While some of the above data is useful, the machine impedances, if furnished, are supplied on a separate data sheet. The machine nameplate data required for short-circuit calculations is noted by an *. Typical data on the nameplate is as follows: The data available from the machine nameplate is not significantly complete for an accurate short-circuit calculation. (Requires voltage level and MVA base, usually 100 MVA)įigure 1 – MV dual fed switchboard with 2/3 type transfer (Requires voltage level at which ohms are calculated) (Requires voltage level at which current is calculated) Fault current with phase angle or X/R ratio.(Requires voltage level at which MVA was calculated) Typical forms of the data received from the utility is given below: If the plant has more than one connection point, then a more complex equivalent is required and part of the utility may have to be represented. When one set of impedances is furnished by the utility, it is generally assumed to be the maximum short-circuit value or the first-cycle value. Generally, the source impedance from neither a separate R and X (R only and X only) nor first-cycle and interrupting time calculations will be available. To explore the effects of changing %impedance on the secondary short circuit current, the following calculator can be used.In addition, the equivalent source impedance will be from a complex R + jX calculation. Isolation transformers with appropriate %Z can be inserted in front of large non-linear loads to control harmonics, reduce voltage notching.In this situation an isolation transformer with required %Z is inserted in front of the drive. Certain type of drives requires a minimum source impedance for functioning properly.Important parameter when paralleling transformers.Lower %Z allows higher secondary fault current.A situation where high leakage reactance (or high %Z) is required is when controlling the magnitude of prospective secondary fault current is desired. The leakage reactance value is under the control of the transformer designer and manufacturers can make custom impedances for customers if required. Higher primary voltage (Thicker insulation between windings).In general leakage reactance increases with: This leakage flux will cause voltage drop on a loaded transformer. V 1* I 1= V 2* I 2 In a practical transformer, not all primary flux will be one hundred percent coupled to the secondary and there is some ‘leakage flux’. Ideally all the magnetic field flux from the primary side couples 100% to the secondary and hence the following ideal equation can be established. A transformer is a nothing but two (or more) coils coupled by magnetic field. There is another way of defining the %Z or % Impedance or leakage reactance. The % of primary voltage that is required to produce the rated secondary current is what is known as %impedance or per-unit impedance. The variable AC source voltage is increased until rated secondary current flows. For short circuit test, the secondary (usually the low voltage side) is short circuited and a variable AC source is connected to the other side (usually the high voltage side). Open circuit test establishes the no-load core losses etc. One is open circuit test and the other is short circuit test. If you are in the same situation, keep reading! There are two key tests for a transformer soon after production. Often times engineers do not have a clear idea on what this parameter is and how it influences the performance characteristics of the transformer. Ever wondered what is the %Z or %impedance or short circuit impedance that everyone refers to when talking about power transformers? %Z is a key parameter in power transformers and its value determine the available short circuit current at the secondary of the transformer (to a large extent), voltage regulation, impedance offered to harmonics originating at the secondary side etc.