) is at its minimum when the forced commutation happens. In 0
α
_{2}
, this can be calculated from Formula (7),
For m=0.9 and T
_{S}
=416.7μs, it is possible to obtain
α
_{1}
≈0.072, which is then substituted into Formula (8),
From the peak value of the gridside phase voltage U
_{S}
≈311V, it is possible to obtain U
_{AC}
≈38.75V. By this calculation, it is possible to obtain the minimum reverse voltage, which is about 38.75V, in the forced commutation of the thyristor. This theoretically meets the condition of adding reverse voltage to the switch off thyristor
[11]
,
[12]
.
In addition, considering the commutation among the sectors,
Fig. 7
shows a diagram of the vector distribution from Sector I to Sector II. In
Fig. 7
, for the nonnatural commutation, the switchingoff time of the thyristor required to be switched off is T
_{OFF}
; for the natural commutation, the reverse voltage imposed on the thyristor to be switched off is far greater than the minimum, 38.75V. In other words, it can ensure normal conduction of the current source space vectors of the circuit. This is validated by simulation and experiment in the following sections
[10]

[15]
.
Sequence diagram of vectors from Sector I to Sector II of an optimal SVPWM
V. SIMULATION RESULTS
A Simulink verification is conducted for the optimal space vector method control strategy of the threephase currentmode PWM rectifier with a hybrid switch. For the simulation, the power is 280kW, the working direct current is 700A, the gridside filter’s resonance frequency is about 750Hz, the AC inductance of the threephase filter is 125μH, the equivalent series resistance is 0.05Ω, the gridside threephase filter capacitance is 360μF, the quality factor of the filter is about 10, and the working frequency of the circuit is 2.4kHz.
Fig. 8
shows the gridside current waveform.
Gridside current waveform under the optimal control strategy.
(a) Gridside PWM current waveform of Phase a before filtering; (b) Gridside current waveform of various phases after filtering
For the Fourier analysis of the gridside current, shown in
Fig. 8
(b), the THD of the gridside current Isa is approximately 19.53%. This current mainly contains the 13
^{th}
and 19
^{th}
harmonics, which account for around 15.07% and 7.85% of the fundamental component, respectively. This is due to the vector distribution. As shown in
Fig. 6
, vector I
_{6}
is distributed continuously around
α
=0, which leads to gridside current distortion. After filtering the 13
^{th}
and 19
^{th}
harmonics of the power grid by adding a resonance absorption circuit, the gridside threephase current waveform is changed as shown in
Fig. 9
[11]
,
[14]
.
Gridside current waveform. (a) Gridside current waveform after absorption of the 13^{th} harmonic. (b) Gridside current waveform after absorption of the 13^{th} and 19^{th} harmonics.
In
Fig. 9
(a), after filtering the 13
^{th}
current harmonic, the THD of the gridside current Isa is about 12.3%. In
Figure 9
(b), after simultaneous resonance absorption of the 13
^{th}
and 19
^{th}
current harmonics, the THD of the gridside current Isa is around 9.60%.
Compared with the traditional current source space vectors, with the same thyristor switchingoff time and circuit modulation ratio, the highest working frequency of the circuit is 1.2kHz. When using the same circuit topology parameters for the simulation, the gridside current waveform will be as shown in
Fig. 10
. Since the corner frequency of the gridside filter is very close to the working frequency of the circuit, the harmonics of the switching frequency and its marginal frequency are not easy to filter. Upon using the Fourier analysis, it can be seen that the amplitude of the switching frequency harmonic in the gridside current Isa takes up about 27.88% of the fundamental harmonic, while the THD of the gridside current Isa is around 34.3%. In addition, it is impractical to absorb the harmonic of the switching frequency by adding resonance absorption current. It is only feasible to attenuate the harmonics by adding a filter to the main circuit. However, this increases the cost and size of the whole circuit
[10]

[12]
.
Gridside current waveform under the traditional control strategy. (a) Gridside PWM current waveform of Phase a before filtering. (b) Gridside current and voltage waveforms of Phase a after filtering.
RESULTS FOR HARMONIC ANALYSIS OF GRIDSIDE CURRENT WITH ADVANCED CONTROL METHOD
RESULTS FOR HARMONIC ANALYSIS OF GRIDSIDE CURRENT WITH ADVANCED CONTROL METHOD
RESULTS FOR HARMONIC ANALYSIS OF LINE CURRENT WITHOUT ADVANCED CONTROL METHOD
RESULTS FOR HARMONIC ANALYSIS OF LINE CURRENT WITHOUT ADVANCED CONTROL METHOD
Judging by the simulation results, improving the switching frequency by using the optimal current source space vector control will cause certain gridside current distortions. However, by adopting a resonance absorption circuit, it is possible to suppress the gridside current harmonics, while the traditional space vector control methods cannot achieve an equivalent harmonic suppression effect.
