Rick Crispo
Masters Thesis:
A Motor-Generator and Supercapacitor Based
System for Microgrid Frequency Stabilization
Designed an energy storage system to deliver microgrid frequency response. Coupled a doubly-fed induction generator (DFIG) and a squirrel cage induction machine (SCIM) together as a motor/generator set to deliver an immediate inertial response to a change of frequency, and used an inverter and supercapacitors to deliver short term power. The block diagram is shown in Figure 1.
Figure 1. A block diagram of the frequency stabilization system. A control system measures the microgrid frequency and activates the inverter to deliver power.
Created a control system to monitor the grid frequency and activate the inverter to either charge or discharge a bank of supercapacitors depending on the measured conditions. For example, when the frequency is measured below 60 Hz, the system injects power into the microgrid to arrest the frequency deviation; above 60 Hz the energy storage system absorbs power.
The hardware testing results illustrated in Figure 2 show the effective way that the system stages energy delivery to stabilize microgrid frequency.
Figure 2. A microgrid frequency deviation event is simulated with the change of SCIM speed (blue, top plot), which is tracked by the DFIG speed (green). (The microgrid frequency used to activate the control system is measured at the DFIG stator.) The system responds with energy from the inertia (dark blue, bottom plot) and then the supercapacitors (red).
The system is shown to deliver instantaneous energy from the motor inertia and short-term energy from the supercapacitors.
For a microgrid with a high penetration of distributed renewable generation such as small scale solar and wind systems, additional stabilizing inertia, which is typically provided by large-scale synchronous generators, may be beneficial. This system adds real inertia and short-term energy storage to stabilize frequency.