To increasedecrease the alternating volts in electric power, transformers are used. Transformer short circuit fault current calculator jcalc. Transformer calculator kva calculator full load amps. Transformer fault current calculation electrical engineering portal. Calculates the short circuit fault current level of a transformer on an infinite bus. Before the available fault current for equipment within your facility can be determined, you need to know the available fault current at the secondary terminals on the utility transformer feeding your facility. Perform device duty calculations using etaps short circuit analysis software which allows you to determine fault currents and automatically compare these values against manufacturer short circuit current ratings. An online three phase transformer calculator to find the current in different power units. Shortcircuit calculations using transformer and source. A 3 phase transformer has three magnetic circuits that are interlaced to give a uniform distribution of the. The term inrush current means the current at the time of energising the transformer every time to a healthy down stream circuit. This calculator helps determine the current values on both sides of the transformer, and estimates available short circuit current. Enter the primary and secondary transformer voltages.
Threephase shortcircuit current isc calculation at any. Short circuit analytic sca software and mobile apps perform available short circuit fault current calculations in threephase electric power systems you are working with. I have a question on how to calculate auto transformer secondary fault currents. Those would include such software applications as skm systems analysis tools. Fault current calculations are based on ohms law in which the current i. How to calculate transformer primary current sciencing. If for some reason you need a larger transformer to operate appliances, you still divide the wattage by the voltage to find the current.
How do you calculate the maximum possible available fault current. Lets first consider base kva and kv for ht and lt side. Welcome to schneider electrics fault current calculator. This quick calculation can assist you identify the fault current on the secondary of a transformer for the aim of selecting the proper overcurrent protective devices which can interrupt the available fault current. Calculating shortcircuit current on secondary of main transformer.
Step 2 calculate the short circuit current on the transformer secondary bus. This can also be used as a transformer kva calculator and transformer secondary short circuit current calculator. Short circuit current at transformer calculation 08282010 9. A fault, i fs shown on the secondary, will also been seen on the primary side of the transformer i fp. The fault current of a power transformer will depend on the following. An infinite bus shortcircuit calculation can be used to determine the maximum shortcircuit current on the secondary side of a transformer using only transformer nameplate data. How do you calculate the maximum possible available fault. As i understand it an auto transformer will transform only a portion of the required secondary current and the rest is supplied directly from the primary line.
Replacements that result in a higher possible fault current, than that of the existing equipment, shall be addressed to this department, prior to reconnection of existing service equipment. Once the fault currents have been calculated, you can then select overcurrent. It is interesting to see that increasing the available fault current from the utility, assuming a starting point of 50 ka, doesnt have as great of an impact as one would think. See note under step 3 of basic pointtopoint calculation procedure. Per unit analysis can be used to calculate system three phase fault levels and the current distributions. Calculate fault current at each stage of following electrical system sld having details of. Calculator can be used for step down transformer as well as step up transformer sizing applications.
Transformer fault current calculation blakley electrics. To calculate the maximum fault current that can be achieved in a circuit fed by a transformer we refer to the below formula this formula calculates the loop impedance at the end ofa circuit fed from the secondary winding of a transformer. Short circuit calculations with transformer and source. Simple methods for calculating short circuit current without a. Enter the transformer rating and select the appropriate unit. Short circuit currents impose the most serious general hazard to power distribution. Click on the calculate button to obtain the results. Fault current full load amps % impedance z example. Insert the the kva rating power factor pf and impedance z of the transformer is not required to continue.
Infinite bus short circuit calculation brainfiller, inc. Three phase transformers divide the values for vs secondary voltage by v3 and the va. In a real transformer, due to losses, the power at the secondary coil will always be smaller than the power at the primary coil. Because, the voltage level on the primary of the transformer is different than the secondary, the two values of current will be different. Short circuit current calculations cooper industries. A protective device is appropriate for the application if its secondary fault interrupting rating is equal to or greater than the value for i calculated above. Or if you have the available fault current, either given to you from utility, or if you are using this program to calculate fault currents at other parts of the system where the aic ampere interrupting capacity is known, input this number and it will automatically override the. Transformer capacity calculator three phase transformer. The image shows a simple electrical system around a transformer primary voltage v p, secondary v s. In this situation, we assume cable length to be minimal and therefore neglect its. Input the nameplate transformer impedance as a percentage, not a decimal i. The peak let through with an infinite source worst case scenario would have 208 0.
