A dead-time compensation method for three-level converter considering output filter

Abstract

The present invention relates to a dead zone compensation method for a three-level DC-AC converter considering an output filter, and its technical characteristics are: an IGCT three-level power device is considered together with a dv / dt filter connected to its AC side, so as to achieve more accurate compensation of the dead zone time. The specific method is to analyze a certain phase of the IGCT three-level converter, establish a frequency domain transfer function model and obtain a corresponding current time domain function according to the influence of the LCR filter circuit on the output phase voltage; calculate the maximum change value of the current; collect the load current in the equivalent topology, judge the relationship between the load current and the maximum change value of the current, calculate the dead zone equivalent area and obtain the dead zone compensation time, and introduce it into the output voltage to achieve compensation. The present invention is reasonably designed, takes into account the influence of the LCR filter circuit on the nonlinear dead zone effect, solves the problem that the conventional dead zone compensation method is poorly compensated at the current zero crossing, improves the accuracy of the dead zone compensation, and greatly improves the current harmonics on the AC side.

Description

Technical Field

[0001] The invention belongs to the field of IGCT three-level DC-AC converter (including a three-level rectifier and a three-level inverter) control, in particular to a three-level converter dead zone compensation method considering an output filter. Background Art

[0002] There is a delay in the switching of power devices. In order to prevent the bridge arm from being directly connected, a dead zone needs to be added to the complementary drive signal. However, the dead zone time will cause the actual output voltage to deviate from the given voltage under the action of the inverter output current. The addition of dead zone is an important factor leading to inverter nonlinearity. The performance of the motor controller depends not only on the motor control algorithm, but also on whether the inverter can accurately output the target voltage calculated by the motor control algorithm. The addition of dead zone leads to nonlinear output voltage, which will affect the smoothness of the motor's low-speed torque output and the stability of speed control. It is an important problem that needs to be solved in inverter applications. Traditional dead zone compensation schemes ignore the processing at the current zero crossing point or do not consider the influence of the dvdt filter circuit, making it difficult to achieve accurate compensation at the current zero crossing point. Summary of the invention

[0003] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a dead zone compensation method for a three-level DC-AC converter taking into account the output filter, taking into account the influence of the LCR filter circuit on the nonlinear dead zone effect, overcoming the influence of the current zero crossing and the output end LCR circuit on the conventional dead zone compensation method, improving the accuracy of the dead zone compensation, and greatly improving the current harmonics.

[0004] The present invention solves the technical problem by adopting the following technical solutions:

[0005] A dead zone compensation method for a three-level converter considering an output filter comprises the following steps:

[0006] Step 1, performing equivalent topology analysis on a certain phase in a three-level DC-AC converter;

[0007] Step 2: According to the influence of the LCR filter circuit on the IGCT output voltage, a frequency domain transfer function model is established and the corresponding current time domain function is obtained;

[0008] Step 3, calculating the maximum current change value according to the current time domain function obtained in step 2;

[0009] Step 4: Collect the load current in the equivalent topology of step 1, determine the relationship between the load current and the maximum current change value calculated in step 3 according to the voltage time domain function, calculate the dead zone equivalent area, and then obtain the dead zone compensation time, and then superimpose the compensation time into the output voltage to achieve compensation.

[0010] Moreover, the current time domain function obtained in step 2 is:

[0011]

[0012] Where i(t) is the transient component of current caused by a voltage step change, L is the inductance, r is the resistance, c is the capacitance, and t is the time.

[0013] Moreover, the calculation method of the maximum change value of the current in step 3 is:

[0014] when At this moment, the current variation reaches its maximum value.

[0015]

[0016] Among them, L is the inductance, r is the resistance, and c is the capacitance.

[0017] Moreover, the specific implementation method of step 4 is: extract the transfer functions of u1 and i1, take the Laplace inverse transform according to the step response, and obtain the voltage time domain function:

[0018]

[0019] At 0~t m u(t) decreases approximately linearly over time, and the rate of decrease k≈1 / t m ,

[0020] when i L >i(t) max When the compensation time t c ≈0;

[0021] when i L <i(t) max hour,

[0022] The change of u0 during the time from 0 to t1 is k·t1;

[0023] u 01 =u dc -k·t1

[0024]

[0025] like Then Δt=t D -t1, the equivalent dead zone area is:

[0026]

[0027] in,

[0028] like but The equivalent dead zone area is:

[0029]

[0030] According to the volt-second area equivalent, we get i L <i(t) max Dead zone compensation time

[0031] in, i L is the load current of inductor L0, u0 is the voltage at the common point of resistor r and inductor L, i(t) max is the maximum change value of current, L is inductance, r is resistance, c is capacitance, u 01 is the voltage at time t1, u 02 is the voltage at time t2, t D is the dead time, and Δt is the time when u0 turns into the linear decline stage.

