CN112421979A - Neutral point balancing method of I-type three-level inverter based on zero-sequence component injection - Google Patents
Neutral point balancing method of I-type three-level inverter based on zero-sequence component injection Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/487—Neutral point clamped inverters
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Abstract
The invention belongs to the technical field of I-type three-level inverters, and discloses a neutral point balancing method of an I-type three-level inverter based on zero-sequence component injection, which comprises the following steps: 101. acquiring a zero-sequence component V0_ A taking midpoint balance as a target; 102. acquiring a zero-sequence component V0_ B of the switching frequency optimization SFOPWM; 103. when the unbalanced voltage (Vbus _ u-Vbus _ D) is smaller than the amplitude D, the zero-sequence component adopts V0_ B, that is, the modulated wave voltage Vr _ abc is Vr _ abc1+ V0_ B; when the unbalanced voltage (Vbus _ u-Vbus _ D) is larger than the amplitude D, the zero-sequence component adopts V0_ a, that is, the modulated wave voltage Vr _ abc is Vr _ abc1+ V0_ a; 104. and modulating and controlling the I-type three-level inverter by the sampling value of the modulated wave voltage. The invention realizes the flexible acquisition of the modulated wave voltage and improves the output performance of the inverter.
Description
Technical Field
The invention belongs to the technical field of I-type three-level inverters, and particularly relates to a neutral point balancing method of an I-type three-level inverter based on zero-sequence component injection.
Background
The neutral point clamped three-level inverter technology is widely applied to the fields of grid-connected power generation, alternating current motor driving and the like, the balance problem of the neutral point potential of a direct current bus is an inherent difficult problem of the clamped three-level inverter, and output voltage harmonic waves caused by unbalanced neutral point potential of the bus are increased, voltage stress of a switch tube is increased, and phenomena such as machine damage possibly caused by serious unbalance are required for a neutral point potential balance strategy on the direct current side of the three-level inverter.
In the prior art, the scheme of the neutral point balance of the I-type three-level inverter mainly includes:
1. a processing method based on an additional hardware balancing circuit; the method needs to add extra hardware circuits and devices and software control algorithms, increases the complexity of the system and introduces new hardware risk points.
2. A medium and small vector adjusting method based on space pulse width modulation (SVPWM); the algorithm complexity of the method is high, and the effect of the balance algorithm is greatly influenced by the power factor.
3. A zero-sequence component adjusting method based on zero-sequence injection; the method usually adopts a single midpoint balance objective function, so that zero sequence component jump is large, output burrs are more, and system performance is influenced.
Disclosure of Invention
The embodiment of the invention aims to provide a zero-sequence component injection-based neutral point balancing method for an I-type three-level inverter, which can flexibly obtain the voltage of a modulation wave and improve the output performance of the inverter.
The embodiment of the invention is realized as follows:
the neutral point balance method of the I-type three-level inverter based on zero sequence component injection, the converter topology is composed of I-type three levels; the sampling value of the voltage of the capacitor at the upper half part of the direct current bus is Vbus _ u, and the sampling value of the voltage of the capacitor at the lower half part of the direct current bus is Vbus _ d; the three-phase output currents of the inverter are Ia, Ib and Ic respectively; the method comprises the following steps:
101. acquiring a zero-sequence component V0_ A taking midpoint balance as a target;
