# An Improved Coordinated Control Strategy for PV System Integration with VSC-MVDC Technology

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## Abstract

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## 1. Introduction

## 2. System Topology and Model

_{dc0}, the DC voltage and current at primary side of ith full-bridge isolated DC/DC converter is U

_{dci}and I

_{dci}, and the DC voltage and current at primary side of ijth boost converter is U

_{dcij}and I

_{dcij}. In the following contents, a subsystem composed of the ijth PV array, the ijth boost converter, the ith full-bridge isolated DC/DC converter, the DC/AC converter, and the AC system is selected for analysis and explanations (i = 1, …, M, j = 1, …, N).

#### 2.1. PV Panel and Boost Converter

_{p}, and a series resistance R

_{s}[8]. The relationship between output voltage (U

_{pv}) and current (I

_{pv}) of PV panel is shown as follows.

_{ph}is the PV current, I

_{0}is saturation current, q is the electron charge (1.60217646 × 10

^{−19}C), N

_{s}is the number of PV cells in series, k is the Boltzmann constant (1.3806503 × 10

^{−23}J/K), T (in Kelvin) is the temperature, and A is the diode ideality constant.

_{pv}/dU

_{pv}equals to zero, which can be located by comparing the instantaneous conductance (I

_{pv}/U

_{pv}) to the incremental conductance (∆I

_{pv}/∆U

_{pv}), i.e.,

_{ao}) satisfies

_{bo}is the output voltage, D

_{ij}is the duty cycle, and T

_{s}is the period of the carrier.

_{b}) is expressed as

_{a}is the current of the controlled voltage source. The reference direction of these measurements is shown in Figure 3b.

_{dcij_ref}, is offered by MPPT controller. The outer voltage controller generates reference current (I

_{dcij_ref}) for the inner current controller, and the inner current controller produces duty cycle of the control signal (D

_{ij}) for the semiconductor switches. For the detailed model, the duty cycle D

_{ij}needs to be transformed into trigger pulses by PWM (Pulse Width Modulation) block. For the average model, U

_{ao}is controlled by D

_{ij}according to function (3).

#### 2.2. Full-Bridge Isolated DC/DC Converter

_{cd}) is boosted.

_{c}) is given by

_{a}is the current of the controlled voltage source and D

_{i}is the duty cycle.

_{ab}, is expressed as follows, according to the principle of power conservation. The reference direction of these measurements is shown in Figure 4b.

#### 2.3. DC/AC Converter

_{j}(j = a, b, c) is given by

_{j}

_{_ref}is the reference voltage offered by the inner current controller.

_{d}) is expressed as follows, also according to the principle of power conservation.

_{de}is the voltage of MMC at DC side.

_{dc}-Q mode or P-Q mode. The inner current controller adopts PI controller with current decoupled compensation and voltage feed-forward compensation to compute the reference voltage at converter-side.

#### 2.4. AC System

_{N}, as shown in Figure 7. S

_{N}is defined as

_{s}and f are the Root Mean Square (RMS) voltage and frequency of three-phase voltage source at PCC, and L is the inductance of the internal impedance.

## 3. Coordinated Control Strategy of PV Integration System

#### 3.1. Conventional Control Scheme of PV Integration System

_{dc}-Q mode and is responsible for controlling the stability of MVDC bus voltage (U

_{dc0}). The ith full-bridge isolated DC/DC converter controls the voltage at its lower side (U

_{dci}). The ijth boost converter controls the voltage at solar panels (U

_{dcij}) to achieve MPPT. This method ensures DC buses voltage stability in normal states. However, the MVDC bus voltage is controlled only by the DC/AC converter, which is vulnerable for the safe operation of the system. PV panels operate at MPP to generate maximum power in normal states. However, if the AC system cannot consume so much power (e.g., a fault occurs in AC system), the reference DC voltage of PV panel needs to be adjusted with the aid of fast communication system, so as to reduce the power output of PV panels. The MPPT controller in the ijth boost converter should be blocked and boost converter should be in constant voltage mode with the voltage reference (U

_{dcij_ref}) offered by the control center. The delay of communication system always has a negative influence on system’s fast response. The communication system may also reduce the system reliability.

#### 3.2. Improved Control Scheme

_{ij}and ΔU

_{i}are the dead bands of the ijth boost converter and the ith full-bridge isolated DC/DC converter, respectively. Signal S is a global enable signal of extra loops. When the system starts up, the extra loops should be blocked by setting S to false. When S is set to be logic one, the effectiveness of the extra loops is dependent on the measured voltage of the extra loops. Only the basic conventional control scheme takes effect if the extra measured DC voltage is in the range of the dead band. In Figure 10a, E

_{ij}

_{MPPT}and E

_{ij}are the enable signal of the MPPT controller and the original outer voltage loop controller in the ijth boost converter, respectively. In Figure 10b, E

_{i}is the enable signal of the original outer voltage loop controller in the ith full-bridge isolated DC/DC converter.

_{dc0}, will then rise. When it exceeds the dead band, ΔU

_{i}, E

_{i}becomes logic zero to disable the original control blocks (encircled with blue dot line in Figure 10b). U

_{dc0}will be controlled by all of M full-bridge isolated DC/DC converters. The primary side DC voltage of full-bridge isolated DC/DC converters, U

_{dci}, will also rise. When it exceeds the dead band, ΔU

_{ij}, E

_{ij}

_{MPPT}, and E

_{ij}become logic zero to freeze MPPT controller and original voltage loops (encircled with blue dot line in Figure 10a). U

_{dci}will be regulated by all of N boost converters. DC voltage at PV panel will deviate from the voltage corresponding to MPP, and the power generated by PV panel will be reduced automatically. No communication is needed among converters in this process. As there are many DC/DC converters connected to the DC bus, the droop control strategy should be adopted among DC/DC controllers. K

_{ij}and K

_{i}are the droop coefficients of the ijth boost converter and the ith full-bridge isolated DC/DC converter, respectively.

