Generator rotors are subject to very high stresses as a result of the large rotational forces they undergo at operational speed. The effects of these high stresses are compounded when the rotor “cycles” from standstill to full rotational speed and then back again. This “cycling” can cause accelerated deterioration in generator rotors. The Electric Power Research Institute states that “Cyclic operation can result in an increase in forced outage rates, higher operation and maintenance (O&M) costs, and further wear and tear on components…” There are two primary types of cycling; speed cycling and load cycling. Most industry references do not differentiate between speed cycling and load cycling. Speed cycling is the worst kind of cycling, leading to cracking and fatigue failure of many generator rotor components.
Terminology
A meaningful discussion of cycling first requires a thorough understanding of key terminology.
When applied to generator operation, the term “cycling” is often
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In this mode, the rotor speed stays the same at all times. Load following varies the load to match system demand requirements and requires ramping the unit up and down between unit peak grid demand and minimum grid demand, as required. Many plants that are two-shifting also might be required to load follow (load cycling).
This paper will focus on the generator rotor and how it is affected by speed cycling, which includes an operational technique known as two-shifting. Units operating in this mode are typically employed as peaking units, but even very large generators have been known to be speed cycled on a frequent basis.
IEEE C50.13/ANSI C50.13, “Requirements for Cylindrical Rotor Synchronous Generators Rated 10 MVA and Above,” states that speed cycling is related to the number of start-stop cycles the unit undergoes each year. A start-stop cycle or start is defined as the complete cycle of taking the machine from zero speed, standstill or turning gear, up to rated speed (3,600/3,000 rpm), and then back down to zero speed, standstill or turning gear. This entire cycle is the equivalent of “one start.”
The standard referenced previously provides the minimum numbers for starts during the operating life of the machine. They are different for units operating in base load and peaking: 3,000 starts for base load units and 10,000 starts for peaking units or other frequently cycled units.
Fatigue initiation
The operational reality is different. Units originally designated for base load duty might now be used as peakers. This type of operation can result in premature cracking and failure. Some generator rotor components see crack initiation in as little as 200 starts. Failure can be in as few as 400 starts for some rotor components and within 1,000 starts on others. The “requirement” of 10,000 starts is a good one, but one that often is not reached before component failure occurs. Ten thousand starts are equal to 1 start/day for 27.4 years, or 5 starts/week for 50 weeks for 40 years.
The real issue for generators is the number of starts, because they have a direct correlation to fatigue. Webster defines fatigue as “the tendency for a material to break under stress.” In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading, i.e. repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture.
