Phase unbalance is widespread in the distribution networks in the UK

continental Europe

US

China

and other countries and regions. This paper first reviews the mass scale of phase unbalance and its causes and consequences. Three challenges arise from phase rebalancing: the scalability

data scarcity

and adaptability (towards changing unbalance over time). Solutions to address the challenges are: 1) using retrofit table

maintenance-free

automatic solutions to overcome the scalability challenge; 2) using data analytics to overcome the data-scarcity challenge; and 3) using phase balancers or other online phase rebalancing solutions to overcome the adaptability challenge. This paper categorizes existing phase rebalancing solutions into three classes: 1) load/lateral re-phasing; 2) using phase balancers; 3) controlling energy storage

electric vehicles

distributed generation

and micro-grids for phase rebalancing. Their advantages and limitations are analyzed and ways to overcome their limitations are recommended. Finally

this paper suggests future research topics: 1) long-term forecast of phase unbalance; 2) the whole-system analysis of the unbalance-induced costs; 3) the phase unbalance diagnosis for data-scarce LV networks; 4) techno-commercial solutions to exploit the flexibility from large three-phase customers for phase balancing; 5) the optimal placement of phase balancers; 6) the transition from single-phase customers to three-phase customers.