Patent Application
20080315684
Voltage dip is one of the main concerns of power quality for industry and for the electrical system management in buildings. It is more important than other forms of power quality issues such as harmonics and power factor. This is mainly because of the extensive use of electronic and digital devices in modern electrical distribution as these devices are very sensitive to voltage dips. In a manufacturing plant, it may cause a complete shutdown of machinery and the associated financial losses could be very high. For buildings, the sudden shutdown of an escalator may result in an accident. The shutdown of the other power supplies or the elevators may cause inconvenience to the public. Voltage dip is also the major cause of power equipment shutdown.
Typical reasons for voltage dip are due to the switching of high current load, cable fault, lightning strike, and faults in either the distribution or the transmission networks. In some cases, due to the magnetic coupling or incorrect wiring, fault current from one network could be coupled to the adjacent network. Fault current flowing in finite impedances could also give rise to voltage dips in the distribution systems.
Typical voltage dip is usually of the order of tens to hundreds of milliseconds, but the damage would be extended over a long period and the associated financial loss may be very high. Typical examples affected in industrial processes are those in printing, glassmaking, garment manufacturing, and electroplating. Many of the modern devices trip when the supply voltage is reduced by more that 15% of the nominal value. High-intensity discharge (HID) lamps are being used commonly in stadium, playground, and as public lighting but they could be extinguished easily due to voltage dip and these lamps are unable to be switched on again within 5 min.
Many transient problems may only result in the loss of less than one cycle of the main's frequency. Typical causes are switching transients of heavy load or capacitor banks, electrostatic discharge, and large inverter staring transient. Dynamic voltage restorer (DVR) is now used to compensate for the voltage dip. Generally it is an electronic circuit that defects the voltage dip and produces necessary voltage to compensate for it. The line voltage in the network is then protected and any voltage dip is restored within a very short time. The associated appliances and voltage-sensitive devices are protected from fault.
The configuration of a DVR is to rectify ac into dc and the energy is stored using capacitors. The dc voltage is inverted into the ac, which is programmed to produce a corresponding amount of compensating voltage to offset the dips. Usually a transformer is used for isolation and voltage stepping. A common problem of the DVR is the response time. The device should respond to the voltage dips within a cycle. However, in order to give a sinusoidal voltage output, the dc/dc conversion of the DVR is realized using pulse-width modulation and a low-pass filter is needed. The dynamic performance of the low-pass filter is, however, a main concern. It usually slows down the dynamic response. Some DVRs use pure square wave instead of the sine wave. For these devices, even though the response is quick, as square waves can be generated instantaneously, the associated harmonics are large.
Another concern is isolation. Because a dc/ac converter is connected in series with the line, isolation is needed and the response time of the transformer then becomes a concern as virtually all power transformers operate at the main frequency, which is usually 50/60 Hz. The transformer size is also large and it inevitably increases the total size of the DVR. Moreover, the reliability of the conventional DVR has not been studied thoroughly in reported literatures.
The size, weight, and cost of DVR therefore warrant improvement. Conventionally, because of the use of isolation transformer, the total size of the DVR is dominated by the transformer. The overall weight and cost are therefore not favorable. However, by using high frequency switching techniques, the size of the transformer can be reduced significantly.
It is an object of the present invention to overcome the disadvantages and problems in the prior art.
The present invention proposes a DVR that is capable of very fast reaction while avoiding the generation of wide ranging harmonics.
The present invention has usefulness to address voltage dips for electrical systems in industrial environments
The present invention uses a dc/dc converter with a high frequency transformer whose size is at maximum 2% of the size of transformers used in prior art DVR.
These and other features, aspects, and advantages of the system and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings where:
The following description of certain exemplary embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Throughout this description, the term “nominal” refers to a value or level set by a particular government or entity. The term “nominal supply voltage” shall refer to the supply voltage level or minimum set by a government entity in a particular locale. The term “voltage dip” shall refer to delivery of a voltage below its nominal value for a length of time not exceeding 30 seconds.
