4. Closed-loop control of PMSG-based WECS with fuzzy logic controller

Since there is no rotor coil to provide mechanical damping during transient conditions, the operational behavior of PMSG is poor in open-loop scalar V/Hz control. In order to improve the operational characteristics, to obtain a faster response, to optimize power to a greater extent, and to mobilize load power management, closed-loop control of PMSG has been attempted. Though often conventional PI controllers are preferred, due to their simple operation, easy design, and effectiveness towards linear systems, it generally does not suit for nonlinear systems of higher order, time-delayed, particularly complex, and vague systems that have no precise mathematical models.

To overcome these difficulties, fuzzy logic controllers are introduced along the intermediate stages which lie between the generator and grid.

The proposed system includes wind turbine with PMSG constituting a diode rectifier, two boost converters, two fuzzy logic controllers, an inverter, a battery, and a dump resistor. The generated AC voltage passes through the rectifier and gets converted into corresponding DC voltage. Two boost converters are used to boost the rectified output voltage obtained from PMSG and passed through the inverter through DC link. The block diagram of closed-loop control of PMSG with fuzzy logic controller is shown in Figure 4. The DC voltage is converted into three-phase AC voltage using a voltage source inverter comprising more number of metal-oxide-semiconductor field-effect transistors (MOSFETs) and is connected to RL load. Fuzzy logic controllers are used particularly to track the maximum power point and to promote power management. If fine and favorable wind condition prevails, and the wind speed is within the cutoff region, this autonomous wind system can meet the required load demand. On the other hand, if there is excess wind power meeting the load demand, the surplus power generated would be stored either in a battery or dissipated through dump resistor according to the battery condition.

Two fuzzy logic controllers (FLCs) are used to control the duty cycle ratio of the boost converters located near the stator side of PMSG. The first fuzzy controller is used to get MPPT by varying the duty cycle of the first boost converter, thereby increasing stator voltage of the generator.

By varying the duty cycle of boost converters, the rotor speed of PMSG is controlled to achieve optimum power. To manage energy production, another fuzzy logic controller is included such as to vary the duty cycle ratio of second boost converter and thus to regulate the DC output voltage and decide the moment to either charge/discharge the battery or dissipate

Figure 4. Block diagram of closed-loop control of PMSG-based WECS with FLC.

excess energy to the dumb resistor. With this proposed controller, wind energy is primarily provided directly to load without going through a passive element (battery). As a result, the number of charge/discharge cycles is greatly reduced, thereby extending the battery life. The Simulink model of this closed-loop control of PMSG with fuzzy logic is shown in Figure 5.

The first fuzzy logic controller tracks rotor speed with respect to reference speed to extract the maximum power, that is, in search of suitable generator speed which results in maximum power output. Error in speed (e) and the derivative of speed error are given as inputs and the duty cycle of the first boost converter as output is fed to the FIS editor of the first fuzzy logic controller. Figure 6 shows the sketch of FIS editor used in the first FLC which is of Mamdani type. The duty cycle of the boost converters is changed such that the Ton and Toff periods may either increase or decrease.

Figure 6a shows the sketch of FIS editor used in the first fuzzy logic controller. The fuzzy rules are framed using "If…Then" statements with "and" operator. The membership functions of input and output variables used in the FIS of the first FLC are represented in Figure 6b–d.

The reference speed of the generator rotor has been taken as 1700 rpm. Fuzzy rules are formulated in a way that when the actual speed and reference speed of the rotor are almost the same, there is no need of changing the pulse width of the boost converter. The rule matrix for the first FIS editor is given in Table 1.

This type of FLC is mostly used in closed-loop control system, as it reduces steady-state error to zero to a greater extent. A set of 21 rules has been formulated considering the linguistic terms for the input speed error as NB, N, NS, Z, PS, P, and PB and the derivative of speed error as Negative, Zero, and Positive. Likewise, the linguistic terms used for the output of duty cycle

Figure 5. Model of PMSG-based WECS in closed-loop control mode with FLC.

Dynamic Modeling for Open- and Closed-loop Control of PMSG based WECS with Fuzzy Logic Controllers 31 http://dx.doi.org/10.5772/intechopen.72693

Figure 6. Membership functions used in FIS of the first FLC. (a) Mamdani-based FIS editor of the first FLC, (b) input variable of speed error, (c) input variable of speed error derivative, and (d) output variable, duty cycle 1.


Table 1. Rule matrix for first FLC.

of the first boost converter are PBB, PP, PSS, ZZ, NSS, NN, and NSS. The defuzzified outputs are viewed through rule viewers from the respective FIS editors.

It is explained in a manner that if the actual rotor speed is 700 rpm, the speed error becomes "Negative Big" (NB) and say, the error derivative is "Negative," then the duty cycle of the first boost converter must be "Positive Big Big" (PBB). If the actual rotor speed becomes 1000 rpm, the speed error becomes Negative (N), and the error derivative is "Negative," then the duty cycle of the first boost converter must be "PBB."

