Inverters for Photovoltaic Systems

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Objectives:

  • Identify basic waveform types and properties.
  • Compare applications for static inverters.
  • Explain the basic types of inverters used in PV systems.
  • Describe the operation of a simple square wave inverter.
  • Explain how inverters make sine waves from square waves.
  • Describe the functions and features of power conditioning units.
  • Understand inverter specifications and ratings.

AC POWER

Voltage and current, and therefore power, can be either constant or time varying in magnitude. Moreover, time-varying values can either maintain one direction (positive or negative), or alternate between positive and negative directions. See 1. Direct current (DC) is electrical current that flows in one direction, either positive or negative. DC power may be constant or variable, but always maintains one direction.



Waveforms

A waveform is the shape of an electrical signal that varies over time. Waveforms are used to represent changing electrical current and volt age. A periodic waveform is a waveform that repeats the same pattern at regular intervals. A cycle is the interval of time between the beginnings of each waveform pattern.



Waveforms can't be measured or viewed directly. To view the shape of a waveform, the time-varying values (voltage or current) must be plotted against time. Oscilloscopes, or meters with oscilloscope features, are able to display waveforms.

Waves can take a variety of forms, such as smooth curves for gradually changing values, or stepped patterns for abruptly changing values. Sine waves, square waves, and modified square waves are common AC waveforms produced by inverters. See 2.

1. If voltage and current signals are either always positive or always negative, they are DC waveforms. If the signals switch between positive and negative, they are AC waveforms. DC POWER SIGNALS; AC POWER SIGNALS; DC Power vs. AC Power

PV modules, and some other power- generating technologies, produce variable DC power. However, most electrical loads operate using AC power, so DC power usually must be transformed into AC power in order to be useful. Alternating current (AC) is electrical current that changes between positive and negative directions. AC power is characterized by waveform shape, frequency, and magnitude.

Sine Waves. A sine wave is a periodic wave form the value of which varies over time ac cording to the trigonometric sine function. A sinusoidal waveform is a waveform that is or closely approximates a sine wave. Sine waves may shift in time or vary in amplitude, but if they retain the shape of the sine wave, they are still sinusoidal.

The rotating generators that provide most of the electrical power on the utility grid naturally produce sine waves, so most loads are designed to operate using sinusoidal AC power. There fore, interactive inverters produce sine waves for utility synchronization. Other waveforms may damage some loads. However, sine waves are the most complicated type of AC waveform for inverters to produce, so some less-sophisticated inverters approximate a sine wave with square waves or modified square waves.

2. AC waveforms can take a variety of shapes. AC Waveforms

Square Waves. Square waves and modified square waves are non-sinusoidal waveforms. A square wave is an alternating current wave form that switches between maximum positive and negative values every half period. Square waves are inefficient and are not a common inverter output, but are the basis for the improved modified square wave.

Modified Square Waves. A modified square wave is a synthesized, stepped waveform that approximates a true sine wave. Also called a modified sine wave or a quasi sine wave, this type of waveform is the typical AC output of many stand-alone inverters. In terms of power quality, a modified square wave is a substantial improvement over a simple square wave. Compared to square wave inverters, modified square wave inverters have lower harmonic distortion, higher peak voltage, higher efficiency, and better surge current capability.

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History of Current

In the late 1880s, electricity was first becoming accessible to homes and businesses. The first electrical power generation and distribution systems were developed by Thomas Edison and used direct current (DC) power. DC power worked well for powering loads, but could not be transmitted more than about a mile without significant power loss from voltage drop. Edison advocated distributed generation as a solution—building many power plants close to where electricity was needed—but this was impractical at the time.

As an alternative, George Westinghouse and Nikola Tesla devised a system for generating and distributing alternating current (AC) power. AC had many advantages, including safer connections and disconnections without arcing, and the ability to transmit power at high voltage over hundreds of miles with relatively small losses. Edison and Westinghouse became bitter adversaries as each tried to persuade the public that theirs was the better system. Edison tried to convince people that AC power was more dangerous than DC power, even trying to popularize “Westinghoused” as a term for being electrocuted. This controversy became known as the War of Currents.

The Niagara Falls generation project was the first large-scale electrical generation project, and both Edison and Westinghouse competed for the contract. Ultimately, the Niagara Falls Commission chose the AC system. The project was completed in 1896 and was a huge success. Tesla’s system of generators, transformers, motors, and conductors set the standards for all future AC power generation, including the 60 Hz frequency.

Edison’s DC system did not become the primary electrical distribution system, but some remnants of his attempts lingered for more than a century. Up until 2005, Consolidated Edison, New York City’s electric utility company, supplied DC power to 1600 customers in Manhattan, mainly for operating older elevators. Currently, DC power is used in electronics, telephone networks, and in small power networks for off-grid residences and transportation.

Library of Congress: Thomas Edison’s dynamos were the earliest commercial electricity generators.

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Modified square waves are suitable for many AC loads, including linear and switching power supplies used in electronic equipment, transformers, and some motors. However, a load that is sensitive to either peak voltage or zero crossings could experience problems. These inverters may also produce radio frequency (RF) noise that interferes with devices such as radios and TVs. Also, modified square wave inverters should not be used with motor speed controllers or devices that plug directly into a receptacle to charge a battery without a transformer.

Frequency

Frequency is the number of waveform cycles in one second. In the past, frequency was ex pressed as cycles per second (cps, or simply “cycles”), but it is now commonly expressed in equivalent units of Hertz (Hz). The frequency of the U.S. electric grid is maintained at 60Hz. Frequency establishes the speed of motors, generators, and some clocks and is one of the most important parameters in synchronizing AC electrical systems.

Period is the time it takes a periodic wave form to complete one full cycle before it re peats. See 3. Period is the inverse of frequency. For example, a 60 Hz AC waveform repeats 60 times per second. In this case, the period is 1/60 sec, or 16.7 ms.

There are multiple ways to measure or specify the magnitude of a waveform. Peak is the maximum absolute value of a waveform. For example, peak voltage is the maximum value of an AC voltage waveform. Peak-to-peak is a measure of the difference between positive and negative maximum values of a waveform.

The root-mean-square (RMS) value is a statistical parameter representing the effective value of a waveform. An RMS value is the square root of the mean (average) of the squares of the values. This is not the same as the average value, but it has a special significance. A DC voltage equal to the RMS value of an AC voltage produces the same amount of heat in a resistive circuit as does the AC voltage. Most AC voltage and current measurements are actually RMS values. For ex ample, a typical wall outlet provides about 120V, which is the RMS value. The peak voltage of the waveform is actually about 170 V.