let the three-phase switching functions of the converter be Sa, Sb, Sc, take a value of 1 when the output is connected to the positive input terminal, take a value of-1 when the output is connected to the negative input terminal, take a value of 0 in other cases, and only Sx (x ═ a, b, c) takes a value of 0, the bridge arm output is clamped at the midpoint, the output current will flow into/out of the neutral line through the clamping diode, so the neutral line current instantaneous value can be expressed as:
Inp＝[1-abs(Sa)]×Ia+[1-abs(Sb)]×Ib+[1-abs(Sc)]×Ic＝-abs(Sa)×Ia-abs(Sb)×Ib-abs(Sc)×Ic
abs () is an absolute value function;
for the normalized modulated wave voltage, the expression after injecting the zero sequence is
Vra(t)＝Vra1(t)+V0(t)；
Vrb(t)＝Vrb1(t)+V0(t)；
Vrc(t)＝Vrc1(t)+V0(t)；
In the above formula, Vrx1(t) is an original modulation wave, and Vrx (t) is a final modulation wave after injecting a zero sequence component;
the switching function Sx is equivalent to the modulation voltage in a switching period, so the average neutral current in the switching period Ts is:
Inp＝-abs(Vra)×Ia-abs(Vrb)×Ib-abs(Vrc)×Ic
defining a sign function sgn (Vrx), when Vrx > -0, sgn (Vrx) -1; sgn (Vrx) is-1 when Vrx < 0;
substituting the formula to obtain:
Inp＝-[sgn(Vra)×Vra1×Ia+sgn(Vrb)×Vrb1×Ib+sgn(Vrc)×Vrc1×Ic]-V0×[sgn(Vra)×Ia+sgn(Vrb)×Ib+sgn(Vrc)×Ic]
the neutral line current Inp will flow through the dc bus capacitor, which is the root cause of midpoint voltage fluctuation in a steady state, so the nature of midpoint voltage balance control is to control Inp to be 0, and the zero-sequence component of the zero-sequence injection algorithm with midpoint potential balance as a control target can be:
V0＝-[Inp_con+sgn(Vra)×Vra1×Ia+sgn(Vrb)×Vrb1×Ib+sgn(Vrc)×Vrc1×Ic]/[[sgn(Vra)×Ia+sgn(Vrb)×Ib+sgn(Vrc)×Ic]
in the above formula, Inp _ con is the control compensation amount of the neutral current, and may take the value of-K x (Vbus _ u-Vbus _ d), where K is a constant proportional to the bus capacitance and the switching period;
the zero-sequence component of the zero-sequence injection algorithm with the midpoint potential balance as the control target in the above formula is V0_ A;
102. acquiring a zero-sequence component V0_ B of the switching frequency optimization SFOPWM;
the zero-sequence component of the SFOPWM-based zero-sequence component injection modulation algorithm is optimized based on the switching frequency as follows:
V0＝-[max(Va,Vb,Vc)+min(Va,Vb,Vc)]/2
the modulated wave voltage expression is:
Vra(t)＝Vra1(t)+V0(t)；
Vrb(t)＝Vrb1(t)+V0(t)；
Vrc(t)＝Vrc1(t)+V0(t)；
the zero sequence component is referred to as V0_ B for the purpose of optimizing the switching frequency;
103. when the unbalanced voltage (Vbus _ u-Vbus _ D) is smaller than the amplitude D, the zero-sequence component adopts V0_ B, that is, the modulated wave voltage Vr _ abc is Vr _ abc1+ V0_ B; when the unbalanced voltage (Vbus _ u-Vbus _ D) is larger than the amplitude D, the zero-sequence component adopts V0_ a, that is, the modulated wave voltage Vr _ abc is Vr _ abc1+ V0_ a; wherein the amplitude D is greater than zero;
104. and modulating and controlling the I-type three-level inverter by the sampling value of the modulated wave voltage.
The value of the amplitude D can be 2% -10% of the rated voltage of the direct-current bus.
Compared with a method for introducing an additional hardware balancing circuit, the balancing method reduces the complexity and risk of hardware, reduces the complexity of the algorithm and enlarges the application range compared with a balancing algorithm based on space vector modulation; compared with the zero-sequence component injection algorithm based on a single target, the balance method gives consideration to voltage unbalance, output voltage/current harmonic waves and system efficiency.
Drawings
FIG. 1 is a flow chart of a method for balancing the midpoint of an I-type three-level inverter based on zero-sequence component injection according to the present invention;
fig. 2 is a reference topology of a type I three-level converter of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
On the basis of a three-level zero-sequence component injection modulation algorithm, the scheme of mixing two zero-sequence injection algorithms is adopted, so that the system can simultaneously consider the aims of bus midpoint balance, small output voltage harmonic, less switching times and the like, and the performance of the I-type three-level inverter is greatly improved.
