Active Neutral Point Clamped Three-level Synchronous Space Vector Width Modulation Research

Active neutral point clamped (ANPC) topology is widely studied because it makes loss equalization possible. To reduce the switching frequency of high power traction systems, the multi-mode modulation strategy is used in many modulation methods. To reduce the fluctuation problem when switching between different carrier ratios in multi-mode synchronous space vector pulse width (SSVPWM) modulation algorithm, this paper studies the stator flux under different modes and proposes a multi-mode SSVPWM switching strategy based on the pulse insertion method. In this strategy, the stator flux deviation of two modes before and after the switching point is calculated online and the switch sequence is inserted at the switching time to compensate, and the smooth transition between different modes is realized with experimental verification of validity.


INTRODUCTION
In the three-level high-power system, the ANPC three-level inverter topology has increased the replacement state due to the addition of the controllable clamp switching device, making it possible to change the loss between the switching device through a reasonable selection of the redundant switch state.Therefore, it has more application advantages in high power systems [1]- [5] .
In the existing ANPC three-level modulation strategy, synchronous space vector pulse width modulation technology (SSVPWM) has the advantages of easy computing and is easier to realize online, and is widely used in high-power transmission systems [6]- [7] .The research on the three-level SSVPWM algorithm is mostly based on the switch sequence design in the diode clamping (NPC) topology.Research on the modulation and switching strategy in the ANPC three-level structure is also rare.Considering the multi-mode modulation of the ANPC three-level inverter can meet the goals of the maximum switching frequency and WTHD optimization and mode switching involves changes in pulse mode and sampling point, a suitable switching strategy must ensure that the switching time does not have a large impact.At present, there are already switching methods, some documents have proposed the method of accurately compensating the magnetic chain deviation through the modification of the switch time.This method calculates the magnetic chain vector deviation offline and then modifies the switch to ensure the continuity of the front and rear magnetic chains.However, there are many voltage vectors in three levels.The realization of correction is tedious [8] ; Some literature proposes the concept of pulse insertion.This method is used to insert the voltage pulse that can compensate for the magnetic chain deviation to achieve a smooth transition.The article gives a detailed calculation of the duty cycle [9] ; Some literature proposes a switching method based on the switching angle of the SSVPWM algorithm for multi-mode SSVPWM algorithms.By referencing the concept of synchronous modulation switch angle to the SSVPWM algorithm, re-planning the carriers in different modes ensure the same carrier ratio before and after the switching, but in the case of a low pulse, the size of the carrier ratio is increased and the switching frequency is reduced [10] .
To solve the switching impact phenomenon that exists between different pulse numbers of multimode SSVPWM strategy of ANPC three-level inverter due to different sampling points, the proposed strategy in this paper combines the characteristics of SSVPWM algorithm to obtain stator flux vector deviation online and select ANPC redundant zero state at the switching time.Finally, by inserting the switch state, the flux trajectory is modified to compensate for the deviation to achieve the purpose of a smooth transition between different modes.The multi-mode SSVPWM switching strategy proposes in this article experiment verification, and the results show the correctness.Table 1 lists the switching state forms of three-level ANPC, in which OU1, OU2, OL1 and OL2 correspond to four redundant zero states respectively.Because the ANPC three-level inverter can well balance the loss distribution among power switching devices, this paper takes the three-level ANPC inverter as the research object.

