science

Isolated-phase bus

Isolated-phase bus 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 Isolated-phase bus rather than just read about it. In short: In electrical engineering, isolated-phase bus (IPB), also known as phase-isolated bus (PIB) in some countries, is a method of construction for circuits carrying very large currents, typically between a generator and its step-up transformer in a steam or large hydroelectric power plant. Each phase current is carried on a separate conductor, enclosed in a separate grounded metal housing.

Key takeaways

Isolated-phase bus belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Isolated-phase bus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Isolated-phase bus from memory before moving on to harder problems.

Reference excerpt

In electrical engineering, isolated-phase bus (IPB), also known as phase-isolated bus (PIB) in some countries, is a method of construction for circuits carrying very large currents, typically between a generator and its step-up transformer in a steam or large hydroelectric power plant.

Each phase current is carried on a separate conductor, enclosed in a separate grounded metal housing. Conductors are usually hollow aluminum tubes or aluminum bars, supported within the housing on porcelain or polymer insulators. The metal housings are electrically connected so that induced current, nearly of the magnitude of the phase current, can flow through the housing, in the opposite direction from the phase current. The magnetic field produced by this current nearly exactly cancels the magnetic field produced by the phase current, so there is almost no external magnetic field produced. This also limits the amount of force produced between conductors during a short circuit. The external housings of the conductors remain at a low potential with respect to earth ground and are usually bonded to ground. By enclosing the conductors in separate housings a high degree of protection from two-phase and three-phase faults is obtained. Almost any fault would instead be a single-phase earth fault which does not produce a large fault current. The conductors between the generator and the first circuit breaker are even more important to protect against two- and three-phase faults because there is no breaker that can stop the fault current from the generator. While most modern circuit breakers will interrupt the fault current in less than 50 ms, the fault current from the generator will take several seconds to interrupt because the field current in the rotor takes this amount of time to discharge. The consequences of a two- or three-phase fault between the generator and the first circuit breaker are therefore much more serious and often result in severe damage to the busbars and nearby equipment. Isolated-phase bus is made in ratings from 3000 amperes to 45,000 amperes, and rated for voltages from 5000 volts up to about 35,000 volts. In the larger current ratings, dry air is forced through the enclosures and within the tubular conductors for forced-air cooling of the conductors. The cooling air is recirculated through a heat exchanger. Some items of switchgear, such as circuit breakers and isolating switches, are made in housings compatible with the isolated-phase bus. Accessories such as instrument transformers, surge arresters, and capacitors are also made in compatible housings. Due to the expense of its construction and the energy loss, isolated-phase bus is usually used in short segments; a large underground powerhouse may have isolated-phase bus up to about 250 metres or so to connect generators to transformers in a cavern. Forced-air cooling can approximately double the rating over the same size conductors used in a self-cooled system. The extra cost of losses and cooling fan power consumption must be balanced against the lower capital cost of the bus. Various forms of flexible terminals, expansion joints, and weatherproof or fire-proof bushings and terminals are used with isolated-phase bus. Some types of apparatus such as disconnecting switches, circuit breakers, and instrument transformers are made in enclosures that can be welded to become an integral part of the isolated-phase bus system. Isolated-phase bus is usually custom manufactured for a particular project and requires accurate dimensions of the connected equipment for manufacturing. A smaller type of isolated-phase bus is manufactured for direct-current circuits; this may be used in the field circuit of a generator. Currently, the isolated-phase bus world record current is 52,000 A, for bus manufactured by Alstom Power (since 2015 General Electric Power) and installed at the Civaux Nuclear Power Plant, in 1997.

Illustrations

Worked examples

Example 1 — a first encounter with Isolated-phase bus

Start with the simplest possible case. Write down what Isolated-phase bus 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 Isolated-phase bus 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 Isolated-phase bus 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 Isolated-phase bus

In research: Isolated-phase bus 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 Isolated-phase bus 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: Isolated-phase bus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric power systems components, so understanding it makes those chapters shorter.
In everyday life: Look for Isolated-phase bus 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 Isolated-phase bus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Isolated-phase bus 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 Isolated-phase bus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Isolated-phase bus in simple terms?

Why does Isolated-phase bus 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 Isolated-phase bus?

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 Isolated-phase bus.