Lecture 17-18 for EECE 4333

In the P-F curve used in reliability engineering, what does the P-F interval represent?

The time between the first detectable sign of a problem and the functional failure

Which maintenance strategy forecasts the DATE at which a measured condition will reach its limit, and plans the work before the limit is reached?

Predictive maintenance

According to CIGRE TB 995, which feature is central to a digital twin and separates it from an ordinary model?

Two-way interaction: live data update the model, and its outputs act on the asset

Data flow automatically from a transformer into its model, but every decision is sent back to the asset by hand. In the Kritzinger classification, this is a:

Digital shadow

CIGRE counts the virtual-reality substation model of the Slovenian TSO ELES, built from 20 years of laser scans, as a digital twin.

False

A twin recommends: 'treat the oil in May and limit the load to 120 % for four hours.' Which TB 995 capability level is this?

Level 4, prescriptive

What did the CIGRE survey of 70 experts (TB 995) show about Level 5 (autonomous) transformer twins?

About half (52 %) of the respondents did not plan to reach Level 5

In the data layer of a digital twin, what is a 'virtual sensor'?

A quantity calculated instead of measured, such as the hot spot

How should the update rate of a digital twin be chosen?

It should follow the decision that the twin supports

In the common edge-cloud split (CIGRE TB 999), real-time alarms run at the edge in the substation, while long-term trends and predictions run in the cloud.

True

In the common edge-cloud split (CIGRE TB 999), real-time alarms run at the edge in the substation, while long-term trends and predictions run in the cloud.

True

A transformer is 40 years old, but its digital records reach back only about 5 years. What does this mean for its digital twin?

Its early history is missing, so its present ageing state is uncertain

Why must a transformer twin know metadata such as the breathing system, the tap-changer type and the oil type?

They change how dissolved-gas results should be interpreted

Hydro-Quebec trained an autoencoder on 81,269 tap-changer vibration signals without labels. What were the outliers it found?

Signals from a week of maintenance work that disturbed the sensors

In the lecture's view, an AI anomaly alarm raises the flag, but an engineer and an off-line test must confirm the diagnosis.

True

In the multi-state model of paper insulation shown in the lecture, which marker in the oil rises EARLY, before 2-FAL rises?

Methanol

POWERGRID's health index uses 'severity gates'. What does a severity gate do?

It lets one dangerous sign, like rising acetylene, override a good average

In a risk-based approach such as POWERGRID's, two units with the same health index normally receive the same maintenance priority.

False

Which statement about remaining useful life (RUL) matches the lecture?

It is a distribution, best reported as a range

Two transformers of the same age can be expected to have about the same remaining life, because age is the main driver of paper ageing.

False

Robots found a disconnector's excess temperature rising by about 17 deg C per year. It is now 24 deg C and the action threshold is 50 deg C. If the trend stays linear, about how long until the threshold is reached?

About 1.5 years

Why is a straight-line fit a sensible model for the four disconnector measurements?

Four points are too few for a complex model; a line will not overfit

If a bushing's power factor rises and then falls again, this most likely means that its insulation has recovered.

False

Using the six-degree rule (the ageing rate of ordinary paper doubles for every 6 K above 98 deg C), how much faster does the paper age at a 110 deg C hot spot than at 98 deg C?

4 times

Why does CIGRE call the thermal loading guides (IEC 60076-7, IEEE C57.91) 'early examples of digital twins'?

They turn load and ambient into hot spot and ageing

In the APG and Siemens Energy thermal-twin study of a 300 MVA transformer, what did the twin allow compared with the static 110 % limit?

Up to 135-140 % in windy winter periods, in an offline evaluation

What is the main weakness of a purely data-driven (black-box) model of a transformer?

It needs many examples, failures are rare, and it is weak outside its data

When a data-driven transformer model is trained and tested, all data from one transformer should stay in the same split, because otherwise the test score is too optimistic.

True

What is added to the training loss of a physics-informed neural network (PINN)?

A term that measures how far the prediction breaks the physical law

A thermal model fed with a load of 10 per unit calmly computes a hot spot of about 2,900 deg C. What does this show?

