T & D - 43) Reliability of Compressed Air Energy Storage Plant

Compressed air energy storage (CAES) plant

Introduction to CAES

As the name implies, the compressed air energy storage (CAES) plant uses electricity to compress air which is stored in underground reservoirs. When electricity is needed, this compressed air is withdrawn, heated with gas or oil, and run through an expansion turbine to drive a generator. The compressed air can be stored in several types of underground structures, including caverns in salt or rock formations, aquifers, and depleted natural gas fields.

Typically the compressed air in a CAES plant uses about one third of the premium fuel needed to produce the same amount of electricity as in a conventional plant.

A 290-MW CAES plant has been in operation in Germany since the early 1980s with 90% availability and 99% starting reliability.

In the U.S., the Alabama Electric Cooperative runs a CAES plant that stores compressed air in a 19-million cubic foot cavern mined from a salt dome. This 110-MW plant has a storage capacity of 26 h.

The 290 MW Huntorf CAES station

The fixed-price turnkey cost for this first-of-a-kind plant is about $400/kW in constant 1988 dollars. The turbomachinery of the CAES plant is like a combustion turbine, but the compressor and the expander operate independently. In a combustion turbine, the air that is used to drive the turbine is compressed just prior to combustion and expansion and, as a result, the compressor and the expander must operate at the same time and must have the same air mass flow rate.

In the case of a CAES plant, the compressor and the expander can be sized independently to provide the utility-selected “optimal” MW charge and discharge rate which determines the ratio of hours of compression required for each hour of turbine-generator operation. The MW ratings and time ratio are influenced by the utility’s load curve, and the price of off-peak power.

For example, the CAES plant in Germany requires 4 h of compression per hour of generation. On the other hand, the Alabama plant requires 1.7 h of compression for each hour of generation.

At 110-MW net output, the power ratio is 0.818 kW output for each kilowatt input. The heat rate (LHV) is 4122 BTU/kWh with natural gas fuel and 4089 BTU/kWh with fuel oil.

Due to the storage option, a partial-load operation of the CAES plant is also very flexible.

For example, the heat rate of the expander increases only by 5%, and the airflow decreases nearly linearly when the plant output is turned down to 45% of full load. However, CAES plants have not reached commercial viability beyond some prototypes.

Resource: Saifur Rahman – Advanced Energy Technologies

T & D - 39) An Overview Of Smart Power Grid

An Overview Of Smart Power Grid

Abstract

Figure 1 - Tree limbs create a short circuit during a storm, typically resulting in a power outage

The present electric grids use the technology of1970’s. But with the advancement in various concepts of power generation, problems associated with power outages and thefts, and also due to increase in demand, we require a modernized grid to avail all the needs of customers even in the situations of hype, which can be called a “smart grid”.

The smart grid performs various functions such that it increases grid stability, reliability, efficiency and ultimately reduces line losses.

Also the smart grids are designed to allow the two-way processing of electricity from consumers that have distributed generation. Various technologies like sensing and measurement, usage of advanced components are to be used for successful functioning of the grid. In this paper, smart grid, its functions, technologies used in smart grids are discussed.

Introduction to Electric Grid

The electric grid generally refers to all or the smart grid, in a nutshell, is a way to transmit and distribute electricity by electronic means. The electric grid delivers electricity from points of generation to consumers. The electricity delivery network functions via two primary networks: the transmission system and the distribution system. The transmission systems deliver electricity from power plants to distribution substations, while distribution systems deliver electricity from distribution substations to consumers.

The grid also encompasses myriads of local area networks that use distributed energy resources to several loads and/or to meet specific application requirements for remote power, municipal or district power, premium power, and critical loads protection.

Introduction to Smart Grid

Smart grid lacks a standard definition, but enters on the use of advanced of technology to increase the reliability and efficiency of the grid, from transmission to distribution. The Smart Grid is a vision of a better electricity delivery infrastructure.

Smart Grid implementation dramatically increases the quantity, quality, connectivity, automation and Coordination between the suppliers, consumers and networks, and use of data available from advanced sensing, computing, and communications hardware and software.

In addition to being outdated, power plants and transmission lines are aging, meaning they have difficulty handling current electricity needs, while demand may not be reduced any time, but it can still be increasing continuously. One solution could be to add more power lines, but the aging system would still be overwhelmed.

So instead of a quick fix, a more reliable, permanent solution is needed. Perhaps the most fundamental aspect of transitioning to a smarter electricity system is the smart meter.

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Why Modernization of Electric Grid is required?

The major driving forces to modernize current power grids can be divided in four, general categories:

• Increasing reliability, efficiency and safety of the power grid.
• Enabling decentralized power generation so homes can be both an energy client and supplier (provide consumers with interactive tool to manage energy usage).
• Flexibility of power consumption at the client’s side to allow supplier selection (enables distributed generation, solar, wind, and biomass).
• Increase GDP by creating more new, green collar energy jobs related to renewable energy industry manufacturing, plug-in electric vehicles, solar panel, and wind turbine generation, energy conservation and construction.

