Impact on PQ by Grid Connected Electricity Storage Systems

Published On: Jul 26, 2016

The energy puzzle is regularly on top of the world’s political, environmental, and economic agenda. Pushed by demographic growth, aspiration for better living conditions, and the development of digital technologies, electricity consumption is set to increase rapidly.

In this context, today’s electrical grid and the power systems are experiencing the rise of some disruptive innovations. Tomorrow’s power grid would see a flow of power from both directions and an island of power systems comprising of Distributed Generation (micro grids, mini-grid), Smart Grids, Renewable Energy, etc. The power output of these sources are not as reliable and as easy to manage the energy demand than the traditional power sources. To overcome this challenge of reliable power delivery, electricity storage systems, and its subsequent use in a controlled manner is a crucial factor. The role of such enabling technologies is becoming more important as India moves towards deeper penetrations of innovative power generating systems.

Often renewable energy integration with grid are not considered dependable due to its inherent fluctuating nature. This integration takes place with the aid of power electronic converters, which if not implemented properly, high-frequency switching of inverters can inject additional harmonics to the power systems creating major PQ issues. Hence, PQ analysis will be rewarding for electric utilities since it enables continuous monitoring, early excursion detection, root cause analysis, and timely corrective actions improving overall grid reliability. This blog highlights the impact of poor power quality on utility’s network & power systems by use of energy storage systems and its potential mitigation techniques/solutions.

GRID CONNECTED ELECTRICITY STORAGE SYSTEM MARKET GROWTH

Electricity storage is one of the most effective solutions for addressing high intermittency and can act as reserve and ready for use capacity in the entire energy value chain of generation, transmission, and distribution. Storing electricity on a large scale has been pursued for years by electric utilities in hopes of using the power during peak demand. The ability to store large amounts of power will lend a helping hand to power producers fill the production gaps created by growing amount of intermittent generation from renewable sources such as the wind and solar. After years of limited progress, several capable technologies for storing large amounts of power have emerged for energy storage capacity, while some others are promising.

(Please refer APQI’s recent blog on “Energy Storage Technology for performance enhancement of power systems” that covers these technologies in detail and their qualitative comparison across key parameters, its application, market segments and key drivers. A case study has also been included to understand energy storage solution by one of the utilities for meeting peak load)

The overall grid-connected electricity storage market is set to explode, reaching a total of over 40GW of installations by 2022, ‘’as per market research firm IHS. The United States will be the largest country for installations, accounting more than 1/3rd contribution during that period, followed by Germany and Japan. In India, total grid connected storage in India is expected to be 5GW by 2022 growing to 8GW by 2032 and 10GW by 2042 and 15GW by 2047.

KEY PQ ISSUE DUE TO GRID CONNECTED ELECTRICITY STORAGE SYSTEM

Managing power systems inclusive of storage system requires familiarity with the importance of power quality. While electricity storage deployment throughout electricity systems provides various benefits in terms of improved grid reliability, network loading & shifting, voltage regulation, etc. however, if not properly designed and implemented, then its integration will not be effective with large sized renewables, thereby affecting end user energy management. Some of the key PQ issues that may arise due to improper storage systems are:

Harmonics – Harmonics are distorted forms of electric voltages and currents that appear in the grid as a result of non-linear electric loads. This is caused by the conversion of DC to AC power by the inverter. Another factor that influences harmonic distortion in a power system is the penetration of Distributed Generation (DG) sets that uses renewable energy resources like solar PV, wind generator, etc. connected to the power system. The interaction between grid components and a group of renewable energy DG sets can amplify harmonic distortion.

Sustained Interruptions – To improve the power system reliability and to provide backup power (in case of power interruption) or to cover for contingencies when part of a delivery system is out of service. Unfortunately, with an uncontrolled-inverter and lack of storage capacity might not be capable of operating in stand-alone mode. When the Distributed Generation is interconnected in parallel with the utility distribution system, some operating conflicts might arise that affect the system reliability.

Voltage Flickers – Flicker is commonly seen due to rapid changes in the load or due to switching operations in the system. It is typically caused by the use of large fluctuating loads, i.e. loads that have rapidly fluctuating active and reactive power demand. Flicker effect occurs when one generating source reactive power output increases or decreases faster than the remaining generators can compensate.

POTENTIAL PQ MITIGATION SOLUTIONS

Globally, electricity storage technologies are expected to play a crucial role in grid integration and improving power quality. Utilities will have to make sure that there is good compatibility between the electrical system and the equipment. Some of the potential PQ mitigation solutions along with energy storage technologies are mentioned below:

Energy Storage Static Compensator (E-STATCOM) is normally used by distribution utilities to compensate the reactive as well as active power, which is one of the main cause of increasing distribution system losses and various power quality issues. E-STATCOM is used for balancing source current, power factor correction, harmonic mitigation and has the capacity to maintain bus voltage sags at the required level by supplying or receiving of reactive/active power in the distribution system. (Batteries are connected in general as compared to flywheels and capacitors to the DC-side of an E-STATCOM with a DC/DC converter to control the power exchange between the storage and the interface.)
UPS – For very sensitive low power equipment such as computers and servers, up to 4000 kVA, static UPSs can be used. There are different structures of UPSs, however, common to all structures are that active power can be supplied from an energy storage. Hence, complete mitigation of voltage dips and other power quality issues can be obtained. Depending on the size of the storage, even interruptions can be mitigated.
The Unified Power Quality Compensator (UPQC) has compensation capabilities for the harmonic current, the reactive power compensation, the voltage disturbances, and the power flow control. However, it has no compensation capability for voltage interruption because no energy is stored. When UPQC is configured with energy storage technology connected to the DC link through the DC/DC converter, it can compensate the voltage interruption too. UPQC has the ultimate capability of improving the power quality at the installation point in the distribution system.
Energy storage during periods of peak power generation from end-customer side can reduce the injection of renewable power into the grid to overcome voltages and congestion issues.

Broadly speaking, PQ issues often go unnoticed, but can be controlled with the right mitigating technologies coupled with energy storage technologies to avoid unpleasant surprises. Each energy storage application is unique and required careful consideration – from the selection of storage type to the system sizing, its testing, and servicing of the system.

CONCLUSION

As industry experts, facility managers know, good power quality can boost both productivity and employee safety, while poor power quality can mean a drain on the bottom line, thanks to damaged equipment, lost productivity and product spoilage. In the context of growing innovative power systems, energy storage is seen as a game changer for the power sector. The adoption of energy storage could help utilities and grid managers bring more balance to supply and demand, making the grid more resilient and efficient to be able to meet growing demand of quality power by digital age customers.