Dynamic Transfer is a technology that enables Dynapower Energy Storage Inverters to transition from Grid Tied (current source) mode of operation to Microgrid (voltage source) and back to Grid Tied (current source) mode of operation seamlessly. Dynamic Transfer is most commonly used in microgrids as a method to combine multiple generation resources or to provide backup power to customer loads.
Figure 1 shows a typical implementation of Dynamic Transfer. In Grid Tied mode the contactor is closed, this provides a grid connection to the critical loads as well as a grid tied to the inverter. During operation the inverter is continuously monitoring the grid’s voltage and frequency. If either the grid voltage or grid frequency go out of the allowable range the inverter will implement Dynamic Transfer by opening the grid disconnect device (in this example we are showing a contactor) and transitioning to Microgrid mode. By opening the contactor the inverter has isolated itself and the critical loads from the grid. This type of operation is considered ‘intentional islanding’ and is not be confused with anti-islanding. It should be noted that the Grid Sensing PTs and contactor are to be supplied by the customer. The contactor will be controlled by the inverter.
While operating in Microgrid mode the inverter will continuously monitor the Grid Sensing PTs to determine when grid fault has been resolved and that the grid has returned to stable operating condition. The energy storage inverter will wait for a ‘Grid Ok Timer’ (settable by customer but typically 5 minutes) to ensure that the grid is present and stable before reclosing the contactor and returning to Grid Tied mode of operation.
The transition time from Grid Tied to Microgrid mode of operation depends upon the nature of the grid fault. If the grid fault is a high impedance fault the transition time is < 1 60 Hz cycle, however, if the grid fault is a low impedance fault the transition time will be limited by the opening time of the grid disconnecting device.
Figure 2 shows implementation of Dynamic Transfer at medium voltage. The system integrator should pay special attention to the selection of the disconnect device selection at medium voltage. The medium voltage implementation of Dynamic Transfer is most common with Dynapower’s Utility Scale Energy Storage Inverters from the Compact Power System product line.
Dynapower’s Dynamic Transfer technology establishes a shunt connected interruptible power
supply (UPS), this is in contrast with a traditional ‘on-line’ UPS. Figure 3 provides a notional diagram of a traditional UPS system, which includes a front end charging rectifier, the battery, and an inverter directly servicing the critical load. Dynapower’s Dynamic Transfer most notably that
the critical load is separated from the grid by two stages of power electronics. The disadvantage of traditional the ‘on-line’ UPS is that there is efficiency loss since all power to the critical load must go through two stages of power electronics, we can contract this with the Dynapower Dynamic Transfer implementation which only has a single stage of power electronics between the battery
and critical load. The trade off made going to the shunt connected system is that there will be a flicker if the grid suffers a low impedance (phase to phase short or phase to ground short). However, for high impedance grid faults the transition using Dynamic Transfer there will be little to no flicker. See Figure 4.
Dynapower’s Dynamic Transfer has successfully been utilized in systems with certifications to UL 1741, IEEE 1547, IEC 62019, and IEC 62040. Dynapower has successful implementations of Dynamic Transfer with motorized breakers, contactors, and static switches used as the grid disconnect device.