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Inverters

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New power inverters at a health clinic in Rwanda. (Photo: Walt Ratterman)

Power inverters convert DC power to AC power.  Because batteries and PV modules produce DC power while most common electrical devices require AC power, inverters are nearly always a necessary component of such systems. 

Inverters may be classified in a number of ways, but especially by the type of AC waveform they produce, their ability to interconnect with the electrical grid, and their intended application (e.g. PV, battery charging).  Inverters are essential for an array of different applications, thus a range of capacities and features are available on the market.  The following discussion will focus on important inverter types and applications, especially with regard to renewable energy and battery storage.  This will include technical and practical considerations in the selection and use of power inverters, as well as lessons learned through IHFI’s experience deploying inverter systems in Haiti.

 

IHFI Experience 

IHFI has installed backup power battery banks at a number of health facilities in Haiti.  These battery banks are designed to provide power during grid outages and supply pure sine wave power to sensitive electronics.  Each of the IHFI backup power systems uses two to four inverters.  The particular inverters chosen for these systems are off-grid battery inverters which also have a charge control function, allowing them to play two important roles.  Firstly, the inverters use AC grid or generator power to charge the battery bank, in this case acting as a rectifier (a device that converts AC power to DC power, the opposite of an inverter).  Secondly, they convert DC power from the battery bank to pure sine-wave AC power for use by laboratory equipment and computers, thereby protecting those loads from poor quality grid and generator power.  These systems are configured such that one inverter is dedicated to supplying sensitive loads, while the other inverters handle battery charging.  For more information on the operation and deployment of IHFI’s backup power systems, see the following:

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Inverters, the three black components, are integrated with breakers, switches and communications hardware before being linked into the building's wiring. (Photo: Kim Domptail) In this system, one inverter constantly supplies no-contact loads with pure sine-wave power.  The other two inverters are used to supply contact loads during a power outage. (Photo: Kim Domptail) Once fully connected, the systems must be programmed via the system controller.  This inlcudes setting battery charging setpoints and coordinating inverter operation. (Photo: Kim Domptail)

 

Basic Operation 

Inverters convert DC power to AC power.  DC power, represented by a straight line, can be characterized by its voltage and current.  AC power has voltage and current, but is also characterized by its frequency and waveform – with a pure sine wave as the ideal.  So an inverter must generate a frequency and waveform from a flat, DC source.  Additionally, inverters typically change the voltage of the outgoing AC power because electrical devices are designed to operate at common grid voltages (e.g. 120VAC, 220VAC), while a DC source may operate at up to 600V (residential PV system) or as little as 12V (battery bank).

The first step in the conversion process is to establish the desired frequency.  This is accomplished using a “bridge circuit”, an electronic circuit that is capable of switching DC voltage from positive to negative, thus switching the direction of current and creating a crude AC signal.  In a pure sine-wave inverter, the amplitude and frequency of this AC signal is controlled using a strategy termed Pulse Width Modulation (PWM).  PWM provides a flexible means of generating and adjusting the output AC frequency (note, however, that the PWM frequency itself is different from the final AC frequency output by the inverter).  In a grid-tied inverter PWM follows the AC signal delivered by the grid, thereby ensuring that the inverter’s output can be used by the grid.  

The PWM signal generated up to this point is still far from a perfect sign wave, and does not have the correct frequency, voltage or waveform.  A number of other components are responsible for adjusting and smoothing the PWM waveform into the desired sine wave.  While inverter designs vary, the main components involved in conditioning the AC output signal are inductors, capacitors and transformers.  These components take advantage of various properties of electromagnetism to achieve the correct AC characteristics.

 

Wave Form 

AC electrical power is represented by a sine wave, where the current regularly changes direction.  The rate of this change in direction is the current’s frequency, which is typically 50 or 60Hz (50 or 60 changes per second).  For grid power and generators, the AC sine wave is produced naturally, as electricity is ultimately generated using spinning magnets.  Inverters must convert DC power, which has no frequency, to AC power of a specified frequency.  There are several approaches to creating AC power, each balancing simplicity with output quality.

While some inverter technologies can faithfully replicate a pure AC sine wave, others simply produce an approximation.  These technologies have been categorized in the three ways: square wave, modified sine wave and pure sine wave.  The type of loads being supported, as well as the type of application, will dictate the appropriate technology.

Wave FormDescription

Square wave

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Square wave inverters are the least expensive but their output, a square wave, is suitable only for resistive loads such as resistance heaters.

