computer science

Network protector

Network protector is a computer 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 Network protector rather than just read about it. In short: A network protector is a type of electric protective device used in electricity distribution systems. The network protector automatically disconnect its associated distribution transformer from the secondary network when the power starts flowing in reverse direction.

Key takeaways

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

Reference excerpt

A network protector is a type of electric protective device used in electricity distribution systems. The network protector automatically disconnect its associated distribution transformer from the secondary network when the power starts flowing in reverse direction. Network protectors are used on both spot networks and grid networks. The secondary grid system improves continuity of service for customers, since multiple sources are available to supply the load; a fault with any one supply is automatically isolated by the network protector and does not interrupt service from the other sources. Secondary grids are often used in downtown areas of cities where there are many customers in a small area.

Operation Typically the network protector is set to close when the voltage difference and phase angle are such that the transformer will supply power to the secondary grid, and is set to open when the secondary grid would back-feed through the transformer and supply power to the primary circuit. Network protectors typically have three settings, "automatic", "open", and "close". The top side is fed from multiple protectors and is always energized unless all units on a spot network are in the open position. Grid units will always be energized on the top side from the many other units tied into the grid. A spot network is 2 or more transformers dedicated to a single customer. The grid feeds multiple customers. A network protector has a circuit breaker set of contacts and a controlling protection relay. The components are enclosed in a protective housing; some network protectors are installed on transformers below grade and must be in water-resistant enclosures. The mechanism contains electrical and mechanical parts to switch open and close the secondary contacts. The controlling relay monitors voltage and current in the transformer, and opens or closes the contact mechanism through electrical signals. The relay uses a power/time curve so that small, short term reverse power flow (such as from elevator hoists) are ignored. Spot units will be 277/480 and the grid units will be 120/208.

Application The network protector does not protect the (secondary) network cable from overload. The network protector is installed to protect the stability and reliability of the secondary grid by preventing power flow away from the customers and into the primary feeders. If there is a fault on the primary feeder, the substation circuit-breaker is meant to open, disconnecting the primary feeder from one side. The problem is that this primary cable is also connected to a network transformer, which is interconnected to other network transformers on its secondary side. The secondary network will energize the primary feeder through the network transformer. This can be very dangerous, because a fault will continue to be 'fed' from the secondary network side of transformer. Even without a fault, if the electric utility wants to perform maintenance on that primary cable, they must have a way to fully disconnect that primary cable, without worrying about the cable being energized by the secondary network through the network transformer. Thus, the network protector is designed to open its contacts if the relay senses backwards flowing current. However, if there is a fault on the secondary grid, the network protector is not designed to open its contacts up. The secondary fault will continue to be fed from the primary side of the system. In some cases, networks are designed with cable limiters (like fuses) to melt and disconnect the secondary fault under the right conditions. In other cases, the utility lets cable 'burn clear', in which case the fault is allowed to remain fed until the cables fuse, then the fault is isolated. Analysis of the system is required to ensure that the system can, indeed, supply enough current to fuse the cables, wherever the fault is. This method tends to work well at 120 volts, but it is less reliable at higher voltages. The danger in depending on the cable to 'burn clear' is that some conditions will not cause the cable to burn in this manner and instead, the entire section of cable can be damaged from excessive, long-term overloading, causing fires and damage to the secondary network. Typically, network protectors are contained inside a submersible enclosure which is bolted to the throat of the network transformer and placed in underground vaults. IEEE standard C57.12.44 covers network protectors.

Worked examples

Example 1 — a first encounter with Network protector

Start with the simplest possible case. Write down what Network protector claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer 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 Network protector 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 Network protector 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 Network protector

In research: Network protector appears in computer 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 Network protector 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: Network protector is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electric power distribution, Electrical engineering, so understanding it makes those chapters shorter.
In everyday life: Look for Network protector 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 Network protector in 20 minutes

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

Frequently asked questions

What is Network protector in simple terms?

Why does Network protector matter?

Because it connects several computer 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 Network protector?

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 Network protector.