Transmission Operations - Self-Healing Grid Using Wide Area Control

Regional System Operator

Organization

Organizations responsible for maintaining transmission system reliability and ensuring open access to the grid for all market participants. Regional System Operator responsibilities include: transmission planning, contingency analysis, real-time system operation, and market monitoring and management.

SCADA System

System

Control area supervisory control and data acquisition system

Energy Management System

System

Control area energy management system

Wide Area Measurement System(s)

System

Phasor measurement system covers a wide power system area.

Power Marketer

Entity

Entities who buy and sell electricity in wholesale markets.

PowerSystem

System

Composition of interconnected transmission, generation, distribution power systems

Reliability/security Coordinator

Entity

Entities that are responsible for the reliability of the power grid and have the authority to fulfill that responsibility within  the operating region managed by an Regional System Operator

ControlAreaOperator

Entity

Entities that manually operate and maintain control area facilities and equipment, and execute control orders.

Transmission Level Actuator

System

Power system actuators, which are controlled directly by transmission control area SCADA/EMS

Distribution and Plant Control System

System

Distribution management systems, distributed energy resources and generation plant control systems

IED

Device

Intelligent electronic devices including protective relays, RTUs, sensors.

FACTS Device

System

A power electronic based system and other static equipment (such as Static Var Compensator, Thyristor Controlled Series Compensator, STATCOM) that provide control of one or more ac transmission system parameters to enhance controllability and increase power transfer capability.

RAS

Systems/devices

Local or distributed intelligence remedial action schemes acting under emergency operating conditions in accordance with either pre-set or adaptive settings to protect equipment, prevent  wide-area blackouts, and restore services. 

Distribution Management System

System

A distribution management system is a suite of application software that supports distribution system operations.

ADA

System

Advanced distribution automation is a multifunctional system that supports remote monitoring, coordination and operation of distribution components by taking full advantage of new capabilities in power electronics, information technology and system simulation.

DER Device

Device/System

Distributed energy resource refers to distributed generation, storage, load management, combined heat and power and other sources involved in electricity supply, both in stand-alone and interconnection applications.

Plant Control System

System

A DCS (distributed control system) that operates a generation plant

Protective Relay

Device

A device that responds to faults by tripping a breaker according to control logic, based on the monitoring of current and voltage values, and on communications with other protective relays.

Phasor Measurement Unit

Device

Phasor Measurement Unit – a generic device which produces synchronized phasors from voltage and/or current inputs and synchronizing signals. 

Remote Terminal Unit

Device

 Remote Terminal Unit – A device used to control/monitor/record sensor results in SCADA applications

Real Time Data

Information needed to be updated or exchanged in real time. These data include voltage, current phasor measurements, and frequency, rate of change of frequency, rate of change of voltage calculations. The system flow (both MW and MVAR) can be derived by the voltage and current phasors.

Control Area Network Model

One could partition the power system network model used by a control area into four subnetworks as follows: [1]

Subnetwork 1. Internal transmission network is modeled in detail and it is monitored.

Subnetwork 2. Unmonitored internal transmission network is modeled in detail as well. It is expected that the unmonitored internal network will be minimized in time given the growth in the utility communication infrastructure.

Subnetwork 3. Adjacent external network is modeled in detail because it has significant impact on the security of the internal system. This model will be updated based on the input from adjacent control center.

Subnetwork 4: Distant external network is modeled by reduced equivalents because it has less impact on the internal system. 

Control Area Network Model Parameters

The parameters in the control area network model include facility status (such as generation shifts due to changes in transaction schedules, redispatch and unit outages, and the status of power plant auxiliary equipments), transmission element impedances, control device set points (such as generator and LTC settings), generation response capabilities (MW/min), breaker/switch states (these states are critical to update the topology of the control area network), and bus load.

Controller Settings

These settings include relay protection and load shedding schemes, other remedial action schemes (RAS), and set points for FACTSDevice devices, voltage controller, phase-shifters and other controllers.

Control Actions

The control actions involve real and reactive power generators, controllable shunts in transmission, FACTSDevice devices, phase shifters, Load Tap Changers (LTCs), transmission sectionalizing, and distribution automation functions like Volt/Var control, feeder reconfiguration, and load management functions.

Transmission System Limits

The transmission system limits include the determination of the thermal limits, available capacity, economic constraints, interface limits, steady state, transient and small signal voltage stability limits.

PowerSystem Vulnerability Data

The power system vulnerability data include fault information, environmental data, and other sources of power system vulnerability data

Boundary Conditions

Refer to the power system conditions such as voltage, current, and phase angles at the boundary of the network model that is used to simulate internal system behavior.

Dynamic Model Update

EMS system performs dynamic model update, state estimation, bus load forecast. Dynamic Model Update sub-function updates the system model to reflect the status of the transmission and generation equipment and critical operational parameters in real-time, based on gathering the wide-area synchronized phasor measurements and estimating the missing and inaccurate data; The bus-load model update and forecast is supported by the distribution operation model and analysis; In a multi-area interconnected system, each control area updates its model and exchanges the full or reduced model with neighbor areas.

