U.S. patent application number 10/640091 was filed with the patent office on 2005-02-17 for active filter for multi-phase ac power system.
Invention is credited to Cheng, Louis, Zhao, Qihua.
Application Number | 20050035815 10/640091 |
Document ID | / |
Family ID | 34136017 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050035815 |
Kind Code |
A1 |
Cheng, Louis ; et
al. |
February 17, 2005 |
ACTIVE FILTER FOR MULTI-PHASE AC POWER SYSTEM
Abstract
An active filter (300) generates multi-phase compensating
current in an AC power supply system (10) that supplies a load
(200). The filter (300) includes a compensating current output
device (34) outputting multi-phase compensating current to an AC
power line (50); and a controller (310) for controlling the
compensating current output (340) such that the multi-phase
compensating current compensates for current harmonics and power
factor on said AC power line (50). The controller (310) estimates
current harmonics and power factor compensating values as a
function of multi-phase power measurements.
Inventors: |
Cheng, Louis; (Scarborough,
CA) ; Zhao, Qihua; (Mississauga, CA) |
Correspondence
Address: |
Larry J. Palguta
Honeywell Law Department
3520 Westmoor Street
South Bend
IN
46628
US
|
Family ID: |
34136017 |
Appl. No.: |
10/640091 |
Filed: |
August 13, 2003 |
Current U.S.
Class: |
327/552 |
Current CPC
Class: |
Y02E 40/20 20130101;
H02M 1/15 20130101; Y02E 40/40 20130101; H02M 1/12 20130101; H02J
3/185 20130101; H02J 3/01 20130101; Y02E 40/24 20130101 |
Class at
Publication: |
327/552 |
International
Class: |
H03B 001/00 |
Claims
1. An active filter for generating multi-phase compensating current
in an AC power supply system that supplies a load, said filter
comprising: a compensating current output device outputting
multi-phase compensating current to an AC power line; and a
controller for controlling said compensating current output device
without using a transformer such that the multi-phase compensating
current compensates for current harmonics and power factor on said
AC power line, said controller estimating current harmonics and
power factor compensating values as a function of multi-phase power
measurement.
2. The active filter according to claim 1, wherein said multi-phase
compensating current further compensates for phase imbalance of
multi-phase voltage and multi-phase current of said power supply
system and said controller estimates phase imbalance compensating
values as a function of multi-phase supply voltage and multi-phase
supply current measurements.
3. An active filter for generating multi-phase compensating current
in an AC power supply system that supplies a load, said filter
comprising: a compensating current output device outputting
multi-phase compensating current to an AC power line; and a
controller for controlling said compensating current output such
that the multi-phase compensating current compensates for current
harmonics and power factor on said AC power line, said controller
estimating current harmonics and power factor compensating values
as a function of multi-phase power measurement, wherein said
controller comprises: a digital phase lock loop unit that computes
voltage reference signals in a quadratic domain.
4. The active filter according to claim 3, wherein said controller
further comprises: a compensating current reference estimator that
estimates compensating current reference values based on voltage
reference signals computed by said digital phase lock loop unit,
said compensating current reference values being in the quadratic
domain; and a domain transformer for transforming said compensating
current reference values from said quadratic domain to a
multi-phase current domain by using a look-up table.
5. The active filter according to claim 4, wherein said
compensating current output is an inverter, and said controller
further comprises: an inverter switching control unit that
generates inverter switching control signals in accordance with
calculated compensating current values.
6. The active filter according to claim 3, wherein said digital
phase lock loop unit calculates reference voltage values by:
transforming multi-phase voltage measurement values into a
quadratic (d-q) reference frame signals; filtering the quadratic
(d-q) reference frame signals to extract fundamental d-q voltage
signals; and processing the fundamental d-q voltage signals to
generate said reference voltage values.
7. The active filter according to claim 4, wherein said
compensating current reference estimator transforms multi-phase
current measurements into transformed current values, said
transformed current values having a DC component corresponding to a
fundamental current, an AC component corresponding to current
harmonics, and a zero sequence current corresponding to multi-phase
current balance.
8. The active filter according to claim 4, wherein said
compensating current reference values represent a plurality of
power conditions, including current harmonics, power factor, and
multi-phase current balance.
9. The active filter according to claim 4, wherein said
compensating current reference estimator accesses a look up table
to generate said compensating current reference values and to
achieve desired power quality combinations.
10. The active filter according to claim 1, wherein said power
supply system is a variable frequency system.