[0032] The advantages and positive effects of the present invention are:

[0033] The IGCT three-level power device is considered together with the dvdt filter connected to its AC side, so as to achieve more accurate compensation. A certain phase of the IGCT three-level converter is analyzed, and according to the influence of the LCR filter circuit on the output phase voltage, a frequency domain transfer function model is established and the corresponding current time domain function is obtained; according to the current time domain function, the maximum change value of the current is calculated; the load current in the equivalent topology is collected, and the relationship between the load current and the maximum change value of the current is determined according to the voltage time domain function, and the dead zone equivalent area is calculated, and then the dead zone compensation time is obtained, and then the compensation time is introduced into the output voltage to achieve compensation. The present invention is reasonably designed, taking into account the influence of the LCR filter circuit on the nonlinear dead zone effect, solving the problem of poor compensation of the conventional dead zone compensation method at the current zero crossing, improving the accuracy of the dead zone compensation, and greatly improving the current harmonics on the AC side. At the same time, the present invention is also applicable to IGBT three-level converters with output filters. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is the main circuit topology diagram of IGCT three-level inverter;

[0035] Figure 2 for Figure 1 A schematic diagram of one of the phases in FIG.

[0036] Figure 3 It is a complete resonance waveform diagram of the present invention;

[0037] Figure 4 This is the segmented resonant waveform of the present invention.

[0038] Figure 5It is a schematic diagram of voltage variation trend-load current zero crossing of the present invention;

[0039] Figure 6 This is a comparison diagram of the dead zone compensation effect of the present invention. DETAILED DESCRIPTION

[0040] The present invention will be further described below by taking an IGCT three-level inverter as an example with reference to the accompanying drawings.

[0041] Figure 1 The main circuit topology diagram of the IGCT three-level inverter is shown in Figure 2. Usually, an LCR filter is configured at the output end to suppress the output voltage change rate. From the perspective of the controlled object, its output voltage is actually the voltage u0 at the rear end of the filter inductor, which is inconsistent with the inverter output voltage u1. Figure 2 .

[0042] A dead zone compensation method for a three-level converter considering an output filter comprises the following steps:

[0043] Step 1: Convert the inverter single-phase circuit in the inverter IGCT into an equivalent topology.

[0044] In this step, the inverter single-phase circuit in the inverter IGCT is converted as follows: Figure 2 The equivalent topology shown is: voltage input u dc , voltage input-u dc , the common point of the positive and negative power supplies is 0, switch tube V1, switch tube V2, switch tube V3, switch tube V4, diode D1, diode D2, diode D3, diode D4, diode D5, diode D6, resistor r and capacitor c, voltage input u dc Connect the cathode of diode D1, the anode of diode D1 to the cathode of diode D2, the anode of diode D2 to the cathode of diode D3, the anode of diode D3 to the cathode of diode D4, and the anode of diode D4 to the voltage input -u dc , the switch tube V1 is connected in parallel on both sides of the diode D1, the switch tube V2 is connected in parallel on both sides of the diode D2, the switch tube V3 is connected in parallel on both sides of the diode D3, the switch tube V4 is connected in parallel on both sides of the diode D4, the cathode of the diode D5 is connected to the cathode of the diode D2, the anode of the diode D5 is connected to the cathode of the diode D6, the anode of the diode D6 is connected to the anode of the diode D3, the anode of the diode D2 outputs the current i1 and the voltage u1, the anode of the diode D2 is connected to one end of the inductor L, and the other end of the inductor L is respectively connected to one end of the resistor r and one end of the inductor L0, the voltage at the common point of the resistor r and the inductor L is u0, and the load current of the resistor r is i c , the voltage at the other end of the resistor r is u c And the series capacitor c is connected to the midpoint, and the load current of the inductor L0 is iL .

[0045] Furthermore, when the inverter output voltage u1 changes suddenly, due to the existence of the LRC filter loop, the voltage u0 at the rear end of the filter inductor will not change suddenly, and similarly, the device output current i1 will not change suddenly either.

[0046] Step 2: According to the influence of the LCR filter circuit on the IGCT output voltage, a frequency domain transfer function model is established and the corresponding current time domain function is obtained.

[0047] According to the equivalent topology of step 1, the following equation is established:

[0048]

[0049] Among them, u c0 is u c Initial value at the moment of output voltage jump;

[0050] Take the first and second derivatives of the equation:

[0051]

[0052]

[0053] Get the transfer function of i1, u0 and u1:

[0054]

[0055]

[0056] Extract the transfer function between u1 and i1:

[0057]

[0058] According to the step response, take the Laplace inverse transform and get the current time domain function:

[0059]

[0060] Among them, i(t) is the transient component of current caused by the voltage step change.

[0061] Step 3: Calculate the maximum current change value according to the current time domain function obtained in step 2.

[0062] Analyzing the time domain function, we get At this moment, the current variation reaches its maximum value:

[0063]

[0064] The dynamic characteristics of i1 and u0 depend on the initial value of i1. Land i(t) max Amplitude relationship.

