In the power system, maintaining a balance between electricity generation and demand is critical. To ensure system reliability, operators rely on different types of reserves—backup generation capacity that can be activated during sudden demand changes, generator failures, or unexpected outages.
📌 What is Reserve in Power System?
A reserve is the extra generating capacity available to a power system operator to meet unexpected load increases or generation outages. These reserves help in maintaining grid stability and reliability.
⚙️ Types of Power System Reserves
1️⃣ Spinning Reserve
Definition: Extra generation capacity already connected to the grid and operating below maximum output.
- Example: A thermal unit running at 70% load, ready to increase up to 100%.
- Response Time: Within 10 seconds to a few minutes.
- Source: Synchronous generators, hydro plants.
2️⃣ Non-Spinning Reserve
Definition: Reserve not currently connected but can be started quickly (within 10–30 minutes).
- Example: Idle gas turbine ready to start.
3️⃣ Hot Reserve
Definition: Standby capacity in warmed-up condition to reduce startup time.
- Example: Heated thermal plant boiler.
4️⃣ Cold Reserve
Definition: Reserve completely shut down, requiring hours to start.
- Example: Steam plant that needs full start-up process.
5️⃣ Firm Reserve
Definition: Guaranteed reserve through contracts to be used in peak or emergencies.
6️⃣ Supplementary Reserve
Definition: Used when primary and secondary reserves are not sufficient.
7️⃣ Balancing Reserve (Load-Following Reserve)
Definition: Used to match generation with load variations over a short time (5–30 minutes).
8️⃣ Contingency Reserve
Definition: Used for sudden unexpected failures like generator or transmission line loss.
🧮 Numerical Examples
Numerical 1: Reserve Requirement Calculation
Q: A system has a peak load of 500 MW, spinning reserve of 50 MW and non-spinning of 70 MW. Calculate total reserve and reserve margin.
Answer: Total Reserve = 120 MW, Reserve Margin = (120/500)×100 = 24%
Numerical 2: Activation Time
Q: Hot reserve (100 MW, 20 mins) vs Cold reserve (150 MW, 4 hours). Which is helpful for a sudden outage?
Answer: Hot reserve, due to quick activation.
✅ MCQs
- Q1. Which reserve is already synchronized?
(A) Cold (B) Spinning (C) Non-Spinning (D) Supplementary
✅ Answer: (B) - Q2. Which takes the longest to start?
(A) Hot (B) Spinning (C) Cold (D) Balancing
✅ Answer: (C) - Q3. Which is contractual and guaranteed?
(A) Firm (B) Spinning (C) Cold (D) Balancing
✅ Answer: (A) - Q4. Supplementary reserve used when:
(A) Load drops (B) Minor contingency (C) Primary/Secondary insufficient (D) Low demand
✅ Answer: (C)
📌 Summary Table
| Reserve Type | Connected to Grid | Startup Time | Use Case |
|---|---|---|---|
| Spinning | Yes | Seconds–Minutes | Sudden load/generator trip |
| Non-Spinning | No | 10–30 Minutes | Support after initial reserve |
| Hot | No (Warmed-up) | 15–30 Minutes | Quick start |
| Cold | No (Cold) | Hours | Long-term backup |
| Firm | Maybe | Contractual | Emergency backup |
| Supplementary | Maybe | Minutes–Hours | After primary/secondary |
| Balancing | Yes or No | 5–30 Minutes | Load-following |
| Contingency | Yes or No | Immediate–30 Minutes | Unexpected failure |
🔖 FAQs
- Q1. Difference between spinning and non-spinning reserves?
Ans: Spinning is online; non-spinning is offline. - Q2. Is cold reserve reliable for instant use?
Ans: No, it takes hours to start. - Q3. Which reserve balances real-time load variations?
Ans: Balancing reserve.
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