11. The active filter according to claim 10, wherein said power
system is a variable frequency power system of an aircraft.
12. A controller of an active filter that generates multi-phase
compensating current in an AC power supply system that supplies a
load, said controller comprising: an input for receiving
multi-phase power measurements; a compensating current calculation
unit for calculating multi-phase compensating current based on said
multi-phase power measurements; and an output for outputting
control signals based on the calculated multi-phase compensating
current to control a compensating current output device to
compensate for current harmonics and power factor on an AC power
line without using a transformer.
13. The controller according to claim 12, wherein said multi-phase
compensating current further compensates for phase imbalance of
multi-phase voltage and multi-phase current of said power supply
system and said controller estimates phase imbalance compensating
values as a function of multi-phase supply voltage and multi-phase
supply current measurements.
14. A controller of an active filter that generates multi-phase
compensating current in an AC power supply system that supplies a
load, said controller comprising: an input for receiving
multi-phase power measurements; a compensating current calculation
unit for calculating multi-phase compensating current based on said
multi-phase power measurements; and an output for outputting
control signals based on the calculated multi-phase compensating
current to control a compensating current output device to
compensate for current harmonics and power factor on an AC power
line, wherein said compensating current calculation unit comprises:
a digital phase lock loop unit that computes voltage reference
signals in a quadratic domain.
15. The controller according to claim 14, wherein said compensating
current calculation unit further comprises: a compensating current
reference estimator that estimates compensating current reference
values based on voltage reference signals computed by said digital
phase lock loop unit, said compensating current reference values
being in the quadratic domain; and a domain transformer for
transforming said compensating current reference values from said
quadratic domain to a multi-phase current domain by using a look-up
table.
16. The controller according to claim 15, wherein said compensating
current output is an inverter, and said controller generates
inverter switching control signals in accordance with calculated
compensating current values.
17. The controller according to claim 15, wherein said digital
phase lock loop unit calculates reference voltage values by:
transforming multi-phase voltage measurement values into a
quadratic (d-q) reference frame signals; filtering the quadratic
(d-q) reference frame signals to extract fundamental d-q voltage
signals; and processing the fundamental d-q voltage signals to
generate said reference voltage values.
18. The controller according to claim 15, wherein said compensating
current reference estimator transforms multi-phase current
measurements into transformed current values, said transformed
current values having a DC component corresponding to a fundamental
current, an AC component corresponding to current harmonics, and a
zero sequence current corresponding to multi-phase current
balance.
19. The controller according to claim 15, wherein said compensating
current reference values represent a plurality of power conditions,
including current harmonics, power factor, and multi-phase current
balance.
20. The controller according to claim 15, wherein said compensating
current reference estimator accesses a look up table to generate
said compensating current reference values and to achieve desired
power quality combinations.
21. The controller according to claim 12, wherein said power supply
system is a variable frequency system.
22. The controller according to claim 21, wherein said power system
is a variable frequency power system of an aircraft.
23. A method of generating multi-phase compensating current in an
AC power supply system that supplies a load, said method
comprising: outputting multi-phase compensating current to an AC
power line without using a transformer; and adjusting the
compensating current output to the AC power line without using a
transformer such that the multi-phase compensating current
compensates for current harmonics and power factor on the AC power
line, said adjusting step estimating current harmonics and power
factor compensating values as a function of multi-phase power
measurement.
24. The method according to claim 23, wherein said multi-phase
compensating current further compensates for phase imbalance of
multi-phase voltage and multi-phase current of the power supply
system and said step of adjusting estimates phase imbalance
compensating values as a function of multi-phase supply voltage and
multi-phase supply current measurements.
25. The method of generating multi-phase compensating current in an
AC power supply system that supplies a load, said method
comprising: outputting multi-phase compensating current to an AC
power line; and adjusting the compensating current output to the AC
power line such that the multi-phase compensating current
compensates for current harmonics and power factor on the AC power
line, said adjusting step estimating current harmonics and power
factor compensating values as a function of multi-phase power
measurement, wherein said step of adjusting comprises: executing a
digital phase lock loop function that computes voltage reference
signals in a quadratic domain.
26. The method according to claim 25, wherein said step of
adjusting further comprises: estimating compensating current
reference values based on voltage reference signals computed by
said digital phase lock loop function, said compensating current
reference values being in the quadratic domain; and transforming
said compensating current reference values from said quadratic
domain to a multi-phase current domain by using a look-up
table.
