A quickSTEP Online Course · • Summarize the capabilties of Enlighten web-based monitoring and...

Basics of Solar MicroinvertersA quickSTEP Online Course

www.usa.siemens.com/step© Siemens industry, Inc.

© Siemens Industry, Inc. 2017

Trademarks

Siemens is a trademark of Siemens AG. Product names mentioned may be trademarks or registered trademarks of their respective companies.

National Electrical Code® and NEC® and NFPA 70® are registered trademarks of the National Fire Protection Association.

NEMA® is a registered trademark and service mark of the National Electrical Manufacturers Association.

UL® is a registered trademark of UL, LLC.

Other trademarks are the property of their respective owners.

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Course Topics

Welcome to Basics of Solar Microinverters. This course covers the following topics:Chapter 1 - Introduction

• Overview• Solar Microinverter Systems

Chapter 2 – Siemens Products• System Components

Final ExamIf you do not have an understanding of basic electrical concepts, you should complete Basics of Electricity before attempting this course.

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Course Objectives

Upon completion of this course you will be able to…• Describe the function of a photovoltaic cell• Define the term inverter• State the purpose of important solar inverter functions such as maximum power point

tracking and anti-islanding.• Distinguish between a string inverter and a microinverter.• List the benefits of a solar microinverter system in comparison to other solar inverter

systems.• List and describe the function of the main components of a Siemens microinverter

system.• Identify the two voltage options for Siemens microinverters and describe the cabling

arrangement for each option. • Identify the two AC drop spacing options available for Siemens trunk and drop cabling.• Describe the function of Envoy-S communications gateways.• Summarize the capabilties of Enlighten web-based monitoring and analysis software.

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SITRAIN® Training for Industry

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Online Self-paced Learning – Programs with maximum flexibility so students can easily fit courses into their busy schedules

Virtual Instructor-led Learning - Classroom lectures delivered in the convenience of your home or office

Classroom Learning - Expert and professional instructors, proven courseware, and quality workstations combine for the most effective classroom experience possible at your facility or ours

How-to Video Library - Quick, affordable, task-based learning options for a broad range of automation topics for training or purchase

Simulators - World-class simulation systems available for training or purchase

This course also describes learning options available from the Siemens SITRAIN USA organization and our global SITRAIN partners. For additional information: www.usa.siemens.com/sitrain


Solar Photovoltaic Technologies

There are two broad categories of solar energy applications, solar thermal and solar photovoltaic. This course focuses on solar photovoltaic applications.

Solar photovoltaic (PV) applications vary in scale. For example, on the larger scale, arrays of solar modules, like the ones shown in the accompanying graphic, are used to convert sunlight into electricity for distribution by an electric utility company.

However, this course focuses on PV applications that convert sunlight into electricity for use in a home or commercial facility. In addition, while PV modules are discussed, this course primarily covers Siemens products used with PV modules.

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Photovoltaic Cells

Solar PV installations are made up of many solar cells. A solar cell, also called a photovoltaic (PV) cell, is a device that produces a voltage when light shines on it. Therefore, a PV cell is essentially a battery that is dependent on light to create a voltage. Like a battery, a PV cell is a direct current (DC) source. This means that the direction of the current is constant.

A single PV cell produces a small voltage, up to about half a volt for a typical silicon PV cell. However, the exact voltage varies with temperature and light intensity. Because the voltage provided by a solar cell is variable, the current it provides to the load is also variable. Additionally, just as multiple battery cells can be placed in series to produce a higher voltage, the same is true of PV cells.

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PV Cells, Modules, and Arrays

A single PV cell provides only a small voltage and can supply only a very small current; therefore, a number of PV cells are grouped together on a solar panel.

Because the term panel has multiple uses in the context of a power distribution system, this course uses the term solar module or PV module instead of solar panel.

To supply the necessary energy for a practical installation, solar modules are assembled to form a solar array. The larger the number of modules, the greater the amount of energy the array can provide.

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Types of PV Cells and Modules

There are multiple technologies used to produce solar modules, but these three module types dominate today’s market: monocrystalline silicon, polycrystalline silicon, and thin film.

Monocrystalline, meaning single crystal, silicon modules have an efficiency of 14 to 22 percent. This is pretty good in the solar industry. Efficiency is the percentage of solar energy that is converted into electrical energy. Unfortunately, the manufacturing cost for this type of module is relatively high.

Polycrystalline, meaning many crystals, silicon modules are less expensive to manufacture, but their cell structure has crystal boundaries that reduce the available energy. This translates into an efficiency of 10 to15 percent.

