Unified power quality conditioner (UPQC) with energy storage is commonly based on conventional capacity configuration strategy with power angle control. It has problems such as phase jumping before and after compensation. DC-link cannot continuously emit active power externally. Therefore, this paper presents the compensation strategy of full load voltage magnitude and phase in capacity configuration of UPQC. The topology of UPQC is integrated a series active power filter (SAPF), a shunt active power filter (PAPF) and a photovoltaic-battery energy storage system (PV-BESS). The principle of full load voltage compensation is analyzed based on the PV-BESS-UPQC topology. The magnitude constant of load voltage is maintained by controlling the appropriate shunt compensation current. Then the UPQC capacity configuration is carried out using the full load voltage compensation strategy. The compensation capacity of UPQC series and shunt units are reduced. Finally, the simulation results show that the proposed compensation strategy reduces the capacity configuration by 5.11 kVA (36.4%) compared to the conventional compensation strategy. The proposed strategy can achieve full compensation of the load voltage, which can effectively reduce the capacity allocation and improve the economy of UPQC. It also has the PV-BESS units' ability of providing active power and can stabilize the DC-link voltage.

With the continuous development of the electric power industry, the power quality issue is getting more and more attention. Compared with the traditional electrical loads, the new generation of power distribution system consisting of power electronic devices also requires higher and higher power quality. Power quality becomes a problem that cannot be ignored [

At present, conventional UPQC has the problem of insufficient DC-link energy storage, which limits the use and development of UPQC. Some scholars have proposed adding distributed generation units on the DC-link for solving the problems such as low compensation efficiency of UPQC [

For the UPQC-related compensation strategy with energy storage units on the DC-link, Devassy et al. [

It is thus clear that how to achieve full compensation of load voltage magnitude and phase. While reducing the capacity configuration and cost of UPQC is an urgent problem in current UPQC applications. Therefore, this paper takes the PV-BESS-UPQC as a topology of study. It analyzes the problems of conventional PV-BESS-UPQC power angle control. Then the compensation strategy of full load voltage magnitude and phase in capacity configuration has been proposed. Also, the capacity configuration principles and methods of series and parallel units are analyzed. Finally, the correctness and effectiveness of the proposed strategy are verified by simulation and experiment.

The structure of UPQC mainly consists of a series active power filter (SAPF) and a shunt active power filter (PAPF). The PV-BESS integrated UPQC is added with Boost converter and photovoltaic array. The construction of PV-BESS-UPQC is displayed in

When the UPQC with PV-BESS is in operation, the series compensation unit is equated to a controlled voltage source. The series compensation unit is mainly used to compensate for the sudden change of grid voltage, unbalance and harmonics to ensure the voltage quality on the load. The PV array and BESS are connected to the DC-link through the DC/DC converter. It can suppress the fluctuation of the DC-link voltage, buffer the voltage and power to ensure the stability of the DC-link voltage. PV-BESS also provide active power to the load when necessary. Since the PV array is connected to the UPQC DC-link directly, the PV array is constructed in a way such that the maximum power point (MPP) voltage is equivalent to the reference DC-link voltage. During nominal conditions, the rating of PV array ensures that the load active power is delivered by the PV array and power is supplied to the grid and charging BESS by the PV array as well. Besides, the BESS is designed in a way that, when the PV array generate less power than the DC-link load demand, the BESS provides the insufficient power equivalent to the decrease in DC-link voltage. Moreover, when there is no power produced by PV array, the BESS will supply the total load demand.

In conventional PV integrated UPQC compensation control, the series converter unit only operates when voltage drops occur and only provides active power and does not participate in the compensation of load reactive power. In [

The phase diagram of UPQC-PV operation based on power angle control under voltage drops is displayed in

The magnitude and phase of voltage jump is the most common and the most concerned power quality problem for power users. In response to the problem of load voltage phase which is often not considered in the conventional energy storage UPQC power angle compensation strategy. This paper proposes a full load voltage compensation strategy of magnitude and phase. Its compensation phase diagram is shown in

As shown in

In summary, the full compensation of the load voltage considers the following four factors: The post-fault supply voltage

In

In the full load voltage compensation strategy, the magnitude of the supply current

Specifically, the appropriate shunt compensation current

As shown in

In

In summary,

For the convenience of analysis, the supply voltage drop factor

According to the compensation triangle composed of phase quantities

According to the compensation triangle formed by the phase quantities

The phase angle

The compensation capacity of the series unit is expressed as:

Normally to meet the requirements of high power factor, power factor is generally required to be 0.85 to 0.9. The variation range of

In this conventional compensation strategy, it is stipulated that the amplitude of

From

In

From the selection method of

According to the compensation triangle formed by the phase quantities

From the

Further from the trigonometric relationship, the phase angle of

From the above analysis, it can be seen that the compensation capacity of the shunt unit under the compensation strategy proposed in this paper is shown as follows:

The relationship between the shunt unit compensation capacity

In the conventional power angle compensation strategy shown in

And the power factor is generally required to be 0.85 to 0.9. The variation range of

From

Configuration of PV-BESS unit capacity includes the capacity configuration of PV arrays and the energy storage system. The capacity configuration in PV power generation output is mainly affected by solar radiation illumination, ambient temperature and other factors. The capacity of PV arrays is designed as follows:

The capacity of the energy storage system is mainly determined by the following parameters: the size of the active power of the load, the degree of drop of the supply voltage and the duration of the drop. The capacity is determined by the worst possible working conditions of the user load. The worst working condition of the load considers the situation of power interruption. When the energy storage unit realizes the uninterrupted power supply to the load, that is, the load active power is completely provided by the energy storage unit. The capacity of the energy storage unit is designed as follows:

In summary, the compensation strategy proposed in this paper achieves complete compensation of the load voltage. Compared with the conventional compensation strategy, the compensation capacity of UPQC can be greatly reduced. The capacity cost of UPQC series and shunt units can be saved. Also, the capacity design of PV-BESS unit ensures the uninterrupted power supply to the load.

The

To achieve compensation of possible harmonic components in the supply voltage, the required voltage harmonic compensation value is extracted by a low-pass filter. This harmonic compensation value is used as part of the

The control block diagram of the shunt converter unit is shown in

The PV-BESS configuration displayed in

When the grid voltage drop is deep, the PV-BESS unit can play a limiting role on the grid current. It can reduce the risk of insufficient compensation capacity of UPQC in the case of deep grid voltage drop. In order to meet the fast energy leveling and improve the DC-link voltage constancy, the DC-link adopts the double closed-loop control strategy of voltage external loop control and current internal loop control. It enhances the ability of bus voltage self-regulation.

The PV system output power,

In

In order to verify the correctness and effectiveness of the proposed full load voltage compensation strategy in capacity configuration of UPQC integrated PV-BESS. The series and shunt unit compensation capacities are simulated using the MATLAB/Simulink simulation platform. The conventional compensation strategy and the full load voltage compensation strategy proposed in this paper are compared, respectively. The UPQC-PV related simulation parameters are set in

System parameter | Parameter value |
---|---|

Voltage supply (V) | 380 |

DC voltage (V) | 800 |

Series converter | L1 = 2 |

Shunt converter | L2 = 2 |

Series transformer ratio | 1:1 |

Load power angle (°) | 30 |

Rated load capacity (KV |
10 |

Capacity of energy storage unit (A |
10 |

The simulation is set at 2 s when the phase of the power supply voltage has a transient 20% drop of amplitude (60 V) and a

The comparison of the supply voltage and load voltage waveforms before and after using the conventional power angle compensation strategy is shown in

When using the full load voltage compensation strategy proposed in this paper. The comparison of supply voltage and load voltage waveforms before and after compensation is shown in

The waveforms of supply voltage and load voltage before and after compensation are shown in

When using the conventional power angle compensation strategy. The active and reactive power simulation waveforms of each unit of the system are shown in

The simulation results using the compensation strategy proposed in this paper are shown in

In order to further verify the function of UPQC with PV-BESS to supply continuous power to the load when the supply voltage drops completely under the proposed control strategy. The power supply is set to open circuit at 2 s. The system simulation results are shown in

In summary, when in the case of a fault with a 20% drop in power supply voltage and a jump in phase. The proposed compensation strategy increases the compensation capacity of shunt units by 2.07 kVA compared to the conventional compensation strategy. But it reduces the compensation capacity of series units by 7.18 kVA. That results in a reduction of 5.11 kVA (36.4%) in the total system capacity. Therefore, the proposed compensation strategy reduces the capacity allocation of UPQC by reducing the compensation capacity of series units. The series converter compensates the load voltage to the rated value with the amplitude of 310.26 V in the special case that the supply voltage drops to zero. It ensures the uninterrupted power supply to the load. The PV-BESS unit provides the full active power of the load. At which time the power of the load reactive unit is distributed in a coordinated manner.

In this paper, a full load voltage compensation strategy in capacity configuration is proposed for PV-BESS-UPQC. The problems of phase jump and high compensation capacity of series units after the compensation strategy are pointed out. Then, the compensation strategies proposed in this paper are analyzed. Through the comparative analysis of the two compensation strategies, the main conclusions are as follows:

By reasonably distributing the power of UPQC series unit, shunt unit and PV-BESS unit, the capacity configuration of UPQC can be effectively reduced and the compensation performance of UPQC can be improved.

The PV-BESS unit is used to maintain the power supply current as the active component of the load current under normal conditions. It reduces the feeder current increased when the power supply voltage drops and eliminates the risk of feeder overcurrent in the distribution network.

In the special case that the power supply voltage drops to zero, the PV-BESS unit provides all the active power of the load at this time. It ensures the uninterrupted power supply to the load.

Supported by

The authors declare that they have no conflicts of interest to report regarding the present study.