CN102684533B - Neutral point voltage control method for neutral point clamped (NPC) type three-level inverter based on carrier amplitude shift - Google Patents

Abstract

The NPC type three-level inverter mid-point voltage control method based on carrier wave amplitude shift that the invention discloses a kind of, specifically: enable the amplitude of the upper and lower carrier wave of NPC type three-level inverter be expressed as 1+K × m and 1-K × m, K is carrier amplitude adjustment factor, m is modulation ratio, by adjusting carrier amplitude adjustment factor in real time to adjust carrier amplitude, the carrier wave obtained after amplitude is adjusted is compared output pwm signal with modulating wave. The adjusting method of K are as follows: When, if Unp is greater than Udc/2, -1≤K ＜ 0, if Unp is equal to Udc/2, K=0, if Unp is less than Udc/2,0 K≤1 ＜; When, if Unp is greater than Udc/2,0 K≤1 ＜, if Unp is equal to Udc/2, K=0, if Unp is less than Udc/2, -1≤K ＜ 0; Wherein, For power-factor angle, Unp is inverter mid-point voltage, and Udc is DC bus-bar voltage. The present invention is constant by keeping modulating wave and upper and lower carrier frequency, changes the amplitude of carrier wave up and down, and the balance of Lai Shixian midpoint potential, this method is simple with calculating, and realizes easy advantage.

Description

Carrier amplitude shift-based neutral point voltage control method for NPC type three-level inverter

Technical Field

The invention belongs to the technical field of power electronics, and particularly relates to a carrier amplitude shift-based neutral point voltage control method for an NPC (neutral point clamped) type three-level inverter.

Background

Compared with the traditional two-level inverter, the diode clamped (NPC) three-level inverter has the advantages of low requirement on the withstand voltage grade of a switching device, high equivalent switching frequency and small harmonic wave of an output waveform, and therefore, the three-level inverter is widely applied to a medium-high voltage high-power conversion circuit. But the midpoint potential balancing problem is an inherent outstanding problem. The imbalance of the midpoint potential will increase the harmonics of the output voltage and also damage the switching devices and filter capacitors.

The inconsistency of the dc bus capacitance parameters, load imbalance and different modulation strategies may cause the midpoint voltage imbalance. There are two main control strategies for midpoint voltage balancing: one is Space Vector Pulse Width Modulation (SVPWM) method based on space vector, and the other is Sinusoidal Pulse Width Modulation (SPWM) method injecting zero sequence component. In the SVPWM modulation method, the action time of redistributing the redundant small vectors is the most commonly used midpoint voltage control method, but the method has limited balancing capability, the relation between the redundant vectors and the midpoint voltage is also very complex, and meanwhile, the SVPWM method has complex algorithm and large operation amount and is not beneficial to being expanded to a higher-level inverter. In the SPWM modulation method, the algorithm of midpoint voltage balance is mostly based on a modulation wave, and the balance control of the midpoint voltage is mainly realized by injecting a specific zero sequence component into the modulation wave. However, the selection of the zero sequence component needs a high skill, the calculation of the zero sequence component often needs parameters such as three-phase reference voltage, output current, direct current bus capacitance voltage and capacitance value, and a large number of parameters make the calculation of the zero sequence component complicated, which is not beneficial to the realization of a real-time control system, and increases the cost.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a carrier amplitude shift-based midpoint voltage control method for a three-level inverter, which realizes the balance of midpoint potential by keeping the modulation wave and the upper and lower carrier frequencies unchanged and changing the amplitudes of the upper and lower carriers.

A carrier amplitude shift-based neutral point voltage control method for an NPC type three-level inverter specifically comprises the following steps: adjusting the carrier amplitude adjusting coefficient in real time so as to adjust the carrier amplitude, comparing the carrier obtained after amplitude adjustment with a modulating wave to output a PWM signal so as to control the working state of a switching tube of the inverter;

the carrier amplitude is adjusted as follows:

the amplitudes of an upper carrier and a lower carrier of the NPC type three-level inverter are respectively expressed as 1+ Kxm and 1-Kxm, K is a carrier amplitude adjusting coefficient, and m is a modulation ratio;

