电网规模的扩大、输电电压等级的提高，使得继电保护在电力系统中的地位变得更加重要。
With the expansion of power grid and the increase of the transmission voltage grade, the status of relay protection in power system is becoming more and more important.
1949年我国火电最大机组为1万千瓦，输电电压等级最高为220千伏，且全部为外国设计制造。
In 1949, the largest thermal power unit of China was 10 MW and the highest transmission voltage was 220 kV, all designed and made by foreign countries.
随着电压等级的提高及线路结构的变化，输电线路雷击跳闸率计算法也不断地改进。
With being higher the voltage and varying of transmission line structure, the calculation method of lighting trip-out rates should be improved continually.
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