Voltage Fluctuations
The flickering light sources has a problem since the beginning of the systems of distribution. But with increasing number of customers and installed capacity has grown rapidly flashing the problem ..
Voltage fluctuations in power systems, a variety of technical adverse events causing disruption of production processes and high costs. flicker However, with adverse physiological, may affect the safety of workers and productivity. Man can cause interference of light caused by voltage fluctuations in flash. In general, you can significantly affect the flashing vision and discomfort and fatigue. The physiology of this phenomenon is complex. Overall, the flickering effect our vision and implementation process of the brain, often causes discomfort and affects the quality of work. In some situations it may even lead to accidents, because it affects the ergonomics of the production environment, so that the operator fatigue and reduced concentration.
What is flashing?
Basically varies the impression of instability of visual perception by a light source whose luminance or spectral distribution induced over time. Usually applied to the cyclical variation in light intensity due to voltage fluctuations, which in turn can be caused by changes in power generation, transmission and distribution. However, the flicker often caused by the use of rapid fluctuations much power and reactive phenomenon demand.The flickering light sources has been a problem since the beginning of the systems of distribution. But with increasing number of customers and installed capacity flicker problem has grown rapidly. Consider changing the tension as the cause of the fire, driving fluctuations, mitigation methods and rules in relation to the amount of flicker.
The causes of voltage fluctuations.
The classification of RMS voltage variations shown. 1 as a conspiracy against the voltage of the duration of the disorder. The shaded area corresponds to voltage changes discussed in this article on the blink. In theory, all power, the voltage at the load end is different from the source. We demonstrate this with an equivalent circuit per phase where E is the supply voltage, the RS is the equivalent line resistance, XS equivalent line reactance is, the journal equivalent impedance of the network, I / O stream and UO is the voltage at the load. Depending on the cause of the change in voltage may take the form of a voltage drop that a constant value in a time interval, a rapid change in voltage, or slow, or change in voltage. Voltage fluctuations is defined as a set of variations in RMS voltage or a cyclical variation of the voltage curve envelope.
The defining characteristics of voltage fluctuations are:
• The measurement of voltage changes (difference between the maximum and minimum effective or peak value of voltage that occur during the event);
• The number of voltage fluctuations on a time unit, and
• Indirect effects (such as blinking) changes associated with volt-age diseases.
Until recently, the voltage fluctuations in electrical systems and freight terminals, also constrained by the RMS summit is influenced in a voltage change in the electrical system peak. Fluctuations in supply voltage, its power spectrum (and the power spectrum of voltage fluctuations), and their duration is taken into account in the assessment of voltage fluctuations taken. Currently, the basic parameters that determine the voltage flicker short-term fluctuations, as an index of DVT, the severity of the flicker-and long-term as the index of LTP. These parameters relate to the impact of voltage fluctuations in the illumination and its influence on people.
Sources of voltage fluctuations.
The main cause of voltage fluctuations is the variability of the temporal component of the reactive power load size. The charges include electric arc furnaces, rolling mill drives the primary winding and all those responsible at a high rate of change of power in relation to the ability to short circuit at the PCC (PCC). Small loads of energy, such as starting of induction motors, welders, ovens, electric controls, electric saws and hammers, pumps and compressors, cranes, elevators and may also be sources of fibrillation.
Other causes are changing transformers and capacitors, selector switches, which can change the inductive component of the source impedance. Changes in production capacity of wind turbines can have an effect as well. Sometimes the voltage fluctuations caused by the tension between the low-frequency harmonics.
Reduction of voltage fluctuations in the power grid.
The effects of voltage fluctuations depend for the first time in their scope, by the properties of the electrical system, and secondly, the frequency of occurrence, which is determined by the type of load and affected the character of his operation. In general, to focus on mitigation measures to limit the amplitude of voltage fluctuations. The technological process is influenced rare.
Examples of mitigation measures for different types of devices:
EAF:
The integration of motors of the series (or variable-saturation), the smooth functioning of the control system of electrodes to isolate and to provide temporary heating load.
Welding Plant:
Providing a means for connect processors, welders a single phase to phase 3 network load balancing to distribute between the phases, phase supply welding machines connected to different phases of the lighting technology.
• Frequency
• Use soft-start devices.
• Another way to reduce the amplitude of voltage fluctuations, increase the power of the short circuit to the load power in the PAC, in which the consumer varies.
