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Example - Combined GFM and GFL Control Library Based Microgrid Control

Location

The example model can be found in the Artemis installation folder:

C:\OPAL-RT\ARTEMIS\[ARTEMIS version]\common\Examples\

Description

Model

In this example model, four distributed Generation (DG) unit fed islanded microgrid has been developed. Among the four, three DGs operate in Grid Forming (GFM) mode, whereas one operates in Grid Following (GFL) mode. All the DGs are modeled as dispatchable sources, and two level three phase average model inverters have been used for power conversion. The DGs operate in parallel and connected at the point of interconnection (POI) to feed the loads. During operation, the DGs with grid forming control, forms the grid, regulate the voltage and frequency of the microgrid whereas the DG with grid following control follows the phase angle reference and respond according to the voltage and frequency condition at its control terminal.  The DGs are required to maintain the voltage and frequency of the islanded microgrid by dispatching the required active and reactive power demanded by the loads.

The high-level block diagram of the example model is depicted in the following Figure. In the figure, the green blocks represent the DG units with grid forming control, the DG with grid following control is highlighted in light bule, the orange block represent the POI, and the magenta block represents the loads. The DGs have different name-plate ratings and during operation, they share power proportionally.

 

The grid-forming inverter of the Distributed Generation (DG) unit-1 is shown in the following Figure. A dispatchable DC source feeds the GFM inverter; therefore, with varying operating conditions, the DG can instantly adjust its active and reactive power dispatch in proportion to its nominal capacity.  For the DG, Inner Control loop type has been chosen to be “Double-Loop Control” and “Droop Control” has been chosen to be the Primary Control method. During operation, to compensate any steady state-error in voltage and frequency at the POI, secondary controller has also been adopted to the GFM based DG control.

Based on the functionalities, the controller is divided into five subsystems. The first subsystem corresponds to the Signal Conditioning unit, which measures the signals of interest, filters and conditions them to per-unit values, and transforms the signals into d-q coordinates, as required by the subsequent controllers. Afterward, the Secondary Controller estimates any deviation of voltage and frequency from the nominal and set the references of voltage and angular frequency for the Primary Control Loop. The Primary Control Loop block estimates the phase angle and angular frequency and generates references for the magnitude of the virtual voltage source according to the system’s operating conditions. Next, the Inner Control Loop block executes the references set by the Primary Control Loop and generates the reference control signal in d-q coordinate. At last, the Reference Generation block is used to condition the reference control signal generated by its preceding control block according to the DGs nominal DC voltage and the GFM inverter’s operating voltage. In this example, an average two-level three phase inverter model has been used; therefore, the reference signal is directly connected to the Uref port of the the inverter. At the output of the two-level converter, an LC filter is used to reduce high-order harmonics generated by the switching dynamics of the conversion process.

The Distributed Generation unit (DG4) with grid following control is shown in the following Figure. Similar to the other DGs, this one is also configured as disposable. Its active and reactive power dispatch is regulated according to the “Frequency-watt (Droop)” and “Voltage-Reactive Power (Volt-Var)” grid support functions, respectively, configured at the Secondary Control block. Before the activation of secondary control, active power reference is set to zero for the DG.

Based on the functionality, the controller is divided into four subsystems. The first subsystem corresponds to the Signal Conditioning unit, which measures the signals of interest, and conditions them to per-unit values as required by the controller. Consequent to that, the Secondary Control block generates the active and reactive power references, necessary to implement the grid support functions presented in the IEEE std. 1547-2018 [1]. Next, the Primary Control block generates the reference signals corresponding to active and reactive components of the output current. At last, the Wave Reference block is used to generate the reference signal to drive the two-level converter. In this example, an average converter model is used, and therefore, the reference signal is the duty ratio.

All the DGs are connected with the loads at the Point of Inter-Connection (POI). Connection and disconnection of loads is controlled by a three-phase circuit breaker.

Scenarios

The example model is intended to introduce different dynamic changes to the system presented in the previous section to show the effectiveness of the Smart Inverter toolbox and its associated control blocks. The control blocks are supposed to maintain the microgrid’s voltage and frequency around the nominal value by injecting necessary active and reactive power, respectively. Refer to the supplementary MATLAB code to obtain the control parameters and nominal values used in the example model. The following dynamic changes are introduced to the example model to observe the effectiveness of the controller under varying operating conditions:

  1. At t= 10 second, secondary control of the DG (DG 4) with grid following control is activated.

  2. At t= 15 seconds, secondary control of the DGs (DG 1, 2 & 3) with grid forming control is activated.

  3. At t=20 seconds, a new load gets connected with the islanded microgrid.

  4. At t=25 seconds, the newly connected load gets disconnected from the islanded microgrid.

Simulation and Results

The following figures show the simulation results of the example model at the POI and the DG terminal:

From the beginning of the simulation, droop control is activated as the primary control approach on DGs 1 to 3 (grid forming control). Droop ensures proportional power sharing among the DGs, however deviates the voltage and frequency from their respective nominal values. As no secondary controller is activated during this period of time, primary controller alone cannot compensate for the deviations. In addition, during this period, active and reactive power references for DG 4 (grid following control) are set to zero, hence it dispatches no power.

The secondary controller of DG 4 (Grid following control) is activated at 10s. The controller senses the deviations and as can be seen in the Figure above, immediately starts dispatching active and reactive power to compensate them. However, due to its grid following nature, it cannot fully recover the voltage and frequency.

At 15s, the secondary controller of the DGs with grid forming control (DG 1, 2, & 3) is activated. Sensing the deviations, the controllers increase active and reactive power dispatch from the DGs until the voltage and frequency deviation is compensated. The Volt-Var grid support function of DG 4 (grid following control) is programmed in such a way that it makes no reactive power contribution if the voltage at the point of control remains within 0.98 to 1,02 pu. Moreover, the dead band frequency of the Frequency-watt (Droop) grid support function is 0.036 Hz around the nominal value, which implies that DG 4 contributes no active power if the system frequency remains within this range. The activation of the grid forming secondary controllers (DG 1 to 3) brings the voltage and frequency back to nominal and as a result, DG 4 stops exporting active reactive power (Figure 5).    

At 20s a new load gets connected with the system which subsequently disconnected by 25s. Droop control in DG 1 to 3 ensures that the DGs reacted with the varying operating conditions and share powers proportionally and their secondary controller ensures accurate power dispatch and maintains voltage and frequency at nominal.    

Bibliography

[1]

IEEE, "IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces," IEEE Std 1547-2018 (Revision of IEEE Std 1547-2003), no. doi: 10.1109/IEEESTD.2018.8332112, pp. 1-138, 2018.

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