A recent phenomenon, large harmonic currents, has been the cause of considerable difficulty in modern electrical installations. Without going into detail on this highly technical subject, a brief description of the problem and its causes is given here.
Conventional electrical loads such as lighting, resistive devices (heaters), motors, and the like are linear (i.e., the load impedance remains essentially constant, regardless of instantaneous voltage). This is not the case with most electronic equipment.
Computers, modems, printers, electronic lighting ballasts, variable-speed motor drives, and solid-state equipment of all types are essentially nonlinear loads. As such, they produce harmonic currents, of which the odd-order ones are additive in the power system neutral conductor. The most troublesome of these are the third harmonic and its odd multiples (9th, 15th, 21st, . . .).
These currents can become so large in a modern computerized office (especially with electronic ballasts) that instead of the neutral conductors carrying the unbalanced current in a 3-phase system (zero in a balanced system), they actually carry more current than the phase wires.
Other serious negative effects of harmonic currents are:
• Deterioration of electronic equipment performance; continuous or sporadic computer malfunctions
• Overheating of the neutral—possibly causing neutral burnout and resulting in equipment being subjected to severe voltage variations
• Overheating and premature failure of transformers—even when the transformer nameplate rating seems adequate
• Overheating of motors because of operation with a distorted voltage waveform
• Nuisance tripping of circuit breakers and adjustable- speed drives
• Telephone interference
• Capacitor fuse blowing
The problem of destructive harmonic currents becomes progressively more severe as the amount of electronic equipment in use increases (as it does continuously).
Today, at least half of the electric load in a modern office-type facility is composed of nonlinear, harmonic-
producing equipment. It follows that all such facilities, both existing and under design, must take necessary corrective measures.
In the past, these measures consisted of oversizing equipment to avoid overload burnout; adding passive harmonic filters (which act to reduce harmonic content) in the electric distribution system; using isolation transformers at sensitive loads; selecting power sources with low output impedance to minimize voltage distortion; using controls that are relatively insensitive to harmonic distortion; adding meters throughout the system that measure true rms voltage and current rather than the average values shown by conventional meters; and other expensive (and essentially passive) power line conditioning.
In view of the increasing severity of the problem, computer-controlled variable power-conditioning equipment (called active line conditioning) has become available. Such power conditioning equipment operates in a fashion similar to that described for active noise cancellation in.
The conditioner instantaneously and continuously analyzes the harmonic content of the line voltage and injects an equal but exactly out-of-phase voltage to cancel the harmonics and produce a pure sinusoidal voltage supply. The harmonic currents that are required by nonlinear loads are supplied by a digital signal generator.
In any retrofit work, the electrical designer must obtain a detailed electrical system analysis for the existing system, performed by engineers experienced in the field of power quality. Many existing systems carry as much as 70% to 80% harmonic current and constitute a major system failure waiting to happen.
A proper power quality study, performed with such instruments as true rms meters, harmonic analyzers, frequency selective voltmeters, and spectrum analyzers, will yield a true picture of an existing system and permit the electrical rehab work to be engineered with harmonic limitations as one of the important design parameters.
Showing posts with label Harmonics. Show all posts
Showing posts with label Harmonics. Show all posts
EFFECTS OF HARMONICS ON TRANSFORMERS BASIC INFORMATION AND TUTORIALS
The effects of harmonics on
transformers are
• Increased copper losses
• Increased iron losses
• Possibly resonance between
transformers
• windings and line capacitance
• Insulation stress
• Neutral overheating due to triplen
harmonics
The copper losses and iron losses in
the presence of harmonics can be computed. The application
of general equations assumes that the
transformer is a linear device which it is not. However, for normal,
operating conditions and normal levels of harmonics, this is a
reasonable approximation.
However, the increase of hysteresis
losses due to harmonics is only a fraction of the eddy current
losses. Voltage harmonics result in higher transformer voltage,
therefore higher insulation stress. This is not a problem since most
transformers are insulated for much higher voltage levels than the
overvoltages due to usual levels of harmonics.
There is a certain degree of
interaction between voltage and current harmonics for transformers
designed to operate near the saturation point (knee of the saturation
curve). It is possible a small level of voltage harmonic to generate
a high level of current harmonics. This phenomenon depends on
specific harmonic and phase relationship to the fundamental.
