Ferroresonance and Loading Relationships for DSG Installations

Abstract

Self-excitation of induction generators or small synchronous generators connected to distribution feeders is analyzed during the transient period following the islanding of a generator on the feeder. The relationship of feeder load after separation and the occurrence of ferroresonance with an isolated induction, synchronous or single-phase generator are developed. The results indicate that an isolated distribution system generator (DSG) can theoretically support as much as three times the generator's rated power output in a ferroresonant mode provided the prime mover has the needed inertia or torque available at the abnormal isolated speed. The abnormal voltage and frequency operation during separation modifies load, generator, and prime mover characteristics and may result in an energy balance, although such balance is not obvious before separation. INTRODUCTIONThe resurgence of interest in the application of induction generators in power systems has been driven by the PURPA legislation of 1978. Induction generation has been viewed as an attractive option for the small power producer because it is a seemingly simple device which is relatively inexpensive and easy to maintain. Under normal steady-state conditions this assessment is largely true. However, when switching operations are required on the feeder to which such generation is interconnected the transient response may be far from simple. Under specific feeder conditions the damage that can be sustained by the feeder or the unit during these transient periods can eliminate any savings in first cost or operating cost.The 1935 paper by Bassett and Potter is one of the earliest papers to treat the subject of self-excited operation of the induction generator.1 Their steady-state analysis promoted the concept that stand-alone operation of induction generators was plausible using carefully sized capacitors to provide the source of excitation. In a 1939 paper, Dr. Wagner first analyzed the problem of accidental separation of an induction motor with terminal connected power factor correcting capacitors.2 His quasisteady-state analysis clearly demonstrated that high overvoltage could be produced by high inertia shaft loads or "...applications are known in which gas or gasoline motors are connected to the samne shaft with the induction motor and the utilization device, so that, in the event of the removal of the electric power source, the armature can actually increase in speed and remain at the increased speed until manual readjustments are made." Wagner's work was the first to show that any overspeed during a separation transient will be in the direction to aggravate the overvoltage problem. His paper also gave evidence that a machine will self-excite under load "very nicely" during isolated operation.in more eloquent mathematical form.3 Barkle and Ferguson introduced the use of the induction machine circle diagram to predict overvoltages and the range of parameters that would produce overvoltage operation. However, their analysis was also a quasi-steady-state appr...

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“…Ferroresonance can lead to significant overvoltages; peak voltage may reach 3-4 pu [1], [20]. Ferroresonance may occur when both induction and synchronous machines are connected to the circuit [21].…”

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“…This usually occurs when; the isolated load is less than three times the rating the rating of the DG, there is adequate capacitance, and there is at least one transformer remaining connected to the island. [4] An isolated distribution system generator can theoretically support as much as three times the generator's rated output power in a ferroresonant mode provided the prime mover has the needed inertia or torque available at the abnormal isolated speed. [5] IV.…”