Pv Systems

The inverter is one of the most important and most complex components in an independent energy system. To choose an inverter you should know some basic fu 00004000 nctions, capabilities, and limitations.

All types of off-grid PV electric energy systems share a common bond which is the storage battery, the storage battery absorbs and releases energy in the form of direct current (DC) electricity

In contrast, the national grid supplies alternating current (AC) electricity. AC is the standard form of electricity for anything that "plugs in" to mains power. DC flows in a single direction. AC alternates its direction many times per second. AC is used for grid service because it is more practical for long distance transmission.

An inverter converts DC to AC, and also changes the voltage. In other words, it is a power adapter. It allows a battery-based system to run conventional appliances through conventional wiring. There are appliances able to use DC directly but for the majority of modern appliances you will need an inverter.

There is another type of inverter that we are not concerned with in this article called a grid-interactive inverter. These are used to feed solar (or other renewable) energy into a grid-connected home and to feed excess energy back into the national grid.

Not a simple device

Outwardly, an inverter looks like a box with one or two switches on it, but inside there is a small universe of dynamic activity. A modern inverter must cope with a wide range of loads, from a single night light to the big surge required to start a well pump, a power tool or heavy machinery. The battery voltage of a solar or wind system can vary as much as 35 percent (with varying state of charge and activity).

Through all of this, it is the inverter's job to regulate the quality of its output within narrow constraints, with a minimum of power loss.

Define your needs

To choose an inverter, you should first define your needs. Then you need to learn about the inverters that are available. Inverter manufacturers print everything you need to know on their specification sheets. Here is a list of the factors that you should consider.

Operating Environment

Where is the inverter to be used? Inverters are available for use in buildings, for recreational vehicles, boats, and portable applications. Will it be connected to the utility grid in some way? Electrical conventions and safety standards differ for various applications, it's important to check.

Electrical Standards

The DC input voltage must conform to that of the electrical system and battery bank. 12 volts is no longer the dominant standard for home energy systems, 24 and 48 volts are the common standards now. A higher voltage system carries less current, which makes system wiring cheaper and easier.

The inverter's AC output must conform to the conventional power in the region in order to run locally available appliances. The standard for AC mains service in the UK is 230 volts at a frequency of 50 Hertz (cycles per second).

Safety Certification: An inverter should be certified by an independent testing laboratory and be stamped accordingly. This is your assurance that it will be safe, will meet the manufacturer's specifications, and will be approved in an electrical inspection. There are different design and rating standards for various application environments These also vary from one country to another.

Power Capacity

How much load can an inverter handle? Its power output is rated in watts (watts = amps x volts). There are three levels of power rating a continuous rating, a limited-time rating, and a surge rating. Continuous means the amount of power the inverter can handle for an indefinite period of hours. When an inverter is rated at a certain number of watts, that number generally refers to its continuous rating.

The limited-time rating is a higher number of watts that it can handle for a defined period of time, typically 10 or 20 minutes. The inverter specifications should define these ratings in relation to ambient temperature (the temperature of the surrounding atmosphere). When the inverter gets too hot, it will shut off. This will happen more quickly in a hot atmosphere. The third level of power rating, surge capacity, is critical to its ability to start motors, and is discussed below.

Some inverters are designed to be interconnected or expanded in a modular fashion, in order to increase their capacity. The most common scheme is to "stack" two inverters. A cable connects the two inverters to synchronise them so they perform as one unit.

Power Quality - SINE Wave vs. Modified SINE Wave"

Some inverters produce "cleaner" power than others. Simply stated, "sine wave" is clean; anything else is dirty. A sine wave has a naturally smooth geometry, like the track of a swinging pendulum. It is the ideal form of AC power. The utility grid produces sine wave power in its generators and normally delivers it to the customer relatively free of distortion. A sine wave inverter can deliver cleaner, more stable power than most grid connections.

How clean is a "sine wave"? The manufacturer may use the terms "pure" or "true" to imply a low degree of distortion. The facts are included in the inverter's specifications. Total harmonic distortion (THD) lower than 6 percent should satisfy normal home requirements. Look for less than 3 percent if you have unusually critical or sensitive electronics, as in a recording studio.

Other specs are important too. RMS voltage regulation keeps your lights steady. It should be plus or minus 5 percent or less. Peak voltage (Vp) regulation needs to be plus or minus 10 percent or less.

A "modified sine wave" inverter is less expensive, but it produces a distorted square waveform that resembles the track of a pendulum being slammed back and forth by hammers. In truth, it isn't a sine wave at all. The misleading term "modified sine wave" was invented by advertising people. Engineers prefer to call it "modified square wave."