Reduction of total harmonic distortion of three-phase inverter using alternate switching strategy

Nur Arifah Ramli, Auzani Jidin, Zulhani Rasin, Tole Sutikno Faculty of Electrical Engineering, Universiti Teknikal Malaysia Melaka, Malacca, Malaysia Research Laboratory of Power Electronics and Drives, Universiti Teknikal Malaysia Melaka, Malacca, Malaysia Department of Electrical Engineering, Universitas Ahmad Dahlan, Yogyakarta, Indonesia Embedded System and Power Electronics Research Group, Yogyakarta, Indonesia


INTRODUCTION
A three-phase inverter is the main part in alternating current (AC) electrical drives, which is used to produce desired magnitude and frequency of AC voltages from an unregulated or a regulated direct current (DC) voltage. The DC voltage is obtained from a rectifier or a series of batteries. Along with power electronic controller and electrical machine, AC motor drives is well known through all sectors in the industry. In terms of system efficiency, controllability and overall performance, it can guarantee for excellent performances. Figure 1 illustrates the use of three-phase inverter in AC motor drives, in which the inverter can be performed as voltage source inverter (VSI) and current source inverter (CSI). Several AC drives perform the inverter as VSI, for examples in direct torque control (DTC), space vector modulation (SVM). The application of VSI can be applied in both single-phase and three-phase applications [1]. Usually, VSI takes place of the full converter in most applications [2]. They are widely used in power generation applications, three-phase UPS systems and islanded operation of three-phase system, where the balanced three-phase voltage output is required when the loads are unbalanced [3]. On the other hands, there are many AC motor drives perform the inverter as CSI, such as field-oriented control (FOC) and induction motor. CSI is very rarely implemented alternative to the VSI but quite well known for its function [4]. Nevertherless, the significant practical application of CSI regarding of self-commuted variant is not found yet [5]. Some applications have applied CSI with independent dc link to balance the dc-link currents with proper control of the rectifiers [6]. The output of inverter is generated by the AC voltage that can be a variable or constant depending on the requirements of the applications [7]. The AC output is controlled independently by VSI that behave as a voltage source that is required by many industrial applications. On the other hand, CSI produce high quality current waveforms that are widely used by medium-voltage industrial applications. In order to produce a smooth current waveform, VSI needs to generate AC output voltage waveform that is composed with the discrete values so that the load can be inductive at the harmonic frequencies. VSI will generate large current spikes in a capacitive load that is prevented by inductive filter between the AC side of VSI and the load. In the conjunction of the VSI, CSI produced large voltage spikes in the inductive load. If this happened, capacitive filter will be used between CSI at the AC side and the load [8]. To control the inverter, some aspects need to be taken into account such as the efficiency of the inverter. This include the switching losses and harmonic reduction that principally depended on the modulation strategies [9]. The inverter control techniques are based on the fundamental frequency and high switching frequency.
The regular pulse width modulation (PWM) modulation methods can be divided into two class which are open loop and closed. Open loop is applied for VSI while closed loop is usually used for the CSI. This proposed research is mainly focused on open loop which is VSI. Open loop consists of PWM techniques such as sinusoidal pulse width modulation (SPWM) [10], space vector pulse width modulation (SVPWM) [11] and third harmonic pulse width modulation (THPWM) [12]. The modulation methods developed to control the multilevel inverters that are based on multi-carrier orders with PWM. SPWM is the most common control schemes that control the multilevel inverters. In conventional SPWM, a sinusoidal reference waveform is compared with a triangular carrier waveform to generate switching sequences for power semiconductor in inverter modules. The fundamental frequency SPWM control method was proposed to minimize the switching losses. The multi-carrier SPWM control methods are also implemented to increase the performance of multilevel inverters and are classified according to vertical or horizontal arrangements of carrier signal [13]. Its switching scheme is easy to implement in both analogue and digital circuit. In the simplest form, it involves the high switching frequency triangular carrier voltage with a sinusoidal modulating signal that represents the desired fundamental component of the voltage waveform [14]. The peak magnitude of the modulating signal should remain limited to the peak magnitude of the carrier signal. The proposed method is developed by considering the side band harmonics and modulation bandwidth in Simulink during simulation. The experimental verification of the