Distribution transformer calculations ms excel spreadsheet. Calculating overcurrent protection, transfer switches and. By dwight chestnut circuit breakers protect transformers from overcurrent situations and short circuits. Distribution transformer calculations spreadsheet provides determining the mvlv. The transformer is about 20 feet away, and a 100a fusible disconnect fused at 90a was installed at the transformer. Neplan overcurrent protection selectivity analysis back to overview. How to size an overcurrent device for a transformer. These faults are difficult to detect by the overcurrent relay of the lineterminal circuit.
This transformer calculator helps you to quickly and easily calculate the primary and secondary fullload currents of the transformer. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. This inrush current is dependent upon where in the sine wave the transformer was last turned off in relation to the. Instantaneous, definite time, idmt, directional or nondirectional are types of overcurrent characteristics often used in protection from basic to advanced feeder applications. Electrical calculation tools is a set of 7 computer aided online tools designed to help you. Each relay protection device comprises a definitetime delay overcurrent protection relay in which the trip of the current sensitive element starts the time delay device. There setting methods are same but the only difference is that pick up of 51 is higher than that of 51n. Overcurrent protection for a transformer on the primary side is typically a circuit breaker. Transformer secondary protection is not required for the following conditions. If you need assistance with circuit protection devices such as fuses or breakers, or wire sizing, please consult a licensed electrical contractor or electrician in your area who will be familiar with state and local codes. In some instances where there is not a high voltage panel, there is a fused disconnect instead. These elements are part of the transformer protection in that they limit the accumulated. A large number of calculation provisions are in the national electrical code nec. Calculation of size of transformer, fuse and circuit breaker.
Current transformer requirements for non directional. Section 6 provide the primary transformer fault protection. How to overcurrent protect 600 volt class transformers and associated wiring per nec 4503b and nec 2403. Neplan overcurrent protection selectivity analysis.
How to determine the correct overcurrent protection for. Nec electrical design software performs electrical design. Using overcurrent protection for generator conductors. Circuit breakers protect transformers from overcurrent situations and short circuits. Faster than the heavy full features electrical network calculation software. The bussmann series pointtopoint shortcircuit current calculator is a simple.
Protection and overcurrent coordination part 2 youtube. One of the more confusing areas of the code involves the what, where, and when of overcurrent protective devices protecting conductors originating at generator terminals. I have a distribution panel that is rated at 208120v, 3ph, 4w and need to provide a new 75kva transformer to feed a new 480277, 3ph, 4w panelboard. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead. This module is used for the coordination of various overcurrent protection devices in a given network. Transformer series part 2 calculating the primary and.
Please check our article ct connection to sense earth faultearth faults detection philosophy to know earth fault is detected. Transformer overcurrent protection is required to protect the primary windings from short circuits and overloads and the. To avoid using over sized conductors, overcurrent devices should be selected at about 110 to 125 percent of the transformer fullload current rating. Recommendations for overcurrent protection ul and csa.
Combined overcurrent and earthfault relay spaj 140 c. Calculating overcurrent protection, transfer switches and more. It also determines the turns ratio and type of transformer. Select the number of phases from the dropdown menu. The combined overcurrent and earthfault relay spaj 140 c is used for the selective shortcircuit and earthfault protection of radial feeders in solidlyearthed, resistanceearthed. The bussmann series transformer protection guide program translates nec. In order to calculate the overcurrent protection on the secondary side, most of the same principles apply as with the primary calculations, with a few exceptions. Here is how to calculate the overcurrent protections amps. Such overcurrent protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer. Calculate size of circuit breaker or fuse on primary and secondary side of transformer having following detail transformer detailsp kva primary voltage vp 1 volt secondary voltage vs 430 volt transformer impedance 5% transformer connection delta star transformer is in unsupervised condition.
Introduction for reliable and correct operation of the overcurrent protection the current transformer ct has to be chosen carefully. Transformer calculator kva calculator full load amps. Calculate size of circuit breaker fuse for transformer. Once the circuit breaker opens or trips due to a short circuit or. Overcurrent protection for transformer mike holts forum. Since short circuit current is minimized, a smaller gauge wire may be used between the transformer. As a reminder, low voltage is defined in the canadian electrical code ce code as any voltage exceeding 30 v but not exceeding 750 v. You have a 2kva 2000 va transformer and your primary voltage is 460vac and your secondary voltage is 120vac. Calculation tools for low voltage electrical network. Conductor sizing and overcurrent protection calculations for continuous and noncontinuous loads the type of the load continuous or noncontinuous must be considered in the national electrical code requirements for conductor sizing and overcurrent protection. Transformer overcurrent protection electricity forum. Calculation of idmt over current relay settings 505150n51n calculation model for thermal relay siemens 7sj64. Wiring transformer overcurrent protection for primary.
