ADVANCEMENTS IN INVERTER TECHNOLOGY

P. BHANU TEJA

B100338EE

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT

CONTENTS

1. INTRODUCTION

2. TECHNICAL BACKGROUND ON INVERTERS

3. OVER VIEW OF ADVANCED INVERTER

FUNCTIONS.

4. IMPACTS AND CHALLENGES OF ADVANCED

INVERTERS ADOPTION

5. ADVANCEMENTS IN PV INVERTER

6. SOME ADVANCEMENTS IN APPLICATION

7. MARKET

8. CONCLUSION

INTRODUCTION

Inverters are power electronics-based devices which convert direct current (DC) to alternating current (AC).

This function is fundamental to the integration of power from many sources into the distribution system.

Widely used in photovoltaic, wind turbine generators and energy storage resources.

In these applications, inverters convert a generated or stored DC to a precisely modulated and grid synchronized AC waveform.

Beyond this fundamental purpose, there exist a range of complementary, technologically viable, and demonstrated functions that an inverter may be designed to provide.

As DER (Distribution Energy Resources) become incorporated onto the grid at higher penetration levels, advances in inverter functionalities represent a significant opportunity to improve the stability, reliability, and efficiency of the electric power distribution system.

TECHNICAL BACKGROUND ON INVERTERS

Standard Inverter Key Concepts:

Fundamentally, an inverter is a device which converts a direct current (DC) input to an alternating current (AC) output.

Inverters are used in a range of applications, including consumer power electronics, electric vehicles, and photovoltaic and energy storage interconnections to power distribution systems at the primary (4 kV, 13.8 kV, 27 kV, and 33 kV) and secondary (120/240 V, 120/208 V, 240/480 V) levels.

In distribution applications, these devices produce a sinusoidal waveform of the appropriate frequency.

Inverters may be 1. Stand alone(off-grid): supply generated or

stored power solely to connected loads.2. Grid tie : allow generated or stored power to be

supplied to a utility’s distribution network when not needed by the load.

Standard Inverter Functionalities:

1.Power Transfer Optimization: Inverters are designed to optimize transfer of power

from DER to load, often through a technique called Maximum Power Point Tracking (MPPT).

Based on computation of the ideal equivalent resistance from measurements of current, voltage, and the respective rates of change.

2.Voltage Conversion: In order to supply power to a load or to the distribution

grid, power generated by a distributed energy resource usually must be delivered at a different voltage.

3.Grid Synchronization A central component of an inverter’s efficacy is the

ability to construct an output AC waveform that is synchronized with the utility distribution system.

4.Disconnection When fault conditions are present, a grid-tied

inverter is required to disconnect from the distribution system at the point of common coupling (PCC).

5.Storage Interfacing An inverter may enable the integration of a battery

or other energy storage device with a distributed generator.

6.Anti-islanding protection:

Normally, grid-tied inverters will shut off if they do not detect the presence of the utility grid.

There are load circuits in the electrical system that happen to resonate at the frequency of the utility grid.

The inverter may be fooled into thinking that the grid is still active even after it had been shut down. This is called islanding.

An inverter designed for grid-tie operation will have anti-islanding protection built in; it will inject small pulses that are slightly out of phase with the AC electrical system in order to cancel any stray resonances that may be present when the grid shuts down.

OVERVIEW OF ADVANCED INVERTER FUNCTIONS

Advanced Inverter Key Concepts An advanced inverter has the capacity

1. To supply or absorb reactive power2. To control and modulate frequency and voltage, and 3. Voltage and frequency ride-through.

Capacitors could be installed to either supply or absorb reactive power. Practical limitations include:1. Limited variability of reactive power that can be supplied or absorbed

dependent on the ability to switch on/off various combinations of capacitors at a location.

2. Reactive power supplied or absorbed by capacitors will greatly change with minor changes in voltage level.

As a flexible source and sink of both active and reactive power, advanced inverters provide an opportunity for the extensive control that enables safety and reliability in DER applications.

ADVANCED INVERTER FUNCTIONALITIES

1.Reactive Power Control: Definition: The presence of inductive loads results in a phase

difference between voltage and current waveforms, causing losses which reduce the efficiency of real power distribution.

Less efficient power distribution requires greater current, which magnifies the impact of line losses.

Implementation:

The supply of reactive power via capacitors will cause the phase of the current to lead that of the voltage, while the opposite may be achieved when an inductive load absorbs reactive power.

