Year 9 Science · Unit 3 · Lesson 14
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Learning Goals
1. The grid runs at a frequency of 50 Hz. What happens to the frequency when demand suddenly becomes greater than supply, and why must grid operators act quickly?
Sort it: what kind of generator?
Write each item in the correct box. Each box gets two items.
Baseload
Peaking
Intermittent
Stored and dispatchable
2. A generator produces 400 MW at 20,000 V. A step-up transformer raises this to 500,000 V. Calculate the current (a) before and (b) after the transformer. (c) By what factor does the heat lost in the lines fall?
(a) Current at 20,000 V A
(b) Current at 500,000 V A
(c) Heat loss falls by a factor of
3. The transmission line has a total resistance of 0.20 Ω. Use P = I²R to calculate the power lost as heat at each current in Question 2. Explain what your answers show about sending 400 MW at 20,000 V.
Loss at 500,000 V W
Loss at 20,000 V W
Data: electricity demand on a summer day
Table 1 shows the demand on the grid in one state across a hot summer day. Many homes in the state have rooftop solar panels, which supply part of their own demand during the day.
| Time | Demand on the grid (MW) |
|---|---|
| 12 midnight | 6,000 |
| 3 am | 5,400 |
| 6 am | 6,200 |
| 9 am | 5,600 |
| 12 noon | 4,800 |
| 3 pm | 5,600 |
| 6 pm | 8,400 |
| 9 pm | 7,600 |
Source: sample values for this worksheet.
4. Draw a line graph of demand on the grid against time.
5. (a) Why is demand on the grid lowest at noon on a sunny day? (b) By how many MW does demand rise between 3 pm and 6 pm? (c) Which kinds of generator from your sort can supply that rise, and why can’t the others?
(b) Rise MW
6. At 6 pm on this day, a large coal unit suddenly breaks down. Explain what happens to the grid frequency, and how a grid-scale battery, a gas peaking plant and demand response could each help. Give one limitation of each.