The model was used outside its validity domain

After a transformer's oil is regenerated, its 2-FAL value drops; a twin that is not told about the regeneration will wrongly read this as slower paper ageing.

True

Which term describes the ability of the grid to withstand a sudden disturbance, such as a short circuit or the loss of one line?

Security

In cybersecurity (Lecture 19), the word 'security' means confidentiality, integrity and availability, a different meaning from the one used in power-system operation.

True

Which question does resilience answer?

How much service is lost in an extreme event, and how fast it comes back

A system that settles to a new equilibrium after a disturbance is, for that reason, also secure against the next contingency.

False

At 15:05 on 14 August 2003, one hour before the cascade, the U.S.-Canada Task Force found that the system was:

Still N-1 secure: able to survive any one of 800+ contingencies

What was the main lesson of the Odessa, Texas event of 4 June 2022?

One normally cleared fault led to a large loss through in-plant settings

The ENTSO-E expert panel traced the Spain and Portugal blackout of 28 April 2025 mainly to tree contacts and overloaded lines, as in 2003.

False

In Elgerd's masses-and-strings analogy for a power system (Glover Fig. 12.1), what do the strings represent?

Transmission lines

Why did the Sammis-Star 345 kV line trip at 16:05:57 on 14 August 2003?

Its Zone 3 relay read high current at low voltage as a distant fault

Of the three 345 kV lines that tripped between 15:05 and 15:41, two were loaded at about 44 % and 88 % of their ratings and the third at 93 % of its emergency rating. What does this tell you?

They tripped because the trees were too tall, not because of overload

The Task Force concluded that a cascade, once started, cannot be stopped by human intervention.

True

Which two stability classes were added in the 2021 IEEE PES revision of the stability classification?

Resonance stability and converter-driven stability

Which list goes from the FASTEST to the SLOWEST phenomenon?

Converter controls, rotor-angle first swing, AGC, restoration

Which list gives the five operating states of Glover's textbook in their usual order?

Normal, Alert, Emergency, Blackout, Restorative

In ERCOT's under-frequency load-shedding (UFLS) scheme, what happens when the frequency falls to 59.3 Hz?

The first 5 % of the load is shed automatically

During Winter Storm Uri (15 February 2021), ERCOT frequency stayed below 59.4 Hz for 4 min 23 s. Why was 59.4 Hz so important?

Generators trip if frequency stays below 59.4 Hz for 9 minutes

The lecture's timeline of 14 August 2003 (after Task Force Fig. 5.1) has three lanes. Which three?

The power grid, the control-room computers and the operators

Which event is a resilience problem rather than an ordinary reliability (N-1) problem?

A winter storm that freezes gas wells, plants and wind turbines at once

Why do standard reliability indices such as SAIDI often say little about resilience?

IEEE 1366 excludes major event days such as severe storms

According to CIGRE TB 911, resilience reduces the likelihood of a blackout but cannot eliminate the risk of a blackout and its consequences.

True

On a self-healing feeder, after the faulted section has been isolated, how do the customers DOWNSTREAM of the fault get power back?

A normally open tie switch closes, and a neighbouring feeder supplies them

In the FLISR algorithm shown in the lecture (Elipse video), what happens when something unexpected occurs during switching?

It stops and leaves the situation to the operators

In U.S. DOE Smart Grid Investment Grant results, FLISR cut the customers interrupted by up to 45 % per event, but the damaged section still needed a repair crew.

True

Why does a large thermal power plant need a black-start unit to restart after a total blackout?

Its pumps, fans, controls and excitation need outside power to start

A 500 MW plant needs about 5-10 % of its capacity from outside to restart. How much power must the black-start source supply to it?

25-50 MW

What is a cranking path?

The transmission path from the black-start source to the generators being started

Why do operators pick up load in small blocks during restoration?

Big steps can drive a weak island's frequency and voltage out of limits

What is cold load pickup?

After a long outage, demand at reconnection is several times normal

When two islands are about to be joined, CIGRE recommends running the bigger island slightly faster than the smaller one.

False

Why can rooftop solar make restoration harder during daylight hours?

Thousands of inverters reconnect and push island frequency and voltage