# Coordinated Control of Virtual Power Plants to Improve Power System Short-Term Dynamics

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## Abstract

**:**

## 1. Introduction

#### 1.1. Motivation

#### 1.2. Literature Review

#### 1.2.1. Short-Term Dynamics

#### 1.2.2. Control Structures

#### 1.3. Contributions

- A simple yet effective coordinated control of VPPs for short-term frequency containment, which improves the power system dynamic response and maintains frequency stability.
- A comprehensive study of the impact of different communication networks, focusing in particular on the effect of latency, on the performance and stability of the proposed coordinated control.
- An in-depth discussion of the impact of stochastic sources such as SPVG and WG on the performance of the VPP with and without the inclusion of the proposed coordinated control.

#### 1.4. Organization

## 2. Control of VPPs

#### 2.1. Proposed Coordinated Control

#### 2.2. Frequency Control of Energy Storage Systems

#### 2.3. Frequency Control of Solar Photo-Voltaic Generation

#### 2.4. Frequency Control of Wind Power Plants

#### 2.5. VPP Control Modes

**Mode 1**: DERs and ESSs regulate the frequency but are fully independent (${y}_{p}=0$).**Mode 2**: Only ESSs regulate the frequency. DERs do not include a frequency controller.**Mode 3**: DERs do not include a frequency control. The ESS is regulated to keep constant the power injection ${p}_{\mathrm{inj}}$ of the VPP into the POC. This is a typical VPP operation mode, where TSO schedules the VPP output every 15 min.**Mode 4**: In [5], the weather-driven DERs such as WGs and SPVGs are considered to be non-dispatchable resources due to the stochastic nature of the wind and clouds. The ESS is the only device that regulates frequency. Therefore, in this mode, only the ESS is fed with the signal ${y}_{p}$.**Mode 5**: In [31], it is proposed that wind farms and VPPs can be used for emergency frequency control in smart super grids. Hence, in this mode, both ESS and DERs are coordinated with the signal ${y}_{p}$.**Mode 6**: As with Mode 5, both the ESS and DERs are coordinated in this mode. However, the feedback signal ${y}_{p}$ is used differently for ESSs and DERs. ESSs are always fed with ${y}_{p}$ and, thus, their primary frequency regulation acts immediately after the contingency. On the other hand, DERs are included in the coordinated control and receive the signal ${y}_{p}$ after a given time after the occurrence of the contingency, e.g., 15 s. The timer that activates the feedback signal for the DERs is triggered by the magnitude of the variation of the frequency $\Delta {\omega}_{{\scriptscriptstyle \mathrm{POC}}}$.

## 3. System Modeling

#### 3.1. Stochastic Wind

#### 3.2. Stochastic Load

#### 3.3. Stochastic Solar Irradiance

## 4. Case Study

- The VPP is connected through an Under-Load Tap Changer (ULTC) type step down transformer with TG.
- One SPVG, two WGs, and one ESS are connected at buses D8, D5, D2, and D2, respectively. Each DER uses the bus frequency signal by an Synchronous Reference Frame Phase-Locked Loop (SRF-PLL) installed at Bus D1 for frequency control. The initial active power generation of the WG and the SPVG are 15 MW each, whereas the power rate of the ESS is 10 MW.
- The total active and reactive power consumption of loads in the VPP are 57.8 MW and 11.7 MVar, respectively.
- Since the focus of the case study is to observe the power system short-term transient behavior (a few tens of seconds), the impact of the State of Charge (SoC) of ESS is neglected.

