System Start-up

The start-up of the Dual Mode Controller requires special consideration with regards to the generation of the internal reference signals. The system must be able to start operation without the presence of the grid which is termed ‘Black Start’ as

shown in Figure 7-3. This preliminary operational mode must initialize the system with an internally generated reference. The black start internal reference is used until the islanding process has begun to operate and has reached a valid steady- state (frequency within defined limits). Once the island controller generates a valid voltage reference, the system transitions from using the black start reference to use the island reference. If the grid connection becomes engaged while in black start, the system will transition to the grid connection operation and use the grid as the reference source. If the grid is present but not connected, once the island controller becomes active, the system will utilize the tracking algorithm to bring the island references to be within the tolerances of the grid values to allow the grid connection to be engaged. The system can be forced to the Black Start mode from the Idle and Open Loop modes. This will force a system restart for testing purposes.

7.2.2 Support States

The support states represent modes of operation that are used during the test of the system. The states also provide a state transition that is required when other modes encounter problems. Both the Idle and Fault modes place the inverter into a ‘safe’ operating mode. The transitions from the Idle, Fault and Open Loop states are illustrated in Figure 7-4.

7.2.2.1 Idle

The Idle state places the inverter into a ‘safe’ operating mode such that no power will be transferred to the AC bus. The Idle state is also useful as an interim state that must be used to transition into the special states of Forced Island, Open Loop and Black Start.

7.2.2.2 Fault

The Fault state is a special mode that the controller enters when a fault condition is detected by the system or an erroneous state transition is attempted. The inverter is placed in a ‘safe’ mode and will remain in this state until the operator places the system back into the Idle state.

7.2.2.3 Open-Loop

The Open-Loop state is used during testing of the system which enables the open loop controller. The system will remain in this state until the operator places the system into the Idle state.

7.2.3 Island

The Island states support the operation of the microgrid independent of the main utility. The Load Following and Forced Island states are illustrated in Figure 7-4.

7.2.3.1 Load Following

The Load Following state becomes active when the grid connection is lost. The Islanding (Voltage Control) Controller becomes active in this mode. The system will transition to a grid connected state when the grid connection is re-established.

7.2.3.2 Forced Island

The Forced Island state is a special form of islanding control. The controller that operates is the same as the load following mode. The forced islanding option allows the operator to place the controller into the islanding operation regardless of the connection state. The forced island state will only become active when the controller receives feedback that the PCC has been placed into a forced island. The only valid transition is back to the Idle state (or Fault).

7.2.4 Grid Connected

The microgrid becomes a ‘power producer’ when connected to the grid. The grid connected modes (Grid Idle, Curtailment, Peak Shaving and Export) are illustrated in Figure 7-5. These modes are only valid when the microgrid is connected to the grid.

7.2.4.1 Grid Idle

The Grid Idle state is used as the transition state from an island operation to a grid connected operation. During the transition period, the microgrid transitions from full load support to minimal load support. The grid idle state also provides a default state when no other grid connected mode has been selected. The controller will not deliver any power to the AC bus while in this mode.

7.2.4.2 Curtailment

The operator must enable the curtailment operation using the supervisory user interface. When the curtailment state is disabled the system returns to the Grid Idle state. It is possible for the operator to choose to switch to another grid connected mode.

7.2.4.3 Peak Shaving

The operator must enable the peak shaving operation using the supervisory user interface. The operator must specify a valid peak shaving limit from the user interface. Otherwise, the system remains in the idle state. When the peak shaving

state is disabled, the system returns to the Grid Idle state. It is possible for the operator to choose to switch to another grid connected mode.

7.2.4.4 Export

The operator must enable the export operation using the supervisory user interface. The operator must specify a valid export power amount from the user interface. The export amount must be greater than zero but less than the available generation. If an invalid export level is entered, the system remains in the idle state. When the export state is disabled, the system returns to the Grid Idle state. It is possible for the operator to choose to switch to another grid connected mode.

7.2.5 Shutdown

The shutdown operation requires placing the controller in a state that will stop power flow through the inverter. This state is equivalent to the idle state with the additional requirement that the pulses being sent to the switch are stopped. The operator interface provides a “Pulse Disable” function which will stop pulses and places the switch into a safe state.

7.2.6 Restart

The controller can be restarted from a cold state by forcing the system back to its Black Start operation. The restart function can only be initiated when in Idle and Open Loop modes.

7.3 System Disturbances

The system controllers can be influenced by different sources of disturbances. One of the most drastic operation change that will cause a disturbance is the opening or closing of the point of connection between the microgrid and the grid. As well load changes and changes to operating modes or set point changes used by the different operating modes will introduce disturbances into the system.