A method for neutral point shift of cascaded high-voltage inverter

Abstract

The present invention belongs to the technical field of industrial electric transmission, and specifically relates to a method for neutral point offset of a cascaded high-voltage frequency converter, the method comprising: after a power unit fails, the offset angle of each phase voltage after the neutral point offset of the DC bus voltage difference of each power unit is calculated according to the voltage amplitude of each phase, the main controller generates each phase modulation wave signal according to the offset angle of each phase voltage, the main controller compares the modulation wave signal with the triangular carrier signal to generate a PWM wave by a pulse width method, and sends the PWM wave signal to the power unit control board, the power unit control board drives the inverter circuit semiconductor power device switch to generate the required output voltage. The present invention is suitable for cascaded high-voltage frequency converter power unit fault bypass processing, and can effectively solve the problem of unbalanced three-phase output line voltage after neutral point offset.

Description

Technical Field

[0001] The invention belongs to the technical field of industrial electric transmission, and in particular relates to a method for neutral point offset of a cascade type high voltage frequency converter. Background Art

[0002] With the development of high-voltage, high-current, high-switching-frequency semiconductor power devices and microprocessor technology, power electronic equipment is increasingly widely used in the field of industrial electrical transmission technology, and inverter-driven motor speed regulation is a typical example. In high-voltage, high-power, and high-reliability applications, cascaded high-voltage inverters are widely used because the power units used in power unit cascades are easy to manufacture in modular form, can be replaced by each other, and can increase the output voltage and capacity by increasing the number of power units.

[0003] At present, typical power unit cascade type high voltage inverter, such as Figure 1 As shown, the grid AC power is applied to the R phase, S phase, and T phase of the primary winding of the phase-shifting transformer. The transformer steps down the primary winding voltage and shifts the phase, and then the secondary winding of the transformer inputs the AC power to the power units A1-A4, B1-B4, and C1-C4. The outputs of the power units A1, A2, A3, and A4 are connected in series to serve as the inverter output power phase A, the outputs of the power units B1, B2, B3, and B4 are connected in series to serve as the inverter output power phase B, and the outputs of the power units C1, C2, C3, and C4 are connected in series to serve as the inverter output power phase C, and are connected into a star-shaped three-phase high-voltage power supply to supply power to the motor.

[0004] Typical power unit structure for cascaded inverters, such as Figure 2 As shown, the power unit uses semiconductor power devices to rectify the AC power input into DC power. The DC power is connected to the filter capacitor to form a DC bus. The DC bus is used as the DC input of the inverter circuit. The single-phase AC power output is formed by controlling the switching sequence of the semiconductor power devices Q1, Q2, Q3, and Q4 of the inverter circuit. The main controller generates a PWM wave (pulse width modulation wave) by comparing the modulation signal and the carrier signal, and sends the PWM wave to the power unit controller, which drives the semiconductor power devices Q1, Q2, Q3, and Q4 of the inverter circuit to switch.

[0005] When the power unit cascade type high-voltage inverter is used in high-reliability occasions, power unit redundancy design is usually adopted. After one or more power units fail, the output of all power units is instantly interrupted, the faulty power unit is bypassed, and the PWM wave used to control the output of each power unit is recalculated by the neutral point offset method. After the inverter output is interrupted for a short time, the voltage required by the load is re-output to achieve the purpose of uninterrupted process and continued long-term operation. After the faulty power unit is bypassed, if the neutral point voltage continues to remain unchanged, the output phase voltage and line voltage of the inverter will be asymmetric. For this reason, the neutral point offset can be implemented. Although the phase voltage is still asymmetric, the output line voltage is symmetric, which can meet the stable operation of the motor. Only by adopting a suitable neutral point offset method can the three-phase line voltage output of the inverter be balanced, and abnormal heating and vibration of the motor caused by the unbalanced output three-phase line voltage can be avoided, which may cause equipment damage.

