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This paper proposes control of maximum power tracking system of tidal current energy system. A permanent magnet synchronous generator (PMSG) works as a variable speed generator in the proposed energy system. A controller was applied to achieve the maximum power control of tidal current turbine on a wide range of water current speed change. A dynamic model and simulation of the energy system coupled with current change are presented. The measured DC voltage and DC current are used to determine the position of maximum power point that controls the DC/DC boost converter duty cycle depending on the Hill Climb Search (HCS) algorithm. This algorithm doesn’t require any information or measurements about current’s speed change or generator’s characteristics. A supercapacitor added to fix the load voltage despite of tidal current speed or load variations. Simulation results show the effectiveness of the controller proposed system.

Nowadays, the shortage of traditional energy resources in the near future motives the use of renewable energy resources especially with demand increasing in energy need [

The main problem is that tidal current turbines produce energy depending on the climatic changes “wind speed”, geographical changes “water depth”, and mainly on distance between earth and moon. Therefore, it is necessary to construct a system capable of generating maximum power under all of these constraints.

The use of variable speed tidal current turbine with Permanent Magnet Synchronous Generator (PMSG) will solve the previously mentioned problem with maximum power tracking specially with low current’s speed. There is no need for gearboxes with PMSG. Moreover it improves system reliability and reduces the mechanical stress and maintenance cost [

In this paper, a hybrid tidal current energy-supercapacitor conversion system is proposed. This system consists of a variable speed tidal current turbine with PMSG, three-phase bridge rectifier, DC/DC boost converter and MPPT controller. MPPT algorithm was built to control the boost converter switch duty cycle depending on the rectifier DC current and DC voltage using the hill climb concept. Any change in load or tidal current speed won’t cause any change in load output voltage as used supercapacitor will balance in the voltage change. The integration of tidal current energy system and supercapacitor require a power converter. This converter must be capable of allowing bidirectional power flow between supercapacitor and DC-link and a control done to regulate the load voltage level.

All models developed in this paper were simulated in Matlab/Simulink simulation environment and the simulation results demonstrate the validity and effectiveness of the proposed control scheme.

Tidal current turbine converts tidal current energy into mechanical energy that runs the generator creating electrical energy expressed by

where,

where, R are the rotor blade radius and _{1}, V_{2} and V_{3} at the maximum power points that desired to be tracked.

In PMSG, the output voltage E and current are directly proportional to rotor speed and electromagnetic torque, respectively like the flowing [

where k is the generator constant and

phase voltage equation for PMSG may be as follows:

V, I is the generator phase voltage and L is its inductance. This voltage is rectified using three phase uncontrollable bridge rectifier which converts AC voltage of PMSG to a DC voltage according to Equation (5). For simplification in calculations, equations present output DC voltage expressed dividing the input signal as six equal portions of the sine input signal as [

where,

From the past two equations

where,

For boost converter the relations between input and output voltage and current in the steady state are [

where, D is the control switch duty cycle,

Many control strategies are suggested to cope up with the variation in tidal current energy to ensure three objectives, 1) Energy capture: the overall efficiency of the rotor and transmission must be maximized to provide maximum possible power production; 2) Mechanical loads: guarantee a certain level of resilience of mechanical parts by lightening the variable loads; 3) Safety: safe turbine operation, mitigate damaging, fatigue loads and detect fault conditions.

Among those control strategies that are used in MPPT are the Tip Speed Ratio (TSR) algorithm, Power Signal Feedback (PSF) algorithm and Hill Climb Search (HCS) algorithm. Mechanical variables are feed to the controller in case of TSR control system and electrical ones used in PSF and HCS control algorithms. The Boost DC-DC converter importance appears at low generator speed (low current speed). Moreover, the total harmonic distortion decreases [

The HCS control algorithm continuously searches for the peak output power of the tidal current turbine. It over- comes the previously mentioned TSR and PSF methods problems, as there is no need for tidal current speeds, rotor angular speed or system dynamic characteristics. This tracking algorithm taking into account the current operating point power and compare it with neighbor power points, if the new neighbor power point is greater than current operating point power, the controller will update the operating point to the new one and vice versa.

