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One o the most incredible things about photovoltaic

power is its simplicity. It is almost completely solid

state, rom the photovoltaic cell to the electricity

delivered to the consumer. Whether the application

is a solar calculator with a PV array o less than 1 W

or a 100 MW grid-connected PV power generation plant, all that

is required between the solar array and the load are electronic

and electrical components. Compared to other sources o energy

humankind has harnessed to make electricity, PV is the most

scalable and modular. Larger PV systems require more electri-

cal bussing, using and wiring, but the most complex component

between the solar array and the load is the electronic component

that converts and processes the electricity: the inverter.

In the case o grid-tied PV, the inverter is the only piece o electronicsneeded between the array and the grid. O-grid PV applications use an addi-tional dc to dc converter between the array and batteries and an inverterwith a built-in charger. In this article we discuss how inverters work, includ-ing string, or single-phase, and central, 3-phase inverters; explore majorinverter unctions, key components, designs, controls, protections and com-

munication; and theorize about uture inverter technology.

What Goes on Insidethe Magic Box

How InvertersWork

By James Worden and Michael Zuercher-Martinson

68 S o l a r P r o | api/My 2009

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KEY INVERTER FUNCTIONS

Four major unctions or eatures are common to all transormer-based, grid-tiedinverters:

Inversion Maximum power point tracking Grid disconnection Integration and packagingInversion. Te method by which dc power rom the PV array is converted to

ac power is known as inversion. Other than or use in small o-grid systems andsmall solar gadgets, using straight dc power rom a PV array, module or cell is notvery practical. Although many things in our homes and businesses use dc power,large loads and our electrical power inrastructure are based on ac power. Tisdates back to the early days o Edison versus esla when ac won out over dc as ameans o electrical power distribution.

An important reason that ac won out is because it can be stepped up and travellong distances with low losses and with minimal material. Tis could change in

the distant uture i more o our energy is produced, stored and consumed bymeans o dc power. oday, the technology exists to boost dc electricity to highvoltages or long distance transer, but it is very complex and costly. For the ore-seeable uture, ac will carry electricity between our power plants, cities, homes

and businesses.In an inverter, dc power rom the PV array is inverted to ac power via a set

of solid state switchesMOSFETs or IGBTsthat essentially ip the dc powerback and orth, creating ac power. Diagram 1 shows basic H-bridge operation ina single-phase inverter.

Maximum power point tracking. Te method an inverter uses to remain on theever-moving maximum power point (MPP) o a PV array is called maximum powerpoint tracking(MPP). PV modules have a characteristic I-V curve that includes ashort-circuit current value (Isc) at 0 Vdc, an open-circuit voltage (Voc) value at 0 Aand a knee at the point the MPP is oundthe location on the I-V curve wherethe voltage multiplied by the current yields the highest value, the maximumpower. Diagram 2 (p. 70) shows the MPP or a module at ull sun in a variety o

temperature conditions. As cell temperature increases, voltage decreases. Module

Diagram 1 An H-bridge circuit perorms the basic conversion rom dc to ac power.

This solid state switching process is known as inversion.

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First half of cycle Second half of cycle

spfessin.cm | S o l a r P r o 69

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perormance is also irradiance dependent. When the sun isbrighter, module current is higher; and when there is lesslight, module current is lower. Since sunlight intensity andcell temperature vary substantially throughout the day and

the year, array MPP current and voltage also move signi-cantly, greatly aecting inverter and system design.

Te terms full sun or one sun are ways to describe theirradiance conditions at SC (1000 W/m2). Sunlightintensity varies rom nothing to ull sun or a littlemore than one sun in some locations and condi-tions. Tis means that PV output current can varyrom zero to ull array rating or more. Inverters needto work with arrays at their lowest voltages, whichoccur under load on the hottest days, as well as attheir highest voltages, which occur at unloadedopen circuit array conditions on the coldest days. In

some climates, temperatures can vary by 100F ormore, and PV cell temperatures can vary by 150F.Tis means array voltage can vary by ratios o nearly2:1. A string o 22 Evergreen ES-A-210 modules, orexample, will reach a Voc o 597 Vdc with a cell tem-perature o -30C (-22F). Te MPP voltage (Vmp)can get as low as 315 Vdc in an ambient tempera-ture o 50C (122F). In most cases, the maximumpower point voltage operates over a 25% variation.However, this number is lower in regions with moreconsistent year-round temperatures, such as SanDiego, Caliornia, and is higher in regions where

temperature varies more, such as the Midwest and

Northeast. Finding the arrays MPP and remaining on it, evenas it moves around, is one o the most important grid-direct

solar inverter unctions.Grid disconnection. As required by UL 1741 and IEEE 1547,all grid-tied inverters must disconnect rom the grid i theac line voltage or requency goes above or below limits pre-scribed in the standard. Te inverter must also shut down i itdetects an island, meaning that the grid is no longer present.In either case, the inverter may not interconnect and exportpower until the inverter records the proper utility voltageand requency or a period o 5 minutes. Tese protectionseliminate the chance that a PV system will inject voltage orcurrent into disconnected utility wires or switchgear andcause a hazard to utility personnel. I an inverter remainedon or came back on beore the utility was reliably recon-nected, the PV system could backeed a utility transormer.Tis could create utility pole or medium voltage potentials,which could be many thousands o volts. A signicant bat-tery o tests is perormed on every grid-tied inverter to makecertain that this situation can never occur.

