# Optimal Operation Strategy of ESS for EV Charging Infrastructure for Voltage Stabilization in a Secondary Feeder of a Distribution System

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## Abstract

**:**

## 1. Introduction

## 2. Introduction Necessity of an ESS for Voltage Regulation at the Secondary Feeder with EV Infrastructure

## 3. Line Impedance Decision Algorithm for Operation of Voltage Stabilization ESS

## 4. Optimal Operation Algorithm of ESS for Voltage Regulation

#### 4.1. EVI-Algorithm of ESS for Voltage Regulation

#### 4.2. SOR-Algorithm of ESS for Voltage Regulation

#### 4.3. Coordination Control Strategy between EVI-Algorithm and SOR-Algorithm for ESS

- [Category 1] When the voltage in the secondary feeder violates the upper limit and lower limit.
- Case l. Only one voltage is not kept within the upper limit and lower limit
- Case 2. Multiple voltages are not kept within the upper limit and lower limit

- [Category 2] When the ESS is performed as the SOR-algorithm operation

## 5. Modeling of Electric Vehicle Charging Infrastructure at the Secondary Feeder

#### 5.1. Battery Energy Storage System Modeling for Voltage Stabilization

#### 5.2. Loads (Consumer) and EV Loads (Charging) Modeling

#### 5.3. Photovoltaic Power Generation Modeling

#### 5.4. Control Algorithm Modeling

## 6. Case Studies

#### 6.1. Performance Index

#### 6.2. Simulation Condition

#### 6.3. Analysis of the Secondary Feeder Voltage Characteristic by Voltage Stabilization Operation of ESS

#### 6.3.1. Verification of the EVI Algorithm for Voltage Stabilization Strategy

#### 6.3.2. Analysis of the feeder voltage characteristic by the introduction of ESS

#### 6.4. Analysis of the SOC Characteristic of ESS According to the Application of SOR Algorithm

#### 6.4.1. Verification of the SOR Algorithm for Maintain of Proper SOC Operation Range of ESS

#### 6.4.2. Analysis of the ESS SOC Characteristic by SOR Algorithm

#### 6.5. Performance Caluation for Feeder Voltage Characteristic

## 7. Conclusions

- (1)
- To operate the ESS introduced in the EVS infrastructure, this paper presented a method to determine the line impedance based on the voltage and the passing current on the existing secondary feeder with RES and EV infrastructure. Specifically, based on the least square method, the optimal line impedance value was determined by the relationship between the pole transformer voltage and the power common coupling voltage at the EVS infrastructure, including passing current.
- (2)
- To overcome voltage problems at the EVC infrastructure in the secondary feeder, a voltage compensation method by the ESS operation was proposed. The control characteristic of the ESS was analyzed for the voltage stabilization operation when the feeder voltage was not kept at an allowable limit. In this case, it is confirmed that ESS has been exactly controlled by the operation determination signal.
- (3)
- Therefore, from the simulation results, it was confirmed that all of the feeder voltages at the PCC section, including all sections, could be perfectly maintained within the allowable limits by the EVI-algorithm of the ESS in case the feeder voltage could not be maintained to reasonable conditions (over/under voltage phenomena) by the maximum output of PV system, energy demands of the EVC, and load.
- (4)
- When the ESS did not operate as a voltage stabilization function and the SOC of the ESS violated the set SOR range, the SOC pattern was varied by the operation characteristic of the ESS based on the operation signal of the SOR algorithm. Based on the simulation, it was confirmed the that this strategy is an effective tool to solve the no guarantee problems for operation papacy by the ESS.
- (5)
- Based on the result of the performance index base, it was clear that feeder voltages in the distribution system (secondary feeder) could be maintained within the allowable limits. Therefore, the case study results show that this strategy is an effective tool to solve the voltage problems by an ESS. However, since this proposed scheme operates based on centralized control concepts, it was necessary to well configure and operate the communication system with other components, such as inverters, chargers, and measurement devices.
- (6)
- This study was performed in a simulation environment with certain conditions. Therefore, in our future work, we will implement this algorithm in an EMS system and validate it in a real electric vehicle charging infrastructure. In addition, it is necessary to analyze the difference in the performance index according to the control period of the proposed algorithm and the effect of the change in capacity and combination of the EVC on the proposed algorithm.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

${Z}_{n}$ | line impedance value |

$lr$ | liner regression method |

${U}_{p.tr}$ | pole transformer voltage |

${U}_{ci}$ | power common coupling voltage at EV infrastructure |

${I}_{ci}$ | passing current at the secondary feeder |

$\mathrm{COS}{\theta}_{ci}$ | Power factor at the secondary feeder |

$n$ | ESS site (EV infra) section number of the secondary feeder |

$m$ | measuring time |

$q$ | error function |

$\mathrm{T}$ | total number of the time interval |

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**Figure 1.**Strategy of power stabilization in the secondary feeder by an energy storage system (ESS).

