T & D - 42) The Structure Of Power System

An interconnected power system is a complex enterprise that may be subdivided into the following major subsystems:

• Generation Subsystem
• Transmission and Subtransmission Subsystem
• Distribution Subsystem
• Utilization Subsystem

Generation Subsystem

Generation subsystem includes generators and transformers.

Generators

Three-phase ac generator from around 1895

An essential component of power systems is the three-phase ac generator known as synchronous generator or alternator. Synchronous generators have two synchronously rotating fields: One field is produced by the rotor driven at synchronous speed and excited by dc current. The other field is produced in the stator windings by the three-phase armature currents.

The dc current for the rotor windings is provided by excitation systems. In the older units, the exciters are dc generators mounted on the same shaft, providing excitation through slip rings. Current systems use ac generators with rotating rectifiers, known as brushless excitation systems. The excitation system maintains generator voltage and controls the reactive power flow. Because they lack the commutator, ac generators can generate high power at high voltage, typically 30 kV.

The source of the mechanical power, commonly known as the prime mover, may be hydraulic turbines, steam turbines whose energy comes from the burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal combustion engines burning oil.

A steam turbine used to provide electric power

Steam turbines operate at relatively high speeds of3600 or 1800 rpm. The generators to which they are coupled are cylindrical rotor, two-pole for 3600 rpm, or four-pole for 1800 rpm operation. Hydraulic turbines, particularly those operating with a low pressure, operate at low speed. Their generators are usually a salient type rotor with many poles. In a power station, several generators are operated in parallel in the power grid to provide the total power needed. They are connected at a common point called a bus.

With concerns for the environment and conservation of fossil fuels, many alternate sources are considered for employing the untapped energy sources of the sun and the earth for generation of power. Some alternate sources used are solar power, geothermal power, wind power, tidal power, and biomass.

The motivation for bulk generation of power in the future is the nuclear fusion. If nuclear fusion is harnessed economically, it would provide clean energy from an abundant source of fuel, namely water.

Transformers

High voltage transformer 40MVA

The transformer transfers power with very high efficiency from one level of voltage to another level. The power transferred to the secondary is almost the same as the primary, except for losses in the transformer.

Using a step-up transformer will reduce losses in the line, which makes the transmission of power over long distances possible.

Insulation requirements and other practical design problems limit the generated voltage to low values, usually 30 kV. Thus, step-up transformers are used for transmission of power. At the receiving end of the transmission lines step-down transformers are used to reduce the voltage to suitable values for distribution or utilization.

The electricity in an electric power system may undergo four or five transformations between generator and consumers.

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Transmission and Subtransmission Subsystem

An overhead transmission network transfers electric power from generating units to the distribution system which ultimately supplies the load.

Transmission lines also interconnect neighboring utilities which allow the economic dispatch of power within regions during normal conditions, and the transfer of power between regions during emergencies.

Standard transmission voltages are established in the United States by theAmerican National Standards Institute (ANSI). Transmission voltage lines operating at more than 60 kV are standardized at 69 kV, 115 kV, 138 kV, 161 kV, 230 kV, 345 kV, 500 kV, and 765 kV line-to-line.

Transmission voltages above 230 kV are usually referred to as extra-high voltage (EHV).

High voltage transmission lines are terminated in substations, which are called high-voltage substations, receiving substations, or primary substations.

The function of some substations is switching circuits in and out of service; they are referred to as switching stations. At the primary substations, the voltage is stepped down to a value more suitable for the next part of the trip toward the load. Very large industrial customers may be served from the transmission system.

The portion of the transmission system that connects the high-voltage substations through step-down transformers to the distribution substations is called the subtransmission network. There is no clear distinction between transmission and subtransmission voltage levels.

Typically, the subtransmission voltage level ranges from 69 to 138 kV. Some large industrial customers may be served from the subtransmission system. Capacitor banks and reactor banks are usually installed in the substations for maintaining the transmission line voltage.

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Distribution Subsystem

The distribution system connects the distribution substations to the consumers’ service-entrance equipment. The primary distribution lines from 4 to 34.5 kV and supply the load in a well-defined geographical area.

Some small industrial customers are served directly by the primary feeders. The secondary distribution network reduces the voltage for utilization by commercial and residential consumers. Lines and cables not exceeding a few hundred feet in length then deliver power to the individual consumers.

The secondary distribution serves most of the customers at levels of 240/120 V, single-phase, three-wire; 208Y/120 V, three-phase, four-wire; or 480Y/277 V, three-phase, four-wire. The power for a typical home is derived from a transformer that reduces the primary feeder voltage to 240/120 V using a three-wire line.

Distribution systems are both overhead and underground. The growth of underground distribution has been extremely rapid and as much as 70 percent of new residential construction is via underground systems.

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Load Subsystems

Power systems loads are divided into industrial, commercial, and residential.

Heavy-Duty Single-Phase Capacitor Start And Run Induction Motor

Industrial loads are composite loads, and induction motorsform a high proportion of these loads. These composite loads are functions of voltage and frequency and form a major part of the system load.

Commercial and residential loads consist largely of lighting,heating, and cooking. These loads are independent of frequency and consume negligibly small reactive power. The load varies throughout the day, and power must be available to consumers on demand.

The daily-load curve of a utility is a composite of demands made by various classes of users.

