1 of 13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter

^{Department of Electrical Engineering,} ^{Indian Institute of Technology Bombay}

Jenson Joseph A, Sandeep Anand, Baylon G Fernandes

2 of 13

Presentation Outline

Motivation
System Description

LCL filter Design

Control Scheme
Loss Model
Results and Discussion
Conclusion

Medium Voltage Power Station *

* https://www.sma.de/en/products/system-solutions-packages.html

Inverter

MV T/F

2/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

3 of 13

Motivation

Silicon Carbide (SiC) based high voltage MOSFETs like 3.3 kV, 6.5 kV and 10 kV are being made available by many manufacturers.

These high voltage MOSFETs enable the use of simple converter topologies like three level Neutral Point Clamped (NPC) converters at MV level as well.

Device	Device parameters
Device	Blocking Voltage	Current (T_j=115^oC)	R_dsON (T_j=115^oC)
G2R120MT33J [1]	3300 V	22 A	185 mΩ
G2R300MT65-CAL [2]	6500 V	10 A	650 mΩ

Source: www.genesicsemi.com

G2R120MT33J

G2R300MT65-CAL

Source: www.genesicsemi.com

[1] https://www.genesicsemi.com/sic-mosfet/G2R120MT33J/ G2R120MT33J.pdf

[2] https://www.genesicsemi.com/sic-mosfet/bare-chip/G2R300MT65-CAL/G2R300MT65-CAL.pdf

3/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

4 of 13

Motivation

The conventional PV inverters have a low frequency transformer

Inverter is directly connected to the MV grid.
A medium frequency transformer is used for step up of voltage.
Lower efficiency for system having medium frequency transformer
An adaptive DC bus control is proposed to overcome this lower efficiency.

4/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

The conventional medium voltage inverters targeted at Photovoltaic applications have a low frequency step up transformer at the grid side.

All the commercially available medium voltage PV inverters follow a system configuration similar to the one shown here.

It consists of a 1000V PV panel voltage with an mpp range of 500-800V and an IGBT based inverter.

With the availability of the high voltage SiC mosfets it is possible to realise a system where the inverter is directly connected to MV grid.

Hence a system is proposed as shown, a 1500V PV string voltage with mpp range of 800V-1300V acts as input. Then an isolated DCDC converter is used to boost the PV voltage up to 20 kV to realise a grid connected inverter at 11kV grid.

The choice of DC bus voltage is of importance as a high value will result in low efficiencies when operating at low grid voltages as well as at low power levels.

Hence an adaptively changing DC voltage is proposed here.

The main aspect of the system that will be discussed is the grid connected inverter. To determine the device performance the chosen devices in previous slides are used to realise the MV converter.

�

5 of 13

System Description

Three phase, three level configuration of NPC

Switch realization using 3.3 kV, 22 A MOSFETs

Switch realization using 6.5 kV, 10 A MOSFETs

Series parallel combination of devices done to achieve the required rating of the converter (11kV, 1 MVA).

5/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

6 of 13

System Description (LCL Filter Design)

Inverter side filter inductor calculation

Filter Capacitor Calculation:

Grid side filter inductor calculation

[3] X. Ruan, et. al., Control Techniques for LCL-Type Grid-Connected Inverters, Beijing, China: Springer Press, 2018.

6/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

To calculate the Inverter side filter inductor the maximum and minimum values are obtained using the folowing equations.

The minimum value of inverter side inductor is calculated by considering the maximum ripple current (Δ𝑖) to be 30% of rated.

The maximum value is limited by the voltage drop appearing acroos the inductor ie (𝜆_(𝑉_𝐿𝑖 )≈ 5%)

This gives the range of the inverter side filter inductor to be between 12.54mH and 19.25mH. Hence a value of 15mH is chosen.

Filter capacitor is determined based on the reactive power introduced by the capacitor and limiting it to (𝜆_𝐶) 5% of the rated power.

This gives us that the maximum capacitor value is 0.6microF. Hence a 0.5micro farad is chosen.

moving on to the grid side inductor, its value is determined based on the limit imposed on the switching frequency component in grid current which is to be less than 0.3%. Which implies the Harmonic proportion factor 𝜆ℎ must be less than 0.3% and a value of 0.15% is chosen. The 𝜔_ℎ is the dominant switching frequency component in grid current corresponding to the dominant harmonic component in the switching voltage waveform.

And using the previous calculated values of inverter side inductor and capacitive filter we get a value of 2.5mH for the grid side inductor.

Now that the system and its parameters are discussed we move on to the control aspects

�

7 of 13

Control Scheme

Conventional PQ Control Scheme

Proposed Adaptive DC Voltage Control

7/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

The control scheme shown here is the conventional PQ control used for PV inverters.

