Electricity: power supply or regulation systems – For reactive power control – Using converter
Reexamination Certificate
2002-04-25
2003-12-30
Berhane, Adolf D. (Department: 2838)
Electricity: power supply or regulation systems
For reactive power control
Using converter
Reexamination Certificate
active
06670793
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to alternating current (ac) electric power systems for providing ac electric power. In particular, this invention relates to series voltage compensators for compensating voltage dips in such ac electric power systems and method thereof.
BACKGROUND
As is known in the art, an alternating current (ac) electric power system provides for generation, transmission and subsequent distribution of ac electric power to consumer locations. These consumer locations can be, for example, residential homes, commercial premises and industrial buildings or factories. Typically, an ac electric power system includes, among other things, conductors on which ac electric power is supplied to the consumer locations. Examples of such conductors include underground cables and overhead lines.
Conventionally, voltage of ac electric power generated from a power source is first stepped up for transmission and then stepped down for distribution. Generally, distribution voltages are in the kilovolt (kV) range. Such distribution voltages are then stepped down to a consumer voltage level that is commonly at 400 Volts (V). In most ac electric power systems, ac electric power is supplied to a consumer location via a multiplicity of connections forming an ac power supply network.
In distributing ac electric power to consumer locations, faults may occur within an ac power supply network. These faults can be, for example, an underground cable damaged by civil works or an overhead line failure due to lightning strikes. Such faults adversely affect supply voltages to consumer locations and are commonly referred to as voltage disturbances.
One type of voltage disturbance is known as a voltage dip. A voltage dip is a sudden and momentary reduction in a supply voltage from a normal level. Generally, magnitude and duration of a voltage dip depends on the causes of the voltage dip and also on control measures that are implemented to restore the supply voltage to its normal level. For example, the duration of a voltage dip typically depends on, among other things, the time taken to identify the fault location causing the voltage dip and for circuit breakers to trip and isolate the fault location.
Generally, the magnitude of a voltage dip is greater when nearer, in electrical terms, to the fault location causing the voltage dip. Hence, the magnitude of the voltage dip is usually different at different consumer locations and may range from 10% to 80% of a supply voltage. Also, in ac electric power systems providing supply voltages in two or more phases (polyphase), the magnitude of a voltage dip is generally different in each of these phases. Often, a single-phase line-to-ground fault at a fault location can end up as a voltage dip on all phases at a consumer location This is due to use of star-delta transformers that are known to transfer at least some magnitude of a voltage dip in one phase to the other phase(s).
One technique to compensate for a voltage dip is described in U.S. Pat. No. 5,329,222 issued to Gyugyi et al on Jul. 12
th
, 1994. This patent describes an apparatus and method for compensating utility line transients with a series injection voltage. However, the use of a three-phase inverter and a transformer for coupling the three-phase inverter into a high voltage distribution system results in the series injection voltage on each phase being coupled to each other to some extent. Coupling the series voltage injection voltage as such is not appropriate because a voltage dip may be different on all three phases and varying differently in time for each of these three phases.
Another technique to compensate for voltage dips is described in U.S. Pat. No. 5,883,796 issued to Cheng et al on Mar. 16
th
, 1999. This patent describes an apparatus and method for restoring voltage dips using a three-phase series injection inverter. Consequently, injection voltages provided by the apparatus and method of this patent has a similar limitation as in U.S. Pat. No. 5,329,222 in that the injection voltages are again coupled to some extent.
Yet another technique to compensate for voltage dips in supply voltages is with a current-to-voltage compensator. Current-to-voltage compensators operate on the basis that most voltage disturbances are due to single-phase line-to-ground faults in which the remaining phase(s) is(are) normal. By taking current from the normal phase(s) during a single-phase voltage dip, and by means of a semiconductor inverter converting this current into a series compensation voltage, the phase having the voltage dip can thus be compensated. Consequently, current-to-voltage compensators cannot adequately compensate voltage dips for all phases, particularly when all three phases have voltage dips.
In addition to the difficulty of compensating voltage dips in a single phase for a polyphase ac electric power system, energy storage in the above compensators is also a problem. This is because capacitors are typically used to store energy to provide injection or compensation voltages. Such capacitors can be expensive when a large energy storage capacity is required so as to provide compensation voltages for voltage dips of long durations.
Furthermore, voltage compensators using series injection inverters provide inverter voltages that are typically insufficient in magnitude to compensate voltage dips in ac electric power distribution systems. As such, these inverter voltages have to be stepped up in magnitude using step-up transformers. Use of step-up transformers adds significantly to the cost of conventional voltage compensators and this makes such compensators less desirable for general low-cost applications.
In addition to the above voltage compensators, uninterrupted power supplies (UPSs) can also be used to compensate voltage dips on one or more phases of an ac electric power system. However, UPSs are designed primarily to compensate another type of voltage disturbance known as a voltage collapse. In a voltage collapse, supply voltages to consumer locations are totally absent. Consequently, a UPS has to fully provide the supply voltages over the entire duration of the voltage collapse. This duration is typically much longer than that of voltage dips. Hence, a UPS requires energy storage that is substantially larger in capacity compared to voltage compensators having series injection inverters. Furthermore, inverters of UPSs operate in a continuous high frequency switching mode even in the absence of any voltage disturbance. Attendant losses during the continuous high frequency switching mode makes a UPS inefficient under normal supply voltage conditions.
Voltage dips can cause substantial financial losses especially when commercial or industrial operations are affected. Hence, alleviating voltage dips in an ac electric power system is desirable. Thus, a need clearly exists for a series voltage compensator that addresses the above problems in ac electric power systems to thereby provide supply voltages that are stable and reliable without incurring substantial additional costs.
SUMMARY
In accordance with one aspect of the invention, there is disclosed a dynamic series voltage compensator for compensating voltage dips in an alternating current electric power system providing at least one supply voltage, each of the at least one supply voltage being at a respective phase, the dynamic series voltage compensator including:
means for independently monitoring each of the at least one supply voltage;
means for generating digital signals indicative of voltage magnitude of the each of the at least one supply voltage over a present voltage cycle period;
means for comparing the digital signals with stored data indicative of voltage magnitude of the each of the at least one supply voltage over a preceding voltage cycle period;
means for determining difference between the digital signals and the stored data at corresponding time periods within the present and preceding voltage cycle periods;
and
means for controlling, when the difference exceeds a predetermined value for a corresp
Cornick Keith Joseph
Hee Pak Khian
Li Hongqun
Berhane Adolf D.
Merchant & Gould P.C.
SP Systems PTE LTD
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