[0006] At present, when the neutral point offset method is used to calculate the phase angles α, β, and γ after the neutral point offset, it is assumed that the DC bus voltage of each power unit is consistent and the output phase voltage amplitude of the power unit is consistent, and the three-phase voltage amplitudes of A, B, and C are calculated. However, in actual application, due to the differences in the input impedance and transformation ratio of the secondary windings corresponding to each power unit, the input voltage of each power unit is different, which in turn causes differences in the DC bus voltage inside each power unit and the output phase voltage amplitude of the power unit. In particular, after one or more power units fail to bypass, the impedance of the secondary winding of the transformer changes, and the difference in the input voltage of each power unit, the difference in the DC bus voltage inside each power unit, and the difference in the output phase voltage amplitude of the power unit will increase. When the phase angles α, β, and γ after the neutral point offset calculated by the above method are used to generate modulation waves to control the power unit, a balanced three-phase line voltage cannot be obtained. Summary of the invention

[0007] The object of the present invention is to provide a method for neutral point offset of a cascaded high-voltage inverter, which is suitable for fault bypass processing of power units of cascaded high-voltage inverters and can effectively solve the problem of unbalanced three-phase output line voltage after neutral point offset.

[0008] The technical solution to achieve the purpose of the present invention is:

[0009] A method for neutral point offset of a cascaded high-voltage inverter, the method comprising: after a power unit fails, calculating the offset angle of each phase voltage after the neutral point offset of the DC bus voltage difference of each power unit according to the voltage amplitude of each phase, a main controller generating a modulation wave signal of each phase according to the offset angle of each phase voltage, the main controller compares the modulation wave signal with a triangular carrier signal to generate a PWM wave through a pulse width method, and sends the PWM wave signal to a power unit control board, and the power unit control board drives the inverter circuit semiconductor power device switch to generate the required output voltage.

[0010] The calculation formula of each phase modulation wave signal is:

[0011] UAref=k*cos(theta-ai)

[0012] UBref=k*cos(theta+4π / 3+bi)

[0013] UCref=k*cos(theta+2π / 3+ci)

[0014] in,

[0015] UAref: A phase modulation wave signal,

[0016] UBref: B-phase modulation wave signal,

[0017] UCref: C-phase modulation wave signal,

[0018] Theta: The current phase angle of the modulation wave signal UAref,

[0019] k: current modulation wave peak value,

[0020] ai: Phase A voltage after neutral point offset o ′ A lags behind the offset angle of the A phase voltage OA before the neutral point offset,

[0021] bi: B phase voltage after neutral point offset o ′ B leads the deviation angle of B phase voltage OB before the neutral point deviation.

[0022] ci: C phase voltage after neutral point shift o ′ C leads the offset angle of the C phase voltage OC before the neutral point shifts.

[0023] The calculation formula for the offset angle of each phase voltage after the neutral point of the DC bus voltage difference of each power unit is offset is:

[0024]

[0025] in,

[0026] ai: Phase A voltage after neutral point offset o ′ A lags behind the offset angle of the A phase voltage OA before the neutral point offset,

[0027] bi: B phase voltage after neutral point offset o ′ B leads the deviation angle of B phase voltage OB before the neutral point deviation.

[0028] ci: C phase voltage after neutral point shift o ′ C leads the deviation angle of the C phase voltage OC before the neutral point deviation,

[0029] aj: Phase A voltage after neutral point shift o ′ The angle between A and AC,

[0030] bj: Phase B voltage after neutral point shift o ′ The angle between B and BC,

[0031] cj: Phase voltage of phase C after neutral point shift o ′ The angle between C and AC,

[0032] a: A-phase voltage amplitude after neutral point shift,

[0033] b: B-phase voltage amplitude after neutral point shift,

[0034] c: C phase voltage amplitude after neutral point shift,

[0035] x: Output line voltage amplitude before and after neutral point offset.

