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PRIMARY SECONDARY AND TERTIARY FREQUENCY CONTROL IN POWER SYSTEMS

Primary, Secondary and Tertiary Frequency Control in Power Systems

Author: Engr. Aneel Kumar

Keywords: frequency control, primary frequency control, automatic generation control (AGC), tertiary control, load-frequency control, grid stability.

Frequency control keeps the power grid stable by balancing generation and load. When generation and demand drift apart, system frequency moves away from its nominal value (50 or 60 Hz). Grids rely on three hierarchical control layers — Primary, Secondary (AGC), and Tertiary — to arrest frequency deviation, restore the set-point and optimize generation dispatch.

Related: Power System Stability — causes & mitigation

Primary, Secondary and Tertiary Frequency Control in Power System

Overview of primary, secondary and tertiary frequency control in power systems.

⚡ Primary Frequency Control (Droop Control)

Primary control is a fast, local response implemented by generator governors (droop control). It reacts automatically within seconds to frequency deviations by changing turbine/generator power output proportionally to frequency error. Primary action arrests the frequency fall/rise and stabilizes the system at a new quasi-steady value.

Related: The per-unit system — basics and utility

Typical primary response time is 0–30 seconds. Primary control works off the machines’ kinetic energy and governor droop settings; it does not restore frequency to nominal — that is the role of secondary control.

Related: Power system stability concepts & inertia

Advantages of Primary Control

  • Very fast, automatic local action to arrest frequency deviation.
  • No central communication required — resilient and robust.
  • Immediate support across the grid through many generators sharing the burden.

Disadvantages of Primary Control

  • Leaves frequency at a quasi-steady offset (droop causes steady error).
  • Unequal sharing if generator droops are not coordinated.
  • Relies on stored kinetic energy — limited duration and magnitude.
Dynamic and quasi-steady-state frequency deviation during primary control

Figure 1: Dynamic (Δfdyn) and quasi-steady (Δf) frequency deviation.

🔁 Secondary Frequency Control (AGC)

Secondary control is centralized Automatic Generation Control (AGC). It uses a central regulator to change generator setpoints and restore system frequency to nominal while correcting scheduled tie-line power exchanges between control areas. Secondary action typically begins to be effective after about 30 seconds and completes within minutes (often < 15 min).

Related: Power system protection & operational coordination

Advantages of Secondary Control

  • Restores frequency to nominal and corrects tie-line power flows.
  • Re-allocates generation set-points to release primary reserves.
  • Enables coordinated operation across the control area/region.

Disadvantages of Secondary Control

  • Slower than primary — takes seconds to minutes to act.
  • Requires reliable communications and centralized control systems.
  • Needs accurate measurement of tie-line power and frequency for correct operation.
Generator contribution to primary control based on droop

Figure 2: How generators with different droops contribute to primary control.

🛠️ Tertiary Frequency Control (Dispatch & Reserve Restoration)

Tertiary control is manual or automated dispatch-level action that restores secondary reserves and optimizes generation costs after disturbances. It reallocates generation set-points, starts/stops peaking units, dispatches hydro or storage resources, or modifies interchange schedules.

Advantages of Tertiary Control

  • Restores and re-allocates reserves for sustained operations.
  • Optimizes generation economically (dispatch & unit commitment).
  • Enables strategic use of storage, pumped hydro and market resources.

Disadvantages of Tertiary Control

  • Slow — not suitable for arresting immediate frequency excursions.
  • Often requires market/dispatch coordination and operator decisions.
  • Effectiveness depends on available reserves and start-up times of units.

🔍 Quick Comparison: Primary vs Secondary vs Tertiary

Feature Primary Secondary (AGC) Tertiary
Response time Seconds (0–30 s) 30 s – 15 min 15 min – hours
Main goal Arrest frequency deviation Restore nominal frequency & tie-line flows Restore reserves & economic dispatch
Control type Local / decentralized (governor) Centralized (AGC) Manual or automated dispatch
Requires communications? No Yes (measurements & telemetry) Yes (market/dispatch systems)
Restores frequency to nominal? No (leaves offset) Yes Yes — and restores reserves

⚠️ Emergency Measures & Summary

If frequency excursions exceed permissible limits, emergency actions include load shedding (under-frequency tripping) and generator disconnection (over-frequency). Effective frequency control (primary + secondary + tertiary) reduces the chance of blackouts and equipment damage by ensuring fast arrest, full restoration and economic recovery.

Timing of primary, secondary and tertiary frequency control ranges

Figure 3: Timing of primary, secondary and tertiary control ranges.

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Comments

Unknown said…
when it is suitable to apply whether primary frequency control or secondary frequency control or tertiary frequency control in power system according to the percentage load disturbance
Friday, 26 January, 2018
Unknown said…
yes you are right sir. its very useful article.
Friday, 28 June, 2019
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