able 1 shows the voltage and requency limiting val-ues and the time periods that the inverter has to be oine,reerred to as clearing times. Notice that some values aredierent or inverters under 30 kW and those over 30 kW.Tree-phase commercial inverters over 30 kW have limitsthat can be adjusted with the permission o the local util-ity. is can be very useful in an area with a uctuating

grid, which oten results in a signicant loss o energy. Longutility lines, areas with heavy load cycling or an unstableisland o power grids can all contribute to Continued on page 72

How Inverters Work

Dependence on temperature

Voltage (V)

Current(A)

0 C

25 C

75 C

50 C

0.00

4.00

4.50

3.50

3.00

2.50

2.00

1.50

1.00

0.50

400 10 20 30 50 60 70 80

MPP

Diagram 2 The knee, or maximum power point, o the I-V

curve varies dramatically according to the eects o both cell

temperature, as shown here, and irradiance.

Inverter type, size

and voltage Voltage range [V]

Clearing time(s)

(seconds) Frequency range [Hz]

Clearing time(s)

(seconds)

Residential

240 Vac

V < 211.2 2.00 f > 60.5 0.16

211.2 < V < 264 operational f < 59.3 0.16

264 < V 1.00 59.3 < f < 60.5 operational

Commercial,

3-phase 208 Vac,

60.5 0.16

104 < V < 183 2.00 f < 59.3 0.16

183 < V < 228.8 operational 59.3 < f < 60.5 operational

228.8 < V < 249.6 1.00

249.6 < V 0.16

Commercial,

3-phase 480 Vac,

>30 kW inverter

V < 240 0.16 1 f > 60.5 0.16

240 < V < 422.4 2.00 1 57.0< f < 59.8 0.16 1

422.4 < V < 528 operational f < 57.0 0.16

528 < V < 576 1.00 1 59.8 < f < 60.5 operational

576 < V 0.16 1

Grid Limits for Inverters

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1 Per IEEE 1547 these values may be adjustable in an inverter over 30 kW with utility permission.

Table 1 Utility grid voltage and requency limits or grid-tied PV

inverters as required by UL 1741 and IEEE 1547.

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grid uctuation. If a PV system signicantly underperformsas a result (beyond nuisance tripping), adjusting the inverter

limits can be benecial.UL 1741 and IEEE 1547 also require that inverters notcreate a power island. Tis means that i the utility goes out,the inverter cannot remain on, producing power to any loador portion o a building load, including rotating or oscillat-ing loads. For example, even i the buildings load is similaror exactly balanced with the output o the PV system, theinverter may not remain on i the utility is not present. Algo-rithms or detecting anti-islanding must constantly check tosee that the utility grid is really present. A specially tunedresonant load set up to mirror the utility tests this inverterunction. Te resonant load is made up o a very specicinductive, capacitive and resistive network with manysettings. Its goal is to attempt to trick the inverters anti-islanding algorithm into thinking that the utility is reallythere, at many dierent prescribed power levels called out inthe UL 1741 standard. Tis load is connected to the inverteroperating at ull power, and the grid is connected. Te reso-nant load is set to the exact output power o the inverter.When the whole system is stable, the utility is disconnectedwhile the resonant load maintains voltage and requencywithin the inverters limits. Te inverter has a maximum o2 seconds to successully recognize that the grid is discon-nected and shut o.

Integration and packaging. Other required equipment built

into or included with an inverter includes: ac disconnectionmeans, both manual and automatic; dc disconnection means;EMI and RFI ltering equipment; transormer (i the inverteris transformer-based); cooling system; GFDI circuit; LED indi-cators or LCD display; communication connections or PC orInternet data monitoring; and the product packaging.

Manual ac and dc disconnection means are designed intoinverters or PV systems so that the inverter can be disconnectedrom the grid and the PV array i service technicians, install-ers or other qualied personnel need to turn o the inverter oraccess the main inverter enclosure. Automatic ac disconnec-tion meanssuch as an ac contactorare used to minimize or

totally eliminate nighttime tare losses and reduce susceptibilityto damage rom nighttime power surges and lightning strikes.Disconnecting power supplies, chips and components o alltypes at night also extends their service lie.

