siemens basics of solar and microinverters

The term microinverter simply means that the device is a small inverter. Many solar PV systems use one or more larger inverters. So, 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, inexpensive inverters can be used.

At the heart of an inverter, multiple semiconductor switching devices, such as transistors or silicon controlled rectifiers, convert the applied direct current to alternating current. The number of semiconductor switches required depends 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 most commercial applications. A control circuit that is part of the microinverter determines the  timing of the switching devices. Additional components are also included for protection and signal filtering.Although a solar microinverter is a relatively simple component, in addition to changing variable DC to a relatively constant AC, it must also do some other important things. For example, it must incorporate a capability called multiple 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 must quickly shut 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.

INVERTER SYSTEM

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.

ADVANTAGES OF MICROINVERTER
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 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. This 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 currrent, is also approximately zero. On the extreme right of thecurve 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).

Keep in mind that, 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.

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 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.

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.

RACKING METHODS

There are three primary types of racking methods for solar PV modules with multiple variations of each type.

The bottom two photos in the accompanying graphic have ground mountings. In the photo on the lower left, the mounting incorporates solar tracking capability to optimize the available light.

The photo on the lower right shows a stationary system. This type of system has a low initial cost, but is not able to harvest solar energy as efficiently as the system with tracking capability.

ROOF MOUNYED RACKING SYSTEM

As previously mentioned, roof mounted solar racking systems have a number of key advantages. For example:

They are inexpensive because they are made from simple, light weight components that are readily available.

They are prefabricated for ease of installation and can be adapted as needed to fit the application requirements.

They are durable because they have a high strength-to-weight ratio and are made with corrosion resistant components.

They incorporate a reliable grounding system.

OTHER COMPONENTS

Depending on the type, size, and complexity of the solar PV installation, a variety of additional components may be required.

For example, one or more disconnect switches are required for commercial and some residential applications.

Where microinverters are not employed, one or more DC combiner boxes and multiple DC-to-DC converters may be needed.

Most systems also require multiple junction boxes for interconnecting system components.

All applications also require service entrance equipment. For residential applications, this means a meter mount for the utility meter and a load center to house the main and branch circuit breakers.

An economical solution employed in many applications involves use of a meter load center combination. This enclosure incorporates the bussing and provisions for both the utility meter and the main and branch circuit breakers.

Siemens offers a variety of meter mounts, safety switches, load centers, meter load center combinations as well as many other products for residential and commercial applications.

LOAD SIDE CONNECTION

Service equipment connections can be confusing, but some of the confusion can be eliminated by taking a few basic concepts into consideration.

For many smaller projects, especially existing residential applications, it is often convenient to use existing service equipment to save money and to avoid making changes to the utility connection.

This typically involves making load side connections in a load center or in the load center part of a meter load center combination. This is a perfectly reasonable thing to do as long as the following factors are considered.

First, the load center must have provisions for the additional branch circuit breakers.

120% RULE

Because circuit breakers are designed to trip at 80 percent of their handle rating, the National Electrical Code® (NEC®) allows the main breaker rating to be up to 120 percent of the load center busbar rating. (Note: because code requirements vary by location, please consult the full version of the appropriate code to determine requirements for a specific application.)

When using an existing load center or meter load center combination to support a new solar PV installation, branch breakers are often added to the load center. Because the current from these additional branch circuit breakers does not pass through the main circuit breaker, care must be taken not to overload the busbars. This means that the main breaker rating plus the sum of the solar branch circuit breaker ratings cannot exceed 120 percent of the busbar rating.

As an example, consider a meter load center combination with 200 A rated busbars. In this example, the main breaker rating plus the sum of the solar branch circuit breaker ratings cannot exceed 240 A (120 percent of 200 A). Therefore, if the main circuit breaker rating is 200 A, the sum of the solar branch circuit breaker ratings cannot exceed 40 A.

In situations where the main breaker rating plus the sum of the solar branch circuit breaker ratings exceeds 120 percent of the busbar rating, it may be possible for an electrician to resolve the problem by analyzing the existing branch loads to determine if a lower rated main breaker can be used. If not, either a larger main breaker panel or a second main breaker panel is needed.

LINE SIDE CONNECTIONS

Line side connections are connections on the line side of the main circuit breaker. This type of connection is common for new construction and is required by some electric utilities.

For this type of connection, additional equipment, such as a utility controlled disconnect switch, may be mandated by the utility.

The accompanying graphic shows a wiring diagram for one model of Siemens Solar-Ready Meter Load Center Combo. Note how the connections from the PV system disconnect switch connect on the line side of the load center's main circuit breaker.

Use of line side connections for the PV system avoids the problem of potentially overloading the load center busbars because all power to the load center passes through the main breaker.

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

SIEMENS SOLAR INVERTERS
As previously mentioned, Siemens offers products for use in the full range of solar applications. For example, consider the following Siemens inverters designed for solar PV applications.

SINVERT PVS inverters for 350 kW to more than 2400 kW are ideal choices for medium-sized and large photovoltaic power plants. The special feature of these central inverters is their master-slave mode of operation.

For medium commercial applications, SINVERT PVM inverters are UL listed inverters ranging from 12 kW to 24 kW with peak efficiency greater than 98%.

For residential and light commercial applications, the Siemens Microinverter System features a unique modular setup that is simple, efficient, safe, and scalable from 215 W to well over 50 kW.