Modified sine wave

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Modified sine wave inverters produce a staircase square wave that more closely approximates a sine wave. This type of inverter is common and most AC electronic devices and motors will run on a modified AC sine wave.

Pure sine wave

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These inverters can produce power that is indistinguishable from what comes out of the outlet, but they cost more than the other types of inverters.  A pure sine wave inverter is required in any grid-tie application (as inverter output must match the grid power).  Furthermore, some sensitive electronics, such as computers and laboratory equipment, may not work with a modified AC sine wave and require pure sine wave inverters.

 

Grid Interconnection 

Inverters may also be classified by whether they are able to tie into an electrical grid.  Grid-tied (grid following) inverters are designed to match the frequency and voltage of the electrical grid to which they are connected.  In contrast, off-grid (grid forming) inverters do not adjust their AC output to match an interconnected grid, but rather produce power at a set voltage and frequency.

Grid-tied 

Grid-tied PV systems are among the most cost-effective ways to deploy PV technology.  By interconnecting to the electrical grid, energy generated by the PV system may be used on site, or injected into the grid, ensuring that no PV energy is wasted, while negating the need for battery storage.  The ability to interconnect with the electrical grid varies from one utility to the next; likewise, the manner in which energy is sold to the grid (net metering, feed-in tariff, etc.) is entirely dependent on local regulations.

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Grid-tie inverters are used alongside safety disconnects and a utility's net meter. (Photo: US DOE)

From a technical standpoint, grid-tied PV inverters have two critical capabilities: grid following and grid disconnection.  Grid following allows the inverter to replicate the AC sine wave (voltage and frequency) supplied by the grid.  This ensures that power produced by the PV system is seamlessly synchronized with the existing power supply.  

Automatic grid disconnection, designed to prevent islanding, is an essential safety feature of all grid-tied PV inverters.  Islanding occurs when a grid-connected PV system continues to send power to the grid during a power outage. Although the grid is down, the power lines are still electrified by the connected PV system, creating a potential hazard for utility technicians working under the assumption that the lines are not energized.  Grid-tied inverters will disconnect their AC output whenever the grid power has abnormal voltage or frequency, or is interrupted completely.  According to the applicable standards (UL 1741 and IEEE 1547), once disconnected the grid-tied inverter will only re-connect to the grid after grid power has been within acceptable voltage and frequency limits for five minutes.

Off-grid

Off-grid inverters do not have the ability to tie into the electrical grid, but are designed to serve on-site loads directly, fed from a PV array or battery bank.  Off-grid inverters form their own AC sine wave, independent of any other power source. 

 

Transformers 

Transformers are a common component in inverters that a) help create a pure sine wave, b) ensure proper output voltage, and c) provide isolation between the AC and DC sides of the system.  Inverters can be characterized by the type of transformer they utilize, either: low frequency, high frequency or none (transformerless).  Each topology has its own advantages and disadvantages in terms of safety, efficiency, cost, and weight.

TopologyDescription
Low frequency transformer Inverters will use low-frequency transformers at either 50Hz or 60Hz, depending on the geographic market they are designed for (i.e. 50Hz for Europe, 60Hz for US).  Because low frequency transformers output at a set frequency, they require fewer components than high frequency or transformerless designs, enhancing reliability.
High frequency transformer High frequency transformers are smaller, less expensive and more efficient than low frequency transformers and also have the benefit of tolerating a greater range of input DC voltages.
Transformerless Inverters that do not utilize a transformer are lighter and more efficient than their inverter-based counterparts.  The duties usually required of the transformer - voltage and sine wave regulation - are taken up by other components and circuits not typical of transformer-based designs.  Because such designs do not provide isolation between the AC and DC sides of the inverter, the overall system may require additional safety measures or over-current protection.

  

Applications 

Inverters are used in any application where a DC power source must be converted to an AC power source.  The most common applications are in battery-powered mobile and marine vehicles, uninterruptible power supplies, renewable energy generation systems, and stationary battery banks – these last two applications are described in greater detail.

Photovoltaic systems 

PV inverters differ in their capabilities depending on whether or not they are designed for grid interconnection, as described above.  Regardless of grid suitability, PV inverters should always employ Maximum Power Point Tracking (MPPT), a strategy for maximizing the power production of a PV array.

Battery systems

An important characteristic of battery inverters is their DC input voltage.  Battery systems typically operate at 12, 24 or 48V (compare this to a PV system, which may operate at 600V); appropriate inverters must match this DC voltage.  Also, inverters designed for use with batteries often include charge control capabilities, allowing the inverter to charge the batteries as needed, using power from the grid or another AC source. 