The following detailed description of specific implementations of the present invention is provided in conjunction with specific embodiments:
referring to fig. 1 and 2, the invention achieves the objectives of direct current bus midpoint balance, small output voltage harmonic, and switching frequency optimization by using two zero-sequence component injection methods in a mixed manner based on a midpoint clamping I-type three-level inverter hardware topology:
1. the converter topology consists of three levels of type I;
2. the sampling value of the voltage of the capacitor at the upper half part of the direct current bus is Vbus _ u, and the sampling value of the voltage of the capacitor at the lower half part of the direct current bus is Vbus _ d;
3. three-phase output currents of the inverter are Ia, Ib and Ic respectively;
4. let the three-phase switching functions of the converter be Sa, Sb, Sc, take a value of 1 when the output is connected to the positive input terminal, take a value of-1 when the output is connected to the negative input terminal, take a value of 0 in other cases, and only Sx (x ═ a, b, c) takes a value of 0, the bridge arm output is clamped at the midpoint, the output current will flow into/out of the neutral line through the clamping diode, so the neutral line current instantaneous value can be expressed as:
Inp＝[1-abs(Sa)]×Ia+[1-abs(Sb)]×Ib+[1-abs(Sc)]×Ic＝-abs(Sa)×Ia-abs(Sb)×Ib-abs(Sc)×Ic
abs () is an absolute value function;
for the normalized modulation voltage, the expression after injecting the zero sequence is
Vra(t)＝Vra1(t)+V0(t)；
Vrb(t)＝Vrb1(t)+V0(t)；
Vrc(t)＝Vrc1(t)+V0(t)；
In the above formula, Vrx1(t) is an original modulation wave, and Vrx (t) is a final modulation wave after injecting a zero sequence component;
the switching function Sx is equivalent to the modulation voltage in a switching period, so the average neutral current in the switching period Ts is:
Inp＝-abs(Vra)×Ia-abs(Vrb)×Ib-abs(Vrc)×Ic
defining a sign function sgn (Vrx), when Vrx > -0, sgn (Vrx) -1; sgn (Vrx) is-1 when Vrx < 0;
substituting the formula to obtain:
Inp＝-[sgn(Vra)×Vra1×Ia+sgn(Vrb)×Vrb1×Ib+sgn(Vrc)×Vrc1×Ic]-V0×[sgn(Vra)×Ia+sgn(Vrb)×Ib+sgn(Vrc)×Ic]
the neutral line current Inp will flow through the dc bus capacitor, which is the root cause of midpoint voltage fluctuation in a steady state, so the nature of midpoint voltage balance control is to control Inp to be 0, and the zero-sequence component of the zero-sequence injection algorithm with midpoint potential balance as a control target can be:
V0＝-[Inp_con+sgn(Vra)×Vra1×Ia+sgn(Vrb)×Vrb1×Ib+sgn(Vrc)×Vrc1×Ic]/[[sgn(Vra)×Ia+sgn(Vrb)×Ib+sgn(Vrc)×Ic]
in the above formula, Inp _ con is the control compensation amount of the neutral current, and may take the value of-K x (Vbus _ u-Vbus _ d), where K is a constant proportional to the bus capacitance and the switching period;
the zero-sequence component of the zero-sequence injection algorithm with the midpoint potential balance as the control target in the above formula is V0_ A;
5. the zero-sequence component of the SFOPWM-based zero-sequence component injection modulation algorithm is optimized based on the switching frequency as follows:
v0 ═ max (Va, Vb, Vc) + min (Va, Vb, Vc) ]/2, this zero sequence component was chosen because it greatly reduced the number of switching while increasing the modulation ratio;
the modulated wave expression is:
Vra(t)＝Vra1(t)+V0(t)；
Vrb(t)＝Vrb1(t)+V0(t)；
Vrc(t)＝Vrc1(t)+V0(t)；
the zero sequence component aims at the optimal switching frequency, has good effects of reducing output voltage harmonic and improving system efficiency, and is called as V0_ B;