in that

When it is, if U_npGreater than U_dc/2, then-1 is not less than K < 0 and U_npAnd U_dcThe greater the difference between/2, the greater the absolute value of K, if U_npIs equal to U_dcAnd/2, then K is equal to 0, if U_npLess than U_dc/2, then 0 is more than K and less than or equal to 1 and U_npAnd U_dcThe larger the difference between/2, the larger K;

in thatWhen it is, if U_npGreater than U_dc/2, then 0 is more than K and less than or equal to 1 and U_npAnd U_dcThe greater the difference between/2, the greater K, if U_npIs equal to U_dcAnd/2, then K is equal to 0, if U_npLess than U_dc/2, then-1 is not less than K < 0 and U_npAnd U_dcThe larger the difference between/2, the larger the absolute value of K;

wherein,

is a power factor angle, U_npIs the inverter midpoint voltage, U_dcIs the dc bus voltage.

A carrier amplitude shift-based neutral point voltage control method for an NPC type three-level inverter specifically comprises the following steps: adjusting the modulation wave adjusting coefficient in real time so as to adjust the modulation wave, comparing the modulation wave obtained after adjustment with a carrier wave to output a PWM signal so as to control the working state of a switching tube of the inverter;

the tone isThe damping is adjusted as follows: order modulated wave <math> <mrow> <msubsup> <mi>u</mi> <mi>j</mi> <mo>&prime;</mo> </msubsup> <mo>=</mo> <mfenced open='{' close=''> <mtable> <mtr> <mtd> <mfrac> <mrow> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>+</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>+</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&GreaterEqual;</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mfrac> <mrow> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>+</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&lt;</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> </mtable> </mfenced> <mo>,</mo> </mrow> </math> Wherein u is_jIs an initial modulation wave, K is a carrier amplitude adjusting coefficient, and m is a modulation ratio;

in that

When it is, if U_npGreater than U_dc/2, then-1 is not less than K < 0 and U_npAnd U_dcThe greater the difference between/2, the greater the absolute value of K, if U_npIs equal to U_dcAnd/2, then K is equal to 0, if U_npLess than U_dc/2, then 0 is more than K and less than or equal to 1 and U_npAnd U_dcThe larger the difference between/2, the larger K;

in that

When it is, if U_npGreater than U_dc/2, then 0 is more than K and less than or equal to 1 and U_npAnd U_dcThe greater the difference between/2, the greater K, if U_npIs equal to U_dcAnd/2, then K is equal to 0, if U_npLess than U_dc/2, then-1 is not less than K < 0 and U_npAnd U_dcThe larger the difference between/2, the larger the absolute value of K;

wherein,

is a power factor angle, U_npIs the inverter midpoint voltage, U_dcIs the dc bus voltage. The technical effects of the invention are as follows:

the invention adopts the technical scheme that a carrier amplitude shift-based midpoint voltage control method of a three-level inverter is adopted, and midpoint voltage U is measured in the method_npAnd comparing the amplitude of the carrier wave with a given value, determining the amplitude shift value of the carrier wave according to the comparison result, comparing the carrier wave obtained after amplitude adjustment with a modulation wave to output an SPWM signal so as to control the working state of a switching tube of the inverter, thereby realizing the balance control of the midpoint voltage. The invention realizes the balance of the midpoint potential by keeping the modulated wave and the upper and lower carrier frequencies unchanged and changing the amplitudes of the upper and lower carriers, the method has simple calculation,advantage of easy implementation

Drawings

Fig. 1 shows a topology of an NPC type three-level inverter to be modulated according to the present invention.

Fig. 2 is a schematic diagram of a three-level SPWM modulation scheme, where 2(a) is a carrier and modulation wave schematic diagram, 2(b) -2 (e) are switching tube output states, and 2(f) is an output phase voltage waveform.

Fig. 3 is a block diagram of a control structure for changing the carrier amplitude according to the method of the present invention.

FIG. 4 is a schematic diagram of the method of the present invention for changing the amplitude of the upper and lower carriers.

FIG. 5 shows the modulation ratio m, the midpoint voltage fluctuation dU in the method of the present invention_cAnd a relation graph of the adjusting coefficient K.