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An inverter, directly mounted to the PV system's racking, serves a string of solar panels. (Photo: Claus Ableiter)

Inverters charge and discharge a battery bank. (Photo: US DOE)

 

Additional Considerations 

Phases 

Inverters are designed to output AC power as single-phase, split-phase or three-phase.  The phase requirement for any inverter system depends on the how the existing electrical wiring is configured and on the supported loads.  Note that many brands also allow single phase inverters to be linked into groups of two or three via a communications channel, thus enabling them to synchronize their AC output to create split-phase or three-phase power.

System control and programming

Inverters are responsible for many of the active processes that take place in a PV or battery system, e.g. MPPT, battery charging, grid following.  They are therefore perhaps the most intelligent component of such systems, meaning that they can be programmed to, for example, begin battery charging at specific state of charge or sell power to the grid at certain times of the day.  These processes can have a profound effect on the performance and longevity of the system, thus their proper programming is also essential to system health.

Inverter programming is usually accomplished via a display on the inverter itself or an external system controller.  Since programming requirements and capabilities differ from one manufacturer to the next, it is essential that programming instructions are carefully followed during installation.

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A MATE3 system controller installed at an IHFI backup system in Haiti.  The controller is used to program inverter setpoints, coordinate inter-system communications, and enable remote monitoring.  (Photo: Daria Mashnik)

System communication

The communication and monitoring capabilities of any given inverter will depend entirely on the brand and model.  Inverters may need to communicate with other components within the same system, such as system controllers or other inverters.  As noted above, in order to generate three-phase power from three, single-phase inverters, all three inverters must communicate to coordinate their AC output.  Likewise, AC output must be in sync if power from multiple inverters is to be aggregated.

Monitoring 

Many inverter manufacturers offer some form of internet connectivity for their devices, and in some cases, remote system performance monitoring via the internet.  This capability is useful for ensuring that the system is working properly and for tracking its performance (e.g. PV generation, battery bank state-of-charge).

Inverter size terminology: micro, string, central

Inverter sizes are referred to by their rated AC output, which can range from less than 100W to 1MW or more.  Apart from capacity, inverter sizes can be loosely defined by their applications.  Large inverters designed for utility-scale applications are generally termed “central” inverters, because they often serve large, multi-string solar arrays.  “String” inverters are of low-to-mid-capacity and are designed for residential and commercial applications.  They are called string inverters because they typically serve a single string or small array of PV modules.  “Micro” inverters are designed to serve a single PV module at a time.  These small devices are designed to attach directly to the rear of a solar panel.

 

Inverter Sizing 

As inverters are a common component of many different types of energy systems, there are a wide range of possible sizing criteria.  Generally speaking, inverters should be sized based on electrical load they are serving, ensuring that the rated AC output meets the peak power demand on the system.  PV inverters, on the other hand, are sized based on their DC input capacity and the peak capacity of the PV system.  Because PV systems usually operate below their peak capacity, PV inverters are typically specified to be undersized (a ratio of 1.2:1 PV capacity to inverter capacity is a common rule of thumb, but proper sizing should be done by an experience professional according to site conditions).

DC input voltage is another important consideration to sizing and specifying PV and battery inverters.  A battery inverter’s DC input voltage must match that of the battery bank it is connected to, thus battery inverter models are normally found at 12, 24, or 48VDC.  In a PV inverter, DC input voltage may fall within a large range, such flexibility is necessary as the voltage of a PV array will change significantly depending on the amount of sun available and the temperature.  Typically, PV inverter specifications will provide maximum, minimum and rated DC input voltages, as well as an MPPT voltage range (the range of voltages under which the inverter can perform maximum power point tracking).  It is up to the PV system’s designer to ensure that the system voltage remains within the inverter’s limits under even the most extreme sunlight and temperature conditions expected at the site.

 

Inverter Standards 

 

International Electrotechnical Commission (IEC)
CategoryStandard
 Inverters IEC 62109-1 ed1.0: Safety of power converters for use in photovoltaic power systems - Part 1: General requirements
 Inverters IEC 62109-2 ed1.0: Safety of power converters for use in photovoltaic power systems - Part 2: Particular requirements for inverters

 

Underwriters Laboratory (UL)
CategoryStandard
 Inverters UL 1741: Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources

 

Institute of Electrical and Electronics Engineers (IEEE)
CategoryStandard
Grid-tied Inverters IEEE 1547: Standard for Interconnecting Distributed Resources with Electric Power Systems

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