# Coordinated control for voltage regulation of distribution network voltage regulation by distributed energy storage systems

- Delong Zhang
^{1}, - Jianlin Li
^{2}Email author and - Dong Hui
^{1}

**3**:3

https://doi.org/10.1186/s41601-018-0077-1

© The Author(s) 2018

**Received: **24 May 2017

**Accepted: **10 January 2018

**Published: **26 January 2018

## Abstract

With more and more distributed photovoltaic (PV) plants access to the distribution system, whose structure is changing and becoming an active network. The traditional methods of voltage regulation may hardly adapt to this new situation. To address this problem, this paper presents a coordinated control method of distributed energy storage systems (DESSs) for voltage regulation in a distribution network. The influence of the voltage caused by the PV plant is analyzed in a simple distribution feeder at first. The voltage regulation areas corresponding to DESSs are divided by calculating and comparing the voltage sensitivity matrix. Then, a coordinated voltage control strategy is proposed for the DESSs. Finally, the simulation results of the IEEE 33-bus radial distribution network verify the effectiveness of the proposed coordinated control method.

## Keywords

## 1 Introduction

In the past decade, the development of wind power and PV generation is very fast because of their technical and environmental advantages. They also bring several new challenges to power system. With the improvement of permeability of PV generation in the distribution network, the traditional distribution network becomes an active network, whose voltage bus connected PV generation is higher than other bus [1].

The traditional distribution network has multiple voltage regulation methods, such as on-load tap changer (OLTC) [2], reactive power regulator [3]. However, these methods cannot always regulate the voltage flexibly and effectively, because the voltage is different from the network without PV. The voltage of one node is regulated to the normal range, the others maybe exceed caused by the action of regulator. Moreover, the frequent transformation of the power grid will be very uneconomical. There are usually two ways to adjust voltage using a distributed generation (DG). One is to increase the reserve capacity of the distributed power supply, which obviously cannot maximize the utilization rate of renewable energy. The other is to apply inverter to generate reactive power to regulate voltage, which will increase inverter efficiency. In [4], the authors proposed a voltage regulation method by adjusting the inverter DG injected active power. In [5], the voltage regulation criterion of DG connected to the distribution network is proposed, and the capacity and position of reactive power compensation are determined accordingly. In [6, 7], it is found that DG should reduce the active output according to the actual voltage of the grid. Obviously, this method is not conducive to the full play of DG. In [8], a coordinated control method based on DGs and OLTC voltage regulation is proposed, which divides the DGs and OLTC adjustment area and controls its processing order to reduce the operation times of the on-load tap changer.

The DESS is usually installed with distributed PV plant. Expect for the normal operation, DESS can be used to regulate the voltage. It can not only improve the utilization of PV power, but also improve the voltage quality of power system [9, 10]. Distribution network contains a number of DESSs, if they are in a chaotic state, it cannot play the role in voltage regulation. Therefore, the main challenge using DESSs to regulate voltage is how to coordinate DESSs reasonably and determine the output power [11, 12].

In [13], the authors propose a coordinated control method of DESSs, which includes distributed and localized controls. The consensus algorithm is used to regulate the feeder voltage, while the localized control regulates the state of charge (SoC) of each ESS within the desired SoC range. In [14], the former one is voltage regulation in terms of their installed capacity and the latter one modifies the BESs performance according to their SoC. These papers mainly focus on the voltage regulation of low voltage distribution network that the area is small. They are not suitable for large area medium voltage distribution network.

In this paper, a coordinated control method using distributed energy storage is proposed for adjusting the voltage of a medium voltage distribution network. Firstly, in the three-node simple system, the voltage of system with and without PV plant is compared. The influence caused by PV plant is analyzed. Secondly, a method is proposed to divide the voltage regulation area by calculating and comparing the voltage sensitivity matrix. On this basis, coordinated control sequence of DESSs and the model of the energy storage system is established. Finally, the simulations are carried out on the IEEE 33-node system.

The paper is organized as follows. The theory of voltage deviations in the distribution feeders with high penetration of PV cells is explained in section 2. The proposed coordinated control strategy of distribution network voltage regulation is presented in section 3. Section 4 introduces the IEEE 33-bus test distribution system and presents simulation results. Finally, Section 5 concludes the paper.

### 1.1 Problem description

In the distribution network, the voltage is generally reduced with the extension of the feeder. The access of distributed PV generations changes this situation. In this part, a simple three-node system is used to illustrate the problem.