**Figure 4.**Configuration of the secondary feeder with electric vehicle (EV) infrastructure for the proposed method.

**Figure 12.**Flowchart of coordination control strategy between EVI-algorithm and SOR-algorithm for ESS.

**Figure 13.**Bi-directional grid-connected inverter model with an ESS [16].

**Figure 21.**Analysis of the feeder voltage characteristic at electric vehicle charging (EVC) infrastructure without an ESS.

**Table 1.**Previous researches of stabilization of voltage on applications of an electric vehicle charging infrastructure.

Ref. | Major Contents | Advantages | Disdvantages |
---|---|---|---|

[5,12] | Charging price method is used considering the voltage deviation | User friendly with economic approach Real-time price update | Slow response to voltage variation |

[6,7,8,9,10,11] | Time-of-use pricing (TOU) scheme is introduced to decrease the peak load demand and alleviate overload | User friendly with economic approach | Slow response to voltage variation |

[13,14,15] | Fast-charging process control scheme to control power quality (DC/DC) | Fast response to voltage variation | Customized components (power converter, EV charger) |

Classification | Signal Condition | ||||
---|---|---|---|---|---|

$\mathit{\alpha}$ | $\mathit{\beta}$ | $\mathit{\gamma}$ | $\mathit{\u03f5}$ | $\mathit{\delta}$ | |

Charging operation of voltage stabilization | 1 | 0 | - | - | - |

Discharging operation of voltage stabilization | 0 | 1 | - | - | - |

Charging operation within SOR range | 0 | 0 | 1 | 0 | 1 |

Discharging operation within SOR range | 0 | 0 | 1 | 1 | 0 |

Not operation | 0 | 0 | 0 | 0 | 0 |

0 | 0 | 1 | 0 | 0 | |

0 | 0 | 0 | 1 | 0 | |

0 | 0 | 0 | 0 | 1 |

Section Number | Impedance | Length (km) | Power Factor | Load (kW) | RES (PV System) (kW) | EV Charger (kW) | |
---|---|---|---|---|---|---|---|

R (Ω/km) | X (Ω/km) | ||||||

1 (total) | 0.40 | 0.027 | 1 | 1 | 0–15.0 | 0–30.0 | 0–18.9 |

1-1 (load) | 0.40 | 0.027 | 0.03 | 1 | 0–15.0 | - | - |

1-2 (RES) | 0.40 | 0.027 | 0.05 | 1 | - | 0–30.0 | - |

1-3 (EVC) | 0.40 | 0.027 | 0.05 | 1 | - | - | 0–18.9 |

1-4 (ESS) | 0.40 | 0.027 | 0.08 | 1 | - | - | - |

Not Operation of ESS | ESS Operation | |
---|---|---|

Performance index value | $8.2803\times {10}^{6}$ | $7.0532\times {10}^{6}$ |

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**MDPI and ACS Style**

Kim, D.-J.; Ryu, K.-S.; Ko, H.-S.; Kim, B. Optimal Operation Strategy of ESS for EV Charging Infrastructure for Voltage Stabilization in a Secondary Feeder of a Distribution System. *Energies* **2020**, *13*, 179.
https://doi.org/10.3390/en13010179

**AMA Style**

Kim D-J, Ryu K-S, Ko H-S, Kim B. Optimal Operation Strategy of ESS for EV Charging Infrastructure for Voltage Stabilization in a Secondary Feeder of a Distribution System. *Energies*. 2020; 13(1):179.
https://doi.org/10.3390/en13010179

**Chicago/Turabian Style**

Kim, Dae-Jin, Kyung-Sang Ryu, Hee-Sang Ko, and Byungki Kim. 2020. "Optimal Operation Strategy of ESS for EV Charging Infrastructure for Voltage Stabilization in a Secondary Feeder of a Distribution System" *Energies* 13, no. 1: 179.
https://doi.org/10.3390/en13010179