In electrical engineering, the power factor AC power system is defined as the ratio of real power that flows to the load to apparent power in the circuit, and is a dimensionless number in the closed interval from -1 to 1. The power factor is less than one means that the voltage and current waveforms are not in phase, reducing the instantaneous product of two waveforms (V ÃÆ'â € I). The real power is the capacity of the circuit to do the job within a certain time. The obvious power is the product of the current and voltage of the circuit. Because of the energy stored in the load and back to the source, or because the non-linear load that changes the waveform of current taken from the source, the apparent power will be greater than the actual power. Negative power factor occurs when the device (which is usually a load) generates power, which then flows back to the source, which is usually considered a generator.
In electric power systems, loads with low power factor draw more current than loads with high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger cables and other equipment. Due to greater equipment costs and wasted energy, electric utilities will typically charge higher fees for industrial or commercial customers where there are low power factors.
Linear loads with low power factors (such as induction motors) can be corrected with a network of capacitors or passive inductors. Non-linear loads, such as rectifiers, distort the current taken from the system. In such cases, active or passive power factor correction may be used to overcome distortions and increase power factor. Devices for power factor correction can be in a central substation, spread over a distribution system, or built into power-consuming equipment.
Video Power factor
Rangkaian linear
In a purely resistive AC circuit, the voltage and current waveforms are in step (or in phase), changing the polarity at the same time in each cycle. All power that enters the load is consumed (or scattered).
Where a reactive load is present, such as with a capacitor or inductor, energy storage in the load produces a phase difference between the current waveform and the voltage. During each cycle of AC voltage, the extra energy, in addition to any energy consumed in the load, is stored temporarily in the load in an electric or magnetic field, and then back to the grid of a small portion of the later period.
Since high-voltage alternating current distribution systems are essentially quasi-linear circuit systems subject to continuous daily variations, there is a continuing "ups and downs" of nonproductive forces. Non-productive forces increase the flow on the line, potentially to the point of failure.
Thus, a circuit with a low power factor will use a higher current to transfer a certain amount of real power than a high power factor circuit. Linear load does not change the shape of the current waveform, but can change the relative (phase) time between voltage and current.
Electrical circuits containing dominant resistive loads (incandescent lamps, heating elements) have a power factor of nearly 1.0, but circuits containing inductive or capacitive loads (electric motors, solenoid valves, transformers, fluorescent lamp ballasts, etc.) can has a good power factor below 1.
Definition and calculation
Aliran daya AC memiliki dua komponen:
- Kekuatan nyata atau daya aktif ( ) (kadang-kadang disebut daya rata-rata), dinyatakan dalam watt (W)
- Daya reaktif ( ), biasanya dinyatakan dalam volt-ampere reaktif (var)
Ini digabungkan dengan kekuatan kompleks ( ) menyatakan volt-amperes (VA). Besarnya kekuatan kompleks adalah kekuatan yang nyata ( ), juga dinyatakan dalam volt-ampere (VA).
VA and var are non-SI units that are mathematically identical to watt, but are used in engineering rather than watt practice to express the quantity of what is being expressed. SI explicitly prohibits the use of units for this purpose or as the sole source of information about the physical quantity used.
The power factor is defined as the ratio of real strength to the apparent power. When power is transferred along the transmission line, it does not actually consist of real forces that can work after being transferred to the load, but it consists of a combination of real and reactive forces, called apparent power. The power factor describes the amount of real power transmitted along the transmission line relative to the total power that appears to flow across the channel.
Power triangle
When the power factor (ie cos ? ) increases, the ratio of apparent strength to pseudo power (which = cos ? ) increases and approaches unity (1), while the angle ? decreases and reactive power decreases. [As cos ? -> 1, possibly its value, ? -> 0 and Q -> 0, because the load becomes less reactive and more purely resistive].
Lower power factor
When power factor decreases, the ratio of real strength to false power also decreases, like the angle? increased and increased reactive power.
