Power system control is all about keeping things running smoothly. It's like being a traffic cop for electricity, making sure everything flows where it should and nothing gets out of hand.

The main goals are to keep frequency, voltage, and power flow in check. If these go wonky, you could end up with blackouts, damaged equipment, or even fires. Control systems are the unsung heroes that keep the lights on.

Power System Control Objectives

Maintaining System Parameters within Limits

• Power system control aims to maintain the system's frequency, voltage, and power flow within specified limits to ensure stable and reliable operation
• Frequency control maintains the balance between power generation and load demand, typically targeting a nominal frequency of 50 Hz or 60 Hz
• Voltage control maintains the voltage levels at various nodes in the power system within acceptable ranges, usually ±5% of the nominal voltage
• Power flow control manages the flow of active and reactive power through transmission lines to prevent overloading and ensure optimal power transfer

Consequences of Deviations from Acceptable Limits

• Deviations from the acceptable limits of frequency, voltage, or power flow can lead to system instability, equipment damage, or cascading failures
• Frequency deviations can cause generators to trip offline, leading to further imbalances and potential blackouts (Northeast blackout of 2003)
• Voltage deviations can result in equipment damage, reduced efficiency, and potential voltage collapse (California electricity crisis of 2000-2001)
• Overloaded transmission lines can overheat, sag, and potentially cause fires or widespread outages (2003 Italy blackout)

Control Systems for Stability

Role of Control Systems in Maintaining Stability

• Control systems play a crucial role in maintaining power system stability by continuously monitoring system parameters and initiating corrective actions when necessary
• Power system stability refers to the ability of the system to regain equilibrium after being subjected to disturbances, such as generator outages, line faults, or sudden load changes
• Control systems help in maintaining different types of stability, including rotor angle stability, frequency stability, and voltage stability

Types of Stability and Their Control Mechanisms

• Rotor angle stability is maintained by ensuring that synchronous machines remain in synchronism after a disturbance
  • Power system stabilizers (PSS) provide damping to generator rotor oscillations
  • Excitation systems control generator field currents to maintain synchronism
• Frequency stability is achieved by balancing power generation and load demand, with control systems adjusting generator outputs or shedding loads when necessary
  • Automatic generation control (AGC) adjusts generator setpoints to maintain frequency
  • Under-frequency load shedding (UFLS) schemes disconnect loads to prevent frequency collapse
• Voltage stability is maintained by regulating reactive power flow and controlling voltage levels at critical buses
  • Automatic voltage regulators (AVRs) control generator excitation to maintain voltage
  • Static VAR compensators (SVCs) and static synchronous compensators (STATCOMs) provide fast-acting reactive power support

Importance of Control Systems for Reliable Operation

• Reliable power system operation depends on the effectiveness of control systems in preventing cascading failures and minimizing the impact of disturbances
• Well-designed and coordinated control systems can quickly detect and respond to disturbances, preventing their propagation and limiting their impact
• Advanced control technologies, such as wide-area monitoring and control systems (WAMCS), can provide real-time situational awareness and enable faster and more accurate control actions

Power System Control Hierarchy

Primary Control (Local Control)

• Primary control, also known as local control, acts on a time scale of seconds and is responsible for maintaining the balance between power generation and load demand within a local area
• Primary frequency control is achieved through governor control of generators, which adjusts the mechanical power input to maintain the system frequency
  • Droop control allows generators to share load changes proportionally to their capacities
  • Isochronous control maintains a constant frequency by a single generator or a small isolated system
• Primary voltage control is performed by automatic voltage regulators (AVRs) on generators and by local reactive power compensation devices
  • AVRs adjust generator excitation to maintain terminal voltage
  • Capacitor banks and inductors provide local reactive power support

Secondary Control (Area Control)

• Secondary control, also called area control or automatic generation control (AGC), operates on a time scale of minutes and coordinates the actions of multiple generators within a control area
• Secondary frequency control adjusts the power output of generators to restore the system frequency to its nominal value and maintain the scheduled power exchanges between control areas
  • Area control error (ACE) is used as a performance metric for secondary frequency control
  • PI controllers adjust generator setpoints based on ACE to drive it towards zero
• Secondary voltage control manages reactive power flow and voltage profiles within a control area
  • Pilot bus voltage control scheme uses a centralized controller to maintain voltages at selected buses
  • Reactive power optimization minimizes transmission losses while maintaining voltage constraints

Tertiary Control (Economic Dispatch)

• Tertiary control, also known as economic dispatch, operates on a time scale of tens of minutes to hours and optimizes the power generation mix to minimize costs while meeting system constraints
• Tertiary control considers factors such as generator fuel costs, transmission losses, and security constraints to determine the optimal dispatch of generation resources
  • Economic dispatch algorithms, such as lambda iteration and gradient methods, are used to solve the optimization problem
  • Security-constrained economic dispatch (SCED) incorporates power flow and contingency constraints to ensure secure operation
• Tertiary control also includes unit commitment, which determines the optimal schedule of generator start-ups and shutdowns over a longer time horizon (24-48 hours)

Coordinating Control Actions for Stability

Importance of Control Coordination

• Coordinating control actions across different levels and devices is essential for maintaining overall power system stability
• Uncoordinated or conflicting control actions can lead to system instability, such as oscillations, voltage collapse, or cascading failures
• Control coordination ensures that the actions taken by primary, secondary, and tertiary control systems are compatible and do not counteract each other

Coordination between Different Control Levels and Parameters

• Coordination between frequency and voltage control is necessary to maintain a stable operating point, as changes in one parameter can affect the other
  • Reactive power support from generators can be limited by their active power output, requiring coordination between frequency and voltage control
  • Load shedding schemes should consider both frequency and voltage stability to prevent over-shedding or under-shedding
• Coordination between local and area control systems is required to prevent undesirable interactions and ensure smooth transitions between control modes
  • Primary control actions should not interfere with the objectives of secondary and tertiary control
  • Secondary control should not override or counteract primary control actions unless necessary for system stability

Considering Device Limitations and Time Delays

• Control coordination also involves considering the limitations and time delays of various control devices, such as generator ramp rates, transmission line thermal limits, and communication latencies
  • Generator ramp rates limit the speed at which their output can be adjusted, affecting the response time of frequency control
  • Transmission line thermal limits constrain the amount of power that can be transferred, requiring coordination with power flow control
  • Communication delays can introduce latencies in control actions, necessitating the use of predictive or adaptive control techniques

Advanced Control Techniques for Enhanced Coordination

• Advanced control techniques, such as wide-area monitoring and control systems (WAMCS), can enhance control coordination by providing real-time data and enabling faster and more accurate control actions
  • Phasor measurement units (PMUs) provide synchronized measurements of voltage and current phasors across the power system
  • Wide-area controllers use PMU data to detect and respond to disturbances more quickly and effectively than local controllers
  • Adaptive and intelligent control techniques, such as fuzzy logic and neural networks, can improve control coordination by adapting to changing system conditions and learning from past experiences