3. Waveform Parameters: Certain parameters are integral to defining the characteristics of an AC waveform.

Three-Phase AC Power

Three-phase AC power includes three separate voltage and current waveforms occurring simultaneously 120° apart. See 4. Three-phase AC power is commonly used for motors because they can be more efficient and smaller than single-phase motors of the same power output. Many large PV inverters are designed to produce three-phase AC outputs.

4. Three-Phase Power: Three-phase power is composed of three separate voltage waveforms that are 1200 out of phase.

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True RMS Meters

For a true sine wave, the peak, RMS, and average values of the waveform are always proportional. This means the values can be calculated from one another with simple formulas. These formulas are accurate only for true sine waves.

Calculating the actual RMS value of a waveform is possible with the fast electronics and microprocessors in many meters. A meter that has this capability will be labeled “true RMS.” However, a low-end meter shortcuts this process by measuring the peak value and multiplying that value by 0.707, which is a much faster and easier calculation. If the waveform is a true sine wave, the value will be accurate. That is, the calculation will yield the same result as calculating the RMS value directly.

However, if the waveform is not a sine wave or is a distorted sine wave, this shortcut method produces an incorrect RMS value. For example, the RMS and peak voltages are equal for a square wave. This can lead to an improper conclusion that a modified square wave or square wave inverter is not producing its rated voltage or power. A true RMS meter is required to measure these signals accurately. Fortunately, the prices of true RMS meters are falling and the cost advantage of non-true AMS meters is becoming less significant.

Measuring peak and RMS values with a true RMS meter is an easy way to gain information about the shape of a waveform without using an oscilloscope or graphical multimeter to display the actual waveform. If the calculated RMS value (calculated from the peak value) is less than the actual RMS, then the waveform is wider and flatter, like a square wave. If the calculated RMS value is more than the actual RMS, then the waveform is very narrow.

Power Quality

Excessive variations in circuit parameters can cause damage to loads and distribution the equipment.

Power quality problems can be caused by age, current, harmonics, and power factor. It is common for actual circuit parameters to vary but allowable ranges are typically very small the power source, but they can also be caused, by loads on the electrical system

AC Voltage Conversions

VRMS = 0.707 x Vp = 0.638 x VPEAK = 0.9 x VRMS

Power quality is the measure of how closely can power in an electrical circuit matches the nominal values for parameters such as volt.

It is important to ensure adequate power quality in inverter systems, as in any electrical system. However, most inverters perform much of the power quality monitoring automatically, so problems usually are easy to identify. The inverter may alert the operator to a power quality problem with an alarm or visual display, and may shut down automatically to avoid damage to equipment if the problem is significant. However, an understanding of power quality issues and their common causes is important to avoid problems.

Voltage Variations. Voltage in a power distribution system is typically acceptable within the range of +5% to -10% from the nominal voltage. Small voltage fluctuations typically do not affect equipment performance, but voltage fluctuations outside the normal range can cause circuit and load problems. See 5.

Voltage sags are commonly caused by over loaded transformers, undersized conductors, conductor runs that are too long, too many loads on a circuit, peak power usage periods (brown outs), and high-current loads being turned on. Voltage sags are often followed by voltage swells as voltage regulators overcompensate.

Voltage swells are caused by loads near the beginning of a power distribution system, incorrectly wired transformer taps, and large loads being turned off. Voltage swells are not as common as voltage sags, but are more damaging to electrical equipment.

Transient voltages are temporary undesirable voltages in an electrical circuit, ranging from a few volts to several thousand volts and lasting from a few microseconds up to a few milliseconds. Transient voltages are caused by the sudden release of stored energy due to lightning strikes, unfiltered electrical equipment, contact bounce, arcing, and high current loads being switched ON and OFF. Transient voltages differ from voltage drops and surges by being larger in amplitude, shorter in duration, steeper in rise time, and erratic. High- voltage transients can permanently damage unprotected circuits or electrical equipment.

Voltage Unbalance. Voltage unbalance is the unbalance that occurs when the voltages of a three-phase power supply or the terminals of a three-phase load are not equal. Voltage unbalance also results in a current unbalance. See 6. Voltage unbalance should not be more than 1%. The primary cause of volt age unbalances of less than 2% is too many single-phase loads on one phase of a three-phase system.

Single phasing is the complete loss of one phase on a three-phase power supply. Single phasing is the maximum condition of voltage unbalance. Common causes of single phasing include blown fuses, mechanical switching failure, or a lightning strike on the power lines.

5. Voltage Variations: Voltage variations outside allowable ranges include voltage sags, voltage swells, and transients.

6. Three-Phase Unbalance: Three-phase voltage and current waveforms are unbalanced if they are not equal in magnitude.

Current Unbalance. Current unbalance is the unbalance that occurs when current is not equal on the three power lines of a three-phase system. Small voltage unbalances cause large current unbalances. Large current unbalances cause excessive heat, resulting in insulation breakdown. Typically, for every 1 % of voltage unbalance, current unbalance is 4% to 8%. Current unbalances should never exceed 10%.

Phase Unbalance. When three-phase power is generated and distributed, the three power lines are 1200 out of phase with each other. Phase unbalance is the unbalance that occurs when three-phase power lines are more or less than 120° out of phase. Phase unbalance of a three-phase power system occurs when single-phase loads are applied, causing one or two of the lines to carry more or less of the load. Loads must be balanced on three-phase power systems during installation.

Harmonic Distortion. A harmonic is a waveform component at an integer multiple of the fundamental waveform frequency. For example, the second harmonic frequency of a 60Hz sine wave is 120 Hz, the third is 180 Hz, the fourth is 240 Hz, and so on. These higher- frequency harmonic components superimpose on the fundamental frequency, distorting the waveform. See 7.

Harmonics

7. Harmonics can add to the fundamental frequency to produce distorted waveforms.

Fundamental frequencies and harmonics can add together to form composite wave forms. Total harmonic distortion (THD) is the ratio of the sum of all harmonic components in a waveform to the fundamental frequency component. Total harmonic distortion is ex pressed as a percentage. For example, a current waveform with 5% THD means that 5% of the total current is at frequencies higher than the fundamental.

Harmonics are commonly caused by non linear loads, including variable-frequency drives, switching power supplies, and low- quality inverters. Harmonics are also present in square waves and modified square waves, and can be a significant inverter issue. Harmonics cause extra heat in motors and transformers and sometimes create audible noise.

Power Factor. AC loads are either resistive or reactive loads. A resistive load is a load that keeps voltage and current waveforms in phase. True power is the product of in-phase voltage and current waveforms and produces useful work. See 8. True power is also called real power or active power and is represented in units of watts (W).