VI. EXPERIMENT RESULTS
To validate the aboveanalyzed threephase current source PWM rectifier with a hybrid switch as well as its PWM rectification control method and principle, a lowpower test prototype has been designed. The main circuit parameters for the experiment are as follows: IGBT modules are used for T
_{1}
, T
_{2}
; a common thyristor is used for the rectifier, with the switchingoff time set as the minimum value of 42μs; the gridside filter capacitance is 126.6μF; the filter inductance is l mF; the DCside filter inductance is l mH; the capacitance is 2mF; the load current is 15A; the working frequency of circuit is 2.4kHz; and the amplitude of the modulation ratio is 0.85. The digital control system is based on a DSP chip (TMS320LF28335)
[11]
,
[15]
.
Fig. 11
(a) shows the drive pulse of the upper tube of Phase A and the PWM current waveform of Phase A, which are enlarged in the circle of
Fig. 11
(b). After the minimum zero vector time, 20μs of a cycle, the common thyristor can be realized via the forced commutation. This validates the feasibility of the optimal current source space vector strategy.
Current waveforms under the optimal control strategy. (a) Drive pulse of upper tube and PWM current waveform of Phase a. (b) Drive pulse of upper tube and PWM current waveform of Phase a after amplifying.
Fig. 12
shows the gridside current and gridside voltage waveform of Phase A after filtering. It can be seen that the THD of the gridside current is 3.5%. It can also be seen that the gridside current and voltage are of the same phase, and that the current waveform is basically a sine wave.
Fig. 13
shows the voltage waveform output of the DCside rectifier bridge and that after the filter inductance. It can be seen that the enhancement of the circuit switching frequency can greatly reduce the cost and size of the DC filter
[11]
,
[12]
,
[15]
.
Gridside current i_{as} and voltage waveform u_{as} of Phase a
Output voltage u_{d} of rectifier and filtered DCside voltage u_{dc}
VII. CONCLUSION
This paper analyzes the influence of highpower rectifiers on power grids, and introduces a method for eliminating the interference of power grids by highpower converters. Specifically, a threephase current source PWM rectifier with a hybrid switch that uses a thyristor to replace the highpower controllable switch tube to make a low cost highpower PWM rectifier. While guaranteeing thyristor switchingoff, the use of the optimal space vector control method also enhances the working frequency of the circuit, and ensures that the gridside current harmonics can be better suppressed. Moreover, this paper analyzes both the advantages and disadvantages of the optimal control method by system simulation. The proposed method makes the gridside current harmonics of the rectifier greatly suppressed via circuit optimization. Finally, the feasibility of the threephase current source PWM rectifier with a hybrid switch and the correctness of the optimal current source space vectors are validated by experimental results.
BIO
Yan Xu received her B.S. and M.S. degrees in Optical Communication from the Nanjing University of Posts and Telecommunications, Nanjing, China, in 2004 and 2011, respectively, where she is presently working toward her Ph.D. degree. Her current research interests include microgrid power quality, electric power savings, and relay protection.
Guangxiang Lu was born in Jiangsu, China, on October 3, 1956. He received his B.S., M.S. and Ph.D. degrees in Electrical Engineering from Southteast University, Nanjing, China, in 1982, 1991 and 2000, respectively. He is presently a Professor in the Department of Electrical Engineering, Southteast University. His current research interests include microgrid power quality, electric power savings, and relay protection.
Lijie Jiang was born in Chongqing, China, on May 20, 1983. He received his B.S., M.S. and Ph.D. degrees in Electrical Engineering in 2005, 2008 and 2011, respectively. He is presently working for Monlithicpower System Inc., Hangzhou, China. His current research interests include power electronic digital control and system integration.
GuiPing Yi received his B.S. degree from the College of Electrical and Information Engineering, Nanchang University, Nanchang, China, in 2004, and his M.S. degree from the College of Electric Power and Automation Engineering, Shanghai University of Electric Power, Shanghai, China, in 2010. He is presently working toward his Ph.D. degree. His current research interests include microgrid power quality, electric power savings, reactive power compensation, and active power filters.
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