Hysteresis control method of three-phase four-switch type active filter
Technical Field
The present invention relates to an active filter, and more particularly, to a hysteresis control method for a three-phase four-switch active filter.
Background
As the application of power electronic devices is becoming more and more extensive, and a large number of distributed power supplies are connected to a power grid, the problem of the power quality of a power system becomes more and more serious, and as a dynamic parallel compensation device, an Active Power Filter (APF) can compensate system harmonic waves and reactive power in real time, so that the dynamic parallel compensation device is widely researched and applied. APF has been developed to date, and various topologies of different forms have appeared, such as single-type, multi-converter hybrid, hybrid of active filtering and passive filtering, and the novel three-phase four-switch hybrid active filter according to the present invention.
Because the APF operates in a severe industrial field for a long time, the IGBT of the power switch device is always in a high-power high-frequency high-temperature field environment, and the reliable operation of the power switch device is a necessary factor for ensuring the stability of the APF. Once an IGBT generates overvoltage or overcurrent breakdown, the conventional treatment condition is to take the APF equipment out of the network for maintenance. With the occurrence of three-phase four-switch APF (TFSSAPF), when a single-phase power device in the device fails, the APF can still continue to work effectively and reliably by changing the topological structure thereof, so that the APF has a certain self-healing capability, the time is increased for the effective work of the APF, and more time is won for the maintenance after the APF fails.
Since the 80 s of the last century, domestic and foreign scholars have conducted a great deal of research on conventional three-phase six-switch APFs, and the modulation control algorithms of the conventional three-phase six-switch APFs tend to be perfect, but the modulation control strategies of the conventional three-phase four-switch APFs as fault-tolerant topologies of the conventional three-phase six-switch APFs are not deeply researched. The three-phase four-switch APF is a fault-tolerant circuit topology form after the fault reconstruction of the traditional three-phase six-switch APF, and because the number of the switch devices is reduced, the four-switch APF has the potential of reducing the cost and reducing the operation and maintenance cost, so that the three-phase four-switch APF has important research value.
As a core part of an active filter, current control directly affects compensation performance of the active filter, and in order to realize fast control of current, current control schemes currently used in a conventional three-phase six-switch active filter mainly include hysteresis current control, Pulse Width Modulation (PWM) and Space Vector Modulation (SVM). When the APF has single-phase open circuit fault, the damaged phase is automatically cut off through self fault diagnosis, and the topological structure is cut into a three-phase four-switch. Due to the change of topology, the current control scheme is different from the traditional three-phase six-switch topology. The invention provides a novel hysteresis control method based on an error-tolerant active filter. The method can effectively solve the problem that the direct connection of the three-phase four-switch APF and the capacitor has the advantages that the one-phase output of the midpoint of the capacitor cannot be directly controlled, the adaptability is strong, the dynamic response of the system is excellent, and the harmonic waves in the power grid can be well treated.
Disclosure of Invention
When the APF has single-phase open circuit fault, the fault phase is automatically cut off through self fault diagnosis, and the topology is switched to a three-phase four-switch structure. Due to the change of the topological structure, the current control method is different from the traditional three-phase six-switch control method. The invention provides a novel current hysteresis control method based on a three-phase four-switch fault-tolerant APF topological structure, which is used for respectively carrying out hysteresis control on an alpha-beta shaft under a static two-phase coordinate system, thereby realizing the integral control on abc three-phase current.
The technical scheme adopted by the invention for solving the technical problem comprises the following steps:
firstly, three-phase reference command current i of a filter* ca、i* cb、i* ccAnd the actually output three-phase compensation current ica、icb、iccTransforming the coordinate system to an alpha-beta two-phase orthogonal coordinate system through Park transformation to obtain i* cα、i* cβAnd icα、icβ;
Reference current i converted by Park* cα、i* cβAnd the actual output current icα、icβThe error signal between the two signals passes through a hysteresis comparator to obtain an output signal dαAnd dβ;
And obtaining a corresponding switch state table according to the topological structure of the three-phase four-switch active filter, and obtaining the switching action functions of the two bridge arms by combining the hysteresis comparison result and the switch state table to realize the hysteresis control of the three-phase four-switch active filter.