Power quality problems can be diagnosed with graphical power quality analyzers. (e.g., from Fluke)

A reactive load is an AC load with inductive and /or capacitive elements that cause the current and voltage waveforms to become out of phase. Inductive loads are the most common reactive loads and include motors and transformers. Reactive power is the product of out-of-phase voltage and current waveforms and results in no net power flow. Inductive loads momentarily retard current in the process of building magnetic fields and cause the current waveform to lag the voltage waveform in time. Capacitive loads momentarily store voltage and cause the current waveform to lead the voltage waveform in time. Reactive power is represented in units of volt-amperes reactive (VAR).

Power Factor

Power factor is the ratio of true power to apparent power and describes the displacement of voltage and current waveforms in AC circuits. Apparent power is a combination of true and reactive power and is given in units of volt-amperes (VA). For resistive loads, voltage and current waveforms are in phase and apparent power equals the true power, so the power factor equals 1. Reactive load circuits have a power factor of less than 1 because the true power is less than the apparent power.

Low power factor has important con sequences for both utility and inverter AC power. Because reactive loads return some power to the source, larger conductors, overcurrent protection, switchgear, and other distribution equipment must be provided for lower-power-factor loads. Consequently, maintaining high power factor minimizes the sizes and costs for this equipment. Furthermore, normal utility kilowatt-hour revenue meters record only true power, not apparent power.

INVERTERS

When AC loads and appliances are to be used in a PV system, an inverter is required to convert DC power to AC power. An inverter is a device that converts DC power to AC power. See 9. The AC output is connected to a distribution panel to power the AC loads. Some inverters allow the flexibility to operate both AC and DC electrical loads. However, inverters add to the complexity and cost of a system, and some power is lost in the conversion process.

Early inverters were electromechanical devices that coupled a DC motor to an AC generator. Electromechanical inverters were noisy and inefficient, and have largely become obsolete. Static (solid-state) inverters change DC power to AC power using electronics and have no moving parts. They are more efficient and much less expensive than electromechanical inverters in most applications. Inverters used in PV systems are exclusively static inverters.

 

8. REACTIVE LOADS: Resistive loads keep the voltage and current waveforms in phase, while reactive loads cause the current waveform to lead or lag the voltage waveform.

Early converters were electromechanical devices that combined a motor with a commutator to produce DC output from an AC input Inverting the connections resulted in AC output from DC input The term inverter stems from these inverted converters loads and include motors and transformers. Reactive power is the product of out-of-phase voltage and current waveforms and results in no net power flow. Inductive loads momentarily retard current in the process of building magnetic fields and cause the current waveform to lag the voltage waveform in time. Capacitive loads momentarily store voltage and cause the current waveform to lead the voltage waveform in time. Reactive power is represented in units of volt-amperes reactive (VAR).

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Electromechanical Rota Converters

In the late 1900s, rotating electromechanical devices were developed as the first power converters. Depending on their design, they convert DC power to AC power, AC power to DC power, or AC power to AC power at different voltages, phase, or frequency. The input power runs a motor, which produces mechanical power. The mechanical power is then transferred to an electrical generator, which is conFigr.d for the desired output power.

Discrete rotary converters are separate motor and generator units that are mechanically coupled by a shaft. This coupling offers power and harmonic isolation, voltage output control, and greater surge protection. Integral rotary converters combine the motor and generator into one self-contained unit. Some of the electrical energy flows directly from input to output, bypassing the mechanical power conversion process, which improves efficiency and allows the unit to be much smaller and lighter than discrete systems.

Electromechanical converters can handle very large loads, but are also heavy and inefficient and require considerable maintenance. These systems are largely obsolete, because solid-state power electronics are smaller, lighter, more efficient, and virtually maintenance free, even when several electronic converters must be connected in parallel to handle large loads.

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9. Inverters are available in many different configurations and ratings.

8. REACTIVE LOADS. Resistive loads keep the voltage and current waveforms in phase, while reactive loads cause the current waveform to lead or lag the voltage waveform.

Power factor is the ratio of true power to apparent power and describes the displacement of voltage and current waveforms in AC circuits. Apparent power is a combination of true and reactive power and is given in units of volt-amperes (VA). For resistive loads, voltage and current waveforms are in phase and apparent power equals the true power, so the power factor equals 1. Reactive load circuits have a power factor of less than 1 because the true power is less than the apparent power.

Low power factor has important con sequences for both utility and inverter AC power. Because reactive loads return some power to the source, larger conductors, overcurrent protection, switchgear, and other distribution equipment must be provided for lower-power-factor loads. Consequently, maintaining high power factor minimizes the sizes and costs for this equipment. Furthermore, normal utility kilowatt-hour revenue meters record only true power, not apparent power.

INVERTERS

When AC loads and appliances are to be used in a PV system, an inverter is required to convert DC power to AC power. An inverter is a device that converts DC power to AC power. See 9. The AC output is connected to a distribution panel to power the AC loads. Some inverters allow the flexibility to operate both AC and DC electrical loads. However, inverters add to the complexity and cost of a system, and some power is lost in the conversion process.

Early inverters were electromechanical devices that coupled a DC motor to an AC generator. Electromechanical inverters were noisy and inefficient, and have largely become obsolete. Static (solid-state) inverters change DC power to AC power using electronics and have no moving parts. They are more efficient and much less expensive than electromechanical inverters in most applications. Inverters used in PV systems are exclusively static inverters.

Early converters were electromechanical devices that combined a motor with a commutator to produce DC output from an AC Input Inverting the connections resulted in AC output I from DC input. The term inverter stems from these inverted converters.

Power Factor

PV Inverters

Inverters for PV systems are broadly classified as either stand-alone or interactive in operation. The difference involves whether the inverter input is connected to the PV array or the battery bank. PV arrays and batteries have different characteristics, which affects inverter design.

Stand-Alone Inverters. Stand-alone inverters are connected to batteries as the DC power source and operate independently of the PV array and the utility grid. PV arrays charge the batteries but do not directly influence the operation of the inverter. For stand-alone inverters, it is the electrical load connected to the AC output, rather than the DC power source, that affects the performance of the inverter. See 10. DC loads may also be powered directly from the battery bank.

Stand-alone inverters must be sized to meet the total connected AC load for both steady-state and surge-load requirements. Overloading the output of stand-alone inverters raises the temperature of the unit until it automatically shuts down.

Utility-Interactive Inverters. Utility-interactive PV inverters are connected to, and operate in parallel with, the electric utility grid. Sometimes called grid-connected or grid-tie inverters, these inverters interface between the PV array and the utility grid and convert DC output from a PV array to AC power that is consistent and synchronous with the utility grid. Interactive inverters are loaded by the DC source, not the AC output, so AC loads do not directly impact the operation of the inverter. See 11.

