Abstract:

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.

Key words: synchronous grid size, frequency stability, low frequency oscillation, synchronous power support, direct synchronization, indirect synchronization