Calculating Generator Reactances
Calculating generator reactancesPower topic #6008 | Technical information from Cummins Power Generation
Calculating generator reactances
> White paper By Timothy A. Loehlein, Technical Specialist-Electrical
Generator reactances are used for two distinctly different purposes. One use is to calculate the flow of symmetrical short circuit current in coordination studies. A second use for generator reactances are in specifications that limit the sub-transient reactance to 12% or less in order to limit the voltage distortion induced by non-linear loads. For either short circuit or harmonic distortion analyses, the stated reactances will need to be converted to a common base to make valid comparisons. Typically, generator reactances are published in per unit values on a specified base alternator rating. Where the generator set rating differs from the alternator base rating it will be necessary to convert the per unit values from the alternator base rating to the generator set rating. For selecting circuit breakers with adequate AIC rating the maximum asymmetrical short circuit current to flow in the first half cycle may be approximated from the generator sub-transient reactance (x"d ) and a factor to account for DC offset.
The flow of current in an AC circuit is controlled by impedance. When a short circuit fault occurs in a distribution system the fault current that flows is a function of: 1. the internal voltage of the connected machines in the system (generators and motors),
2. the impedance of those machines, 3. the impedance to the point of the fault, mostly cable impedance, 4. and the impedance of the fault, if arcing. The generator internal voltage and generator impedance determines the current that flows when the terminals of a generator are shorted. The effect of armature reaction on the generator air gap flux causes the current to decay over time from an initial high value to a steady state value dependant on the generator reactances. Since the
Calculating generator reactances
> White paper By Timothy A. Loehlein, Technical Specialist-Electrical
Generator reactances are used for two distinctly different purposes. One use is to calculate the flow of symmetrical short circuit current in coordination studies. A second use for generator reactances are in specifications that limit the sub-transient reactance to 12% or less in order to limit the voltage distortion induced by non-linear loads. For either short circuit or harmonic distortion analyses, the stated reactances will need to be converted to a common base to make valid comparisons. Typically, generator reactances are published in per unit values on a specified base alternator rating. Where the generator set rating differs from the alternator base rating it will be necessary to convert the per unit values from the alternator base rating to the generator set rating. For selecting circuit breakers with adequate AIC rating the maximum asymmetrical short circuit current to flow in the first half cycle may be approximated from the generator sub-transient reactance (x"d ) and a factor to account for DC offset.
The flow of current in an AC circuit is controlled by impedance. When a short circuit fault occurs in a distribution system the fault current that flows is a function of: 1. the internal voltage of the connected machines in the system (generators and motors),
2. the impedance of those machines, 3. the impedance to the point of the fault, mostly cable impedance, 4. and the impedance of the fault, if arcing. The generator internal voltage and generator impedance determines the current that flows when the terminals of a generator are shorted. The effect of armature reaction on the generator air gap flux causes the current to decay over time from an initial high value to a steady state value dependant on the generator reactances. Since the