undefined | unit 4 monitoring and control

PS2

Unit - 4

Monitoring & Control

Q1) What are the features of State Estimation?

A1) The important features of State Estimation are, as follows:

SE runs every 15 minutes or on request for major system changes.

The most important aspect of state estimation is bad data identification. The bad data detection is based on the performance index J. The measurement set consists of real and reactive power flows, real and reactive bus injections and bus voltages. This data is available to companies whose lines are represented in the SE model. This alone could be a worthwhile justification for including state estimation in a system control centre.

The purpose of the SE is for security monitoring and for security analysis of the power system.

The results of the SE are used for security monitoring and the operator is informed of overloads or other critical conditions.

The SE results are stored in a historical file for a (7-day) particular period.

Q2) What is meant by Power System Security?

A2) The power system needs to be secured, we need to protect it from the black out or any internal or external damage. The operation of the power system is set to be normal only when the flow of power and the bus voltages are within the limits even though there is a profitable change in the load or at the generation side. From this we can say that the security of the power system is an important aspect with respect to the continuation of its operation.

Security functions are of two types, as follows:

Security control: It determines the exact and proper security constraint scheduling which is required to obtain the maximized security level.

Security assessment: It gives the security level of the system in the operating state.

Q3) Describe the two components of SCADA.

A3) SCADA (Supervisory Control and Data Acquisition) comprises of two parts namely:

Supervisory control: This indicates that the operator, residing in the energy control centre (ECC), has the ability to control remote equipment. For example, relays located within the RTU, on command from the ECC, open or close selected control circuits to perform a supervisory action. Such actions may include, for example, opening or closing of a circuit breaker or switch, modifying a transformer tap setting, raising or lowering generator MW output or terminal voltage, switching in or out a shunt capacitor or inductor, and the starting or stopping of a synchronous condenser.

Data acquisition: This indicates that information is gathered characterizing the state of the remote equipment and sent to the ECC for monitoring purposes. For example: Information gathered by the RTU and communicated to the ECC includes both analog information and status indicators. Analog information includes, for example, frequency, voltages, currents, and real and reactive power flows. Status indicators include alarm signals (over-temperature, low relay battery voltage, illegal entry) and whether switches and circuit breakers are open or closed. Such information is provided to the ECC through a periodic scan of all RTUs. A 2 second scan cycle is typical.

Q4) Discuss the applications of SCADA in Power Generating Stations.

A4) As the power system deals with power generation, transmission and distribution sectors, monitoring is the main aspect in all these areas. Thus, the SCADA implementation of power system improves the overall efficiency of the system for optimizing, supervising and controlling the generation and transmission systems.

SCADA function in the power system network provides greater system reliability and stability for integrated grid operation.

With the use of Programmable Logic Controllers (PLC) hardware and powerful bus communication links along with SCADA software and hardware’s in power generating stations, delivering an optimal solution for each and every process operation is flexible with advanced control structures.

The functions of SCADA in power generation include:

Continuous monitoring of Speed and Frequency

Geographical monitoring of coal delivery and water treatment processes

Supervising the status of circuit breakers, protective relays and other safety related operations

Generation operations planning

Active and reactive power control

Turbine protection

Load scheduling

Historical data processing of all generation related parameters

Q5) Describe State Estimation.

A5)

State estimator is an important tool for online monitoring, analysis and control of power systems. State estimation is used in all Energy Management Systems (EMS) to identify the present operating state of a system.

Here, SE= State Estimator

LP=Limit Checking Program

ED=Economic Dispatch

MD=Mimic Display

CAP=Contingency Analysis Programs

State Estimation (SE) is mainly used to filter redundant data, to eliminate incorrect measurements and to produce reliable state estimates. It allows the determination of the power flows in parts of the power system which are not directly metered.

The bus voltage magnitude, real power injections, reactive power injections, active power flow, reactive power flow and line current flows are common measurements available in SCADA systems. SE is a very useful tool for the economic and secure operation of transmission networks.

From early days of Schweppe, developments of SE are done as a notion of robust estimation, hierarchical estimation, with and without the inclusion of current measurements, etc.

The state variables of State estimation are the voltages and phase angles. Once the estimates of the state variables are known proper actions, if required (during emergency, normal insecure states), can be taken to bring the system back to its normal secure state.

For proper monitoring of the system, the intervals at which the telemetry data are collected and the estimates of the state variables made must be very less.

As the size of a power system increases, collecting data and solving the state estimation problem in a very short time in one control centre, not only becomes very difficult, but also requires extra investment in setting up long telemetry and communication lines.

Q6) Define Phasor Measurement Unit.

A6)

The Phasor Measurement Unit (PMU) is a microprocessor-based device that uses the ability of digital signal processors in order to measure 50/60Hz AC waveforms (voltages and currents) at a typical rate of 48 samples per cycle (2400/2880 samples per second).

PMUs are used in the transmission side of the grid. These are installed at various places on the grid and are time synchronized using Global Positioning System (GPS).

Thus, synchronised real time measurements are obtained from multiple measurement points on the grid.

PMUs provide power system automation in the grid: First, the analog AC waveforms are synchronously sampled by an A/D converter for each phase. In order to provide synchronous clock for the entire system, the time from GPS satellites are used as input for a phase-lock oscillator and thereby, waveforms of the entire system are sampled with 1 microsecond accuracy. In the next step, PMU uses digital signal processing techniques to calculate the voltage and current phasors.

Also, line frequencies can be calculated by PMU at each site. By using this technique, a high degree of resolution and accuracy can be achieved. The measured phasors are tagged by GPS time stamps and are transmitted to a PDC (Phasor Data Concentrator) at the rates 30-60 samples per second.