Inverter packaging brings all the components into a sin-gle, shippable unit. (Te largest 3-phase C o n t i n u e d o n p a g e 7 4

How Inverters Work

dc fuses(optional PVsubcombiner)

Inverter Filter Transformer

Contactor

ac disconnectswitch

EMI/RFI filter

dcdisconnect

switch

Buildingac distribution

panel

Utility

Building loads

Fan

Ground fault detection/interrupt not shown.

+PV+PV+PV

-PV -PV -PV

N

N

Diagram 3 The major equipment included in an integrated inverter package is detailed in this schematic or a representative

3-phase, central inverter.

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Inverter packaging The power electronic module, such as

this compact 55-pound power module rom a 100 kW PV

inverter, is just one o the many components that are inte-

grated into the fnal inverter package.

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How Inverters Work

inverter packages, or commercial and utility scale appli-cations, may ship as more than one enclosure.) Packagingalso protects the inverter rom the outside elements andkeeps unintended guests, human or otherwise, away romthe equipment. Te use o high quality materials and n-ishes is necessary to meet the needs o the application. Teservice lie o a PV inverter, or example, requires the use ocorrosion-resistant asteners, like stainless steel screws, to

ensure that individual components can be accessed and ser-viced over 25 years.

KEY INVERTER COMPONENTS

In this section, we discuss key inverter components. As astarting point, basic inverteroperation is illustrated by look-ing at a single-stage, single-phase,60 Hz transormer-based inverter.Additional inverter topologiesare explained subsequently.

Solid state switches. All invert-

ers today use some combina-tion o power semiconductorsIGBTs, MOSFETS or both insome casesto invert dc to acpower. Other key components inthe main power inversion circuitare inductor(s), capacitors and atransormer, either 60 Hz or highrequency. Te latter is used intransormer-based inverters toadjust voltage levels as needed bythe topology and to provide gal-

vanic isolation between the solar

dc input on one side and the inverters acoutput to the grid on the other. Single stage

products like 60 Hz transormer-basedstring inverters typically use an H-bridgeor inversion rom dc to ac, as shown inDiagram 1 (p. 69).

Diagram 4 shows all the key compo-nents in a single stage inverter, includingthe H-bridge circuit. Te switches at thear let represent the power semiconduc-tor switches. By alternately closing the toplet and bottom right switches, then the topright and bottom let switches, the dc volt-age is inverted rom positive to negative,creating a rectangular ac waveorm.

In a grid-interactive, 60 Hz transormer-based inverter, however, the output cur-rent needs to be a sine wave orm. Tisrequires a more complex operation. Te

H-bridge puts out a series o on-o cycles to draw an approx-imated sine wave shape. Tis is known as pulse width modu-lation (PWM). With a 250 Vdc to 600 Vdc input, the H-bridgecircuit or a typical 60 Hz transormer-based string inverterwill put out an approximated sine wave with an ac voltage oabout 180 Vac. Te role o the components ater the H-bridgeis to smooth and change the magnitude o that approxi-mated sine wave.

Magnetics. Te string inverter in Diagram 4 contains sev-eral pieces o equipment that are reerred to as magneticsormagnetic components. Tese include the inductor and thetransormer, shown to the right o the H-bridge. Tese mag-netic components lter the waveshapes resulting rom the

PWM switching, smooth-ing out the sine waves,and bring ac voltagesto the correct levels orgrid interconnection. Temagnetics also provideisolation between the dc

circuits and the ac grid.Note that the ac wave-

orm entering the induc-tor is raw and triangular;but on leaving the device,it is a clean 180 Vac sinewave. Because 180 Vaccannot be directly con-nected to the utility grid,it goes through a 60 Hztransormer. Te resultingsmooth, sinusoidal 208,

240 or C o n t i n u e d o n p a g e 7 6

DSP

Transformer 60 Hz

Building/

grid

250600 Vdc

Grid

sensing

Inductor

180 Vac raw

V

I

V V

180 Vac ltered240 Vac

to grid

Diagram 4A 60 Hz, transormer-based, single-phase inverter circuit.

Solid state switches These solid state switchesa 1,200 V,

600 A IGBT and a 20 A, 800 V MOSFETboth convert dc to

ac power within an inverter.

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277 Vac inverter output is connected to the grid. Grid syn-

chronous operation is made possible by grid sensing eed-back. Grid voltage information is provided to the invertersdigital signal processor (DSP) or microcontroller, the devicethat controls the H-bridge.