[0036] The inverter is an inverter with 4 power units connected in series per phase, and the calculation formula for the voltage amplitude of each phase is:

[0037]

[0038] in,

[0039] UdcA1, UdcA2, UdcA3, UdcA4: DC bus voltages of power units A1, A2, A3, A4 respectively,

[0040] UdcB1, UdcB2, ​​UdcB3, UdcB4: DC bus voltages of power units B1, B2, B3, B4 respectively,

[0041] UdcC1, UdcC2, UdcC3, UdcC4: DC bus voltages of power units C1, C2, C3, C4 respectively,

[0042] Udcnom: inverter output phase voltage peak base value,

[0043] Ue: Rated output line voltage effective value of the inverter.

[0044] The frequency converter is a cascade frequency converter in which n power units per phase are connected in series, and n is an integer.

[0045] The method is applied to a VF controlled power unit cascade inverter, and the transformation between a two-phase stationary coordinate system and a three-phase stationary coordinate system is performed through the calculated three-phase offset angle, and is applied to a vector controlled power unit cascade inverter.

[0046] The beneficial technical effects of the present invention are:

[0047] The present invention provides a method for neutral point offset of a cascaded high-voltage inverter, which uses the voltage amplitudes a, b, and c of each phase after considering the DC bus voltage difference factor to calculate the A phase voltage after the neutral point offset. ′ The offset angle ai of the A phase voltage OA before the neutral point offset, and the B phase voltage o ′ The offset angle bi of the B phase voltage OB before B is offset from the neutral point, and the C phase voltage o ′ C is relative to the offset angle ci of the C phase voltage OC before the neutral point offset, and the offset angles ai, bi, ci are used to participate in the calculation of the modulation wave generated by the main controller to control the power unit. After the power unit of the cascaded high-voltage inverter is bypassed due to a fault, especially when there is a difference in the DC bus voltage of each power unit, the three-phase output line voltage imbalance rate is lower than that of the prior art, and the problem of three-phase output line voltage imbalance after the neutral point offset can be effectively solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a schematic diagram of the power circuit of the power unit cascade type high voltage inverter;

[0049] Figure 2 is a schematic diagram of a power unit for a cascaded frequency converter;

[0050] Figure 3 Schematic diagram of the neutral point offset principle.

[0051] In the figure: 1. AC input terminal of phase-shifting transformer; 2. Phase-shifting transformer; 3. Primary winding of phase-shifting transformer; 4-15. Secondary winding of phase-shifting transformer; 16-27. Output three-phase copper bar of secondary winding of phase-shifting transformer; 28-39. Completely identical power unit; 40. AC asynchronous motor; 41. AC power input terminal of power unit; 42-47. Semiconductor devices of power unit rectifier circuit; 48-51. Semiconductor devices of power unit inverter circuit; 52-53. Output copper bar of power unit; T1-T2. Output terminal of power unit; 56-57. Drive signal wiring; 58. Power unit control board; 59. Optical fiber; 60. Main controller; 61. DC bus of power unit; 62. Bypass contactor. Udcxx is DC bus voltage of xx power unit, xx refers to A1, A2...C4. O, o ′ are the neutral points before and after the power unit is bypassed, a, b, c are the phase voltage amplitudes of phase A, phase B, and phase C after the neutral point is offset, x is the output line voltage amplitude before and after the neutral point is offset, OA is the phase voltage of phase A before the neutral point is offset, OB is the phase voltage of phase B before the neutral point is offset, OC is the phase voltage of phase C before the neutral point is offset, o ′ A is the phase voltage of phase A after the neutral point is offset, o ′ B is the phase voltage of phase B after the neutral point is offset, o′ C is the phase voltage of phase C after the neutral point is offset, α is o ′ A and o ′ The angle between C and β is o ′ A and o ′ The angle between B and γ is o ′ B and o ′ The angle between C and aj is o ′ The angle between A and AC, bj is o ′ The angle between B and BC, cj is o ′ The angle between C and AC, ai, bi, ci are calculated quantities, and their meaning is ai is the voltage of phase A after the neutral point is offset o ′ A is the offset angle of the A phase voltage OA before the neutral point offset, and bi is the B phase voltage o ′ B is the deviation angle of the B phase voltage OB before the neutral point deviation, ci is the C phase voltage o ′ C is the offset angle ci of the C phase voltage OC before the neutral point offset, the angle between OA and OB is 120°, and the angle between OA and OC is 120°. DETAILED DESCRIPTION

[0052] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0053] Figure 1 This is a typical schematic diagram of a power unit cascade high-voltage inverter. When the inverter is operating normally, the grid AC power is applied to the R phase, S phase, and T phase of the primary winding side of the phase-shifting transformer. The transformer steps down the primary winding voltage and shifts the phase. Then the transformer secondary winding sends the AC power to the input ends of the power units A1-A4, B1-B4, and C1-C4.

