###### Abstract  
The high penetration of photovoltaic (PV) systems in low-voltage distribution networks relieves the pressure of energy shortages. However, the severe intermittency and variability in the output of PVs also bring a series of voltage quality problems, such as over-voltage, under-voltage, and voltage fluctuations. In this paper, a novel voltage control strategy based on air conditioners (ACs) and an on-load-tap-changer (OLTC) is proposed. By regulating the operating power of the ACs, voltage regulation powers are provided for the distribution network during the regulation time delays of the OLTC, which can effectively avoid the occurrence of regulation lag by the OLTC. The thermal model and electrical model are established to evaluate the operating performance of voltage regulation potential of the ACs. Furthermore, the voltage regulation strategy is formulated based on the regulation potential of the ACs, which it can make full use of. The effectiveness of the proposed voltage control strategy is illustrated in numerical studies.  
distribution network; voltage regulation; on-load-tap-changer (OLTC); air conditioners (ACs); time delays +
Footnote †: journal: Applied Sciences  
With the development of the smart grid, a flexible load can be remotely controlled to optimize the voltage of a distribution network through demand response technology [9]. Among the numerous flexible loads, air conditioners (ACs) contribute to almost 40% of the total power demand in the United States [10]. In addition, the insulation of buildings has improved, allowing the ACs to change the operating power with little effect on room temperature. The development of microelectronic technology and frequency conversion technology makes the compressor speed of the AC adjust continuously [11]. In this way, the inverter AC can flexibly participate in voltage regulation. Therefore, inverter ACs are suitable and have enormous potential to participate in voltage regulation for the distribution network.  
[PERSON] et al. equivalent an inverter AC to the virtual battery and regulate the low-voltage distribution network voltage by coordinating distributed VESSs [12]. Additionally, [PERSON] et al. provide a hierarchical control scheme to coordinate multiple groups of AC controlled loads to regulate network loading and voltage in a distribution network [13]. However, due to the limited quality of ACs on the user side, the ACs cannot afford massive regulation power. In addition, changing the operating power of ACs would affect the room temperature, negatively impacting the comfort of occupants [14]. Therefore, the ACs cannot provide a voltage regulation service for long.  
To address the above problems, a novel voltage regulation strategy with coordinated control of an OLTC and ACs is proposed. When the distribution network encounters a voltage problem, the ACs provide a voltage regulation service during the time delays of the OLTC. After the regulation action of the OLTC finishes, the ACs quit the regulation service. In this manner, the voltage regulation can be responded to quickly. The voltage regulation potential of the ACs and OLTC can be fully utilized. The contributions of this paper can be described as follows:  
1. A detailed thermal and electrical model is developed for a voltage regulation service. Based on the models, the voltage regulation potential can be evaluated to formulate the voltage regulation strategy of the ACs, making full use of the voltage regulation potential.
2. A novel voltage regulation method is proposed by coordinating the control of the OLTC and ACs. In the regulation process, the ACs take priority in voltage regulation during the time delays of the OLTC. In this manner, the slow action defect of the OLTC can be offset.
3. A voltage ranking optimization algorithm is proposed. The algorithm considers preferentially regulating the node with the most severe voltage problem, which can utilize less regulation power for voltage optimization.