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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control circuit of power converter, and
particularly to a converter control circuit which controls the d.c. output
voltage to be constant and the input current to be in phase with the power
voltage.
2. Description of the Prior Art
FIG. 1 shows in block diagram a conventional power converter control
circuit disclosed in an article entitled "Review of Control
Characteristics of PWM Converters", in the proceeding of the 1985 national
convention of The Institute of Electrical Engineers of Japan. In the
figure, indicated by 1 is a power converter, which is a PWM converter in
this example. Indicated by 2 is an a.c. filter reactor provided at the
a.c. input of the converter 1, 3 is an a.c. power source which supplies an
input current I.sub.S, 4 is a d.c. filter capacitor provided at the d.c.
output of the converter 1, 5 is a load, 6a is a power voltage V.sub.S
detecting circuit, 6b is a d.c. output voltage V.sub.D detecting circuit,
6c is an input current I.sub.S detecting circuit, and 6d is a load current
I.sub.L detecting circuit.
Indicated in FIG. 1 are sections of a control circuit, in which are
included a reference voltage generating circuit 101 for producing a
reference voltage V.sub.DR' a subtracter 102 which calculates the
difference between the detected voltage V.sub.D provided by the detecting
circuit 6b and the reference voltage V.sub.DR to evaluate the voltage
difference, a voltage control circuit 103 which produces a voltage control
signal depending on the voltage difference, a feed-forward control circuit
107a which produces a feed-forward signal derived from the detected value
I.sub.L from the detecting circuit 6d multiplied by a constant K.sub.L, an
adder 107 which calculates the difference between the voltage control
signal and the feed-forward signal to produce an input current peak
command I.sub.m, a sinusoidal wave generating circuit 108 which produces a
sinusoidal waveform sin .theta. in phase with the power source voltage
V.sub.S based on the detected value V.sub.S of the detecting circuit 6a, a
multiplier 109 which multiplies the peak command I.sub.m and the
sinusoidal waveform sin .theta. to produce an input current command
I.sub.SS, a subtracter 111 which calculates the difference between the
detected value I.sub.S from the detecting circuit 6c and the input current
command I.sub.SS to evaluate the current difference, a current control
circuit 112 for producing a current control signal depending on the
current difference, an adder 113 which adds the detected value V.sub.S of
the detecting circuit 6a to the current control signal to compensate the
disturbance of the power voltage V.sub.S, a carrier wave generating
circuit 115 which produces a carrier signal, e.g., a triangular wave, a
PWM (pulse-width modulation) circuit 114 which compares the output of
adder 113 with the carrier wave to time the switching operation of
switching devices (not shown) constituting the converter 1, and drive
circuit 116 which activates the converter 1 depending on the output pulse
width provided by the PWM circuit 114.
Next, the operation of the above converter system will be described. The
converter 1 converts a.c. input power into d.c. power and supplies it to
the load 5. The capacitor 4 is provided for absorbing the variation in the
d.c. output voltage V.sub.D of the converter 1. The control circuit
controls the d.c. output voltage V.sub.D so that it is equal to the
reference voltage V.sub.DR, and also causes the input current I.sub.S to
be sinusoidal in phase with the power voltage V.sub.S so that the system
operates at a 100% power factor, with less harmonics and lower distortion
factor.
In order to maintain a constant d.c. output voltage V.sub.D, the voltage
control circuit 103 provides a voltage control signal for modifying the
peak value of the input current I.sub.S. If the voltage control has a
laggard response, an abrupt fall of the d.c. output voltage V.sub.D across
the capacitor 4 could cause a control disability, and this problem is
overcome by adding a feed-forward signal with a value of K.sub.L I.sub.L
to the voltage control signal on the adder 107 so that the peak value
command I.sub.m is instantaneously responsive to a load variation.
The input current command I.sub.SS is produced through the multiplication
of the peak value command I.sub.m and the sinusoidal waveform sin .theta.
in phase with the power voltage V.sub.S on the multiplier 109. The input
current command I.sub.SS is subtracted by the input current I.sub.S on the
subtracter 111 to evaluate the current difference, which is followed by
the current control circuit 112 to produce the current control signal. The
current control signal is added by the power voltage V.sub.S on the adder
113 so that the disturbance by the power voltage V.sub.S is compensated,
and then the resulting signal is fed to the PWM circuit 114. The PWM
circuit 114 compares the current control signal with a carrier wave, e.g.,
a triangular wave at 1-2 kHz, provided by the carrier generating circuit
115 to produce a PWM signal with a pulse width dependent on the values of
voltage difference and current difference. The PWM signal is fed to the
drive circuit 116, which operates the switching devices of the converter 1
accordingly.
