Question

Explain the choice of size and number of generator units in a power plant.

Answer 1

Selection of Size and Number of Generating Units:
1. The size/rating and number of generating units in such way that they approximately
match with the load curve/load duration curve as closely as possible.
2. In order to calculate the size of the units, the station auxiliary load should be taken in to account.
3. Also the transmission line losses should be considered. It can be approximately taken
as 20 % of the consumer load.
4. The future demand and expansion should also be considered as the load on the station
always increases.
5. The plant must have some reverse capacity at least 15-20 % more than M.D. under
abnormal conditions.
6. Select size/rating of generating units in such way that reliability to maintain supply
will be more.
7. Select size/rating of generating units in such way that the plant capacity factor, load
factor diversity factor, plant use factor will be more.
8. Select size/rating of generating units in such way that unit almost run at full load or at
load which gives maximum efficiency.
9. Select size/rating of generating units in such way that power generation will be
economical.
10. Initial and operating cost also to be taken in to account
11. Space required also to be considered.
12. The minimum number of units should be two.
13. As far as possible, the units of equal capacities are selected which will have
following advantages.
i) The parts can be interchanged.
ii) The maintenance will be easier.
iii) The working time of each plant regulated.
iv)The spare parts required to be stored are less.
14. While selecting the size/rating and number of generating units there are two options
i) To select single generating unit of large capacity
ii) To select more numbers of small capacity generating unit either of same
ratings or different ratings.
Both options have its own advantages and disadvantages.
15. In summary,
Load on the power system is variable where reliability of supply is important so
it is neither practicable nor economical to use a single unit of large capacity.
But, if power plant is connected to grid system then generating unit of
higher capacity can be installed.

Answer 2

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The Fundamental Trade-Off

At its core, the decision boils down to a trade-off between two main approaches:

1. A Few Large Units: This approach leverages economies of scale. A single, large generator is generally more efficient and has a lower capital cost per megawatt (MW) than multiple smaller units.
2. Many Small Units: This approach prioritizes flexibility, reliability, and redundancy. A plant with multiple smaller units can better adapt to changing demand and is less vulnerable to a single point of failure.

Let's break down the key factors that influence this trade-off.

1. Load Demand Characteristics (The Plant's Purpose)

The primary driver for the design is the type of electrical load the power plant is intended to serve. This is best understood by looking at a load curve, a graph of power demand over time.

• Baseload Plants:
  Purpose: To meet the minimum, constant demand on the grid, operating 24/7 at or near full capacity.
  Examples: Nuclear, large coal, some large combined-cycle gas turbine (CCGT) plants.
  Choice: Few, very large units.
  Reasoning:

  *   **Efficiency is King:** Since these plants run continuously, fuel cost is a major operational expense. Large steam turbines (used in nuclear and coal plants) are most efficient when they are large and running at full power.
  *   **Economies of Scale:** The capital cost per MW for a 1,200 MW nuclear reactor is significantly lower than for twelve 100 MW reactors.
  *   **Flexibility is Not a Priority:** These plants are not designed to start, stop, or change output quickly.
  

• Intermediate/Cycling Plants:
  Purpose: To follow the daily or weekly variations in demand, ramping up in the morning and down at night.
  Examples: Combined-Cycle Gas Turbine (CCGT) plants.
  Choice: A moderate number of medium-to-large sized units. (e.g., a CCGT plant with two gas turbines and one steam turbine).
  Reasoning:

  *   **Balance of Efficiency and Flexibility:** CCGTs are highly efficient but can also ramp their output more quickly than a baseload plant.
  *   **Phased Operation:** Having multiple units (e.g., two gas turbines) allows the plant to run at partial load more efficiently by shutting down one unit instead of running both at a low, inefficient level.
  