Integrated thyristor-switched capacitors and capacitors, functioning together as a Flexible AC Transmission System (FACTS), Solid-state- and power electronics-based compensators, allow increasingly rapid and exact provision of reactive power.

Advanced inverters, combined with existing FACTS infrastructure and control Systems.

A capability curve prescribes the output reactive power, which is diminished at lower voltage levels and at higher output active power.

These inverters control power factor according to the characteristic capability curve in order to match the mix of resistive and inductive loads on the circuit.

Impacts: significant potential to increase efficiency and flexibility of

power distribution. providing sufficient resolution in controlling reactive power. precise modulation of reactive power supplied to the

conductor and load.

2.Voltage and Frequency Ride-Through

Definition:

Ride-through may be defined as the ability of an electronic device to respond appropriately to a temporary fault in the distribution line to which the device is connected.

Standard inverters are required to identify a typical fault and disconnect from the circuit when a fault is detected.

This course of action will inhibit the DER’s operation and prevent it from functioning under the restored normal conditions.

Implementation:

Ride-through capabilities are tied to measurements of the distribution system’s AC frequency and voltage.

Ride-through functionality is highly dependent on monitoring, processing, and algorithmic response.

The controlling algorithm will implement a response, such as an increase in power in response to a low voltage.

If the condition persists and the inverter fails to reach sufficient parameters within the IEEE 1547 disconnection time frame, the disconnection will take place as with the standard inverter, ceasing all ride-through responses.

Sags and swells in voltage levels can be remedied by the injection of reactive power into the line.

Disadvantage: In non-utility scale DER applications such as residential and small commercial, if ride-through is permitted by standards to prolong the presence of a fault, people will use a fault circuit to greater risk of damage or injury.

IMPACTS & CHALLENGES OF ADVANCED INVERTERS ADOPTION

Impacts:1) reactive power control increases efficiency of

power distribution by reducing line losses.

2) The voltage and frequency ride-through functionalities provide dynamic grid support in the presence of a fault along the interconnected line.

3) Avoiding “unnecessary” disconnection, especially of large distributed energy resources, could improve grid reliability.

Challenges:

1. There is ongoing work to develop interoperability standards for DER devices including inverters and inverter controllers.

Therefore, limitations are there to how much these advanced functionalities can be used autonomously without adversely impacting the grid or other customers’ equipment

2. Different safety requirements and standards are to be implemented for residential and small commercial applications.

3. EPRI (Electric Power Research Institute) conducted a study, indicating that over 69% of downtime events are caused by the PV inverters.

The main contributors to these failures were software bugs and material failures, which indicates a need for significant refining of the inverter technologies being deployed.

ADVANCEMENTS IN PV INVERTER

Over the last 40 years, solar panels are connected together into strings and the DC power is wired to a large inverter in a central location called string inverter.

In 1990s, Micro inverter technology came into existence, in which inverter installed behind each solar module. All the inverters connected through busbar.

PROS: lower initial cost per

peak watt price. easier to install,

maintain.

CONS: problems with one

panel are felt across the entire string.

difficult to fix. takes more space.

CONS: cost more per peak

watt. difficult to install,

maintain.

PROS: one panel won’t

impact other. easy to fix. takes less space.

STRING INVERTER MICRO INVERTER

SOME ADVANCEMENTS IN APPLICATION1. In Air conditioner,

Compressor motor is driven by inverter to control its speed.

Inverter technology provides a more precise room temperature without the temperature fluctuations.

A microwave inverter is a system used in microwave powering which uses inverter power supply as opposed to traditional magnetic coils or transformers. It is more efficient and powerful.

Other applications include welding, HVDC, UPS, LCD screen, Electric tasers, Hybrid vehicles etc.

MARKET Enphase is one of leading suppliers of micro inverters.

World leading central inverters suppliers are Ingeteam, ABB, SMA, Eltek, Sungrow etc.

In India, research and development of inverter businesses are Sukhila Power Electronics, APLAB, APD Global, laito infotech etc.

CONCLUSION Advanced inverter functionalities may lend

significant improvement to the stability, reliability, and efficiency, of the electric power distribution system.

Distribution automation systems implemented by utilities will be central to the integration of these functionalities, which require protection, control, and communication to reach full efficacy.

Standards for interoperability and performance are being revised to consider safe and reliable augmentation of inverter functionality to support increased penetration of DER.

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