A Comparison of the Transient Response of Two and Four Stroke Diesel Powered Generator Sets 810920

The response of two and four-stroke diesel powered electric generator sets to transient loading was investigated. The effects of addition and removal of load on generator output voltage and frequency were determined for comparison of the transient response characteristics of two-stroke diesel-generator sets operating at 1200 and 1800 rpm, and a four-stroke diesel-generator set operating at 1800 rpm. Three engine-generator sets of approximately 55 KW_e capacity, representing a cross-section of the many available, were connected to a load cell used to instantaneously change the load on the diesel generators. Instrumentation was provided to record output frequency and voltage, electric power, and instant of load change (current monitor). Maximum steady state load was established for each engine-generator set, and the load was changed in increments of this maximum, from various steady state base loads. The time required for the frequency to return to steady state, and the magnitude of the maximum frequency deviation resulting from a load change were determined, and compared for the three generator sets.

The response time (time to return to frequency stability following a load change) was found to be the shortest for the 1800 rpm two-stroke generator set, and longest for the 1800 rpm four-stroke set, with the response of the lower speed two-stroke set in between. The response time of the four-stroke set was two to three times as long as that of the generator sets equipped with two-stroke engines. Response times for all the generator sets increased, as the magnitude of the load change increased, up to a point, and thereafter remained relatively constant with load change. The relationship among the generator sets' response times held over the entire range of load additions and removals, as well as for all base loads from which the load changes were initiated.

Maximum frequency deviations resulting from a load change were dependent upon the response rates of the engines' governors, and were determined to increase linearly with the magnitude of load change. The low speed (1200 rpm) two-stroke generator set experienced the largest frequency deviations, whereas the deviations of the other two sets were approximately the same with the high speed (1800 rpm) two-stroke having a slight advantage.

Output voltage deviations resulting from a load change were found to be negligible over the entire range of base loads and load changes investigated.