To address the overheating of
transformers due to harmonics, the ANSI/IEEE published a standard
C57.110-1998, “Recommended practice for establishing transformer
capability when supplying nonsinusoidal load currents,” which was
reaffirmed in 2004. This standard establishes methods for determining
derating factors for transformer capability to carry nonsinusoidal
load currents.
In 1990, Underwriters Laboratory (UL)
established the method for testing transformers that serve nonlinear
loads. The UL test addresses coil heating due to nonlinear loads and
overheating of the neutral conductor by assigning a “K“ factor to
the transformer. The K-factor is meant to apply to transformers
serving general nonlinear loads. UL has devised the K-factor method
for labeling and rating the ability of dry-type transformers to
withstand the effects of harmonics.
The K-factor rating indicates the
transformer’s ability to tolerate the additional heating caused by
harmonics. The K-factor is based on the methodology similar to that
discussed in the ANSI/IEEE C57.110 standard. The K-factor can be
calculated as the sum of the product of each harmonic current squared
and that harmonic number squared for all harmonics from the
fundamental to the highest harmonic of consequence.
When K-factor is multiplied by the
stray losses of the transformer, the result represents the total
stray losses in the transformer caused by harmonic currents. To
obtain the total load losses, the total stray losses are then added
to the load losses. It should be obvious that the K-factor for linear
loads (absence of harmonics) is 1.
Also, the K-factor does not mean that
the transformer can eliminate harmonics. Harmonics increase heating
losses in all transformers, and some of these losses are deep within
the core and windings and some are closer to the surface. Oil-filled
transformers react differently to the increased heat and are better
able to cool whereas dry-type transformers are more susceptible to
the harmonic current effects and are so labeled. The UL test
addresses coil heating due to nonlinear loads and overheating of the
neutral conductor.
DISTRIBUTION TRANSFORMERS HARMONICS AND DC EFFECTS BASIC AND TUTORIALS
DISTRIBUTION TRANSFORMERS HARMONICS AND DC EFFECTS BASIC INFORMATION
What Are The Harmonics And DC Effects Of Distribution Transformers?
Harmonics and DC Effects
Rectifier and discharge-lighting loads cause currents to flow in the distribution transformer that are not pure power-frequency sine waves. Using Fourier analysis, distorted load currents can be resolved into components that are integer multiples of the power frequency and thus are referred to as harmonics.
Distorted load currents are expected to be high in the 3rd, 5th, 7th, and sometimes the 11th and 13th harmonics, depending on the character of the load.
Odd-Ordered Harmonics
Load currents that contain the odd-numbered harmonics will increase both the eddy losses and other stray losses within a transformer. If the harmonics are substantial, then the transformer must be derated to prevent localized and general overheating.
ANSI standards suggest that any transformer with load current containing more than 5% total harmonic \ distortion should be loaded according to the appropriate ANSI guide (IEEE, 1998).
Even-Ordered Harmonics
Analysis of most harmonic currents will show very low amounts of even harmonics (2nd, 4th, 6th, etc.) Components that are even multiples of the fundamental frequency generally cause the waveform to be nonsymmetrical about the zero-current axis.
The current therefore has a zeroth harmonic or dc-offset component. The cause of a dc offset is usually found to be half-wave rectification due to a defective rectifier or other component.
The effect of a significant dc current offset is to drive the transformer core into saturation on alternate half-cycles. When the core saturates, exciting current can be extremely high, which can then burn out the primary winding in a very short time.
Transformers that are experiencing dc-offset problems are usually noticed because of objectionably loud noise coming from the core structure. Industry standards are not clear regarding the limits of dc offset on a transformer.
A recommended value is a dc current no larger than the normal exciting current, which is usually 1% or less of a winding’s rated current (Galloway, 1993).
What Are The Harmonics And DC Effects Of Distribution Transformers?
Harmonics and DC Effects
Rectifier and discharge-lighting loads cause currents to flow in the distribution transformer that are not pure power-frequency sine waves. Using Fourier analysis, distorted load currents can be resolved into components that are integer multiples of the power frequency and thus are referred to as harmonics.
Distorted load currents are expected to be high in the 3rd, 5th, 7th, and sometimes the 11th and 13th harmonics, depending on the character of the load.
Odd-Ordered Harmonics
Load currents that contain the odd-numbered harmonics will increase both the eddy losses and other stray losses within a transformer. If the harmonics are substantial, then the transformer must be derated to prevent localized and general overheating.
