Advanced space vector modulation with “fractional” power cells

The paper develops a model and studies various operating modes of a 5-phase multi-level cascade inverter as part of high-voltage powerful variable-frequency drives (VFD) with the most typical fan load for this type of electric drives. The aim of the work is to test the efficiency of the balanced spatial-vector pulse-width modulation method in a multi-phase high-voltage multi-level inverter in emergency modes by simulating the electric drive in the MATLAB/SIMULINK/Simscape Electrical environment. The paper studies the features of using the balanced spatial-vector pulse-width modulation method in a five-phase frequency converter in normal and emergency modes and, especially, the differences from a three-phase inverter with a similar control principle. The model with a 5-phase synchronous motor from the MATLAB ac8_example.slx database is taken as a basis, which allows us to consider the simulation results reliable. The rules for calculating phase voltages in normal and emergency modes are formulated and the corresponding blocks of the inverter control system model are prepared - a block for calculating basic vectors and a block of spatial vector pulse-width modulation (PWM). The output signals of this block are used for further processing in the modules for controlling power cells with 2- or 3-level PWM. Such a model for a 5-phase electric drive has been built for the first time, which is a scientific novelty of the work. Calculations of transient processes showed the absence of any oscillations, shocks when the system switches from normal to emergency mode and vice versa. That is, the proposed method of balanced spatial vector modulation preserves the symmetry of electromagnetic fields in the engine when individual H-bridges are damaged. It is shown that in normal mode, a 5-phase inverter using the spatial vector modulation method allows increasing the utilization factor of voltage sources by 23.1%. When individual power cells in phases fail, the utilization factor may decrease, but in any case it remains greater than 1. The method ensures compensation for emergency damage even in the event of a short circuit of one or two inverter phases. This significantly increases the service life of the electric drive, which is especially important in critical mechanisms and technological processes, where such complex inverters are actually used. The study found that when using the proposed method in a 5-phase VFD, unlike a 3-phase one, the current balance in the load is disrupted, which leads to increased heating of individual phases of both the motor and the power section of the inverter. The second point of scientific novelty of the work is that in order to reduce this negative effect, it is proposed to improve the algorithm for calculating the output coordinates of the regulator by using the so-called “fractional” power cells with a voltage that is not a multiple of their rated voltage. This leads to a smooth change in phase shifts and asymmetry when individual H-bridges and even one or two phases are damaged. This, in turn, reduces the current imbalance and the corresponding overheating from 2...3 to 12...15%, which gives confidence in the efficiency of using this improvement. It is also important to note that such a change in the control algorithm does not require any changes in the circuit and does not increase the requirements for the computing power of the processors used. • It was developed a model in the paper and various modes of operation of a 5-phase multilevel cascade inverter as a part of high voltage powerful Variable Frequency Drives (VFD) were studied. The purpose of the work is a verification of the efficiency of the balanced space vector pulse-width modulation method in a polyphase high-voltage multilevel inverter in emergency modes by simulating an electric drive in MATLAB/SIMULINK/Simscape Electrical. • The paper presents the features of using the method of balanced space-vector pulse-width modulation in a five-phase frequency converter in normal and emergency modes, taking into account the mathematical and geometric interpretation of the motor stator as a symmetrical 5-phase load. The model with a 5-phase synchronous motor from the MATLAB ac8_example.slx database was used as a basis, which allows the simulation results to be considered reliable. The rules for calculating phase voltages in regular and emergency modes have been formulated and the corresponding blocks of the model of the inverter control system have been prepared. These values are used for further processing in power cells control modules with 2- or 3-level pulse-width modulation (PWM). Such a model for a 5-phase electric drive was built for the first time, which is a scientific novelty of the work. • Calculations of transient processes showed the absence of any oscillations, shocks when the system transitions from normal mode to emergency mode and vice versa. That is, the proposed method of balanced space-vector modulation preserves the symmetry of electromagnetic fields in the engine when individual H-bridges are damaged. It is shown that in regular mode, a 5-phase inverter using the space vector modulation method allows to increase the utilization factor of voltage sources by 23.1%. If individual power cells in phases fail, the utilization factor may decrease, but in any case, remains greater than 1. The method provides compensation for emergency damages even in case of short-circuiting of one or two phases of the inverter. This significantly increases the durability of the electric drive, which is especially important in responsible mechanisms and technological processes, where, in fact, such complex inverters are used. • In the course of the study, it was established that when using the proposed method in a 5-phase VFD, unlike a 3-phase one, the balancing of currents in the load is disturbed, which leads to increased heating of individual phases of both the motor and the power part of the inverter. The scientific novelty of the work is that in order to reduce this negative effect, it is proposed to improve the algorithm for calculating the output coordinates of the regulator by using the so-called ”fractional” power cells with a voltage not multiple of their nominal voltage. This leads to a smooth change of shifts and asymmetry of phases in case of damage to individual H-bridges and even phases. This, in turn, reduces the imbalance of currents and the corresponding overheating from 2...3 to 12...15%, which gives confidence in the effectiveness of using this improvement.