FIG. 6 is a waveform diagram of PWM output with carrier amplitude changed by the method of the present invention. Wherein, (a) is a schematic diagram of a carrier wave and a modulated wave after the amplitude of the carrier wave is changed, (b) to (e) are output states of a switching tube, and (f) is an output phase voltage waveform.

Fig. 7 is a block diagram of the control structure for changing the modulation wave in the method of the present invention.

FIG. 8 is a diagram of an equivalent modified modulated wave according to the present invention.

Detailed Description

The following describes a specific embodiment of the present invention with reference to the drawings.

Fig. 1 is a main circuit configuration diagram of a three-phase NPC type three-level inverter. The input DC voltage is U_dcThe intermediate DC support capacitors are respectively C1 and C2, and the voltage at two ends of the capacitor is U_c1，U_c2. Midpoint voltage of U_npIts value is equal to U_c2Same, under normal conditions, the midpoint voltage does not occurGenerating an offset, U_np＝U_dc/2. However, due to the inconsistency of the parameters of the support capacitor, the imbalance of the load, the modulation strategy and the like, the charge and discharge of the capacitor are unbalanced, the fluctuation of the midpoint voltage is caused, and the fluctuation value dU of the midpoint voltage is caused_cIs defined as dU_c＝U_c1-U_c2. Fig. 2 shows a conventional three-level SPWM modulation method. Wherein (a) is a schematic diagram of an up-down carrier wave and a modulation wave, (b) to (e) are output states of the switching tube, and (f) is an output phase voltage waveform. However, the conventional SPWM modulation method cannot eliminate or suppress the midpoint voltage offset when the midpoint voltage is offset.

Fig. 3 shows a control schematic diagram of the present invention, and the flow of the control method is specifically as follows:

step 1, measuring midpoint voltage U in real time_npAnd compared to a given value;

and 2, determining the amplitude shift value of the carrier wave according to the comparison result in the step 1.

The amplitudes of the upper carrier and the lower carrier are respectively expressed as 1+ Kxm and 1-Kxm, wherein the Kxm is the variation of the carrier amplitude, and the K is the carrier amplitude adjusting coefficient, and the K is more than or equal to-1 and less than or equal to 1.

The expression for the neutral point voltage fluctuation of the NPC type three-level inverter is:

${\mathrm{dU}}_c=U_{c1}-{\mathrm{<figure-callout\; id="2"\; label="U\; c"\; filenames="HDA00001614868800022.png"\; state="\{\{state\}\}">U</figure-callout>}}_c=\frac{i_o}{C}=\frac{i_a{d}_{\mathrm{ao}}+i_b{d}_{\mathrm{bo}}+i_c{d}_{\mathrm{co}}}{C}---\left(1\right)$

wherein dU_cFor mid-point voltage fluctuations, U_c1And U_c2The voltage values of two ends of the DC bus capacitor C1 and C2 are respectively, C is the DC bus capacitance value i_oIs a midpoint current, i_a，i_b，i_cRespectively, three phase current value, d_ao，d_bo，d_coThe duty cycles of the three-phase "O" state action, respectively.

The duty cycle of the three-phase "O" state action as the carrier amplitude shifts can be expressed as:

<math> <mrow> <msub> <mi>d</mi> <mi>jo</mi> </msub> <mo>=</mo> <mfenced open='{' close='' separators=''> <mtable> <mtr> <mtd> <mfrac> <mrow> <mn>1</mn> <mo>-</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>+</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&GreaterEqual;</mo> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mfrac> <mrow> <mn>1</mn> <mo>+</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&lt;</mo> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> </mtable> <mrow> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>2</mn> <mo>)</mo> </mrow> </mrow> </mfenced> </mrow> </math>

wherein d is_jo(j ═ a, b, c) is the duty cycle of the three-phase "O" state action, u_j(j ═ a, b, c) is a three-phase modulated wave, m is a modulation ratio, and m is 0. ltoreq. m.ltoreq.1.

From (1) and (2), the midpoint voltage fluctuation value dU in one modulation wave period_cThe relation with the carrier amplitude adjustment coefficient K is as follows:

wherein I_mFor the magnitude of the phase current,

is the power factor.