THE SMART HOME

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

The current Siemens products available here include the Siemens Microinverter System, the VersiCharge electric vehicle charging station, and Solar-Ready Meter Combo. These products help to safely increase the electricity generated and consumed by the owner.

Products in the future will include smart thermostats (to control air conditioning and heating loads), smart switches (to control lighting and outlets), and smart appliances (such as washing machines, refrigerators, dryers) that will turn on automatically when the price for electricity is lowest and can save owners the most money.

MICROINVERTER SYSTEM COMPONENTS

As shown in the accompanying graphic, the Siemens Microinverter System includes 215 W microinverters with various connector options, Trunk-and-Drop Cabling and related items, the Envoy communications gateway, and the Enlighten web monitoring application.

Siemens can also provide the load center and meter mount or meter load center combination. Various versions of these products are available to meet every region and utility requirement. Siemens Solar-Ready Meter Combos meets utility requirements for many areas, so contact Siemens to verify that your local utility has approved them.

SIEMENS MICROINVERTER SYSTEM


The most critical component of the Siemens Microinverter System is the microinverter. This unit contains all the power circuitry and processors to invert and measure the electricity generated. The microinverter also has a weatherproof enclosure that protects it from the high temperatures and harsh weather conditions. This enclosure is easy to attach to the racking and to the grounding conductor.

The inverter has two sets of connectors to allow the flow of electricity. On the left side are the positive and negative quick connectors that attach to the solar module terminals. These are either an MC-4 or Tyco style. On the right side is an AC output connector that attaches to the Trunk-and-Drop Cabling.

MICROINVERTER CATALOG NUMBER

Siemens Microinverter system has relatively few part numbers. This is an advantage both to distributors and contractors because it minimizes inventory requirements.

The accompanying graphic shows the catalog number logic for microinverters. Four part numbers are available, and the only variables are the type of DC connector used (MC-4 or Tyco) and the country of manufacture (Canada or China).

TRUNK AND DROP CABLING


The Trunk and Drop Cabling transmits the AC electricity from the inverter to the load center or other utility connection point. The quick connectors and easy to use accesories provide a plug-and-play system which greatly reduces wiring time and overall costs (no wire nuts, no crimping, no hassles).

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 1.025 or 1.7 meters. Each drop provides a connector for plugging in a microinverter's AC connector.

The 1.025 meter (40 inches) and 1.7 meter distances correspond to the dimensions of a standard 60 cell solar module. When modules are said to be arranged in portrait orientation, the short dimension (1.025 m) is parallel to the roofline and when the modules are in landscape orientation, the long dimension (1.7 m) is parallel.

Two voltage/phase options are 240 VAC, single-phase (for most  residential applications) and 208 VAC, three-phase (for commercial applications). These voltage/phase options have different internal wiring schemes, 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 or 40 and 240 drops.Trunk-and-Drop Cabling part numbers have only three variables: length between drops, voltage and phasing type, and the number of drops.

TRUNK AND DROP CABLE PHASING


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 happens in 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 215 W (about the same as two light bulbs), this imbalance is quite small and does not cause problems.

TRUNK AND DROP CABLE ACCESSORIES

The following Trunk-and-drop cable accessories are listed in the accompanying graphic.

Terminator Cap:  provides a waterproof seal at the exposed end of a Trunk-and-Drop cable run.

Seal Cap: provides a waterproof seal for unused drops in the Trunk-and-Drop cable.

Disconnect Tool: used to remove the Seal Cap or to disconnect the microinverter AC connector from the Trunk-and-Drop cable. Also has tools to seperate Tyco and MC-4 quick connectors.

Cable Clips : used mount cable to racking.

Installation Kit: includes 4 Terminator Caps, 5 Seal Caps, and 1 Disconnect Tool.

THE ENVOY COMMUNICATION GATEWAY

The Envoy Communications Gateway is critical for unlocking the intelligence of the microinverter system.  Each Microinverter sends a Power Line Carrier signal onto the AC wiring of the structure.  The Envoy converts the Power Line Carrier signals from the microinverter network to Ethernet signals compatible with an Internet-connected broadband router. Included in the Envoy Communications Gateway (IEMU03) is a Powerline Carrier to Ethernet Bridge (EPLC01).

Power Line Carrier or PLC uses the Trunk-and-Drop cable's copper wires that also carry 60 Hz AC to communicate messages using very high frequency signals (>100 kHz).

The Envoy's Ethernet connection to a broadband router permits Internet communication to set up, monitor, and troubleshoot microinverters and to monitor overall system perfomance. Because system information is available via the Internet, it can be also be monitored remotely via a smart phone or computer. Owners can also check system status using the liquid crystal display (LCD) mounted on the front of the Envoy.


ENLIGHTEN WEB BASED MONITIRING AND ANALYSING SYSTEM


Enlighten is web monitoring 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. Enlighten also has a web services API for those looking to display data on their own website or to bring the data into their own data aquisition system.

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.

As stated previously, Siemens can provide many of the products needed to complete an electrical installation. For example, let's take a brief look our Solar-Ready Meter Combo.

Where applicable, this is an excellent solution for many new installations because it provides 60 A rated line-side connections ahead of the main breaker on the load side of the enclosure. This avoids the potential for bus overloading as discussed earlier.

A variety of configuratons with a range of meter provisions are available to meet utility requirements in selected states. Contact Siemens or your utility prior to purchase or instalation to determine if this product is approved in your area.