Skip to main content

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.

Labels:

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
Post a Comment

Popular posts from this blog

Breaker Schemes in Substations

Breaker Schemes in Substations — Types, Design, Advantages, Disadvantages, and Comparison Author: Engr. Aneel Kumar Figure 1: Infographic overview of breaker schemes commonly used in substations. Introduction The breaker scheme or busbar arrangement in a substation defines how incoming feeders, outgoing feeders, and power transformers are connected to the bus. The choice of scheme has a direct impact on system reliability, maintainability, safety, and cost . A simple bus scheme is economical but vulnerable to outages, while advanced schemes such as breaker-and-a-half or double-bus/double-breaker provide very high reliability but at much higher cost and design complexity. Engineers select breaker schemes considering fault tolerance, maintenance needs, space requirements, expansion possibilities, protection coordination, and capital investment . Below, we explain eac...
Post a Comment
Read more

SYMMETRICAL COMPONENT ANALYSIS

Unbalanced three phase systems can be split into three balanced components, namely Positive Sequence (balanced and having the same phase sequence as the unbalanced supply), Negative Sequence (balanced and having the opposite phase sequence to the unbalanced supply) and Zero Sequence (balanced but having the same phase and hence no phase sequence). These are known as the Symmetrical Components or the Sequence Components and are shown in figure 2.10. The phase components are the addition of the symmetrical components and can be written as follows.  a = a 1 + a 2 + a 0 b = b 1 + b 2 + b 0 c = c 1 + c 2 + c 0 The unknown unbalanced system has three unknown magnitudes and three unknown angles with respect to the reference direction. Similarly, the combination of the 3 sequence components will also have three unknown magnitudes and three unknown angles with respect to the reference direction. Thus the original unbalanced system effectively has 3 complex unknown quan...
Post a Comment
Read more

TYPES OF ELECTRIC LOADS

Devices that are connected to the power system are referred to as electrical loads. Toasters, refrigerators, bug zappers, and so on are considered electrical loads. There are three types of electrical loads. They vary according to their leading or lagging time relationship between voltage and current. The three load types are resistive, inductive, and capacitive. Each type has specific characteristics that make them unique. Understanding the differences between these load types will help explain how power systems can operate efficiently. Power system engineers, system operators, maintenance personnel, and others try to maximize system efficiency on a continuous basis by having a good understanding of the three types of loads. They understand how having them work together can minimize system losses, provide additional equipment capacity, and maximize system reliability. The three different types of load are summarized below. 1) RESISTIVE LOAD: The resistance in a wire (i.e., cond...
Post a Comment
Read more

SOLIDLY GROUNDED NEUTRAL SYSTEMS

Solidly grounded systems are usually used in low voltage applications at 600 volts or less. In solidly grounded system, the neutral point is connected to earth. Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the system three phase fault current.. However, if the reactance of the generator or transformer is too great, the problem of transient over voltages will not be solved. While solidly grounded systems are an improvement over ungrounded systems, and speed up the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides. To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be extend from the source to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain very low impedance to ground faults so...
Post a Comment
Read more

ESSENTIAL ELEMENTS OF DIESEL POWER PLANT

FUEL SUPPLY SYSTEM OF DIESEL POWER PLANT It consists of storage tank, strainers, fuel transfer pump and all day fuel tanks. The fuel oil is supplied at the plant site by rail or road. The oil is stored in the storage tank. From the storage tank, oil is pumped to smaller all day tank at daily or short intervals. From this tank, fuel oil is passed through strainers to remove suspended impurities. The clean oil is injected into the engine by fuel injection pump. AIR INTAKE SYSTEM OF DIESEL POWER PLANT This system supplies necessary air to the engine for fuel combustion. It consists of pipes for the supply of fresh air to the engine manifold. Filters are provided to remove dust particles from air which may act as abrasive in the engine cylinder. Because a diesel engine requires close tolerances to achieve its compression ratio, and because most diesel engines are either turbocharged or supercharged, the air entering the engine must be clean, free of debris, and as cool as possible. ...
Post a Comment
Read more

ADVANTAGES AND DISADVANTAGES OF CORONA EFFECT IN TRANSMISSION LINES | ELECTRICAL ENGINEERING GUIDE

Advantages and Disadvantages of Corona Effect in Power Systems In high-voltage overhead transmission lines , the corona effect plays a critical role in system performance. Corona occurs when the air around a conductor becomes ionized due to high electric stress. While often seen as a drawback because of power losses and interference , it also provides certain engineering benefits . This article explains the advantages and disadvantages of corona effect in detail, with examples relevant to modern electrical power systems. ✅ Advantages of Corona Effect Increase in Virtual Conductor Diameter Due to corona formation, the surrounding air becomes partially conductive, increasing the virtual diameter of the conductor. This reduces electrostatic stress between conductors and minimizes insulation breakdown risks. Related Reading: Electrostatic Fields in High Voltage Engineering Reduction of Transient Surges Corona acts like a natural cushion for sudden ...
Read more