Converters in power systems
A perturb-and-observe tracker with 2 V steps quickly finds the maximum power point, but it oscillates and loses almost 2 %. With 0.1 V steps it is accurate, but slow: after 40 iterations it has not arrived yet.
01Converters in generation
Renewable generation reaches the grid through converters. A photovoltaic panel delivers direct current with a current-voltage characteristic that depends on irradiance and temperature; a DC-DC converter makes it work at its maximum power point and an inverter injects that power into the grid. A modern wind turbine produces AC of variable frequency, which a back-to-back converter —active rectifier, DC bus and inverter— adapts to the grid frequency. Storage systems based on batteries use bidirectional converters that charge or discharge according to the demand of the grid.
02The maximum power point
With the one-diode model, without parasitic resistances, the current of a panel of \( N_s \) cells in series is:
A 60-cell panel with Isc = 9 A and Voc = 38 V at 1000 W/m², with n = 1.3, at 25 °C. The tracker starts at 20 V and takes 80 steps: the first 40 with the initial irradiance and the other 40 with the final one. The curves are the power as a function of voltage; the dots, the path of the tracker.
When there is partial shading, the curve of a set of panels has several maxima and the simple algorithm can get stuck at a local one; commercial inverters periodically sweep the whole curve.
03The grid-connected inverter
A grid-connected inverter does not set the voltage, which the grid imposes, but the current it injects. A phase-locked loop (PLL) measures the grid phase and the control generates a sinusoidal current reference in phase with it, with the amplitude that corresponds to the available power; an L or LCL filter attenuates the switching harmonics. It must detect the loss of the grid and disconnect, the anti-islanding protection, and meet the connection requirements of the utility.
04Transmission and compensation
| System | Function |
|---|---|
| High-voltage direct-current transmission (HVDC) | Long or submarine links and links between grids of different frequency, with thyristor rectifiers and inverters or with voltage-source converters |
| Static VAR compensator (SVC) | Thyristor-controlled reactors and switched capacitors that regulate the voltage |
| STATCOM | An inverter that injects capacitive or inductive reactive current, fast and compact |
| Active filter | An inverter that injects the harmonics opposite to those of a nonlinear load |
The problems that this equipment solves were described in power quality.
05In the laboratory
Measure the current-voltage curve of a panel with a variable electronic load, in full sun and with partial shading.
Program a perturb-and-observe tracker on a microcontroller driving a buck converter, and record the power.
Implement a digital PLL that locks onto the attenuated grid voltage and measure its response to a phase jump.
06Common mistakes
- Choosing a voltage step without considering the trade-off. A large one oscillates; a small one is slow to follow changes in irradiance.
- Trusting a simple tracker with partial shading. It can get stuck at a local maximum.
- Voltage-controlling a grid-connected inverter. The grid imposes the voltage; the current must be controlled.
- Omitting the anti-islanding protection. It endangers anyone working on a line believed to be de-energized.
07Self-assessment
Where is the maximum power point of the lab panel at 1000 W/m²?
At about 32.3 V, with 273.8 W.
And at 500 W/m²?
At about 31.0 V, with 131 W: the voltage hardly changes, the current does.
What does a grid-connected inverter control?
The current it injects, in phase with the grid voltage.
What does a STATCOM do?
It injects reactive power with an inverter to regulate the grid voltage.
08Further reading
- Remus Teodorescu, Marco Liserre and Pedro Rodríguez. Grid Converters for Photovoltaic and Wind Power Systems. Wiley-IEEE Press, 2011. Control of grid-connected inverters, synchronization and anti-islanding.
- Narain G. Hingorani and Laszlo Gyugyi. Understanding FACTS. Wiley-IEEE Press, 2000. Static compensators and flexible transmission systems.
- Gilbert M. Masters. Renewable and Efficient Electric Power Systems. 2nd ed., Wiley, 2013. Photovoltaic and wind generation from the resource to the grid.