modeled system is analyzed by controlling the IGBT modules with developed SPWM algorithm generated by high switching strategy. SVM or SVPWM was originally developed as vector approach to PWM for three phase inverters. It is a more sophisticated technique for generating sine wave that provides a higher voltage to the motor with lower total harmonic distortion. The main aim of any modulation technique is to obtain variable output having a maximum fundamental component with minimum harmonics. SVPWM method is an advanced computation intensive PWM method [15]. Despite the advancement, the analysis of the SVPWM contained a complex implement in the software compared to SPWM [16]. Even though the SVPWM has more advantage of less harmonic content, the differences between the two modulations are small. Thus, with intangible differences, SPWM is chosen as the circuit is much simpler than SVPWM when executed in the software.
The sinusoidal PWM is the simplest modulation scheme to understand but it is unable to fully utilize the available DC bus supply voltage. Due to this problem, the THPWM technique was developed to improve the inverter performance. The sinusoidal PWM technique causes decrease maximum achievable output voltage. In this case, an increase of maximum achievable output voltage is studied. Hence, by simply adding a third harmonic signal to each of the reference signals, it is possible to obtain a significant amplitude increase at the output voltage without loss of quality [17]. It is remarkable that the reference signal resulting from the addition of the third (V3) and first harmonic (V1) is smaller in amplitude than the first harmonic. At the output, the obtained amplitude of the first harmonic is equal to the amplitude of the first harmonic reference. Note also that the third harmonic is not seen at the output voltage. However, the phase of the third harmonic injected must be strictly synchronous with the phase of the reference voltage. The synchronization will cause the difficulty of the injected third harmonic generation [18]. Thus, SPWM technique is more lenient compared to the THPWM to obtain the THD voltage of the motor.
The growth in AC motor drives is drastically as over the past years due to the following factors: -Rapid technology development of microprocessor in calculating/executing complex algorithms [19], -Rapid technology development of solid state switching devices which able to operate the switching at higher power and switching frequencies [20], -High demand for industrial applications, e.g. electric vehicle application fraction drives, automation industry [21]. Based on the factors above, in addition, there are several researchers used the fully optimization of the inverters has invented the topologies that undergone constant change which the first was CSI followed by VSI based on application [22]. AC motor drives give benefits in term of process control, system stress and energy savings [23]. It enables the users to control the speed of motor at a constant speed. The process output needs to match the different synchronizing part as to lock the flow become smooth between sub processes in the process control. AC drives do have its own disadvantages. It tends to have complexity in the circuit and costly compared to other motor. The horsepower rating expensive is expensive as it has a modern technology build inside the drives [24]. For an example, if the AC motor tends to rotate faster than the command AC drive, the motor itself will acts as regenerates that it can pump energy into drive again. The drive will short circuit if the energy that are regenerated cannot absorb this energy which can lead audible noise.
In this research method, the main concern is to minimize the Total Harmonic Distortion (THD) of output voltage as well as output current, and filter size. Based on the research report in [25], THD can be minimize by operating the inverter at high switching frequency without degrading IGBT switching performance . In such way, the undesired harmonic components can be located at higher frequencies and hence small filter size can be effectively eliminate the harmonic components. In this paper, IGBT is chosen due to its advantage which can operate at higher power capability [26]. However, IGBT cannot be operated at very high switching frequency, i.e. above 50 kHz due to its thermal restriction as reported in [27]. This paper provides a simple solution to enable the use of IGBT to operate at very high switching frequency without degrading its switching performances.
The objective of this paper is to study, implement and improve the performances of AC drives using alternate switching and high switching frequency strategy. The main objectives of this research are: -To reduce THD of output voltage and current and filter size by operating the inverter at high switching frequency without degrading IGBT switching performances. -To design an 'Alternate Switching Strategy' using analog and digital ICs, which allows IGBT based inverter to operate at high switching frequency. -To verify the improvements of the inverter (i.e. reduction of THDs) through simulation and hardware results.