The conventional power converter control circuit arranged as described
above is intended to be responsive to an abrupt fall of the d.c. output
voltage V.sub.D through the addition of the load current I.sub.L signal on
a feed-forward basis to the voltage control signal for producing the input
current command I.sub.SS. Consequently, in case of a single-phase inverter
for the load 5, the load current I.sub.L has a significant amount of
ripple, which appears in the input current command I.sub.SS' resulting
disadvantageously in an increased harmonics included in the input current
I.sub.S waveform.
SUMMARY OF THE INVENTION
This invention is intended to overcome the foregoing prior art deficiency,
and its prime object is to provide a power converter control circuit which
alleviates the fall of d.c. output voltage caused by an abrupt change in
the load and also relieves the control system from the influence of load
current ripples.
The invention resides in the power converter control circuit which
evaluates the mean value of load current and produces a feed-forward
signal on the basis of the mean value.
Other objects and advantages of this invention will become more apparent
from the following detailed description of specific embodiments taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the conventional power converter control
circuit;
FIG. 2 is a block diagram of the power converter control circuit embodying
the present invention;
FIG. 3 is a schematic diagram of the converter according to the embodiment;
FIG. 4 is a block diagram of the voltage control circuit;
FIG. 5 is a diagram used to explain the principle of moving average in the
sampling control;
FIG. 6 is a flowchart showing the creation of the feed-forward signal
derived from the load current;
FIGS. 7(a)-7(c) are a set of waveform diagrams obtained by simulation;
FIG. 8 is a block diagram of the current control circuit; and
FIG. 9 is a block diagram showing another embodiment of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of this invention will be described with reference to the
drawings. In FIG. 2, where portions identical to those in FIG. 1 are given
the common symbols and explanation thereof is not repeated, indicated by
104 is an averaging circuit placed between the detection circuit 6b and
subtracter 102, 105 is an averaging circuit placed between the detection
circuit 6d and adder 107, 106 is a filter which passes the output of the
averaging circuit 105, 118 is a switch which delivers selectively the
output of averaging circuit 105 or the output of filter 106 to the adder
107, 117 is a limiter placed between the adder 107 and multiplier 109, and
110 is a low-pass filter placed between the detection circuit 6c and
subtracter 111.
FIG. 3 shows a specific circuit arrangement of the converter 1. The
converter 1 comprises rectifying diodes D1-D4 in a bridge configuration
and switching devices, e.g., transistors, S1-S4 in anti-parallel
connection with the respective diodes D1-D4. The converter 1 is of the
voltage type, in which the switching devices S1-S4 operate several times
in a period of power frequency in response to the dive signal from the
drive circuit 116.
Next, the operation of the inventive control circuit shown in FIG. 2 will
be described. The current command I.sub.SS is produced from the load
current feed-forward signal I.sub.LS and power voltage V.sub.S so that the
input current I.sub.S has an instantaneous response to the load by a minor
current loop, and the d.c. output voltage is controlled to the reference
voltage V.sub.DR by a major voltage loop.
First, the major voltage control loop will be described. The voltage
control system consists of the circuitries 101-109, 117 and 118, and it is
shown in more detail in FIG. 4. Moving average circuits are employed for
the averaging circuits 104 and 105. Moving average is a technique of
digital control in which detected values are sampled at a sampling
interval Ts and a certain number of sampled data of new sampled values are
averaged. For example, when the d.c. output voltage V.sub.D including
periodic ripples is averaged for every six data, the result of moving
average of d.c. output voltage V.sub.D at a time between KTs and (K+1)Ts
is given as follows.
##EQU1##
The moving average at a time between (K+1)Ts and (K+2)Ts is as follows.
##EQU2##
By determining the number of data for averaging depending on the period of
ripple in this way, the moving average value becomes a virtually constant
value, and the influence of ripples on the control operation is
eliminated.
In FIG. 4, the subtracter 102 calculates the voltage difference from the
mean value of d.c. output voltage V.sub.D provided by the moving average
circuit 104 and the reference d.c. voltage V.sub.DR(K)' and delivers the
result to the voltage control circuit 103. The voltage control circuit 103
consists of a proportional operator 103a which multiplies a constant
K.sub.P to the input, and an integral operator 103b which sums the input
multiplied by a constant K.sub.I and the multiplied result retarded by a
delay element Z.sup.-1. The voltage control signal produced by the voltage
control circuit 103 is added to the feed-forward signal I.sub.LS(K) by the
adder 107.