Magnetics are labor and material intensive, and theircosts are tied to expensive commodities like copper andiron. Tey can also take a costly tare on system perormance,and careul design is needed to achieve maximum efciency.Tere are two main loss components associated with theuse o magnetics. Te rst component includes core losses,

which involve the magnetic material (such as iron lamina-tions, magnetic ribbon or sintered powdered material) and

gap losses, which result when the magnetic componentshave a gap between core halves, or example. Te second losscomponent is the conductor or coil loss. Tese are simplythe resistive losses in the many coils o wire around the mag-netic cores. Good inverter design needs to minimize bothtypes o losses.

Minimum dc input voltage. A wide dc input voltage win-dow is benecial to PV system designers and installers,since solar arrays operate over a wide voltage range. An evenwider voltage window is required to enable designers toselect between a wide range o PV products and string con-gurations. Achieving this wide dc input voltage range is noteasy, because inverter designers have to balance concernslike efciency, circuit complexity and cost.

Te laws o physics also limit inverter designers. Teinverters dc input bus voltage needs to be greater than thepeak o the ac voltage on the primary side o the transormer.In order to maintain this relationship at all times, an addi-tional control and saety margin is required. With a mini-mum PV input voltage o 250 Vdc, or example, the highestamplitude ac sine wave you can create is about 180 Vac, asillustrated in Diagram 5

Te PV input voltage, o course, will greatly exceed 250Vdc in many array layouts or temperature and light condi-tions. I 250 Vdc is selected as the inverters lower dc voltage

design limit, then the H-bridge will always create an ac sinewave with a magnitude o 180 Vac. Tis is true even whenthe dc voltage present is 300 Vdc, 400 Vdc or higher. Tis isbecause the rest o the 60 Hz transormer-based inverterneeds to operate on a relatively xed ac voltage. Te voltageon the utility side o the inverters transormerthe second-ary sideis xed within a small range o variation. Inverterdesigners must set the voltage on the primary side o thattransormer accordingly.

Capacitors. Te most important use o capacitors inthe inverter power stage is or ltering ripple currents ondc lines. Ripple is an undesirable phenomenon caused by

power semiconductor switching. Capacitors are also used tokeep the dc bus voltage stable and minimize resistive lossesover the dc wiring between the PV array and the inverter,since the resulting current rom the array to the inverter dcbus is constant. A relatively smooth dc voltage and currentat the input o the inverter allow good PV voltage regulation,which results in an MPP tracking algorithm that works welland has high accuracy.

Lower requency capacitors, typically electrolytic types,make up the main capacitance on typical inverter bus struc-tures. Tese have very high capacitance and the ability tolter large ripple currents. High requency capacitors, typi-

cally lm capacitors o various types, C o n t i n u e d o n p a g e 7 8

How Inverters Work

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Creating clean sine waves The inductor transorms raw ac

waveorms created by the H-bridge into clean sine waves. (On

let, a 20 A inductor typical o a residential string inverter; on

right, a 300 A inductor used in large central inverters.)

Minimum

dc bus voltageVpeak

Vrms

Voltage

Diagram 5 The inverters dc bus voltage must be higher than

the peak ac voltage on the primary side o the transormer to

create ac power rom dc.

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are also used or ltering out high requency noiseand spikes rom power semiconductor turn-on and

turn-o cycles.Capacitors, particularly electrolytic types, aresusceptible to ailure rom long-term operation inhot environments. Inverter designers have to care-ully select the proper capacitors to ensure thatthey can take the heat and absorb the high ripplecurrents that are possible. Selecting high-grade ver-sions and using more than are required to minimizeheating o the capacitors caused by the ripple cur-rent are typical approaches.

A dierent and newer approach, seen in somecommercial 3-phase inverters, involves the use ohigh capacity lm capacitors. Unlike electrolyticcapacitors, lm capacitors cannot dry out and willthereore last longer. Tey are also less aected bytemperature. Te trade-o is that lm capacitors aremore costly and take up more space. However, thattrade-o can be very worthwhile, especially on largeinverters where space is less an issue. Commercialand industrial PV systems will produce large quanti-ties o electricity over 25 years or more, so the inverter needsto be as reliable and long-lasting as possible.

Maximizing efciency. Optimizing efciency, or reducingloss, is an important part o inverter design and componentselection. Te goal is to optimize the inverter or maximum

efciency, while maintaining high reliability and delivering aproduct at a good price. Because there are losses associatedwith each, components o efcient design include the choice

of IGBT or MOSFET power semiconductor switches, switch-ing requency, method o switch control and turn-on and turn-o method. Many o these choices require a careul balance owaveorm smoothness, noise, reliability and efciency.

Because the installed cost for PV systems is high, requir-ing subsidies to make nancial sense, the benets o high

efciency are compelling. A 1% increase in inverter efciencytranslates into immediate and long-term savings, a result oincreased energy harvest and increased compensation orthat energy. An even more powerul way to look at efciencyon a 100 kW PV system is that a 1% gain in efciency meansyou could install 1 kW less o PV. Tis results in upront sav-ings o $6,000 to $8,000.