Optimal Power Flow (OPF)

EMS system performs optimal power flow analysis, recommends optimization actions: Optimal Power flow provides operations personnel with recommended system changes to correct limit violations while optimizing the system for pre-defined objectives including minimizing losses, maximizing MW capacity via optimal Mvar control, minimizing the number of controls moved, or minimizing the movement in all available controls. OPF uses bus load models supported by Distribution Management System applications and includes the bus dispatchable load in its variables. OPF issues sets of actions for multiple local controllers, distributed intelligence schemes, and Distribution Management System applications.

Stability Analyses

EMS system performs stability study of network to: Determine the dynamic stability limits and Determine whether network is operating close to limits of stability

Real Time Contingency Analysis

EMS system performs contingency analysis (CA), recommends preventive and corrective actions:

Contingency Analysis and post-contingency analysis of remedial action provides the ability to correct problems caused by harmful disturbances

Result from contingency analysis is analyzed by post contingency optimal power flow

The post contingency optimal power flow simulates the behavior of relay protection, load shedding schemes, other remedial action schemes (RAS), FACTS devices, voltage controllers, phase-shifters, and other local controllers.  Status and set points are obtained from the dynamic model update, and are applied to probabilistic models of power system operations.

CA sub- function considers multiple sets of independent and dependent contingencies and provides risk assessment and severity evaluation of the sets

CA sub-function develops and implements preventive actions to reduce the risk and severity of anticipated contingencies, including generation-constrained optimal power flow implemented in closed-loop mode, blocking of some controls, and pre-arming of RAS and other distributed intelligence schemes.

CA checks the success of execution of the preventive actions and changes the input criteria in case of failure.

This activity is further elaborated in the “Contingency Analysis” use cases.

Real-time Emergency Operations (system protection level)

Protects power system facilities from damage

Automatically sheds load under conditions of low frequency, based on pre-defined or real-time computed settings, modes of operations, and priorities of connected groups of customers. Should be made adaptive to the conditions of the interconnected self-healing grid and non-intentional and intentional islanding.

Automatically sheds or reduces generation to preserve load balance over the transmission lines and power system stability

Automatically sheds load under conditions of low voltage, based on pre-defined or real-time computed settings, modes of operations, and priorities of connected groups of customers. Should be made adaptive to the conditions of the interconnected self-healing grid and non-intentional and intentional islanding.

Automatically sheds load under specific conditions, based on pre-defined or real-time computed settings, modes of operations, and priorities of connected groups of customers. Should be made adaptive to the conditions of the interconnected self-healing grid and non-intentional and intentional islanding.

Restores load based on real-time power system restoration capabilities. Should be made adaptive to the changing conditions.

Fast control of LTC to prevent voltage instability (Fast = 10 to 100ms – depending on the size of the control area)

Fast control of shunts to prevent voltage instability

Fast control of series compensation devices to prevent system instability and critical overloads

Balanced separation of the power system into near balanced islands to prevent cascading development of severe contingencies into wide-area blackout.

Filters and summarizes multiple alarms into a conclusive message about the core cause of the contingency. Uses centralized alarm reduction based on events from multiple substations

Automatically locates faults based on high-speed synchronized measurements

Provides field crews with real-time information by using mobile computing

Provides pre-fault, fault, and post-fault data for fault location, alarm processing, and analyses of the emergency operating conditions.

Provides other EMS applications and the operators with near-real time stability limits

Changes the modes of operation, the settings, and the priorities of RAS, based on evaluation of the developing or expected emergency conditions.

Issues summary requirements to DISCOs for changing distribution operations, based on evaluation of the developing or expected emergency conditions.

Changes the modes of operation, the objectives, constraints, and the priorities of Distribution Management System Volt/var control  application, based on evaluation of the developing or expected emergency conditions

Changes the modes of operation, the objectives, constraints, and the priorities of Distribution Management System feeder reconfiguration application, based on evaluation of the developing or expected emergency conditions

Issues summary requirements (amount and timing) to DISCOs for activating the interruptible/curtailable load systems. DISCO defines the specifics of implementation.

Issues summary requirements (amount and timing) to DISCOs for activating the direct load control systems. DISCO defines the specifics of implementation.

Issues summary requirements (amount and timing) to DISCOs for activating the DER Device reserves.  DISCO defines the specifics of implementation.

Issues summary requirements (amount and timing) to DISCOs for activating the load managements systems. DISCO defines the specifics of implementation.

This activity is further elaborated in the “Emergency Operation” use cases.

System Restoration

Operators perform system restorations based on system restoration plans prepared (authorized) by operation management. System restoration to normal state, in addition to automatic restoration, if needed.

Unit starts, auto-synchronization, load energization, based on the power system recovery capability monitored and coordinated by EMS

This activity is further elaborated in the “Advanced Auto Restoration” use case.