According to the formula, when the phase current amplitude and the power factor are fixed, the unique relation between the carrier amplitude adjusting coefficient and the midpoint voltage fluctuation value can be obtained. In that

If K is greater than zero, then dU_cLess than zero, the midpoint voltage rises; if K is less than zero, dU_cAbove zero, the midpoint voltage will drop. In that

If K is greater than zero, then dU_cAbove zero, the midpoint voltage drops; if K is less than zero, dU_cLess than zero, the midpoint voltage will rise.

By changing the direction and the size of the carrier amplitude adjustment coefficient K, the midpoint voltage can be adjusted, and the balance of the midpoint voltage is realized. In that

When it is, if U_npGreater than U_dcIf/2, K is less than zero, if U_npLess than U_dcAnd/2, K is greater than zero. In that

When it is, if U_npGreater than U_dcIf/2, K is greater than zero, if U_npLess than U_dcAnd/2, K is less than zero. U shape_npIs the inverter midpoint voltage. U shape_npAnd U_dcThe larger the difference between/2, the larger the absolute value of K.

And 3, comparing the carrier wave with the modulated wave to output a switching signal, if the amplitude of the modulated wave is greater than that of the carrier wave above, switching on the corresponding switching tube 1 and switching off the switching tube 3, otherwise, switching off the switching tube 1 and switching on the switching tube 3. If the amplitude of the modulated wave is larger than that of the carrier wave below, the corresponding switch tube 2 is switched on, the corresponding switch tube 4 is switched off, otherwise, the switch tube 2 is switched off, and the corresponding switch tube 4 is switched on, wherein the switch tubes 1-4 are four switch tubes from top to bottom in the same bridge arm.

The neutral point voltage of the NPC type three-level inverter can be controlled according to the steps. The invention changes the amplitude of the upper carrier wave and the lower carrier wave by keeping the modulated wave and the upper carrier wave and the lower carrier wave unchanged, and well solves the problem of midpoint potential balance.

Example 1

The carrier amplitude shift-based midpoint voltage control method of the three-level inverter can effectively realize midpoint voltage control. FIG. 3 shows an algorithm implementation block diagram of the method of the present invention, u'_tri1，u′_tri2Respectively, the carrier values after the amplitude changes. Fcn1 is the adjustment function for the carrier amplitude given by:

<math> <mrow> <msubsup> <mi>u</mi> <mrow> <mi>tri</mi> <mn>1</mn> </mrow> <mo>&prime;</mo> </msubsup> <mo>=</mo> <mfenced open='{' close='' separators=''> <mtable> <mtr> <mtd> <mfrac> <mn>2</mn> <mi>T</mi> </mfrac> <mo>&times;</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> <mo>&times;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mrow> <mo>(</mo> <mn>0</mn> <mo>&le;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mo>&lt;</mo> <mi>T</mi> <mo>/</mo> <mn>2</mn> <mo>)</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mn>2</mn> <mo>+</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>-</mo> <mfrac> <mn>2</mn> <mi>T</mi> </mfrac> <mo>&times;</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>+</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> <mo>&times;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mi>T</mi> <mo>/</mo> <mn>2</mn> <mo>&le;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mo>&lt;</mo> <mi>T</mi> <mo>)</mo> </mrow> </mtd> </mtr> </mtable> <mrow> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>4</mn> <mo>)</mo> </mrow> </mrow> </mfenced> </mrow> </math>

<math> <mrow> <msubsup> <mi>u</mi> <mrow> <mi>tri</mi> <mn>2</mn> </mrow> <mo>&prime;</mo> </msubsup> <mo>=</mo> <mfenced open='{' close='' separators=''> <mtable> <mtr> <mtd> <mfrac> <mn>2</mn> <mi>T</mi> </mfrac> <mo>&times;</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> <mo>&times;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mo>-</mo> <mn>1</mn> <mrow> <mo>(</mo> <mn>0</mn> <mo>&le;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mo>&lt;</mo> <mi>T</mi> <mo>/</mo> <mn>2</mn> <mo>)</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mn>1</mn> <mo>-</mo> <mn>2</mn> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>-</mo> <mfrac> <mn>2</mn> <mi>T</mi> </mfrac> <mo>&times;</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> <mo>&times;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <mi>T</mi> <mo>/</mo> <mn>2</mn> <mo>&le;</mo> <mi>rem</mi> <mrow> <mo>(</mo> <mi>t</mi> <mo>/</mo> <mi>T</mi> <mo>)</mo> </mrow> <mo>&lt;</mo> <mi>T</mi> <mo>)</mo> </mrow> </mtd> </mtr> </mtable> <mrow> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>5</mn> <mo>)</mo> </mrow> </mrow> </mfenced> </mrow> </math>

where T is the carrier period and rem () is the remainder function.

