Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2000-05-30
2001-02-27
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S040000, C363S044000
Reexamination Certificate
active
06195274
ABSTRACT:
BACKGROUND OF INVENTION
The invention relates to a method according to the preambles of independent claims
1
and
5
for defining an instantaneous value of a current of a pulse-controlled inductive load when the impedance of the load is known, the method comprising the steps of
measuring the output voltage of a pulsed voltage source, and
measuring the output current of the pulsed voltage source.
The current or magnetic flux of the inductive load is normally controlled by changing the voltage affecting over the load, the voltage typically consisting of single or multi-level voltage pulses generated by semiconductor switches. The simplest solution is that the switches operate at a fixed switching frequency, whereby the pulse width of the voltage pulses determines the average voltage level in the load. An example for such a pulsed voltage source is a pulse-width-modulated (PWM) voltage source. The current of the load is here quite well controlled dynamically, if the switching frequency is sufficiently high.
On account of the switching losses of the semiconductor switches, the switching frequency is kept as low as possible, particularly in connection with high-power apparatus, such as frequency converters that control squirrel-cage induction motors. Accurate dynamic control of the current of the load can here be achieved only if the switching of the switches is based directly on the instantaneous values of the current of the load. The pulsed voltage sources based on the use of instantaneous values of the current of the load include, for example, voltage sources that are based on the Direct Torque Control (DTC) and tolerance band control of the current. The methods work well when the load is close to the voltage source that generates voltage pulses, and when there are no capacitive components between the load and the pulsed voltage source, whereby the current of the load can be measured in undisturbed conditions.
In practice, however, the load is often situated at a relatively long distance from the voltage source supplying it. The instantaneous current of the pulsed voltage source then differs from the instantaneous current of the load on account of the currents passing through the stray capacitances in the supply cable. The reason for this is that the transfer impedance of the cable is usually much lower than the impedance of the inductive load. Because the impedances are not equal, each voltage pulse supplied generates voltage oscillation at that end of the supply cable which is close to the load, and current oscillation at that end of the supply cable which is close to the voltage source.
The modulation basis used is usually the current measured at the end of the voltage source; above a certain length of cable the measurements are so inaccurate due to the current oscillation that the instantaneous value of the load current can no longer be controlled. When long cables are used, either the dynamics of the control has to be compromised or the current of the load has to be measured separately at the load, whereby expensive separate cabling has to be installed to enable the transfer of the measuring signal. Apart from the long cables, capacitive components between the load and the voltage source also inhibit the above kind of modulation that is based on the current measured at the end of the voltage source. Such a capacitive component can be, for example, an LC low pass filter.
BRIEF DESCRIPTION OF INVENTION
The object of the invention is to provide a method in which the above drawbacks are avoided and to enable more reliable defining of the instantaneous value of the current of the pulse-controlled inductive load. The object is achieved with a method of the invention, which is characterized by comprising the steps of
low-pass filtering the measured output current of the pulsed voltage source to produce a fundamental wave current,
defining a load current estimate by computation on the basis of the measured output voltage of the pulsed voltage source and the impedance of the load,
high-pass filtering the load current estimate, and
defining the instantaneous value of the load current by adding the high-pass-filtered load current estimate to the fundamental wave current.
The preferred embodiments of the invention are claimed in the dependent claims.
The method of the invention is based on estimating the load current on the basis of the measured voltage of the voltage source and the estimated impedance of the load. The estimated load current is obtained by combining the fundamental wave component of the measured current and that component in the estimated current which changes when the semiconductor switches are switched.
The advantage of the claimed method is that a highly reliable estimate of the load current is obtained by a simple method. Although the cables are long, the load current can be estimated accurately by the method, whereby the dynamic control of the pulsed voltage source can be maintained at a high level under all conditions. The DTC or any current modulation whatsoever can then also be made to work in connection with long cables.
High cabling costs, which would result if the measurement were performed at that end of the supply cable which is close to the load, are avoided by the method of the invention. Further, since the method enables reliable estimation of the current, capacitive components, such as LC low-pass filters or the like, can also be connected to the supply cable where necessary.
The method allows the current of the output of the pulsed voltage source to be measured using a more cost-effective current detector than in the prior art. The highest-frequency components are filtered from the measured current in the method, whereby the bandwidth requirement of the current measurement is notably lower than in the prior art. For the same reason, an AD converter used to modify the current data can be much more cost-effective than in the prior art solutions, in which all the frequencies of the current have been measured as accurately as possible.
The problems caused by the long cables, for example, have made it necessary to restrict the switching frequency of the DTC. On account of the method of the present invention, the length of the minimum pulse of the pulsed voltage source need not be restricted, but due to the accurate current data the switching frequency can be raised arbitrarily high if so desired. The raising of the switching frequency makes it possible to further improve the dynamic accuracy of the control.
The invention further relates to a method which is characterized by comprising the steps of
adding a correction term to the output voltage of the pulsed voltage source to produce an estimation voltage;
defining a load current estimate by computation on the basis of the estimation voltage and the impedance of the load, whereby the load current estimate provides the instantaneous value of the load current;
low-pass filtering the load current estimate;
low-pass filtering the measured output current of the pulsed voltage source to produce a fundamental wave current;
comparing the fundamental wave current with the low-pass-filtered load current estimate to produce an error parameter proportional to the difference of the currents; and
multiplying the error parameter by a coefficient to produce the correction term.
The method of the invention is based on comparing the low-pass-filtered components of the load current estimate and the output current of the pulsed voltage source with each other, and on the basis of the comparison, adding a correction term proportional to the difference of the currents to the voltage used to compute the load current estimate. In the method, the load current estimate provides the instantaneous value of the load current.
REFERENCES:
patent: 4350943 (1982-09-01), Pritchard
patent: 4575668 (1986-03-01), Baker
patent: 5079498 (1992-01-01), Cleasby et al.
patent: 5381328 (1995-01-01), Umezawa et al.
patent: 5390070 (1995-02-01), Niedermeier
patent: 5391976 (1995-02-01), Farrington et al.
patent: 5446647 (1995-08-01), Ikeda et al.
p
Heikkila Samuli
Schroderus Petri
ABB Industry OY
Dykema Gossett PLLC
Laxton Gary L.
Wong Peter S.
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