PROPOSED ALTERNATE SWITCHING STRATEGY
In order to improve AC drive performances, specifically in reducing output phase voltage THD, it is necessary to operate the inverter at very high switching frequency. As discussed above, the use of IGBT in the inverter circuit is strongly recommended due its high-power operation capability. However, the IGBT cannot be operated at very high switching frequency, i.e. above 50 kHz. To enable the three-phase inverter using IGBTs to operate at very high switching frequency, an alternate switching strategy is proposed. Generally, in this strategy, the conventional inverter circuit needs to be modified and the switching frequency as well as switching states for switching each IGBT is splitted such that the IGBT switching frequency is one-fourth of the inverter switching frequency.

Modification of three-phase inverter circuit
The modification of the inverter circuit with additional IGBTs can be illustrated in Figure 2. For example, the top IGBT in Leg 3 (or c-phase) of the conventional three-phase inverter, i.e. Q5 is represented as a group of four IGBTs in the modified inverter which are connected in parallel, i.e. Q5a, Q5b, Q5c and Q5d. This means every single IGBT in the original inverter circuit is now being replaced with a group of four IGBTs. Therefore, the modified inverter circuit consists of 24 IGBTs. In this way, the inverter switching frequency can be divided into four, so that every IGBT in a group has a one-fourth of inverter switching frequency. For examples, if the inverter switching frequency is chosen as 200 kHz, therefore every IGBT in each group has a switching frequency of 50 kHz. It shoud be noted that the inverter still operates at switching frequency of 200 kHz as each IGBT in a group is switched alternately, i.e. one-by-one and one IGBT is switched at one time. Therefore, the switching operation of IGBT is prevented to operate beyond its limitation and at the same time the reduction of high frequencies components of output voltage as well as output current can be obtained with smaller size of filters.

Generation of switching states for alternate switching
This section explains the proposed alternate switching strategy that allows the inverter to operate at very high switching frequency for improving AC drive performances. For simplification, the explanation of the proposed strategy will only consider for switching alternately for four IGBTs in a group, e.g. as illustrated in Figure 2. Figure 3 shows the block diagram and timing diagram of the proposed alternate switching strategy for switching IGBTs Q5a, Q5b, Q5c and Q5d in the modified inverter with their respective switching states S5a, S5b, S5c, and S5d. The block diagram of the proposed strategy has two main parts, i.e. two-bit counter and demultiplexer. The proposed alternate switching strategy employs a 1-to-4 demultiplexer and a two-bit counter, where the input PWM signal Sx=S5 is obtained from PWM generator and the output signals, i.e. S5a, S5b, S5c, and S5d are used to control the switching operation of IGBTs. The operation of the proposed 1602 alternate switching strategy to produce alternate swiching states can be illustrated by timing diagram in Figure 3(b). Figure 4 shows a simulation model of two-bit counter using Gray Code and 1-to-4 demultiplexer for producing alternate switching states, e.g. S5a, S5b, S5c, and S5d. The simulation model requires two inputs which are a S5 (PWM) signal and enabling signal (i.e. designated as ENB), to produce two-bit output, i.e. A1 (MSB) and A0 (LSB). These two-bit outputs are used to index the selection table given in Table 1, for splitting a PWM signal into four switching states in demultiplexer operation. Note that the enabling signal is set to logic 1 to activate the counting operation.  The two-bit counter employs two D Flip-flop and a NOT logic gate, and 1-to-4 demultiplexer employs a set of logic gates which is constructed based on Boolean expression given in (1).
Where the Boolean expression for each switching state is functioned of PWM signal and two-bit address, which can be obtained from the demultiplexer operation given in Table 1.