The method requires sampling the midpoint voltage U_npAnd will U_npAnd U_dcAnd/2, comparing, and determining the change amount of the carrier wave amplitude according to the comparison result. FIG. 4 is a schematic diagram of the variation of the carrier amplitude when K > 0 in the method of the present invention. In the method, the modulation wave does not change, the carrier period does not change, the amplitude of the carrier wave is changed, the amplitudes of the upper carrier wave and the lower carrier wave are respectively changed into 1+ Kxm and 1-Kxm, wherein the Kxm is the variable quantity of the amplitude of the carrier wave.

Mid-point voltage fluctuation value dU in one modulation wave period_cThe relationship with the carrier amplitude adjustment coefficient K is given by equation (3). FIG. 5 shows the drawing taken from I_mThe ratio of/C is a unit amount,

dU of time_cK, m. Modulation ratio m is constant, midpoint fluctuation value dU_cProportional to the adjustment coefficient K. When K is greater than zero, dU_cLess than zero, the midpoint voltage rises; if K is less than zero, dU_cAbove zero, the midpoint voltage drops. By applying a midpoint voltage U_npAnd U_dcAnd/2, comparing to obtain a carrier amplitude adjusting coefficient K, and obtaining the adjusted carrier by the formula (4) and the formula (5). Comparing the adjusted carrier wave with the modulated wave to output a switching signal, FIG. 6 shows the method of the present invention changing the amplitude of the carrier waveAnd outputting a waveform diagram by PWM. Wherein, (a) is a schematic diagram of a carrier wave and a modulated wave after the amplitude of the carrier wave is changed, (b) to (e) are output states of a switching tube, and (f) is an output phase voltage waveform. For the A-phase bridge arm, if the amplitude of the modulation wave is greater than that of the upper carrier wave, the corresponding switch tube S_a1Switching on and off tube S_a3Turn-off, otherwise, switch tube S_a1Turn-off, switch tube S_a3And (4) opening. If the amplitude of the modulation wave is larger than that of the lower carrier wave, the corresponding switch tube S_a2Switching on and off tube S_a4Turn-off, otherwise, switch tube S_a2Turn-off, switch tube S_a4And (4) opening.

The adjustment of the amplitude of the upper carrier and the lower carrier can be conveniently realized through a control chip (a singlechip, an ARM, a DSP and the like) and a programmable logic device (for example, an FPGA).

Example 2

For a system which is inconvenient to realize the change of the carrier amplitude, the change of the carrier amplitude can be equivalent to the modulation wave, and the premise is that the switching sequence and the switching action time are kept unchanged before and after the equivalence. Fig. 7 is a block diagram showing an algorithm implementation of the equivalent modulation wave in the method of the present invention, and compared with the above method, the modulation wave is adjusted only by keeping the carrier wave unchanged. Fcn2 is the adjustment function of the modulated wave, given by:

<math> <mrow> <msubsup> <mi>u</mi> <mi>j</mi> <mo>&prime;</mo> </msubsup> <mo>=</mo> <mfenced open='{' close='' separators=''> <mtable> <mtr> <mtd> <mfrac> <mrow> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>+</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>+</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&GreaterEqual;</mo> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mfrac> <mrow> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>+</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&lt;</mo> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> </mtable> <mrow> <mo>-</mo> <mo>-</mo> <mo>-</mo> <mrow> <mo>(</mo> <mn>6</mn> <mo>)</mo> </mrow> </mrow> </mfenced> </mrow> </math>

wherein is u'_jAnd (4) the equivalent modulated wave. FIG. 8 is a schematic diagram of the variation of the equivalent modulation wave when K > 0 in the method of the present invention. Sampling midpoint voltage U_npAnd will U_npAnd U_dcComparing with 2 to obtain carrier amplitude adjusting coefficient K, keeping the upper and lower carrier frequencies equal and amplitude, adjusting the modulated wave according to formula (6), comparing the adjusted modulated wave with the carrier wave to output switching signal, or obtaining the midpoint voltage same as the adjusted carrier amplitudeAnd controlling the effect of pressure.

