Table 7.4.2 Droop control for regulating voltages and frequency Control Level Timescale Function Implementation level Primary Milliseconds Realize proportional sharing of real and reactive power demand LC Secondary Seconds Restore frequency and voltages to set‐points LC Tertiary Minutes or event‐driven Optimize set‐points for frequency and voltages MC Figure 7.4.5 illustrates the hierarchical control strategy applied to a grid‐forming DER for regulating microgrid frequency. In this case, DER is initially operated at point “a”, representing its initial set‐point. Once the microgrid suffers a real‐power shortage (e.g., switched to the islanded mode), the real‐power output of DER will be adjusted for primary control to supply the power deficit according to the droop characteristic. Subsequently, the operating point will be shifted to point “b”, representing an increase in the DER power output at a reduced frequency. The secondary control will then take place for mitigating the regulation error introduced by the primary control. The secondary control restores the DER operating frequency at its set‐point by maintaining the adjusted active power output at point “c”. The tertiary control identifies point “d” as the optimal DER power output without re‐adjusting the microgrid frequency. The secondary and tertiary controls shift up/down the droop characteristics by applying proportional, integral, and derivative (PID) controllers for error compensations. Microgrid Frequency Real Power Output a b c d Primary Control Secondary Control Tertiary Control Figure 7.4.5 Hierarchical microgrid control for network frequency regulation
47 b) Upper‐Layer Control: Regulating Power Exchanges Between Networked Microgrids When the IIT‐Bronzeville networked microgrids are islanded from the main grid, the operation of the two microgrids is coordinated by enabling power exchanges via IC. The MC in BCM is responsible for issuing supervisory commands that regulate power exchanges between the two microgrids. When one microgrid encounters a power imbalance, the other microgrid will provide support by exercising the power exchange determined by the MC in BCM. Without the loss of generality, we assume an adequate level of reactive power is already supplied at each microgrid level; therefore, IC will only consider active power transfer for adjusting operating frequencies of both microgrids. The islanded networked microgrids coordinate the operation of the two networked microgrids by enabling power exchanges via IC. The MC in BCM is responsible for issuing supervisory commands that regulate inter‐microgrid power exchanges. When one microgrid encounters a power imbalance, the other microgrid will provide support by exercising the desired power exchange for balancing the power in each microgrid. Without the loss of generality, we assume an adequate level of reactive power is already supplied at each microgrid level; therefore, IC will only consider the active power transfer for adjusting the frequency.
You've reached the end of your free preview.
Want to read all 61 pages?
- Spring '14