When the power network breaks down or connected to the high-power unbalanced load

the voltage of the power network will be asymmetric

which will affect the stable operation of the power system. So

it is particularly important to compensate the reactive power quickly. Delta-connected cascaded static synchronous compensator（STATCOM） can compensate reactive power and negative sequence current comprehensively

which is one of the most effective power quality solutions for high voltage and large capacity applications. However

the calculation method of phase current reference signals is complex and slow

as a result

the fast compensation is hard to achieved. This paper proposes a fast unbalance compensation method based on sliding mode control for delta-connected cascaded STATCOM. Sliding mode control is introduced and combined with the PI controller in the current reference loop

therefore

the references of the compensation currents can be obtained fast. Then

the response speed of unbalance compensation is improved. Combined with voltage regulation method

phase to phase balance control and current inner loop control of delta-connected cascaded STATCOM

the fast support of grid voltage is realized. Finally

the simulation results verify the correctness and effectiveness of the proposed control method.