engineering

Loss of load

Loss of load is a engineering 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 Loss of load rather than just read about it. In short: Loss of load in an electrical grid is a term used to describe the situation when the available generation capacity is less than the system load. Multiple probabilistic reliability indices for the generation systems are using loss of load in their definitions, with the more popular being Loss of Load Probability (LOLP) that characterizes a probability of a loss of load occurring within a year.

Key takeaways

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

Reference excerpt

Loss of load in an electrical grid is a term used to describe the situation when the available generation capacity is less than the system load. Multiple probabilistic reliability indices for the generation systems are using loss of load in their definitions, with the more popular being Loss of Load Probability (LOLP) that characterizes a probability of a loss of load occurring within a year. Loss of load events are calculated before the mitigating actions (purchasing electricity from other systems, load shedding) are taken, so a loss of load does not necessarily cause a blackout. The concept of probabilistic assessment of power resource adequacy dates back to the 1930s. A foundational paper was published by Calabrese in 1947, which introduced a method to calculate the expected number of days when peak daily electricity demand would exceed the available generating capacity. This paper also started the tradition of describing the reliability metrics with multiple different, and loose, phrases like “loss of load duration” and “expected total number of days of loss of load".

Loss-of-load-based reliability indices Multiple reliability indices for the electrical generation are based on the loss of load being observed/calculated over a long interval (one or multiple years) in relatively small increments (an hour or a day). The total number of increments inside the long interval is designated as N {\displaystyle N} (e.g., for a yearlong interval N = 365 {\displaystyle N=365} if the increment is a day, N = 8760 {\displaystyle N=8760} if the increment is an hour):

Loss of load probability (LOLP) is a probability of an occurrence of an increment with a loss of load condition. LOLP can also be considered as a probability of involuntary load shedding; Loss of load expectation (LOLE) is the total duration of increments when the loss of load is expected to occur, L O L E = L O L P ⋅ N {\displaystyle {LOLE}={LOLP}\cdot N} . Frequently LOLE is specified in days, if the increment is an hour, not a day, a term loss of load hours (LOLH) is sometimes used. Since LOLE uses the daily peak value for the whole day, LOLH (that uses different peak values for each hour) cannot be obtained by simply multiplying LOLE by 24; although in practice the relationship is close to linear, the coefficients vary from network to network; Loss of load events (LOLEV) a.k.a. loss of load frequency (LOLF) is the number of loss of load events within the interval (an event can occupy several contiguous increments); Loss of load duration (LOLD) characterizes the average duration of a loss of load event: L O L D = L O L E L O L F {\displaystyle {LOLD}={\frac {LOLE}{LOLF}}}

One-day-in-ten-years criterion A typically accepted design goal for L O L E {\displaystyle LOLE} is 0.1 day per year ("one-day-in-ten-years criterion" a.k.a. "1 in 10"), corresponding to L O L P = 1 10 ⋅ 365 ≈ 0.000274 {\displaystyle {LOLP}={\frac {1}{10\cdot 365}}\approx 0.000274} . In the US, the threshold is set by the regional entities, like Northeast Power Coordinating Council: resources will be planned in such a manner that ... the probability of disconnecting non-interruptible customers will be no more than once in ten years The "1 in 10" value was gradually accepted as the norm in the 1960s.

Worked examples

Example 1 — a first encounter with Loss of load

Start with the simplest possible case. Write down what Loss of load claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Loss of load 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 Loss of load 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 Loss of load

In research: Loss of load appears in engineering 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 Loss of load 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: Loss of load is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical engineering, Reliability indices, so understanding it makes those chapters shorter.
In everyday life: Look for Loss of load 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Loss of load in 20 minutes

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

Frequently asked questions

What is Loss of load in simple terms?

Why does Loss of load matter?

Because it connects several engineering 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 Loss of load?

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 Loss of load.