27. The method according to claim 25, wherein said digital phase
lock loop function calculates reference voltage values by:
transforming multi-phase voltage measurement values into a
quadratic (d-q) reference frame signals; filtering the quadratic
(d-q) reference frame signals to extract fundamental d-q voltage
signals; and processing the fundamental d-q voltage signals to
generate said reference voltage values.
28. The method according to claim 26, wherein said compensating
current reference values represent a plurality of power conditions,
including current harmonics, power factor, and multi-phase current
balance.
29. The method according to claim 26, wherein said step of
estimating compensating current reference values accesses a look up
table to generate said compensating current reference values and to
achieve desired power quality combinations.
30. The method according to claim 23, wherein said power supply
system is a variable frequency system.
31. The method according to claim 30, wherein said power system is
a variable frequency power system of an aircraft.
32. The active filter according to claim 1, wherein the controller
estimates the current harmonics and power factor compensating
values as a function of a multi-phase voltage measurement.
33. The controller according to claim 12, wherein the compensating
current calculation unit calculates the multi-phase compensating
current based on a multi-phase voltage measurement.
34. The method according to claim 23, wherein the current harmonics
and power factor compensating values are estimated as a function of
a multi-phase voltage measurement.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to power factor and current harmonics
compensation in a multi-phase AC power system.
[0003] 2. Related Art
[0004] Electrical power distributions systems that deliver AC power
are known to be affected by characteristics of the associated
load(s). More specifically, non-linear loads, such as motor drives,
generate substantial current harmonics in the power supply lines.
Furthermore, such loads may cause power factor displacement and
imbalance between phases of the current and voltage supplied in the
power system. It has been recognized that mitigating the effects of
load-generated harmonics can improve the performance of the power
system. To this end, one conventional technique for improving
quality of the supplied multi-phase power has been to install
passive filters inside the loads to suppress the current harmonics
being generated by the loads themselves.
[0005] Although the use of passive filters can effectively remove
some of the major harmonics, such filters are source impedance
dependant. Also, this technique does not address other power
quality issues caused by the load(s). More specifically, the use of
passive filters does not compensate for power factor displacement
or maintain balance between phases of the supply voltage and supply
current. Thus, conventional techniques offer only an incomplete
solution for power quality in a multi-phase AC power system.
SUMMARY OF THE INVENTION
[0006] In one aspect, the present invention provides an active
filter that suppresses current harmonics and improves power factor
in a variable or constant frequency multi-phase power system. In
another aspect, the present invention provides a controller of an
active filter that generates a multi-phase compensating current
that compensates the power factor and current harmonics in a
variable or constant frequency multi-phase power system.
[0007] In one embodiment, the present invention is an active filter
for generating multi-phase compensating current in an AC power
supply system that supplies a load, the active filter comprising: a
compensating current output device outputting multi-phase
compensating current to an AC power line; and a controller for
controlling the compensating current output such that the
multi-phase compensating current compensates for current harmonics
and power factor on the AC power line. The controller estimates
current harmonics and power factor compensating values as a
function of multi-phase supply measurements.
[0008] In another embodiment, the present invention is a controller
of an active filter that generates multi-phase compensating current
in an AC power supply system that supplies a load, the controller
comprising: an input for receiving multi-phase power supply
measurements; a compensating current calculation unit for
calculating multi-phase compensating current based on the
multi-phase power supply measurements; and an output for outputting
control signals based on the calculated multi-phase compensating
current to control a compensating current output device to
compensate for current harmonics and power factor on an AC power
line.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Other aspects and advantages of the present invention will
become apparent upon reading the following detailed description and
upon reference to the drawings, in which:
[0010] FIG. 1 illustrates an exemplary implementation of an active
filter that compensates for power factor and current harmonics in a
multi-phase AC power system in accordance with the principles of
the present invention;
[0011] FIG. 2 illustrates an active filter controller configuration
for controlling the generation of multi-phase compensating current
in accordance with an embodiment of the present invention;
[0012] FIG. 3 is a flow diagram illustrating an operation performed
by the active filter controller to control the multi-phase
compensating current generated to compensate for power factor and
current harmonics in accordance with an embodiment of the present
invention;
[0013] FIG. 4 is a block diagram illustrating components of a
digital phase lock loop unit that generates voltage reference
values used to calculate current compensating values in accordance
with an embodiment of the present invention;
[0014] FIG. 5 illustrates a series of waveforms to demonstrate the
operating principles of the active filter in accordance with an
embodiment of the present invention; and
[0015] FIGS. 6A-6C illustrate a series of waveforms that further
demonstrate the operating principles of the active filter for
compensating power factor and current harmonics in a multi-phase
power system in accordance with the principles of the present
invention.
