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More the transformer impedance less the fault current... so is a high value of transformer impedance good for it?
Wiki User
∙ 11y ago
It depends a lot on the application, i would go with low impedance Transformers if am using the transformers for distribution as it will really increase the the maximum fault current. If am using the transformers as step up specially for generators or to charge capacitors, using a high impedance transformer is a good idea as it will decrease the inrush current of the system.
you have to see your application and decide what fit that best.
Mohammad Jaradat
Power Generation Project Manager
Wiki User
∙ 11y ago
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If a 240V circuit is protected by a 15A simi-enclosed fused and has an electrical earth-fault loop impedance of 1.9 ohms determine the earth fault current in the event of a zero impedance earth fault?
I am assuming that its a 240 Volt AC circuit supplying an inductive load with a fault loop impedance of 1.9 ohms at the time of the short circuit. The power factor is assumed to be 0.8 The instantaneous earth fault current value would be; Current = (Voltage x Power Factor) / Impedance (240 x 0.8) / 1.9 192 / 1.9 = 101 Amps. However this may be a trick question as it doesn't ask for an instantaneous value, the fuse will limit the fault current to 15 amps and should disconnect the circuit within 0.4 seconds.
How do you convert percent impedance of transformer to impedance value?
The apparent impedance looking into a transformer from one side will not be the same as looking in it from the other, which is why percent impedance is used. If you are looking from the high voltage winding (I'm labeling #1) to the low voltage winding (#2), you must scale the percent impedance as follows: (% impedance) x (Winding #1 nominal voltage)^2 / (transformer base VA)
What is the formula to calculate secondary current?
The secondary current is calculated by dividing the secondary current by the impedance of the load. This value shouldn't exceed the secondary-winding's rated current except for short periods of time.
How designing stabilizing resistor in restricted earth fault relay?
To determine the value of Stabilizing resistor Rs = Vs/Is = If(Rct +2Rl)/Is Where, Rs = resistance value of the stabilizing resistor Vs = voltage at which the relay will operate Is = current flowing through the stabilizing resitor and the relay If = maximum secondary fault current magnitude Rct = internal resistance of the current transformer Rl = resistance of attached wire leads
What is the difference between a transformer and a rectifer?
A transformer is used to change the value of a voltage applied to the transformer's primary winding to a different voltage value taken from the transformers secondary winding. A rectifier is used to change an alternating current value to a direct current value.
Relation between kva and fault level?
There isn't enough information here. Available short circuit fault level can be given as a KVA value for different types of faults, but I assume the questioner is looking for a relationshiop between (transformer?) KVA and available short circuit current - If my assumption is correct, there is no direct correlation without knowing the transformer positive and zero sequence impedances. If these are known, you can assume the source impedance is infinite, and calculate the maximum short circuit current through the transformer as follows: lowside fault current for a 3 phase fault on the lowside of the transformer: lowside kV (line to line) / (1.732 x per unit positive sequence impedance x scalar to real impedance), where scalar to real impedance is equivalent to lowside kV (line to line) ^2 / base kVA. For a L-G fault, do the same with zero sequence impedance.
What is the reason for using a neutral current transformer on a transformer?
A current transformer is primarily used at the neutral point of a transformer for earth fault protection. A neutral current transformer will measure any ground fault current which will essentially flow from the star point of the transformer. A fault-detection device other devices is connected to the current transformer and, if the fault current exceeds a certain trigger value, the fault-detection device will give a trip command to an earth-fault relay to disconnect the supply of electricity to the transformer.
What is neutral earth compensator?
For earth fault protection on the windings of a delta connected transformer. Used in MV distribution. An earth fault current return path is provided by connecting a Neutral Earthing Compensator (NEC) between the three phases of the power system and the earth system. This is done at the source of the supply. The NEC transformer winding has a Zig-zag configuration with no secondary winding. The impedance of the winding is high when there is no fault on the system resulting in only a small magnetising current in the transformer windings. The Zig-zag winding configuration results in a low impedance when an earth fault condition occurs. By inserting resistance between the neutral of the Zig-zag transformer and earth, the earth fault currents can be limited to any desired value. The resistance value and rating has been standardised to allow an earth fault current of 300 amp for 10 seconds, although some older installations may still operate at the old standard of 600 amps.