There are a variety of approaches for producing thin film modules. One of the more promising technologies uses Cadmium-Telluride (CdTe) or CadTel . Thin film modules are becoming more popular because they are less expensive to manufacture. However, these types of modules have an efficiency of only 8 to 13 percent. Page 1-9


Units of Measure and Other Terms

As a review of some of the terminology used in the Basics of Electricity course, the top chart in the accompanying graphic shows some commonly used electrical quantities, their symbols, and units of measure.

These units of measure are often preceded by a metric unit prefix that scales the quantity by a power of ten. (For example, 1 kilowatt is equal to 1000 watts.) The middle chart shows some of the more common metric unit prefixes.

The bottom chart provides a brief description of some additional terms appropriate for the discussion of solar energy systems.

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Electrical Components of a Small Scale PV System

The accompanying graphic shows an example of the electrical components of a typical small scale PV system.

This graphic shows only three PV modules in one branch circuit, but a typical system includes multiple branches with more modules per branch.

This example also shows one microinverter per PV module. Other approaches for converting the direct current (DC) provided by PV modules to the alternating current (AC) needed by the electrical system are also used, but this course covers microinverter systems.

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Balance of System

Additional components shown include a kilowatt-hour meter, load center or panelboard, and a communication gateway, which communicates important information for system monitoring via computer.

One concept often used with PV systems is refer to all of the components except for the PV modules as the balance of system (BOS). This includes not only the electrical components shown in the illustration (minus the PV modules), but also the mechanical components used to mount the PV modules.

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Online Self-paced Learning

With Siemens online self-paced learning, you select the topics and set your own pace for completing chosen courses. All course material can be accessed online.Instruction starts upon completing the purchase of a subscription.

You can choose from over 500 courses consisting of high-quality graphics, on-screen text, supporting voiceover narration, and interactive exercises. Features includeprintable course content for reference and underlined key vocabulary terms with definitions displayed with a simple mouse-over action.

Depending on the subscription purchased, you can choose any 10 or 25 courses or select the entire online self-paced course catalog.

These courses are offered 24/7/365, so you can begin your subscription at any time. From the date of registration,you have one year to complete your course selections.

For additional information: www.usa.siemens.com/sitrain

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Chapter 1 – Introduction

This chapter covers the following topics:

• Overview

• Solar Microinverter Systems

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What is an Inverter?

Quite simply, an inverter is a device that converts direct current (DC) to alternating current (AC). Many inverters are used to control motors and require complex circuits to accurately control the motor. For solar PV applications, however, simpler, less expensive inverters can be used.

At the heart of an inverter, multiple semiconductor switching devices, such as insulated gate bipolar transistors or silicon controlled rectifiers, convert the applied direct current to alternating current. The number of semiconductor switches required depends in part on whether the output of the inverter is single-phase or three-phase AC. Single-phase AC is needed for most residential applications, and three-phase AC is needed for many commercial applications.

Most inverters require a control circuit that controls the timing of the switching devices. Additional components are also included for protection and signal filtering.

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Solar Microinverters

As will be explained later, many solar PV systems use one or more larger inverters. However, the PV systems covered in this course use one microinverter for each solar PV module. The term microinverter simply means that the device is a small inverter.

Although a solar microinverter is a relatively simple component, in addition to changing variable DC to a constant frequency AC, it must also do some other important things.

For example, it must incorporate a capability called maximum power point tracking (MPPT). As will be described in more detail later in this course, MPPT maximizes the power output of the microinverter.

Also, because a solar microinverter must provide grid-compatible power, it quickly shuts itself off in the event that its own output is no longer grid compatible or if a utility power outage is sensed. This last feature, anti-islanding, will also be discussed later.

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Solar Inverter Systems

Some solar PV systems use a single inverter sized to channel all the power available from all PV modules to the load and power grid. However, many solar PV applications use multiple string inverters. A string inverter, as shown in the left in the accompanying graphic, is simply an inverter that is connected to multiple PV modules.

Keep in mind that the accompanying graphic has been simplified for explanation purposes and a string inverter system typically has additional components. For example, one or more DC combiner boxes may be used to reduce wiring cost and complexity.

Also, because a string inverter provides power for multiple modules, failure of an inverter results in a significant loss of power available to the load.

In contrast, the microinverter system, as shown on the right in the accompanying graphic, uses one micoinverter per PV module. While at first glance this may appear more complex, in reality, this approach has a number of important advantages.

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Advantages of a Solar Microinverter System

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A solar microinverter system has a number of important advantages in comparison to a central inverter or string inverter system.

