调差系数和强励电压倍数对发电机暂态稳定的影响

Effect of Reactive Current Compensation and Excitation System Ceiling Voltage Ratio on Transient Stability of Generator

该文首先将调差系数和励磁电压强励倍数对暂态稳定的影响转化为研究两者对同步转矩的影响,以便于数学推导。然后对调差系数和励磁电压强励倍数影响发电机同步转矩的机理进行推导和数值计算,继之以单机、两机系统和实际系统的仿真。计算和仿真结果表明,水轮机大负荷下,负调差提供负同步转矩而正调差提供正同步转矩;小负荷下,调差系数提供的同步转矩呈抛物线规律变化。汽轮发电机则不同,大小负荷下,均有负调差提供正同步转矩而正调差提供负同步转矩。但短路故障时,励磁电压达到输出上限,与调差系数无关,所以调差系数不会影响发电机的暂态稳定。与一般认识不同,研究发现,虽然励磁强励顶值电压E_(FDMAX)越大越有利于暂态功角稳定,但影响幅度微小。E_(FDMAX)绝对值的变化对发电机q轴暂态电势E_(q’0)和发电机同步转矩KS的影响在短路故障期间十分微小,在短路故障消失后影响虽有所增大,但仍然微小。短路故障消失后,E_(FDMAX)增大对发电机无功输出和高压侧母线电压提升明显,这是增大E_(FDMAX)的主要作用所在。

Firstly,the relationship between transient stability and synchronous torque of generator was analyzed,the influence of reactive current compensation(RCC)and excitation ceiling voltage ratio on transient stability was transformed into that on synchronous torque for mathematical derivation.Then,the mechanism of RCC and excitation ceiling voltage ratio affecting synchronous torque of generator was deduced and calculated by an example,followed by the simulation of the single-machine system,two-machine system and actual system.The results of calculation and simulation show that minus RCC coefficient provided minus synchronous torque and plus RCC coefficient provides plus synchronous torque for hydro-generator under heavy load,and the synchronous torque provided by RCC changes in parabola for hydro-generator under light load.For turbine-generator under all loads,minus RCC coefficient provided plus synchronous torque and plus RCC coefficient provides minus synchronous torque.Since the excitation voltage reached the upper limit in case of short circuit fault,it has nothing to do with the RCC coefficient,so the RCC coefficient would not affect the transient stability of the generator.Different from the general understanding,it can be found that the larger the excitation ceiling voltage,i.e.E_(FDMAX)is,the better the transient power angle stability is,but the impact on the transient power angle stability of the units is small.During the short-circuit fault,the absolute value change of E_(FDMAX)has little effect on the transient potential of q-axis,i.e.E_(q’0)and the synchronous torque,i.e.KS.After the short-circuit fault disappears,the influence increases,but it is still small.After the short-circuit fault disappears,the increase of E_(FDMAX)has obvious effect on the reactive power output of generator and the voltage of high side bus,which is the main function of increasing E_(FDMAX).