Interactive PV systems are interconnected with the utility at the distribution panel or on the supply side of service entrance equipment. In a sense, the utility acts as an infinitely large energy storage system that accepts excess energy from the interactive system and supplies extra energy when needed. This allows AC power produced by the PV system to either supply on-site electrical loads or to back- feed power to the grid when the PV system output is greater than the site load demand. At night and during other low insolation periods when the electrical loads are greater than the PV system output, the balance of power required by the loads is received from the electric utility.

p212 Large PV systems may include multiple inverters with their outputs in parallel.

10. Stand-Alone Inverters: Stand-alone inverters are connected to the battery bank and supply AC power to a distribution panel that is independent of the utility grid.

11. Interactive inverters are connected to the PV array and supply AC power that is synchronized with the utility grid.

Bimodal Inverters. Bimodal inverters can operate in either interactive or stand-alone mode (though not simultaneously). These inverters are connected to the battery bank like stand-alone inverters, while the PV array charges the batteries. Sometimes referred to as battery-based interactive or multimode inverters, bimodal inverters are popular for small electrical systems where a back-up power supply is required for critical loads such as computers, refrigerators, and water pumps when a utility power outage occurs.

Under normal circumstances and when connected to an energized utility grid, bimodal PV systems operate in interactive mode, serving on-site loads or sending excess power to the grid while keeping the battery fully charged. If a grid outage occurs, the inverter opens the connection with the utility and operates from the battery bank to supply power to loads at a dedicated subpanel. The subpanel loads must not exceed the inverter power rating.

AC Module Inverters. An AC module is a PV module that outputs AC power through an interactive inverter attached in place of the normal DC junction box. AC modules are only permitted for interactive operation. The module has no accessible DC wiring and is not subject to normal DC circuit requirements for PV systems, which simplifies system design and installation because far fewer components are needed. The modularity also makes sys tem changes and expansion relatively easy.

Multiple AC module inverters are connected in parallel to form AC branch circuits. Since the inverter electronics must be designed to withstand outdoor weather and temperature extremes, the units can be relatively expensive, but in some cases, the savings from not purchasing and installing DC components may outweigh the additional costs.

Inverters are also available that operate like the inverter portion of fully integrated AC modules, but are separate components that must be added to individual modules. See 12. However, while the DC portion of the system is minimized, it is still accessible and may be subject to some requirements.

12. AC Module lnverters -- AC module inverters are small interactive inverters that are supplied by a single PV module.

Switching Devices

Despite their small size, AC module inverters include many of the sophisticated features of larger inverters, such as anti-islanding protection, maximum power point tracking, and sine wave output. Systems with multiple AC module inverters may even keep track of the operating status and output of individual modules over a data network or power line communications interface, though this data monitoring requires additional equipment.

Solid-state inverters use electronics to switch DC power and produce AC power. There are many types of electronic components that can perform switching functions. Continuous improvements in semiconductor manufacturing technology and performance are yielding lower-cost, higher-power, and higher-speed electronic power devices. See 13.

Thyristors. Some basic solid-state inverter designs use thyristors, usually of a type called silicon-controlled rectifiers (SCRs). Thyristors have three leads. When a small current is applied to one lead, the thyristor is turned ON and a larger current is allowed to flow between the other two, like closing a switch. Otherwise, the thyristor is like an open switch and allows no current flow. Like a mechanical switch, thyristors can only be completely ON or completely OFF. Large thyristors are used in high-power applications up to several megawatts, such as in HVDC power transmission.

Transistors. Most inverters use transistors, which are similar in switching capability to thyristors, with two main differences. First, a small voltage, rather than a current as in thyristors, activates the transistor. Second, the magnitude of the activating voltage varies the transistor’s resistance. This means that in addition to completely ON or completely OFF, transistors can allow every point in between, like a dimmer switch.

There are many kinds of transistors, though inverters commonly use metal-oxide semiconductor field-effect transistors (MOS-FETs) or insulated gate bipolar transistors (IGBTs). Power MOS-FETs operate at lower voltages with higher efficiency and lower resistance than IGBTs. MOS-FETs switch at very high speeds, up to 800 kHz, and are generally used in 1 kW to 10 kW applications. IGBTs handle high current and voltage, but switch at lower speeds, up to 20 kHz, and are more common for large, high-voltage applications that may exceed 100 kW.

Switching Control

Each switching device in an inverter requires a control circuit to activate the switching function. Some circuits are designed to regulate switching automatically from an external signal, while others use microprocessors for timing and control.

Line Commutation. In the simplest inverter designs, switching is controlled automatically by an external source, such as utility power. A line-commutated inverter is an inverter whose switching devices are triggered by an external source. Line-commutated inverters alternately turn the switches ON and OFF by the positive and negative half-cycles of the utility voltage, automatically synchronizing the inverter output to the utility. See 14. Line-commutated inverters use a simple and effective design, but can't operate independently of the grid.

13. Switching Devices: Solid-state switching devices used in PV inverters include transistors and thyristors.

Self-Commutation. A self-commutated inverter is an inverter that can internally control the activation and duration of its switching. Self-commutated inverters can operate either interconnected with or independent of utility power. This is accomplished by incorporating microprocessors for precise timing and control. Compared to line-commutated inverters, self-commutated inverters can better control AC waveform output, adjust power factor, and suppress harmonics. Most PV inverters manufactured today are self-commutated inverters.

Self-commutated inverters are either voltage-source or current-source types. A voltage-source self-commutated inverter treats the DC input as a voltage source and produces an AC voltage output. A current- source self-commutated inverter treats the DC input as a current source and produces an AC current output. Stand-alone and bimodal inverters are typically of the voltage-source type. Most interactive-only inverters are of the current-source type.

Square Wave Inverters

There can be multiple processing steps to convert DC power into sine wave AC power. The first step in the inverting process is to switch the DC power back and forth to create AC power, producing a square wave. If an inverter performs only this step, it is a square wave inverter. Square wave output is not very efficient and can be detrimental to some loads. More-sophisticated inverters add further processing after this step to produce modified square waves or sine waves.

Switching circuits are typically designed symmetrically. One side of the circuit includes devices to switch and control DC input into the positive half-wave of the AC output. The other side switches and controls a reversed polarity of the DC input into the negative half-wave of the AC output. Two circuit designs for producing square wave output are H-bridge circuits and push-pull circuits.

H-Bridge Inverter Circuit. An H-bridge inverter circuit is very similar to a full-wave rectifying circuit, but the two circuits perform opposite functions. An H-bridge inverter circuit is a circuit that switches DC input into square wave AC output by using two pairs of switching devices. One pair is open while the other pair is closed. The two pairs alternate states to change the direction of the DC current flow through the circuit’s output. See 1g. This design is known as an H-bridge inverter because the switching array can be drawn in an “H” shape in a circuit diagram.

14. Line Commutation: Line-commutated inverters use an external AC signal to activate and deactivate the inverter switching devices.

p215 lnverters control high-power switching devices with low-power electronics to produce AC output.