DETAILED DESCRIPTION
[0016] FIG. 1 illustrates a multi-phase AC power system
implementing an active filter for compensating power factor and
current harmonics in accordance with the principles of the present
invention. As shown in FIG. 1, the power system 10 includes: a
three-phase voltage source 100 that supplies constant or variable
frequency power via power lines 50; a load 200 that draws
three-phase current from the power lines 50; a voltage measuring
unit 400 for sampling the three-phase voltage supplied by the
three-phase voltage source 100 via the power lines 50; a load
current measuring unit 500 for sampling the three-phase current
being supplied to the load 200 via the power lines 50; an active
filter 300 for outputting a three-phase compensating current to the
power lines 50 to compensate for power factor and harmonics; and a
compensating current measuring unit 600 for measuring the
compensating current output by the active filter 300. The load 200
may be a non-linear load (e.g., motor drives), which creates
current harmonics in the AC power system 10 and creates power
factor displacement and phase imbalance. It should be recognized,
however, that applicability of the active filter 300 is not limited
to such load types.
[0017] As shown in FIG. 1, the active filter 300 includes a
controller 310; an inverter 340; and an inductor-based filtering
unit 350. In accordance with switching control signals output by
the controller 310, the inverter 340, which is a voltage-source
inverter, generates a multi-phase compensating current that is
injected to the power lines 50 via the filtering unit 350. An
Insulated Gate Bipolar Transistor (IGBT) voltage-source inverter is
a well-known device, and, as such, details of the inverter
configuration/operation are not provided herein.
[0018] In a manner described in detail below, the controller 310 of
the active filter 300 receives multi-phase voltage measurements
from the voltage measuring unit 400 and multi-phase current
measurement values from the current measuring unit 500 and
calculates a compensating current for compensating for current
harmonics, power factor displacement, and phase imbalance power
conditions. As a result of this calculation, the controller 310
outputs gating control signals to the inverter 340, generating the
multi-phase compensating current output to the power lines 50.
Thus, based on the computations performed by the controller 310,
the active filter 300 injects counter current harmonics into the
power system 10, as well as compensates for power factor
displacement and phase imbalance. The power system 10 illustrated
in FIG. 1 may be a variable frequency system or a constant
frequency system, such as an aircraft 400 Hz system.
[0019] FIG. 2 is a block diagram illustrating a configuration of
the controller 310 according to an embodiment of the present
invention. As shown in FIG. 2, the controller 310 includes: a
digital phase lock loop unit 320; a compensating current reference
estimator 312; a domain transformer 314; and an inverter switching
control unit 316. Operation of and functional interaction between
the components illustrated in FIG. 2 will become apparent from the
following discussion. Initially, although the various components of
FIG. 2 are illustrated as discrete elements, such an illustration
is for ease of explanation and it should be recognized that certain
operations of the various components may be performed by the same
physical device, e.g., one or more digital controllers. General
operation of the controller 310 illustrated in FIG. 2 will be
described with reference to the flow diagram of FIG. 3.
[0020] Initially, the controller 310 receives multi-phase voltage
measurements from the voltage measuring unit 400 and multi-phase
current measurements from the current measuring unit 500 (S410).
The voltage measurements are supplied to the digital phase lock
loop unit 320. In a manner discussed in more detail below, the
digital phase lock loop unit 320 calculates reference voltage
values from the multi-phase voltage measurements in the quadratic
(d-q) reference frame domain (S412). The compensating current
reference estimator 312 receives the multi-phase current
measurement values and the reference voltage values calculated by
the digital phase lock loop unit 320 and estimates compensating
current reference values (S416). The compensating current reference
estimator 312 outputs compensating current reference values (Icd,
Icq, Ic0) in the d-q axis domain, which are transformed by the
domain transformer 314 into the multi-phase abc domain (Ica, Icb,
Icc) (S418). The inverter switching control unit 316 compares
values Ica, Icb, Icc to a reference (e.g., triangular) waveform to
generate switching control values for the inverter 340 (S420).
These operations will be explained in greater detail below.