Smart grid delivery

Smart Grid Functions

The integrated system of the smart grid has two scopes.

One scope is transmission monitoring and reliability and includes the following capabilities:

• Real time monitoring of grid conditions.
• Improved automated diagnosis of grid disturbances, and better aids for the operators who must respond to grid problems.
• Automated responses to grid failure that will isolate disturbed zones and prevent or limit cascading blackouts that can spread over a wide area.
• “Plug and play” ability to connect new generating plants to the grid, reducing the need for the time consuming interconnection studies and physical upgrades.
• The automatic restoration of power would be accomplished by a combination of sensors, computer analysis and advanced substation components, as well as by the ability to reroute power to outage locations.
• Enhancing ability to manage large amounts of solar and wind power.

The second scope is consumer energy management:

• At a minimum, the ability to signal homeowners and businesses that power is expensive and/or tight in supply. This can be done, via special indicators or through web browsers or personal computer software. The expectation is that the customer will respond by reducing its power demand.
• The next level of implementation would allow the utility to automatically reduce the consumer’s electricity consumption when power is expensive or scarce. This would be managed through the link between the smart meters and customer’s equipment or appliances.
• The smart grid system would automatically detect distribution line failures, identify the specific failed equipment, and help determine the optimal plans for dispatching crews to restore service. The smart grid would automatically attempt to isolate failures to prevent local blackouts to spread over that area.
• The smart grid would make it easier to install distributed generation such as rooftop solar panels, and to allow “net metering”, a rate making approach that allows operators of distributed generators to sell surplus power to utilities. The smart grid would also manage the connection of millions of plug-in hybrid electric vehicles into the power system.

Hence the functions of smart grid can be summarized into the following terms as self healing, consumer participation, resist attack, high quality power accommodate generation options, enable electricity markets, optimize assets, enable high penetration of intermittent generation options.

Technology- Initial Focus

Smart Grids rely on information technology advancements across telecommunications and operations. Utilities apply these technologies both to grid operations – transmission and distribution wires and associated equipment and to the customer site-meters, customer owned energy technology equipment and appliances, and home area networks (HANs).

Wires

High temperature superconductor (HTS) wire enables power transmission and distribution cables with three to five times the capacity of conventional underground AC cables and up to ten times the capacity of DC cables. Fault current management capability when using Fault Blocker cable systems.

Wires-focused Smart Grid projects commonly involve:

• One of the components to smart grid would be the replacement of the aging power lines with high-temperature superconducting lines.
• The new wires could be installed underground to avoid cluttering up the already congested cityscapes.
• New telecommunications and operational (sense and control) technologies: These improve delivery performance and resilience.
• New sensor and control technologies. These, when combined with distributed intelligence, make it possible to report and resolve grid issues in real time (self healing).
• Transmission and distribution intelligent electronic devices. These alert operators, automatically respond to problems, and integrate generation from renewable resources.

Sensing and Measurement

Smart Grid - Advanced Metering Infrastructure (AMI)

Core duties are evaluating congestion and grid stability, monitoring equipment health, energy theft prevention, and control strategies support. Technologies include smart meters, sensing systems, advanced switches and cables, digital protective relays etc… In all these, smart meters play a vital role.

In Smart Metering, an Advanced Metering Infrastructure (AMI) of interval meters and two-way communications systems serves as a gateway for utility/customer interaction. Smart Metering has the potential to reduce both customer and utility costs.

If you take a look at your current electricity meter, you will see that it is very mechanical, humming along blindly, waiting to be read by a technician, to determine the amount of electricity used in a given month, at the end of which you receive a bill. A smart meter utilizes what is known as real-time monitoring (RTM). A display lets the consumer know how much electricity is used and even when it is less expensive to use it.

“Studies have shown that when people are made aware of how much power they are using, they reduce their use by about 7%.” A smart grid also prevents the entire system from becoming overloaded, lessening the chance for a power outage.

Advanced Components

Innovations in superconductivity, fault tolerance, storage, power electronics, and diagnostics components are changing fundamental abilities and characteristics of grids.

Technologies within these broad R&D categories include: flexible alternating current transmission system devices, high voltage direct current, first and second generation superconducting wire, high temperature superconducting cable, distributed energy generation and storage devices, composite conductors, and “intelligent” appliances.

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Renewable Energy and the Smart Grid

Renewable Energy and the Smart Grid

The smart grid can be seen as an alternative energy source, certainly a change from the current way of doing things. In addition to rerouting electricity, the smart grid would be able to fill in the gaps of these alternative energy power sources. One way this could be accomplished, surprisingly enough, is with another alternative energy technology – the electric car, specifically, the plug-in electric hybrid (PHEV).

This would work through the concept of energy storage, in the case of the PHEV, specifically referred to as V2G or vehicle to grid. This use of alternative energy sources, like wind and solar reduces the nation’s dependence on foreign oil and helps keep pollution from car exhaust and power plants to a minimum.

Other Technologies

Integrated communications will allow for real-time control, information and data exchange to optimize system reliability, asset utilization, and security.