Display the timecurrent curves of 1 to 8 circuitbreakers. The secondary winding overcurrent protection is typically set with two steps. These variations in voltage and current on the output of a photovoltaic power system result in more complex code calculations for fault current protection and overcurrent protection than is normally found in the typical ac circuit calculations for the same parameters. These are the two most common ways to provide overcurrent protection on the primary side. Multiwinding current differential protection protect power transformers with as many as four windings by using a combination of single or dualslope percentage differential characteristics, overcurrent protection, and restricted earth fault ref protection. A collection of really good online calculators for. Transformer conductors and overcurrent protection iaei. Primary protection only is required if the transformer is singlephase and the secondary has only two wires. The rej 523 is based on a microprocessor environment. The suite of overcurrent elements 51, section 8 is generally considered backup transformer protection, or for protection of the bus and backup protection for the feeder relays. Nec guidelines for transformer and transformer feeder. Calculating overcurrent protection device rating on the low voltage side of the. For a 480v208v 3o transformer, the calculations will use 208v x v. Electrical calculator mc group electrical calculation software.
The distortion of the secondary current of a saturated ct may endanger the operation, selectivity and coordination of protection. Im working on overcurrent relay setting of primary transformer protection 2000kva with 6% impedance with 2005 ct ratio. Electrical calculator mc group electrical calculation. Quick commissioning of electrical vfd using siemens starter software duration. Faults close to generator terminals may result in voltage drop and fault current reduction, especially if the generators are isolated and the faults are severe. Since im coming off a 208120v distribution panel and increasing the voltage to 480277v am. What does the nec actually require in the way of overcurrent protection for conductors supplied by a generator. Calculation of phase fault overcurrent relay settings 9. Overcurrent protection overcurrent protection can be used for primary protection, or as a backup for phase overcurrent or earth fault protection. The response of the overcurrent protection devices is visualized in the timecurrent characteristic tcc curve. Due to the long lead time for repair of and replacement of transformers, a major goal of transformer protection is limiting the damage to a faulted transformer. The overcurrent protection required for transformers is consider for protection of transformer only. Overcurrent protection fundamentals r ced engineering. Introduction transformers are a critical and expensive component of the power system.
Recommendations for overcurrent protection ul and csa north american standards ul and csa north american standards north american standards, including ul 508, national electric code 450, and the canadian electrical code, part 1, require overcurrent protection on all control circuit transformers. A selfs upervision system continuously monitors the operation of the relay. Up to date with schneider electric low voltage protection devices and circuit breakers. Transformer secondary protection although ocpds are required as primary protection for transformers with no exception, there are a few scenarios where overcurrent protection is not required at the transformer secondary, as per tables 450. Wiring transformer overcurrent protection for primary and. Knowing where to find those calculation provisions and how to perform them is essential to electricians, engineers, inspectors and others involved in the electrical industry.
Transformer calculations before beginning to discuss the. They also protect circuits downstream from the transformer. Overcurrent protection rating and location are shown in diagram a. Overcurrent protection fusing and circuit breaker protection. Enter the transformer rating and select the appropriate unit. Based upon basic information about a transformer, the program will take you through a flow chart to arrive at the maximum fuse value that can be used for your particular transformer. Mohammed tawfeeq overcurrent protective relays overcurrent relays overcurrent relaying is the simplest and cheapest type of protection used for protection of lines, transformers. Definitions of terms used in this white paper can be found in the technical application guide section of the powrgardcatalog. Overcurrent coordination setting guidelines transformers. The xspider software system is a graphically oriented design system for. The overcurrent relay rej 523 is intended for selective shortcircuit protection in medium voltage distribution networks but can also be used for protection of generators, motors and transformers. More conductor sizing and overcurrent protection calculations. Selecting overload protection and wires for transformers involves number of rules that change with transformer size, type, and placement of the overload protection. Software support blog about me my protection guide.
Once the circuit breaker opens or trips due to a short circuit or some other overcurrent scenario, the circuit breaker blocks current flow to the circuit. How to determine the correct overcurrent protection for transformers primary overcurrent protection a transformer has all the same component parts as a motor, and like a motor, exhibits an inrush when energized. The sel387a current differential and overcurrent relay is a lowcost solution for twowinding transformer protection. Relay setting of idmt and instantaneous over current and. This calculator helps determine the current values on both sides of the transformer, and estimates available short circuit current.
The table shows the maximum over current as a percentage of the transformer current. Overcurrent relay protection is given at b, c, d and e, that is, at the infeed position of each part of the electrical power system. Calculate idmt over current relay setting 5051 october 11. Based on system operating and equipment sizing practices equipment protection is not always possible. Calculation of size of tran sformer, fuse and circuit breaker.
Secondary overcurrent protection overcurrent devices are used between the output terminals of the transformer and the load for three reasons. To start we will look at transformer overcurrent protection. Protect the transformer from load electrical anomalies. Overcurrent protection can be used for primary protection, or as a backup for phase overcurrent or earth fault protection. Sir please clear that,in higher over current setting calculation,how you have considered relay plug setting is 2. Overcurrent relays with voltage control engineering360.