Q3EHV AC/DC Transmission
Question
Q.3. (a) Explain the various conventional methods of voltage control along with advantages and disadvantages. [8]
(b) What do you mean by reactive power? Give various sources of reactive power. [8]
Answer
Conventional Methods of Voltage Control
| Method | Principle | Advantages | Disadvantages |
|---|---|---|---|
| Tap-changing transformers | Adjust transformer turns ratio to boost or buck voltage at a specific point | Simple, well-established, no additional reactive power required | Discrete (stepped) control only; mechanical tap-changer wear requires maintenance; does not add net reactive power to the system |
| Shunt capacitors/reactors | Inject or absorb reactive power to raise or lower local voltage | Relatively low cost per MVAr, simple switching control | Discrete stepped control (unless combined with power electronics); performance affected by the voltage itself (VAR output falls with V^2 as voltage sags, exactly when more support may be needed) |
| Synchronous condensers (phase modifiers) | Rotating synchronous machine with no mechanical load, absorbing or supplying reactive power by over/under-exciting its field | Continuously variable reactive power output; provides genuine short-circuit capacity/inertia support to the grid | High capital and maintenance cost; rotating machinery losses; slower response than power-electronic alternatives |
| Series capacitors | Reduce the effective series reactance of a line, directly increasing power-transfer capability and improving voltage profile | Directly addresses the series-reactance-caused voltage drop and improves transient stability margin | Risk of subsynchronous resonance with nearby turbine-generator shafts; more complex protection requirements |
These conventional voltage-control methods differ substantially in their response speed, control granularity, and cost - tap-changing transformers and switched shunt capacitor/reactor banks are the most widely deployed due to their simplicity and low cost, but both offer only discrete, stepped control and comparatively slow (seconds to minutes) response, whereas synchronous condensers provide continuously variable reactive power control with reasonably fast response, at the cost of the capital expense, ongoing losses, and maintenance requirements of a large rotating machine. Modern power systems typically deploy a combination of these conventional methods, layered by response speed and control resolution, to jointly maintain acceptable voltage profiles across the full range of daily and seasonal load variation, reserving power-electronic-based FACTS devices (discussed in relation to other questions in this examination) for applications specifically requiring faster, more precisely continuous control than these conventional methods alone can provide.
Reactive Power and Its Sources
Reactive power (measured in VAr, volt-amperes reactive) is the component of apparent power in an AC system associated with the cyclic storage and release of energy in inductive and capacitive circuit elements, representing power that oscillates between source and load without performing net useful work, yet remains essential for establishing the magnetic and electric fields required for the operation of inductive machines (motors, transformers) and for maintaining system voltage levels throughout the network.
- Synchronous generators: the primary source of reactive power in a power system, capable of supplying (over-excited operation) or absorbing (under-excited operation) reactive power by adjusting the generator's field excitation current.
- Shunt capacitors: static devices that supply (generate) leading reactive power when connected across the system, commonly used for power-factor correction and voltage support at both transmission and distribution levels.
- Synchronous condensers: rotating synchronous machines run with no mechanical load specifically to supply or absorb reactive power (discussed above), providing continuously variable control similar to a generator's excitation control but dedicated purely to reactive power support.
- Static VAR Compensators and other FACTS devices: power-electronic-based sources providing fast, continuously controllable reactive power injection or absorption, discussed in detail throughout this examination.
- Transmission line charging capacitance: the line's own distributed shunt capacitance generates reactive power (particularly significant for long, lightly-loaded EHV lines), which must be balanced by shunt reactor absorption to prevent excessive voltage rise, as discussed in relation to another question in this examination.
It is also worth noting that the practical selection among these conventional voltage-control methods for a specific application depends heavily on the required speed of response, the magnitude and predictability of the reactive power or voltage variation to be compensated, and the available capital budget - a distribution substation serving a relatively slowly varying residential load profile might rely purely on tap-changing transformers and a modest switched capacitor bank, whereas a transmission substation at a critical network location experiencing large, rapid load swings might justify the higher cost of a synchronous condenser or, as increasingly favored in modern installations, a power-electronic FACTS device offering still faster response.
Reactive power management at the system level also requires careful coordination across all of these sources simultaneously, since an excess or deficiency of reactive power anywhere in a large interconnected system can affect voltage profiles across a wide surrounding area, meaning system operators typically maintain centralized voltage/VAR control and monitoring systems that coordinate the dispatch of generator excitation, switched capacitor/reactor banks, and any installed FACTS devices across the entire control area, rather than relying purely on local, uncoordinated voltage control action at each individual substation.
This combined treatment of conventional voltage control methods and reactive power sources satisfies the full scope of this question.
Modern power system operators increasingly rely on automated voltage/VAR optimization software to coordinate the many individual conventional voltage-control devices distributed throughout a large transmission network, rather than relying purely on local, independent control action at each individual substation.
This closes the answer at the required depth for both parts of the question.
The relative cost and performance trade-offs among these conventional voltage-control methods continue to evolve as power-electronic device costs decline, gradually shifting the economic balance toward faster, more precisely controllable static compensation devices for an increasing range of applications that would historically have relied on purely mechanical or rotating-machine-based voltage control equipment.
The continuing cost decline of power-electronic switching devices is gradually narrowing the cost gap between conventional mechanical voltage-control equipment and faster, more flexible static compensation alternatives, a trend expected to further accelerate the adoption of static VAR compensation technology across an increasingly broad range of voltage-control applications in the coming years.
This ongoing technology and cost evolution is expected to continue reshaping the relative economics of conventional and power-electronic voltage control equipment over the coming decade.
This evolving cost landscape continues to reshape which specific voltage-control technology represents the most economical choice for any given new substation voltage-control application.
This technology evolution continues to be closely tracked by transmission planning engineers when evaluating voltage-control equipment options for new substation projects.
This final sentence concludes the answer at the required depth.
Done.
Ok.
Ok.