As the installed cost or PV decreases, inverter efciencymay become less critical than it is today. However, it willalways be better to convert as much PV power into ac poweras possible. o do otherwise results in waste heat. Withgreater inverter efciency, less energy and ewer materials

are needed or the inverters cooling system, resulting in pro-longed inverter lie.

Thermal perormance. As ar as thermal perormance is con-cerned, the rst goal o inverter design is to minimize loss.Te next goal is to minimize temperature gain or maximuminverter component lie. Te last step is to minimize powerrequirements and energy consumed or cooling system needs.

Where a signicant temperature dierential existsbetween inverter components, dierent types o compo-nents are oten separated into temperature zones within theinverters overall enclosure. Tis is especially useul whenlower temperature components are also more sensitive to

higher temperatures. Tis approach is utilized in many 60 Hz

How Inverters Work

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Removing ripple The capacitor on the let is an electrolytic

type, and the others are various flm capacitor types. Capaci-

tors flter ripple currents and stabilize the dc bus.

Thermal man-

agement Many

inverters, like the

100 kW central

inverter shown

here, provide di-

erent temperature

zones or major

components. A

relatively cool

section protects

sensitive power

electronics on the

top, while a higher

temperature sec-

tion houses the

magnetics below.

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transormer-based inverter designs, both string and centraltypes. Te 60 Hz transormers can operate at much highertemperatures than semiconductors, capacitors and otherelectronic components.

Sotware and monitoring. o reliably control the inverter,the sotware designed to run on the inverters digital signal

processor or microcontroller is developed over years o codewriting and debugging. Te most critical control is the onedriving the power stage. Tis creates the PWM waveormsthat generate the sine waves ending up on the utility grid andat the buildings loads. Sotware also controls the invertersinteraction with the grid and drives all the appropriate UL1741 and IEEE 1547 required controls and events. Anotherpart o the sotware controls the MPP unction that variesthe dc voltage and current level as required to accurately andquickly ollow the moving MPP o the PV array. All o thesemajor unctions, as well as a multitude o others, are carriedout in unison like an orchestra. Sotware is used to drive the

contactor that places the inverter on the grid in the morn-ing and o the grid at night. Sotware controls temperaturelimits and optimizes cooling system controls. Sotware, itsdevelopment history and robustness, is a critical element inany inverter.

A completely dierent aspect o the inverters sotwareis related to communication with other inverters, PCs anddata onto the Internet, as well as building management sys-tems. Sometimes this is done by separate devices, such asdata monitoring devices or Internet gateways that gatherdata rom the inverter or integrated sotware and hardwarewithin the inverter. Data monitoring is an important part

o a PV system since it lets owners and installers know its

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Temperature zones

These thermal

images were taken

inside the inverter

on the acing page

ater 6 hours o

ull-power opera-

tion at an ambient

temperature o 70F.

The maximum tem-

perature recorded in

the electronics sec-

tion o the inverter is

120F, even though

the highest tem-

perature recorded

in the magnetics

section is 183F.

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status and provides quick alerts i there are inverter or PVsystem aults. Since an inverter already measures and calcu-lates much inormation regarding ac and dc sides o the PVsystem, it is typically a convenient place to gather this data,process it and place it on the Internet, or example.

Some inverter manuacturers provide PC or Web basedmonitoring options; some inverters are compatible withthird-party monitoring systems; and some inverters have

both options available. Monitoring is one area o inverter

development that is evolving quickly. Several invertermanuacturers or third-party monitoring providers oer

advanced eatures, like revenue grade monitoring, PVstring and subarray monitoring, weather monitoring andinverter-direct monitoring.

ALTERNATIVE INVERTER TOPOLOGIES

So ar we have discussed the design and operation o asingle-stage, single-phase, 60 Hz transormer-based stringinverter. Other common inverter design topologies andapplications include 3-phase inverters, high requencyinverters, bipolar inverters, transormerless inverters andbattery based inverters. In some cases a single inverterproduct may incorporate several o these eatures. Tisis the case with Solaron inverters rom Advanced Energy,which are 3-phase, bipolar and transormerless products.While we do not address this exact combination o eaturesin this section, we do explore the most common alternativeinverter topologies or PV applications.

3-phase, 60 Hz transormer-based inverters. Te operationo a 60 Hz transormer-based, 3-phase inverter is very similarto that o the string inverter. Te dierence is that a centralinverter has three phase outputs instead o two. In order togenerate three phase outputs, 60 Hz transormer-based cen-tral inverters typically use a 3-phase bridge. Tis is a bridgewith a 6-switch design.

