# Modelling and Control of a VIENNA Smart Rectifier-I for Wind Power Systems Integrated Under Transient Conditions

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## Abstract

An improved topology with a fault ride through (FRT) capability when subjected to a DC-link fault-based wind power plant (WPP) employing a Vienna active rectifier-I is proposed in this paper. The proposed system is capable of mitigating fault occurring on the DC-link side using the PWM-controller technique implemented on the Vienna active rectifier. FRT capability analysis is conducted in this paper, simulation results demonstrate the suitability of the control strategy. Actually, use of proposed wind energy conversion unit (WECU) topology has led to the improvement of system stability by maintaining constant output voltage. Furthermore, the WECU integrating Vienna active rectifier-I is also proven as a feasible technology that can be employed in a large-scale WPP or renewable power generations to realize technical and economical efficient grids integration with high voltage direct current (HVDC) transmission systems.

## Keywords

Voltage control Fault ride through Vienna rectifier Wind power Transient stability DC collection grid Fault clearance Smart rectifiers## Introduction

Due to the increased demand of electricity nowadays, the world is moving towards the use of renewable energy sources because of high efficiency, resource availability, cost competitiveness and environmental adequateness. In this respect, recent researches have been developed towards the improvement of renewable energy technology [1, 2, 3]. In fact, for an AC collection grid, each wind energy conversion system (WECS) on a WPP includes: a wind-turbine plus mechanical parts (e.g. gearbox), a generator (e.g. doubly fed induction generator (DFIG), permanent magnet synchronous generator (PMSG), squirrel cage induction generator (SCIG)), and a huge 50- or 60 Hz power transformer including controller circuit. In this paper a wind farm with DC collection grid is considered. For a wind farm employing DC collection grid, the huge power transformers in the WECSs are replaced by the power electronic converters. The power electronic converter is significantly compact and small in size compared to the power transformer of identical features.

Most of studies being done on the design of DC collection grids for wind farm employ the conventional full-bridge active rectifiers as the topology of power converters in WECS. The conventional full-bridge active rectifier contain large numbers of controlled-switches, which increase complexity of controller circuit and affect the system efficiency because of the high switching frequency operation of the controlled switch. The wind DC collection grid in this paper is built out using the parallel connection of Vienna-I active rectifier, and thus reduces the numbers of controlled switches in the power converter compared with conventional active rectifiers.

- 1.
Fixed speed WECU: the wind-turbine with a SCIG.

- 2.Variable speed WECU: this group is classified into two types:
- (i)
PMSG based wind-turbine with a full rating converter.

- (ii)
DFIG based wind-turbine with a partial rating converter.

- (i)

- 1.
Can ride-through several grid-faults.

- 2.
Able to cut out from the grid immediately in case of a fault and resume rapidly in normal operation after the fault.

- 3.
Has the capability to smooth out the intermittent power fluctuation from the wind.

- 1.
Capability of double boosting effect because it consists of two inductors, which can improve the rectifier features.

- 2.
Ability of buck-boost operation.

- 3.
Allows transmitting the power to the DC capacitors even if the supply voltage is positive or negative.

- 4.
Involvement of a single switch per power cell that permits to construct a high stable and reliable rectifier.

- 5.
While it is usually used for a three-phase rectification, it can also be used for a single phase applications.

- 6.
Ability of obtaining a two DC-output voltages so that DC-ripples decrease for a given capacitor value.

- 7.
It has the possibility of continuous current and power factor correction operation modes.

However, the disadvantage of using this topology is the difficulty of implementing two inductors [19]. Remaining part of this article is organized as follows. “Modelling and Control of a Vienna Active Rectifier-I” presents modelling and control of Vienna active rectifier-I including rectifier analysis and design of the rectifier input side filter. The AC-bus controller design including design equations and investigation for stability using bode plots have been presented in details in “Design Criteria of the Rectifier Controller”. “Fault Effect Elimination and Analysis” discusses the fault effect elimination and analysis which discusses the type of connection and the controller that has been used for ride through the fault. Simulation results and discussions are presented in “Simulation Results and Discussions” Conclusion of the paper is presented in “Conclusion”.

