The Ultimate Guide to Inverters

Introduction

This guide will explore the importance of inverters and the part they play in converting a direct current (DC) to a usable alternating current (AC), in order to power a number of different applications within our homes. This includes renewable energy systems, which can help to reduce our carbon footprints and save on our energy bills.

We will investigate how inverters work, the different types, and their role in supplying efficient electricity. We will also look into solar inverters, wind energy inverters, and those that are utilised in electric vehicles. As well as how to use them safely, how to manage them, and how to keep them maintained.

Browse the guide at your own pace or open the dropdown menu to click the links to jump to the sections you need the most.

Section 1 – What are Inverters?

Inverters are electronic devices that convert a direct current (DC) to a usable alternating current (AC). This conversion is important as even though many electronic devices and systems operate on DC electricity, there are other electronic devices, applications, and systems that rely on AC electricity, such as those within most households. The electrical grid also relies on AC electricity.

The DC electricity, needed to be converted to AC, is often sourced from batteries (including solar), solar panels, wind turbines, and other DC generators. The inverter makes this conversion by changing the voltage, frequency, and magnitude of the electrical signal to match the required AC characteristics. This electricity can then be used within homes or fed back into the grid.

Section 2 – Types of Inverters

Grid-Tied Inverters

Grid-tied inverters are designed to connect renewable energy sources generated at a home to the electrical grid, enabling seamless and efficient electricity transferral between the two systems. Their key features are:

Synchronisation with the Grid: To ensure seamless integration of renewable energy to the grid, inverters synchronise their AC output with the frequency and voltage of the electrical grid. Preventing disruptions in the grid’s operations and maintaining power quality.

Export and Import of Electricity: Inverters allow a bidirectional power flow. If the energy demand within the home is higher than the renewable energy system produces, electricity can be imported from the grid.

Monitoring and Communication: These inverters often have monitoring and communication capabilities, that allows the monitoring of the system’s performance, tracking the energy production, and ensuring it is functioning properly.

Safety Features: These inverters often have safety features such as anti-islanding which disconnects the inverter during a power outage to protect those fixing the power lines.

Off-Grid Inverters

Off-grid inverters are designed to convert DC electricity, from (solar) batteries or generators into AC electricity, to power electrical appliances and devices in off-grid or standalone systems. Their key features are:

Standalone Power Systems: These are commonly used in rural or isolated locations where electricity from the grid is unavailable or difficult to access. These systems tend to include solar panels, wind turbines, batteries, or hydroelectric generators.

Battery Charging: Any excess energy that is generated from renewable sources during periods of high production can be stored in batteries for later use, when demand is higher, or when energy is no longer being generated.

AC Coupling and Backup Generators: Some off-grid inverters support AC coupling and allow for energy to be drawn from the backup generator during times of insufficient renewable energy generation.

Inverter Sizing and Load Management: Make sure the inverter can handle peak power demands of the connected electrical loads. Load management systems can also be connected to prioritise critical loads and optimise energy use.

Remote Monitoring and Control: Many off-grid inverters have monitoring and control features so users can check the system’s performance, battery status, and energy usage remotely and make informed decisions on any issues.

Off-Grid Self Sufficiency: Off-grid inverters allow the user to be self-sufficient, as they can produce their own electricity from renewable sources. Without having to depend on the electrical grid.

Hybrid Inverters

Hybrid, or multi-mode, inverters combine the features of both grid-tied and off-grid inverters. Their key features are:

Grid Interaction: If in grid-tied mode, hybrid inverters allow user to feed excess energy back into the grid. By doing this, homeowners can earn credits or gain payments through net metering schemes.

Off-Grid Capabilities: If in off-grid mode, hybrid inverters can supply energy from renewable sources or stored energy in a battery. This can help increase energy independence and enhance resilience during power outages.

Battery Integration: Any excess renewable energy produced can be used to charge batteries, and the energy can be discharged when renewable energy production is low or during peak demand periods.

