CN104601026A - Suspended capacitor voltage control method of five-level ANPC (Active Neutral-Point-Clamped) converter - Google Patents

Abstract

The present invention relates to a five-level ANPC converter, in particular to a method for controlling the suspension capacitor voltage of a five-level ANPC converter, comprising the following steps: the SHEPWM pulse wave acts on the five-level ANPC converter; real-time detection of the five-level ANPC The voltage value of the floating capacitor of the converter is compared with the reference value of the floating capacitor voltage to determine whether the floating capacitor needs to be charged or discharged; detect the positive and negative of the load current of the five-level ANPC converter, and determine the positive and negative values according to the positive and negative sum of the load current of the ANPC converter The suspension capacitor needs to be charged and discharged in the state of charging and discharging, and the redundant switch state of the appropriate ANPC converter is selected; the suspension capacitor of the five-level ANPC converter is charged and discharged through the selected switch state, until the change of the suspension capacitor voltage in a sampling period is 0, repeat detection of floating capacitor voltage value. After adopting the above method, an appropriate redundant switch state is selected according to the direction of the load current and the voltage of the floating capacitor, so that the voltage of the floating capacitor can fluctuate within a small range.

Description

Voltage Control Method of Floating Capacitor in Five-level ANPC Converter

technical field

The invention relates to a five-level ANPC converter, in particular to a method for controlling the suspension capacitor voltage of the five-level ANPC converter.

Background technique

With the advancement of technology and the improvement of living standards, people's requirements for power quality are also constantly improving. The emergence of active neutral-point-clamped (ANPC) topology makes up for the diode-clamped topology and capacitor The shortcomings of the clamping topology are better suited for the high-voltage frequency conversion fields of 6 and 10kV. ANPC multi-level converters are more and more used in the fields of industry and social life to provide higher quality electric energy. At present, domestic selection of SHEPWM modulation method for five-level ANPC converters has not been published yet. Selecting SHEPWM control can effectively reduce low-order harmonics at high frequencies, improve waveform quality, and reduce requirements for filters. One of the prerequisites for five-level and above ANPC topologies to work normally is that the floating capacitor voltage must remain stable. During SHEPWM control, if the key issue of the floating capacitor voltage is not considered, if the obtained SHEPWM switch state causes the floating capacitor voltage to be large fluctuations will lose practical significance.

Contents of the invention

The technical problem to be solved by the present invention is to provide a control method for keeping the floating capacitor voltage stable when SHEPWM modulates the five-level ANPC converter.

In order to solve the above-mentioned technical problems, the five-level ANPC converter suspension capacitor voltage control method of the present invention comprises the following steps:

Step S101: Determine the switch angle to be set according to the modulation degree m of SHEPWM, determine the three-phase output modulated by SHEPWM, and output the three-phase five-level SHEPWM pulse wave to act on the five-level ANPC converter;

Step S102: Detect the voltage value of the floating capacitor voltage of the five-level ANPC converter in real time, and compare it with the reference value of the floating capacitor voltage to determine whether the floating capacitor needs to be charged or discharged;

Step S103: Detect the positive and negative of the load current of the five-level ANPC converter, and select the appropriate redundant switch state of the ANPC converter according to the positive and negative of the load current of the ANPC converter and the required charge and discharge state of the floating capacitor determined in step S102;

Step S104: Charge and discharge the suspension capacitor of the five-level ANPC converter according to the selected switch state until the variation of the voltage of the suspension capacitor is 0 within one sampling period, and return to step S102.

Further, the current flowing through the floating capacitor in the step S104 can be expressed as I _cf =I _S3 -I _S1 , and the current flowing through each switch tube can be expressed as I _Si =X _i I _load , where _Xi represents each switch tube When the switching tube S _i is turned on, the value of Xi _i is 1, and when the switching tube S _i is turned off, the value of Xi _i is 0; I _cf =(X ₃ -X ₁ )I _load , in Within a sampling period, the variation of the floating capacitor voltage is ${\mathrm{\ΔV}}_{\mathrm{cf}}={\∫}_0^T\frac{1}{\mathrm{cf}}(x_3-x_1)I_{\mathrm{load}}\mathrm{dt}.$

Further, in the step S101, the five-level SHEPWM pulse wave acts on the five-level ANPC converter to output five levels which are 2E, E, 0, -E and -2E respectively.

Further, in step S102, when the intermediate level E or -E is detected each time, the flag position is 1, and the logic judgment is started. When the real-time detected floating capacitor voltage value is greater than the reference voltage value E, select Discharge the floating capacitor; otherwise, when the real-time detected voltage value of the floating capacitor is lower than the reference voltage value E, choose to charge the floating capacitor.

Furthermore, in step S103 , the load current flows from the single-phase bridge arm of the five-level ANPC converter to the load as the positive direction, and the load current is positive, otherwise it is negative.

Furthermore, according to the need to select the appropriate redundant switch state for the charging and discharging state of the floating capacitor and the positive and negative of the load current in step S102, after selecting the appropriate redundant switch state, the flag position is 0.

