This study enhances a capacity expansion planning model to study the system-level impacts of carbon-free electricity procurement by voluntary actors in the western United States, accounting for changes in both system operations and installed capacity. We assess multiple proposed strategies for voluntary procurement of new, locally generated carbon-free electricity, including those that match a participating consumer's demand with carbon-free generation on an annual basis ("volumetric matching''), on an hourly basis ("temporal matching''), or aim to eliminate a consumer's emissions impact as measured via short-run marginal emissions accounting ("emissions matching''). We find that in the current U.S. policy environment, voluntary carbon-free electricity procurements made under volumetric or emissions matching strategies have zero or near-zero long-run impact on system-level CO2 emissions. Carbon-free electricity procurements made under these strategies reduce deployment of similar carbon-free resources by independent developers, but have little impact on fossil-fired generation. By contrast, temporal matching drives significant reductions in system-level CO2 emissions by requiring generation of carbon-free electricity even in hours when fossil-based resources would normally be preferred. Temporal matching also incentivizes procurement of advanced clean firm generation and long-duration storage technologies that would not otherwise see market uptake. Electricity cost premiums for voluntary participants are near-zero under volumetric and emissions matching strategies and can exceed $20/MWh under temporal matching, but are reduced when a larger portfolio of advanced technologies is available for procurement. These outcomes are sensitive to changes in policy: while volumetric matching has near-zero impact under current federal and state policies, it is the most cost-effective means of incremental CO2 mitigation in a scenario with a binding system-wide clean electricity standard, although total emissions reductions remain modest.