The DC bus voltage is maintained constant at a reference value such that the inverter is operating at 0.9 modulation index at nominal voltages.

The DC bus voltage required for this operation will be at 20.2kV. This results in high switching losses in the inverter. In order to to reduce the switching losses in the inverter an adaptive dc voltage control is proposed.

An additional PI control is added to the conventional control which is used to maintain the modulation index of the inverter at 0.95 by varying the DC bus voltage reference.

this results in a DC bus variation from 16.5kV- 20.2kV for varying grid conditions and at nominal operating conditions the dc link voltage is only 18kV which is substantially lower than 20.2 kV in case of conventional control.

The control is achieved by calculating the real time modulation index of the converter and changing the DC voltage in response to the deviation of modulation index from set point.

�

8 of 13

Controller Performance

Grid Voltage and Grid current for varying grid conditions

Per unit magnitude representation of grid voltage and current; DC bus voltage.

Modulation Index of the inverter

8/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

9 of 13

Loss Model of SiC MOSFETs

The loss modelling of SiC MOSFETs can be done in two ways:

Curve fitting based modelling:

Based on SPICE or similar simulation software.

Analytical modelling:

Based on device parameters from datasheet.

[4] M. Shen and S. Krishnamurthy, "Simplified loss analysis for high speed SiC MOSFET inverter," 2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC), 2012, pp. 1682-1687.

[5] S. Eskandari, K. Peng, B. Tian and E. Santi, "Accurate Analytical Switching Loss Model for High Voltage SiC MOSFETs Includes Parasitics and Body Diode Reverse Recovery Effects," 2018 IEEE Energy Conversion Congress and Exposition (ECCE), 2018, pp. 1867-1874.

9/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

Now to understand the improvement of performance of the converter, loss analysis is done.

The SIC mosfets being relatively new the loss models can be made by two methods 1 curve fitting based modelling usng SPICE or similar softwares.

2 using the Analytical model which are derived based on the device behaviour and characteristics.

Two different analytical models are used to determine the loss energy curve of 3.3kV MOSFET and compared with the actual data sheet energy loss curve.

1 A device capacitor stored energy model and a 2 two capacitor model for cgd and cds is used for device loss estimation from literature.

It is found that the analytical loss model based on the stored capacitor energy of MOSFETs gives us accurate results for the device loss.

Hence the stored capacitor energy model is used for estimating the device losses.

�

10 of 13

Results and Discussion

Loss distribution in a 3L-NPC phase leg

Converter efficiency for varying power levels at different grid voltages

The power loss is non uniform in the converter.
The loss in converter realized using 3.3 kV devices is lower than the one with 6.5 kV

3.3 kV

6.5 kV

At lower power levels the switching loss dominate the device losses.
At Higher power levels the conduction loss dominate the device losses

10/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

The figure gives the loss distribution in one leg of the inverter realised using 3.3kV and 6.5kV operating at 1 MVA output power and nominal grid voltage. the loss in M1 and M4 is considerably higher than the other switches.

these switches limit the maximum rating of the inverter.

This shows that using higher voltage MOSFETs to realize a switch will lead to higher losses in the converter.

This is due to the longer turn on/off time for the 6500 V device caused by the larger parasitic capacitances which need to be charged and discharged in each cycle.

The second figure shows the efficiency vs output power. It can be seen that at lower power levels the switching loss dominates because of which lower DC bus voltage is preferred. Also it is seen that the inverter realised using the 3.3kV device has better efficiency in comparison.

�

11 of 13

Results and Discussion

Device	Fixed DC voltage control Loss (kWh)				Adaptive DC voltage control Loss (kWh)
Device	Switching	Conduction	Filter	Total	Switching	Conduction	Filter	Total
G2R120MT33J	12.84	16.83	33.10	62.77	11.53	16.28	32.89	60.70
G2R300MT65-CAL	41.42	10.19	33.11	84.72	36.94	10.17	32.89	80.02

Fixed DC voltage control

Adaptive DC voltage control

Fixed DC voltage control

Adaptive DC voltage control

11/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

12 of 13

Conclusion

The proposed control is achievable by small changes in the conventional PQ control scheme.
Using the proposed control, losses in the converter are reduced up to 5.5%.
Based on a 3L-NPC converter realized using high voltage SiC MOSFETs of different current and voltage ratings it can be concluded that the converter realized using 3.3 kV voltage SiC MOSFETs have lower losses.
Utilizing low voltage 3300 V MOSFETs for realizing the Medium Voltage inverter along with the proposed adaptive dc voltage control gives the maximum efficiency for the MV grid connected PV inverter.

12/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand

13 of 13

Thank you for your kind attention.

13/13

An Adaptive DC Voltage Control for SiC based Medium Voltage Photovoltaic Inverter - Sandeep Anand