• This can be done by:
• Connecting the load to a higher level of nominal voltage;
• The supply of this category of expense lines dedication cated;
• Separation of power fluctuations in the fixed cost burden, with separate windings of a transformer with three windings;
• The increase of the nominal value of the transformer load or fluctuating
• The installation of capacitors in series.
Solutions for voltage stabilization.
Another way to reduce the amplitude of the voltage fluctuations is to reduce the change of power system reactive. You can do this through the installation of surge protection dynamic. Its effectiveness depends mainly on their rated power and speed of response.
The call of the reactive power at the fundamental frequency and dynamic voltage stabilizers to produce voltage drops in the impedances of the network. After leading or lagging reactive power, the effective voltage can be increased or reduced at the PCC. Figure 3 shows the ranking of the alternatives on the dynamics of voltage stabilizers. There are three main phases of the nominal power systems designed to stabilize the voltage to the essentials of a distribution system or a load or a load in a CCR.
Synchronous machines.
These are the traditional sources of fundamental reactive power (lag or lead) which is supplied continuously. It can also be the source of mechanical energy in the use of an equalizer and a motor. The use of a synchronous machine excitation current control is meaningless, because the standard to reach threshold of voltage changes, the machine has a capacity several times the power of the required load stabilization. This fact and the necessary process parameters needed for dynamic stabilization of the synchronous machine in a control system with excitation control circuit voltage of power quickly, as used in Fig. 4. This solution enables a fast rise time of the reactive power of the machine.
static compensators.
These devices (non-STATCOM) used inductive or capacitive passive components are selected, the control phase or in combination with controlled core saturation. Enter the required stabilization of the reactive current or separate steps or, more often, continuously variable. static compensators than the best solution for improving power quality, both technical and economic.
Saturable reactor compensation.
These devices use magnetic circuit saturation voltage stabilization. Two of these solutions have found wide practical application: the self-organization saturable reactor (SR) and the reactor control circuit CC. SR was one of the first application of SVC on an industrial scale to mitigate the effects of voltage fluctuations. They are designed so that the range of the minimum voltage, the core just below the saturation magnetization and the current flows, similar to a load transformer. In this state, which has practically no influence on the level of tension. At rated voltage, the reactor is saturated, so a small change in the impact of the power supply to a significant change of course. The trim is usually connected to the network without transformers.
Reactors with control loops are often parallel with a bank of capacitors, a filter for higher harmonics is operated. Basically it works like a transformer that controls adjusts the DC magnetizing current, the size of the majority. Continuous monitoring of the liquidation is usually full by a thyristor converter powered not exceeding 1% of the rated power stabilizer. This solution enables the forces of the transient current, a faster system performance. By adjusting the magnetizing current, reactor primary current changes from practically zero (unsaturated core) the maximum (saturated core) over the entire range of current variations.
A major disadvantage of this solution is the generation of harmonics. In version 3 phases, allowing a greater number of slots and the appropriate participation of many high-order harmonic coil currents are virtually eliminated, but at the expense of the slow response of the system. The use of stabilizers in three phases allows the correction of the imbalance.
Thyristor switched capacitor (TSC).
In this solution, cut capacitor between phase and phase to each switch section (on or off) with thyristor AC switch. Therefore, change the values ??of the susceptance equivalent compensatory rest periods in a discreet manner, depending on the number of active sections. By providing a sufficient number of small sections, the resolution requires a change in the susceptance a step can be achieved. Switching Synchronization and first prevent bias associated overcurrent and overvoltage capacitor normally with the change of the capacitors. The reaction time for balanced operation no longer than 20 milliseconds.
FC / TCR compensator.
This solution is an example of indirect compensation. Depending on the desired function (voltage control and reactive power compensation), the value of the sum is controlled by the two components of current. For example, to control the fundamental current of the capacitor, the capacitor acts like a filter capacitor stage or strain (TCR / TSC). To control the fundamental harmonic power reactor is used a thyristor AC switch.
commutated voltage source converter reactive power and energy.