The greatest value of load during a 24-hr period is called the peak or maximum demand. To assess the usefulness of the generating plant the load factoris defined. The load factor is the ratio of average load over a designated period of time to the peak load occurring in that period. Load factors may be given for a day, a month, or a year.

The yearly, or annual load factor is the most useful since a year represents a full cycle of time.

The daily load factor is:

Daily L.F. = average load / peak load            (1.1)

Multiplying the numerator and denominator of (1.1) by a time period of 24 hr, we obtain:

Daily L.F. = average load x 24 hr / peak load

Daily L.F. = energy consumed during 24 hr / peak load x 24 hr          (1.2)

The annual load factor is:

Annual L. F. =  total annual energy / peak load x 8760 hr          (1.3)

Generally there is diversity in the peak load between different classes of loads, which improves the overall system load factor.

In order for a power plant to operate economically, it must have a high system load factor. Today’s typical system load factors are in the range of 55 to 70 percent. Load-forecasting at all levels is an important function in the operation, operational planning, and planning of an electric power system. Other devices and systems are required for the satisfactory operation and protection of a power system.

Some of the protective devices directly connected to the circuits are called switchgear. They include instrument transformers, circuit breakers, disconnect switches, fuses and lightning arresters. These devices are necessary to deenergize either for normal operation or on the occurrence of faults.

The associated control equipment and protective relays are placed on switchboards in control houses.

Reference: Electrical Energy Systems by Mohamed E. El-Hawary (Dalhousie University)

T & D - 40) Delayed Auto-Reclosing On EHV Systems

Figure 1 - Delayed auto-reclose scheme logic

On highly interconnected transmission systems, where the loss of a single line is unlikely to cause two sections of the system to drift apart significantly and lose synchronism, delayed auto-reclosing can be employed.

Dead times of the order of 5s-60s are commonly used.

No problems are presented by fault arc de-ionisation times and circuit breaker operating characteristics, and power swings on the system decay before reclosing. In addition, all tripping and reclose schemes can be three-phase only, simplifying control circuits in comparison with single-phase schemes.

In systems on which delayed auto-reclosing is permissible, the chances of a reclosure being successful are somewhat greater with delayed reclosing than would be the case with high-speed reclosing.

Scheme Operation

The sequence of operations of a delayed auto-reclose scheme can be best understood by reference to Figure 1. This shows a transmission line connecting two substations A and B, with the circuit beakers at A and B tripping out in the event of a line fault.

Synchronism is unlikely to be lost in a system that employs delayed auto-reclose.

However, the transfer of power through the remaining tie-lines on the system could result in the development of an excessive phase difference between the voltages at points A and B. The result, if reclosure takes place, is an unacceptable shock to the system.

It is therefore usual practice to incorporate a synchronism check relay into the reclosing system to determine whether auto-reclosing should take place.

After tripping on a fault, it is normal procedure to reclose the breaker at one end first, a process known as ‘live bus/dead line charging’. Reclosing at the other and is then under the control of a synchronism check relay element for what is known as ‘live bus/live line reclosing’.

Example

For example, if it were decided to charge the line initially from station A, the dead time in the auto-reclose relay at A would be set at, say, 5 seconds, while the corresponding timer in the auto-reclose relay at B would be set at, say, 15 seconds. The circuit beaker at A would then reclose after 5 seconds provided that voltage monitoring relays at Aindicated that the busbars were alive and the line dead.

With the line recharged, the circuit breaker at B would then reclose with a synchronism check, after a 2 second delay imposed by the synchronism check relay element.

If for any reason the line fails to ‘dead line charge’ from end A, reclosure from end Bwould take place after 15 seconds. The circuit breaker at A would then be given the opportunity to reclose with a synchronism check.

Synchronism Check Relays

The synchronism check relay element commonly provides a three-fold check:

1. Phase angle difference
2. Voltage
3. Frequency difference

The phase angle setting is usually set to between 20° - 45°, and reclosure is inhibited if the phase difference exceeds this value. The scheme waits for a reclosing opportunity with the phase angle within the set value, but locks out if reclosure does not occur within a defined period, typically 5s.

A voltage check is incorporated to prevent reclosure under various circumstances. A number of different modes may be available. These are typically undervoltage on either of the two measured voltages, differential voltage, or both of these conditions.

The logic also incorporates a frequency difference check, either by direct measurement or by using a timer in conjunction with the phase angle check. In the latter case, if a 2 second timer is employed, the logic gives an output only if the phase difference does not exceed the phase angle setting over a period of 2 seconds. This limits the frequency difference (in the case of a phase angle setting of 20°) to a maximum of 0.11% of 50Hz, corresponding to a phase swing from +20° to -20° over the measured 2 seconds.

While a significant frequency difference is unlikely to arise during a delayed auto-reclose sequence, the time available allows this check to be carried out as an additional safeguard.

As well as ‘live bus/dead line’ and ‘live bus/live line’ reclosing, sometimes ‘live line/dead bus’ reclosing may need to be implemented. A numerical relay will typically allow any combination of these modes to be implemented. The voltage settings for distinguishing between ‘live’ and ‘dead’ conditions must be carefully chosen.

In addition, the locations of the VT’s must be known and checked so that the correct voltage signals are connected to the ‘line’ and ‘bus’ inputs.

Reference: Network Protection & Automation – Areva

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.