Load Management: These allow the user to prioritise specific electrical loads during times of limited power supply.

Energy Management and Control: These systems allow the user to optimise energy usage, allowing them to monitor and control their consumption and production.

Backup Power: Hybrid inverters can switch between off-grid and grid-tied modes automatically. When the grid power fails, it will switch to a renewable source or energy stored in a battery.

Microinverters

Microinverters are installed directly onto each solar panel, converting DC to AC at the source, known as at panel-level. This can improve the system’s performance, increase its energy harvest, and enhance fault tolerance. Other key features include:

Increased Energy Production: They eliminate the negative impact from partial shading as each panel operates independently, allowing efficient energy production.

Module-Level Monitoring: Individual panel monitoring is provided, enabling the monitoring of real-time performance for each module. Due to this, issues can be detected quickly.

Enhanced Safety: There is no high DC voltage present in the system at panel level, increasing the safety for installers, maintenance personnel, and homeowners.

Simplified System Design: There installation process is simple compared to other inverter systems, as there is no need for complex string system requirements.

Scalability: The system can be easily expanded by adding individual solar panels with their own microinverter. These are advantageous for homeowners looking to start with a smaller system and expand.

Higher Reliability: As each panel operates independently, if one microinverter experiences an issue, the rest of the system will continue to operate.

Section 3 – Inverter Topologies

Voltage Source Inverters (VSI)

VSIs act as a controllable voltage source, that converts DC to AC with adjustable frequency and magnitude. This means that they can transform the electrical power to meet the specific requirements in electrical systems, both in grid-tied and off-grid systems.

The main components are the DC input source which is usually a capacitor or a battery, power switches, and control circuitry (regulates the switching of the power switches to produce the required AC output voltage). The operating principle requires the use of pulse-width modulation (PWM) techniques.

They can be used in various applications including:

• Renewable energy systems (grid-tied solar and wind energy systems) to make the AC power compatible with the grid.
• Motor drives and adjustable-speed drives to control the speed of motors efficiently.
• Uninterruptible Power Supplies (UPS) to act as a backup power source.

Current Source Inverters (CSI)

These are used when a controllable current source is required to convert DC into AC. The main components are the DC input source which is usually a capacitor or a battery, power switches, and control circuitry (regulates the switching of the power switches to produce a constant output current). The operating principle requires the use of pulse-width modulation (PWM) techniques. Which efficiently switches the power switch on and off, to maintain a constant output current.

They can be used in various applications including:

• Induction heating to ensure a constant current reaches the induction coils, to efficiently control heating applications.
• Grid-tied applications, such as solar panels.
• Variable frequency drives, such as motors to control speed.

PWM (Pulse Width Modulation) Inverters

These inverters are designed to efficiently convert DC to AC by adjusting the width of the voltage pulses in a controlled way. This then controls the output voltage, frequency, and the waveform, allowing versatility.

They work by switching the power semiconductor devices on and off at high frequencies and by varying the width of the voltage pulses. By doing this the average voltage across the load is controlled.

They can be used in various applications including:

• Solar PV inverters, both grid-tied and off-grid.
• Motor drives, to control the speed.
• Uninterruptible power supplies (UPS), to provide backup power.

Multilevel Inverters

These inverters convert DC into AC with multiple voltage levels, such as high voltage levels, to improve efficiency. To achieve this several DC voltage levels have to be combined through power semiconductor switches and capacitors, or other energy storage elements.

They can be used in various applications including:

• High power motor drives to control speed.
• Grid-connected solar inverters.

Resonant Inverters

These inverters are designed to efficiently convert DC to AC by utilising the resonant behaviour of inductive and capacity elements within the circuit. There are two types including Series Resonant Inverters (SRI) and Parallel Resonant Inverters (PRI).

