Q7Power System Engineering

(a) Separately Excited A.C. Excitation System

In a separately excited AC excitation system, the field current of the main alternator is supplied not from the main alternator's own output (as in a self-excited system) but from an independent, separate AC exciter - typically a smaller AC generator (often a rotating-armature or rotating-field alternator) mounted on the same shaft as the main alternator, or in modern designs, a brushless AC exciter whose rotating armature output is rectified directly on the rotating shaft assembly itself, avoiding the need for brushes and slip rings altogether.

The AC exciter's own three-phase (or single-phase, in smaller designs) AC output is converted to the DC current required by the main alternator's field winding using a rectifier - in a conventional (brush-type) separately excited system, this rectifier is a stationary, non-rotating rectifier bridge, with the resulting DC current fed to the main alternator's rotating field winding through brushes and slip rings; in a modern brushless excitation system, the AC exciter's armature is instead mounted directly on the rotating shaft along with a rotating rectifier assembly, so the rectified DC output is fed directly to the main field winding without any need for brushes or slip rings at all, eliminating the maintenance, wear, and reliability concerns associated with brush-and-slip-ring contact.

The AC exciter's own field current, in turn, is controlled by a pilot exciter and automatic voltage regulator (AVR) system, which senses the main alternator's terminal voltage and adjusts the AC exciter's field current (and hence, through the excitation chain, the main alternator's own field current and terminal voltage) to maintain the desired voltage setpoint despite changes in load or other system disturbances - this multi-stage excitation arrangement (pilot exciter controlling the AC exciter's field, which in turn supplies the main field via rectification) provides a robust, self-contained excitation source that does not depend on the main alternator's own output being already established, which is particularly valuable during black-start conditions (starting a generating station from a completely de-energized state) where a self-excited system, lacking any initial field current source, would have no way to establish its own excitation without an external supply.

(b) Advantages of Interconnected Power System; Cold Reserve

An interconnected power system links multiple individual generating stations and load areas (potentially spanning multiple utilities, states, or even countries) together through a common transmission network, rather than operating each generating station and its local load area as an isolated system, offering several significant advantages.

• Improved reliability: if a generating unit fails in one part of the interconnected system, neighboring interconnected areas can supply the resulting shortfall, reducing the likelihood of local supply interruption compared to an isolated system with no such backup capability.
• Reduced total required reserve capacity: because peak demands in different interconnected areas do not all occur at exactly the same time (due to time-zone differences, differing local load patterns, or simply statistical diversity), the total reserve generating capacity required across the whole interconnected system is generally less than the sum of the reserve capacity each area would need to hold individually if operated in isolation.
• Economic benefits from load and generation diversity: interconnection allows economical power exchange between areas with different generation cost structures or different daily/seasonal load patterns, allowing lower-cost generation in one area to serve load in another area during periods of favorable price/availability difference (economy energy interchange).
• Improved frequency and voltage regulation: a larger interconnected system has greater overall inertia and generation capacity available to respond to load or generation disturbances, generally resulting in smaller frequency deviations for a given disturbance magnitude than would occur in a smaller, isolated system.
• Ability to share large, efficient generating units: very large, highly efficient (and hence lower per-unit-cost) generating units may only be economically justifiable if they can serve a sufficiently large combined load, which interconnection makes possible even for individual areas whose own local load alone might be too small to economically justify such a large unit.

Cold reserve refers to standby generating capacity that is available to be brought into service but is not currently running or synchronized to the system, and requires a comparatively long start-up time (from a completely shut-down, cold condition, potentially several hours for a large thermal unit needing gradual boiler warm-up) before it can begin supplying power - this distinguishes cold reserve from spinning (hot) reserve, which refers to already-synchronized, already-running generating capacity that can respond to a load increase or generation loss within seconds, and from quick-start reserve (units, typically gas turbines, capable of achieving full output within a few minutes from a stopped condition), reflecting the general classification of reserve capacity by how quickly it can actually be made available to the system in response to a contingency.

It is also worth noting the historical progression of excitation system technology that separately-excited AC systems represent a middle stage of: starting from the earliest DC excitation systems (discussed in relation to the alternate question in this examination), through separately excited (and eventually brushless) AC exciter systems as described here, and finally to today's fully static excitation systems, which dispense with any rotating exciter machine entirely and instead derive the field current directly from a thyristor rectifier bridge fed from the main generator's own terminal voltage (or a dedicated excitation transformer) - this progression has been driven consistently by the goals of faster excitation response (important for power system stability, since faster field-forcing capability during a disturbance directly improves the equal-area-criterion decelerating-area margin examined elsewhere in this examination), reduced maintenance burden, and improved overall reliability.

The interconnected power system advantages listed above also carry an important corresponding responsibility: because interconnection links the fortunes of multiple areas together, a sufficiently severe disturbance in one part of a large interconnected system can, if not properly contained by adequate protection and control systems, potentially propagate and cascade into other interconnected areas that would otherwise have been entirely unaffected - this is precisely why interconnected system operation requires carefully coordinated protection schemes, adequate inter-area transmission capacity, and well-defined operating procedures and reserve-sharing agreements among the interconnected utilities, to ensure that the genuine reliability and economic benefits of interconnection are realized without introducing new cascading-failure vulnerability that would not exist in smaller, isolated systems.