[0021] FIG. 4 is a block diagram illustrating components of the
digital phase lock loop (DPLL) unit 320 of the controller 310 in
accordance with one embodiment of the present invention. Again, the
functional elements are illustrated as a plurality of discrete
elements, but it should be recognized that these functions may be
combined in one or more processing elements. As shown in FIG. 4,
the DPPL unit 320 includes a park transformation unit 321; band
pass filters 322, 323; a magnitude normalizer 324; a polarity
detector 325; a phase transformation unit 326; low pass filters
327, 328; and a phase estimation unit 329. Operation and functional
interaction between these elements will next be described.
[0022] The DPLL unit 320 computes the voltage reference signals
Cosine, Sine, which are subsequently used to estimate the
multi-phase compensating current. First, the park transformer unit
321 transforms the three-phase power line voltages into the
quadratic (d-q) reference frame. The transformation performed by
the park transformation unit 321 is defined as below: 1 [ Vd Vq ] =
2 3 [ 1 - 1 2 - 1 2 0 - 3 2 3 2 ] [ Va Vb Vc ] ( equation 1 )
[0023] where:
[0024] Va, Vb, Vc=Phase voltages;
[0025] Vd=Transformed voltage in d-axis; and
[0026] Vq=Transformed voltage in q-axis.
[0027] Second, d-q axis voltage outputs, Vd and Vq, are filtered by
band pass filters 322, 323, respectively, to extract the
fundamental components therefrom. The band pass filter outputs are
normalized by the magnitude normalizer unit 324 by scaling these
values to unity magnitude to generate signals U_Imag_1 and
U_Real_1. The magnitude normalizer 324, connected to the outputs of
band pass filters 322, 323, scales the voltage signals (Vd, Vq) to
unity magnitude (U_Real_1 and U_Imag_1) by using the following
equation: 2 [ U_Real _ 1 U_Imag _ 1 ] = 1 Vd 2 + Vq 2 [ Vd Vq ] (
equation 2 )
[0028] The polarity detector 325 generates a square wave in
accordance with the voltage signals (Vd, Vq) output by the park
transformation unit 321.
[0029] This operation is defined as below:
If Vd>=0 then Sq_Real=1 Else Sq_Real=0
If Vq>=0 then Sq_Imag=1 Else Sq_Imag=0. (equation 3)
[0030] Next, the phase transformation unit 326 multiples U_Imag_1
and U_Real_1 by un-filtered d-q voltage signals, Sq_Real and
Sq_Imag (output by the polarity detector 325), to form the
modulated product signals, sqc and sqs. By multiplying the unity
voltage signals (U_Real_1, U_Imag_1) with the square wave
references (Sq_Real, Sq_Imag), the phase transformation unit 326
produces phase shifting signals (sqs, sqc) of the two signals. The
equation for this operation is: 3 [ sqs sqc ] = [ - Sq_Imag Sq_Real
Sq_Real Sq_Imag ] [ U_Real _ 1 U_Imag _ 1 ] ( equation 4 )
[0031] These products contain DC and AC components. The DC
components correspond to the phase shift of the filtered
references, and the AC components represent the harmonics contents
in the power line 50.
[0032] The low pass filters 327, 328 receive the outputs of the
phase transformation unit 326, sqc and sqs, respectively. The low
pass filters 327, 328 filter out the DC component of the phase
shifting signals (sqs, sqc), outputting signals cos a and sin a,
respectively. The DC components represent the phase shift of the
voltage references (U_Real_1, U_Imag_1) with respect to the
transformed voltages (Vd, Vq). The phase estimation unit 329 shifts
the voltage references (U_Real_1, U_Imag_1) to generate the
reference signals (Sine, Cosine): 4 [ Sine Cosine ] = [ cos sin sin
- cos ] [ U_Real _ 1 U_Imag _ 1 ] ( equation 5 )
[0033] The reference signals (Sine, Cosine) will be used in
transforming the measured voltage and current into the synchronous
frame as discussed below. As described above, the band pass filters
322, 323 extract the fundamental voltage components in the d-q
axis. However, this filtering causes phase shifting to the
function. The polarity detector 325, the phase transformation unit
326, and the low pass filters 327, 328 compute the phase shifting.
Based on the estimated phase shift, the phase estimation unit 329
corrects the voltage references by adjusting its phase in
accordance with equation 5.