Advantages to these inverters include improved efficiency, reduced losses, and lower electromagnetic interference. This makes them suitable for various high-frequency and high-power applications. Applications include:

• Induction heating, to efficiently control the heating process.
• Fluorescent lamp ballasts to ensure stable lighting.
• Wireless power transfer to enable efficient and contactless power transmission for charging electronic devices.

Section 4 – Inverter Applications

Solar Inverters (Photovoltaic (PV) Inverters)

They are essential in solar energy systems for converting the generated DC electricity to usable AC electricity. They ensure solar energy is utilised efficiently and that there is seamless integration with the grid. Many solar inverters come with monitoring and data logging features to track performance, energy production, and consumption. Incorporating Maximum Power Point Tracking (MPPT) can increase efficiency and optimise the efficiency of the solar energy system.

Solar inverters are most commonly used in grid-tied systems to allow users to earn credits or payment through net metering for any excess energy fed back into the grid. In an off-grid system, solar inverters are used alongside batteries to store excess solar energy for later use or when energy is not being generated. Hybrid inverters can also be used to combine grid-tied and off-grid functionalities.

String Inverters: These solar inverters are designed to convert DC electricity generated by a string of solar panels into usable AC electricity.

Wind Energy Inverters

They convert the variable frequency and voltage generated by wind turbines into a usable AC electricity. Therefore, they are designed to handle the wind’s variable nature and fluctuating frequency. Many turbines produce an alternating current and the inverter converts it to a stable usable frequency.

Grid-tied inverters allow the generated electricity to be fed back into the grid. Off-grid inverters work alongside batteries to supply power to off-grid locations. Hybrid inverters combine both functionalities, to provide backup power in an outage.

Uninterruptible Power Supply (UPS) Inverters

These are important components in backup power systems as they provide a reliable and continuous electrical power in the event of an outage, by converting the direct current from batteries or other energy storage devices into an alternating current. They provide an uninterrupted and stable power supply to connected electrical loads.

When utility power is available, the incoming AC is sent to the electrical load, whilst also charging the batteries. When the utility power is not available power is drawn from the battery to the connected loads.

Electric Vehicle (EV) Inverters

These are responsible for converting the direct current in the vehicle’s battery to a usable alternating current required to drive the electric motor. They are crucial to controlling speed, ensuring efficient power conversion, and contributing the overall performance and energy efficiency.

EV inverters also allow regenerative braking. Whilst braking, the motor converts kinetic energy into electric energy. The inverter turns the AC generated into DC to be stored in the battery.

Motor Drives and Control Inverters

These inverters play an important role in controlling the speed, torque, and direction of the motor, in order to optimise the motor’s performance and improve energy efficiency. They convert a fixed-frequency AC from a power supply to a variable-frequency AC to control the motor speed.

Types include variable frequency drives (VFDs), servo drives, and vector control drives.

Section 5 – Control Techniques

Open-Loop Control

An open-loop control system does not directly measure the output or feed it back to the controller. The controller, therefore, relies on a predetermined setpoint or reference to generate control actions. This means that the performance of the system is not monitored or corrected based on the actual output, making it less precise and susceptible to errors.

This control is often used for systems that are simple and stable, and where precise control is not critical.

Closed-Loop Control (Feedback Control)

This control system monitors the output of the system continuously with a sensor and compares it to a desired setpoint. The controller can then use this information to adjust the control actions (using an actuator) and the system output to make it closer to the desired value. These control systems ensure stability, accuracy, and resilience to any disturbances, by using precise regulation and monitoring.

Maximum Power Point Tracking (MPPT) in Solar Inverters

These optimise the energy harvest from PV solar panels. The maximum power point (MPPT) of a solar panel varies depending on the changes in environmental conditions, including sunlight intensity and temperature. MPPT algorithms are put in place to track these changes and adjust the inverter’s operating point to maximise energy generation and increase efficiency. This technology also allows more solar panels to be added without compromising the system’s efficiency.