[0065] Step 4: Collect the load current in the equivalent topology of step 1, determine the relationship between the load current and the maximum current change value calculated in step 3 according to the voltage time domain function, calculate the dead zone equivalent area, and then obtain the dead zone compensation time.

[0066] Since the dynamic characteristics of i1 and u0 depend on the initial value of i1 L and i(t) max Amplitude relationship, so Figure 3 As shown, when i L >i(t) max When , the output u0 will drop to zero within the dead time; Figure 4 As shown, when i L <i(t) max When the output u0 drops to 0 after i1, due to i c The current remains constant, and u0 enters the linear decline stage;

[0067] Therefore, according to the switch change time i1=i L Initial value, calculate the subsequent change trend, and then calculate the dead zone compensation value according to the volt-second equivalent principle:

[0068] When i1=i L When , extract the transfer function of u1 and i1:

[0069]

[0070] According to the step response, take the Laplace inverse transform and get the voltage time domain function:

[0071]

[0072] At 0~t m u(t) decreases approximately linearly over time, and the rate of decrease k≈1 / t m ,

[0073] Therefore, when i L >i(t) max When u(t) drops rapidly to zero, this period of time can be ignored. The compensation time t c ≈0;

[0074] like Figure 5 As shown, when i L <i(t) max hour, The change of u0 during the time from 0 to t1 is k·t1:

[0075] u 01 =u dc -k·t1,

[0076]

[0077] like Then Δt=t D -t1, the equivalent dead zone area is:

[0078]

[0079] in,

[0080] like but The equivalent dead zone area is:

[0081]

[0082] According to volt-second area equivalent, dead zone compensation time Among them, Δt needs to be discussed in categories:

[0083] like Then Δt=t D -t1,

[0084] like but

[0085] in, t m is the time it takes for the current to reach its maximum value.

[0086] According to the derivation conclusion of the above-mentioned dead zone compensation method of the three-level converter considering the output filter, the dead zone compensation time is classified and discussed.

[0087] When the output voltage changes from zero voltage to positive voltage:

[0088] Load current i L >0,t c =t D ;

[0089] Load current i L <0,t c =0;

[0090] When the output voltage changes from positive voltage to zero voltage:

[0091] Load current i L >0,

[0092] Load current i L <0,t c =t D .

[0093] like Figure 6 The following are the current waveforms before and after compensation. The dead zone effect will cause the output voltage to deviate from the given value, resulting in a smaller current amplitude and distortion at the zero crossing point. After compensation, the dead zone effect is improved, the current amplitude is increased, and the current harmonics are also greatly improved.

[0094] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive, so the present invention includes but is not limited to the embodiments described in the specific embodiments, and any other embodiments derived by those skilled in the art based on the technical solution of the present invention also fall within the scope of protection of the present invention. At the same time, the present invention is also applicable to IGBT converters with output filters.

Claims

1. A dead zone compensation method for a three-level DC-AC converter considering an output filter, characterized in that: The following steps are involved: Step 1, performing an equivalent topology analysis on a certain phase power circuit in a three-phase three-level DC-AC converter; Step 2: According to the influence of the LCR filter circuit on the output voltage of the converter, a frequency domain transfer function model is established and the corresponding current time domain function is obtained; The current time domain function obtained in step 2 is: Where i(t) is the transient component of current caused by the voltage step change, L is the inductance on the device side, r is the damping resistor, c is the filter capacitor, and t is the time; Step 3, calculating the maximum current change value according to the current time domain function obtained in step 2; The calculation method for the maximum change value of current in step 3 is: The time point corresponding to the maximum current value, that is, the time from the moment when the phase module output voltage u1 jumps to the time when the phase module output current i1 is the maximum, is: Among them, L is the inductance on the device side, r is the damping resistor, and c is the filter capacitor; Step 4: Collect the load current in the equivalent topology of step 1, determine the relationship between the load current and the maximum current change value calculated in step 3 according to the voltage time domain function, calculate the dead zone equivalent area, and then obtain the dead zone compensation time, and then superimpose the compensation time into the output voltage to achieve compensation; The specific implementation method of step 4 is: through the phase module output voltage u1 and output current i1 transfer function, take the Laplace inverse transform according to the step response, and obtain the voltage time domain function: At 0~t m u(t) decreases approximately linearly over time, and the slope of the decrease is k≈1 / t m , when i L >i(t) max When the compensation time t c ≈0; when i L <i(t) max hour, The change of u0 during the time from 0 to t1 is k·t1; in 01 =in dc -k·t1 like Then Δt=t D -t1, the equivalent dead zone area is: in, like but The equivalent dead zone area is: According to the volt-second area equivalent, we get i L <i(t) max Dead zone compensation time in, i L is the current of the load side inductor L0, i.e., the load current, u0 is the voltage at the common point of the resistor r and the inductor L, i(t) max is the maximum change value of the current, L is the inductance on the device side, r is the damping resistor, c is the filter capacitor, u 01 is the voltage at time t1, u 02 is the voltage at time t2, t D is the dead time, and Δt is the time when u0 turns into the linear decline stage.

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