#### 4.1. Monte Carlo Analysis

#### 4.2. Impact of Communication Delays

## 5. Conclusions

- 1.
- The proposed coordinated control approach for ESS and DERs in VPP can significantly improve power system frequency stability. The proposed control approach performs better than either conventional VPPs that do not regulate the frequency, i.e., use a constant power set-point, and VPPs that regulate the frequency through the independent controllers of ESSs and DERs.
- 2.
- Communication delays have a significant impact on the proposed coordinated control approach. This had to be expected, as the proposed strategy works as a sort of fast secondary frequency control. To reduce the negative impact of communication networks without increasing the bandwidth, a two-phase coordinated control is proposed. In this operating mode, the ESS acts first whereas DERs are included in the coordinated control in a second phase. This reduces the impact of the limited capacity of the ESS and, in turn, improves the transient stability.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Conflicts of Interest

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**Figure 6.**Trajectories of the frequency of the Center of Inertia (COI) without VPP frequency control and without ESS. The mean and the standard deviation of the frequency at $t=50$ s are ${\mu}_{\mathrm{COI}}=1.002$ pu(Hz) and ${\sigma}_{\mathrm{COI}}=6.16\xb7{10}^{-5}$ pu(Hz), respectively.

**Figure 8.**Trajectories of the frequency of the COI using: (

**a**–

**c**) proportional gain; (

**d**–

**f**) lead-lag controller; and (

**g**–

**i**) PI controller.

**Figure 9.**Frequency of the COI following a three-phase fault occurs in the TG, where the measurements $\Delta {\omega}_{{\scriptscriptstyle \mathrm{POC}}}$ and ${p}_{\mathrm{inj}}$ are transmitted through high/medium/low-speed communication networks, respectively.

**Table 1.**Mean frequency ${\mu}_{\mathrm{COI}}$ and standard deviation ${\sigma}_{\mathrm{COI}}$ for different VPP control modes.

Statistics | Mode 1 | Mode 2 | Mode 3 | |
---|---|---|---|---|

${\mu}_{\mathrm{COI}}$ | $1.001431$ | $1.001511$ | $1.002144$ | |

${\sigma}_{\mathrm{COI}}\times {10}^{-5}$ [pu(Hz)] | $23.6$ | $28.1$ | $9.73$ | |

Control Type | Statistics | Mode 4 | Mode 5 | Mode 6 |

Prop. Control | ${\mu}_{\mathrm{COI}}$ [pu(Hz)] | $1.000328$ | $1.000323$ | $1.000321$ |

${\sigma}_{\mathrm{COI}}\times {10}^{-5}$ [pu(Hz)] | $1.83$ | $1.58$ | $1.43$ | |

Lead-Lag | ${\mu}_{\mathrm{COI}}$ [pu(Hz)] | $1.000198$ | $1.000155$ | $1.000135$ |

${\sigma}_{\mathrm{COI}}\times {10}^{-5}$ [pu(Hz)] | $8.62$ | $2.24$ | $3.27$ | |

PI | ${\mu}_{\mathrm{COI}}$ [pu(Hz)] | $1.000267$ | $1.000020$ | $1.000023$ |

${\sigma}_{\mathrm{COI}}\times {10}^{-5}$ [pu(Hz)] | $12.4$ | $1.37$ | $1.60$ |

Levels | Bandwidth | PMU Data Rate | Background Traffic |
---|---|---|---|

High Speed | 20 Mbps | 25 frames/s | RTU, Video Stream |

Medium Speed | 5 Mbps | 25 frames/s | RTU, Video Stream |

Low Speed | 1 Mbps | 25 frames/s | N/A |

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**MDPI and ACS Style**

Zhong, W.; Chen, J.; Liu, M.; Murad, M.A.A.; Milano, F.
Coordinated Control of Virtual Power Plants to Improve Power System Short-Term Dynamics. *Energies* **2021**, *14*, 1182.
https://doi.org/10.3390/en14041182

**AMA Style**

Zhong W, Chen J, Liu M, Murad MAA, Milano F.
Coordinated Control of Virtual Power Plants to Improve Power System Short-Term Dynamics. *Energies*. 2021; 14(4):1182.
https://doi.org/10.3390/en14041182

**Chicago/Turabian Style**

Zhong, Weilin, Junru Chen, Muyang Liu, Mohammed Ahsan Adib Murad, and Federico Milano.
2021. "Coordinated Control of Virtual Power Plants to Improve Power System Short-Term Dynamics" *Energies* 14, no. 4: 1182.
https://doi.org/10.3390/en14041182