Cascaded STATCOM under high voltage is easily balanced during heavy loads but probably out of control at empty loads. Research on the mechanism of out-of-control at empty loads is the key to solution of this problem.Consequently

we emphatically research the main contents include as follows: ①Equivalent circuit diagram of serial impedance

parallel impedance and control power consumption are simplified and drawn

respectively. ②According to the average modeling theory

the DC capacitor voltage is decomposed into the direct current component and the alternating current component

the alternating current is decomposed into the fundamental wave and the harmonic component

and the modulation degree equivalent switch function is used to list the mathematical expression of the DC voltage of the single power module. ③According to the small signal method of stability analysis

the steady-state component and the disturbance component of the DC capacitor voltage of a power module are calculated. ④In the two cases whether the DC voltage balance control is adopted or not

the DC voltage balance equation of any two power modules of the convert chain at the heavy load and the light load is deduced and calculated

and the DC voltage balance is related to the modulation de-gree

switching frequency

and other factors.（5）An improved DC voltage balance control strategy is proposed. The switching frequency is increased to improve the voltage balancing performance at empty load. In the end

simulations and tests are used to compare the results under four typical operating conditions. The results show that the voltage-sharing control strategy proposed under empty-load conditions can achieve DC capacitor voltage equalization of the power module to increase reliability of the cascaded STATCOM.