[0054] When all power units are fault-free, the bypass contactor contacts corresponding to each power unit are in Figure 2 In the state shown, the bypass contactor contacts short-circuit the lead-out wiring between Q1 and Q2 and the T2 terminal. T1 and T2 are the output terminals of the power unit. The outputs of power units A1, A2, A3, and A4 are connected in series to serve as the inverter output power phase A, the outputs of power units B1, B2, B3, and B4 are connected in series to serve as the inverter output power phase B, and the outputs of power units C1, C2, C3, and C4 are connected in series to serve as the inverter output power phase C, and are connected to form a star-shaped three-phase high-voltage power supply to supply power to the motor. Figure 2This is a typical schematic diagram of a power unit for a cascade inverter. The power unit uses semiconductor power devices to rectify the AC power input into a DC power supply. The DC power supply is connected to the filter capacitor to form a DC bus. The DC bus is used as the DC input of the inverter circuit. By controlling the switching sequence of the semiconductor power devices Q1, Q2, Q3, and Q4 in the inverter circuit, a single-phase AC power output is formed. The main controller generates a PWM wave (pulse width modulation wave) by comparing the modulation signal and the carrier signal, and sends the PWM wave to the power unit controller, which drives the semiconductor power devices Q1, Q2, Q3, and Q4 in the inverter circuit to switch.

[0055] The inverter operates normally and all power units are free of faults. The main controller calculates the modulation wave signal UAref for controlling the A-phase power units A1, A2, A3, and A4 according to the motor operation requirements. The main controller calculates the modulation wave signal UBref for controlling the B-phase power units B1, B2, B3, and B4 according to the motor operation requirements. The main controller calculates the modulation wave signal UCref for controlling the C-phase power units C1, C2, C3, and C4 according to the motor operation requirements.

[0056] UAref=k*cos(theta)

[0057] UBref=k*cos(theta+4π / 3)

[0058] UCref=k*cos(theta+2π / 3)

[0059] Wherein, theta represents the current phase angle of the modulation wave signal UAref, and k represents the current modulation wave peak value.

[0060] The main controller compares the modulation wave signal with the triangle carrier signal in the main controller to generate the PWM wave (pulse width modulation wave) for controlling the power unit, and sends the PWM wave to the power unit control board, which drives the 12 power unit internal inverter circuit semiconductor devices Q1, Q2, Q3, Q4 switches to form a single-phase AC output for each power unit. The PWM wave (pulse width modulation wave) generated by comparing the modulation wave signal with the triangle carrier signal in the main controller to control the power unit is a pulse width modulation method widely used in power electronics. In the design of cascade inverter products, various manufacturers widely use carrier phase-shifted pulse width modulation and carrier stacked pulse width modulation, in which the triangle carrier signal is a fixed frequency waveform, and the modulation wave signal is calculated and generated by the controller according to the motor operation requirements, which is particularly important for generating the final PWM wave.

[0061] Take the case where power unit A1 fails and needs to be bypassed as an example. After power unit A1 fails, the power unit A1 control board detects the power unit A1 failure and sends the failure information to the main controller through optical fiber. The main controller immediately sends a pulse signal to all 12 power units, shuts off the semiconductor devices Q1, Q2, Q3, and Q4 in the inverter circuits inside all power units, and reduces the inverter output to 0. The main controller then activates the bypass contactor of power unit A1 through the peripheral circuit, disconnects the connection between the lead-out wiring between Q1 and Q2 and the T2 terminal, and short-circuits the T1 and T2 terminals through the bypass contactor contacts.

