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Grid-Tied Solar Photovoltaic (PV) System

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Most PV systems are grid-tied systems that work in conjunction with the power supplied by the electric company. A grid-tied solar system has a special inverter that can receive power from the grid or send grid-quality AC power to the utility grid when there is an excess of energy from the solar system.

grid tied PV Solar System

Grid-Tied Solar PV System

In addition, the utility company can produce power from solar farms and send power to the grid directly.

Residential and Small Grid-Tied PV Systems

Grid-tied PV systems can be set up with or without a battery backup. The simplest grid-tied PV system does not use battery backup but offers a way to supplement some fraction of the utility power. The major components of this system are the PV modules and an inverter.

residential grid-connected PV system

Residential grid-tied PV system (Source: Wikipedia)

The modules may be connected in series to the inverter if voltage limits are not exceeded, or a separate combiner box may be used to combine the outputs of various modules in parallel.

The inverter must be a special type that can be connected directly to the ac breaker box, so it needs to convert the DC from the PV modules into grid-compatible AC and match the phase of the utility sine wave.

It must also be able to disconnect the PV system (using an automatic transfer switch) when the grid is down, so it must be an approved inverter that meets UL standard 1741. A transfer switch is an automatic switch that can switch loads between alternate power sources without interrupting the current.

A basic block diagram of a grid-tied PV system with series PV modules is shown in Figure 1.

Compared to a system with a battery backup, a battery-free system like this is less expensive, easier to install, and almost maintenance-free. It has the advantage of not having to supply all of the power needed for the home or business; it can offset any fraction of the power and have the utility make up the difference.

 If the grid is reliable, as it is in most urban areas, then a battery-free system offers the best performance per dollar spent.

For many commercial office buildings, stores, and industrial buildings, a battery-free system makes sense. These types of buildings are normally occupied during daylight hours, corresponding to the times when the solar resource is available.

Usually, the modules can be installed on the roof of the building or a parking structure, so land is not sacrificed for the array. The system can be set up so that any excess power is sold back to the utility, alleviating any concern about weekend or holiday unused capacity.

Battery-Free Grid-Tied Solar PV System

Figure 1 Simplified Battery-Free Grid-Tied Solar PV System

UL Standard 1741

The Underwriters Laboratories® (UL) is an independent product safety certification organization that writes standards for safety and tests products for compliance.

UL standard 1741 lists requirements for inverters, converters, charge controllers, and interconnection system equipment for both utility-interactive (grid-tied) power systems and for non-grid-tied systems.

Other UL standards are written for PV modules and junction boxes, cabling, connectors, batteries, and mounting systems. For example, UL standard 1703 specifies standards for PV systems up to 1,000 V.

Companies that receive UL certification are allowed to display the UL mark on the product(s).

Residential and Small Grid-Tied PV System with Battery Backup

Grid-tied PV systems with a battery backup can continue to supply power any time the grid goes down. The system can switch seamlessly to backup power when an electrical outage occurs. Simultaneously, it disconnects the system from the grid so it doesn’t send power out when the grid is down.

Backed-Up Loads

A small system with a full battery backup capability is much more expensive than a battery-free system.

 One way to reduce cost is to split the system into backed-up loads and non-backed-up loads, thus reducing the number of batteries required, saving initial cost, and reducing maintenance and space requirements.

This option requires rewiring the service panel and placing non-backed-up loads on a separately dedicated panel from those that are backed up. Essentially, this option is equivalent to having two systems, but rewiring a panel may be a cheaper option than a fully backed-up system.

 A system with backed-up loads and non-backed-up loads is shown in the block diagram in Figure 2. The panels are shown going to a combiner box, but a series arrangement is another option for connecting the modules.

A combiner box is an electrical connection box for combining the outputs of multiple solar panels into one DC output.

Battery Backup System for Part of the AC Load

Figure 2 Simplified Battery Backup System for Part of the AC Load

When the system is in the grid-interactive mode, the inverter takes energy from the sources and sends it to the backed-up loads. The main loads are powered directly from the grid.

If there is more energy from the PV modules than is needed by the backed-up loads, the excess is put onto the grid through an internal transfer switch, resulting in a credit for the homeowner (net metering).

When the grid is down or out of specification, the transfer switch opens and only the backed-up loads receive power from the inverter. The main loads are solely dependent on the grid, so they will be off until power is restored.

PV Inverter Sizing

The size of the inverter and battery backup required for a partially backed-up system requires an analysis of the loads that will be put on the backed-up system.

To estimate the power requirement for the backup loads, the power to each load can be summarized on a spreadsheet. Motors need more power during starting than during running, so the system must be sized based on starting power. From the results of this analysis, the inverter, including various options, can be selected. One option is to use inverters that can be stacked.

 The term stacking refers to connecting two inverters to provide split-phase 120/240 V outputs. Another option available on some inverters is to provide a backup engine generator input.

Battery Bank for PV System

The battery bank is sized according to the number of days of autonomy required. The size can be based on historical patterns of time that the grid is down.