After adopting the above method, the five-level ANPC converter can effectively suppress low-order harmonics in the entire working range and obtain a better output waveform. This method has better dynamic performance and can control the floating capacitor voltage under ideal steady-state conditions. fluctuate within a small range. Under dynamic conditions, the appropriate redundant switch state can also be selected according to the direction of the load current and the magnitude of the floating capacitor voltage, so that the floating capacitor voltage can fluctuate within a small range.

Description of drawings

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is the single-phase topology structure of the five-level ANPC converter of the present invention.

Fig. 2 is the flow chart of the five-level ANPC converter floating capacitor voltage control method of the present invention

Fig. 3 is a schematic diagram of logical judgment of the method for controlling the floating capacitor voltage of the five-level ANPC converter according to the present invention.

Fig. 4 is a schematic diagram for logic judgment to select an appropriate redundant switch state.

Fig. 5 is a switch state table of the five-level ANPC converter of the present invention.

Detailed ways

SHEPWM control is to set a "gap" at a specific position of the voltage waveform. Through the optimal selection of the switching time, the pulse width modulation voltage waveform is properly controlled, so that there are no specific orders of harmonics in the output voltage of the converter. SHEPWM can eliminate certain low-order harmonics in the case of high switching frequency, while the remaining high-order harmonics can be easily filtered out by filter design. Selecting SHEPWM control can effectively reduce low-order harmonics at high frequencies, improve waveform quality, and reduce requirements for filters. The output waveform quality of the five-level ANPC converter is better, and it can be better used in motor control, power electronics, and power quality control. The key point of modulation is whether the suspension capacitor voltage can be kept in balance.

As shown in Fig. 2 and Fig. 3, a kind of five-level ANPC converter SHEPWM of the present invention and the control method of floating capacitor voltage comprise the following steps:

Step S101: Determine the switching angle to be set according to the modulation degree m of the SHEPWM, determine the three-phase output of the SHEPWM modulation, and output the three-phase five-level SHEPWM pulse wave to act on the five-level ANPC converter. The single-phase topology diagram of the five-level ANPC converter is shown in Figure 1. It can be seen that the five-level ANPC converter includes three capacitors, including a floating capacitor, and the current flowing through the floating capacitor can be expressed as _Icf = I _S3 -I _S1 , the current flowing through each switch tube can be expressed as I _Si =X _i I _load , where Xi _i represents the switching state of each switch tube, when the switch tube S _i is turned on, the value of Xi _i is 1, when When the switch tube S _i is turned off, the value of Xi _i is 0; I _cf =(X ₃ -X ₁ )I _load , within one sampling period, the variation of the floating capacitor voltage is The five-level SHEPWM wave acting on the five-level ANPC converter will output five level values, namely 2E, E, 0, -E, -2E. The five-level ANPC has 8 switch states, as shown in Table 1, except that the two level values of 2E and -2E have only one switch state, and the other three level values have two redundant switch states. However, in the four switching states of V1, V4, V5, and V8 in Table 1, the current does not flow through the floating capacitor, so it has no effect on the voltage of the floating capacitor. That is, the corresponding switch states V8 and V1 are respectively used at the two level values of 2E and -2E in each phase, and the two redundant switch states V4 and V5 at the 0 level are equally distributed and used at the 0 level. For the two intermediate level values E and -E, the switching states V2, V3, V6, and V7 are inconsistent with the direction of the load current, and have different effects on the suspension capacitor voltage, which will be dynamically selected by the next step.

As shown in Figure 4, the five-level ANPC can work normally, and the floating capacitor voltage must be kept at a quarter of the DC bus voltage, that is, E, so that the output phase voltage can output five-level values normally. When the first intermediate level E or -E comes, the flag position is 1, and a logical judgment is made. After selecting the appropriate redundant switch state, the flag position is 0, and the logical judgment is stopped. When the next intermediate level comes, The flag position is 1, and then logical judgment is made to select the appropriate redundant switch state, and the process is repeated accordingly. The logical judgment includes two aspects, one is to judge the voltage value of the floating capacitor, and the other is to judge whether the load current is positive or negative. Steps S102 and S103 are to judge the above two aspects respectively.

Step S102: Detect the voltage value of the suspension capacitor voltage of the five-level ANPC converter in real time, and compare it with a reference value of the suspension capacitor voltage to determine whether the suspension capacitor needs to be charged or discharged.

Step S103: Detect whether the load current of the five-level ANPC converter is positive or negative, and select an appropriate redundant switch state of the ANPC converter according to the positive or negative of the load current of the ANPC converter and the required charge and discharge state of the floating capacitor determined in step S102. Wherein, the direction of the load current flowing from the single-phase bridge arm of the five-level ANPC converter to the load is the positive direction.

Step S104: Charge and discharge the suspension capacitor of the five-level ANPC converter according to the selected switch state until the variation of the voltage of the suspension capacitor is 0 within one sampling period, and return to step S102.