6. v0_ a aims at midpoint balance, but the calculation result is greatly affected by the output voltage and current Vabc, Iabc, especially the zero sequence component is greatly jumped due to repeated symbol jump at the zero crossing point of the voltage and current, and further the waveform of the output voltage/current is affected; v0_ B aims at optimizing the switching times and the output waveform, but it does not have the suppression capability of midpoint voltage balance; in view of the above, the present invention combines two zero-sequence components, wherein when the unbalanced voltage (Vbus _ u-Vbus _ d) is smaller than a certain magnitude, the zero-sequence component is V0_ B, and when the unbalanced voltage is larger than the certain magnitude, the zero-sequence component is V0_ a; simulation results and product test results show that V0_ B does not have midpoint balancing capability, but does not cause imbalance under steady-state conditions, but requires V0_ A to suppress imbalance at dynamic jump because: ideally, the average midpoint current Inp obtained by calculation is 0 due to the symmetry of the three-phase current and the modulated wave, so that the midpoint voltage balance is not affected. However, when the voltage jumps dynamically, due to factors such as control accuracy, delay and disturbance, the three-phase current and the modulation wave are easy to be asymmetric, so that Inp is not 0, and the midpoint voltage is affected.
As shown in fig. 1, the balancing method in the present invention first calculates a zero sequence component V0_ a targeting midpoint balancing, calculates a zero sequence component V0_ B of the SFOPWM with optimized switching frequency, and when the unbalanced voltage (Vbus _ u-Vbus _ D) is smaller than a certain magnitude D, the zero sequence component adopts V0_ B, that is, Vr _ abc is Vr _ abc1+ V0_ B, and when the unbalanced voltage is larger than the certain magnitude D, the zero sequence component adopts V0_ a, that is, Vr _ abc is Vr _ abc1+ V0_ a, and is executed by different algorithms, and the value of the voltage unbalance D in the flowchart can be determined according to actual requirements.
In addition, the value of D can be 2-10% of the rated voltage of the direct current bus.
Fig. 2 is a reference topology of a type I three-level converter of the present invention. The circuit topology diagram is a circuit topology commonly used in the prior art, and in order to make the balancing method of the present invention easier to understand, various modifications may be made in the prior art, such as replacing different electronic components to achieve the same function, but any three-level converter is suitable for the adjustment of the balancing method of the present invention.
On the basis of a three-level zero-sequence component injection modulation algorithm, the scheme of mixing two zero-sequence injection algorithms is adopted, so that the system can simultaneously consider the aims of bus midpoint balance, small output voltage harmonic, less switching times and the like, and the performance of the I-type three-level inverter is greatly improved. Compared with a method for introducing an additional hardware balancing circuit, the balancing method reduces the complexity and risk of hardware, reduces the complexity of the algorithm and enlarges the application range compared with a balancing algorithm based on space vector modulation; compared with the zero-sequence component injection algorithm based on a single target, the balance method gives consideration to voltage unbalance, output voltage/current harmonic waves and system efficiency.
