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Reactive Power Control In Electric Systems MILLER TIMOTHY J. E.. A unified compensation and the electric arc furnaceharmonicsreactive power coordinationselected bibliographyindex. Download ebook PDF download. Reactive Power. Reactive Power Control in Electric Systems by T. J. E. Miller, , available at Book Depository with free delivery worldwide. Reactive Power Control in Electric Systems [Timothy J. E. Miller] on medical-site.info *FREE* shipping on qualifying offers. A unified approach to the fundamental.
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The proper selection and coordination of equipment for controlling reactive power and voltage stability are among the major challenges of power system engineering .
These challenges gave birth to some selected devices to control or compensate reactive power. In recent decades there has been significant progress in terms of equipment designed to improve the stability of voltage in power systems. This is mainly due to the development of power supply systems in the world, which requires seeking better ways of adjusting and controlling power flows and voltage levels. Almost all power transported or consumed in alternating current AC networks, supply or consume two of powers: real power and reactive power.
Real power accomplishes useful work while reactive power supports the voltage that must be controlled for system reliability. Reactive power is essential to move active power through the transmission and distribution system to the customer. For AC systems voltage and current www. Although AC voltage and current pulsate at same frequency, they peak at different time power is the algebraic product of voltage and current. Real power is the average of power over cycle and measured by volt-amperes or watt.
The portion of power with zero average value called reactive power measured in volt-amperes reactive or vars. Transmission connected generators are generally required to support reactive power flow. For example, Transmission system generators are required by the Grid Code Requirements to supply their rated power between the limits of 0.
The system operator will perform switching actions to maintain a secure and economical voltage profile while maintaining a reactive power balance equation: 1. By making decisive switching actions in the early morning before the demand increases, the system gain can be maximized early on, helping to secure the system for the whole day. To balance the equation some pre-fault reactive generator use will be required. Other sources of reactive power that will also be used include shunt capacitors, shunt reactors, Static VAR Compensators and voltage control circuits 1.
A convention is also adopted stating that the reactive energy should be positive when the current is leading the voltage inductive load. In an electrical system containing purely sinusoidal voltage and current waveforms at a fixed frequency, the measurement of reactive power is easy and can be accomplished using several methods without errors.
However, in the presence of non-sinusoidal waveforms, the energy contained in the harmonics causes measurement errors.
According to the Fourier theorem any periodic waveform can be written as a sum of sin and cosine waves.
As energy meters deal with periodic signals at the line frequency both current and voltage inputs of a single phase meter can be described by: …………………………………………………… 3 ……………………………… Active power the average active power is defined as: …………………….. Apparent power The apparent power is the maximum real power that can be delivered to a load.
In general terms, decreasing reactive power causing voltage to fall while increasing it causing voltage to rise. A voltage collapse occurs when the system try to serve much more load than the voltage can support. When reactive power supply lower voltage, as voltage drops current must increase to maintain power supplied, causing system to consume more reactive power and the voltage drops further. If the current increases too much, transmission lines go off line, overloading other lines and potentially causing cascading failures.
If the voltage drops too low, some generators will disconnect automatically to protect themselves. Voltage collapse occurs when an increase in load or less generation or transmission facilities causes dropping voltage, which causes a further reduction in reactive power from capacitor and line charging, and still there further voltage reductions.
If voltage reduction continues, these will cause additional elements to trip, leading further reduction in voltage and loss of the load. The result in these entire progressive and uncontrollable declines in voltage is that the system unable to provide the reactive power required supplying the reactive power demands. Since the current flowing through your electrical system is higher than that necessary to do the required work, excess power dissipates in the form of heat as the reactive current flows through resistive components like wires, switches and transformers.
Keep in mind that whenever energy is expended, you pay. It makes no difference whether the energy is expended in the form of heat or useful work. We can determine how much reactive power your electrical devices use by measuring their power factor, the ratio between real power and true power.
A power factor of 1 i. Newer homes with the latest in energy efficient appliances can have an overall power factor in the nineties. The typical residential power meter only reads real power, i.
To begin with, electric companies correct for power factor around industrial complexes, or they will request the offending customer to do so at his expense, or they will charge more for reactive power. Clearly electric companies benefit from power factor correction, since transmission lines carrying the additional reactive current to heavily industrialized areas costs them money.
Most importantly, you pay for reactive power in the form of energy losses created by the reactive current flowing in your home. These losses are in the form of heat and cannot be returned to the grid. Hence you pay. The fewer kilowatts expended in the home, whether from heat dissipation or not, the lower the electric bill. Since power factor correction reduces the energy losses, you save. As stated earlier, electric companies correct for power factor around industrial complexes, or they will request the offending customer to do so, or they will charge for reactive power.
However, it is true that power factor correction assists the electric company by reducing demand for electricity, thereby allowing them to satisfy service needs elsewhere. But who cares? Power factor correction lowers your electric bill by reducing the number of kilowatts expended, and without it your electric bill will be very exorbitant. In the most cases, PFC is used for economic reasons. Using compensating device, one can save on electricity bill as well as keep certain grid parameters determined by the energy provider.
Power factor correction gives even more profits, than only savings. PFC also allows to decrease transmission losses and limits voltage drops. Generally, reliability of the network gets better.
Different voltage and reactive power control methods have been proposed. Properly locating and sizing shunt capacitors will decrease power losses. As an improvement to the capacitor planning based on the load size, methods to include customer load profiles and characteristics in the capacitor planning are proposed in -. Proper capacitor planning will also improve the voltage profile in the distribution system.
