Abstract: Three level rectifiers as a result of its unique merit, have received more and more values. Introduced three level bridge-type rectifier’s principle of work, and carried on with the digital signal processor to its control system has realized, showed the entire numerical control system’s hardware design and the software design method. The simulation and the experimental result have confirmed the fundamental research result.
Key word: Digital signal processor; Three levels; PWM rectifier; Power factor adjustment
Introduction
Three levels (ThreeLevel, TL) the rectifier is one kind may use in the high-pressured high efficiency the PWM rectifier, has the power factor close 1, and the switching voltage stress ratio two levels reduce half the merit. Literature [1] and [2] mentioned that one kind of three level Boost electric circuit, uses to the rectification bridge carries on the power factor to adjust, but as a result of the diode rectification electric circuit’s irreversibility, is unable to realize the power class two-way flow. Literature [3],[4] and [5] mentioned several kind of three level PWM rectifier, although has realized three levels, but on the switching valve the voltage stress reduction half’s merit has not realized. Three level rectifiers are freely more than two level rectifier switch quantity, the control is complex, but? Has characteristic which two level rectifiers do not have:
1) the level number increases enables it to have the smaller direct-current side voltage pulsation and a better dynamic property, when the turn-on frequency is very low, if 300~500Hz can satisfy to the current harmonics request;
2) the level number increases also causes in the power source side current ratio two levels electric current closer sines, and along with level number’s increase, the sine is better, the power factor is higher;
3) the switch increases is also advantageous in reduces on switching valve’s voltage compressed stress, enhances the installment work the stability, is suitable to the voltage request high situation.
1 TL rectifier principle of work
TL rectifier main circuit as shown in Figure 1, is composed three level bridge circuits by 8 switching valve V11~V42. Supposes u1=u2=ud/2, then each switching valve will undertake direct-current side voltage half.
Take the left half bridge arm as the example, when 1 condition, when electric current is just, the electric current flows through VD11 and VD12 from A spot to the out-port; When is a negative value, the electric current flows through V11 and V12 from A spot arrives at the out-port, therefore, regardless of being is what value, had uAG=uCG= ud/2, D1 to prevent electric capacity C1 by V11(VD11) pipe nipple. Likewise, when 0 condition, 
has uAG=0; When - 1 condition, had uAG=uDG=-ud/2, D2 to prevent electric capacity C2 by V22(VD22) pipe nipple.
The right half bridge arm principle is similar, therefore A and B terminal voltage profile as shown in Figure 2, thus has five levels on exchange side voltage uAB: ud, ud/2,0,-ud/2,-ud.
Each half bridge has three kind of active statuses, the entire TL bridge altogether has a 32=9 condition. The distinction is as follows:
The condition 0(1,1) switching valve V11, V12, V31, V32 clear, converter exchange side voltage uAB is equal to 0, the electric capacity through the direct-current side load electric discharge, the line current is size increases along with the main circuit voltage us change or reduces.
The condition 1(1,0) switching valve V11, V12, V32, V41 clear, exchange side input voltage uAB is equal to ud/2, the input end inductance voltage is equal to us-u1. Electric capacity C1 voltage by forward (or reverse) electric current charge (u1<us, or discharges us<u1), C2 through direct-current side load electric discharge.
Condition 2(1,-1) switching valve V11, V12, V41, V42 clear, input voltage uAB=ud, forward (either reverse) electric current to electric capacity C1 and C2 charge (or electric discharge), because the input inductance voltage is reverse, electric current is reduces gradually.
The condition 3(0,1) switching valve V12, V21, V31, V32 clear, exchange side input voltage uAB is equal to - ud/2, inputs in the inductance the voltage to be equal to us u1. Electric capacity voltage by forward (either reverse) electric current charge (or electric discharge).
The condition 4(0,0) switching valve V12, V21, V32, V41 clear, inputs the terminal voltage is 0, the electric capacity through the direct-current side load electric discharge, the line current is size increases along with the main circuit voltage us change or reduces.
The condition 5(0,-1) switching valve V12, V21, V41, V42 clear, the exchange side voltage is ud/2, forward (either reverse) electric current to the electric capacity C2 charge (or electric discharge), electric capacity C1 through load current electric discharge.
Condition 6(-1,1) switching valve V21, V22, V31, V32 clear, uAB=-ud, forward (either reverse) line electric current to two electric capacity C1 and C2 charge (or electric discharge), because pressure-rise inductance voltage forward, the line electric current will increase gradually.
Condition 7(-1,0) switching valve V21, V22, V32, V41 clear, exchange side voltage level for - ud/2, forward (either reverse) electric current to the electric capacity C2 charge (or electric discharge), electric capacity C1 through load current electric discharge.
The condition 8(-1,-1) switching valve V21, V22, V41, V42 clear, inputs the terminal voltage is 0, the pressure-rise inductance voltage is equal to us, two electric capacity C1 and C2 through load current electric discharge. Electric current is increases according to the voltage us change (or reduces).