The details of steps S102-S104 are as follows: As shown in Figure 4, in this embodiment, when it is detected that the intermediate level E is approaching, the flag position is 1, and the floating capacitor voltage detected in real time by step S102 is compared with the floating capacitor voltage reference As a result, it is determined whether to charge or discharge the floating capacitance. When the real-time detected floating capacitor voltage value is greater than the reference voltage value E, select the redundant switch state to discharge the floating capacitor; if the real-time detected floating capacitor voltage value is lower than the reference voltage value E, then select the redundant switch state The floating capacitor is charged, and then the flow direction of the load current detected in step S103 is comprehensively judged to select an appropriate switch state to act on the five-level ANPC converter. As shown in FIG. 5 , when the floating capacitor needs to be charged, if the load current is greater than 0, the switch state V7 is selected, and if the load current is less than 0, the switch state V6 is selected. When it is necessary to discharge the floating capacitor, if the load current is greater than 0, select the switch state V6, and if the load current is less than 0, select the switch state V7. Similarly, when it is detected that the intermediate level-E is coming, the flag position is 1, and the result of comparing the floating capacitor voltage detected in real time with the floating capacitor voltage reference in step S102 determines whether to charge or discharge the floating capacitor; When the floating capacitor voltage value is greater than the reference voltage value E, the redundant switch state is selected to discharge the floating capacitor; if the real-time detected floating capacitor voltage value is lower than the reference voltage value E, the redundant switch state is selected to discharge the floating capacitor charging, and then comprehensively judge the flow direction of the load current detected in step 4 to select an appropriate switch state. As shown in FIG. 5 , when the floating capacitor needs to be charged, if the load current is greater than 0, the switch state V3 is selected, and if the load current is less than 0, the switch state V2 is selected. When it is necessary to discharge the suspension capacitor, if the load current is greater than 0, select the switch state V2, and if the load current is less than 0, select the switch state V3.

Although the specific embodiments of the present invention have been described above, those skilled in the art should understand that these are only examples, and various changes or modifications can be made to the embodiments without departing from the principle and essence of the present invention. The scope of protection is limited only by the appended claims.

Claims (6)

1. a five-level ANPC converter floating capacitor voltage control method is characterized in that, comprising the following steps: Step S101: Determine the switch angle to be set according to the modulation degree m of SHEPWM, determine the three-phase output modulated by SHEPWM, and output the three-phase five-level SHEPWM pulse wave to act on the five-level ANPC converter; Step S102: Detect the floating capacitor voltage value of the five-level ANPC converter in real time, and compare it with the reference value of the floating capacitor voltage to determine whether the floating capacitor needs to be charged or discharged; Step S103: Detect the positive and negative of the load current of the five-level ANPC converter, and select the appropriate redundant switch state of the ANPC converter according to the positive and negative of the load current of the ANPC converter and the required charge and discharge state of the floating capacitor determined in step S102; Step S104: Charge and discharge the suspension capacitor of the five-level ANPC converter through the selected switch state until the variation of the voltage of the suspension capacitor is 0 within one sampling period, and return to step S102. 2. According to the five-level ANPC converter floating capacitor voltage control method according to claim 1, it is characterized in that: the current flowing through the floating capacitor in the step S104 can be expressed as I _cf =I _S3 -I _S1 , and flows through each The current of the switch tube can be expressed as I _Si =X _i I _load , wherein _Xi represents the switching state of each switch tube, when the switch tube S _i is turned on, the value of Xi _i is 1, and when the switch tube S _i is turned off , the value of Xi _i is 0; I _cf =(X ₃ -X ₁ )I _load , within one sampling period, the variation of the floating capacitor voltage is $\mathrm{\Δ}{V}_{\mathrm{cf}}={\∫}_0^T\frac{1}{\mathrm{cf}}(x_3-x_1)I_{\mathrm{load}}\mathrm{dt}.$ 3. According to the five-level ANPC converter suspension capacitor voltage control method according to claim 1, it is characterized in that: in the step S101, the five-level SHEPWM pulse wave acts on the five-level ANPC converter to output 5 levels respectively for 2E, E, 0, -E and -2E. 4. According to the five-level ANPC converter floating capacitor voltage control method according to claim 3, it is characterized in that: in step S102, when detecting that the intermediate level E or -E comes each time, the flag position is 1, when When the real-time detected floating capacitor voltage value is greater than the reference voltage value E, the floating capacitor is selected to be discharged; otherwise, when the real-time detected floating capacitor voltage value is lower than the reference voltage value E, the floating capacitor is selected to be charged. 5. according to the five-level ANPC converter suspension capacitance voltage control method described in claim 4, it is characterized in that: in step S103, load current is positive direction with the direction that flows to load from the single-phase bridge arm of five-level ANPC converter , the load current is positive, otherwise it is negative. 6. according to the five-level ANPC converter suspension capacitance voltage control method described in claim 5, it is characterized in that: according to needing in step S102 to the positive and negative selection suitable redundant switch state of suspension capacitance charging and discharging state and load current, The flag is set to 0 after selecting the appropriate redundant switch state.