*a*,

*b*,

*c*are three nodes.

*Z*

_{1}and

*Z*

_{2}are the impedance of line 1 and line 2 respectively. \( {S}_1^{\prime } \) and \( {S}_1^{{\prime\prime} } \) are the power of the start and the end of line 1 respectively.\( {S}_2^{\prime } \) and \( {S}_2^{{\prime\prime} } \) the power at the start and end of line 2 respectively.

*S*

_{ b }and

*S*

_{ c }are the load of node

*b*and node

*c*respectively.

In the power system without PV generation, some equations can be obtained according to the simple power flow calculation method.

It is assumed that the voltages of node *b*, *c* have meet the error requirements. It is obviously that the voltage of node *c* is smaller than *b*, *b* is smaller than *a*.

When adding the PV station on node *c*, in order to clarify the problem more clearly, we assume that the reactive power of the PV plant *Q*_{
PV
} and *Q*_{
c
} are both zero, active power *P*_{
PV
} is larger than *P*_{
c
}.

We can see that PV plant is not only to meet the load demand of node *c*, but also provide power to the grid. \( {S}_2^{{\prime\prime} } \) and \( {S}_2^{\prime } \) are negative and have the opposite direction with original direction without PV. \( {S}_1^{"} \) and \( {S}_1^{\hbox{'}} \) become smaller than before. Therefore, Δ*V*_{1} becomes smaller and *V*_{
b
} becomes larger, Δ*V*_{2}becomes negative. The voltage of node *c* is larger than node *b*.

From the above analysis, we can see that the voltage of node *c* is larger than node *b* after the access of PV plant. This is only a simple three-node system analysis. In fact, the number of network nodes and PV plants are more so that the situation of voltage is more complicated. The traditional way of voltage regulation, for example, the on-load tap changer, may lead to make a node to return to normal, while the other nodes are beyond the allowable range.

With the increasing configuration of DESSs on PV plants, the use of DESS for voltage regulation becomes a new regulated method. DESS can regulates voltage by a variety of ways, such as absorption or generation of active power, absorption or generation of reactive power, combination of active power and reactive power [15–17].

### 1.2 Proposed coordinated control strategy

### 1.3 Division of voltage regulation area

The distribution network usually contains a number of DESSs. If many DESSs act at the same time when a node voltage changes, this kind of disorder will not only play a good voltage regulator effect, but will deteriorate the voltage quality of other nodes. Therefore, for a distribution network with multiple DESSs, it is necessary to determine which nodes does the DESS have a greater effect on the voltage regulation. The voltage of these nodes is divided into an area. When the voltage of a node in this area is out of its range, the DESS in this area will act for voltage regulation.

Δ*P* and Δ*Q* are the output of active power and reactive power changes of DESS respectively. Δ*V* and Δ*δ*are the voltage amplitude and phase angle changes respectively.

\( \left[\frac{\partial V}{\partial P}\right] \) is the voltage-active sensitivity matrix and \( \left[\frac{\partial V}{\partial Q}\right] \) is the voltage-reactive power sensitivity matrix. They indicate respectively the amount of voltage change caused by the change of active and reactive power of DESS.

*i*, caused by the active and reactive power disturbance of node j, is:

If the change of DESS is constant, the higher the voltage sensitivity, the stronger the support effect of the DESS on the voltage. In other words, the lower the voltage sensitivity, the weaker the support effect of the DESS on the voltage.

- (1)
Calculate the power flow based on the monitoring information. Obtain the Jacobian matrix of the distribution system and calculate the inverse matrix of Jacobian matrix.

- (2)
When the DESS

*x*act, Δ*V*_{i − x}can be obtained according to the formula (15). When the DESS*y*acts, Δ*V*_{i − y}can be obtained according to the formula (15). Compare Δ*V*_{i − x}and Δ*V*_{i − y}, if Δ*V*_{i − x}is larger, node*i*belongs to the area of DESS*x*, where*x*and*y*are the mark number of DESSs. Every DESS charges or discharges to calculate the Δ*V*_{ i }, and all of them are compared to select the maximum. - (3)
Since the Jacobian matrix is real-time and needs to be recalculated at each sampling time, the voltage regulation region also changes.