This operation principle for one tidal current speed can be modified for more than several currents speed. A three different tidal current speeds as in

_{1} the controller will search for the maximum power then force the turbine to operate at ω_{1} that corresponding to maximum power point P_{1}. If a sudden increase occurs in tidal current speed to be v_{3 }then the generated power no longer the maximum then the controller continues searching the maximum power point by comparing the current generated power with the previously generated one until reaching P_{3} the force the turbine to run with ω_{3}. Typically, the reverse occurs when the speed decrease to v_{2}.

The DC/DC boost converter controller block diagram is as shown in _{dc} generated from the rectifier circuit and the error signal is then fed to a PI controller.

A comparator then generate a duty cycle signal that control the switching on and off, This duty cycle generated from comparing the PI output signal with a fixed frequency repetitive saw tooth wave form, so in many cases this control algorithm called pulse width modulation PWM algorithm [

A super capacitor is a high-capacity electrochemicalcapacitor with capacitance values in the range of around 100,000 µF up to some 1000 F. It preferred over other storage devices due to its good electrical behaviour, low initial cost in comparison to battery, long life time and basically low charge time [

The converter itself is composed of the inductor, the supercapacitor, 2-IGBTs; 2-Diodes. A 20 kHz fixed-fre- quency PWM is applied on either IGBT to transfer energy back and forth. In bidirectional DC-DC converters, there are two modes of operation. The first mode is the boost mode (IGBT 1-ON and IGBT 2-OFF), where the supercapacitor is discharged to a higher voltage level at the DC link; in the second mode, buck mode (IGBT 1-OFF and IGBT 2-ON); here the excess power from the tidal current turbine charges supercapacitor [

To ensure converter operating in continuous conduction mood, inductor value must be higher than that calculated from the following equations [

where,

The whole system described in

Simulation is performed using Matlab/Simulink simulation environment. Two main controllers are performed: MPPT control using HCS principle, comparing DC boost converter input voltage and current with output ones. Depending on the desired output voltage level and the actual input voltage, the boost converter is designed.

The second controller is for load voltage regulation. Two inverted signals are provided to the bidirectional IGBT switches depending on the controller signals with buck and boost moods.

Variation in tidal current speed and load change are applied to the two controllers giving the following results:

The simulation is performed on three different load values (100 Ω - 200 Ω - 300 Ω), no rule is used to select those value except that the boost converter parameter are sized at 300 Ω. Any increase or decrease over and under those values want effect on the controllers’ effectiveness.

As shown in Figures 8-10, change in tidal current speed (

Parameter | Value |
---|---|

TIDAL CURRENT TURBINE | |

Rated nominal power | 1.5 kW |

Rated current speed | 1.55 m/sec |

Rated rotor speed | 104.7 rad/sec |

Turbine rotor radius | 1 m |

PMSG | |

Number of pole pairs | 12 |

Stator resistance | 0.45 ohm |

Inductance | 0.95 mH |

DC/DC Boost converter | |

Input capacitor | 100 µF |

Inductor | 1 mH |

Output capacitor or DC-link | 400 µF |

Design output voltage | 400 V |

Switching frequency | 20 kHz |

SUPERCAPACITOR | |

Rated capacitance | 150 F |

BI-DIRECTIONAL CONVERTER | |

Inductance | 1 mH |

in load or tidal current speed due to closed loop control. The super capacitor and bidirectional converter compensate the change in output voltage increasing or decreasing the voltage level to maintain the desired voltage.

This paper presented a simple control algorithm to track the maximum power from tidal currents in spite of speed variation. The whole system was modelled in Matlab/Simulink simulation program applying the simple Hill Climb Search principle by sensing the rectified voltage and current. Any change in tidal current speed and

desired output voltage will force the controller to change the duty cycle of the boost converter and gates pulse of the bi-directional converter. Results analysis showed the good behaviour of the two controllers to achieve MPPT and voltage regulation without any knowledge of turbine characteristics or measure changes in current’s speed.

Marwa M. Elzalabani,Faten H. Fahmy,Abd El-Shafy A. Nafeh,Gaber Allam, (2016) Maximum Power Point Tracking of Hybrid Tidal Current—Supercapacitor Energy Conversion System. Journal of Power and Energy Engineering,04,35-44. doi: 10.4236/jpee.2016.44004