# Arizona Solar Power Society

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Arizona Solar Power Society
Photovoltaic Systems Inverters AC Power • Inverters • Power Conditioning Units • Inverter Features and Specifications Arizona Solar Power Society

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. 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 Figure 8-1. 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.

AC waveforms can take a variety of shapes.
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 Figure 8-2.

Certain parameters are integral to defining the characteristics of an AC waveform.
Period is the time it takes a periodic waveform to complete one full cycle before it repeats. See Figure 8-3. 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.

Three-phase power is composed of three separate voltage waveforms that are 120° out of phase.
Three-phase AC power includes three separate voltage and current waveforms occurring simultaneously 120° apart. See Figure 8-4. 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.

Voltage variations outside allowable ranges include voltage sags, voltage swells, and transients.
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 Figure 8-5.

Three-phase voltage and current waveforms are unbalanced if they are not equal in magnitude.
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 Figure 8-6. Voltage unbalance should not be more than 1%. The primary cause of voltage unbalances of less than 2% is too many single-phase loads on one phase of a three-phase system.

Harmonics can add to the fundamental frequency to produce distorted waveforms.
A harmonic is a waveform component at an integer multiple of the fundamental waveform frequency. For example, the second harmonic frequency of a 60 Hz 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 Figure 8-7.

Resistive loads keep the voltage and current waveforms in phase, while reactive loads cause the current waveform to lead or lag the voltage waveform. 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 Figure 8-8. True power is also called real power or active power and is represented in units of watts (W).

Inverters are available in many different configurations and ratings.
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 Figure 8-9. 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.

Stand-alone inverters are connected to the battery bank and supply AC power to a distribution panel that is independent of the utility grid. 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 Figure DC loads may also be powered directly from the battery bank.

Interactive inverters are connected to the PV array and supply AC power that is synchronized with the utility grid. 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 Figure 8-11.

AC module inverters are small interactive inverters that are supplied by a single PV module.
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 Figure However, while the DC portion of the system is minimized, it is still accessible and may be subject to some requirements.

Solid-state switching devices used in PV inverters include transistors and thyristors.
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 Figure 8-13.

Line-commutated inverters use an external AC signal to activate and deactivate the inverter switching devices. 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 Figure Line-commutated inverters use a simple and effective design, but cannot operate independently of the grid.

H-bridge inverter circuits use two pairs of switching devices to direct a DC input to the output in both directions. 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 Figure This design is known as an H-bridge inverter because the switching array can be drawn in an “H” shape in a circuit diagram.

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

Square waves can be modified by adjusting the duration and magnitude of the pulses.
The simple square wave output from inverter circuits can be 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 voltage is stepped higher. See Figure 8-17.

Combining multiple modified square waves with different magnitudes and durations results in a multistepped modified square wave that more closely approximates a sine wave. To create a multistepped 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 Figure 8-18.

Pulse-width modulation at high frequencies generates the truest approximation of a sine wave.
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 Figure Some PWM methods also adjust frequency by spacing narrow pulses farther apart and wide pulses closer together.

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

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. 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 Figure 8-21.

The primary and secondary windings in an autotransformer share some of the same windings.
An autotransformer is a transformer with only one winding and three or more taps. See Figure 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.

Inverter nameplates include much of the needed information for sizing and operating the inverter.
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 Figure Inverters not marked as interactive inverters are not permitted to operate in utility-interconnected applications.

At high temperatures, an interactive inverter may limit current input by raising the input voltage, which also lowers power input and output. 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 Figure 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.

Most inverters operate from a relatively wide range of input voltages, but the range for MPPT operation is smaller. 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 Figure 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.

In order to output AC voltage within the specified range, the DC input voltage must meet certain minimum values. 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 Figure 8-26.

Inverters may limit maximum DC input current with increasing DC input voltage.
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 current for some interactive inverters decreases with increasing DC input voltage in order to limit inverter output power. See Figure 8-27.

Most sine wave inverters maintain high efficiency over a wide operating-power range.
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 Figure 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.

Inverter enclosures may include protective devices such as circuit breakers.
Most inverters include devices to protect the inverter and connected equipment from damage from excessive temperatures, currents, or power levels. See Figure For example, stand-alone inverters disconnect themselves if DC input voltages become too low, such as from a discharged battery, preserving AC output power quality and preventing the inverter from drawing excessive currents. Nearly all interactive inverters include ground-fault protection. Since most inverters include transformers, 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.

Inverter interfaces include on-board screens, remote data monitors, and computerized data acquisition and processing software. Most modern inverters incorporate microprocessors, and many provide features for data monitoring and communications. Interfaces 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 Figure 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.