• A microinverter system converts more of the energy available from the PV modules. This results in a significant savings to the system owner.

• A microinverter system is easier to install. No extra enclosures are required for DC disconnects or combiner boxes. The cabling is simple and easy to connect. The mounting hardware installs quickly.

• A microinverter system is more reliable. Microinverters handle very small amounts of energy, tend to run cooler, have simpler designs, and a significantly longer mean time between failure.

• If a microinverter does fail, system troubleshooting is simpler, the replacement cost is much lower, and less energy is lost to the system during the malfunction. Keep in mind that a single inverter system provides an expensive, single point of failure, and, if a string inverter fails, the power from multiple PV modules is lost.

• Finally, a microinverter system is safer. For example, in a string inverter system, the inverter and cabling must handle a larger DC voltage, up to 600 VDC in some systems, compared to up to only 45 VDC for a microinverter system. The higher DC voltage creates a potential fire hazard if wiring or components fail or are not installed properly. Additionally, contractors and inspectors are sometimes not used to handling a high DC voltage and may be at greater risk of injury.


Maximum Power Point Tracking

As previously mentioned, maximum power point tracking (MPPT) is a feature of a solar inverter that maximizes the power output of the inverter. The maximum power point tracker in the inverter does this by regulating the current and voltage on the DC side of the inverter to optimize the power output on the AC side.

Consider the accompanying illustration. The blue curve shows the possible combinations of current and voltage from a PV module under optimal sunlight and temperature conditions. On the left extreme of the curve is the short circuit current of the module (Isc). Because the voltage is approximately zero at this point, the direct current power, which is the product of voltage and current, is also approximately zero. On the extreme right of the curve is the open circuit voltage (Voc). Because the current is zero at this point, the power is also zero. Given this curve, the product of current and voltage is maximum at the maximum power point (MPP).

At any given time, a PV module is unlikely to be operating at its optimal conditions of maximum sunlight and low temperature. In fact, the actual power curve is constantly changing over the course of a day, moving closer to the maximum power curve or further away from it as the sunlight and temperature change. This means that the maximum power point tracker in a solar inverter must also be continually making adjustments.

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String Inverter MPPT

One of the advantages previously mentioned for a solar microinverter system is greater power available. This is in comparison to either a single inverter system or a string inverter system.

In order to understand this critical point, consider how MPPT works in a string inverter system. For simplicity, only three PV modules (A, B, and C) are shown in this example. Modules A and B are each receiving full sunlight and the power available from each module is shown by the green square.

However, module C is receiving reduced sunlight due to shading or dirt, leaves, or other debris on the module. This has the effect of reducing the current available from the module. The voltage is also reduced, but to a lesser degree. The power available from module C is shown by the blue square.

Because the maximum power point tracker in the string inverter cannot compensate for each module individually, the resulting power is less than the sum of the total power available from each module.

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Microinverter MPPT

Using the same example as on the previous page, but with a microinverter for each PV module, the maximum power point tracker in each microinverter is able to compensate for the conditions of each module.

As a result, the total power available is equal to the sum of the powers available from the three modules. This will always be greater than or equal to the power delivered by a string inverter.

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Anti-islanding

Anti-islanding is a critical safety feature required of all solar inverters connected to the main utility grid (grid-tied). From the perspective of the electrical utility, a power island provides power to the grid during an electrical outage. Because a power island is a safety hazard to utility maintenance personnel, this condition must be prevented.

For this reason, solar inverters in grid-tied systems are required to have an anti-islanding feature that prevents the inverter from providing power to the grid within two cycles of 60 hertz power once grid power is lost.

Additionally, a grid-tied inverter also has the requirement to rapidly shut down if the voltage or frequency it provides is out of tolerance for utility requirements.

Requirements for grid-tied inverters are defined in the UL 1741 standard.

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Microinverter Grounding

National Electrical Code® (NEC®) Article 690 covers solar photovoltaic (PV) systems. This article identifies solar microinverters systems as either having an ungrounded PV source or a grounded PV source. This terminology can be confusing, so some explanation is required.

A solar microinverter has AC connections that are wired as described later in this course. It also has DC connections to a PV module (the PV source). Some solar microinverters, including the first generation of Siemens solar microinverters, do not have a DC integrated ground. This means that they require a grounding electrode conductor connected to each of the solar microinverters.

Siemens solar microinverters with a DC integrated ground do not require a grounding electrode conductor to interconnect microinverters, reducing the time and money required to install a system. As required by NEC Article 690 Siemens solar microinverters with a DC integrated ground must also have built-in ground fault protection.

Refer to NEC Article 690 for additional information.