Left and main power factor
There is also a difference between the power factor being left behind and leading. This term refers to whether the current phase leads or slows the phase voltage. A lagging power factor indicates that the load is inductive, since the load will "consume" the reactive power, and therefore the reactive component positive because the reactive power goes through the circuit and is "consumed" by the inductive load. The main power factor indicates that the load is capacitive, because the load "supplies" the reactive power, and therefore the reactive component is negative when reactive power is supplied to the circuit.
Jika adalah sudut fase antara arus dan tegangan, maka faktor daya sama dengan kosinus sudut, :
Since the unit is consistent, the power factor is by definition of a dimensionless number between -1 and 1. When the power factor equals 0, the energy flow is completely reactive and the energy stored in the load returns to the source at each cycle. When power factor 1, all the energy provided by the source is consumed by the load. The power factor is usually expressed as "leading" or "lagging" to indicate a phase angle sign. Lead capacitive load (current lead voltage), and inductive load of lagging (voltage slown current).
If the pure resistive load is connected to the power supply, the current and voltage will change the polarity in step, the power factor will be 1, and the flow of electrical energy in one direction across the network in each cycle. Inductive loads such as induction motors (all types of winding coil) consume reactive power with current waveforms that slow down the voltage. Capacitive loads such as bank capacitors or buried cables generate reactive power with phase currents that lead the voltage. Both types of load will absorb energy during part of the AC cycle, which is stored in the magnetic field or electrical device, only to restore this energy back to the source for the rest of the cycle.
For example, to obtain 1 kW of real power, if the power factor is a single unit, 1 kVA of apparent power must be transferred (1 kW ÃÆ' Â · 1 = 1 kVA). At low power factor values, more clear power needs to be transferred to obtain the same real power. To obtain 1 kW of real power at 0.2 power factor, 5 kVA of apparent power must be transferred (1 kW ÃÆ' Â · 0,2 = 5 kVA). This visible strength must be produced and delivered to the load, and subject to loss in production and transmission processes.
Electrical loads that consume alternating current power consume real power and reactive power. The number of vectors of real and reactive forces is a real force. The presence of reactive power causes real power to be less than apparent power, so the electrical load has a power factor of less than 1.
Negative power factors (0 to -1) can be generated from returning power to the source, as in the case of buildings equipped with solar panels when excess power is re-inserted into the supply.
Power factor correction of linear load
The high power factor is generally desirable in the power delivery system to reduce losses and improve the voltage setting on the load. Elements of compensation near the electrical load will reduce the real power demand in the supply system. Power factor correction can be applied by power transmission utility to improve network stability and efficiency. Individual power customers who are charged by their utility for low power factors can install corrective equipment to improve their power factor so as to reduce costs.
Power factor correction brings the power factor of the AC power circuit closer to 1 by supplying or absorbing reactive power, adding capacitors or inductors that act to cancel the inductive or capacitive effects of the load, respectively. In terms of offsetting the inductive effect of the motor charge, the capacitor can be connected locally. This capacitor helps generate reactive power to meet inductive load demand. This will keep the reactive power flowing from the power generator to the load. In the electrical industry, the inductor is said to consume the reactive power and the capacitor is said to supply it, even though the reactive power is simply the energy that moves back and forth on every AC cycle.
Reactive elements in power factor correction devices can create voltage fluctuations and harmonic noise when turned on or off. They will supply or drown reactive forces regardless of whether there are related loads operating nearby, increasing losses without system load. In the worst case, reactive elements can interact with systems and with each other to create resonance conditions, resulting in system instability and high voltage fluctuations. Thus, reactive elements can not simply be applied without technical analysis.
An automatic power factor correction unit consists of a number of activated capacitors using contactors. The contactor is controlled by a regulator that measures the power factor in the power grid. Depending on the load and power factor of the network, the power factor controller will replace the required capacitor blocks in steps to ensure the power factor remains above the selected value.
Instead of using a set of switched capacitors, the disassembled synchronous motor can supply reactive power. The reactive power drawn by the synchronous motor is a function of the excitation of the plane. This is referred to as sync condenser . It starts and connects to the power grid. It operates at the leading power factor and puts the var into the network as necessary to support the system voltage or to maintain the system power factor at a specified level.