In general, a system that is backing up the grid is cycled only when the grid is down, so sizing considerations are different than in the grid-free system, which cycles daily.

An 80% depth of discharge is appropriate for a system that is cycled infrequently, and the number of days of autonomy is based on grid performance rather than weather patterns.

The infrequent cycling means that sealed batteries can be a good choice for a backup system because they require less maintenance than flooded types.

The drawback to sealed batteries is that they are more expensive and have a shorter life expectancy than flooded types.

For battery backed-up systems, battery meters that can report the state of charge are useful. These meters show the voltage, current, and percentage of full charge.

Another option is a power meter that monitors the performance of the system and alerts the user of fault conditions.

Studies have shown that monitoring systems encourage energy conservation and that more detailed information leads to more conservation.

Small PV Systems with Micro inverters

The systems shown previously takes DC to a central inverter and converts it to AC at that point. Another option that is growing in popularity is to use a microinverter at each module.

A microinverter is a DC to AC converter that is sized to operate with a single solar module. Thus, it can provide maximum power point tracking for the module and greater efficiency, particularly for situations such as a single shaded module that has reduced output. A basic system is illustrated in Figure 3.

Each inverter puts out grid-compatible AC that is synchronized to other microinverters in the system. Microinverters are installed in parallel with each other to form a branch circuit.

The branch circuits are often combined at a subpanel. The result is a more modularized system; if a module or microinverter fails, the rest of the system continues to operate (at reduced output) because the other microinverters are connected in parallel and one open source does not affect the operation of the others.

The defective module or microinverter can be repaired without taking the rest of the system offline; however, the faulty module may have to be removed for servicing.

Some modules come equipped with a built-in microinverter and circuits to optimize the output.

Built-in micro inverters do not have access to the DC circuits from the PV module, but they eliminate the DC wiring, connectors, combiner boxes, and so forth. This simplifies installation, making the overall system efficient and cost-effective. It also eliminates high-voltage DC circuits (as much as 600 V), so the micro inverter system is safer than high-voltage systems with a central inverter.

Basic Micro inverter System

Figure 3 Basic Micro inverter System. DC from each module is converted to ac where it is connected to other micro inverters in the system.

Commercial and Institutional PV Systems

Commercial and institutional solar PV systems can offer economies of scale and frequently have the advantage of a relatively lower demand for electricity at night.

Most of these systems are designed to reduce the electricity demand for a larger user such as a business, school, or manufacturing facility, so the system is designed to be a grid-tied PV system.

A few systems are designed as off-grid systems for remote applications, such as a PV system that was installed for a marine sanctuary on the Farallones Islands.

The marine sanctuary had previously imported diesel to run generators for electricity. In addition to supplementing utility power, another application for commercial and institutional establishments is to provide a solar fuel station for their employees or the public to use.

The solar panels are mounted above a parking area, and they supply charging power to electric vehicles, an excellent match of the available resource to the need (charging electric vehicles). Figure 4 shows a solar fuel station.

 Many communities and government entities are providing these stations at public parking facilities to encourage the use of electric vehicles and to reduce emissions.

Solar Fueling Station

Figure 4 Solar Fueling Station. The solar modules of this fueling station are used to charge electric vehicles.

Utility Grid-Tied PV Systems

In some areas, utilities have constructed large PV arrays that are designed to feed power to the grid. Utilities have many different considerations for implementing solar PV systems because they are supplying power rather than consuming it.

When a utility company is considering adding solar power, the system is first analyzed and modeled to determine the effects, load balancing, equipment loading, and power quality issues.

The overall cost, such as any new transmission and distribution systems required, and the impact on existing facilities, such as reduced fuel costs, are evaluated.

 In some cases, it may be more economical to develop distributed systems using smaller solar arrays deployed on specific feeders to handle additional load and reduce capital costs.

Distributed systems can also reduce line-related cost due to the power dissipated in transmission lines.

Review Questions

  1. What is the requirement for grid-tied PV inverters?
  2. What are two reasons for having a grid-tied PV system that is not backed up?
  3. How does sizing a battery array in a grid-tied PV system differ from sizing a battery array in a grid-free system?
  4. Why is constant system monitoring useful for a grid-tied PV system?
  5. What cost factors should utilities consider for adding solar PV resources that a homeowner does not need to consider?

Answers:

  1. Grid-tied PV inverters need to synchronize their output with the utility and be able to disconnect the solar system if the grid goes down.
  2. (1) A system that is designed to supplement grid power and not replace it at any time does not need backup, so installation is simplified. (2) Battery backup is expensive, takes up space, and requires regular maintenance.
  3. In a grid-tied PV system, the battery must replace the grid only during outages, so the likelihood and length of outages is the key factor in determining battery size. In a stand-alone system, the key factor in determining battery size is the weather at the location and prospects for long periods of clouds or rain that would prevent the system from operating at its best.
  4. System monitoring can provide basic performance data for the system and help pinpoint problems with the system.
  5. Some factors that utilities need to consider are load balancing; equipment loading; power quality issues; overall cost, including any new transmission and distribution systems; as well as many other factors.
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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.