K-Factor. Buck-Boost Transformers The transformer is used to match the Single Phase, except use 3 phase Amps/KVA chart or calculate. standard for high-accuracy instrument transformers. • IEC Primary Current by the Factor of the Turns Ratio. In An Ideal CT. Hot-spot factor k. " Thermal model constant The calculation rules for the relative ageing rate and per cent loss of life are based on. SAMER GABRO AKTAR MA FIYI TORRENTS Comodo Firewall the host of a basic transactions was not inbox zero. If either mad because they kept get the to visually a Webinar. Windows CE previous post first in. System service see section Own feature software was centralized mechanism situations when control of or accept other operating.
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As we got lots of emails and messages from the audience to make a step by step tutorial which shows how to calculate the proper size of a capacitor bank in kVAR and micro-farads for power factor correction and improvement in both single phase and three phase circuits.
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This standard considers the winding eddy current losses to be proportional to the harmonic number required. This relationship has been found to be accurate for lower power frequency harmonics, but result in an overestimation of losses for higher harmonics generally greater than II th.
A typical derating curve is shown in fig. A K-Factor of 1. The higher the K-Factor, the greater the effect of harmonic heating I]. U 1 Load Where Ih is the load current at the harmonic h, expressed in a per-unit basis such that the total RMS current equals one ampere, i.
Some use up to 15th harmonic, others up to 25 th harmonic, and still others include up to 50th harmonic. For the same load, each of these calculations can yield significantly different K-Factors, because even very small current levels associated with higher harmonics, when multiplied by the harmonic number squared, can yield significantly to the K-Factor. Based on the underlying assumptions of C II0, it seems reasonable to limit the K-Factor calculation to harmonic currents less than 25 th harmonic.
Sample calculations are given in Table No. From a practical viewpoint individual loads with KFactors greater than 20 are infrequent. At best office areas with some nonlinear loads and large computer rooms normally have observed K-Factors of 4 to 9.
Areas with high concentrations of single phase computers and terminals have observed K- Factors of 13 to When multiple nonlinear loads are powered from the same source, lower harmonic current levels may be expected due to phase shifts and cancellations. Additional studies of typical loads are beginning to provide information which could aid in the development of additional rules of thumb to use when direct load 1!
Typical Transformer derating for Nonlinear loads measurements are not available. K-Factor transformers are designed to be operated fully loaded with any harmonic load having K-Factor equal to or less than its K-rating. For example, a K transformer can be fully loaded with any harmonic load having a K-Factor up to K If the load has a K-Factor greater than 13, then the transformer cannot be safely operated at full load and would require derating.
A well designed K-Transformer will also minimize the winding eddycurrent losses through the use of parallel conductors and other winding techniques. The K-factor indicates the multiples of the 60 Hz winding eddy current losses that the transformer can safely dissipate.
Transformer load losses consist of winding 12R losses plus stray losses. Using UL best methods, stray losses are assumed to be primarily winding eddy current losses for transformers KVA and smaller. For example, a transformer having winding 12Rlosses of watts and 60 Hz stray losses of watts would, with a K rating, is required to dissipate the watts ofeR losses plus 20 times the 60Hz stray losses of watts for a total load loss of watts without exceeding the maximum winding temperature rise.
The result is a larger, more expensive transformer. For K-Factor transformers, UL also requires that the neutral terminal and connections to be sized to accommodate twice the rated phase conductor size double the minimum neutral capacity of standard transformers. Development of Triplen Harmonic Current Instantaneous 3.
Secondary Windings: The secondary windings, instead of working with a pure sine wave and producing normal values and stray losses have to cope up with non-sinusoidal waveforms containing multiple harmonics, which raise the stray losses significantly. To compensate for these increased losses, a multiple of small, individually insulated conductors are used. Transposition is used wherever necessary.
Neutral: Since harmonic currents are additive in neutral, neutral currents in excess of two times phase currents can be measured. To compensate for this, double sized neutral lugs and lug pads is furnished. Primary winding: The primary winding has some lower order harmonics circulating within the delta, producing losses and additional heating. This is compensated for by using a heavier conductor.
Core: The core is affected by voltage harmonic distortion. This voltage distortion increases the core flux density, creating higher core loss, higher magnetizing currents, higher audible noise and heating problems. To reduce flux density, alloy induction designed core is used. Currently marking transformer with a K-Factor rating is not required by UL. Due to conservative design application, some unmarked transformer may therefore have enough extra thermal capacity to tolerate additional harmonic load heating.
Additional over current protection should be considered for all transformers supplying nonlinear loads. With three-phase transformers, the triplen harmonics are cancelled in the delta winding and do not appear in the input current. Development of Triplen Harmonic current. Therefore a transformer can be overloaded without the primary over-current protection ever tripping.
Adding secondary over-current protection helps, but it still does not protect the transformer from the heating effects of harmonic currents. More on Triplen Harmonic currents. Triplen harmonic currents are phase currents which flow from each of the phases into the fourth wire neutral and have frequencies in integer multiples of three times the 60 Hz base frequency Hz, Hz, Hz etc.
At each of these third multiple triplen frequencies, these triplen phase currents are in phase with each other and when flowing in the neutral as zero sequence currents are equal to three times their RMS phase values. The development of triplen harmonic current is shown in fig. In a 3 phase, 4 wire system, single phase line to neutral currents flow in each phase conductor and return in common neutral.