EXPERIMENTAL SETUP
This section discusses the experimental setup of the proposed alternate switching strategy for minimizing THD of output voltage. Figure 5 shows an electronic circuit of the proposed alternate switching strategy for controlling four switches in a group, i.e. S5a, S5b, S5c, and S5d. The two-bit counter with Gray code is implemented using integrated circuit (IC) model: MC74HC74ADG, and 1-to-4 demultiplexer is implemented using IC model: MC74AC139DG. The input of the circuit is a PWM signal, e.g. S5 and this signal is splitted into four switching states, i.e. S5a, S5b, S5c, and S5d, where each switching state performs at one-fourth of the PWM signal frequency. Figure 6 shows two photos of the experimental setup of the proposed alternate switching strategy. Specifically, the modified three-phase inverter circuit consist of 6x4 IGBTs, gate driver and the proposed alternate switching circuits is shown in Figure 6(a), while the complete experiment setup is shown in Figure  6(b). A three-phase resistor bank is used as a loading unit and the PWM signals are generated from dSPACE1104 controller board.

RESULTS AND DISCUSSION
Some experiments are carried out to validate the THD reduction in the proposed method. The experiments are carried out by applying Sinusoidal pulsewidth modulation (SPWM) technique for three different inverter switching frequencies, i.e. fsw=5 kHz, fsw=10 kHz and fsw=50 kHz. Each experiment applies a dc voltage of 50 V and a simple low pass filter with cut-off frequency, fc=10 kHz. Figures 7 and 8 show experiment results of output phase voltages and their frequency spectrums, before and after filtering actions, for three different switching frequencies. From these figures, some observations can be made which can be stated are: -The wave shape of output phase voltage is closely to sinusoidal wave shape after filtering action applied, particularly at the highest inverter switching frequency, i.e. fsw=50 kHz, -Harmonic components are located at higher frequencies when the inverter switching frequency increases, before and after filtering action applied, -Output phase voltage seems to contain insignificant harmonic components after filtering action is applied for each inverter switching frequency, and -The fundamental component of output phase voltage after filter is applied is less than that produced before the filter is applied due to low pass filter gain. Thus, it obviously shows that the quality of output phase voltage waveform improved if the inverter operates at higher switching frequency, and it is significantly improved after filter applied. Table 2 shows the THD of output phase voltages corresponding to the results obtained in frequency spectrum shown in Figure 8. From the table, the THD of the output phase voltage decreases when the inverter switching frequency increases. Obviously, the THD of output phase voltage is significantly minimized to 10.15% after the filter is applied and when the inverter operates at the highest inverter switching frequency. This is because the filter with low cut-off frequency can effectively eliminate and minimize harmonics at higher frequencies. Therefore, this clearly shows that the improvement of output phase voltage waveform is due to low THD as given in Table 2. Moreover, this also highlights that less filter size is required to minimize THD for improving output phase voltage waveform if the inverter operates at the highest switching frequency.

CONCLUSION
This paper has proposed an improvement of THD of output phase voltage by operating the inverter at high switching frequency. In practice, IGBT power switch is not recommended to be applied at high switching frequency operation due to its limitations, i.e. it cannot operate above 50 kHz and thermal restriction. This paper has suggested to apply the proposed alternate switching strategy so that IGBTs can be employed for high inverter switching frequency. In this strategy, the inverter operates at high switching frequency, e.g. 200 kHz, however, each IGBT in a group of four switches in the modified inverter circuit is switched alternately, and as a result each IGBT operates at one-fourth of the inverter switching frequency, e.g. 50 kHz which is still below its maximum frequency. Some experiment results have verified the effectiveness of the proposed alternate switching operation to improve output phase voltage wave shape as well as THD performance. The main benefit of the proposed method is to facilitate the use of IGBT in high power applications, such that it can be used for operating the inverter at high switching frequency for improving THD performances.