The compensator includes a voltage converter (VSC). The switching of semiconductor devices (pulse width modulation) to determine the value and nature of the reactive power (inductive or capacitive). The compensator is used for STATCOM, which is a new generation of static compensators for switching semiconductor devices. Static Synchronous Compensator Its name derives from the principle of the operation, which is analogous to the operation of the synchronous compensator. The basic element of the scale is an AC / DC converter, with the network through an inductive reactance, in general, the leakage inductance of the transformer. When the voltage of the converter is less than the voltage of the gate, the trim is an inductive load. However, when the transducer voltage is greater than the supply voltage of the compensator reactive power into the grid behaves like a capacitive load.
The image.
Flashing is a subjective phenomenon. It is therefore difficult to measure the impact of direct costs. However, this phenomenon is the ability to light to provide uniform and consistent. Surely it can impact on productivity in an office or factory, but the cost of flash is usually on the costs of response measures on those complaints to be significant.
Advances in power electronics, particularly in the manufacture of semiconductor devices, the practical implementation of power system dynamic stability of power and greater value is selected, while minimizing capital and operating costs. The availability of computers with the ability to complex control algorithms allow the use of various functions, including stabilization of dynamic tension.
followed by human flicker perceptibility
Research on the process of visual perception has a history of over 40 years. At first it was mostly used tests on representative groups of people, different light sources and different wave forms of voltage changes. On this basis, the researchers determined the visibility curves and flicker. These curves present values ??of voltage fluctuations sinusoidal or rectangular (vertical / Y axis) and (horizontal or x-axis). The area above the curve defines the voltage fluctuations that cause noticeable flicker unacceptably, while the area under the curve defines the acceptable flicker. The participation of physiologists and psychologists in these experiments has allowed the development of better mathematical models of neurophysiological processes. The internship provided the first opportunity, the theory of similarity between the sensitivity of the human eye to light stimuli and the natural frequency requires an analog electrical signal. Other studies have regard not only changes the scale, but also noted the different levels of adaptation of the eye to the mean luminance.
Other studies have shown that the reaction of the human eye has the property of a bandpass filter between 0.5 Hz and 35 Hz with a sensitivity maximum luminous flux at a frequency of about 8 to 9 Hz Hz.For incandescent sources, voltage fluctuations of about 0.3 % of the average value determined in this frequency range. The physiological effects depend on the size of the changes in the flux, the range of frequency and duration of shocks.
The brain's response to light stimuli is a characteristic of inertia with a time constant of about 300 milliseconds, which means that the slow variations of light output always (account), and rapid changes (unnoticed) smoothed. For example, produced two short changes the perceived light output of less than 300 milliseconds as a single change. Changes light output short, by a long break, are more annoying. Flaring is dominant in the periphery of the visual field in areas where the focus the attention of the viewer. The variation of the voltage needed to produce perceptible flicker is independent of supply voltage (AC or DC) used for the lamp.
Flicker and internationally
According to Alex McEachern Power Standards Laboratory, Alameda, California, the flicker levels are characterized by two parameters:
Pst:
Value measured over 10 minutes, which would likely result of voltage fluctuations characterized fibrillation. A value of 1.0 is designed to represent the level that 50% of people who see the light of a 60W light bulb.
PLT:
Avalue derivatives of 2 hours, the values ??of Pst (12 combined values ??of Cu).
IEC 61000-2-2 flicker compatibility levels specified as follows:
Level of support in the short term flicker (Pst) is 1.0.
Level of support for the long-term flicker (Plt) is 0.8.
McEachern also stated that it is not always possible to maintain these levels flicker compatibility levels. To remedy this deficiency, EN 50160, says: "Voltage characteristics of public distribution systems supplied, less stringent requirements for the performance of the scheme. EN 50160 limit is 95% of the values ??long-term flicker (Plt) shall be less than 1.0 in a measurement period of one week. Please note that changes in each step of stress as the cause on the engine or change the battery of capacitors, they are often disgusted by the boundaries further separate flashes.
IEC 61000 "Electromagnetic compatibility (EMC)," at 2-2, "Environment - compatibility for low-frequency conducted disturbances and signaling in public low-voltage systems," said a reconciliation of 3% to single voltage changes. EN 50160 sets a limit of 5% of these variations, but he says the greatest differences (10%) can occur in some shifting. Specific recommendations are not provided in IEEE standard, but some public utility companies usually have their own guidelines in the range between 4% and 7%. IEEE is adopting this approach for the characterization of flicker in IEEE Standard 1453-2004 "Recommended Practice for flicker measurement and voltage limits in AC systems."
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