If a 240V circuit is protected by a 15A simi-enclosed fused and has an electrical earth-fault loop impedance of 1.9 ohms determine the earth fault current in the event of a zero impedance earth fault?
I am assuming that its a 240 Volt AC circuit supplying an inductive load with a fault loop impedance of 1.9 ohms at the time of the short circuit. The power factor is assumed to be 0.8 The instantaneous earth fault current value would be; Current = (Voltage x Power Factor) / Impedance (240 x 0.8) / 1.9 192 / 1.9 = 101 Amps. However this may be a trick question as it doesn't ask for an instantaneous value, the fuse will limit the fault current to 15 amps and should disconnect the circuit within 0.4 seconds.
When 440 volts and 300 amps is given as an input to the transformer the output obtained is 11 kva in this secondary current will decrease or not?
Your question reveals a misunderstanding of how a transformer works.The primary current of a transformer is determined by the secondary current, not the other way around. When the secondary voltage is applied to a load, a secondary current flows, and its value is determined by the secondary voltage and the load impedance. This secondary current then determines the value of the primary current.
If impedance of power transformer is increase from 8 to 10 percentage then what will effect?
if the designed value of percentage impedance is change, for general this should affect tow things * if the percentage impedance is decrease this should increase the fualt level current *if the percentage impedance is increase this should increase the transformer losses and tempreture rise so the designed value of percentage impedance is determined according IEC if it is less than or equal 10% the margin should be + or_ 15%, if it is more than 10% the margin should be + or _ 15% so the percentage impedance of this transformer is not accepted according IEC standers
How do you convert percent impedance of transformer to impedance value?
The apparent impedance looking into a transformer from one side will not be the same as looking in it from the other, which is why percent impedance is used. If you are looking from the high voltage winding (I'm labeling #1) to the low voltage winding (#2), you must scale the percent impedance as follows: (% impedance) x (Winding #1 nominal voltage)^2 / (transformer base VA)
Why do you need to measure ground resistance value?
The whole point of earthing or grounding is to provide a low-impedance path back to the supply transformer in the event of an earth fault within a wiring installation, which will then enable sufficient earth-fault current to flow in order to operate the circuit's protective device (circuit breaker or fuse). When designing the circuit's protection system, it is therefore necessary to determine the resistance of the earth (ground) as it determines the value of this current.
HOW TO CALCULATE THE IMPEDANCE FOR THE TRANSFORMER?
The term, 'percentage impedance', is a little misleading, as it is defined as 'the value of primary voltage that will cause rated current to flow in the secondary winding, expressed as a percentage of the rated primary voltage'. So, the test is carried out as follows: the secondary winding is short-circuited through an ammeter capable of reading the rated secondary current. A variable voltage is applied to the primary winding. The primary voltage is gradually increased until the ammeter indicates rated secondary current. That primary voltage is then expressed as a percentage of the rated primary voltage -and that value is the transformer's 'percentage impedance'.
Why impedance relay is called as distance relay?
because the distance is propotional to the impedance of the line ,so the operation of the impedance relay comes into picture when the impedance seen by the relay is less than the pre-setting value.When a fault occurs ,the current increases to a high value and so the Impedence decreases and the relay actuates
Why ELCB are measured in terms of milliampere. how will we measure the capacity required for any application. how do we calculate it?
ELCB's are "earth leakage circuit breakers". They are used in situations where high impedance grounding is used, meaning a phase to ground fault has very low current levels. This results in standard overcurrent/breaker protection not necessarily "seeing" the fault. And I do not believe ELCBs are usually rated in milliamperes. Their interrupting rating, and load is usually similar to MCCBs. They include a leakage current rating, which is in mA (leakage current is current to ground). You calculate how much ground current you will have from a fault study. If you are intentially high impedance grounding (such as for a generator), then you should know the value of impedance you are using, and this value is usually chosen to limit ground current to a specific current (such as 5 amps). If you are high impedance grounded for some other reason, you need to determine the impedance to ground (the best method to do so will depend on your situation); once you know this, you also know your normal line to ground voltage, and expected current flow is a simple calculation.
How do you size zigzag transformer in ETAP software how to find the impedance value in the device coorination mode?
http://www.etap.com/faqs.htm