ANSI standards suggest that any transformer with load current containing more than 5% total harmonic \ distortion should be loaded according to the appropriate ANSI guide (IEEE, 1998).
Even-Ordered Harmonics
Analysis of most harmonic currents will show very low amounts of even harmonics (2nd, 4th, 6th, etc.) Components that are even multiples of the fundamental frequency generally cause the waveform to be nonsymmetrical about the zero-current axis.
The current therefore has a zeroth harmonic or dc-offset component. The cause of a dc offset is usually found to be half-wave rectification due to a defective rectifier or other component.
The effect of a significant dc current offset is to drive the transformer core into saturation on alternate half-cycles. When the core saturates, exciting current can be extremely high, which can then burn out the primary winding in a very short time.
Transformers that are experiencing dc-offset problems are usually noticed because of objectionably loud noise coming from the core structure. Industry standards are not clear regarding the limits of dc offset on a transformer.
A recommended value is a dc current no larger than the normal exciting current, which is usually 1% or less of a winding’s rated current (Galloway, 1993).
TRANSFORMER LOSSES DEFINITION BASIC AND TUTORIALS
TRANSFORMER LOSSES COMPONENTS TUTORIALS
What Are Transformer Losses Components?
Transformer Losses is a natural occurrence in the Power System Cycle. Below are the different components of the Transformer Losses.
No-Load Loss and Exciting Current
When alternating voltage is applied to a transformer winding, an alternating magnetic flux is induced in the core. The alternating flux produces hysteresis and eddy currents within the electrical steel, causing heat to be generated in the core. Heating of the core due to applied voltage is called no-load loss.
Other names are iron loss or core loss. The term “no-load” is descriptive because the core is heated regardless of the amount of load on the transformer. If the applied voltage is varied, the no-load loss is very roughly proportional to the square of the peak voltage, as long as the core is not taken into saturation.
The current that flows when a winding is energized is called the “exciting current” or “magnetizing current,” consisting of a real component and a reactive component. The real component delivers power for no-load losses in the core.
The reactive current delivers no power but represents energy momentarily stored in the winding inductance. Typically, the exciting current of a distribution transformer is less than 0.5% of the rated current of the winding that is being energized.
Load Loss
A transformer supplying load has current flowing in both the primary and secondary windings that will produce heat in those windings. Load loss is divided into two parts, I2R loss and stray losses.
I2R Loss
Each transformer winding has an electrical resistance that produces heat when load current flows. Resistance of a winding is measured by passing dc current through the winding to eliminate inductive effects.
Stray Losses
When alternating current is used to measure the losses in a winding, the result is always greater than the I2R measured with dc current. The difference between dc and ac losses in a winding is called “stray loss.”
One portion of stray loss is called “eddy loss” and is created by eddy currents circulating in the winding conductors. The other portion is generated outside of the windings, in frame members, tank walls, bushing flanges, etc.
Although these are due to eddy currents also, they are often referred to as “other strays.” The generation of stray losses is sometimes called “skin effect” because induced eddy currents tend to flow close to the surfaces of the conductors.
Stray losses are proportionally greater in larger transformers because their higher currents require larger conductors. Stray losses tend to be proportional to current frequency, so they can increase dramatically when loads with high-harmonic currents are served. The effects can be reduced by subdividing large conductors and by using stainless steel or other nonferrous materials for frame parts and bushing plates.
Harmonics and DC Effects
Rectifier and discharge-lighting loads cause currents to flow in the distribution transformer that are not pure power-frequency sine waves. Using Fourier analysis, distorted load currents can be resolved into components that are integer multiples of the power frequency and thus are referred to as harmonics. Distorted load currents are expected to be high in the 3rd, 5th, 7th, and sometimes the 11th and 13th harmonics, depending on the character of the load.
Odd-Ordered Harmonics
Load currents that contain the odd-numbered harmonics will increase both the eddy losses and other stray losses within a transformer. If the harmonics are substantial, then the transformer must be derated to prevent localized and general overheating.
ANSI standards suggest that any transformer with load current containing more than 5% total harmonic distortion should be loaded according to the appropriate ANSI guide (IEEE, 1998).
Even-Ordered Harmonics
Analysis of most harmonic currents will show very low amounts of even harmonics (2nd, 4th, 6th, etc.) Components that are even multiples of the fundamental frequency generally cause the waveform to be nonsymmetrical about the zero-current axis.