The converter 200 also contains diode circuitry 205, for example, rectifier diodes. The diodes 205 are connected in antiparallel format. The diode circuitry 205 can further include switching devices, capacitors, and other electronic components.
The converter 200 is isolated from the DC link and the load 211. The DVR further contains an inverter 207 said inverter connected in line with the load 211. The inverter is preferably an H-bridge inverter. The inverter 207 is used for delivering a voltage from the dc/dc converter 200 to the load 211. Through the inverter 207, the square wave generated from the converter 200 is modulated via sinusoidal signal. The inverter 207 through impedances and circuit breakers provides a compensated square wave delivered to the load 211.
In the event that a voltage dip is not incurred, the voltage is not to be taken through the inverter 207 but rather through the static bypass 209 to the load 211. Upon a voltage dip occurring, the voltage will be analyzed, voltage dip will be detected, and taken through the inverter 207.
Measuring the voltage from a source 401 can contain the steps of delivering the current through a comparator circuit and comparing the actual signal against a reference signal, the reference signal being the voltage at an appropriate level. The comparator can, for example, be a subtractor circuit. Comparing the actual signal against the reference signal can generate an error signal. In the event the error signal is greater than or less than a set value, a determination will be made as to whether the stored energy should be tapped.
Determining whether the voltage (actual) is outside an allowable range 403 includes the steps of comparing the error signal against a set value. The set value can be created by establishing a percentage of a nominal supply voltage. The percentage can be between ±1 to 5% of the nominal supply voltage. In another embodiment, the percentage to set the value at is 1 to 10% of the nominal supply voltage.
In the event the error value is above or below the set value, which can represent a dip or surge in the voltage, the stored energy may be tapped and the energy, i.e., voltage, is delivered 405. The device to be tapped can be a capacitor. In the present invention, the power is delivered to the dc/dc converter. In one embodiment, the voltage is firstly delivered to the inverter of the converter. Following, the voltage is then passed to the high frequency transformer, for stepping up or stepping down as needed. As the transformer is of a high frequency nature, it is designed to process the high frequency switching waveform to diode circuitry. The dc/dc converter generates one or more square wave(s) 407. If the system of the present invention possesses one converter, one square wave will be generated. If the system possesses two converters, two square wave are generated, and so on.
Upon passage to a offset inverter, the generated square wave(s) is/are used to modulate a sinusoidal signal 409. In essence, the square wave is added to the sinusoidal signal, allowing the generation of a modulated wave. The modulated wave possesses the characteristics of a square wave with decreased harmonics, characteristic of a sinusoidal signal.
The modulated wave is then delivered through impedances and circuit breakers, to a load 411.
In other embodiments, n number of waveforms can be generated, as shown in
An H-bridge converter is used to generate the square waves for the compensation. The compensated voltage has an amplitude of 30 V and a pulse width of 5.1 ms. The test condition is based on the ac source voltage being sagged to 90Vrms.
The dual quasi-square voltage is applied to the DVR. The aim of this part of the test is to see how the THD varies under the same compensated voltage Vcom.
The following description of certain exemplary embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Throughout this description, the term ir conditioner? refers to a device for the cooling and often dehumidification of air in an enclosed environment.
Having described embodiments of the present system with reference to the accompanying drawings, it is to be understood that the present system is not limited to the precise embodiments, and that various changes and modifications may be effected therein by one having ordinary skill in the art without departing from the scope or spirit as defined in the appended claims.
In interpreting the appended claims, it should be understood that:
a) the word omprising? does not exclude the presence of other elements or acts than those listed in the given claim;
b) the word ? or
n? preceding an element does not exclude the presence of a plurality of such elements;
c) any reference signs in the claims do not limit their scope;
d) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise; and
e) no specific sequence of acts or steps is intended to be required unless specifically indicated.