2 hardware circuit design
May see from Figure 2, in the input voltage frequency constant situation, must have a three level voltage waveform in the converter exchange side, input voltage one cycle planted agent defines two operating areas: The region 1 and the region 2, as shown in Figure 3.
In the region 1, the voltage is bigger than - ud/2, and is smaller than ud/2, has three levels on voltage uAB: - ud/2,0, ud/2. Likewise, in the region 2, the voltage absolute value is bigger than ud/2, and is smaller than direct-current side voltage ud, (or negative alternation) on has two levels in the voltage alternation: ud/2 and ud (or - ud/2 and - ud). Corresponding level’s working space like table 1 arranges in order.
Table 1 corresponding level working space
|
Working space |
1 |
2 |
1 |
2 |
|
us>0 |
us<0 |
us>0 |
us<0 |
|
|
High level |
ud/2 |
0 |
ud |
- ud/2 |
|
Low level |
0 |
- ud/2 |
ud/2 |
- ud |
For the convenience control, here defines two controlled variable SA and SB,
May design a switch questionnaire according to Table 1, like Table 2 arrange in order, its memory in DSP, when carries on the real-time control, then may act according to the input voltage, the electric current signal, inquires the switch strategy which from the table must adopt.
Table 2 questionnaires
|
SA |
SB |
V11 |
V12 |
V21 |
V22 |
V31 |
V32 |
V41 |
V42 |
uAB |
|
1 |
1 |
1 |
1 |
0 |
0 |
1 |
1 |
0 |
0 |
0 |
|
1 |
0 |
1 |
1 |
0 |
0 |
0 |
1 |
1 |
0 |
ud/2 |
|
1 |
- 1 |
1 |
1 |
0 |
0 |
0 |
0 |
1 |
1 |
ud |
|
0 |
1 |
0 |
1 |
1 |
0 |
1 |
1 |
0 |
0 |
- ud/2 |
|
0 |
0 |
0 |
1 |
1 |
0 |
0 |
1 |
1 |
0 |
0 |
|
0 |
- 1 |
0 |
1 |
1 |
0 |
0 |
0 |
1 |
1 |
ud/2 |
|
- 1 |
1 |
0 |
0 |
1 |
1 |
1 |
1 |
0 |
0 |
- ud |
|
- 1 |
0 |
0 |
0 |
1 |
1 |
0 |
1 |
1 |
0 |
- ud/2 |
|
- 1 |
- 1 |
0 |
0 |
1 |
1 |
0 |
0 |
1 |
1 |
0 |
The entire control system take piece of DSP as the core, control diagram as shown in Figure 4.
The phase-locked loop electric circuit produces one with the supply voltage with the phase unit sinusoidal waveform, the ud sampled signal carries on the adjustment through the low speed voltage outer ring regulator, the electric current is sampled signal carries on the adjustment through high speed electric current inner rim G1, the electric capacity C1 straightforward discharge presses u1 and the electric capacity C2 straightforward discharge presses u2 to carry on the adjustment separately through two PI regulators, compensated loop G2 uses in compensating two electric capacity voltages not to be balanced.
The examination line electric current orders is and reference current is* compares, the current error signal which produces delivers to electric current inner rim G1, tracks the supply current change, produces line current waveform with principal voltage with phase.
3 software designs
The system uses two general timer GPT1 and GPT2 has the periodic CPU interrupt, GPT1 uses in the PWM signal having, the ADC sampling and the high-frequency current link controls (20kHz), GPT2 to use in the low frequency electricity pressure ring’s control (10kHz), both use rise/to fall the counting pattern continuously. The low speed electricity pressure ring’s sampling time is 100μs, the high speed electric current link sampling time is 50μs. Interrupt mask register IMR, EVIMRA and EVIMRB cause GPT1 have the interrupt in the drop along and the specific cycle, GPT2 only along have the interrupt in the drop.
The entire procedure divides into the master routine module, the initialization module, the current control link computation module, the voltage control link computation module, the PWM signal to have the module and so on five major parts. Procedure flow as shown in Figure 5.
4 simulation results and experiment
The simulation parameter is as follows: Input voltage us exchanges 220V,50Hz, output 1kW, switching valve GTO, turn-on frequency 500Hz. Under the rectification condition and the contravariant condition supply voltage us, supply current is, exchange side voltage uAB profile distinction like chart 6 and shown in Figure 7. The experimental result also confirmed the design accuracy, when uses the GTO tube, the turn-on frequency is low (500Hz), the input side current waveform still the very close sine, the installment obtained the close 1 power factor, simultaneously on switch’s voltage stress reduced half.
5 conclusions
Will use the entire numerical control three level PWM rectifier the control system peripheral circuit to reduce to few, may obtain the very high performance under the low cost. Is better than based on the DSP three level rectifier the tradition power factor compensating circuit dynamic property, may obtain the quite good sine current waveform under the low turn-on frequency, and may use in switch components and so on GTO. Like uses in domains and so on high-pressured, high efficiency three-phase circuit, VVVF power source, motor control, this plan superiority is more obvious.
