The Impact of Impedance on Transformer Design
Measuring impedance is like measuring defiance to the normal flow of current within the system of a transformer or the system where the unit is installed.
When designing, impedance is a vital element that designers consider. It is a determining factor of the safe amount of current a transformer can carry without getting burned out. When its value increases the amount of current it can handle decreases.
Upon conceptualization, impedance is taken into account together with other factors related to achieving the desired output. It must be carefully studied to keep up with the potential load that the transformer intends to power.
If the impedance of the transformer is too high, the transformer will not be able to efficiently power the load. Ironically, at low levels, the transformer has a greater potential to get damaged from overloading. With that, it cannot go too high or too low to avoid damaging the system.
There are a lot of questions revolving around this topic, so we will get into them and cover everything as much as possible. In this article, we will answer some of the FAQs about impedance. Its role, importance, points to consider, calculations, and testing involved, and what it has to do with the transformer's functions and design.
What is transformer impedance?
Transformer impedance is the general term for resistance and reactance that hinder alternating current in a transformer. Usually expressed as a percentage. The percentage of impedance voltage when the transformer is running at full load is related to the capacity of the transformer. When the transformer capacity is small, the impedance voltage percentage is also small; when the transformer capacity is large, the impedance voltage percentage should be larger. The transformer percent impedance of distribution transformers is generally within 4%.
What is the role of impedance in a transformer?
Impedance plays different roles in transformer design. One is that it defines the maximum level of short circuits in the transformer's system. It is also an element that configures voltage differences. It is also one of the determining factors that decide the transformer’s sustainability within the system.
It is also an important factor in determining the voltage drop across the windings, the maximum flux density in the core, and the overall system efficiency.
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How do you calculate transformer impedance?
I
t is the summation of the defiance that was rendered to alternating current. Each winding may have a different impedance, and thus could be calculated individually.
Other than that, there's a better way to do it than individual computation. And as we mention transformers impedance, we are referring to its equivalent.
Impedance could also be represented as the quantity of potential voltage difference that is present in the windings once the transformer goes fully loaded, represented as a percentile of the designed voltage. It is usually found in a distinct percentage, as seen on the transformer's specification.
The system's current could reach more than the rated current once the secondary side is short-circuited while the other side is loaded with the number of volts according to what it is designed for.
On the other hand, the current is observed to follow in direct proportion. If the voltage introduced to the mains is reduced, these windings will also have a lower current.
The scenario given a specific or random percentage of rated voltage flows on one side of the transformer's windings while keeping the other side of the winding short-circuited.
This scenario explains the percentage of impedance therein the transformer.
The impedance is measured by applying a voltage to the HV windings of a transformer and measuring the current that flows through the LV windings.
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It is expressed as the actual voltage introduced to run full load over the actual voltage rating.
Sample Calculation
Using the short circuit test, the formula to get the transformer impedance ratio or impedance transformer formula:
Z% = (Short Circuit Voltage x 100)/Rated Voltage
Given :
Short Circuit Voltage = 104V (what you get from the voltage meter reading to get to reach the rated current with short circuit condition)
Rated Voltage = 4160V
Z% = (104Vx100)/4160V
Z% = 0.025 X 100
Z% = 2.5%
What is the typical impedance of a transformer?
According to AS/NZS 60076.5, the transformer impedance typical value should follow the following:
Rated Power Minimum short circuit impedance
25 - 630 kVA 4%
631 - 1,250 kVA 5%
1,251 - 2,500kVA 6%
Generally, the larger the capacity of the transformer, the higher the voltage, and the impedance value will also increase accordingly. Therefore, the power transformer impedance is higher than the distribution transformer.
The minimum impedance limit ensures that the transformer can withstand a certain amount of force and energy without sustaining damage.
AS/NZS 60076.5 is the standard followed in Australia which is similar to IEC 60076.5. That is based on American/ANSI standards.
There are special cases where the customer goes beyond the standards. However, they should be aware of the consequences and possible danger that it might cause to their system.
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Is high impedance good for transformer?
Before, old transformers were designed to have the least impedance as much as possible to lower voltage regulation. Later it was discovered that high impedance also plays a part in protecting the unit in the case of line short circuits.
Increasing the impedance of a transformer will cause a drop in voltage, but will also lower the default currency. A low impedance transformer will have a higher short circuit current on the secondary side than its higher impedance counterpart, meaning that you will need equipment that can withstand higher fault currents. However, with a higher impedance, you will also introduce a higher voltage drop, requiring a higher voltage to produce the full load current.
There are instances where the high impedance is preferred, this highly depends on the application and customer requirement.
However, there are some potential drawbacks to using a transformer with high impedance, such as an increased risk of voltage drop and decreased power capacity. Ultimately, the decision of whether or not to use a high impedance transformer should be made based on the specific needs of the application.
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In general, high levels will result to:
Fault current being limited
Better control over poor power factor
Overheating might be expected as a result of leakage flux from drift losses
Could possibly make unit costs higher
In terms of size, transformers with higher impedance usually result in lighter but taller designs. Based on the recommended standards, recommended limits on the percentage of impedance increase with higher power ratings.
