Abstract

An isolated new hybrid scheme employing a doubly fed induction generator (DFIG) fed by solar power and grid integrated with a simple three-phase squarewave neutral point clamped (NPC) inverter has been proposed. This works as an uninterruptable power source that can feed a DFIG in all cases. The primary source of power is SPV and shortage power can be provided by the grid. The distinctive "integrated" strategy removes the complexity of designing a boost converter, battery controller, rectifier and natural point clamped (NPC) inverter and reduces the number of sensors and electronic components. In this approach, the simplest way of modeling is introduced. The presence of a grid-connected rectifier in parallel (with the SPV source) ameliorates the quality of power fed into the DFIG by curtailing the voltage dip in the SPV output. Also, the other benefits of the proposed system are high-reliability, compact size and low cost, which makes the system suitable for real-time simulation. All the analytical, simulation results of the present research are presented.

Keywords: solar PV, MPPT, converters, NPC inverter, doubly fed induction generator

## 1. Introduction

It will be very painful for you to spend slightly more on your electricity bill if you knew it was sourced from the solar photovoltaic panel of your neighbor's roof. Since the quantity of energy presented from non-renewable sources is finite, renewable sources can be an alternative option for power generation [1]. Solar photovoltaic cells and Doubly Fed Induction Generators are green energy efficient sources with minimal environmental impact [2]. The wound rotor is usually fed by the stator or rotor, which is why it is frequently named a doubly fed induction generator (DFIG) in the literature [3, 4]. The different aspects of the hybrid system are that power conversion efficiency can be improved, overall cost of the system can be reduced, the power quality is improved and more reliable and utilization of power is optimized. Variation in grid injection power can be optimized. The rating of the SPV cell is taken higher so that the variation of grid injected power and reverse power flow can be minimized at the point of common coupling (PCC). Since this system does not allow for reverse

An Integrated Hybrid Renewable Energy System Based on Doubly Fed Induction Generator… DOI: http://dx.doi.org/10.5772/intechopen.81083

power flow at the PCC, because the boost converter is situated at the output of the PV and the rectifier is situated at the output of grid, the boost converter will not allow current in the reverse direction [2].

## 2. Power conversion system

The schematic diagram in Figure 1 shows the complete technique. The MPPT can enhance up to 30% of energy compared to the standard photovoltaic panel and it is externally connected to the circuit. The main purpose of an MPPT is to change its input voltage, which is also the PV panel input voltage, so that it corresponds to the voltage at which the panel delivers maximum power. At its output, the MPPT always provides the voltage required by the battery or machine pump load [2]. The perturb and observe based MPPT has the following limitations.


The proposed system planned to use the software Typhoon HIL [4]. Depending on application and requirement many types of induction generator are available in the Market. The designing of the induction generator is not a tedious task but controlling of Wound rotor induction generator, for wind energy conversion system, is a challenging task for electrical engineering fraternity [5].

#### 2.1 Design aspect in proposed wind energy conversion system configuration (WECS)

The unpredictable nature of wind energy has forced us to design a hybrid system so that the stored energy from the solar power or grid can be fed to the rotor of doubly fed induction generator at the time of sub-synchronous mode. The relationship between wind velocity and output power of turbine is nonlinear. The output power can be represented as follows [3].

$$P\_m = \mathbf{0}.\mathbf{5} \ast \mathbf{C}\_p(\boldsymbol{\lambda}, \boldsymbol{\beta}) \ast \mathbf{\hat{p}} \ast \mathbf{A} \ast \boldsymbol{\nu}^{\boldsymbol{\beta}} \tag{1}$$

Figure 1. Complete representation of proposed technique.

where Pm is the output power of the turbine, ƥ is air density, Cp is the power coefficient, ν is the velocity of wind, β is the pitch angle, λ is the tip speed:

#### 2.2 Doubly fed induction generator

Since wind is an unpredictable source of energy, the slip s and rotor voltage amplitude can be defined as:

$$\mathbf{S} = \frac{(\mathbf{N}\_s - \mathbf{N}\_r)}{\mathbf{N}\_t} \tag{2}$$

$$V\_r = \mathbb{S} \ast T\_{sr} \ast V\_s \tag{3}$$

where Tsr is voltage transformation ratio between stator and rotor, S is a slip, Vs is the stator voltage, Vr is the rotor voltage, N<sup>s</sup> is the synchronous speed, N<sup>r</sup> is the rotor speed.

The active power delivered to the rotor and the mechanical power delivered to the shaft of the generator can be calculated as:

$$P\_r = -\mathbf{S} \ast \mathbf{P}\_\mathbf{s} \tag{4}$$

$$P\_m = (\mathbf{1} - \mathbf{s}) \ast P\_s. \tag{5}$$

where Pr is the rotor power, Ps is the stator power and Pm is the mechanical power:

The universally accepted method of driving a mathematical model of DFIG is to convert the synchronously rotating stator flux vector in terms of quadrature axes and direct axes. A simplified mathematical model would help with behavior analysis.

