level (primary frequency response). The grid frequency is then corrected to its prefault level so the balancing authority area's control error is reduced to zero with slower responsive reserves (secondary frequency response). Tertiary reserves would be needed to protect the system against a possible subsequent event when both primary and secondary reserves have already been depleted. VRE is capable of providing any type of operating reserves; however, the challenge here is that accurate resource forecasts are needed at different time horizons to ensure the availability of such reserves from wind and solar generation. Energy storage can also be used to provide additional reserves and help correct possible imbalances due to forecast errors. Table 1 describes the characteristics of several types of generators and their ability to provide various aspects of grid stability. Normally, power electronic converters provide full decoupling of the wind and solar generator from the grid table 1. Generation types and capability for grid stability. Generation Type Inertia Conventional synchronous generation √ √ √ DFIG wind turbine generator with partial power conversion √* √ √ √ √* √ √ √ √** √ √ √ √* √ √ √ G-Box DFIG Reactive Power, Voltage Control Fault RideThrough Active Power Control Collector T Turbine Transformer Bus Grid id Power Converter Wind turbine generator with full-size power conversion Collector Bus PWM Converter PMSG T Turbine Transformer Grid Collector Array ay Bus Inve In verr ter Transformer = ~ Grid id PV Array Plant Transformer = ~ Collector Bus Grid id Battery Storage *Synthetic rotational inertia-like response possible at any operating conditions. **Synthetic rotational inertia-like response possible if curtailed with headroom. march/april 2017 ieee power & energy magazine 69