NPC PV Inverter
Description of the legacy 3level NPC PV Inverter component.
This Schematic Editor component block from the Microgrid category models a lowvoltage, photovoltaic solar inverter implemented with a threephase threelevel neutral point clamped inverter, rated at 1 MVA @ 480V LL.
Cascaded control loops consist of a current controller (resonant control in alphabeta coordinates with 5th, 7th and 11th harmonic rejection), an openloop power controller (based on instantaneous grid voltage), and an energybased DC voltage controller with a fixedstep MPPT algorithm.
The DC link is open for connection to a Photovoltaic panel component with a maximum power point around the rated value of 1000 V_{DC}. This inverter implements a modulation strategy to internally keep the capacitorcreated DC midpoint voltage at half of the total DC voltage.
The reactive power is defined in terms of a desired power factor and a direction (inductive or capacitive). The power controller allows for P, Q, or power factor priority in situations of power saturation (references greater than the maximum allowed power).
The startup procedure is implemented as follows: once grid voltages and frequency are in the allowable range, the grid contactor will be closed. After that, if the inverter enable input is true, the inverter will start switching and the system will look for the maximum power point for the connected PV panel. This inverter will ramp up the references on startup.
This NPC PV inverter implements voltage and frequency ridethrough functionality according to UL 1741 SA with Rule 21 requirements. See Voltage and Frequency ridethrough for details.
The following sections describe in more details the component parameters, inputs, and outputs.
component  component dialog window  component parameters 

NPC PV Inverter 

Number  Name  Description 

0  Inverter Enable  Enables operation of the inverter. Note that grid contactor control is independent from this command. 
1  Power Saturation Priority  Defines priority when requested setpoints are higher than maximum allowed power. Setting = 0 ensures active power priority. Setting = 1 ensures reactive power priority. Setting = 2 preserves power factor (reduces P and Q proportionally). 
2  Reference Power Factor  Desired power factor for inverter output power (between 0 and 1). 
3  Reactive Output Power direction  Direction for the reactive output power. Setting +1 means output reactive power is capacitive (supplying to the grid), 1 means it is inductive (consuming from the grid). 
Number  Name  Description 

0  DC link Voltage  Measurement of total DC link voltage. 
1  DC link midpoint Capacitor Voltage  Measurement of midpoint capacitor DC link voltage. In normal operation should be half of total voltage. 
2  Precharging Contactor  Digital signal active when precharging contactor is closed. 
3  Main grid Contactor  Digital signal active when main grid contactor is closed. 
4  Operating  Digital signal active when all startup procedures are finished and inverter is operating with all control functions. 
5  Frequency ridethrough inverter enable  Digital signal active when inverter should not be subject to a trip because of frequency ridethrough protection. 
6  Voltage ridethrough inverter enable  Digital signal active when inverter should not be subject to a trip because of voltage ridethrough protection. 
7  Voltage ridethrough references enable  Digital signal active when inverter should not zero the power injected to the grid because of voltage ridethrough protection. 
8  Grid RMS Voltage phase A   
9  Grid RMS Voltage phase B   
10  Grid RMS Voltage phase C   
11  Grid RMS Current phase A   
12  Grid RMS Current phase B   
13  Grid RMS Current phase C   
14  Grid frequency   
15  Grid injected Active Power (P)   
16  Grid injected Reactive Power (Q)   
17  Grid injected Apparent Power (S)   
18  Grid injected Power Factor   
Voltage and Frequency ridethrough
This inverter implements frequency and voltage ride through according to UL 1741 SA with Rule 21 requirements. A summary of the settings can be found in the following tables:
Region  Voltage (% Nominal Voltage)  RideThrough Duration (s)  Operating Mode  Maximum Trip Time (s) 

High Voltage 2 (HV2)  V ≥ 120 %  0.16  
High Voltage 1 (HV1)  110 % < V < 120 %  12 
Momentary Cessation (gate blocking) 
13 
Near Nominal (NN)  88 % ≤ V ≤ 110 %  Indefinite  Continuous Operation  Not Applicable 
Low Voltage 1 (LV1)  70 % ≤ V < 88 %  20  Mandatory Operation  21 
Low Voltage 2 (LV2)  50 % ≤ V < 70 %  10  Mandatory Operation  11 
Low Voltage 3 (LV3)  V < 50 %  1 
Momentary Cessation (gate blocking ) 
1.5 
Region  System Frequency Default Settings  Frequency in pu  RideThrough Duration (s)  RideThrough Operational Mode  Trip Time (s) 

High Frequency 2 (HF2)  f > 62  f >1.0083  No RideThrough  Not Applicable  0.16 
High Frequency 1 (HF1)  60.5 < f < 62  1.033 < f < 1.0083  299  Mandatory Operation  300 
Near Nominal (NN)  58.5 < f < 60.5  0.975 < f < 1.0083  Indefinite  Continuous Operation  Not Applicable 
Low Frequency 1 (LF1)  57.0 < f < 58.5  0.95 < f < 0.975  299  Mandatory Operation  300 
Low Frequency 2 (LF2)  f < 57.0  f < 0.95  No RideThrough  Not Applicable  0.16 
Example
Overall behavior and control methodologies can be better understood with the use of the given example:
Model name: npc_pv_inverter.tse
SCADA interface: npc_pv_inverter.cus
Path: /examples/models/microgrid/npc_pv_inverter/
Acknowledgements
This component was developed with partnership with Federal University of Santa Maria's Power Electronics and Control group (GEPOC) in Brazil, represented by Prof. Humberto Pinheiro, Ph.D and team.