The creation of the feed-forward signal I.sub.LS(K) will be described on
the flowchart of FIG. 6. A moving average circuit is assumed for the
averaging circuit 105. Initially, step ST1 evaluates the moving average
value I.sub.LA(K) from the load current detected value I.sub.L(K). Next
step ST2 calculates the difference between the moving average value
I.sub.LA(K) and the feed-forward signal I.sub.LS(K-1) of the preceding
sample point, and tests whether the difference is greater than a certain
preset value I.sub.LO. If the difference is greater than I.sub.LO, step
ST3 operates on the switch 118 to select the b-position so that the then
moving average value I.sub.LA(K) is delivered as a feed-forward signal
I.sub.LS(K) to the adder 107. In another case when the difference does not
exceed I.sub.LO' step ST4 operates on the switch 118 to select the
a-position so that the moving average value I.sub.LA(K) is fed through a
filter 106 having certain 1-order time lag characteristics, and step ST5
delivers the resulting feed-forward signal I.sub.LF(K) as a feed-forward
signal I.sub.LS(K) to the adder 107.
FIGS. 7(a)-7(c) show simulated waveforms for a single-phase inverter for
the load 5, with the preset value I.sub.LO being 25% of the rated current.
Shown by 7(a) is the load current I.sub.L(K) at the input of the inverter,
and it includes many harmonics. Shown by 7(b) is the moving-averaged load
current I.sub.LA(K), and shown by 7(c) is the feed-forward signal
I.sub.LS(K). It is observed on the waveform that the feed-forward signal
I.sub.LS(K) promptly follows a sharp fall of the load current I.sub.L(K).
Referring back to FIG. 4, the adder 107 has its output representing the
effective current command I.sub.me(K) for the d.c. output of the converter
1. In order to convert the effective current command I.sub.me(K) to the
input current command I.sub.SS(K) for the a.c. converter input, a
multiplier 119 multiplies the mean value V.sub.D(K) of d.c. output voltage
V.sub.D by the I.sub.me(K) and divides the result by the effective value
V.sub.se(K) of power voltage V.sub.S. The output of the multiplier 119 is
clamped by the limiter 117 to the allowable current of the converter 1,
and then applied to the multiplier 109. The multiplier 109 multiplies a
waveform .sqroot.2 sin .theta. in phase with the power voltage V.sub.S to
the output of the limiter 117, and the input current command I.sub.SS(K)
is obtained.
Next, the minor current loop will be described. This current control system
is formed by sections 111-113 in FIG. 2, and it is shown in more detail in
FIG. 8. In the figure, the input current I.sub.S detected value is rid of
ripple components by a low-pass filter 110 (see FIG. 1). The resulting
signal I.sub.S(K) is applied to the subtracter 111 for subtraction from
the input current command I.sub.SS(K) and the current difference is
evaluated, which is delivered to the current control circuit 112. The
current control circuit 112 consists of an integral operator 112a which
sums the output of the subtracter 111 multiplied by a constant G.sub.I and
the multiplied result retarded by a delay element Z.sup.-1, and a
proportional operator 112b which multiplies a constant G.sub.P to the
detected value I.sub.S(K). The integral operator 112a and proportional
operator 112b have their outputs merged and then added to the detected
value V.sub.S(K) of power voltage V.sub.S by the adder 113, resulting in a
control signal V.sub.CS(K). The control signal V.sub.CS(K) is delivered to
the succeeding PWM circuit 114, by which it is compared with the carrier,
and the switching devices S1-S4 of the converter 1 are controlled for
their switching operation. The foregoing minor current control loop
reduces the retardation, allowing an increased loop gain, whereby the
total system has an instantaneous response.
Through the foregoing operations, the d.c. output voltage V.sub.D is
maintained constant, and the input current I.sub.S is controlled to be a
sinusoidal current with lower distortion factor and in phase with the
power voltage V.sub.S. By employment of moving average circuits for the
averaging circuits 104 and 105, the ripple component of the 2-fold output
frequency is eliminated in the case of a single-phase inverter or the like
for the load 5. The feed-forward signal I.sub.LS, by being fed through the
filter 106, is rid of the ripple caused by sampling for the averaging
process, whereby the input current command I.sub.SS includes fewer
harmonics.
Although in the foregoing embodiment the feed-forward control system for
providing the load current I.sub.L detected value for the control circuit
is designed to use the moving average value I.sub.LA directly as a load
current feed-forward signal I.sub.LS instead of passing it through the
filter 106 when the I.sub.LA exceeds a certain preset value I.sub.LO, this
treatment of direct feed-forward signal is not necessary in cases where
the moving average value of load current does not vary greatly.
Accordingly, the switch circuit 118 as shown in FIG. 9 can be removed, and
yet the same effectiveness as the preceding embodiment is achieved.
Although in the foregoing embodiment the voltage control circuit 103 and
current control circuit 112 are formed in a digital control system, the
whole or part of the inventive control circuit may be configured by analog
control circuitries, and yet the same effectiveness as the preceding
embodiment is achieved.
According to this invention, as described above, the load current
feed-forward control system is designed to use a detected value averaging
circuit, and it alleviates the influence of load current ripples on the
control system and is also effective for controlling the converter input
current to a sinusoidal current with lower distortion factor and in phase
with the power voltage.
* * * * *
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