15. H-Bridge Inverter Circuits -- H-bridge inverter circuits use two pairs of switching devices to direct a DC input to the output in both directions.

Push-Pull Inverter Circuit. A push-pull inverter circuit is a circuit that switches DC input into AC output by using one pair of switching devices and a center-tapped transformer. The circuit gets its name from the backward and forward current flow through the circuit. See 16. First, the top switch closes, allowing current to flow from the DC source through the transformer and back in a clockwise loop. Then, the top switch is opened and the bottom switch is closed. The current flows again from the DC source, this time in a counter clockwise loop. The alternating current in the primary winding of the transformer is in the shape of a square wave, which induces a similar AC output in the secondary winding.

A switching device can be shorted across the output winding to zero the transformer output during part of each half-cycle. This creates an additional step in the output waveform, resulting in a modified square wave.

16. Push-Pull Inverter Circuits -- Push-pull inverter circuits use one pair of switching devices and a transformer to alternate the direction of direct current.

Low-Frequency Waveform Control

The simple square wave output from inverter circuits can he further refined to improve sine wave approximation. By adjusting the duration of the alternating square pulses, the output becomes a modified square wave. Transformers are used to step the input voltage up to output voltage levels. For shorter pulses, the peak volt age is stepped higher. See 17. To create a multi-stepped modified square wave, multiple square wave inverter stages are operated in parallel. The outputs are then combined to produce a stepped waveform that more closely matches a true sine wave. See 18.

Inverters that use this method are called low-frequency inverters because the frequencies of these waveforms are still only 60 Hz. While they are simple, low-frequency inverters use very heavy transformers and are less efficient than higher-frequency designs.

High-frequency Waveform Control

Pulse-width modulation (PWM) control is used to create a sine wave inverter output. Pulse-width modulation (PWM) is a method of simulating waveforms by switching a series device ON and OFF at high frequency and for variable lengths of time. When the pulses are narrow, the current is OFF most of the time, which simulates a low voltage. When pulses are wide, the current is ON most of the time, which simulates a high voltage. See 19. Some PWM methods also adjust frequency by spacing narrow pulses farther apart and wide pulses closer together.

By using very high frequencies and gradual changes in pulse width, loads are less affected by abrupt changes in voltage. Most loads operate as if PWM output were a true sine wave. High- frequency switching and power conversion also reduces harmonics in output current, which reduces the size and weight of the transformers. Pulse-width modulation, along with advancements in digital controls and microprocessors, has resulted in very efficient inverter designs.

High-frequency inverters use a DC-DC converter to step DC input voltage up to higher levels or use higher-voltage DC arrays. The DC power is then inverted to AC power at high frequency without the need for a large transformer. Pulse-width modulation is then the final switching stage to produce 60 Hz sine wave AC power.

17. Low-Frequency Control: Square waves can be modified by adjusting the duration and magnitude of the pulses.

19. High-Frequency Pulse-Width Modulation: Pulse-width modulation at high frequencies generates the truest approximation of a sine wave.

18. Multi-stepped Modified Square Waves: Combining multiple modified square waves with different magnitudes and durations results in a multi-stepped modified square wave that more closely approximates a sine wave.

 

POWER CONDITIONING UNITS

The physical enclosure that is referred to as an inverter is actually often a power conditioning unit (PCU). Power conditioning units perform one or more power processing and control functions in addition to inverting, such as rectification, transformation, DC-DC conversion, and maximum power point tracking. See 20. These functions can also be performed by separate components, but this is usually not necessary. Power conditioning units may include system-monitoring capabilities and protective features such as disconnects and overcurrent protection equipment.

20. Power conditioning units are inverters that also perform other power control and conversion functions.

Rectifiers

A rectifier is a device that converts AC power to DC power. Rectifiers are used in battery chargers and DC power supplies operating from AC power. Many stand-alone and hybrid PV inverters include multifunction, programmable battery chargers with the same charge control and regulation algorithms as separate battery charge controllers.

Transformers

A transformer is a device that transfers energy from one circuit to another through magnetic coupling. A transformer consists of two or more coupled windings and a core. Current in one winding creates a magnetic flux in the core, which induces voltage in the other winding. Transformers are used to convert between high and low AC voltages, change impedance, and provide electrical isolation and voltage regulation. See 21.

21. Transformers use induced magnetic fields to transfer AC power from one circuit to another while transforming the power to higher or lower voltages or providing electrical isolation.

Transformers are used within inverters and power conditioning units to raise or lower AC voltages and provide electrical isolation.

Transformers can't convert between DC and AC, change the voltage or current of a DC source, or change the frequency of an AC source. However, transformers are integral components of devices that perform these functions, including inverters.

The ratio of the number of turns in the windings of a transformer (turns ratio) determines how voltage and current are stepped up or down. For example, a transformer with a 1:10 turns ratio will step an input voltage up 10 times. Since the power must remain the same (neglecting small losses), the cur rent must be stepped down to one-tenth of the input current. Therefore, a 12 V input at 20 A would be transformed to a 120 V output at2A.

An autotransformer is a transformer with only one winding and three or more taps. See 22. The voltage source is applied to two taps and the load is connected to two taps, one of which is a common connection with the source. Each tap corresponds to a different source or load voltage. In an autotransformer, a portion of the same winding acts as part of both the primary and secondary windings. Autotransformers are an economical and compact way to adjust a voltage up or down slightly. For example, an autotransformer can be used to convert 240 V output from an inverter to 208 V for interconnection to a residential system. Unlike transformers with multiple windings, autotransformers do not provide electrical isolation.

DC-DC Converters

A DC-DC converter is a device that changes DC power from one voltage to another. Modern switched-mode DC-DC converters use high-frequency switching and transformers to convert DC power to a different voltage, either higher or lower than the source voltage. These devices are lightweight and efficient, and because they use transformers, they provide circuit isolation. A buck converter is a step- down DC-DC converter. A boost converter is a step-up DC-DC converter.

Many PV inverters use DC-DC converters to change the DC input from low voltage to high voltage prior to the power-inverting process. Also, external DC-DC converters may be used on battery-based systems to deliver DC voltage at levels other than the nominal battery voltage. Maximum power point trackers are a form of DC-DC converter. DC-DC converters are characterized by their power rating, input and output voltages, and their power conversion efficiency.

Maximum Power Point Trackers

A maximum power point tracker (MPPT) is a device or circuit that uses electronics to continuously adjust the load on a PV device under changing temperature and irradiance conditions to keep it operating at its maximum power point. Since they are connected directly to the array. all interactive inverters include MPPT circuits.