After dividing the voltage regulation area, it is also necessary to arrange the sequence of the operation of the DESS. The local DESS will act firstly, then the non-local DESS with higher Δ*V*_{
i
} will act. If there are non-local DESSs with same value of Δ*V*_{
i
}, the one that has the closer distance to the root node will act. Because root node is a slack node, the DESS closed to it has less impact on other nodes.

### 1.4 Energy storage model and judgment of energy storage status.

*E*_{
rate
} is the rated capacity of energy storage. *η*_{
ch
} and *η*_{
disch
}are the energy storage charge and discharge efficiency, respectively. Δ*E* = *P*Δ*t* is the energy charge or discharge energy. *P* is the charge or discharge power. Δ*t* is the sampling time.

As mentioned earlier, we needs to determine whether there is distributed storage cannot participate in voltage regulation. This step is very important. If a DESS cannot respond to voltage regulation needs, but the system give a command to act, the DESS maybe break down.

- (1)
It is assumed that the voltage of the node i exceeds the minimum value, then the energy storage should be discharged. If the discharge power of corresponding DESS is rated power, it cannot increase more discharge power. If not, step (2) will be executed.

- (2)
Determine whether the SoC is the minimum, if so, then exclude the energy storage, if not, then the calculate the Jacobian matrix;

- (3)
The power flow will be calculated considering the charge or discharge power with the interval of 5 kW until find the minimum power value to make the voltage normal.

- (4)
When the voltage at node

*i*exceeds the maximum value, the judgment of the energy storage state is similar.

### 1.5 Case study

#### 1.5.1 Voltage analysis of the system without energy storage

#### 1.5.2 Voltage analysis of the system with energy storage

Two 300 kW/600kWh DESS are installed at node 9 and 15. They are DESS 1 and 2 respectively. One 300 kW/800kWh DESS is installed at node 30. It is DESS 3. After the DESSs are installed, the next step is to divide the voltage regulated area. Each DESS will have obvious effect on the nodes its own area.

*V*caused by DESS 2 is bigger. Therefore, the node 10~ 18 belong to the area of DESS 2. The node 26~ 33 are effected by DESS 3 more obviously. Other nodes are effect by three DESSs similarly. It can be judged that the regulation is more effective on the nodes near the DESS. The areas are shown in Fig. 3.

In the above simulation analysis, the voltage regulation area can be obtained by calculating the Δ*V* caused by the output power change of DESS. When the voltages of some nodes exceed the limit, but the SoC of corresponding DESS is not enough, the other DESS will be coordinated to act. The voltage will be returned to the normal range, although the non-local DESS will charge or discharge more power than the local DESS. In these simulations, we can see that the voltage can be regulated use DESSs. When the voltage is lower or higher than the limit value, DESSs will charge or discharge power according to the different voltage regulated areas and the different values exceeded the normal value. When the SoC of local DESS exceeds the allowed range or is not enough, the non-local DESS will coordinately act to regulate voltage.

## 2 Conclusions

- (1)
The voltage will become more disordered if more PV stations access the distribution network.

- (2)
The proposed method is able to divide the voltage regulation areas. The DESSs can charge or discharge when the voltage of their own area exceed the limit.

- (3)
If the voltages of two or more areas exceed the limitation at the same time, the DESSs will act. The effect of two DESSs is larger than one. Taking 8:00 for instance, DESS 2 and 3 discharge respectively 60 kW to make the voltage of their own area back to the normal value, while 45 kW and 40 kW discharged by DESS2 and 3 can achieve this effect when they act both.

- (4)
When the voltages of some nodes exceed the limit, but the SoC of corresponding DESS is not enough, the other DESS can be coordinated to act. And it can achieve the regulation effect too.

The proposed coordinated control strategy is suitable for medium voltage distribution network or larger system. The future work will deal with adapting active and reactive power to regulate voltage and optimize the output power of multiple DESSs.

## Declarations

### Acknowledgements

Not applicable.

### Funding

This paper is supported by The National Key Research and Development Plan, Energy Storage Technology of 10MW Level Redox Battery, 2017YFB0903504.

### Availability of data and materials

Not applicable.

### Authors’ contributions

DZ contributed significantly to analysis, manuscript preparation and manuscript submission as a corresponding author; JL and DH helped perform the study analysis with constructive discussions, professional advice and revised the manuscript. All authors read and approved the final manuscript.

### Ethics approval and consent to participate

Not applicable.

### Consent for publication

Not applicable.

### Competing interests

The authors declare that they have no competing interests.

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

## Authors’ Affiliations

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