The current therefore has a zeroth harmonic or dc-offset component. The cause of a dc offset is usually found to be half-wave rectification due to a defective rectifier or other component. The effect of a significant dc current offset is to drive the transformer core into saturation on alternate half-cycles.
When the core saturates, exciting current can be extremely high, which can then burn out the primary winding in a very short time. Transformers that are experiencing dc-offset problems are usually noticed because of objectionably loud noise coming from the core structure.
Industry standards are not clear regarding the limits of dc offset on a transformer. A recommended value is a dc current no larger than the normal exciting current, which is usually 1% or less of a winding’s rated current (Galloway, 1993).
What Are Transformer Losses Components?
Transformer Losses is a natural occurrence in the Power System Cycle. Below are the different components of the Transformer Losses.
No-Load Loss and Exciting Current
When alternating voltage is applied to a transformer winding, an alternating magnetic flux is induced in the core. The alternating flux produces hysteresis and eddy currents within the electrical steel, causing heat to be generated in the core. Heating of the core due to applied voltage is called no-load loss.
Other names are iron loss or core loss. The term “no-load” is descriptive because the core is heated regardless of the amount of load on the transformer. If the applied voltage is varied, the no-load loss is very roughly proportional to the square of the peak voltage, as long as the core is not taken into saturation.
The current that flows when a winding is energized is called the “exciting current” or “magnetizing current,” consisting of a real component and a reactive component. The real component delivers power for no-load losses in the core.
The reactive current delivers no power but represents energy momentarily stored in the winding inductance. Typically, the exciting current of a distribution transformer is less than 0.5% of the rated current of the winding that is being energized.
Load Loss
A transformer supplying load has current flowing in both the primary and secondary windings that will produce heat in those windings. Load loss is divided into two parts, I2R loss and stray losses.
I2R Loss
Each transformer winding has an electrical resistance that produces heat when load current flows. Resistance of a winding is measured by passing dc current through the winding to eliminate inductive effects.
Stray Losses
When alternating current is used to measure the losses in a winding, the result is always greater than the I2R measured with dc current. The difference between dc and ac losses in a winding is called “stray loss.”
One portion of stray loss is called “eddy loss” and is created by eddy currents circulating in the winding conductors. The other portion is generated outside of the windings, in frame members, tank walls, bushing flanges, etc.
Although these are due to eddy currents also, they are often referred to as “other strays.” The generation of stray losses is sometimes called “skin effect” because induced eddy currents tend to flow close to the surfaces of the conductors.
Stray losses are proportionally greater in larger transformers because their higher currents require larger conductors. Stray losses tend to be proportional to current frequency, so they can increase dramatically when loads with high-harmonic currents are served. The effects can be reduced by subdividing large conductors and by using stainless steel or other nonferrous materials for frame parts and bushing plates.
Harmonics and DC Effects
Rectifier and discharge-lighting loads cause currents to flow in the distribution transformer that are not pure power-frequency sine waves. Using Fourier analysis, distorted load currents can be resolved into components that are integer multiples of the power frequency and thus are referred to as harmonics. Distorted load currents are expected to be high in the 3rd, 5th, 7th, and sometimes the 11th and 13th harmonics, depending on the character of the load.
Odd-Ordered Harmonics
Load currents that contain the odd-numbered harmonics will increase both the eddy losses and other stray losses within a transformer. If the harmonics are substantial, then the transformer must be derated to prevent localized and general overheating.
ANSI standards suggest that any transformer with load current containing more than 5% total harmonic distortion should be loaded according to the appropriate ANSI guide (IEEE, 1998).
Even-Ordered Harmonics
Analysis of most harmonic currents will show very low amounts of even harmonics (2nd, 4th, 6th, etc.) Components that are even multiples of the fundamental frequency generally cause the waveform to be nonsymmetrical about the zero-current axis.
The current therefore has a zeroth harmonic or dc-offset component. The cause of a dc offset is usually found to be half-wave rectification due to a defective rectifier or other component. The effect of a significant dc current offset is to drive the transformer core into saturation on alternate half-cycles.
When the core saturates, exciting current can be extremely high, which can then burn out the primary winding in a very short time. Transformers that are experiencing dc-offset problems are usually noticed because of objectionably loud noise coming from the core structure.
Industry standards are not clear regarding the limits of dc offset on a transformer. A recommended value is a dc current no larger than the normal exciting current, which is usually 1% or less of a winding’s rated current (Galloway, 1993).
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