Common MPPT methods include Perturb and Observe (P&O), Incremental Conductance (IncCond), and Fractional Open-Circuit Voltage (FOCV).

Section 6 – Efficiency and Power Loss

Inverter Efficiency

Inverter efficiency measures how effectively an inverter converts the input power (DC) to the output power (AC) without significant losses. It can help to determine the overall performance and energy efficiency of various applications. The formula to calculate inverter efficiency is:

Efficiency (%) = (Output Power / Input Power) × 100

There are several things that can affect the inverter efficiency including:

• Conversion Technology: The type of semiconductor device used.
• Operating Range: Inverter efficiency can vary at different load levels.
• Cooling and Heating Dissipation: Inverters generate heat during operation, cooling mechanisms are needed to prevent thermal issues.
• Control Algorithms: Such as MPPT or Vector Control.

A higher inverter efficiency will reduce energy loss and therefore reduce both energy consumption and costs. They also allow the battery to have a longer life, which also reduces the need for frequent replacement. A high inverter efficiency is the most sustainable option as they lower carbon footprints and reduce greenhouse gas emissions.

Power Loss Analysis

This examines the energy loss that occurs during the conversion of a direct current (DC) to an alternating current (AC). Analysing energy loss is important to assess the efficiency and performance of the inverter, as well as for identifying areas that need improving. They also allow for the selecting of the most suitable semiconductor device and other components.

Power loss can occur through:

• Switching Losses: Occurs in the power semi-conductor device, during their switching operations.
• Conduction Losses: Power is dissipated when the power semiconductor is on, due to material resistance.
• Copper Losses: Due to the resistance of the copper material in conductors and windings.
• Core (Iron) Losses: Occurs in magnetic components (e.g., transformers and inductors).
• Heating and Cooling Dissipation: Heat generation can affect the inverter’s performance.

Analysis can be carried out through computer-aided simulation tools (e.g., SPICE (Simulation Program with Integrated Circuit Emphasis)) or experimental measurements (power analysers or thermal sensor).

Thermal Management in Inverters

Inverters generate heat during the conversion process, due to power losses, thermal management ensures that there are safe temperature limits to maximise performance, safety, reliability, and lifespan. Proper cooling and heat dissipation techniques are essential to prevent overheating and damage to components, to maintain efficiency and functionality.

Temperature sensors can also be used to monitor the inverter’s internal temperature. Information is then fed back to the control system to adjust parameters to improve the thermal performance.

Section 7 – Inverter Protection and Safety

Overcurrent Protection

Overcurrent protection safeguards against excessive current flow within the circuit. This prevents damage to the components, equipment, and wiring, which can occur due to overloading or short circuits. Without these safety features, these issues can cause fire hazards and electrical accidents.

These protective features also allow the system to be reliable, as it quickly stops excessive currents, ensuring that the electrical system can carry on working.

Protection can include:

• Circuit Breakers: Automatically opens the circuit if the current exceeds the limit. The current flow is disrupted and damage to the circuit is prevented.
• Fuses: Made of a thin wire or metal element that melts when subjected to an excessive current, which disrupts the current flow. Fuses can be easily replaced.
• Current Sensing: To monitor the current flow, if the current exceeds the limit the protection device starts working.

Overvoltage Protection

Overvoltage protection safeguards against voltage surges or spikes that exceed the normal operating levels. This can be caused by lightning strikes, switching operations, or power disturbances. These surges/spikes can cause significant damage to the equipment and components within systems. Overprotection devices are designed to detect and divert excess voltage away from the circuit to prevent damage to and ensure the reliability of the system.

This protection can help to prevent costly repairs and replacements of damaged equipment, as well as preventing data loss and corruption to protect important information in electronic systems.

Protection can include:

• Surge Protectors: They divert excess voltage to a protective ground path.
• Voltage Clamping: They clamp the voltage to a safe level. A low-impedance path is used when the voltage exceeds safe levels.
• Fast Response Time: This ensures that the protection is activated quickly to protect the equipment and components.