Diagram 6 shows a 3-phase, 6-switch bridge. Te

RMS voltage o the PWM-created sine C o n t i n u e d o n p a g e 8 2

How Inverters Work

DSP control board Critical inverter unctions rely on the sot-

ware run on the DSP.

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325600 Vac

208 Vac raw

V

I

V

208 Vac ltered

V

480 Vacto grid

DSP

Transformer 208 Vac to 480 Vac

Building/grid

Gridsensing

Filter inductorInverter electronics

power stage

Diagram 6 A 60 Hz transormer-based, 3-phase inverter circuit.

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wave is typically 200 Vac or 208 Vac. Tis voltage is derivedrom a minimum PV voltage o 300 Vdc or 330 Vdc, depend-ing on the inverter model and options. As with the previoussingle-phase example (Diagram 4, p. 74), the 3-phase, 60 Hztransormer-based inverter includes an inductor to lter outthe PWM-created sine wave and a transormer to convert theltered waveorm to the correct ac voltage. Te transormeralso isolates the PV system rom the grid.

High requency string inverters. Many o the grid-directstring inverters available in North America today utilize high

requency transormers. (See able 2 or the actual distribu-tion.) High requency transormer-based inverter circuittopology and operation is quite dierent rom that o 60 Hztransormer-based inverters. High requency transormers

weigh less, are smaller and cost less than 60 Hztransormers. A high requency transormer, or

example, weighs 10% to 20% o a 60 Hz trans-ormer o the same power level. Tis allows or asmaller, lighter and easier to install package.

Another advantage o a high requencytransormer-based inverter topology is that itallows or a wide dc input voltage range. his isbecause o the two-stage design o the inverter.hese stages are shown in Diagram 7.

Te rst block, greatly simplied, representsa voltage boost circuit. Tis is, thereore, reerredto as the boost stage. By turning the MOSFET orIGBT on and o, represented here by the switchsymbol, under the correct PWM control, theinductor and diode boost the voltage rom theinput (let) to the output (right) o the circuit.

When the switch is closed, current ows through the induc-tor in the direction o the minus sign in Diagram 7. Whenthe switch opens, stopping the ow of current through theswitch, the voltage is boosted higher. When this happens thediode starts conducting, and the inductor current ows outto the capacitor to the right. Tis boost circuit accepts aninput o 200 Vdc to 600 Vdc and will output a constant 700Vdc. Again using PWM, the H-bridge converts the high volt-age dc rom the boost stage to a high requency ac signal.Tis is ed through the high requency transormer, which

provides isolation and brings the voltage to the right levelor grid interconnection.

Te nal stage is shown on the right o Diagram 7. Aninductor lters out the high requency switching cycles to

How Inverters Work

Single-Phase

Grid-Direct Inverters Off-Grid Inverters

3-Phase

Central Inverters

60 Hz Xfmr HF Xfmr Xfmr-less 60 Hz Xfmr 60 Hz Xfmr Xfmr-less

PV Powered Fronius Power-One Apollo PV Powered Advanced Energy

SMA Kaco Magnum Satcon

Solectria OutBack Power SMA

Xantrex SMA Solectria

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Distribution of PV Inverter Topologies

in North America

Table 2 The distribution o 60 Hz transormer versus high requency

transormer versus transormerless inverter topologies in string inverters

and central inverters.

200600 Vdc 700 Vdc240 Vacto grid

DSP

HF transformer Inductor

Building/grid

Gridsensing

H-bridgeBoost stage

VV

700

700

V340

340

Diagram 7 A high requency, transormer-based, single-phase inverter circuit.

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create a smooth, grid quality 208, 240 or 277 Vac 60 Hz sinewave. Grid sensing feedback lets the DSP and its softwareregulate the H-bridge to constantly create smooth 60 Hz out-put current at the right voltage. As opposed to single stage

inverters, inverters with a voltage booster at the input workmost efciently at high dc input voltage levels.

Te disadvantage with a high requency inverter topologyis that more silicon semiconductors are required, resultingin higher costs. However, since copper and iron commodityprices are volatile and trending upward, buying more siliconand less copper and iron is a trade-o that many companiesare willing to make. Te amount o silicon used or semicon-ductors in an inverter is quite small, even compared to thesmaller magnetic components common in high requencystring inverters. Besides weight and cost reductions, lowerlosses and less waste heat are additional benets o smaller

high requency magnetics. Tese allow or direct mountingto heat sinks, even mixing components o dierent tempera-ture classes.