## Modelling and Control of a Vienna Active Rectifier-I

### The Rectifier Analysis

The converter topology has been shown in Fig. 1. There are three levels of output voltage, *V*_{C1} and *V*_{C2} each is half of the total output voltage. And the third level is the total (*V*_{C1} + *V*_{C2}) and should be greater than the maximum supply voltage. It is a boost converter, because the switching frequency is too high compared with the supply frequency, and the output voltage is always greater than the input voltage. The converter injects ripple in the generator, so ripple filter should be considered in the converter input side design. The operation details of the considered converter have been explained in [11, 12, 13]. It consists of three active switches (IGBT or MOSFET) and eighteen diodes. The control of the three switches ensures sinusoidal waves input current and desired balanced output DC voltage. The output voltage of the considered rectifier depends on the polarity of the input AC current as well as the switching states [12, 19, 20, 21].

### Filter Design

*i*

_{g}into the power source

*V*

_{g}as shown in Fig. 2.

The filter also attenuates the EMI and hence enhances the system stability by protecting the converter from transient voltages and currents [22].

#### Inductor Design

Parameters used for filter design

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

Generator rms voltage | | 500 V |

Rectifier output voltage | | 2400 V |

Switching frequency | | 4 kHz |

Desired output current ripple | Δ | 1.2 A |

Duty ratio | D | 0.8 |

Desired output voltage ripple | Δ | 48 V |

Rectifier output current | | 50 A |

#### Capacitor Design

*μ*

*F*to meet the system filter specifications. The inductor and capacitor have been designed to decrease the rectifier input current and voltage ripples, such that it will affect the output current and voltage wave forms.

### Switching Losses

*v*

_{a}=

*V*

_{g}, then the diode becomes forward biased and when the transistor is in ON state the energy loss can be given by:

*W*

_{ON}+

*W*

_{OFF}

#### Efficiency Versus Switching Frequency

*η*is the electrical system efficiency and losses are given by Eq. 9.

*P*

_{cond.}is conduction losses,

*P*

_{fixed}is fixed losses and

*P*

_{sw}is switching losses

## Design Criteria of the Rectifier Controller

The controller aims to ensure constant output voltage and current, and the governor of the control circuit is an output voltage and sinusoidal supply input current, the output voltage is sensed and subtracted from the reference voltage to produce the error signal. It is desirable to minimize the error signal, and the current reference is used to maintain the supply sinusoidal current.

### Current Loop Controller Design and Analysis

*ω*

_{0}is the AC source frequency and

*θ*

_{0}is the initial rotor angle. The nonlinear system in Fig. 4, can be described by:

*i*

_{d},

*i*

_{q}and

*ρ*are the state variables.

*V*

_{td},

*V*

_{tq}and

*ω*are the control inputs.

*V*

_{sd}and

*V*

_{sq}are disturbance inputs. The relationship between a modulating signal and the corresponding AC side terminal voltage can be given by:

*U*

_{d}and

*U*

_{q}are control inputs. By substituting for the

*m*

_{d}and

*m*

_{q}from Eqs. 21 and 22 in Eqs. 19 and 20, yields

*i*

_{d}and

*i*

_{q}can be controlled by

*U*

_{d}and

*U*

_{q}, respectively, as shown in Fig. 5a and b.

Where *G*_{v}(s) and *G*_{i}(s) and are the voltage and current compensators respectively, \(V_{dq}^{\ast }\) and \(i_{dq}^{\ast }\) are the reference voltage and current vectors, *L*_{f} is the filter inductor, *R*_{f} is the equivalent series resistance of the inductor, *C*_{f} is the filter capacitor value. *G*_{p wm} (*s*) is the transfer function related to PWM delays, and = [1 − (*T*_{d}/2)]/[1 + (*T*_{d}/2)*s*], where *T*_{d} is the delay time of the system and = 1.5 *T**s* = 375 *μ**s*.