## 4. Simulation and Results

_{dc}-Q mode and regulates MVDC bus voltage at 60 kV.

_{dc0}) and power injected into AC grid (P

_{ac}and Q

_{ac}) are depicted in Figure 12. U

_{dc0}is regulated at 1 pu during 0~15 s. The power injected into AC grid is smoothly increased to maximum power about 4MW during 6~10 s. When DC/AC converter switches into P-Q mode at 15 s, U

_{dc0}will rise and it is a signal to reduce the power generated from PV system.

_{1}, is depicted in Figure 13. When U

_{dc0}exceeds the dead band (ΔU

_{1}= 0.02 pu), E

_{1}becomes logic zero to disable the primary side DC voltage control of full-bridge isolated DC/DC converter 1. The isolated DC/DC converter 1 is then in control of MVDC bus voltage (U

_{dc0}). The switchover of the controller occurs at 15.02 s when U

_{dc0}is greater than 1.02 pu.

_{dc1~4}, are regulated by isolated DC/DC converters at 1 pu during 0~6 s. During 6~22 s, DC voltages, U

_{dc1~4}, are controlled by isolated DC/DC converter 1~4 when they are in the range of 0.97~1.03 pu (the dead band is 0.03 pu). They are controlled by boost converters connected with the isolated DC/DC converter, if DC voltage exceeds the dead band.

_{11}, and enable signal of MPPT control, E

_{11MPPT}, of boost converter 11 are depicted in Figure 15. The primary side DC voltage and power waveforms of boost converter 11~12 are shown in Figure 16. The extra voltage controller and MPPT controller of boost converter 11 are blocked during 0~6 s. The controller of boost converter 11 regulates the voltage at PV panel 11 (U

_{dc11}) at 1.035 pu, which is very close to PV panel’s open circuit voltage (1.05 pu). Extra voltage controller and MPPT controller are unblocked during 6~22 s. The dead band of extra voltage loop in boost converter 11 (ΔU

_{11}) is set to be 0.03 pu. As shown in Figure 15, E

_{11}and E

_{11_MPPT}are zero during 7.08~7.66 s, and 15.23~22 s, as the secondary side voltage (U

_{dc1}) of boost converter 11 exceeds 1.03 pu during these periods. The controller of primary side voltage of boost converter 11 is then disabled, and boost converter 11 begins to regulate U

_{dc1}. U

_{dc1}is stabilized at 1.08 pu during 15~18 s, and at 1.06 pu during 18~22 s. The MPPT controller is also disabled, and PV panels operate at a higher voltage with less power output.

_{dc0}) and power injected into AC grid (P

_{ac}and Q

_{ac}) in two cases are depicted in Figure 17. It is clear that the proposed control strategy has a faster response than the conventional method. Although the DC voltage deviates from 1 pu with the proposed control strategy, it is still in safe range to keep the system in stable operation.

## 5. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 3.**(

**a**) Boost converter topology; (

**b**) Average model of boost converter; and, (

**c**) Controller design of boost converter.

**Figure 4.**(

**a**) Full-bridge isolated Direct Current (DC)/DC converter topology; (

**b**) Average model of full-bridge isolated DC/DC converter; and, (

**c**) Controller design of full-bridge isolated DC/DC converter.

**Figure 6.**(

**a**) DC/Alternating Current (AC) topology; (

**b**) Average model of DC/AC; and, (

**c**) Controller design of DC/AC converter.

**Figure 10.**(

**a**) Improved controller design of boost converter ij; (

**b**) Improved controller design of full-bridge isolated DC/DC converter i.

**Figure 13.**MVDC bus voltage and enable signal of the original voltage loop in isolated DC/DC converter 1.

**Figure 15.**Primary side DC voltage and enable signal of original voltage loop and MPPT controller in boost converter 11.

Devices | Parameters | Values |
---|---|---|

PV panel | open circuit voltage | 1050 V |

short circuit current | 1270 A | |

maximum power point voltage | 835 V | |

maximum power | 500 kW | |

Boost converter | rated power | 500 kW |

rated voltage at primary side | 1 kV | |

rated voltage at secondary side | 2 kV | |

Full-Bridge Isolated DC/DC converter | rated power | 1 MW/1.5 MW |

rated voltage at primary side | 2 kV | |

rated voltage at secondary side | 60 kV | |

DC/AC converter | rated power | 5 MVA |

rated dc voltage | 60 kV | |

rated ac voltage | 35 kV | |

AC system | SCL | 500 MVA |

rated ac voltage | 35 kV |

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**MDPI and ACS Style**

Che, Y.; Li, W.; Li, X.; Zhou, J.; Li, S.; Xi, X.
An Improved Coordinated Control Strategy for PV System Integration with VSC-MVDC Technology. *Energies* **2017**, *10*, 1670.
https://doi.org/10.3390/en10101670

**AMA Style**

Che Y, Li W, Li X, Zhou J, Li S, Xi X.
An Improved Coordinated Control Strategy for PV System Integration with VSC-MVDC Technology. *Energies*. 2017; 10(10):1670.
https://doi.org/10.3390/en10101670

**Chicago/Turabian Style**

Che, Yanbo, Wenxun Li, Xialin Li, Jinhuan Zhou, Shengnan Li, and Xinze Xi.
2017. "An Improved Coordinated Control Strategy for PV System Integration with VSC-MVDC Technology" *Energies* 10, no. 10: 1670.
https://doi.org/10.3390/en10101670