Utilities today devote considerable attention to managing voltage levels and reactive power (VAR) throughout the power transmission and distribution systems. Loads that contain capacitors and inductors — such as electric motors, pool pumps, and the power supplies in modern electronics — put additional strain on the grid, as the reactive portion of these loads causes them to draw more current than an otherwise comparable resistive load (such as a light bulb) would draw for the same amount of kilowatts transferred. This extra current causes the transformer to heat up, which wastes energy and reduces service lifetime. Uncorrected reactive power makes it harder to stabilize grid voltage. It also drives additional cost throughout the entire grid since everything — wires, transformers, and generators — must be sized to carry the total current.
The more reactive power flowing on a line, the less “room” there is for real power, and the less efficient the transmission and/or distribution system will be.
Improving the efficiency of power transmission and distribution comes down to two choices: you can reduce the resistance of the wires by making them larger or using better materials (not a practical solution), or you can improve the effectiveness of the flow of electricity. To address the latter, it’s important to understand one technical concept and that is the difference between active and reactive power.
In the last 20 years, this problem has attracted the interest from both academia and industry and this has produced many special devices and algorithms. This paper first introduces reactive power sources and secondly the role of smart grid to control and improve the efficiency of power flows.
REACTIVE POWER SOURCES
The controllable reactive power sources include generators, shunt reactors, shunt capacitors and On Load Tap Changers of transformers (OLTC).
To optimize the movement of electric energy along power line, we would ideally like to eliminate reactive power flows, or at least minimize them. Utilities do this on their local distribution systems using devices such as capacitor banks or special transformers, typically located at substations or on feeders. These devices work to keep reactive power flows down, making the full capacity of the conductor available for the real power.
Generators:can generate or absorb reactive power depending on the excitation. When over-excited they supply the reactive power, and when under-excited they absorb reactive power. The automatic voltage regulators of generators can continually adjust the excitation.
Reactors, shunt capacitors and OLTC:are traditionally switched on/off through circuit breakers on command from the operator. Since the early eighties, advances in Flexible AC Transmission Systems (FACTS) controllers in power systems have led to their application to improve voltage profiles of power networks. The most frequently used devices are: Reactive Power Controller (RPC) and Static Var Compensator (SVC).
The RPC connects or disconnects capacitor stages automatically by detecting the phase divergence between the fundamentals of current and voltage. The measured divergence is compared with several segmental set phase divergence regions, capacitor contactors will be switched on or off according to it.
Compared with RPC, the SVC is more advanced electronics equipment. It can provide continuous capacitive and inductive reactive supply to the power system. The SVC typically consists of a Thyristor Controlled Reactor (TCR), a Thyristor Switched Capacitor (TSC) and AC Filters (ACF). From the viewpoint of power system operation, an SVC is equivalent to a controllable reactor and a fixed capacitor. Its output can vary depending on the level of generation and absorption of reactive power so as to maintain its terminal voltage at a certain level.
In both the techniques described above, the volt/VAr control devices have operated autonomously, independent of one another and with-out centralized coordination. This approach worked, but it left a good deal of efficiency on the table since actions taken by one device might have less-than-optimal results for another location on the grid or for the system as a whole. Hence modern and intelligent approach to Volt/Var control is needed to optimise the grid performance, reduce losses and enhance consumer satisfaction. This can be achieved through Volt/Var automation.
Advances in automation and communications have laid the foundation to make centralized, coordinated volt/VAr control possible and the emergence of Volt/VAr automation using today’s faster computers lead to volt/VAr optimization. This is one of the functions of intelligent/smart grid.
As it is known, is an advanced application that runs periodically or in response to operator demand at the utility control centre or in substation automation systems. Combined with two-way communication infrastructure and remote control capability for capacitor banks and voltage regulating transformers. The capacitor banks are equipped with sensors that automatically trigger the need for reactive power compensation in case of any loss, by sending signal to the grid or substation automation system. The real time information makes it possible to optimize the energy delivery efficiency on distribution systems.
The Role of Smart Grid
The “smart grid” is based on the usage of smart energy technologies—the application of power control by means of digital information systems (smart meters and smart appliances) that communicate through the advanced communication technology (i.e. internet) with the electricity utility—to optimize electrical power system generation, delivery, and end-use energy demands.
Smart Grid will integrate all the components of power system to enhance the performance of the grid. Much of the integration of components relates to communication systems, IT systems, and business processes. And to achieve this, Smart grid, real time data and active grid management, requires fast and two-way digital communication.
Electric utilities use a wide variety of telecommunications including:
• Wired and wireless telephone
• Voice and data dispatch radio
• Fiber optics
• Power line carrier
• The internet
_ Hybrid fiber cable (HFC)
_ Digital subscriber line (DSL)
_ Broad band over power lines (BPL)
_ Wireless (wi-fi and wi-max), and
The real breakthrough here is in the communication speed and quality of the computation since the smart grid network uses advanced algorithms to identify the optimal operation strategy from millions, or even billions of possibilities, arriving at a result fast enough to apply it in real-time utility networks.
Smart grid plays a vital role in the utilities quest to deal with reactive power control in a smarter, faster and efficient manner. This result improved grid efficiency, reduces the amount of power that must be generated, hence dealt with the reactive power effect on the grid, it also improves power factor and can result in substantial savings in cost of energy and infrastructure utilizationand with it the emissions of CO2 and other pollutants associated with power generation can be minimise significantly.