In some system designs with multiple arrays, individual MPPTs are connected to each input source circuit to allow the inverter to extract maximum output. This improves performance if the arrays have different I-V characteristics or are oriented in different directions. As the cur rent output for these arrays varies, the individual MPPTs optimize each array output.

Stand-alone inverters do not directly operate or control the array, so they do not normally include MPPT circuits. However, MPPT battery charge controllers can interface between an array and the battery bank that is powering a stand-alone inverter. This MPPT application can improve array battery-charging potential, but does not affect operation of the inverter.

22. Autotransformers -- The primary and secondary windings in an autotransformer share some of the same windings.

INVERTER FEATURES and SPECIFICATIONS

PV system inverters include a number of basic and optional features. Inverter specifications include performance data, operating limits, installation requirements, safety, and maintenance. This information is found on the inverter nameplate and in the product manuals.

Listing and Certifications

Inverters installed in PV systems are required to conform to certain standards for product listing and certifications. These include the safety standard UL 1741 as well as certifications for EMI under FCC Part 15. Inverters must include a listing mark on their nameplate label. See 23. Inverters not marked as interactive inverters are not permitted to operate in utility- interconnected applications.

23. Inverter nameplates include much of the needed in formation for sizing and operating the inverter.

UL 1741 inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources addresses requirements for all types of distributed generation equipment, including inverters and charge controllers for PV systems. IEEE 1547 Standard for Interconnecting Distributed Re sources with Electric Power Systems, and IEEE 1547.1 Standard Conformance Test Procedures for Equipment Interconnecting Distributed Resources with Electric Power Systems are the basis for UL 1741 listing for interactive inverters.

Installation

National codes and standards dictate the safety and installation requirements for inverters used in PV systems. Inverter installation requirements are governed by the NEC® Article 690, “Solar Photovoltaic Systems” and other applicable sections of the NEC®, including overcurrent protection devices, disconnects, grounding, and utility-interconnection. Many of these requirements are based on equipment standards and listing requirements for interactive inverters under UL 1741. Most inverter manufacturers also provide details on requirements for code-compliant installation in their product manuals.

Power Ratings

The principal inverter specification is the out put power rating. In the case of stand-alone inverters, the power rating limits the power it can deliver to AC loads. For an interactive inverter, the output power rating limits the power it can handle at its DC input, which limits the size of the PV array.

Interactive inverters are available from many manufacturers in power ratings from 700 W for small residential units to over 500 kW for large commercial and utility-scale installations. Stand-alone inverters are available in sizes down to about 50 W, but are commonly in the 3 kW to 6 kW range. Exceedingly large battery banks and high currents limit stand-alone inverters to a maximum of about 50 kW for large remote- power installations.

Temperature Limitations. Solid-state switching devices are capable of handling only so much current before they overheat and fail. Paralleling switching devices in the design increases the power rating of an inverter. However, thermal management in electronic inverters is still a major concern, and temperature is the primary limiting factor for inverter power ratings.

Manufacturers specify a permissible operating temperature range for their inverters, such as -20°C to +50°C. Inverters may deal with excessive operating temperature in several ways. Many inverters use heat sinks and /or ventilation fans to regulate temperature.

Interactive inverters control high temperatures by limiting the array power delivered to the inverter. The inverter forces the array operating point from maximum power to a higher operating voltage (toward open-circuit voltage), reducing power and current levels. See 24. Once the inverter temperatures stabilize, the array is again loaded to its full power output. At the higher input voltage, the power output falls below the inverter’s rating.

24. Power Output Limiting: At high temperatures, an interactive inverter may limit current input by raising the input voltage, which also lowers power input and output. RATED POWER OUTPUT; INVERTER ADJUSTS INPUT VOLTAGE TO LIMIT POWER AT HIGH TEMPERATURES

If the temperature or load limits are exceeded, inverters limit or disconnect their output. Most stand-alone inverters shut down or internally disconnect the AC output if load limits are exceeded. Otherwise overcurrent devices trip if the AC output is excessively loaded. The load must then be decreased (manually) before the inverter can be restarted.

Voltage Ratings

Inverter performance is strongly associated with operating voltages. Voltage ratings are given for the AC output and DC input circuits, and may apply to stand-alone and interactive inverters in different ways. Voltage specifications are typically given as a range, with mini mum and maximum limits for operation.

AC Output. Inverter AC output interfaces with either the utility grid or with electrical loads and appliances, so inverter voltage ratings are consistent with normal utility voltage standards. Smaller inverters (less than 6 kW) typically produce an AC output voltage of 120 V or 240 V nominal single-phase. Larger inverters produce 208 V, 277 V, or 480 V nominal three-phase AC output. Some inverters can be conFigr.d for a variety of output voltages at the time of installation.

For interactive inverters, AC voltage output must be maintained at -10% to +5% of the nominal system voltage. In the case of 120 V nominal output, this is a range from 108 V to 126 V. Variations may occur in supply voltage. If the inverter senses that voltage is out of range for more than 30 seconds (1800 cycles), the AC output disconnects within 10 cycles. If the voltage varies more than —30% or +10% for 10 cycles, the inverter must disconnect within 10 cycles. This standard reduces the potential for islanding. Additionally, voltage flicker shall not be allowed to exceed a 3% voltage sag, in accordance with IEEE 519 Recommended Practice and Requirements for Harmonic Control in Electrical Power Systems. Inverter specifications may include RMS output volt age regulation.

p221 MOSFET switching devices can become so hot when switching high currents that they must be bonded to large metal heatsinks.

There is often a fixed relationship between the utility voltage and the array voltage that the inverter MPPT will track. The required array voltage increases with increasing grid voltage, and the array must have maximum power voltage in this range to permit MPPT operation. See 26.

DC Input. DC input voltage ratings are based on the operating characteristics of either a battery bank (for stand-alone inverters) or a PV array (for interactive inverters). Small stand-alone inverters are designed to operate from nominal 12 V lead-acid batteries, or multiples thereof. These inverters operate within a relatively narrow voltage range, between 11 V and 16 V, based on actual battery volt ages during normal charging and discharging conditions. For AC output power levels over 1 kW, 12 V inputs are impractical, because increased currents require larger and more expensive conductors and switchgear. Instead, battery banks for larger systems are designed to deliver nominal 24 V (actual range 22 V to 32 V) or 48 V (44 V to 64 V) or higher.

For interactive inverters, the DC input voltage requirements are more complex. Minimum and maximum voltage limits are given for inverter operation and another, narrower voltage range within which the inverter will properly track maximum power from the array. See 25. Minimum operating voltages are required to perform basic inverter functions and produce an output with sufficient peak and RMS voltage. Below this level, the inverter will not start or operate. Maximum DC voltage avoids exceeding the inverter’s voltage-handling capability, and most are limited to less than 600 V for product listing and code compliance reasons.