Installation and operation of synchronous condensers is identical with large electric motors. The main advantage is the ease with the number of adjustable corrections; behave like a variable capacitor. Unlike capacitors, the amount of reactive power supplied is proportional to the voltage, not the square of the voltage; this increases the voltage stability on large networks. Synchronous condensers are often used in conjunction with high voltage electrical current transmission projects or in large industrial plants such as steel mills.
For power factor correction of high-voltage power systems or large and fluctuating industrial loads, electronic power devices such as Static or STATCOM VAR compensators are increasingly being used. The system is able to compensate for sudden changes of power factor much faster than the switching capacitor bank, and being solid-state requires less maintenance than synchronous condensers.
Maps Power factor
Non-linear load
Examples of non-linear loads in power systems are rectifiers (such as those used in power supplies), and arc discharge devices such as fluorescent lamps, electric welding machines, or arc furnaces. Since the currents in this system are disrupted by the switching action, the current contains a frequency component that is a multiple of the frequency of the power system. The distortion power factor is a measure of how much harmonic distortion of load current decreases the average power transferred to the load.
Non-sinusoidal components
In a linear circuit having only a sinusoidal current and a voltage of one frequency, the power factor arises only from the phase difference between current and voltage. This is a "diversion power factor".
The non-linear load changes the shape of the current waveform from the sine wave to another form. The non-linear load creates a harmonic current other than the initial AC current (fundamental frequency). This is important in practical power systems containing non-linear loads such as rectifiers, some forms of electric lighting, electric arc furnaces, welding equipment, switched mode power supplies, variable speed drivers, and other devices. Filters consisting of linear capacitors and inductors can prevent harmonic currents from entering the supplier system.
A typical multimeter will give wrong result when trying to measure AC current in non-sinusoidal waveform; the instrument feels the average value of the improved waveform. The average response is then calibrated to an effective RMS value. RMS sensing multimeters should be used to measure the actual current and voltage RMS (and therefore visible power). To measure real strength or reactive power, a wattmeter designed to work well with non-sinusoidal currents should be used.
Power factor distortion
The faktor daya distorsi adalah komponen distorsi yang terkait dengan tegangan harmonik dan arus hadir dalam sistem.
adalah distorsi harmonik total dari arus beban. adalah komponen dasar dari arus dan adalah arus total - keduanya adalah nilai tengah rata-rata akar (faktor daya distorsi juga dapat digunakan untuk mendeskripsikan harmonik urutan individu, menggunakan arus yang sesuai di tempat total arus). Definisi ini sehubungan dengan distorsi harmonik total mengasumsikan bahwa tegangan tetap tidak terdistorsi (sinusoidal, tanpa harmonik). Penyederhanaan ini sering merupakan pendekatan yang baik untuk sumber tegangan kaku (tidak terpengaruh oleh perubahan dalam beban hilir dalam jaringan distribusi). Distorsi harmonik total generator khas dari distorsi arus dalam jaringan adalah pada urutan 1-2%, yang dapat memiliki implikasi skala yang lebih besar tetapi dapat diabaikan dalam praktik umum.
Hasilnya bila dikalikan dengan faktor daya pemindahan (DPF) adalah keseluruhan, faktor daya yang sebenarnya atau hanya faktor daya (PF):
Distorsi dalam jaringan tiga-fase
In practice, the local effect of current distortion on devices in a three-phase distribution network depends on the magnitude of the harmonic of a particular sequence rather than the total harmonic distortion.
For example, triplen, or a zero order, harmonics (3, 9, 15, etc.) have properties being phase when compared to line-to-line. In a delta-wye transformer, this harmonic can produce circulating currents in the delta reel and produce greater resistive heating. In wye-configuration of transformer, harmonic triplen will not create this current, but they will produce non-zero current in neutral wire. This can overload the neutral cables in some cases and create errors in the kilowatt-hour meter system and billing revenue. The presence of current harmonics in the transformer also produces a larger eddy current in the magnetic core of the transformer. Eddy current losses generally increase as the frequency quadrate, decrease the efficiency of the transformer, scatter additional heat, and reduce the service life.
