The possible control modes are: Load Following, Curtailment, Peak Shaving, Export and Idle. The Curtailment, Peak Shaving and Export modes are only valid when the system is connected to the grid [7.1]. Five additional states are also supported: Open Loop, Black Start, Idle, Forced Island and Fault.
7.5.1 Load Following
In Load Following (Islanded) mode, there is no grid available. The local generation source must provide the power required by the load such that PRef = PLoad. In the
example shown in Figure 7-6, the system operates in Load Following Mode between
t1 and t2 and in Peak Shaving Mode before t1 and after t2. The example also shows
that the local generation is only capable of producing to the Maximum Generation level. When the load exceeds the maximum generation, a load shedding strategy must be employed to reduce the local load.
7.5.2 Curtailment
Curtailment operation is valid for a defined time interval when the local generation is required to supply energy for the local load attempting to eliminate the amount of power required from the grid. In Curtailment Mode, a connection to the utility is present. The motivation for supporting this mode is that some utilities offer a price discount if a customer can remove all load during a specified interval of time. Penalties are incurred if the customer does not remove itself from the grid at the time specified and stay disconnected for the required duration.
In the example shown in Figure 7-7, the microgrid is requested to remove itself from the grid between t1 and t2. The local generation operates between t1 and t2
producing to its maximum generation level. The load beyond the maximum generation level between t1 and t2 is provided by the utility supply.
7.5.3 Peak Shaving
Peak Shaving operation specifies that the amount of power consumed by a customer from the grid is set to a fixed maximum such that any additional load above the maximum is supplied by the local generation source. In a Peak Shaving Mode, a connection to the utility is present. The Peak Shaving objective is to minimize demand charges for a customer by reducing the peak load experienced by the utility. Failure to peak shave on one day can have severe economic consequences in cases where a customer’s rate is based on monthly peak demand. In this type of application reliability becomes a key consideration.
In the example shown in Figure 7-8 the local generation operates between t1 and t2
when the load (PLoad) rises above the specified Peak Load (Ppeak). The remainder of
the load is supplied by the utility.
7.5.4 Export
Export operation is valid for a defined time interval such that the local generation is operated at a specified generation capacity to deliver a desired surplus generation (after satisfying the local load) to the grid. In Export Mode, a connection to the utility is present. The Export objective is to send a fixed amount of power to the grid. In the example shown in Figure 7-9, the system is required to export a specified amount of power between t1 and t2. Before t1 and after t2 the system
operates in Peak Shaving Mode in this example. At time t1 the local generation
begins operating to supply the local load and to produce a surplus of power to provide the desired export amount to the grid. As the local generation increases the amount of surplus power requested to export may decrease. The total amount of generation available limits the power produced by the microgrid.
7.5.5 Idle
Idle Mode is valid regardless of the connection state. The Idle objective is to force the output from the system to zero. No output current is produced. All of the power required for the local load must be provided by the grid. If the grid is not available in the idle mode, the local load is starved of power because no local generation is available.
7.5.6 Open-Loop
The Open-Loop Mode is a special testing mode used during the validation of the simulation model and experimental facilities. No feedback controls are in place so the inverter output voltage and frequency are manually regulated.
Figure 7-6 Load Following Example
Figure 7-7 Curtailment Example
Figure 7-8 Peak Shaving Example
7.5.7 Forced Island
The Forced Island Mode is a special testing mode used to test the islanding operation of the inverter. In this mode, the system is kept in an islanded state as long as the microgrid remains isolated from the grid.
7.5.8 Fault
The Fault Mode is a special mode that is utilized when the system detects abnormal behaviour, such as an improper state transition or an un-supported state. The inverter is placed into a ‘safe’ mode such that no power is to be delivered by the inverter.
7.5.9 Black Start
The Black Start Mode is the initial mode that the system takes on start-up. This mode is necessary because at system start-up it is not possible to immediately begin operation of either the Voltage Controller or the Current Controller. The black start mode is responsible for placing the system into a known good operating condition (similar function to islanded operation) and to activate either the voltage or the current controllers as soon as possible based on the system conditions at the end of the black start time interval.
7.6 Dynamic Controller
The Dynamic Controller is responsible for the activation of the correct controller based on current operating conditions. The Dynamic Controller operates with four controllers operating simultaneously as illustrated in Figure 7-10. The design of the specific mode controllers are detailed in the Chapter 6.
7.6.1 Controller Selection
The controller selection is based on the active control mode. The Open-Loop and Idle controllers operate independently of all inputs and can be activated at any time.
The Island controller is activated when the connection state changes to “Island” to regulate the voltage and the frequency. The Grid Connection controller is activated when the connection state changes to “Grid Connected” and to regulate the current through the inverter.
Figure 7-10 Dynamic Controller
7.6.2 Controller Activation
As discussed previously, the transition between modes is of special concern. One specific concern is the operation of the PI controllers within the island and grid connection algorithms. When the connection state changes, it is desired for the PI controllers in each algorithm to be reset. This forces the PI to operate in a ‘clean’ state with no residual influence of the previous mode of operation.
7.7 References
[7.1] J. Barton, D. Infield, Energy Storage and Its Use With Intermittent Renewable Energy, IEEE Trans. on Energy Conversion, Vol. 19, No. 2, June 2004, pp. 441-448
8 MICROGRID SIMULATION MODELS
The approach to building the simulation model is to construct the components of the system separately and integrate the working models into the full system model. The full simulation is composed of the following components: Phase Locked Loop, Power Electronic Interface (Open-Loop Model), Fuel Cell Power Source, Current Controller, Voltage Controller, Supervisory Controller, Frequency Tracking and Black Start including Synchronization. The models are evaluated individually as well as collectively to meet the defined control objectives.