1. Basics of interfacing PV to the Grid
Dr John Fletcher

2. What is a photovoltaic cell?
A semiconductor junction that absorbs photon (light) energy and converts it to electrical energy by elevating electrons across the band-gap.
It is a static device (no moving parts) which makes it reliable.
It generates a DC voltage which is useful in some applications, but a hindrance in others.
It is an expensive generation technology.

3. Example PV Cell Characteristics
More sunlight increases current and marginally increases voltage
Increase in array temperature reduces cell voltage and marginally increases short circuit current

4. Maximum power point
At every instant in time there is an operating point (v,i) which results in maximum power generation.
The voltage (hence current) that provides maximum power changes with sunlight and temperature.
Partial shading of an array also impacts on the maximum power that can be generated.
More on MPP later!

5. Series-connecting cells
Series connection of multiple cells multiplies the output voltage.
This allows the voltage to be increases to practical values.
Note that all cells must conduct the same current.

6. Shading effects
With no shading, module would output P1. The v-I characteristic is a multiple of the fully irradiated cell.
If one cell is shaded then that cell has a v-i characteristic that is different to the other cells.
The resultant MPP of the module is P2 which is much less than P1.
Points 1, 1(a) and 1(b) show one operating point (not the MPP).
They indicate that if operating with a shaded cell at module operating point, the shaded cell operates at 1(a) – it is consuming power, not generating.
This can lead to array hotspots and possible destruction/fusing of cells.
To avoid this bypass diodes are used.

7. Shading and bypass diodes
Two arrays with two bypass diodes.
Bypass diode improves MPP with partial cell shading.
Note double ‘hump’ on ¾ shaded cell. This can confuse MPP tracker.

8. Shading and bypass diodes
More bypass diodes improves performance (MPP) under shaded conditions.
Usually one bypass diode is used across cells for economic reasons.

9. Aim of the interfacing system
To interface a source of electrical energy to the grid
Supply power at the correct voltage and frequency.
Extract the maximum power available from the PV array at any instant in time.
Cope with faults at the grid.

10. Basic System Diagram
DC-DC converter processes the variable output voltage from the PV array and converts it to a stable voltage high enough to integrate to the grid.
The DC-DC inverter converts the DC voltage across CDC into a grid-compliant AC voltage.
The DC-DC converter typically performs MPP tracking.
The system is likely to include a transformer (though some countries/states allow transformerless systems).

11. System classifications
Inverter systems can be classified according to
Number of stages of power processing
Transformer or transformerless
Type of inverter stage (CSI or VSI etc.)

12. Maximum Power Point Tracking
MPP tracker is used to maximise the extracted power in real-time (as sunlight, shading and temperature change).
Two methods typically used:
Hill-climbing (perturb and observe)
dP/dV - Rate of change of power with voltage

13. Coordinating inverter and converter
DC-DC converter maximises power (PPV) from PV and delivers to CDC.
The DC-AC converter must ensure that PPV is delivered to the grid.
If there is an imbalance, the voltage across CDC will rise/fall depending on whether the exported power is less than or greater than PPV.
The controller for the DC-AC converter uses the DC voltage across CDC to decide whether to export more or less power.
The synchronisation and power control techniques used by the DC-AC converter are the same as those used for grid-tie inverters.

14. Other types of system topologies
A single stage can be used in some instances (e.g. high enough voltage).
The single power processing stage:
MPP tracking
Grid current (hence power) control
Voltage and frequency compliance

15. Transformer location Typically a transformer is required.
On the low frequency-side (at line frequency 50/60Hz)
Or at an intermediate high-frequency location

16. Transformerless Systems
Example system – with parasitic capacitances and common-mode filters.

17. Some other examples
CSI-based inverter stage

20. Past and Future

21. Standards Inverter systems must satisfy standards:
harmonic limits
power quality,
island detection,
Grounding,
DC injection limits.
Voltage and frequency ranges.
What should happen during grid faults.