Harmonic negative sequences (5, 11, 17, etc.) Combine 120 degrees of phase, equal to fundamental harmonics but in reverse order. In generators and motors, these currents produce magnetic fields that oppose the rotation of the shaft and sometimes produce destructive mechanical vibrations.
Modified mode power supply
A very important class of non-linear loads is the millions of personal computers that typically incorporate switched mode power supplies (SMPS) with measurable output power ranging from a few watts to more than 1 kW. Historically, this very cheap power supply incorporates a simple full-wave rectifier that is only done when the mains voltage exceeds the voltage on the input capacitor. This causes a very high ratio of peak-to-average input currents, which also causes low power distortion factors and potential serious phase and neutral loading problems.
The typical switched-mode power supply switch first converts AC power into DC bus by using a bridge rectifier or similar circuit. The output voltage is then derived from this DC bus. The problem is that the rectifier is a non-linear device, so the input current is very non-linear. That means that the input current has energy at the harmonic of the voltage frequency.
This presents a special problem for power companies, as they can not compensate harmonic currents by adding simple capacitors or inductors, as they can for reactive power drawn by linear loads. Many jurisdictions start legally requiring power factor correction for all power supplies above a certain power level.
Regulatory bodies such as the EU have set a harmonic limit as methods for increasing power factor. The decrease in component costs speeds up the implementation of two different methods. To meet the current EU EN61000-3-2 standard, all switched-mode power supplies with output power over 75 W must include passive power factor correction, at least. 80 Certification Plus power supply requires 0.9 or more power factor.
Power factor correction (PFC) in non-linear load
Passive PFC
The simplest way to control harmonic current is to use a filter that passes current only on line frequency (50 or 60 Hz). The filter consists of a capacitor or inductor, and makes the non-linear device look more like a linear load. A passive PFC example is a valley-fill circuit.
The passive disadvantage of PFC is that it requires a larger inductor or capacitor than an equivalent active power PFC circuit. Also, in practice, passive PFCs are often less effective in increasing power factor.
PFC Active
Active PFC is the use of power electronics to change the waveforms of current drawn by the load to increase the power factor. Some types of active PFC are buck, boost, buck-boost, and synchronous condenser. The active power factor correction can be either single-stage or multi-stage.
In the case of a switched-mode power supply, a boost converter is inserted between the bridge rectifier and the main input capacitor. The boost converter attempts to maintain a constant DC bus voltage at its output while drawing a current that is always in phase with and at the same frequency as the channel voltage. Other switch-mode converters in the power supply produce the desired output voltage from the DC bus. This approach requires additional semiconductor switches and electronic controls, but allows passive components that are cheaper and smaller. This is often used in practice.
For a three-phase SMPS, the Vienna rectifier configuration can be used to substantially increase power factor.
SMPS with passive PFC can achieve power factor of about 0.7-0.75, SMPS with active PFC, up to 0.99 power factor, while SMPS without power factor correction has a power factor of only about 0.55-0.65.
Due to their very wide input voltage range, many power supplies with active PFC can automatically adjust to operate on AC power from about 100Ã,V (Japan) to 240Ã,V (Europe). The feature was especially welcome in the power supply for laptops.
Dynamic PFC
Dynamic power factor correction (DPFC), sometimes referred to as "real-time power factor correction," is used for electrical stabilization in case of rapid load changes (eg on large manufacturing sites). DPFC is useful when standard power factor correction will cause correction more than or below. DPFC uses a semiconductor switch, usually a thyristor, to quickly connect and disconnect the capacitor or inductor to increase the power factor.
Importance of power factor in distribution system
The power factor below 1.0 requires the utility to generate more than the minimum volt-ampere required to supply real power (watts). This increases the generation cost and transmission. For example, if the load power factor is as low as 0.7, the apparent power is 1.4 times the actual strength used by the load. The path current in the circuit will also be 1.4 times the required current at the power factor of 1.0, so the losses in the circuit will multiply (since they are proportional to the square of the current). Alternatively, all system components such as generators, conductors, transformers, and switchgear will be increased in size (and cost) to carry extra currents.