[0062] The main controller calculates the phase A voltage after the neutral point shift as follows: ′ The offset angle ai of the A phase voltage OA before the neutral point offset, and the B phase voltage o ′ The offset angle bi of the B phase voltage OB before B is offset from the neutral point, and the C phase voltage o ′ C is the offset angle ci of the C phase voltage OC before the neutral point is offset. The main controller receives the fault information transmitted by the power unit A1 control board through optical fiber, determines that the power unit A1 is faulty, and then assigns the DC bus voltage of the power unit A1 to 0, that is, UdcA1 is 0. The main controller receives information transmitted by the other 11 power unit control boards through optical fiber, including the DC bus voltages of power units A2, A3, and A4, respectively, UdcA1, UdcA2, UdcA3, and UdcA4, the DC bus voltages of power units B1, B2, B3, and B4, respectively, UdcB1, UdcB2, ​​UdcB3, and UdcB4, and the DC bus voltages of power units C1, C2, C3, and C4, respectively, UdcC1, UdcC2, UdcC3, and UdcC4. Udcnom is the base value of the inverter output phase voltage peak value, and the voltage amplitudes of each phase are calculated as follows.

[0063] Phase A voltage amplitude

[0064] Phase B voltage amplitude

[0065] C phase voltage amplitude

[0066] Where Ue is the effective value of the inverter's rated output line voltage.

[0067] O, o ′ They are the neutral points before and after the power unit is bypassed. The principle of neutral point offset is shown as follows: Figure 3 ,according to Figure 3 , using the voltage amplitudes a, b, c of each phase after considering the DC bus voltage difference factor, the phase-to-phase angles α, β, γ after each neutral point offset can be calculated.

[0068]

[0069] From (1) and (2), we can get:

[0070]

[0071] γ=360°-α-β (6)

[0072] Furthermore, in order to facilitate the generation of modulated wave signals, the offset angle of each phase voltage after the neutral point offset considering the DC bus voltage difference of each power unit is calculated, with the phase angle increasing direction being counterclockwise. Figure 3 For example, after the neutral point is offset, the voltage of phase A is o ′ A lags behind the A phase voltage OA before the neutral point shift by an angle of ai, and the B phase voltage after the neutral point shift is o ′ B leads the B phase voltage OB before the neutral point shift by an angle of bi, and the C phase voltage after the neutral point shift is o ′ The angle of C phase voltage OC before C leads the neutral point shift is ci.

[0073]

[0074] The main controller generates three-phase modulation wave signals UAref, UBref, UCref using the offset angles ai, bi, ci.

[0075] UAref=k*cos(theta-ai)

[0076] UBref=k*cos(theta+4π / 3+bi)

[0077] UCref=k*cos(theta+2π / 3+ci)

[0078] The main controller compares the above-mentioned modulated wave signal with the triangular carrier signal to generate a PWM wave through a pulse width method, and sends the PWM wave signal to the power unit control board. The power unit control board drives the inverter circuit semiconductor power devices Q1, Q2, Q3, and Q4 switches to generate the required output voltage.

[0079] It should be noted that the voltage amplitudes a, b, and c of each phase after considering the DC bus voltage difference factor can be used to calculate the voltage of phase A after the neutral point is offset. ′ The offset angle ai of the A phase voltage OA before the neutral point offset, and the B phase voltage o ′ The offset angle bi of the B phase voltage OB before B is offset from the neutral point, and the C phase voltage o ′C is the offset angle ci of the C phase voltage OC before the neutral point offset, and the offset angles ai, bi, ci are used to participate in the calculation of the modulation wave in the main controller. The above embodiment is a frequency converter with 4 power units in series per phase, and the present invention can be extended to a frequency converter with n power units cascaded; the above embodiment is a calculation case of the modulation wave and can be directly applied to a power unit cascade frequency converter with VF control (voltage-frequency ratio control). Technicians in this field can also use the above calculated three-phase offset angle to transform the two-phase stationary coordinate system and the three-phase stationary coordinate system to apply to the power unit cascade frequency converter with vector control.

