电网互联可以充分发挥电网的经济效益，而同步电网规模充分扩大后将带来电网调度、低频振荡等问题。从纯技术的角度，提出决定同步电网合理规模的3个基本约束：频率稳定约束、低频振荡最低频率约束和同步功率支援效应消失约束。研究表明：稳态频率偏差指标比暂态频率偏差指标对同步电网规模的要求更高，因而同步电网频率稳定约束可根据稳态频率偏差指标计算；快速励磁控制在低频振荡频段内对发电机阻尼的贡献是负的，且振荡频率越低负阻尼越大，配备有电力系统稳定器的发电机总电气阻尼通常在振荡频率低于0.3 Hz后变负，且振荡频率越低负阻尼越大；同步功率系数是刻画同步功率支援效应的指标，若同步电网内2台机组间稳态功角差超过90°，同步功率支援效应将消失。从同步电网一般性结构出发进行研究，其研究结论对实际电网的互联方式与电网规模确定具有指导意义。

Power system interconnections bring primary economic advantages. However， certain technical problems occur with the size expansion of synchronous grids， such as grid dispatching and low frequency oscillation. From technical viewpoints， this paper proposed three basic constraints to determine reasonable size of synchronous grids， including the frequency stability constraint， the minimum frequency constraint of low frequency oscillations and the synchronous support effect disappearance constraint. The research results indicate that steady frequency deviation index has higher requirements for synchronous grid size than that of transient frequency deviation index. Therefore the frequency stability constraint of synchronous grid can be calculated according to the deviation index of steady frequency. Fast excitation control contributes negatively to generator damping in low frequency spectrum， and the lower oscillation frequency the worse of damping effect is. The total damping of generators with power system stabilizer （PSS） tends to be negative when the frequency is lower than 0.3 Hz， and the lower oscillation frequency the worse of damping effect is. The synchronous power coefficient represents synchronous support effect. The synchronous support effect disappears when the steady angle difference of any two generators is larger than 90° within the synchronous grid. Based on the general structures of synchronous grids， the research conclusion in this paper has guiding significance to the interconnection method of actual power grids and grid scale determination.