低频减载（Under Frequency Load Shedding，简记作UFLS）是一种制止事故状态下频率的快速下降,维持系统频率稳定的有效措施，目前已被各国电力系统普遍采用。本论文重点讨论低频减载方案的设计方法。传统的、但也是目前在实际系统中应用最为广泛的低频减载方案是用以频率偏移量Df作为减载判据的方法来整定的。在这种方法的早期应用中，由于受到很多原因的限制（如频率继电器的性能较差,对电力系统功频动态过程的了解不够充分等），设计得到的UFLS方案往往级数少、级差大，因而方案的适应性较差。如澳门电网的原UFLS方案只有四级，级差为0.3Hz和0.5Hz两种，减载效果很不理想。随着电力系统的发展和继电器性能的提高，按Df启动和整定的UFLS方案逐渐向小级差、多级数、每级减载量较少的方向发展，设计方法也变得很多样化。本论文介绍了用于设计按Df启动的减载方案的一种比较合理的方法--最大可用容量切除法并将其应用于澳门电网UFLS的实际设计，总体上取得了比较令人满意的结果。但由于Df只能静态地反映系统中有功缺额的大小，而无法动态地描述其数值，因此，这种按Df来设计的UFLS方案还存在很大的局限性，其适应能力还远远不能满足系统运行的要求，尤其在“小系统大事故”的情况下，这种UFLS方案显得无能为力。引入频率下降率df/dt作为减载判据并将其与Df判据相结合进行UFLS设计是一个众所周知的提高UFLS适应性的理想解决方案，但从目前情况来看，这种双判据UFLS方案尚缺乏较为系统的整定设计方法。本论文根据电力系统功频动态过程的特点，提出了一种按Df和频率级间平均df/dt（Df/Dt）来整定的减载方案，使UFLS的适应能力得到较大提高。为使设计方法系统化、简单化并保持方案良好的适应性，本论文在上述方案思路的基础上又进一步引入了模糊控制（Fuzzy Control）的理论和方法，对基于模糊控制的Df+df/dt双判据UFLS的设计进行了深入探讨。数字仿真表明，模糊控制减载方案具有很强的适应性。 现代电力系统除了存在频率稳定问题之外，其电压稳定性问题也变得日益突出。虽然机理不同，但对电压稳定问题可以象对频率稳定问题的处理一样，用切负荷的方法来防止系统失稳，因此将低频减载的设计方法推广至低电压减载是一个值得研究的课题。本论文对此也进行了一些有益的尝试。
Under frequency load shedding (UFLS), which has been adopted in modern electric power systems worldwide, is an effective means to stop rapid frequency decay and to restore power system stability following heavy losses of generation. In this paper, the design of UFLS is presented.In the traditional UFLS schemes, the observed frequency deviation (△f) following the system disturbance is adopted for determining the amount of load to be shed. The inchoate UFLS schemes designed by use of △f, e.g. The UFLS scheme on Macau Power System, have fewer steps and big step-length and then can not acquire satisfactory effects. At present the tendency of UFLS of such type is to adopt more steps, smaller step-length and less cut-off load of each step. An improved method, the maximum available capacity cut-off load method, and its application to Macau System are introduced in the paper.The UFLS schemes designed by use of △f have some drawbacks, because △f only reflects the static active power deficiency. Although it is attractive to use the variation rate of frequency (df/dt) as an additional criterion for load shedding, there is no systematized method. This paper proposes a new UFLS scheme in which △f and df/dt are adopted as combinative criteria. It has been found in simulation that the new scheme has good adaptability. In order to systematize and simplify the △f+df/dt scheme and keep its adaptability, the fuzzy control has been introduced into the design. This paper serves as an introduction to new UFLS scheme with △f+df/dt dual criteria based on fuzzy control. The simulation results show that the performance of the fuzzy UFLS scheme is excellent.In addition, it has been discussed in this paper when the above-mentioned UFLS design method is adopted in undervoltage load shedding.