Figure 2 represent an equivalent circuit for the DFIG in the synchronous reference frame [3].

$$\mathbf{v\_{qds}} = \mathbf{r\_s} \mathbf{i\_{qds}} + \frac{\mathbf{d} \left(\boldsymbol{\Psi\_{qds}}\right)}{\mathbf{d}t} + \mathbf{j \,\,\alpha\_e \,\,\boldsymbol{\Psi\_{qds}}} \tag{6}$$

$$\mathbf{v\_{qdr}} = \mathbf{r\_r} \mathbf{i\_{qdr}} + \frac{d\left(\Psi\_{qdr}\right)}{dt} + \mathbf{j} \,\mathrm{o\_e} \,\Psi\_{qdr} \tag{7}$$

$$\Psi\_{\rm qds} = \mathbf{L\_m}\mathbf{i\_{qdr}} + \mathbf{L\_m}\mathbf{i\_{qds}}\tag{8}$$

$$
\Psi \mathbf{\dot{q}\_{qdr}} = \mathbf{L\_m} \mathbf{i\_{qds}} + \mathbf{L\_m} \mathbf{i\_{qdr}} \tag{9}
$$

$$\begin{split} \mathbf{Te} &= \frac{3}{2} \frac{p}{2} \text{ Re} \left[ \mathbf{j } \boldsymbol{\Psi}\_{\text{qds}} . \overline{i\_{qds}} \right] \\ &= \frac{3}{2} \frac{p}{2} \text{ Re} \left[ \mathbf{j } \boldsymbol{\Psi}\_{\text{qdr}} . \overline{i\_{qdr}} \right] \end{split} \tag{10}$$

Figure 2. Complex synchronous equivalent DFIG.

An Integrated Hybrid Renewable Energy System Based on Doubly Fed Induction Generator… DOI: http://dx.doi.org/10.5772/intechopen.81083

Figure 3. Basic schematic boost converter with ideal switch.

The stator side active and reactive powers are given as:

$$P\_s = \frac{3}{2} \text{ Re} \left[ \mathbf{V\_{qds}}, \overline{\mathbf{i\_{qds}}} \right] = \frac{3}{2} \left( \mathbf{v\_{qs}} \mathbf{i\_{qs}} + \mathbf{v\_{ds}} \mathbf{i\_{ds}} \right) \tag{11}$$

$$Q\_s = \frac{3}{2} \text{ Im} \left[ \mathbf{V\_{qds}}, \overline{i\_{qds}} \right] = \frac{3}{2} \left( \mathbf{v\_{qs}} \,\mathbf{i\_{qs}} - \mathbf{v\_{ds}} \,\mathbf{i\_{ds}} \right) \tag{12}$$

where. iqdr and iqds are the complex conjugates of the rotor-current and statorcurrent space vectors. rs, rr are stator, rotor resistances (ohms), ψqds, ψqdr are d and q axes stator and rotor flux (wb), vqds, vqdr, are d and q axes stator and rotor voltages (Volt), iqds, iqdr, are d and q axes stator and rotor current (Amp), torque is denoted by Te, Ls, Lm is self and magnetizing inductance (henry), p is the number of poles per phase. Ps is the stator side active power, Qs is the rotor side reactive power.

#### 2.3 Power electronics converters

The rectifier and NPC inverter can be used to convert DC in to AC and AC in to DC and the boost regulator can boost the output of the photo voltaic cell. The boost converter is shown in Figure 3.

The output of the boost can be given by Vo.

$$V\_o = \frac{V\_s}{(1-\alpha)}\tag{13}$$

α is duty cycle, and Vs is input voltage, here it is solar panel voltage.

#### 3. Simulation results and discussion

The model of Figure 1 was developed and the results are presented in Figure 4 as a real time output of the complete model. The simulation ran for time interval t = 1 s and the sample rate was 1MSPS sample per second. The various observations were recorded, such as the wave form of stator voltage Vs (abc), stator current Is (abc), rotor side converter current Ir (abc), output voltage of solar photo voltaic cell

Figure 4. Simulation result real time complete output.

#### Figure 5.

Simulation result stator side output voltage.

Figure 6. Simulation result stator side output current.

Figure 7. Simulation result rotor side converter current.

Figure 8. Output voltage of NPC Inverter.

boost converter output voltage and EMF of battery. The wave form of the stator side voltage also showed that the output voltage MPPT was 45.60 Volt. The output voltage of the battery was 540 Volt and the battery current was 2.37 amp and the power provided by the battery was 1.3 kW (Figures 5–8).
