage distortions in the power system voltage which are generated by harmonics can be kept away from the power supply voltage of a load with that compensation device.
That paper also states that a pulse-width-modulated IGBT power converter, which has a d.c. voltage capacitor, is provided as a pulse-controlled power converter of that compensation device. The connection to the power system is made through the use of a matching filter, for example an LCL combination. The method of coupling the compensation device determines its method of operation. The serial method of coupling optimizes the voltage quality which is supplied to a load from the outside. In contrast, a parallel method of coupling cleans up the currents which go from a load into a power system. Correspondingly, the compensation device with serial coupling corresponds to a controlled voltage source, whereas the compensation device with parallel coupling corresponds to a controlled power source.
Voltage changes in a power supply system arise, for example, due to power system faults or switching operations. The changes can leave the permitted voltage range and thus lead to a failure of loads (for example a voltage dip to 50% of the rated value causes a contactor to be dropped or a rotational-speed-variable drive to switch off) or even to loads being destroyed (20% overvoltage). Therefore, for fault-free operation it is necessary to compensate those changes in the power system voltage. Studies have shown that the most frequent cause of voltage dips are faults in the transmission and distribution power system. The period of time until a fault is detected can extend between a few cycles and a few seconds. Those voltage dips (for example below 70% for a few cycles) can disrupt automated processes because the functioning of computers, robots and drives depends heavily on the voltage quality.
The increasing use of nonlinear loads (in particular diode rectifiers such as are located, for example, in power supply units of PCs, television sets, etc.) in power supply systems distorts the voltage increasingly. Their currents have, in fact, high harmonic levels and cause voltage drops across the power system impedances which are superimposed on the originally sinusoidal power system voltage. At excessively high values, those voltage distortions can lead to overloading of the power system operating equipment (e.g. transformers, compensation systems) and disrupt the orderly operation of other loads.
Public power companies and national working groups (for example IEC) have therefore issued recommendations relating to the maximum permissible voltage distortion which a load may cause. Those recommendations have been used as a basis for the EN standards which came into force in January 1996. So-called compatibility levels for individual harmonics in low-voltage power system have been defined, for example. Equipment manufacturers must develop their products in such a way that equipment can still function without faults with those distortion values. Power companies must ensure that the compatibility levels are not exceeded in their power system.
However, in many power systems the power system voltage distortion has already reached the compatibility level and a further increase is expected. For that reason, it is important to protect sensitive equipment against harmonics present in the power system voltage. That problem also includes the undesired filtering out of a ripple control signal into secondary power supply units.
Heretofore the problem of power system supply harmonics and of the filtering out of a ripple control signal has been solved by using conventional blocking filter circuits. Since the middle of the 80s, active filters have also been used which have control methods that operate both in the time domain and frequency domain. In the conference report entitled “New Trends in Active Filters” by H. Akagi, reprinted in “Conference Proceedings of EPE '95” in Seville, pages 0.017 to 0.026, various active filters have been proposed.
An ideal, three-phase power supply system supplies the load with three purely sinusoidal voltages which have a constant frequency and are displaced by 120° el. with respect to one another and have constant, identical peak values, i.e. a pure positive phase-sequence system space-vector with a rated voltage as amplitude. The ideal power system currents for that power system are proportional to the corresponding conductor/ground power system voltage in each phase. The proportionality factor is equal in all three phases and is constant with steady-state loads. That is because a required quantity of energy or active power is then transferred with the minimum collective current r.m.s value and thus with the lowest-possible capacity utilization of the power system. Those currents are defined as active currents. Such an ideal load displays a steady-state characteristic for the power supply system, like a three-phase balanced resistive impedance.
Any load which deviates from that characteristic produces current components which contribute nothing to the transmission of active power. Those are referred to as reactive currents. Assuming that the power supply voltages approximately correspond to the above-mentioned ideal case, those reactive currents contain the harmonic currents (including a d.c. component), the frequency of which is a multiple of the power system frequency, the fundamental displacement reactive currents, which are produced by the phase displacement between the power system voltage fundamental and power system current fundamental, and the fundamental negative phase-sequence system currents which are due to asymmetrical loads. The harmonic currents are generally divided into harmonics (harmonic frequency as an integral multiple of the power system frequency), interharmonics (harmonic frequency as a rational multiple of the power system frequency) and quasi-harmonics (harmonic frequency as an irrational multiple of the power system frequency).
Those reactive current components give rise to an undesired voltage drop at the power system impedances and cause distorted power system voltages for other loads. Likewise, loads which are switched statistically (nonperiodically) or power system errors give rise to distorted voltages for other loads.
Generally, the power system voltage is composed of the required
Voss Leon
Weinhold Michael
Zurowski Rainer
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Vu Bao Q.
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