The above description is only exemplary of the present invention and should not be taken as limiting the invention, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (2)
1. A neutral point balancing method of an I-type three-level inverter based on zero-sequence component injection is characterized by comprising the following steps: the converter topology consists of three levels of type I; the sampling value of the voltage of the capacitor at the upper half part of the direct current bus is Vbus _ u, and the sampling value of the voltage of the capacitor at the lower half part of the direct current bus is Vbus _ d; the three-phase output currents of the inverter are Ia, Ib and Ic respectively; the method comprises the following steps:
101. acquiring a zero-sequence component V0_ A taking midpoint balance as a target;
let the three-phase switching functions of the converter be Sa, Sb, Sc, take a value of 1 when the output is connected to the positive input terminal, take a value of-1 when the output is connected to the negative input terminal, take a value of 0 in other cases, and only Sx (x ═ a, b, c) takes a value of 0, the bridge arm output is clamped at the midpoint, the output current will flow into/out of the neutral line through the clamping diode, so the neutral line current instantaneous value can be expressed as:
Inp＝[1-abs(Sa)]×Ia+[1-abs(Sb)]×Ib+[1-abs(Sc)]×Ic＝-abs(Sa)×Ia-abs(Sb)×Ib-abs(Sc)×Ic
abs () is an absolute value function;
for the normalized modulated wave voltage, the expression after injecting the zero sequence is
Vra(t)＝Vra1(t)+V0(t)；
Vrb(t)＝Vrb1(t)+V0(t)；
Vrc(t)＝Vrc1(t)+V0(t)；
In the above formula, Vrx1(t) is an original modulation wave, and Vrx (t) is a final modulation wave after injecting a zero sequence component;
the switching function Sx is equivalent to the modulation voltage in a switching period, so the average neutral current in the switching period Ts is:
Inp＝-abs(Vra)×Ia-abs(Vrb)×Ib-abs(Vrc)×Ic
defining a sign function sgn (Vrx), when Vrx > -0, sgn (Vrx) -1; sgn (Vrx) is-1 when Vrx < 0;
substituting the formula to obtain:
Inp＝-[sgn(Vra)×Vra1×Ia+sgn(Vrb)×Vrb1×Ib+sgn(Vrc)×Vrc1×Ic]-V0×[sgn(Vra)×Ia+sgn(Vrb)×Ib+sgn(Vrc)×Ic]
the neutral line current Inp will flow through the dc bus capacitor, which is the root cause of midpoint voltage fluctuation in a steady state, so the nature of midpoint voltage balance control is to control Inp to be 0, and the zero-sequence component of the zero-sequence injection algorithm with midpoint potential balance as a control target can be:
V0＝-[Inp_con+sgn(Vra)×Vra1×Ia+sgn(Vrb)×Vrb1×Ib+sgn(Vrc)×Vrc1×Ic]/[[sgn(Vra)×Ia+sgn(Vrb)×Ib+sgn(Vrc)×Ic]
in the above formula, Inp _ con is the control compensation amount of the neutral current, and may take the value of-K x (Vbus _ u-Vbus _ d), where K is a constant proportional to the bus capacitance and the switching period;
the zero-sequence component of the zero-sequence injection algorithm with the midpoint potential balance as the control target in the above formula is V0_ A;
102. acquiring a zero-sequence component V0_ B of the switching frequency optimization SFOPWM;
the zero-sequence component of the SFOPWM-based zero-sequence component injection modulation algorithm is optimized based on the switching frequency as follows:
V0＝-[max(Va,Vb,Vc)+min(Va,Vb,Vc)]/2
the modulated wave voltage expression is:
Vra(t)＝Vra1(t)+V0(t)；
Vrb(t)＝Vrb1(t)+V0(t)；
Vrc(t)＝Vrc1(t)+V0(t)；
the zero sequence component is referred to as V0_ B for the purpose of optimizing the switching frequency;
103. when the unbalanced voltage (Vbus _ u-Vbus _ D) is smaller than the amplitude D, the zero-sequence component adopts V0_ B, that is, the modulated wave voltage Vr _ abc is Vr _ abc1+ V0_ B; when the unbalanced voltage (Vbus _ u-Vbus _ D) is larger than the amplitude D, the zero-sequence component adopts V0_ a, that is, the modulated wave voltage Vr _ abc is Vr _ abc1+ V0_ a; wherein the amplitude D is greater than zero;
104. and modulating and controlling the I-type three-level inverter by the sampling value of the modulated wave voltage.
2. The zero sequence component injection-based type-I three-level inverter midpoint balancing method according to claim 1, characterized in that: the value of the amplitude D can be 2% -10% of the rated voltage of the direct current bus.
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Application publication date: 20210226 |