Overtemperature Protection

Overtemperature protection prevents the overheating of electrical systems and devices, to prevent damage. Overheating can occur due to high temperatures, heavy loads, faulty components, or inadequate cooling. This protection is put in place to detect abnormal temperature rises and take action to mitigate the risks and ensure safe operations.

Overheating can lead to catastrophic failures that can endanger both people and the environment, as well as causing data loss and corruption to important information in electronic systems.

Protection can include:

• Temperature Sensing: Sensors include thermocouples or thermistors, that monitor the temperature of critical components and the surrounding environment and provide feedback to the protection circuitry.
• Trip Point: The trip point is set to a safe temperature and is activated when the sensed temperature exceeds the threshold.
• Activation Actions: After the protection is triggered, various actions are then initiated to prevent further temperature rise. For example, shut down, load reduction, or activating the cooling mechanisms.
• Delay Circuit: Allows a brief tolerance for temperature fluctuations before the protection is activated. This prevents false activations.

Anti-Islanding Protection in Grid-Tied Inverters

Anti-islanding protection is used in solar photovoltaic (PV) systems, to prevent islanding. This is where the grid-tied inverter continues to supply power to the local electrical grid during a grid outage. This can be incredibly dangerous for electricians working on the grid, as the electricity can flow back into the grid and cause an electrical hazard.

Protection can include:

• Grid Monitoring: The voltage, frequency, and phase angle is continuously monitored. If the parameters deviate from the norm, protective measures are triggered.
• Frequency Shift: The protection detects frequency deviations and acts accordingly.
• Voltage Fluctuations: The protection detects voltage deviations and acts accordingly.
• Disconnection: When abnormal grid conditions are detected, the inverter is disconnected from the grid.
• Specific Grid Standards: Compliance with these standards ensures grid safety.

Section 8 – Grid Integration

Grid Synchronisation (Grid Connection/Grid Integration)

Grid synchronisation is the process where a power generation source (e.g., renewable energy system) is connected and synchronised with the electrical grid. This synchronisation ensures seamless integration of the generated power into the existing grid infrastructure, to not cause disturbances or disruptions and enhance stability.

To efficiently carry out grid synchronisation, the frequency and voltage of the power generated and within the grid must match. Phase alignment also needs to be synchronised to allow a smooth power transfer. One of the most important features of grid synchronisation is to have protective systems in place to monitor and control the connection process, to ensure safety.

Power Factor Correction

Power factor correction is used to improve the power factor of an electrical system. The power factor is the ratio of real power (watts) to apparent power (volt-amperes). This is an important factor that affects the efficiency of power utilisation in an electrical system.

In an AC circuit the power factor falls within a 0-1 range. If it is 1 the real and apparent powers are equal, which is an ideal condition. If it is less than 1, this shows there is a loss to the apparent power, known as reactive power, which leads to inefficient power consumption.

To prevent this, power factor correction capacitors, also known as capacitors banks, can be used to supply reactive power. Power factor correction controllers can also be used to monitor the power factor and switch the capacitors of and on when needed.

Reactive Power Compensation

Reactive power compensation is used to manage and mitigate the effects of reactive power in electrical systems. Reactive power is the power that is consumed by the inductive and capacitive loads in an AC circuit. It creates a magnetic and electrical field that is not doing anything useful.

To correct this, power factor correction techniques should be used to improve the power factor to make it closer to 1. Capacitors can also help to provide reactive power to offset inductive loads and inductors can be used to offset capacitive loads. These devices are strategically placed in the electrical system to supply or absorb reactive power as needed.

There are two types of reactive power compensation, which are:

• Static, that uses a static device such as a capacitor or an inductor, that can be switched on or off.
• Dynamic, which uses synchronous condensers or static VAR compensators (SVCs), that continuously adjust their reactive power output in real-time.