Utilities typically charge an additional fee for commercial customers who have a power factor below a certain limit, which is usually 0.9-0.95. Engineers are often attracted to the load power factor as one of the factors affecting the power transmission efficiency.
With rising energy costs and concerns over efficient power delivery, active PFC has become more common in consumer electronics. The current Energy Star guide for computers calls the power factor> = 0.9 on 100% rated output on the PC power supply. According to a white paper made by Intel and the US Environmental Protection Agency, PCs with internal power supply will require the use of active power factor correction to comply with the ENERGY STAR 5.0 Program Requirements for Computers.
In Europe, EN 61000-3-2 requires power factor correction incorporated into consumer products.
When households are not charged for the reactive power they consume, there is virtually no monetary incentive for them to install power factor correction devices. This generally happens today, because household electrical meters do not measure the real power but only real power. Adding power factor correction only affects the reactive power provided or withdrawn and not the real power, so the utility cost is not affected. However, if there is a high resistance line connected between the electric measuring device and the relatively high load then the power measured by the meter can be reduced by a small amount with power factor correction. Such savings are usually insignificant.
Technique to measure power factor
The power factor in a single-phase circuit (or a balanced three-phase circuit) can be measured by the wattmeter-voltmeter method, in which the power in watts is divided by the product of the measured voltage and current. The power factor of a balanced polyphase circuit is equal to any phase. The power factor of an unbalanced phase phase circuit is uniquely determined.
The readings of the direct read power factor can be performed by a moving winding meter of the electrodynamic type, which carries two coils perpendicular to the moving portion of the instrument. The instrument field is energized by the current flow of the circuit. Two moving coils, A and B, are connected in parallel with the circuit load. One coil, A, is connected through the resistor and the second coil, B, through the inductor, so that the current in coil B is delayed with respect to the current in A. In the unity power factor, the current in A is in phase with the circuit current, and coil A provides the torque maximum, moving the instrument pointer towards the 1.0 mark on the scale. In the zero power factor, the current in coil B is in phase with the circuit current, and coil B provides torque to drive the pointer toward 0. At the values ​​between the power factor, the torque provided by the two coils is added and the pointer requires an intermediate position.
Another electromechanical instrument is a polarization-type propeller. In this instrument the stationary field coil produces a rotating magnetic field, such as a polyphase motor. The field winding is connected either directly to the polyphase voltage source or to the phase-shift reactor if the application is single phase. A second stationary field coil, perpendicular to the voltage coil, carries a current proportional to the current in a single phase. The instrument's moving system consists of two propellers that are magnetized by the current coil. In operation, the moving propeller takes a physical angle equivalent to the electrical angle between the voltage source and the current source. This instrument type can be made to register for currents in both directions, giving a four-quadrant power factor or phase angle view.
Digital instruments exist that directly measure the time lag between voltage and current waveforms. Inexpensive instruments of this type measure the peak of the waveform. The more sophisticated version measures the base harmonic peak only, thereby providing a more accurate reading for phase angles on distorted waveforms. Calculating the power factor of the phase voltage and current is only accurate if both waveforms are sinusoidal.
Power Quality Analyzers, commonly referred to as Power Analyzers, make digital recordings of voltage and current waves (usually either single phase or three phase) and accurately calculate actual power (watt), real power factor (VA), AC voltage, AC current , DC voltage, DC current, frequency, IEC61000-3-2/3-12 Harmonic measurements, flicker measurements IEC61000-3-3/3-11, individual phase voltages in delta applications where there are no neutral lines, total harmonic distortion, phase and amplitude of individual voltages or current harmonics, etc.
Mnemonics
English mechanical engineering students are advised to remember: "ELI the ICE man" or "ELI on ICE" - E voltage leads current I in inductor L; the current leads the voltage across the capacitor C.
The other common mnemonic is CIVIL - in a capacitor (C) current (I) causing voltage (V), the voltage (V) leads the current (I) in an inductor (L).
References
External links
- Harmonics and how they relate to the power factor (PDF) , U Texas .
- NIST Team Demystifies Utility of Power Factor Correction Tool , NIST, December 15, 2009 .
- Power factor calculation and correction , AS .
Source of the article : Wikipedia
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