Conclusion

The major source of energy for human beings is electricity. Without electricity, no technology or science could have been possibly developed. But there are many problems associated with effective functioning of the electric grids which cause a serious loss of power and may even create severe scarcity in future. Also, the latest advancements in generation of electricity from renewable sources also require a means for effective utilization.

So, keeping in view of these, for better performance of the grid, smart grids should be developed all over the world So that we have a more transparent, reliable system that allows consumers to save money and utility companies to more accurately control electricity.

Thus Smart Grid technology paves way for increased utilization of green power.

T & D - 9) Using HVDC Technology For Transmitting Electricity

An alternate means of transmitting electricity is to use high-voltage direct current (HVDC) technology. As the name implies, HVDC uses direct current to transmit power. Direct current facilities are connected to HVAC systems by means of rectifiers, which convert alternating current to direct current, and inverters, which convert direct current to alternating current. Early applications used mercury arc valves for the rectifiers and inverters but, starting in the 1970s, thyristors became the valve type of choice.

Thyristors are controllable semiconductors that can carry very high currents and can block very high voltages. They are connected is series to form a thyristor valve, which allows electricity to flow during the positive half of the alter-nating current voltage cycle but not during the negative half.

Since all three phases of the HVAC system are connected to the valves, the resultant voltage is unidirectional but with some residual oscillation. Smoothing reactors are provided to dampen this oscillation.

HVDC transmission lines can either be single pole or bipolar, although most are bipolar, that is, they use two conductors operating at different polarities such as +/-500 kV.

HVDC submarine cables are either of the solid type with oil-impregnated paper insulation or of the self-contained oil-filled type. New applications also use cables with extruded insulation, cross-linked polyethylene.

Although synchronous HVAC transmission is normally preferred because of its flexibility, historically there have been a number of applications where HVDC technology has advantages:

1 The need to transmit large amounts of power (>500 mW) over very long distances ( >500 km), where the large electrical angle across long HVAC transmission lines (due to their impedances) would result in an unstable system.

Examples of this application are the 1,800 mW Nelson River Project, where the transmission delivers the power to Winnipeg, Canada, approximately 930 km away; the 3,000 mW system from the Three Gorges project to Shanghai in China, approximately 1,000 km distant; and the 1,456 km long, 1,920 mW line from the Cabora Bassa project in Mozambique to Apollo, in South Africa. In the United States the 3,100 mW Pacific HVDC Intertie (PDCI) connects the Pacific Northwest (Celilo Converter Station) with the Los Angeles area (Sylmar Converter Station) by a 1,361 km line.

2 The need to transmit power across long distances of water, where there is no method of providing the intermediate voltage compensation that HVAC requires. An example is the 64 km Moyle interconnector, from Northern Ireland to Scotland.

3 When HVAC interties would not have enough capacity to withstand the electrical swings that would occur between two systems. An example is the ties from Hydro Quebec to the United States.

4 The need to connect two existing systems in an asynchronous manner to prevent losses of a block of generation in one system from causing transmission overloads in the other system if connected with HVAC. An example is the HVDC ties between Texas and the other regional systems.

5 Connection of electrical systems that operate at different frequencies. These applications are referred to as back-to-back ties. An example is HVDC ties between England and France.

6 Provision of isolation from short-circuit contributors from adjacent systems since dc does not transmit short-circuit currents from one system to another.

With the deregulation of the wholesale power market in the United States, there is increasing interest in the use of HVDC technology to facilitate the new markets.

HVDC provides direct control of the power flow and is there-fore a better way for providing contractual transmission services. Some have suggested that dividing the large synchronous areas in the United States into smaller areas interconnected by HVDC will eliminate coordination problems between regions, will provide better local control, and will reduce short-circuit duties, significantly reducing costs.

HVDC PLUS – Maximum power in the smallest space

HVDC PLUS is an advanced and flexible solution for power transmission in fields where space is at a premium. The innovative technical converter concept allows power transmission from remote offshore platforms and wind farms to the onshore grid.

Advantages of HVDC

As the technology has developed, the breakeven distance for HVDC versus HVAC transmission lines has decreased. Some studies indicate a breakeven distance of 60 km using modern HVDC technology.

Some of the advantages identified are:

• No technical limits in transmitted distance; increasing losses provide an economic limit;
• Very fast control of power flow, which allows improvements in system stability;
• The direction of power flow can be changed very quickly (bi-directionality);
• An HVDC link does not increase the short-circuit currents at the connecting points. This means that it will not be necessary to change the circuit breakers in the existing network;
• HVDC can carry more power than HVAC for a given size of conductor;
• The need for ROW is much smaller for HVDC than for HVAC, for the same transmitted power.

Disadvantages of HVDC

The primary disadvantages of HVDC are its higher costs and that it remains a technology that can only be applied in point-to-point applications because of the lack of an economic and reliable HVDC circuit breaker.

The lack of an HVDC circuit breaker reflects the technological problem that a direct current system does not have a point where its voltage is zero as in an alternating current system. An HVAC circuit breaker utilizes this characteristic when it opens an HVAC circuit.