To evaluate competition among firms, it is important to look at the substitution pattern between the firms. In 2013, the Taiwan Fair Trade Commission (Taiwan FTC) fined 9 independent power producers (IPPs) 6.3 billion NTD for their collusive behavior during their contract negotiation process with the utility company (Taiwan Power Company). The IPPs argued that they did not compete with each other in the electricity generation sector because they were already under long-term contracts with the utility company. In this paper, I use unit-level electricity generation data to examine the substitution pattern between IPPs. Using an exogenous event in which He-Ping Power Plant failed to provide power to the utility because its transmission tower had collapsed due to a typhoon, I examine changes in power supply by other IPPs. I find that the event caused other IPPs to increase the amount of electricity supplied to the utility per hour by 350 MWh. During hours when the take-or-pay clauses did not apply, the diversion ratio between IPPs' gas-generating units and the He-Ping Power Plant was about 0.45. My results reject the hypothesis that IPPs did not compete with each other in the electricity generation sector.