• Peaking Plants (Peakers):
  Purpose: To operate only during periods of highest ("peak") demand, which may only be a few hours per day or a few hundred hours per year.
  Examples: Simple-cycle gas turbines, reciprocating engines, hydropower, battery storage.
  Choice: Many, small, fast-acting units.
  Reasoning:

  *   **Flexibility is King:** The most important attribute is the ability to start up in minutes.
  *   **Low Capital Cost:** Since they don't run often, it's crucial to minimize the initial investment. Small, mass-produced units like aeroderivative gas turbines are ideal.
  *   **Efficiency is Less Important:** Fuel costs are secondary to being available when needed, as their total run-time is low.
  

2. Reliability and Redundancy (The N-1 Criterion)

The "N-1 Criterion" is a fundamental principle in power system planning. It states that the system must remain stable even after the loss of its single largest component (be it a generator, transformer, or transmission line).

• Impact of a Few Large Units: If a 1,000 MW plant consists of a single 1,000 MW generator, its failure means a total plant outage. This is a massive shock to the grid, potentially causing widespread blackouts.
• Impact of Many Small Units: If the same 1,000 MW plant consists of four 250 MW units, the failure of one unit results in only a 25% loss of capacity. This is much easier for the grid to manage by calling on other power plants.

Maintenance: Having multiple units also allows for planned maintenance to be performed on one unit while the others continue to operate, ensuring the plant remains available, albeit at reduced capacity.

3. Economic Factors

• Capital Cost (CAPEX):
  Economies of Scale: As mentioned, a single 1000 MW unit is cheaper to build per MW than ten 100 MW units due to shared infrastructure, less land, and simpler control systems.
  Total Investment: However, the total upfront capital required for the single large unit is a massive lump sum, which can be difficult to finance. Building smaller units in phases can be more financially manageable.

• Operating Cost (OPEX):
  Efficiency: Larger units are typically more efficient, leading to lower fuel costs over their lifetime (critical for baseload).
  Staffing & Maintenance: A single large unit might require fewer operators than multiple smaller units. However, a major failure on a giant, bespoke unit can be catastrophic and require very expensive, specialized repairs. Spare parts for smaller, common units are often cheaper and more readily available.

4. Operational Flexibility

• Ramp Rate: How quickly a unit can increase or decrease its power output. Smaller units, especially gas turbines, can ramp much faster than large steam turbines. This is vital for grids with high penetration of variable renewables like solar and wind, as the plant needs to quickly compensate for a drop in wind or a cloud passing over a solar farm.
• Turndown Ratio: The minimum power level a unit can operate at stably. A large unit might only be able to turn down to 50% of its maximum capacity. Below this, it becomes unstable or inefficient. A plant with four smaller units can achieve a much lower turndown by simply shutting down three of them, providing much greater operational granularity.
• Start-up Time: The time it takes to go from a cold state to full power. A peaker plant needs a start-up time of minutes. A large baseload coal or nuclear plant can take many hours or even days.

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Summary Table: Few Large Units vs. Many Small Units

| Feature | Few, Large Units (e.g., 1 x 1000 MW) | Many, Small Units (e.g., 10 x 100 MW) |
| :--- | :--- | :--- |
| Ideal Role | Baseload | Peaking, Ancillary Services, Grid Support |
| Efficiency | Higher (at full load) | Lower |
| Capital Cost (per MW)| Lower (due to economies of scale) | Higher |
| Reliability (N-1) | Lower (single point of failure is catastrophic) | Higher (loss of one unit is manageable) |
| Flexibility | Lower (slow to ramp, poor turndown) | Higher (fast to ramp, excellent turndown) |
| Maintenance | Entire plant may go offline | More flexible, units taken offline individually |
| Footprint | More compact for the same total capacity | More spread out |
| Grid Impact | A sudden trip has a major impact | A sudden trip has a minor impact |

Conclusion

The decision on generator size and number is a complex optimization problem.

• For constant, predictable power, the efficiency and economic advantages of a few large units make them the clear choice for baseload generation.
• For responding to variable demand and providing grid stability, the reliability, speed, and flexibility of many smaller units are essential.

The modern trend, driven by the rise of intermittent renewable energy, is a shift towards more flexible assets. This means that even new "intermediate" plants are being designed with multiple, fast-ramping units to complement the variability of wind and solar power on the grid.