26. Minimum DC Input Voltages: In order to output AC voltage within the specified range, the DC input voltage must meet certain minimum values.

Some interactive inverters have a large range of DC input voltages, allowing many array sizes and configurations. The DC volt age from an array is also affected by ambient temperature, which complicates sizing. Many inverter manufacturers offer string sizing soft ware to determine the minimum and maximum number of series-connected modules for proper inverter operation. These tools, many available online, include databases of commercial PV module specifications and the manufacturer’s inverter ratings. Based on the chosen module information and user-inputted minimum and maximum ambient temperatures for the location, the allowable array configurations are displayed. The highest possible array voltages for the site’s record low temperature influence the maximum number of modules. Conversely, a minimum number of modules must be connected in series to achieve sufficient operating voltages under the hottest conditions. A table of results shows how array maximum open-circuit voltage varies by number of modules and ambient temperature.

25. DC Input Voltage Ranges: Most inverters operate from a relatively wide range of input voltages, but the range for MPPT operation is smaller.

Frequency Ratings

Inverters for North American markets are designed to produce 60 Hz, while inverters for Europe, Asia, and other parts of the world produce 50 Hz. Loads and systems are more sensitive to variations in frequency than volt age, so only a small variation is allowed from the nominal operating frequency. For nominal 60Hz operation, the AC output frequency must be maintained between 59.3 Hz and 60.5 Hz.

Current Ratings

Inverters are rated for operating and maximum allowable AC and DC currents, which are determined by the current-handling capabilities of the switching devices. For the DC side, current ratings limit the PV array or battery current that can be applied to the inverter. On the AC side, current ratings limit the AC load for stand-alone inverters and the AC current output for interactive inverters.

Maximum continuous AC output and DC input current ratings are given at a reference temperature, and are the basis for sizing conductors, switchgear, and overcurrent protection for the input and output circuits. DC input cur rent for some interactive inverters decreases with increasing DC input voltage in order to limit inverter output power. See 27.

27. DC Input Current Inverters may limit maximum DC input current with increasing DC input voltage.

Maximum AC output fault currents may also be specified, and represent the maximum current that an inverter can deliver into a shorted or overloaded circuit. Inverter specifications sometimes include requirements for external AC and DC overcurrent protection de vices, usually based on 125% of the maximum continuous rated currents. This equipment is provided for some inverters as part of the power conditioning unit.

Stand-alone inverters draw high DC input currents from low-voltage battery banks, especially at low battery voltages (state of charge). Special circuit design requirements apply to these inverters.

Surge Capability. Inverters operating from batteries can deliver high surge and in-rush currents for short periods. Many motors re quire surge current at startup of as much as 6 to 10 times the normal operating current. Surge capability is given as a maximum AC output current for an amount of time, for example 78 A for 100 ms. This may contrast with a continuous rated output current, such as 33 A. Interactive inverters operating from PV arrays can't produce surge current because they are

Inverters for large, building-integrated PV arrays, such as curtain-wall skylights, are fully incorporated into the electrical infrastructure of the building.

Harmonics

Utility-interactive inverters must maintain acceptable limits of harmonic distortion at the utility interconnection. In accordance with IEEE 519, total harmonic distortion shall not exceed 5%, and any individual harmonic shall not exceed 3%. Interactive inverters are required to de-energize from the grid if these limits are exceeded.

Power Factor

Interconnection standards require interactive inverters to maintain an output power factor between 0.95 leading and 0.95 lagging. Most interactive inverters produce AC output with a power factor of 1 under all conditions. Output power factor for stand-alone inverters is a function of the load, which may or may not operate with a power factor of I.

Efficiency

Inverter efficiency is the effectiveness of an inverter at converting DC power to AC power. Some input power is used to operate the inverter and some is lost as heat in the switching and power conversion processes. Stand-by losses are the power required to operate inverter electronics and keep the inverter in a powered state. Output power depends on the waveform shape. Therefore, the output power is always less than the input power. Inverter efficiency is calculated with the following formula:

Most interactive PV inverters are rated 90% to 95% efficient. Quality stand-alone inverters producing sine wave output have peak efficiencies of about 90%. Lesser-quality modified sine wave inverters may have efficiencies as low as 75% to 85%. In general, high-frequency and high-voltage inverters are more efficient than lower-voltage inverters operating at low frequency.

Inverter efficiency is primarily affected by the inverter load. In stand-alone inverters, the AC load defines the inverter load, and for interactive inverters, the PV array defines the load. Efficiency can also be affected by inverter temperature and DC voltage input. Inverter stand-by losses are nearly constant for all output power levels, so efficiency is lower for low power outputs. See 28. Some modified square wave stand-alone inverters reach peak efficiency at levels well below their maximum rated power output. Most interactive inverters maintain high efficiency over a wide operating range.

Harmonic distortion is used to create waveforms of certain shapes The sum of a series of each integer sine wave harmonic produces a waveform with a sawtooth shape The sum of a series of odd-numbered harmonics produces a square waveform The sum of a series of odd numbered harmonics with every (4n - 1 )th harmonic inversed produces a waveform with a triangle shape.

28. Inverter Efficiencies: Most sine wave inverters maintain high efficiency over a wide operating-power range. PERCENT OF OUTPUT POWER RATING (%); DC = DC power input (in W)

Protective Devices

Most inverters include devices to protect the inverter and connected equipment from dam age from excessive temperatures, currents, or power levels. See 29. For example, stand-alone inverters disconnect themselves if DC input voltages become too low, such as from a discharged battery, preserving AC out put power quality and preventing the inverter from drawing excessive currents. Nearly all interactive inverters include ground-fault protection. Since most inverters include trans formers, they also provide isolation between the DC power source and utility grid or AC output. Many include voltage surge suppression on the DC and /or AC sides. Even when inverters do include these devices, additional equipment may still be required.

All interactive inverters must employ protective devices for the utility interface, based on the specified limits of grid operations. These inverters monitor voltage, frequency, power factor, and other parameters, and control output accordingly. If the voltage or frequency of the utility power exceeds preset limits, or a potential islanding condition exists, interactive inverters are required to cease interconnected operations. Once the utility parameters have stabilized within acceptable limits for at least 5 minutes, the inverter automatically resumes operation.

Physical Characteristics

Physical specifications include the size, weight, and mounting requirements. Low- voltage, low-frequency inverter designs use heavy transformers, so 5 kW inverters may weigh as much as 200 lbs. Special mounting requirements may apply to these inverters. High-frequency inverters use smaller transformers, so they weigh considerably less for equivalent power ratings. Other physical and mechanical characteristics may be provided and applicable to installation.