Offline restructuring and switching
Formula 1 gives the relationship between the motor stator magnetic chain and the torque: According to motor knowledge, the stator magnetic chain of the motor has the following relationship with the electronic voltage: s () where s u is the stator voltage vector, and s i is the stator current vector and is the stator flux.When the speed of the motor is high, the stator resistance of the motor can be ignored, and the stator flux can be determined by the stator voltage.Finally, the stator flux trajectory of the steady-state optimization is 0 ( ) (0) ( ) where is the initial value of the stator flux vector.If the initial value of the stator flux trajectory is not on the steady-state optimized stator flux trajectory, it needs to be a biased correction, and its correction size is where  is the fundamental wave angular frequency of the motor, and err ψ is the stator flux track offset.
After correcting the reconstructed stator flux track offset, the final stator flux track formula is 0 ( ) (0) ( ) Similarly, if we take = ref s uu, the fundamental wave flux locus of the stator can be obtained according to Formula (6): The steady-state stator flux locus with different pulse numbers after optimization is obtained by reconstruction from Formula (6), as shown in Figure 2. To accurately compensate for the magnetic chain deviation, the magnetic chain information should be calculated online based on the SSVPWM characteristics.
The current magnetic chain amplitude can be calculated by the current phase voltage peak value and the current motor frequency, as shown in Formula (8).The phase is obtained vertically by the magnetic chain vector and voltage vector.
In the formula, m is the adjustment system; dc U is the DC bus voltage.When switching at k time, the current magnetic chain information k  is an ideal magnetic chain, and the sampling point information of the +1 k time is 1 k   .The magnetic chain vector deviation  is calculated from the two modulation mode: SSVPWM_3 mode and SSVPWM_2 mode are switched to explain the online compensation method proposed in this article.The magnetic chain information is brought in two modes (9).The magnetic chain vector deviation is calculated: In the formula, k moment SSVPWM_2 stator magnetic chain vector.After the target flux vector is obtained, if the voltage vector can be inserted at the k moment and allowed to act at the appropriate sampling time, the target flux trajectory can jump from the SSVPWM_3 mode k sampling point to the SSVPWM_2 mode +1 k sampling point, the front and rear flux can be compensated to be continuous and the impact problem can be reduced.Formula (11) explains that, in the SSVPWM method, the relationship between the deviation amplitude of the magnetic chain vector s   and the voltage vector deviation amplitude U  and s T can determine the amplitude of the voltage vector that needs to be inserted.The angle of the inserted voltage vector is the same as the angle of flux vector deviation.is the bias value of the magnetic chain.The voltage vector insertion under the SSVPWM strategy is mainly determined by the switch state.The bottom layer of the SSVPWM method for inserting the voltage vector is to adjust the number or time of the basic voltage vector synthesis, so the inserted switch state should be required to (1) avoid introducing additional flux deviation; (2) avoid simultaneous two-phase switch action.The essence that is, if the current sampling point switch status is (POO-PNO-ONO), the POP state should be selected for insertion to avoid two the same movement of the front and rear switches.

EXPERIMENTAL RESULTS
To verify the correctness and feasibility of SSVPWM online switching strategies, this article conducts experimental research.The motor parameters are in Table 1.The platform used in this article is an ANPC three-level inverter platform based on DSP28377.When the motor frequency is lower than 33.3 Hz, the asynchronous modulation is used and then switch to the synchronous modulation.The motor control method is controlled by V/F, and the bus voltage is 100 V. Due to the limitations of the experimental table, the fluctuations of the torque cannot be directly measured.Therefore, the experimental results only give the voltage current waveform.6 the current has an obvious impact and continues multiple base wave cycles.After using the online compensation switching algorithm, the waveform is shown in Figure 4.The results show that the current impact is suppressed and can achieve a smooth transition, indicating that the online compensation switching algorithm to the ANPC three-level inverter multi-mode SSVPWM strategy numbers sampling point number of the switch effectiveness of the working conditions at different times.
The above simulation results show that when switching between different sampling points of the multi-mode SSVPWM strategy, the flux vector deviation between different modes can be calculated online, and the switching state insertion method based on pulse insertion has good performance.

CONCLUSION
The ANPC three-level inverter improved the problem of uneven switching loss by clamping the controllable device.This article designs the ANPC three-level multi-mode SSVPWM switching strategy.For the multi-mode SSVPWM strategy in the transmission system, due to the different sampling points, the switching impact problem occurred between different modes when switching is between different modes which is an online fast calculation compensation method.The experiment shows that the online compensation method has a nice switch compensation effect under the switching of the multi -mode SSVPWM strategy sampling point.

Figure 1 (
a) shows the active neutral point clamped three-level topology.Figure 1 (b) shows the space vector plane diagram.

Figure 1 .
Figure 1.Anpc three-level inverter doi:10.1088/1742-6596/2625/1/012029 3 where e T is the electromagnetic torque of the motor, p n is the pole logarithm of the motor, s L is the inductance of the motor stator, r L is the inductance of the rotor, m L is mutual inductance, r  is the rotor flux, s  is the stator flux, and  is the included angle of the constant rotor flux.

Figure 2 .
Figure 2. Stator flux trajectory under different pulse numbers

Figure 4 .
Figure 4. SSVPWM_3 Switch to SSVPWM_2 with a compensation strategy

Table 1 .
Switching states of three-level ANPC converter

Table 2 .
Experimental motor parameters