The invention is further explained below with reference to the drawings in which:
as shown in FIG. 1, assuming that a c-phase IGBT open-circuit fault occurs in a three-phase six-switch active filter, a hardware circuit generates a driving signal to drive an AC contactor to cut off the fault phase, and an output end of the fault phase is directly connected to a middle point of a DC side capacitor, namely a system topology circuit (shown in FIG. 2) is switched to a three-phase four-switch state (shown in FIG. 3). When the voltage control on the DC side is stable and the capacitors are voltage-sharing, i.e. uc1=uc2=udcIn the case of 2, the C-phase bridge arm is directly connected to the midpoint of the DC-side capacitor, so that the directly-controllable output line voltage is uao、ubo. Each switching state is as shown in fig. 4, the dead zone effect is ignored, and it is assumed that the upper tube and the lower tube of each bridge arm are complementarily turned on and off, the switching function value is 1 when the upper tube is turned on, and the switching function value is 0 when the lower tube is turned on. S in FIG. 4a、SbIs considered to be 1 and 0, respectively, from the four switch states of TFSSAPF it can be written
Projecting a static three-phase coordinate system to alpha-beta coordinates
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Writing equation (2) in matrix form
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In the formula (3)
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Due to ucoThe output is always 0, and the combined formula (1) and the formula (3) have
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The corresponding relation between the switching signal and the voltage space vector under the alpha-beta coordinate system is obtained according to the formula (5), and the following steps are carried out:
since the new APF topological structure can not control the c-phase output current independently, three independent hysteresis comparators can not be used for controlling the three-phase current respectively. The invention provides a hysteresis control method under an alpha-beta coordinate of a three-phase four-wire system, which respectively performs hysteresis control on an alpha-beta axis under a two-phase orthogonal coordinate system, thereby realizing the integral control of abc three-phase current, and the basic flow is shown in figure 5.
In conjunction with the above table and FIG. 5, illustrated by the α axis, d is when the actual output current is greater (less) than the command valueαIs-1 (+1), so the α axis is required to output a negative (positive) voltage to decrease (increase) the actual output current. Therefore, the corresponding relation between the switching signal and the voltage space vector and the hysteresis output state d under the alpha-beta coordinate are combinedαAnd dβThe switch state U at the current moment can be obtainedkAs shown in the following table:
dα |
dβ |
Uk |
Sa |
Sb |
-1 |
-1 |
U0 |
0 |
0 |
-1 |
+1 |
U1 |
0 |
1 |
+1 |
-1 |
U2 |
1 |
0 |
+1 |
+1 |
U3 |
1 |
1 |
the problem that the middle point one-phase output of a three-phase four-switch fault-tolerant APF topological structure directly connected with a capacitor cannot be directly controlled is effectively solved based on hysteresis control under an alpha-beta coordinate, so that the APF has certain self-healing capacity, the method is simple and reliable, the dynamic response of the system is excellent, the time is prolonged for the effective work of the APF, more time is won for the maintenance after the APF is in fault, and the method has good economic significance and market prospect.
Drawings
FIG. 1 is a fault tolerant APF topology;
FIG. 2 is an APF normal state topology;
FIG. 3 is a three-phase four-switch APF topology;
fig. 4 shows the switching states of a three-phase four-switch APF;
FIG. 5 is a block diagram of hysteresis control in α - β coordinates;
FIG. 6 is a basic structure diagram of a system connected to a three-phase four-switch APF;
fig. 7 is a current waveform of the compensated rear network side by adopting a hysteresis control method under an alpha-beta coordinate.
Detailed Description
Fig. 6 is a diagram of a system for switching in a three-phase four-switch active filter, the system comprising two parts: APF main circuit and measurement and control circuit. The APF main circuit is connected with a power grid through a series inductor and has the function of generating actually required compensation current so as to compensate harmonic waves and reactive current in the power grid. The measurement and control circuit has the functions of detecting voltage and current on the power grid side, current on the load side, APF compensation current, capacitance voltage on the direct current side and the switching state of each contactor, completing protection of the circuit and generating a driving signal of the main circuit IGBT. In the APF main circuit: QF1 and QF2 are air switches; KM1, KM2, KM3 and KM4 are AC contactors; r1, R2 and R3 are buffer resistors; r4 and R5 are discharge resistors; c1 and C2 are direct current bus capacitors; l is an alternating current side connection inductor. The measurement and control circuit part mainly comprises five parts: the device comprises a main control circuit taking a DSP chip as a core, an analog quantity acquisition/AD conversion circuit, a zero-crossing comparison circuit, an overvoltage and overcurrent detection circuit and a switching value input/output circuit.
