science

Voltage control and reactive power management

Voltage control and reactive power management is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Voltage control and reactive power management rather than just read about it. In short: Voltage control and reactive power management are two facets of an ancillary service that enables reliability of the transmission networks and facilitates the electricity market on these networks. Both aspects of this activity are intertwined (voltage change in an alternating current (AC) network is effected through production or absorption of reactive power), so within this article the term voltage control will be…

Key takeaways

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Connect Voltage control and reactive power management to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Voltage control and reactive power management from memory before moving on to harder problems.

Reference excerpt

Voltage control and reactive power management are two facets of an ancillary service that enables reliability of the transmission networks and facilitates the electricity market on these networks. Both aspects of this activity are intertwined (voltage change in an alternating current (AC) network is effected through production or absorption of reactive power), so within this article the term voltage control will be primarily used to designate this essentially single activity, as suggested by Kirby & Hirst (1997). Voltage control does not include reactive power injections to dampen the grid oscillations; these are a part of a separate ancillary service, so-called system stability service. The transmission of reactive power is limited by nature (loss of VARs along a high-voltage transmission line can be an order of magnitude higher than loss of watts, "VARs do not travel well"), so the voltage control is provided through pieces of equipment distributed throughout the power grid, unlike the frequency control that is based on maintaining the overall active power balance in the system. Generally, an increase in production of reactive power corresponds to higher line voltage, while increase of absorption of the reactive power lowers the voltage. In wholesale electricity market, the independent system operator, together with the owners of transmission lines, defines the voltage schedule, a target value or a range of acceptable reference voltages for each generator (typically defined as voltage on the transmission bus). The schedule is typically used as a parameter for the automatic voltage control, although sometimes the control is using the target reactive power ("MVAR") or power factor as a setpoint.

Need for voltage control Kirby & Hirst indicate three reasons behind the need for voltage control:

the power network equipment is designed for a narrow voltage range, so is the power consuming equipment on the customer side. Operation outside of this range will cause the equipment to fail; reactive power causes heating in the generators and the transmission lines, thermal limits will require restricting the production and the flow of real (active) power; injection of reactive power into transmission lines causes losses that waste power, forcing an increase in power supplied by the prime mover. Use of specialized voltage control devices in the grid also improves the power system stability by reducing the fluctuations of the rotor angle of a synchronous generator (that are caused by generators sourcing or sinking the reactive power). Power buses and systems that exhibit large changes in voltage when the reactive power conditions change are called weak systems, while the ones that have relatively smaller changes are strong (numerically, the strength is expressed as a short circuit ratio that is higher for the stronger systems).

Absorption and production of reactive power Electric loads absorb reactive energy if they have lagging power factor (are inductor-like) and produce reactive energy if they have a leading power factor (are capacitor-like). For generator the definition of the current direction is reversed, thus leading generator will absorb reactive power, and lagging will produce it. Due to possible − purely definitional − confusion, it might be convenient to avoid leading/lagging terminology when discussing the production/absorption of the reactive power. Electric grid equipment units typically either supply or consume the reactive power:

Synchronous generators will provide reactive power if overexcited and absorb it if underexcited, subject to the limits of the generator capability curve. Transformers will always absorb the reactive power. Power lines will either absorb or provide reactive power: overhead power lines will provide reactive power at low load, but as the load increases past the surge impedance of the line, the lines start consuming an increasing amount of reactive power. Underground power lines are capacitive, so they are loaded below the surge impedance and provide reactive power. Electrical loads usually absorb the reactive power, with the power factor for typical appliances ranging from 0.65 (household equipment with electrical motors, like a washing machine) to 1.0 (purely resistive loads like incandescent lamps). In a typical electrical grid, the basics of the voltage control are provided by the synchronous generators. These generators are equipped with automatic voltage regulators that adjust the excitation field keeping the voltage at the generator's terminals within the target range. The task of additional reactive power compensation (also known as voltage compensation) is assigned to compensating devices:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Voltage control and reactive power management

Start with the simplest possible case. Write down what Voltage control and reactive power management claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Voltage control and reactive power management before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Voltage control and reactive power management ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Voltage control and reactive power management

In research: Voltage control and reactive power management appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Voltage control and reactive power management in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Voltage control and reactive power management is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric power transmission, so understanding it makes those chapters shorter.
In everyday life: Look for Voltage control and reactive power management outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Voltage control and reactive power management in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Voltage control and reactive power management means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Voltage control and reactive power management out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Voltage control and reactive power management in simple terms?

Why does Voltage control and reactive power management matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Voltage control and reactive power management?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Voltage control and reactive power management.