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Roof Mounted Racking System

Some solar installations are ground-mounted. This approach is often used for large commercial applications.

The mounting system covered in this course is shown in the accompanying graphic. This is a roof mount with microinverters. This type of system is made primarily from extruded aluminum and has a number of desirable characteristics.

• It is inexpensive because it is made from simple, light-weight components that are readily available.

• It is prefabricated for ease of installation and can be adapted as needed to fit the application requirements.

• It is durable because it has a high strength-to-weight ratio and is made with corrosion resistant components.

• It incorporates a reliable grounding system.

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Virtual Instructor-led Learning

Siemens virtual instructor-led courses offer you a live, classroom experience with the convenience and cost savings of online learning. These courses provide hands-oninstruction and live interaction, delivered anywhere an internet connection is available.

Scheduled courses are typically 10-hour agendas presented Monday through Friday in two-hour sessions. These sessions provide you with lecture, demonstration, lab exercises, and Q&A sessions – all presented by Siemens subject matter experts.

For the full course duration, you can complete assignments and reinforce classroom instruction using a virtual cloud-based application providing 24/7 access to fully functional Siemens software such as SIMATIC STEP 7 and PLCSIM.


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Chapter 2 – Siemens Products

This chapter covers the following topics:

• System Components

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The Smart Home

Solar energy is part of a much larger movement towards smart homes and smart buildings. The main idea here is that homes and buildings are no longer simple consumers of electricity but rather can intelligently consume and supply electricity. As electricity becomes more expensive in the future, these technologies will help owners maintain comfort and reduce costs.

Related Siemens products include the Siemens microinverter system, VersiCharge electric vehicle charging stations, and solar ready meter load center combinations. Additional Siemens solar ready products are available for commercial applications.

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Microinverter System Components

As shown in the accompanying graphic, the Siemens microinvertersystem includes 215 W or 250 W microinverters with various connector options, trunk and drop cabling and related items, an Envoy-S communications gateway, and Enlighten web-based monitoring and analysis software.

While this graphic shows the load center and meter in separate enclosures, Siemens can also provide a meter load center combination as described later in this course.

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Siemens Microinverters

Siemens microinverters with DC integrated ground are available in two power ratings, 215 W and 250 W.

The microinverter‘s enclosure protects it from high temperatures and harsh weather conditions and is easy to attach to the racking and to the grounding conductor.

The inverter has two sets of connectors. On the left side are the positive and negative quick connectors that attach to the solar module terminals. On the right side is an AC output connector that attaches to the trunk-and-drop cabling.

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Trunk and Drop Cabling

The trunk and drop cabling transmits the AC electricity from the microinverters to the load center or other utility connection point. The quick connectors and easy to use accesories greatly reduce wiring time and overall cost.

The cable consists of a bundle of either 4 wires (for single-phase systems) or 5 wires (for three-phase systems) wrapped in a rain-proof and UV-light-proof jacket with drops every 1 or 1.7 meters. Each drop provides a connector for plugging in a microinverter‘s AC connector.

The 1 meter and 1.7 meter distances correspond to the dimensions of a standard 60 cell PV module. When modules are said to be arranged in portrait orientation, the short dimension (1 m) is parallel to the roofline and when the modules are in landscape orientation, the long dimension (1.7 m) is parallel.

Two voltage options are 240 VAC, single-phase (for most residential applications) and 208 VAC, three-phase (for commercial applications). These voltage options have different internal wiring, so care must be taken when ordering.

The trunk and drop cable is meant to be cut for the number of inverters on the roof and comes in units of 30, 40, or 240 drops.

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Cabling and Accessories

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Trunk and Drop Cable Phasing

Siemens microinverters have two different cabling schemes (one for single-phase applications and one for three-phase applications).

For 208 VAC, three-phase applications, drops are alternated in an A-B-C-A-B-C pattern. All of this alternating is handled by the trunk and drop cable to ensure that the phases are balanced.

For single-phase applications, each inverter supplies120 VAC to neutral with alternate A-B-A-B connections to provide the necessary 240 VAC for residential applications.

For applications where the number of inverters is not an even multiple, a slight phase imbalance is present. However, because a microinverter puts out a maximum of 250 W, this imbalance is quite small and does not cause problems.

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Envoy-S Communications Gateways

An integral part of a Siemens residential or commercial solar microinverter application is the Envoy-S communications gateway which communicates information to a computer equipped with Enlighten web-based monitoring and analysis software.

Two versions of Envoy-S gateways are available:Envoy-S standard gateway and Envoy-S metered gateway. Both devices collect energy and performance information via AC power lines from system microinverters and communicate the information via WiFi to a computer equipped with Enlighten software.