[0034] The measured current is transformed into the synchronous
frame with respect to the voltage references signals Sine, Cosine
output by the DPPL 320. The reference signals are computed from the
measured voltages as described above. In accordance with these
reference signals (Sine, Cosine), the compensating current
reference estimator 312 transforms the measured currents are
transformed with the equation defined as below: 5 [ Id Iq I0 ] = 2
3 [ Sine - 1 2 Sine + 3 2 Cosine - 1 2 Sine + - 3 2 Cosine Cosine -
1 2 Cosine - 3 2 Sine - 1 2 Cosine + 3 2 Sine 1 1 1 ] [ Ia Ib Ic ]
( equat . 6 )
[0035] The transformed current signals (Id, Iq, I0) consist of DC
and AC components. The DC component corresponds to the fundamental
current component and the AC component corresponds to the current
harmonics. The power factor displacement constitutes the reactive
current in the q-axis, and it is corresponded to the imaginary
current Iq. Table 1 below shows the variations of transformed
current component combinations that are used to estimate the
compensating current reference.
1 Compensating Item Description Constituents Current 1 Current
Harmonics AC Components of Icd = -Id(ac) the transformed Icq =
-Iq(ac) currents (Id, Iq) Ic0 = 0 2 Power Factor Current in q-axis
Iq Icd = 0 Icq = -Iq Ic0 = 0 3 Balance of three-phase Zero Sequence
Icd = 0 current current I0 Icq = 0 Ic0 = -I0 4 Current Harmonics
and AC Components of Icd = -Id(ac) Power Factor the transformed Icq
= -Iq current Id + current Ic0 = -I0 in q-axis Iq 5 Current
Harmonics, Power AC Components of Icd = -Id(ac) Factor and
Balancing the transformed Icq = -Iq three-phase current current Id
+ current Ic0 = -I0 in q-axis Iq + Zero Sequence current I0 6 Power
Factor and balance Current in q-axis Icd = 0 of three-phase current
Iq + Zero Sequence Icq = -Iq current I0 Ic0 = -I0 7 Current
Harmonics and AC Components of Icd = -Id(ac) Balancing three phase
the transformed Icq = -Iq(ac) current current Id + Zero Ic0 = -I0
Sequence current I0
[0036] In accordance with Table 1, the transformed current
constituents represent the power quality of three-phase power
system 10. Thus, the compensating current reference estimator 312
uses Id, Iq, and I0 (the table defines the combinations in the
above equations to improve the power quality) to calculate
compensating values, Icd, Icq, and Ic0 in the d-q axis. The domain
transformation unit 314 inversely transforms the compensating
current (Icd, Icq, Ic0) in dq-axis into abc domain (Ica, Icb, Icc)
and the inverter switching control unit 316 compares the resulting
values to a triangle waveform to generate the switching control
signals to the inverter. The following is the equation of the
inverse transformation: 6 [ Ica Icb Icc ] = [ Sine Cosine 1 2 - 1 2
Sine + 3 2 Cosine - 1 2 Cosine - 3 2 Sine 1 2 - 1 2 Sine - 3 2
Cosine - 1 2 Cosine + 3 2 Sine 1 2 ] [ Icd Icq Ic0 ] . ( equation 7
)
[0037] According to the present invention, the active filter 300
uses the characteristics of the transformed current to extract the
power quality information and to inject a compensating current into
the system 10. The controller 310 further receives a measure of the
compensating current from the compensating current measuring unit
600 to adjust the switching control signals output to the inverter
340.
[0038] FIG. 5 shows the simulated load with current harmonics and
phase shifting. In FIG. 5, waveform (a) illustrates the source
voltage (for one phase), waveform (b) illustrates the simulated
effects of power conditions on the system, and waveform (c)
illustrates the result of compensation. As seen from a comparison
of waveforms (b) and (c), the active filter 300 injects a
compensating current into the system to suppress the current
harmonics. FIGS. 6A-6C illustrate the effect of the active filter
300 on multi-phase power conditions, where waveforms (1)(a),
(1)(b), and (1)(c) in FIG. 6A illustrate the effects of power
conditions on three-phases of the load current (uncompensated);
waveforms (2)(a), (2)(b), and (2)(c) in FIG. 6B illustrates
three-phase input currents with active filter compensation; and
waveforms (3)(a), (3)(b), and (3)(c) in FIG. 6C illustrates
three-phase input voltage. As seen from a comparison of waveforms
(2)(a), (2)(b), and (2)(c) with waveforms (1)(a), (1)(b), and
(1)(c), the active filter 300 in accordance with the principles of
the present invention achieves harmonic, power factor, and phase
compensation to improve total power quality in the AC system 10.
This compensation is applicable to both constant and variable
frequency environments.
* * * * *