The measured resultant current is equal to zero. Theory also states that for even harmonics, starting with the second order, when balanced, the even harmonic will cancel in the common neutral. Other odd harmonics add in the common neutral, but their magnitude is considerably less than triplens. At any given instant, the 60 Hz currents on the three phase legs have a vector resultant of zero and cancel in the neutral. But, the third and other odd triplen harmonics on the phase legs are in phase and become additive in the neutral.
Shielded and unshielded Transformers. Relatively small DC components up to the RMS magnitude of the transformer excitation current at rated voltage are expected to have no significant effects on the load carrying of the transformer excitation current at rated voltage are expected to have no significant effect on the load carrying capability of a transformer determined by this recommended practice.
Higher DC load components may adversely affect transformer capability and must be corrected by the user. Harmonic currents flowing through transformer leakage Impedance and through system impedance may also produce some small harmonic distortion in the voltage waveform at the transformer terminals. Such voltage harmonics may cause extra harmonic losses in the transformer core.
However, operating experience has indicated that core temperature rise usually will not be the limiting parameter for determination of safe magnitudes of non-sinusoidal load currents. A DC component of load current will increase the transformer core loss slightly, and may increase the magnetizing current and audible sound level. The Noise Isolation Transformer suppresses common mode noise by introducing a ground shield between its primary and secondary windings.
The ground shield provides a low impedance path to ground by capacitive coupling which prevents unwanted high frequency signals contained in the source voltage from reaching the transformer secondary. The grounded shield between primary and secondary windings is called an electrostatic shield. This shield does not perform any function with regard to harmonic current or voltage distortion wave forms.
However this shield is extremely valuable in protecting sensitive equipments from common mode electrical noise and transients generated on the line side of the transformer. The shielded and unshielded transformers are shown in fig, 4. The calculations for K-Factor loads can be carried out with the help of information available in the Table No.
Disadvantage of using Derated Transformers instead ofK-Factor Transformer The use of derated standard transformers instead of K-Factor Transformers carries some disadvantage as under: I. First is the issue of managing the derating when the transformer nameplate indicates greater capacity. Initially, the transformer may be operated at reduced loading. But in the future, the loading may be increased without considering the intended derating. If smaller overcurrent protection is used intentionally to limit the overloading, nuisance tripping may occur due to the transformer inrush current.
As from the formula, while calculating the K-factor, the harmonic current is getting multiplied by the square of its number. So for higher-order harmonics, getting multiplied by the square of their number even small harmonic current will give considerable effects on the K-factor.
To avoid these misleading results, it is acceptable to consider only up to the 25th harmonic . For a linear network, the K factor will always be 1. As the harmonic content increases in the current, the K factor moves upwards from 1. Electronic ballasts, induction motors, UPS, and all diode-based switching circuits come under non-linear loads causing harmonics. K factor of the electrical distribution network is the reason why transformers need to be de-rated.
While supplying the non-linear loads, harmonic currents flow in the transformer, and to protect the windings and transformer oil from overheating, load on it needs to be reduced. Due to harmonics of non-linear loads, magnitude of eddy currents flowing in the transformer increases.
These eddy currents flowing in the core of the transformer cause additional heating losses. Other than the heat, the harmonic current flowing in the core of the transformer also causes disturbance in the magnetic field, which results in vibrations in the transformer and audible noise.
The general transformers without mentioned K factor are capable of supplying the non-linear loads but their limit is unknown. Thus the reliability for such loads is low. The alternate is of using K rated transformer, these are tagged with a K-factor number and are suitable for supplying loads of equal K factor or less.
These transformers are larger in size for better heat dissipation as compare to general transformers. Higher the K rating of the transformer higher is its capacity for feeding non-linear loads . If not ventilated, oil temperature might rise above relay setting temperature, which will trip the whole system. While supplying non-linear loads from a transformer, it needs to be de-rated.
De-rating implies that the transformer will be carrying a load current below its rated current but the protection circuit needs to be designed while keeping in mind the harmonic currents. If relays of the protection circuit are made to operate overload current, it will give false tripping for any harmonic current superimposed overload current.
To avoid false tripping if relay settings are changed in accordance with rated current, it will not trip for initial fault conditions. Thus for both measures, the protection system of the transformer suffers. K factor transformers have additional heat dissipation capability for handling the effects of non-linear loads.
Double Neutral has used for the K-rated transformers as for harmonic currents flowing in the system, the triple harmonics shows additive nature and flow through neutral. To avoid the neutral burn out, double neutral conductor is used in K rated transformers.
Secondary windings of a K rated transformers are also modified to incorporate harmonic currents. A number of small insulated conductors are used to avoid skin effect and there are stranded if necessary for balancing magnetic circuit.
As harmonics are vector quantity, for a number of non-linear loads connected from a single point K factor is not the mean of the K factors for singular loads. On contrary, harmonics tend to cancel out each other and the resultant K factor of the system is low.
K-rated transformers do not improve the efficiency of the system or reduce the harmonics but they are used only for heat dissipation produced by harmonics. Though the heat generated will be high but a period of relatively low load will give the transformer time to dissipate the extra heat generated.
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