Section 9 – Inverter Communication and Monitoring

Communication Protocols           

Modbus: Serial communication protocol used in solar inverters for monitoring and control purposes. These provide essential data about the inverter’s performance and status to the monitoring system, data loggers, and other devices.

Modbus TCP/IP: An extension of Modbus that uses an ethernet to provide real-time data between the inverter and the monitoring system using LANs or the internet.

OPC-UA: Platform-independent communication protocol used in UK solar inverters. Provides a secure and reliable data exchange between devices and systems.

MQTT: Lightweight and efficient communication protocol used in modern inverters. It allows data exchange between inverters and cloud-based monitoring platforms.

Remote Monitoring and Control

Real-Time Data Monitoring: This allows operators and system owners to access real-time data from solar inverters. Including power output, energy production, voltage, current, and other vital performance parameters. This can help to detect issues quickly.

Fault Detection and Diagnostics: Identifies and alerts users about faults and malfunctions.

Performance Analysis: Provides historical data and performance trends of a solar PV system. Helps to assess overall efficiency, identify areas that need improvement, and optimises energy production.

Remote Control and Configuration: System operators can adjust settings, update firmware, and change parameters without being near the inverter.

Energy Management and Grid Integration: Remote monitoring allows better energy management when integrating power to the grid. This is because it uses demand-response strategies.

Alarms and Notifications: Can be sent via email, SMS, or a mobile app to inform users of faults or issues.

Cloud-Based Platforms: They provide a secure and scalable solution for storing data, analysing the data, and can be accessed anywhere with the internet.

Data Logging and Analytics

Performance Data Collection: Collects a wide range of performance data from inverters, including real-time power output, energy production, voltage, current, and temperature. The data is recorded at regular intervals and stored.

Cloud-Bases Data Storage: Secure and scalable storage that ensures historical performance data is readily accessible anywhere with internet.

Data Analytics Algorithms: These are used to identify performance trends, irregularities, and potential issues. They can also use predictive analysis and anomaly detection.

Performance Monitoring and Reporting: System operators and owners can access reports and dashboards to track energy production, system efficiency, and other critical performance metrics.

Performance Diagnostics: Data patterns and trends are analysed to identify the cause of faults to allow timely maintenance and corrective actions.

Performance Optimisation: Adjustments can be made to maximise the energy yield, improve power generation, and increase overall efficiency.

Predictive Maintenance: Data analytics can predict potential equipment failures based on performance trends. This allows quick maintenance and prevents unplanned downtime.

Section 10 – Inverter Testing and Standards

Efficiency Standards

Efficiency Standards: Inverter efficiency standards specify the minimum efficiency levels that inverters need to meet. They show, as a percentage, how effectively an inverter converts DC power (solar panel) to AC power (electrical grid or local load). The higher the efficiency rating, the less energy is lost.

CEC (California Energy Commission) Efficiency: Used globally. It measures the efficiency of the inverter at various output levels to provide the inverter’s performance across different load conditions.

EN 50530 Standard: Used to measure the efficiency of grid-connected inverters for testing and rating.

MCS Certification: The Microgeneration Certification Scheme (MCS) ensures that renewable energy products meet quality and performance standards.

Standby Power Consumption: Inverter efficiency standards may include guidelines for minimising standby power consumption when the inverter is not actively converting power.

Safety Certifications

CE Marking: Mandatory certification required for many products sold within the European Economic Area (EEA). To show the product complies with relevant EU safety, health, and environmental protection directives.

Microgeneration Certification Scheme (MCS) Certification: Essential for inverter manufacturers and installers looking for access to UK Government grants and incentives. The certification ensures that the inverter meets safety and performance standards.

International Electrotechnical Commission (IEC) Standards: Safety standards specific to inverters include – IEC 62109 for safety of power converters for use in solar PV systems and IEC 61683 for inverter performance testing and safety requirements.

European Norm (EN) Standards: They define the safety and performance requirements that inverters must meet to ensure safe operation.