The EnergyPlus team advocates using the HeatExchanger:FluidToFluid object to model primary/secondary chilled water systems.

Check out the Plant Application Guide that comes with the EnergyPlus documentation here: (C:\EnergyPlusV8-2-0\Documentation\PlantApplicationGuide.pdf) and the example file (C:\EnergyPlusV8-2-0\ExampleFiles\PlantApplicationsGuide_Example3.idf) to see how the heat exchanger settings are configured.

You can build this system directly in OpenStudio (see screenshots below). Note that the heat exchanger on the primary and secondary loops are the same object. You can connect a heat exchanger that is already in the model connected to the first loop to a second loop by going to "My Model" on the right and dragging it onto the second loop.