Many countries around the world have started the modernization, upgrade and renovation of electrical power networks with the use of renewable energy, advanced demand side management and distributed generation management, the installation of new meters (called smart–meters), microcontrollers and advanced control methods for the nonlinear loads. The resulting network, with a greater degree of flexibility, better control and more efficiency, is called a “Smart Grid” (others names used are Intelligrid, GridWise, FutureGrid, etc.).
A SmartGrid is composed of smart–networks (transmission and distribution), smart–substations, smart–loads, smart–meters, microgrids, etc. A general SmartGrid topology is given in Figure 1.
Figure 1: A general SmartGrid topology
There are two important parts in the development of SmartGrids, on one side the transmission system (on the other side the distribution system), this includes:
Smart Control Centers.
Smart Transmission Networks.
Smart Substations.
The actual development of smart–control centres, smart–transmission networks and smart–substations is built on existing infrastructure. The main reasons for developing a smart transmission system are that in many countries these systems are working near their operational limits and the available and suitable space for adding new transmission lines has been decreased considerably and consequently it is difficult to acquire additional rights of way.
The actual developments related to SmartGrids are mainly aimed at improving the operation of the distribution system. Since a large amount of renewable energy is being added on the distribution network, it is expected that in the near future, much of the distribution system will be composed of systems not too dissimilar to microgrids.
1. Microgrids
A microgrid is a system formed by distributed energy resources (e.g. microsources), interconnected loads and storage devices, see Figure 1. Microgrids can operate in parallel with the main network or in islanded mode during disturbances on the main network. Different benefits can be obtained from these particular modes of operation, such as to improve power quality to the customer. However, a full microgrid is an expensive option; cheaper versions that use some of its functionality only may be preferred.
2. Power Electronic Interfaces
One of the devices increasingly being integrated into electric power networks are power electronic interfaces such as rectifiers, inverters and the back–to–back (AC/DC/AC) converter. Such devices will be important components in future SmartGrids, since they provide many advantages. However, certain problems also increase with the addition of such interfaces. These advantages and disadvantages are:
Advantages:
Add controllability.
Increase unit’s speed of response.
Can add robustness to the system in combination with local energy storage.
Disadvantages:
Increased Harmonic Pollution.
Can be quite sensitive to systems disturbances.
Limited fault current and overvoltage withstand.
Distribution network impose new requirements on converter control.
3. Smart Loads
The increase of global communication infrastructure and the use of embedded intelligence in power networks will allow adding a certain degree of adaptivity to the load. Smart–Loads will need to be interfaced to the network and these interfaces are more likely to be power electronic converters.
4. Smart Meters
Smart meters will probably become one of the most important devices in the SmartGrids. One reason is that with the increase of distributed generation, electric utilities can sell and buy energy from the customers’ side, and these transactions require correct energy measurements in real–time. Also the control system, which is another important part, requires correct energy measurements in order to handle the system variations. Therefore, high accuracy voltage and current meters will be required.
5. Microsources
Microsources are an important part of future SmartGrids. They can give energy support to a particular part of the system. A principal problem of such sources is energy storage, since they do not have the ability to storage energy in their structure. However, there are energy storage devices which contribute to the efficient use of the energy provided by microsources. Many types of microsources have been developed in order to use clean energy and reduce the pollution emissions. Basically, there are three types of microsources; DC sources, high frequency AC sources and low frequency AC sources, these are listed below:
DC Microsources:
Fuel Cells
Photovoltaic Cells
Battery Storage
High Frequency AC microsources:
Micro – turbines
Low Frequency AC Sources:
Wind turbines
Micro hydro turbines
6. Benefits of a SmartGrids
Achieving a complete system that can be called a SmartGrid would offer capability to:
Reduce load peak demand using advanced demand side management.
Integrate a great deal of clean energy sources and reduce pollution emission.
Improve reliability and power quality to both utilities and customers.
Improve overall efficiency of the system in all its aspects.
7. Potential Problems in SmartGrids
There are many problems in electric power networks, and some of them are potentially to be inherited by SmartGrids. These problems will potentially be:
Power balance
Proper voltage profiles in the network nodes.
Time response of protection to faults
Also, the increased use of power electronic interfaces is likely to increase the harmonic pollution in the system.
8. Potential Solutions in SmartGrids
a) Advanced Demand Side Management
At present, demand side management only switches loads in and out: a crude level of control insufficient for the very high levels of control required in ‘SmartGrids’. Since the capacity of transmission networks in many countries is reaching their limits, advanced demand side management will be required soon in the network operation. Advanced demand side management, in addition to the use of smart loads, also promotes the use of distributed generation in order that local generation can be used immediately when it is available to give support to local loads and avoid the stress of the transmission network. Advanced demand side management involves many aspects, such energy efficiency; the exchanging of old incandescent light bulbs with compact fluorescents lights (CFLs), the reconfiguration of misconfigured controls, etc.
b) Active Filters for Harmonic Pollution
Since motor loads together with power electronic interfaces are no longer a linear load, a great deal of harmonic pollution is drawn from the network. Such a situation demands a proper harmonic filter for the load as well as for the network. This might be deal with the use of active filters.
c) Storage Energy Devices
In the case of microsources it is possible to use batteries or supercapacitors for every microsource connected to the DC node or the direct connection of AC storage devices such as flywheels.
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