If the actual speed reaches 1500 rpm, the speed error is Negative Small (NS), the error derivative is "Negative," the duty cycle of the first boost converter must be "Positive" (P). If the actual speed exceeds the reference speed, and hence the speed error becomes "Positive Small" and the error derivative is "Negative," then the duty cycle must be "Positive Small Small" (PSS).

The second fuzzy logic controller is exclusively used for regulating the DC-link voltage by properly changing the duty cycle ratio of inverters and changing ON and OFF periods of power switches, and thus raising the power salvage. As in Figure 7a, the two inputs used in the second FIS editor are change in power and State of charge (SoC) of the battery. Three outputs considered are duty cycle of the second boost converter, position of switch S1, and the position of switch S2. The position of switch S1 decides whether to connect or disconnect the battery to the system according to the status of the battery and the amount of excess power generated.

Position of switch S2 determines the addition of dump resistor in the main circuit. The PWM output from each of the fuzzy logic controller serve as gating pulses for these boost converters and then to the switches too. The rules were formulated for the FIS editor of second fuzzy logic controller in such a way that if there is a change between generated power and the load power, along with the change in the SoC of the battery, duty cycle of the boost converter, position of switch S1 and the position of switch S2 are to be changed.

For example, If the change in power is "Negative" and SoC of the battery is "empty," then the duty cycle of the second boost converter is "VB" (Ton period is large and Toff is small), the

Figure 7. Membership functions used in FIS of the second FLC. (a) Mamdani-based FIS editor of the second FLC, (b) input variable delta P, (c) input variable, SoC of battery, (d) output variable, duty cycle 2, (e) output variable, position of switch S1, (f) output variable, position of switch S2.

position of the switch S1 should be "opened," and the position of switch S2 should also be "opened." If the change in power is "Negative" and the SoC of the battery is "average," then also, the duty cycle of the second boost converter is "VB," the position of the switch S1 should be "opened," and switch S2 should be "opened." If the change in power is "Negative" and the SoC of the battery is "full," then the duty cycle of the converter is "VB," the position of switch S1 should be "opened" and position of switch S2 is "opened."

If not, for other conditions, the change in power is either "Positive," or "Positive small," or "Positive big," the duty cycle of the second converter, position of switch S1, and position of switch S2 as "opened" or "closed" and varied correspondingly. The linguistic terms used as inputs and outputs are stated as follows:

For the inputs of change in power as N, PS, P, and PS, and battery's state-of-charge (SoC) as empty, average, and full, the corresponding outputs, namely duty cycle of second boost converter as VS, S, M,B, and VB, position of switch S1 and switch S2 as opened and closed with their membership functions are shown in Figure 7.

By framing 36 fuzzy "if-then" rules, the duty cycle of the second boost converter is changed and position of switches being altered to prevent wastage and dissipation of power in loads. For analysis, it has been taken such that the load power to be met is around 6 kW and the wind power fluctuates between 5.1 and 6.5 kW. Initially, it is assumed that the wind power is 5.1 kW. If the change in power is "negative" and the SoC of the battery is "empty," then the duty cycle of the second converter should be "Very Big" (VB). If the change in power is "negative" and SoC of the battery is "average," then the duty cycle of the second converter should be "VB."


Table 2. Rule matrix for the second FLC.

Figure 8. Simulated outputs for closed-loop control of PMSG-based WECS. (a) stator voltage Vabc in volts, (b) stator current Iabc in amps, (c) DC-link voltage in the intermediate stages of boost converter, (d) battery parameters, (e) mechanical power and battery power, (f) gate pulses to inverter (ton and toff), (g) generated power and load power at bus 1 and bus 2 and (h) generated output power w.r.t wind speed variations.

If the change in power is "negative" and the SoC of the battery is "full," then the duty cycle of the second converter should be "VB". Conversely, if the generated wind power is very less compared to load power, it is taken that the change in power is "Negative" (5.1–6 kW).

If the obtained wind power exceeds load power, that is, 6.1 kW, then it is understood that the change in power is "Positive Small" (PS) (6.1–6 kW). Similarly, if the wind power and the load power are equal, then it is taken that the change in power is "Positive" (6.0–6 kW) and, accordingly, the other such rules are formulated. Again if the wind power is 6.5 kW, then it is implicit that the change in power is "Positive Big" (6.5–6 kW), and hence the duty cycle is "small," switch S1 is closed to connect the system with battery and switch S2 is opened (Table 2).

The steady- and transient-state characteristics of the closed-loop control mode of PMSG with fuzzy logic controller through which the most important parameters derived are depicted in Figure 8 from (a–f).