Secondary fault current tables effective october 24, 20 page 4 of 27 this information is only for use in determining service equipment ratings and is not to be used for selecting personal protective equipment. If you put a dead short on the secondary and slowly increase the primary voltage from zero until the secondary current is at the rated maximum. Overstressed device alarms are displayed on the oneline diagram and included in short circuit analyzer and study reports. This design solves the fault current problem however the transformation costs. Short circuit analysis short circuit software etap. The ln fault current is higher than the ll fault current at the secondary terminals of a singlephase centertapped transformer. It has two types, single phase and three phase transformer. Calculate the winding impedances for cases 3 and 5 listed in table 1, and then. Electrical calculator mc group electrical calculation. This is a good and simple method for determining the worstcase maximum shortcircuit current through the transformer since it ignores the sourceutility impedance. Calculate the maximum fault current for a transformer rated. Single phase transformer fullload current amps kva. Insert available fault current ampere rating from your local utility step 1b.
For calculation of the maximum fault current that can be potentially achieved within a transformer fed circuit, we use the formula shown in the tool screenshots beginning with. Is a the current at the secondary coil ip a the current at the primary coil note that the electrical power in the primary coil and secondary coil are the same. Use the 3 phase and single phase transformer sizing calculator below to calculate primary and secondary full load current. There can be a variation in the transformer impedance values, but generally the transformers that blakley electrics use will be in the 45% range of primary voltage. Distribution transformer impedance and fault currents. Current of protective circuit breaker short circuit fault current short circuit faultvoltage losses of the distribution transformer. Calculator this calculation is based on the infinite source method at the primary of the transformer, the following calculates the fault current at the transformer secondary terminals. For a 120volt primary, 2000watt transformer, divide 2000 by 120 for the current 2000 watts 120 volts 16. Transfomer rating kva secondary transformer voltage v transfomer. Fault level at ht incoming power supply is 360 mva. Short circuit current at transformer calculation cr4. The inherent secondary fault current for a 204050mva, 115kv, 8%impedance transformer would be.
Where one transformer is larger than the others in the bank, use the fault current value for the larger transformer. Lets say we have a transformer rated 1500 kva with a secondary voltage of 480y. Customers should be advised that the fault current calculations were performed using infinite bus at the transformer secondary terminals. One criterion in selecting a distribution substation transformer protective device is its ability to protect the transformer from secondaryside limited faults or. Transformer rating in kvamva per unit impedence of the transformer %p. The transformer calculator uses the following formulas.
The programs take into consideration all of the key electrical parameters of power distribution system including power supply, cables, transformers, motors and generators. This is usually provided by the utility and gives the maximum possible fault current. Three phase fault example three phase fault example. This is how they determine the impedance of power transformers without destroying them. For example, a 75kva transformer with a 208y120 secondary and 5% impedance has a capacity of 208 amps on the secondary. Fault current calculations are a critical piece of the electrical designengineering puzzle for electrical distribution systems in commercial and industrial installations.
How to calculate the fault current at the primary side of. Step 3 calculate the short circuit current on the transformer secondary bus, but this time we use the transformer impedance and the source impedance. Transformer maximum fault current allowing for loog impedance. Distribution transformer fault current tables for transformers in the downtown network area contact a network design representative to obtain the fault current. Selecting overload protection and wires for transformers involves number of rules that change with transformer size, type, and placement of the overload protection. Transformer current calculator voltage disturbance. How to calculate fault current for transformer answers. Easy and simple methods for calculating short circuit curr. Shortcircuit current calculations cooper industries. This quick calculation can help you determine the fault current on the secondary of a transformer for the purpose of selecting the correct overcurrent protective devices that can interrupt the available fault current.