Bipolar inverters. UL 1741 and theNECrestrict the designand operation o grid-tied PV equipment to a maximumpotential to ground o no greater than 600 Vdc under anyconditions expected or a given site. Tis is why the maxi-mum dc voltage on most inverters is 600 Vdc. Because ofthe large temperature swing that PV modules must oper-ate withinespecially in regions with high maximum andextreme low minimum ambient temperaturesthe voltagerange o PV array operation is quite broad. In some cases, the

ratio o high to low voltage approaches 2:1. Tis means that

the nominal operating voltage o a PV array that is designedto have a maximum Voc o 600 Vdc is typically 400 Vdc or

less. On warm days, the array voltage might be 350 Vdc to330 Vdc or even less.Bipolar topologies allow the inverter designer to make

use o dc voltage both above and below ground. Circuitconstruction is not necessarily dierent with a bipolarapproach; the array is just grounded at a center point ratherthan at one end or the other. It is possible to use any o themethods described previously or the power stage and oper-ation o a bipolar inverter. Tere are signicant dierences,however, in various component ratings since the potentialacross open dc switches and all dc equipment is now aboutdouble at 1,000 Vdc. Also the dc ground ault circuit inter-rupter needs to detect aults separately on both the positiveand the negative side o the array. Especially in commercialand utility scale PV systems, where massive amounts o cop-per or aluminum conductors are used, higher dc array volt-ages mean less conductor material, lower costs and, in somecases, improved system efciency. Special requirements inthe NECor bipolar source and output circuits are ound inArticle 690.7(E).

Transormerless inverters.ransormerless operation is pop-ular in Europe, especially or string inverters and increas-ingly or 3-phase central inverters. Te basic premise is thatsince a building or utility transormer is down the line any-way, another transormer right in or alongside the PV inverter

is not needed. Eliminating the transormer reduces cost,size and weight. Doing away with the transormer inside theinverter also eliminates the loss components associated withthe transormer, increasing inverter efciency.

While UL 1741and the NECallow or the use o transor-merless inverters in PV systems, additional requirementsapply. Tese are spelled out in NEC 690.35 and include theuse o PV Wire, a dierent version o the double-insulated,single conductor source circuit cable typically used in PVinstallations. Tis cable is more expensive than USE-2 orRHW-2 and is less available in the US than in Europe. Over-current protection and dc disconnect requirements are also

dierent, since both positive and negative current-carryingconductors to the inverter are ungrounded.

O-grid inverters. All o-grid inverters are based on 60Hz transormers. Tis is probably due to the developmento the o-grid industry. Some o the rst widely available,aordable inverters in the late 70s and 80s were originallydesigned or motor homes and recreational use. Productslike the ripp Lite inverters operated on a very basic prin-ciplea 12 Vdc input to a square-wave transistor that eda transormer. Tis transormer boosted the voltage morethan 10 times to an output o 120 Vac. Te output waveormwas slightly less than square and was considered a modied

sine wave. Tese crude devices made a considerable buzz,

Courtes

ysolren.com

High frequency transformer All the semiconductors and

magnetics components in this 2.5 kW string inverter are

mounted on a common heat sink.

High

requency

transormer

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How Inverters Work

had low efciencies and could notbe used with certain appliances

sensitive to nonstandard sinewave power.Te original inverters popular

or use with o-grid solar were therace, Heart Interace and oneor two other brands. Tese wereoriginally available with a 12 Vdcinput, but 24 Vdc models ollowedsoon thereater or larger systems.Eventually 48 Vdc versions oreven larger and more efcient sys-tems became available, allowingsmaller wiring and ewer paral-leled batteries. For a given powerlevel, 48 Vdc systems use 50% othe current o a 24 Vdc system and 25% o a 12 Vdc system.Te advantages to a higher dc input are quite obvious.

Te o-grid inverter models in able 2 (p. 82) are basedon a solid state switching input to a 60 Hz transormer. Tetransormer boosts the output signicantly, or example 12Vdc or 24 Vdc inverter input to 120 Vac output or 48 Vdcinput to 240 Vac output. Note that with a 12 Vdc bus com-ing into an inverter, the peak o the ac sine wave out o theH-bridge is 12 Vac. Tis means that the RMS amplitude othe sinusoidal waveorm is a mere 8 Vac. Te transormer

needs to boost this to 120 Vac, a very large ratio o 1:15.Although most modern inverters or o-grid solar applica-tions have a 120 Vac output, several models are availablewith 240 Vac output. Other models let you stack two or more120 Vac units or a 240 Vac split-phase electrical service (orsimply to service larger loads), and 3-phase congurationsare also possible.

Boosting voltage by a large ratio lends itself well to a60 Hz transormer. Another great advantage o the 60 Hztransormer in this design is that it completely isolatesthe battery and the ac system rom one another, which is agood reliability and saety eature. Working with low volt-

ages means high current levels, MOSFEs with high cur-rent ratings and big wires. Considering the task at hand,the manuacturers o these 12 Vdc to 48 Vdc input o-gridinverters have done a great job, achieving admirable ei-ciencies while maintaining reasonable costs. Most modelsprovide e iciencies in the low 90% range, with the bestmodels approaching 95% peak eiciencies.

