By David Herres
Wind turbines and solar arrays produce power only when the wind is blowing or the sun is shining, of course, so many of these renewable energy systems incorporate a means for storing electrical energy during peak production and releasing it as needed. A battery bank will serve the purpose and in a stand-alone system, is generally the way to go.
But there are some disadvantages. The initial cost of batteries is high. And because their life is finite, they must be scrapped and replaced every few years. Batteries are also not 100% efficient. Their internal impedance consumes a small but constant amount of power continuously. Moreover, the battery bank takes up space, and its cost must be figured as part of the installation expense along with the cost of maintenance needed to combat the effects of dc corrosion at terminations. Additionally, there are safety issues because the charging process generates hydrogen and because of an awesome arc-flash potential in a large installation.
For these reasons, the usual practice is to go with cogeneration if utility power is available at the site. Excess renewable energy power is fed into the grid. The alternate energy supply is connected in parallel with the utility supply. The power meter will turn in the direction and at the speed necessary to calculate the balance. This part of the process is automatic.
Usually, unless encumbered with a social conscience, the utility would rather not be bothered with power routed back from renewable sources. After all, utilities are in the business of selling power. But public utilities commissions generally mandate cogeneration and have the authority to determine whether the utility must buy back locally generated power at the retail or wholesale rate, and other pertinent details.
It is necessary to convert the electrical energy renewable sources generate to a form that is compatible with the utility power. AC power that feeds into the grid must match the voltage and frequency of the utility power. In addition, the waveform must be a high-quality sine wave with the positive and negative peaks locked in synch with those of the power signal of the grid.
A solar array produces a dc voltage that varies with the intensity of the solar radiation. An electrical wind turbine produces dc or, more commonly in newer installations, ac that fluctuates in voltage and frequency as the wind shifts speed and direction. This is known as wild ac, and the first step in configuring utility-compatible power is to rectify and filter it to create a uniform dc.
All of this may seem like a complex process, but in reality the equipment that makes the utility match-up possible, known as a synchronous inverter, is not far out of the ordinary in our electronic age. The synchronous inverter contains a CPU that samples the utility power on a continuous basis and produces the compatible output. The cost of this equipment is always less than a full-scale battery bank.
The synchronous (also sometimes called a grid-tie) inverter typically synchronizes its frequency with that of the grid using a local oscillator. It must also limit its output voltage to no higher than the grid voltage. Modern inverters used for this purpose have a fixed unity power factor, meaning the phase angle between the current and voltage generated is within 1 ° of the ac power grid. The inverter has an on-board computer which senses the grid waveform and outputs a voltage in sync with it.
Grid-tie inverters are also designed to quickly disconnect from the utility grid if it goes down. This is an NEC requirement designed to ensure energy from the inverter won’t harm line workers trying to restore grid power.
Modern grid-tie inverters can use several different circuit topologies. For example, they can be found using newer high-frequency transformers, conventional low-frequency transformers, and without any transformer. Instead of converting direct current directly to 120 or 240 Vac, inverters based on high-frequency transformers use a computerized multi-step process that converts generated dc power to high-frequency ac and then back to dc. This “dc link” voltage then feeds a dc/ac converter which produces the grid ac.
Transformerless inverters are lighter and more efficient than their counterparts with transformers. Though widely used in Europe for solar arrays, transformerless inverters have been slow to enter the U.S. The reason is there is no galvanic isolation between the dc and ac circuits. The fear has been that this lack of isolation could let dangerous dc faults pass to the ac side. However, transformerless (or non-galvanically isolated) inverters are now permitted in the U.S. thanks to the use of new safety requirements and the removal of the requirement that all solar electric systems be negative grounded. The safety mechanisms primarily detect residual or ground currents as an indication of a possible fault condition. Manufacturers must also perform isolation tests to ensure separation of dc from ac.
The concept of sinusoidal PWM (pulse width modulation) was introduced in synchronous inverters as a way to reduce the harmonics in the output voltage. Most grid-tie inverters aimed at the solar market include a maximum power point tracker on the input side that lets the inverter to extract a maximum amount of power from the solar array as clouds block part of the array, accumulated dirt reduce its output, and so forth. There are also MPPT algorithms for wind turbines, but they are quite different than those for solar applications, so the inverters for use with wind turbines differ to a great degree from those for solar arrays.