**1. Introduction**

The PV-grid connected power inverter is a necessary part of the PV to electrical energy conversion system [1]. The quality of the voltage depends upon three phenomenons of voltage harmonics, voltage dips or swells and flicker [2]. In the present day, the intense use of electrical loads driven by power electronics (e.g., personal computers) has led to a severe increase of current harmonics drawn from the distribution system. These current harmonics, due to the impedance of the network, induce voltage harmonics into the utility. Voltage dips originate from fault currents in the electrical system or inrush currents of electrical motors and transformers [3]. The common mode circuit is formed in between Photo Voltaiacs (PVs) and the grid, as well as ground due to parasitic capacitance and deficiencies in galvanic isolation between the grid and PVs [1,4]. Electromagnetic interference (EMI) is the main source of unexpected transition at the output port of variable frequency drive (VFD). The fall time and rise time of semiconductor devices (employed in the converter section of VFD) are used to determine voltage transition times. These voltage transition times are around 100ns which is very fast. As a result, high *dv*/*dt* occurs. In the stray line to ground cables and capacitor, the magnitude of common mode noise current is higher if *dv*/*dt* is higher [4]. These noise currents a ffect the control signals and are the main source of EMI.

The instability and disturbances occur at the supply side due to the utilization of a higher number of power devices and components for energy conversion [5]. Mainly, non-linear devices are

responsible for this instability and disturbances. Due to this, harmonics are introduced in the power system. These harmonics causes EMI-related problems, overheating in the equipment, and damage the devices, etc. Inverter common mode voltage (CMV) and its leakage current are the primary concerns of radiated EMI. Noise with high-frequency components is emitted in the form of electromagnetic energy and may interfere with other components and equipment at the common coupling point [6]. To minimize common currents, commonly used methods are [1] improved power inverter structures with common mode current (CMC) suppression capabilities and advanced pulse width modulation (PWM) schemes [7], and (2) the addition of EMI filters [8] and bridge inverter topology based on DC and AC bypass [9]. It is also notable that the circuitry with fast switching semiconductors produces a very high amount *di*/*dt* and *dv*/*dt* and which is one of the reasons for the cause of EMI [10]. Decoupling effect-based configurations with constant CMV [11] and transformerless power converter configurations [12] are proposed to suppress CMC. The CM loop impedance can be increased in order to suppress current in CM loop effectively. In [13], the mid-point of DC and AC side voltage nodes are connected to the proposed new CM internal loop scheme in order to suppress CMC. In [14], a novel modulation scheme is presented to control inverter power switches in order to reduce CMC. In [15], a CM internal loop is formed by employing the RC branch in between the negative bus of DC and output terminals. However, detailed analysis of the CMC and its effect on CMC high-frequency components are not presented. In [16], a new scheme based on a dual CM internal loop for a PV grid-connected transformer-less system is proposed to suppress CMC high-frequency components. In [17] characteristics and analysis of the CMV based on the simplified modelling of a cascaded H-bridge power transformer and PWM strategy are presented under the fault grid condition and balanced condition. In [18], in order to reduce CM leakage current, a new hybrid modulation strategy is suggested. The suggested method is efficient and has reactive power provision with low distortion in the current. Filters at the input/output terminal are employed to suppress this unwanted emission or electromagnetic interference (EMI) [19,20]. Generally, filters are employed at the connector of power supply in order to restrict disturbance signals [21,22]. Generally, classical filters are designed by utilizing passive components, i.e., inductor and capacitor values to attenuate high-frequency voltage and current components [23,24]. However, the performance of the filter is dependent on the value of L and C and has limited achievable insertion loss that should be improved to meet the necessary condition. Moreover, passive filters are bulky, costly, and their volume mainly depends on the inductor size which is approximately directly proportional to the required attenuation. Moreover, there is always uncertainty in parasitic components. In order to reduce CMV and current effect, active [25] and passive filters [26] are suggested. However, these filters increase the size, cost of the system, and control of the equipment. As a result, it is good to advance control strategies in order to reduce the CMV's effect. In order to eliminate or reduce CMV, numerous control schemes are proposed based on the modulation strategy, such as the Sine PWM for three-level neutral point clamped (NPC) inverter [27], PWM based on non-nearest vectors, Space Vector PWM for high-level inverters [28,29], etc. In [30], a detailed comparison of SPWM and SVPWM techniques are presented for the three-phase inverter. Nevertheless, similar PWM schemes presented in [31,32] are restricted to 3–5 levels inverters. Synchronous reference frame and feedforward reference frame-based dynamic voltage restorer comparative study are presented in [33]. In [34], a new SVPWM scheme is presented with advanced features, such as the proposed scheme, which is suitable for inverters with any number of levels, zero CMVs can be achieved at any modulation index, the simple realization of CMV vectors, fast control scheme, etc. In [35], a new methodology called the spread spectrum (SS) technique is presented in order to reduce EMI of power converters over a wide range of frequency. However, this SS technique is competent to reduce EMI levels around 5–10 dB [36]. Based on the output voltage waveform and its alignment, another technique is suggested in [37,38]. However, this methodology has a specific application. In [39], a new technique based on software is presented to reduce EMI. The methodology is suitable for single- and three-phase power inverters. In [40], analysis on the CM EMI and methodology to mitigate EMI in power inverters is discussed by using a delay compensation technique. In order to mitigate EMI, numerous passive filter methods are proposed based on the phases of the noise signal. In [41], a common mode-coupled inductor is designed in order to mitigate common mode noise. Nevertheless, di fferential mode noise is not able to be reduced by using this technique. Hence, later, a new method based on an integrated choke is presented in order to reduce di fferential, as well as common mode, noise at the same time [42].

