The situation is when farmers have to sell their produce at low prices after harvest, when supplies are greatest.

本句中is不是系词，而是不及物动词，等于arises, 出现、发生。

第一个when为连词，引导时间状语从句修饰is。

第二个when也是连词，引导时间状语从句修饰have to sell...。 have to sell...被二个时间状语从句同时修饰，after从句和when从句。

when没有介词用法。