Brattle experts assisted the Hawaiian Electric Companies (HECO) in developing its Power Supply Improvement Plan. Our support focused on how to maintain system security as renewable penetration increases toward 100% and displaces traditional synchronous generation. We evaluated the ancillary services needs to support system frequency following a contingency. We defined technology-neutral requirements that may be met by demand response, distributed resources, and new technologies as well as traditional resources. Finally, we evaluated how battery storage, curtailment of variable energy resources, and more flexible operation of remaining traditional resources could be used in real-time operations.