The capacitor locating and sizing is studied and executed in the planning stage of the distribution system. In order to enhance the distribution system further, the capacitor should also be switched properly in the operation stage of the distribution system , using different types of available capacitor control. Most recently, many researchers have addressed the problem of voltage and reactive power control in distribution systems by focusing on automated distribution systems, such as in -.
At the moment, the voltage and reactive power control based on automated distribution systems can be divided into two categories: off-line setting control and real time control . The off-line setting control, - for instance, aims to find a dispatch schedule for the capacitor switching and the OLTC movement based on a one day ahead load forecast.
Meanwhile, the real time control, - for instance, aims to control the capacitor and OLTC based on real time measurements and experiences. The application of dispatch schedule based load forecasting is motivated by the fact that although there is a random fluctuation in the load variation, the major component of the load variations is related to weather conditions.
Furthermore, there is a deterministic load pattern during the day due to social activities . Therefore, the load profile is quite predictable. Different objective functions and operating constraints have been proposed in voltage and reactive power control with automated distribution systems.
Nevertheless, all researchers - still consider loss Minimization and keeping the voltage within the acceptable range as the main objective and constraint in the voltage and reactive power control. Another objective that is commonly proposed is flattering the voltage profile , . Commonly added operating constraints include the maximum number of OLTC operations and capacitor switchings , -, and the minimum distribution system of .
Other references, such as , consider minimization of OLTC operations and capacitor switching as the objective function. The automated control with off-line setting proposed in - fully replaces the local control operation of the conventional OLTC and capacitor operations with a remotely controlled operation.
The main obstacle application of this method is its dependency on communication links and remote control to all capacitors.
However, many DNOs do not have communication links downstream to the feeder capacitor locations 2. This objective is formulated as an optimization problem that is solved off-line, based on nominal load patterns.
The optimization variables are the locations, sizes and control dead bands of capacitors and tap changer voltage regulators.
Tap changers are normally automatically controlled by a relay controller that measures and regulates the secondary side voltage of the transformer. The control of transformers operating in parallel in the same substation must be coordinated to minimize circulating reactive power flows Lakervi and Holmes, In fact, the control of voltage is a major issue in power system operation.
Kundur It identifies the main objectives of voltage control as: Voltage at the terminals of all equipment in the system should be kept within acceptable limits, to avoid malfunction of and damage to the equipment.
Keeping voltages close to the values for which stabilizing controls are designed, to enhance system stability and allow maximal utilization of the transmission system. Whereas distribution systems as a rule are operated in radial configuration. Consequently, more sophisticated control schemes than those used in distribution systems are necessary. The control of voltage is often divided into the normal, preventive and emergency state control.
A brief overview of the strategies used in the different operating states follows:- Primary Control Primary voltage controllers are used in all power systems to keep the terminal voltages of the generators close to reference values given by the operator or generated by a secondary controller. An automatic voltage regulator AVR acts on the exciter of a synchronous machine, which supplies the field voltage and consequently the current in the field winding of the machine and can thereby regulate its terminal voltage.
The response time of the primary controller is short, typically fractions of a second for generators with modern excitation systems. Furthermore, many generators use a so-called power system stabilizer PSS to modulate the terminal voltage of the machine based on a local frequency measurement to contribute to damping of electromechanical oscillations.
Although the power system stabilizer in most cases is integrated in the AVR, it only introduces fast oscillations around the mean value given by the AVR as long as the generator remains synchronized with the rest of the network. Secondary Control Secondary voltage control acts on a time-scale of seconds to a minute and within regions of the network. The aim of secondary voltage control is to keep an appropriate voltage profile in a region of the system and to minimize circulating reactive power flows and maximize reactive reserves.
Normally, the network is divided in a number of geographic regions. One or a few so-called pilot nodes, which are assumed to be representative of the voltage situation in the region, are selected for voltage regulation by the secondary controller. The main actuators are the set point voltages of the primary controllers of the generators within a region, although the French implementation also uses static compensation devices such as capacitor banks and reactors.
The set point values are calculated by an optimization procedure using a linearized static network model to make each generator in the region contribute to the control of the pilot node voltage s. Tertiary Control Tertiary voltage control acts system-wide on a time scale of about ten to thirty minutes.
The traditional method of tertiary control is so-called reactive power optimal power flow OPF Carpentier, , Dommel and Tinney, The desired operating conditions are specified in the form of a cost function, which is minimized using nonlinear optimization techniques.
Usually, the main goal is to minimize losses and to keep voltages close to rated values. The reduction in the individual voltage harmonics is clearly visible when compared to the scenarios where plain PFC were installed on the 11 kV network shown in Figure 8. The sizing of the switched PFC filter banks makes provision for future network expansions and ramp loading to ensure that the PFC is effective for a wide range of plant loading.
It is proposed that the switched PFC filter banks tuned with 4. The proposed bank consists of four stages: 1x1.
The proposed bank consists of three stages: 1x1. The proposed bank consists of two stages: 1x1. Conclusions This research consisted of a study of power factor correction PFC , load requirements, load flow, fault analyses, harmonic frequency scans and harmonic voltage distortion analyses. The introduction of plain PFCs on the substations resulted in severe parallel resonant peaks that were well above the 2.
However, the installation of switched PFC filter banks tuned with fifth harmonic order resulted in a viable solution. The introduction of the switched PFC banks tuned with fifth order resonant circuit harmonic filters decreased the harmonic impedance peaks, consequently reducing the harmonic voltage distortion effects below the NRS compatibility levels.
Therefore, switched PFC filter banks tuned with fifth harmonic order are the preferred solution. The installation of switched PFC filter banks tuned at the fifth harmonic order resulted in a viable solution with no harmonic voltage distortion violations present in the system. The systems overall power factor was improved to above 0.
Future work to further investigation includes the following. References  Acha, E.
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