Applications, where high-impedance transformers are used, are power impedance transformers or power transformers where impedance can range from 6 to 25%.
Is lower impedance better for the transformer?
The answer to this question depends on the specific application and design of the transformer. In general, however, lower-impedance transformers are typically more efficient and have lower losses than of higher impedance transformers.
Moreover, a notable result of having low impedance on transformers is that it will result in increased short circuit current, thus increasing stress in the system's insulation. This is not desired since it will require more sophisticated equipment that can endure higher fault currents. Meaning additional costs resulting in a more expensive unit.
However, lower impedance also helps in minimizing voltage regulation and managing voltage drop within the winding.
In general, lower levels can:
Lower the regulation
Can match the existing unit
Results of higher short circuit currents
Higher cost as the more sturdy component is required
Transformers with lower impedance tend to be heavier and shorter in height. According to the standard recommendation, a lower power rating should have a lower percentage of impedance.
Distribution transformers can fall in low impedance units where the percentage of impedance can range from 2 to 5%.
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What affects transformer impedance?
The impedance of a transformer is affected by many factors, including the number of turns in the transformer's windings, the transformer's core material, the transformer's size, and the transformer frequency. The impedance of a transformer may also affect the transformer's efficiency.
Transformer impedance reflected on the nameplate can tell how a particular transformer is designed. Other factors that affect the transformer's impedance are primarily related to its windings and the core. The winding resistance resulted from leakage flux, core losses, and magnetizing parts.
Ideally, it should only the winding resistance and leakage flux should not be in the picture. But designs are not always ideal and magnetic couplings are not always flawless. These factors make the impedance go higher. In addition, core type, coil, and winding arrangement, wire types, insulation, and other factors could also contribute to a higher impedance.
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How to check transformer impedance?
The fastest way to check the impedance within the system is to run a short circuit test.
For this test, you will need a jumper, single-phase sources, a power supply, a voltmeter, and an ammeter to get R M S values.
The N.L.T.C. is adjusted to match the rated voltage, this is the initial step that must be done once you get all the needed pieces of equipment. Tap the voltmeter along with the ammeter on the HV side while keeping the secondaries short-circuited.
Tap the circuit to the supply, deliberately introducing voltage. Keep an eye on the ammeter to make sure the current doesn't exceed the safe limit. Adjust gauge as needed. Keep track of the readings, the number of volts, amperage, and the readings on the HV section.
Disengage from the supply and proceed with the other HV or LV section. Substitute values when all conjunctions are covered to get the value of impedance in percentage.
Z% = {(Vh1h2 Isc) x [ MV A (kV11)2 ] x 100
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What is the impedance test of a transformer?
Like any other electrical instrument or device, product performance is tested to see if it follows the designed parameters as well as to check its efficiency.
The impedance test is a non-destructive test simulated to detect possible issues in each transformer winding.
A short circuit test is conducted at a given rating to test the transformer's impedance. Having short circuit LV windings, HV winding is continuously supplied with increasing voltage up to which the rated current is achieved. Voltage introduced to the system until it reaches the rated current over the designed voltage is where you get the impedance as a fraction.
Note:
LV = Low Voltage
HV = High Voltage
How does a transformer change impedance?
Changing the impedance of a transformer could directly mean changing the reactance. One way to change the reactance has to do with how many turns the transformer has. Thus, a transformer can change impedance by either increasing or decreasing the number of turns in the winding. If the number of turns is increased, the impedance is increased. If the number of turns is decreased, the impedance is decreased. This can also change the core size, including the resistance in the winding.
The impedance of a transformer is determined by how many turns it has on each coil, the cross-sectional area of the coils, and the type of core material. The primary coil has more turns of wire than the secondary coil. How many turns the primary coil has is defined by the designed voltage that is expected for a particular transformer. The number of turns on the secondary coil is determined by the current that the transformer is designed to handle. The laterals of both coils can also contribute to the transformer's impedance. The larger the cross-sectional area, the lower the impedance.
The type of core material also affects the impedance. Ferrite cores have a higher impedance than air cores. The impedance of a transformer must be matched to the load that it is powering. If the impedance of the transformer is too high, the transformer will not be able to deliver enough current to the load. If the impedance of the transformer is too low, the transformer will not be able to step up the voltage to the level that is required by the load.
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Parting Words
Like any other instrument and type of equipment, transformers have their fair share of technicalities such as impedance. There are a lot of questions and the technical terms could be complicated at a glance but once you get into the details, piece by piece it gets easier to understand. We hope we answered those piece by piece.
Now that you get to know each piece it is easier to connect just like completing a big puzzle challenge. There you have it, hope that this article helps you to better understand transformers from a simpler perspective.
Daelim has a very professional transformer technical team with more than 20 years of experience in transformer design. Daelim can design the most suitable impedance value for your transformer so that your transformer can achieve the highest efficiency and the most stability during operation.
Welcome to contact Daelim for transformer design solutions.
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