*G*

_{i}(s) that was designed and analysed is proportional resonance PR compensator and can be given using this equation:

*k*

_{p}= 2

*π*

*f*

_{bw}

*L*and \(k_{i}=\frac {R_{f}}{L_{f}}k_{p}\),

*f*

_{bw}assumed to be 600 Hz,

*ω*

*o*= 2

*π*60 = 377 rad/s is the fundamental resonant frequency,

*ω*

_{C1}is a damping frequency and assumed to be 10 rad/s. The current closed-loop transfer function (TF) Fig. 7, can be determined in Eq. 28

### Voltage-Mode Control Using Phase Locked Loop (PLL)

*ρ*is the phase shift and,

*ω*(

*t*) is the input to the voltage controlled oscillator (VCO) as shown in Fig. 9, and it is integrated to produce the desired value of

*ρ*.

*H*(

*p*) is a compensator transfer function and

*p*is a differentiation operator

*p*

_{1}is the filter pole,

*α*is a real constant and should be greater than 1. The maximum phase margin of the filter is given by Eq. 37.

*ω*

_{m}can be equal to the crossover frequency

*ω*

_{c}. The loop gain can be solved using the equation,

^{6}. Using parameters given in Table 2, the loop gain can be got and plots to determine the frequency response.

Parameters used for compensator design

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

Phase margin | | 60 |

Crossover frequency | | 250 rad/s |

Supply frequency | | 377 rad/s |

Peak value of phase voltage | \(\hat {V}_{s}\) | 577 V |

From the bode plot Fig. 10, it is found that L(s) declines with the slope of −20 dB when *ω* is greater than *ω*_{C}. The gain margin and the phase margin are 75 dB and \({90}^{^{\circ }}\)respectively, which is indication of a good stability.

## Fault Effect Elimination and Analysis

The controller output voltage V_{cont.}(t) is sensed with H(s), the sensor output will be H(s) × *V*_{cont.}(s) and is compared with a reference input voltage V_{ref.,} the aim is to let H(s)×*V*_{cont.}(s) closer to V_{ref}. The variation between the reference input V_{ref}., and the sensor output H(s)×*V*_{out}(s) is an error signal. If the feedback system is perfect then the error signal will be zero. In practice, the error signal is commonly not zero but none the less very small. In any case of disturbance the compensator should compensate the error signal to the desired signal.

## Simulation Results and Discussions

Parameters used for simulation

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

Generator voltage rating | | 500 V |

Generator current rating | | 20 A |

Turbine voltage rating | | 500 V |

Turbine current rating | | 20 A |

Generator filter capacitance | | 208 |

Generator filter inductance | | 82mH |

DC link capacitance |
| 1000 |

Load | R | 50 Ω |

Howell et al. [27] they did a similar work of using a fault blocking diode in a DC transmission line to block the fault current and they built a HVDC system topology to transmit unidirectional power into weak AC system while retaining the ability to clear DC faults. They discussed Control strategies including fault blocking diode and its performances under DC and AC faults using electromagnetic transient simulations, and they concluded that the topology is viable candidate. The difference between this work and their work is they used a line commutated converter LCC and a modular multilevel converter MMC as a rotor side converter RSC and a grid side converter GSC, respectively. Furthermore, this control strategy using fault blocking diode was performed in [28, 29, 30, 31, 32] and they concluded and recommended that it is a practical and effective controller that participate in the power system stability.

## Conclusion

This paper has presented DC-link fault analysis to ride through the transient faults. The simulation results have shown the superiority of the proposed FRT capability controller, which involving sub-models consisting of controller and fault blocking diode. The fault blocking diode has detected the fault very fast and protected power tools from thermal over stresses with the controller assistance by creating new current path to eliminate the fault current effect. This paper proved that the unidirectional HVDC scheme is able to protect the transmission DC line from the transient faults that can occur due to short circuit. The current and voltage loop controllers have been designed to maintain constant voltage at steady state conditions. Also, the supply side filter that consists of inductor and capacitor has been designed to decrease the input voltage and current ripple values It has been concluded that the power transmission is found to be rapid and continual for the non-permanent faults and the controller has improved the stability and reliability of the wind power HVDC system.

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