Study On Steady-State Power Limit,Point-to-Point Characteristics And Impedance Matching Of Half-wavelength Power Transmission Line

As the preferred optimal scheme of long-distance and large-capacity transmission,the half-wavelength AC power transmission(HWACT)line has a broad application prospect.The steady-state power limit of the HWACT line is infinite when the loss is neglected.Moreover,the HWACT line fully conforms to the point-to-point characteristics:the sending-to-receiving voltage ratio and current ratio are both fixed at-1,while the reactive power of the whole half-wavelength transmission line is self-balancing.However,a realistic line is lossy,and the total length of the line may not be exactly half-wavelength.Therefore,it is necessary to study the transmission characteristics of the HWACT line under the non-ideal condition.In this paper,the steady-state power limit and the point-to-point characteristics are investigated after considering the line losses and the length offset.In addition,an impedance matching scheme is proposed for relaxing the overvoltage problem of the HWACT line.In the aspect of the steady-state power limit,there are two different restrictive conditions:the phase angle difference is adjustable when the sending-to-receiving voltage amplitude is given,and the load impedance is adjustable when the first end voltage is given.Firstly,the formulas for the steady-state power limit of the HWACT line under the non-ideal condition is derived from the two different restrictive conditions.Then the influence of line losses,length offset and frequency offset on the steady-state power limit is analyzed.When the sending-to-receiving voltage amplitude is given and the load is adjustable,the steady-state power limit of the half-wavelength transmission line reduces to a finite value after the line losses considered,but it can still reach several times of the natural power.When the transmission line transfers the steady-state power limit,the overvoltage along the line is very serious(up to several times of the rated voltage).Hence the factor limiting the transmission power of the half-wavelength transmission line is not the steady-state power limit,but the overvoltage of the line.The length offset or frequency offset(less than 10%)will make the steady state power limit reduce by about 30%.In the aspect of the point-to-point characteristics,firstly,the formulas of the point-to-point characteristic are derived under the non-ideal condition.Then,the influence of the line losses and the length offset on the point-to-point characteristics is calculated and analyzed.The HWACT line is no longer completely comply with the point-to-point characteristics under the non-ideal condition.However,the point-to-point characteristics are good on the whole.According to the calculation results for a UHV transmission line,when the transmission line transfers the natural power with the line losses considered,the maximum offset of point-to-point characteristics is 6%.When the transmission power is less than or greater than the natural power,the point-to-point characteristics will be worse.In the aspect of the point-to-point characteristics,firstly,the variation of the overvoltage along the half-wavelength transmission line is analyzed.When the load impedance of a half-wavelength line is less than the characteristic impedance,there will be an obvious overvoltage problem and it will influence power transmission.Therefore,the load impedance should be controlled not less than the characteristic impedance.Then the impedance matching scheme of paralleled compensating capacitor is put forward.In addition,the analytical formula of the compensating capacitor is deduced.The simulation results show that the paralleled compensating capacitor can effectively reduce the overvoltage.