Date of Award
Open Access Dissertation
Roger A Dougal
High Frequency AC (HFAC) power systems - systems having frequencies higher than the usual 60 Hz - may have advantages in some applications, especially where small size and weight are important (aircraft, ships, etc), or where variable operating speed increases efficiency. While 400 Hz systems are widely used in aircraft, these generally do not include parallel-connected generators that operate at megawatt power levels, which is our domain of interest in this research. In particular, we are interested in micro-grid power systems in the 10-100 MW range, consisting of several generators operating in parallel at system frequencies above 60 Hz. This type of system is of interest for many industrial and commercial applications, especially ship and marine systems.
There is little historic precedent for HFAC power systems, and the operating frequency limits of these systems are not well defined, especially in regard to how intrinsic stability depends on physical factors such as the inertias of rotating machines, the impedances of power buswork, and the operating speeds of circuit protection devices.
In this research, we first explored the benefits of higher-frequency systems, including how weight and volume of equipment such as synchronous generators are reduced, and found that generator power density scales proportionally with frequency. But as the power density increases, the inertia constant of a rotating machine decreases, and can easily become smaller than two seconds, which threatens stability since stability often depends on a large inertia constant.
Increasing frequency was found to deteriorate rotor angle stability under both large-signal and small-signal conditions. For large perturbations caused by short circuits, the Critical Clearing Time (CCT) was found to scale proportionally to the inverse square-root of system frequency. Thus, successful use of HFAC systems requires development of faster-acting circuit protection devices. The upper limit of operating frequency occurs where the operating time of available short-circuit protection devices equals the CCT. Existing circuit breaker technologies appear to support system frequencies as high as 800 Hz. Large-signal stability, studied via extensive simulation tests, confirmed conclusions drawn from the fundamental analysis -- the low inertias in typical micro-grids aggravate the stability problem in higher frequency systems. At the higher angular speeds associated with higher system frequencies, rotor angles can diverge and then quickly exceed the critical value, resulting in the faster loss of synchronism in HFAC systems. This emphasizes the necessity for developing fast-acting circuit protection devices. On the other hand, small-signal stability, studied by eigen-analysis, showed the sensitivity of, and dependency of, stability on some key parameters such as generator damping coefficients or inertia constants. For example, larger inertia constants tend to benefit transient stability but deteriorate the small-signal stability, especially at system frequencies above 1000 Hz. The higher the frequency, the greater the sensitivity; the range of system parameters that permits stable operation of a 3000 Hz system is much narrower than the range of parameters acceptable in a 60 Hz system.
Zheng, H.(2014). High Frequency AC Power Systems. (Doctoral dissertation). Retrieved from http://scholarcommons.sc.edu/etd/2684