In [43], a new method based on the parasitic component's determination is presented in order to predict EMI noise. In [44,45], computer-based three-dimensional modelling is presented for the noise current prediction by determining the value of parasitic components. A novel EMI filter is discussed with su fficient attenuation with a limited LC value in order to CM EMI [46]. Nevertheless, filters are additional components and increase the volume and cost of the system. Moreover, the implementation of the e ffective filter is important and the e ffort for the mitigation of EMI without knowing the system may degrade the performance and require additional cost. To reduce CM voltage, a new impedance balancing method is presented instead of impedance mismatching [47]. The three-phase phase-lock-loop for a distorted utility is discussed in [48]. In [49], controllable devices are used and active filters are presented to suppress CMC generated by the CMV method, called the active noise cancellation scheme, in order to mitigate the noise signal [50]. In [51], a new wavelet transform-based technique to mitigate EMI noise in power converter is presented with a frequency band of 3–30 MHz.

In this paper, EMI mitigation techniques are investigated with the aim to reduce CMV and CMC in a PV-grid tied power inverter. The e ffect of modulation techniques over CMV and CMC are investigated for two-level and three-level inverters to observe the mitigation of EMI. The modified third harmonic injection method reduced the CMV and CMC by 60% in the system. In order to reduce distortion and to improve the overall e fficiency of the system, the modified PWM technique is employed along with EMI chokes.

The article is organized in the seven sections, discussing explicitly the important aspects for investigations and design of EMI filters for the mitigation of CMC and CMV in grid-tied inverter system. In Section 2, the modulation techniques for high power two-level and three-level inverters, are discussed in brief. Section 3 explains the concept of CMV of the inverters and design of filters or EMI chokes. Section 3 also deals with the comparative study of space vector-based and sine-based pulse width modulation (SVPWM and SPWM) techniques. A modified PWM strategy is discussed in Section 4. The results obtained through simulation and experimental works are presented in Section 5. Finally, the conclusion is given in Section 6.

### **2. Modulation Techniques for High-Power Inverters**

Space vector-based and sine wave-based PWM techniques are the common techniques used to generate pulses for the switches of the inverter [29,30]. In the SPWM technique, high-frequency triangular carrier waves (typically several kHz) are compared with the modulating signal (50 Hz or 60 Hz) to generate pulses for a three-phase inverter. In the SVPWM technique, instead of modulating signals, a rotating vector reference is used to generate pulses of the inverter [29,30]. The prime objective of this control and pulse generation scheme is to generate a sinusoidal AC output whose magnitude is limited. The PWM switching scheme not only helps to achieve reduced Total Harmonic Distortion (THD), better harmonic spectrum, and maximum utilization of DC bus but also provides a solution to reduce EMI, switching loss. Figure 1 depicts the power circuitry of three-phase two-level inverter and neutral point clamped (NPC) three-level inverters [21] and its PWM strategies shown in Figures 2–5.

**Figure 1.** Power circuitries of (**a**) three-phase two-level inverter, and (**b**) neutral point clamped (NPC) three-level inverter.

 **Figure 2.** Reference generation through Min–Max modulation strategy (**a**) associate waveform, and (**b**) mathematical model.

The Min–Max modulation strategy is supposed to be equivalent to the space vector modulation strategy. Thus, using this approach, the modulation index can be extended up to 1.15. Figure 2a,b explains the associated waveforms and mathematical model of the reference waveform generation technique through Min–Max modulation strategy, respectively [48]:

$$offset\,\text{in}\,(\text{V}) = -\left[\frac{\max(V\_{A\prime}, V\_{B\prime}, V\_{C}) + \min(V\_{A\prime}, V\_{B\prime}, V\_{C})}{2}\right] \tag{1}$$

where:

$$\begin{cases} V\_A = V\_m \cos(\omega t) \\ V\_B = V\_m \cos\left(\omega t - \frac{2\pi}{3}\right) \\ V\_C = V\_m \cos\left(\omega t + \frac{2\pi}{3}\right) \end{cases} \tag{2}$$

$$V\_{ref} = V\_A + V\_B + V\_C + offset \tag{3}$$

The reference modulating signals are mathematically defined as follows:

$$\begin{cases} V\_{Am} = V\_m \cos(\omega t) + offset \\ V\_{Bm} = V\_m \cos(\omega t - \frac{2\pi}{3}) + offset \\ V\_{Cm} = V\_m \cos(\omega t + \frac{2\pi}{3}) + offset \end{cases} \tag{4}$$

**Figure 3.** Modulating signal of third harmonic injection control technique (**a**) modulating signal, and (**b**) mathematical model.

In the third harmonic injection method [52,53], the third harmonic is injected in a modulation scheme to improve the gain of the pulse width modulator in the inverter. Figure 3a,b shows the third harmonic injection modulating signal control reference waveform and mathematical model, respectively.

The reference modulating signals are mathematically defined as follows:

$$\begin{cases} V\_{Am} = V\_m \cos(\omega t) + V\_{m3} \cos(3\omega t) \\ V\_{Bm} = V\_m \cos(\omega t - \frac{2\pi}{3}) + V\_{m3} \cos(3\omega t) \\ V\_{Cm} = V\_m \cos(\omega t + \frac{2\pi}{3}) + V\_{m3} \cos(3\omega t) \end{cases} \tag{5}$$

In this technique, approximately 17% of third harmonics components are added in the reference waveform of classical SPWM [29,30,34]. The reference waveform of the method third harmonics injection can be also expressed as follows:

$$f(\omega t) = (1.15M\_d \times \sin(\omega t) + 0.19M\_d \times \sin(\omega t)), \ 0 \le \omega t \le 2\pi \tag{6}$$

where *Ma* is the modulation index ratio.