Data and Control Interfaces

Most modern inverters incorporate micro processors, and many provide features for data monitoring and communications. Inter faces may include displays and controls on the inverter itself, while others interface with remote units or computers. Status or values can be indicated by LEDs, alphanumeric LCD displays, or graphical LCD displays. Some systems interface with computer software for processing raw data and automatically generating charts, graphs, or graphical displays. See 30. When connected to a web server, this information can be published on a web site. These systems are particularly flexible for storing and processing system data.

Inverter interfaces typically provide basic system information, including interconnection status, AC output voltage and power, DC input voltage, MPPT status, error codes, fault conditions, and other parameters. Many inverters record energy production on daily and cumulative bases.

Inverter operation parameters are generally not field-adjusted, since this could affect critical safety features and operating limits. How ever, other power conditioning functions may allow operator control or adjustment, such as battery charger and charge controller settings and the operation and control of generators and other power sources, allowing flexibility for a variety of applications.

29. Inverter enclosures may include protective devices such as circuit breakers.

30. Inverter Interfaces: Inverter interfaces include on-board screens, remote data monitors, and computerized data acquisition and processing software. ON-BOARD DISPLAY ; COMPUTERIZED DATA ACQUISITION; REMOTE DATA MONITOR

SUMMARY:

• Sine waves, square waves, and modified square waves are common AC waveforms produced by inverters.

• Sine waves are the most complicated type of AC waveform for inverters to produce, so some less-sophisticated inverters approximate the sine wave with square waves and modified square waves.

• Static (solid-state) inverters change DC power to AC power using electronics, so they have no moving parts and are very efficient.

• AC module inverters are small inverters that take the place of a DC junction box on a PV module.

• Inverters use thyristors and transistors to switch DC power and produce AC power.

• Line-commutated inverters use an external signal to control the switching devices, while self-commutated inverters include microprocessors for precise timing and control of the switching devices.

• By adjusting the duration of the alternating pulses, a square wave becomes a modified square wave.

• Pulse-width modulation (PWM) is used to construct the closest approximation of a sine wave.

• Power conditioning units perform one or more power processing and control functions in addition to inverting, such as rectification, transformation, DC-DC conversion, and maximum power point tracking.

• Transformers are used to convert between high and low AC voltages, change impedance, and provide electrical isolation and voltage regulation.

• The principal inverter specification is the power rating.

• Temperature is the primary limiting factor for inverter power ratings.

• DC input voltage ratings are based on the operating characteristics of either a battery bank (for stand-alone inverters) or a PV array (for interactive inverters).

• Operating and maximum allowable AC and DC current ratings are determined by the current-handling capabilities of switching devices used in the inverter.

• Most inverters include features to protect the inverter and connected equipment from damage from excessive temperatures, currents, or power levels.

• Direct current (DC) is electrical current that flows in one direction, either positive or negative.

• Alternating current (AC) is electrical current that changes between positive and negative directions.

• A waveform is the shape of an electrical signal that varies over time.

• A periodic waveform is a waveform that repeats the same pattern at regular intervals.

• A cycle is the interval of time between the beginnings of each waveform pattern.

• A sine wave is a periodic waveform the value of which varies over time according to the trigonometric sine function.

• A sinusoidal waveform is a waveform that is or closely approximates a sine wave.

• A square wave is an alternating current waveform that switches between maximum positive and negative values every half period.

• A modified square wave is a synthesized, stepped waveform that approximates a true sine wave.

• Frequency is the number of waveform cycles in one second.

• Period is the time it takes a periodic waveform to complete one full cycle before it repeats.

• Peak is the maximum absolute value of a waveform.

• Peak-to-peak is a measure of the difference between positive and negative maximum values of a waveform.

• The root-mean-square (RMS) value is a statistical parameter representing the effective value of a waveform.

• Power quality is the measure of how closely the power in an electrical circuit matches the nominal values for parameters such as voltage, current, harmonics, and power factor.

• Voltage unbalance is the unbalance that occurs when the voltages of a three-phase power supply or the terminals of a three-phase load are not equal.

• Single phasing is the complete loss of one phase on a three-phase power supply.

• Current unbalance is the unbalance that occurs when current is not equal on the three power lines of a three-phase system.

• Phase unbalance is the unbalance that occurs when three-phase power lines are more or less than 1200 out of phase.

• A harmonic is a waveform component at an integer multiple of the fundamental waveform frequency.

• Total harmonic distortion (THD) is the ratio of the sum of all harmonic components in a waveform to the fundamental frequency component.

• A resistive load is a load that keeps voltage and current waveforms in phase.

• True power is the product of in-phase voltage and current waveforms and produces useful work.

• A reactive load is an AC load with inductive and /or capacitive elements that cause the current and voltage waveforms to become out of phase.

• Reactive power is the product of out-of-phase voltage and current waveforms and results in no net power flow.

• Power factor is the ratio of true power to apparent power and describes the displacement of voltage and current waveforms in AC circuits.

• Apparent power is a combination of true and reactive power and is given in units of volt-amperes (VA).

• An inverter is a device that converts DC power to AC power.

• An AC module is a PV module that outputs AC power through an interactive inverter attached in place of the normal DC junction box.

• A line-commutated inverter is an inverter whose switching devices are triggered by an external source.

• A self-commutated inverter is an inverter that can internally control the activation and duration of its switching.

• An H-bridge inverter circuit is a circuit that switches DC input into square wave AC output by using two pairs of switching devices.

• A push-pull inverter circuit is a circuit that switches DC input into AC output by using one pair of switching devices and a center-tapped transformer.

• Pulse-width modulation (PWM) is a method of simulating waveforms by switching a series device ON and OFF at high frequency and for variable lengths of time.

• A rectifier is a device that converts AC power to DC power.

• A transformer is a device that transfers energy from one circuit to another through magnetic coupling.

• A DC-DC converter is a device that changes DC power from one voltage to another.

• A maximum power point tracker (MPPT) is a device or circuit that uses electronics to continually adjust the load on a PV device under changing temperature and irradiance conditions to keep it operating at its maximum power point.

• Inverter efficiency is the effectiveness of an inverter at converting DC power to AC power.

• Stand-by losses are the power required to operate inverter electronics and keep the inverter in a powered state.

1. Why is sine wave output the preferred inverter output?

2. How do modified square wave inverters compare with square wave inverters in terms of power quality?

3. How is power quality relevant to inverter output?

4. Compare the basic system configurations of stand-alone, interactive, and bimodal inverters.

5. What are the similarities and differences between thyristors and transistors?

6. Why can line-commutated inverters not be used in stand-alone systems?

7. How does higher-frequency control result in a better approximation of a true sine wave?

8. How are inverters different from power conditioning units?

9. Why is it important for inverters to manage their temperature?

10. How do stand-by losses affect inverter efficiency for various power levels?

11. What types of system operating information are typically provided by inverter interfaces?

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