The Envoy-S standard gateway communicates PV production information and the Envoy-S metered gateway senses energy consumption from precision current transformers and communicates both PV production and energy consumption information.

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Enlighten Web-based Monitoring and Analysis

Enlighten is a web-based monitoring and analysis application used to view the data coming from each microinverter. This application stores historical data for each microinverter and allows it to be viewed locally or remotely by computer or smartphone.

You can also mimic the layout of PV modules to make the system more intuitive, allowing the user to easily determine if a module or inverter is faulty. For example, the accompanying graphic shows two faulty modules. Note how they are shaded in black for easy recognition.

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Solar Ready Meter Load Center Combinations

Siemens solar ready meter load center combinations are available with a variety of meter socket configurations including lever bypass and EUSERC approved versions with the features shown in the accompanying graphic.

These meter load center combinations have a dedicated alternative energy input rated for up to 60 amps.

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Additional Siemens Solar Ready Products

In addition to the products previously described, Siemens offers additional solar ready products. For example, Siemens solar ready switchboards provide a solution for both AC and DC commercial solar applications.

In addition to all standard switchboard features, optional viewing windows are also available for an additional level of safety when working with inverter inputs. Siemens switchboards meet all utility and code requirements.

Siemens solar disconnect switches are designed for use in DC photovoltaic power generation circuits. These circuits are defined by article 690 of the NEC® which requires the grounded conductor to be at ground potential at all times, preventing it from being switched.

Siemens solar disconnect switches incorporate powerful magnets that assist the double break switching action that quickly dissipates the very hot arcs generated when a 600 VDC circuit is opened under load.

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Classroom Learning

Studies indicate that when students practice what they have learned in a classroom setting they retain 75% of the lesson, as compared with lecture-only settings wherethey retain just 20% of the lesson.

Our learning content is reviewed and approved by Siemens technical and operational experts to ensure compliancewith the highest industry, health, safety, and environmental standards. Siemens simulator workstations provide a safe and risk-free platform for job training, project testing, design engineering, and troubleshooting.

We combine technology and industry experience to deliver highly effective, customized learning programs.• Job targeted courses• Hands-on learning and skill building• System-level training approach• Extensive schedule of classes• Various media and course length options• On-site and custom courses• Multiple training center locations• Packaged services and products


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How-to Video Library

This extensive library of short videos was created by our instructional experts to meet the real-world needs of industry, with all levels of experience in mind. By providingon-demand, how-to instruction in easy-to-understand bites, the How-to Video Library helps maintain the critical industrial and manufacturing knowledge and skills developed during instructor-led training courses. Videos are typically three-minutes long and conveniently available via any computer or mobile device with Internet access.

Learning begins once you’ve completed registration.• Start your subscription at any time. Videos are available

24/7/365.• Purchase one, three, six, or 12-month subscriptions by

technology or in one complete bundle.• Take advantage of our most-flexible option – ultimate

access with a full, one-year subscription.


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Simulators

Engineered to provide a real-world experience, Siemens simulators are fully functional, ready-to-use systemsavailable in a variety of configurations.

System-level design makes the simulators an invaluable tool for program testing and debugging, reinforcing learning, shop floor troubleshooting, and more. With portable construction and hard-shell cases, they can be easily transported. Custom-built systems are also available.


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SITRAIN® Training for Industry

Online Self-paced Learning – Programs with maximum flexibility so students can easily fit courses into their busy schedules

Virtual Instructor-led Learning - Classroom lectures delivered in the convenience of your home or office

Classroom Learning - Expert and professional instructors, proven courseware, and quality workstations combine for the most effective classroom experience possible at your facility or ours

How-to Video Library - Quick, affordable, task-based learning options for a broad range of automation topics for training or purchase

Simulators - World-class simulation systems available for training or purchase


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SITRAIN World

From the basics to advanced specialist skills, Siemens SITRAIN courses deliver extensive expertise directly from the manufacturer and encompass the entire spectrum of Siemens Industry products and systems.

Worldwide, SITRAIN courses are available in over 200 locations in over 60 countries.

For additional information including a SITRAIN world map and SITRAIN contacts worldwide: http://sitrain.automation.siemens.com/sitrainworld/Default.aspx

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Course Completion

This course covered the following topics:Chapter 1 - Introduction

• Overview• Solar Microinverter Sysems

Chapter 2 – Siemens Products• System Components

This course has covered the topics shown on the left. Thank you for your efforts. You can complete this course by taking the final exam and scoring at least 70%.

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