The present invention is not limited to the above embodiments, but rather, the present invention may be embodied in many other forms without departing from the spirit or essential characteristics thereof.

Claims (2)

1. A carrier amplitude shift-based neutral point voltage control method for an NPC type three-level inverter specifically comprises the following steps: adjusting the carrier amplitude adjusting coefficient in real time so as to adjust the carrier amplitude, comparing the carrier obtained after amplitude adjustment with a modulating wave to output a PWM signal so as to control the working state of a switching tube of the inverter;

the carrier amplitude is adjusted as follows:

the amplitudes of an upper carrier and a lower carrier of the NPC type three-level inverter are respectively expressed as 1+ Kxm and 1-Kxm, K is a carrier amplitude adjusting coefficient, and m is a modulation ratio;

in that

When it is, if U_npGreater than U_dc/2, then-1 is not more than K<0 and U_npAnd U_dcThe greater the difference between/2, the greater the absolute value of K, if U_npIs equal to U_dcK =0 if/2, if U_npLess than U_dc2, then 0<K is less than or equal to 1 and U_npAnd U_dcThe larger the difference between/2, the larger K;

in that

When it is, if U_npGreater than U_dc2, then 0<K is less than or equal to 1 and U_npAnd U_dcThe greater the difference between/2, the greater K, if U_npIs equal to U_dcK =0 if/2, if U_npLess than U_dc/2, then-1 is not more than K<0 and U_npAnd U_dcThe larger the difference between/2, the larger the absolute value of K;

wherein,

is a power factor angle, U_npIs the inverter midpoint voltage, U_dcIs the dc bus voltage.

2. A carrier amplitude shift-based neutral point voltage control method for an NPC type three-level inverter specifically comprises the following steps: adjusting the carrier amplitude adjustment coefficient in real time so as to adjust the modulation wave, comparing the modulation wave obtained after adjustment with the carrier wave to output a PWM signal so as to control the working state of a switching tube of the inverter;

the modulated wave is modulated as follows: order modulated wave <math> <mrow> <msubsup> <mi>u</mi> <mi>j</mi> <mo>&prime;</mo> </msubsup> <mo>=</mo> <mfenced open='{' close=''> <mtable> <mtr> <mtd> <mfrac> <mrow> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>+</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>+</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> </mtd> <mtd> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&GreaterEqual;</mo> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> <mtr> <mtd> <mfrac> <mrow> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>+</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> </mrow> <mrow> <mn>1</mn> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> </mrow> </mfrac> </mtd> <mtd> <mrow> <mo>(</mo> <msub> <mi>u</mi> <mi>j</mi> </msub> <mo>&lt;</mo> <mo>-</mo> <mi>K</mi> <mo>&times;</mo> <mi>m</mi> <mo>)</mo> </mrow> </mtd> </mtr> </mtable> </mfenced> <mo>,</mo> </mrow> </math> Wherein u is_jIs an initial modulation wave, K is a carrier amplitude adjusting coefficient, and m is a modulation ratio;

in that

When the temperature of the water is higher than the set temperature,if U is_npGreater than U_dc/2, then-1 is not more than K<0 and U_npAnd U_dcThe greater the difference between/2, the greater the absolute value of K, if U_npIs equal to U_dcK =0 if/2, if U_npLess than U_dc2, then 0<K is less than or equal to 1 and U_npAnd U_dcThe larger the difference between/2, the larger K;

in that

When it is, if U_npGreater than U_dc2, then 0<K is less than or equal to 1 and U_npAnd U_dcThe greater the difference between/2, the greater K, if U_npIs equal to U_dcK =0 if/2, if U_npLess than U_dc/2, then-1 is not more than K<0 and U_npAnd U_dcThe larger the difference between/2, the larger the absolute value of K;

wherein,

is a power factor angle, U_npIs the inverter midpoint voltage, U_dcIs the dc bus voltage.

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