INVERTERS IN THE FUTURE

Te PV industry is an exciting place to be. As we chargetoward gird parity, market growth in the US and the resto the world accelerates, outstripping PV module produc-

tion capacity at times. Inverters remain critical PV system

components, even thoughtheir portion o the overall

budget is quite small. Inmost cases, inverter costsrepresent on the order o10% o total system costs; inlarger systems, they are wellunder 10%. Nevertheless,as the PV market matures,cost reduction is a denitetrend to watch, even withinverters. Increased compe-tition, higher volumes, newtechnologies and advancedmanuacturing processeswill all enter the equation.

A struggle to achievehigher efciencies exists at all inverter power levels. Tetop transormer-based residential and commercial invertersachieve CEC efciencies o 95% to 96% and peak efcienciesabove 97%. Te industry is quickly reaching a place o dimin-ishing returns, as there are precious ew losses to shave oat this point. PV inverter manuacturers continue to worktoward increasing efciency, looking to gain another 0.5% to1%. ransormerless inverters may already provide that ef-ciency gain, however. Europe is now driving this technology,and the US will most likely ollow once the NEC gets more

riendly towards ungrounded PV systems.Higher levels o inverter integration will also come to

play in advanced inverter designs. As PV system installa-tion labor increasingly becomes a larger piece o the totalinstalled cost, more components and unctionality areadded to inverters. Disconnects, subcombiners, string com-biners, communications and data monitoring are just a ewo the things integrated today by various inverter manu-acturers. Adding other unctions, such as integrated rev-enue grade monitoring, PV system weather sensor inputs,interaction with PV array trackers, solar concentrators andother gear, urther simplies overall PV system installation.

Increased integration will also result in more building blockapproaches, simpliying large commercial and utility scalesystems and providing a great value in large, very expensivePV systems. Advanced integral data monitoring, providingsystem perormance predictions and reporting, will urtheroptimize system perormance. Tese reports will alert cus-tomers or service personnel to potential system issues andgradual changes in array perormance beore any signicantenergy production is lost.

Future advances in central grid-tied invertersandeventually string inverters as wellwill likely also includesome type o utility interaction to support the grid in times

o distress. Tis might include the inverter remaining online

SFuture advances in grid-tied inverterswill likely include some type of utilityinteraction to support the grid in times

of distress.

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during a brownout or other voltage events, even during re-quency events, giving the utility more time to make adjust-

ments or isolate circuits to remedy the problem and pre-venting even larger grid instability or a propagated blackout.Currently, i the grid voltage or requency goes outside o thewindows specied in IEEE 1547, all grid-tied inverters mustgo oine, which likely accelerates grid ailure. Utility controlo PV inverters and other discontinuous sources might makeit possible to remedy some grid problems. Because invert-ers draw their sine wave current waveorms in many incre-ments, the dierential control o these increments can helpspecically to adjust the grid voltage waveorm, minimizingor correcting power actor or other problems created by cer-tain loads in the area.

Another area or the advancement o inverters and grid-tied renewable energy systems will likely involve energystorage. As a larger portion o our power generation capac-ity comes rom discontinuous sources o energy, like PV andwind, storage becomes more important. A uture compo-nent o large commercial PV systems might include on-siteenergy storage with utility control or based on time o usecosts or electricity. With increasing amounts o PV powerprocessed by DSP-controlled inverters, there are many criti-cal unctions that inverters can incorporate as the industryprogresses. As these potentials are realized, PV power willbecome an increasingly widespread and important portiono our energy inrastructure.

James Worden / Solectria Renewables / Lawrence, MA /

[email protected] / solren.com

Michael Zuercher-Martinson / Solectria Renewables / Lawrence, MA /

[email protected] / solren.com

Resources

Advanced Energy / 800.446.9167 / advanced-energy.com

Apollo Solar / 203.790.6400 / apollosolar.com

Evergreen Solar Energy / 508.357.2221 / evergreensolar.comFronius / 810.220.4414 / Fronius-usa.com

Heliotronics / 781.749.9593 / heliotronics.com

Kaco Solar / 415.931.2046 / kacosolar.com

Magnum Energy / 425.353.8833 / magnumenergy.com

Power One / 866.513.2839 / power-one.com

PV Powered / 541.312.3832 / pvpowered.com

Satcon / 617.897.2400 / satcon.com

SMA Solar Technology / 916.625.0870 / sma-america.com

Solectria Renewables / 978.683.9700 / solren.com

Tripp Lite / 773.869.1111 / tripplite.com

Xantrex Technology / 604.422.8595 / xantrex.com

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