As shown in fig. 1, when an IGBT open circuit fault of a certain phase occurs in an active filter, a hardware circuit generates a driving signal to drive an ac contactor to remove the fault phase, and an output end of the fault phase is directly connected to a midpoint of a dc side capacitor, and since a new APF topology structure cannot independently control a fault phase output current, three independent hysteresis comparators cannot be used to control three-phase currents respectively, a control algorithm is also switched from an original hysteresis control algorithm applicable to a three-phase six-switch active filter to a novel hysteresis control applicable to a three-phase four-switch, and the algorithm includes the following steps:
1) firstly, three-phase reference instruction current i of an active filter is measured* ca、i* cb、i* ccTransforming the coordinate system to an alpha-beta two-phase orthogonal coordinate system through Park transformation to obtain i* cα、i* cβ:
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Wherein:
2) the three-phase compensation current i actually output in the active filter is compensatedca、icb、iccAlso converting the coordinate system into an alpha-beta coordinate system through Park transformation to obtain icα、icβ:
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Wherein:
3) an error signal between the actual output current and the reference current of the converter passes through a hysteresis comparator to obtain an output signal dαAnd dβ。
If the current error of alpha axis icα-icαIf the current is larger (smaller) than the set positive (negative) loop width, the compensation current actually output by the converter is too small (too large), and the output current of the converter should be increased (reduced) by adjustment. In this case, a positive (negative) voltage vector u should be selectedα(ii) a If it is notαAxial current error icα-icαWithin a set loop width, the output signal d of the hysteresis comparison output deviceαAnd = 0. At the moment, the switching action of the converter is kept unchanged, so that the output current of an alpha axis can track the reference instruction current, the tracking control of the current in an alpha-beta coordinate system is realized, and the tracking control of the current in an abc coordinate system is further realized.
4) The output voltage of each phase of the APF depends on the state S of the three-bridge armabcSwitching function S of three bridge armsi(i = a, b, c) can independently take 0 and 1, and since the three-phase four-switch type APF is in one arm fault (assuming c-phase), the three-phase four-switch has four switch states. Four combinations of switching functions are substituted:
corresponding APF output phase voltage values can be obtained, and the bridge arm phase is usedVoltage ua、ubAnd ucConverting the u into an alpha-beta coordinate system to obtain corresponding u in each switch stateαAnd uβThe table below gives the 4 switch states and the respective voltage values (voltage in U)dcAs a reference):
and selecting a voltage vector and a switch action state according to the hysteresis comparison result, wherein the selection process is as follows:
dα |
dβ |
Uk |
Sa |
Sb |
-1 |
-1 |
U0 |
0 |
0 |
-1 |
+1 |
U1 |
0 |
1 |
+1 |
-1 |
U2 |
1 |
0 |
+1 |
+1 |
U3 |
1 |
1 |
and finally, comparing the switching state tables according to hysteresis comparison results d alpha and d beta to obtain the switching action states of the two bridge arms, so as to realize hysteresis control of the three-phase four-switch active filter.
In order to verify the feasibility and the correctness of the method provided by the invention, a TFSSAPF system model is established based on Matlab/Simulink, and a c-phase bridge arm is directly connected to a direct-current side capacitor neutral point as a fault processing result on the assumption of c-phase open circuit fault. The simulation system parameters are as follows: 380V power frequency three-phase alternating current power supply, the system impedance is ignored; the non-linear load being a three-phase uncontrolled rectifier bridge, RL=23 Ω, 6800 μ F electrolytic capacity is adopted on the direct current side of APF, the set voltage on the direct current side is about twice of the set voltage on the direct current side of the three-phase six-switch, and the set voltage is 1400V in the invention; the output filter is an L filter, L =1 mH.
FIG. 7 is a waveform of the grid current at steady state after compensation by TFSSAPF using the hysteresis control method proposed herein under α - β coordinates. As can be seen from fig. 7, the hysteresis control method under the α - β coordinate provided by the present invention can effectively control TFSSAPF, has a good compensation effect, has universality for a three-phase four-switch active filter, and has a certain self-healing capability for the APF.