[0080] The present invention is described in detail above with reference to the accompanying drawings and embodiments, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of ordinary technicians in the field without departing from the purpose of the present invention. The contents not described in detail in the present invention can adopt the existing technology.

Claims

1. A method for neutral point offset of a cascaded high-voltage inverter, characterized in that: The method comprises: after a power unit fails, the offset angle of each phase voltage after the neutral point offset of the DC bus voltage difference of each power unit is calculated according to the voltage amplitude of each phase, a main controller generates a modulation wave signal of each phase according to the offset angle of each phase voltage, the main controller compares the modulation wave signal with the triangular carrier signal to generate a PWM wave through a pulse width method, and sends the PWM wave signal to the power unit control board, and the power unit control board drives the switch of the semiconductor power device of the inverter circuit to generate the required output voltage.

2. The method for neutral point shift of a cascaded high-voltage inverter according to claim 1, characterized in that: The calculation formula of each phase modulation wave signal is: UAref=k*cos(theta-ai) UBref=k*cos(theta+4π / 3+bi) UCref=k*cos(theta+2π / 3+ci) in, UAref: A phase modulation wave signal, UBref: B-phase modulation wave signal, UCref: C-phase modulation wave signal, Theta: The current phase angle of the modulation wave signal UAref, k: current modulation wave peak value, ai: Phase A voltage after neutral point offset o ′ A lags behind the offset angle of the A phase voltage OA before the neutral point offset, bi: B phase voltage after neutral point offset o ′ B leads the deviation angle of B phase voltage OB before the neutral point deviation. ci: C phase voltage after neutral point shift o ′ C leads the offset angle of the C phase voltage OC before the neutral point shifts.

3. The method for neutral point shift of a cascaded high-voltage inverter according to claim 1, characterized in that: The calculation formula for the offset angle of each phase voltage after the neutral point of the DC bus voltage difference of each power unit is offset is: in, ai: Phase A voltage after neutral point offset o ′ A lags behind the offset angle of the A phase voltage OA before the neutral point offset, bi: B phase voltage after neutral point offset o ′ B leads the deviation angle of B phase voltage OB before the neutral point deviation. ci: C phase voltage after neutral point shift o ′ C leads the deviation angle of the C phase voltage OC before the neutral point deviation, aj: Phase A voltage after neutral point offset o ′ The angle between A and AC, bj: Phase B voltage after neutral point shift o ′ The angle between B and BC, cj: Phase voltage of phase C after neutral point shift o ′ The angle between C and AC, a: A-phase voltage amplitude after neutral point shift, b: B-phase voltage amplitude after neutral point shift, c: C phase voltage amplitude after neutral point shift, x: Output line voltage amplitude before and after neutral point offset.

4. The method for neutral point shift of a cascaded high-voltage inverter according to claim 1, characterized in that: The inverter is an inverter with 4 power units connected in series per phase, and the calculation formula for the voltage amplitude of each phase is: in, UdcA1, UdcA2, UdcA3, UdcA4: DC bus voltages of power units A1, A2, A3, A4 respectively, UdcB1, UdcB2, ​​UdcB3, UdcB4: DC bus voltages of power units B1, B2, B3, B4 respectively, UdcC1, UdcC2, UdcC3, UdcC4: DC bus voltages of power units C1, C2, C3, C4 respectively, Udcnom: inverter output phase voltage peak base value, Ue: Rated output line voltage effective value of the inverter.

5. The method for neutral point shift of a cascaded high-voltage inverter according to claim 1, characterized in that: The frequency converter is a cascade frequency converter in which n power units per phase are connected in series, and n is an integer.

6. The method for neutral point shift of a cascaded high-voltage inverter according to claim 1, characterized in that: The method is applied to a VF controlled power unit cascade inverter, and the transformation between a two-phase stationary coordinate system and a three-phase stationary coordinate system is performed through the calculated three-phase offset angle, and is applied to a vector controlled power unit cascade inverter.

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