A network of PMUs is called Wide Area Measurement System (WAMS) that can be used for large scale monitoring of the grid.

Q7) What is EMS in power systems?

A7)

An energy management system (EMS) is a system of computer-aided tools used by operators of electric utility grids to monitor, control, and optimize the performance of the generation and/or transmission system.

The monitor and control functions are known as Supervisory Control and Data Acquisition (SCADA), followed by several on-line application functions.

Energy Management Software (EMS) is a general term referring to a variety of energy-related software applications which may provide utility bill tracking, real-time metering and lighting control systems, building simulation and modeling, carbon and sustainability reporting, demand response, and/or energy audits. Managing energy can require a system of systems approach.

Electrical energy management systems (EMS) are an important function for the reliable and efficient operation of power systems. EMS is related to the real time monitoring, operation and control of a power system.

The information from the power system is read through Remote Terminal Units (RTUs), an integral part of SCADA to an EMS or Energy Control Centre (ECC).

EMS consists of both hardware and software. Hardware part of EMS consists of RTU, Intelligent Electronic Device (IED), Protection, Computer networking, etc. Software part of EMS consists of Application programs for network analysis of power systems. In EMS, application programs are run in a real time as well as extended real time environment to keep the power system in a secure operating state.

Now-days, EMS is an integral part of any power system. It is used as a part of Substation Automation System (SAS), Demand Side Management (DSM), Protection, and Distribution Management Systems (DMS) for renewable energy and so-on. In the next few years, EMS-DMA will change the role of power systems, monitoring and control.

Q8) Describe the evolution of EMS.

A8) The evolution of EMS has a long past. It has started with control centers in 1960s to fully developed energy management systems:

1960 - Control Centres (CC): These control centers were initially termed a load dispatch centres. The important task was to control the power generation and load demand as to match the generation with load demand. Even today, the term load dispatch centre’s are widely used in various state electricity boards as well as energy control centre’s.

1970 - Energy Control Centres (ECC): Here the main task was to control the energy rather than the power. Here energy monitoring is of main concern the matching of energy of power demand from that of power generation is of main concern.

1990 - Energy Management Systems (EMS): In EMS, the main task was to manage the energy through various techniques like load management (LM), demand side management (DSM), distribution management systems (DMS). EMS are computer-based programs hat perform both computational tasks as well as decision making tasks so as to assist the operator for real time operation and control.

Q9) What are the functions of EMS?

A9) The important benefits of an EMS can be addresses as the following functions:

Control functions:

Real time monitoring and control functions

Automatic Control and automation of a power system like Automated interfaces and electronic tagging

Efficient automatic generation control and load frequency control

Optimal automatic generation control across multiple areas

Tie -line control

Operating functions:

Economic and optimal Operation of the generating system

Efficient operator Decision Making Improved quality of supply

Optimization functions:

Optimal utilization of the transmission network

Power scheduling interchange between areas

Optimal allocation of resources

Immediate overview of the power generation, interchanges and reserves

Planning functions:

Improved quality of supply and system reliability

Forecasting of loads and load patterns

Generation scheduling based on load forecast and trading schedules

Maintaining reserves and committed transactions

Calculation of fuel consumption, production costs and emissions

Q10) What is PLC?

A10)

PLC stands for ‘Programmable Logic Controller’, which is installed to monitor system sensors, by collecting data and critical information about the flow and input within the system.

A PLC will also perform basic interventions, triggering outputs when the pre-programmed limitations are met. A PLC is a versatile piece of hardware; able to perform under challenging conditions where advanced options and real-time usage are necessary. For instance, PLCs can control some of the more complex processes within industrial operations, such as monitoring running motors and machinery.

These devices are very flexible and easy to programme, which means they can function in a wide range of solutions. Modern PLCs were manufactured to upgrade the relays and timers which were previously used in industrial machinery.

Q11) What is the difference between PLC & SCADA?

A11)

The primary difference between a PLC and SCADA is the technology. For example, a PLC is a physical hardware, whereas SCADA is software. This means that a PLC can be picked up and physically inspected, whereas SCADA works on a computer system, and is comparable to that of an operating system, like Windows for example.

SCADA is designed to operate on a much broader scale since it can monitor and collect information from every output of a system. A PLC, on the other hand, will only focus on monitoring only one element within the system.

Q12) How do PLC & SCADA work together?

A12)

Since these technologies are so different, it’s easy to think that PLC and SCADA aren’t connected. However, the association between the two technologies is crucial. Both PLCs and SCADA are used within the same industrial context.

This means that the two work together to support safe and effective operation within a plant. SCADA can be looked upon as the broad software structure that supports the overall system. Whereas PLCs operate within the system that SCADA oversees.

The PLCs require SCADA to control their operation, whereas the SCADA needs the data collected by the PLCs to do this job effectively.

For instance, if the system is monitoring a piece of machinery, the PLC may retrieve data that suggests there too much vibration. The PLC will send this data to the SCADA software, which will then inspect the readout data and decide whether adjustments must be made to the operation of the system. If change is needed, SCADA will send the instruction back to the PLCs, which will then enable the change.

Q13) Discuss the functions of ECC?

A13)

ECC is the decision centre for an inter-connected electric transmission & generation system.

It helps in the minute-by-minute monitoring of physical & economic operations of the interconnected power system grid.

It involves a two-level hierarchy comprising of:

a) Independent System Operator (ISO) to perform higher-level decision making

b) Transmission Owner ECC to do lower-level decision making

ECC comprises if 4 types of control centers:

a) Level 1: Local Control Center – Power stations and substations - monitor & control of load, Protect & break circuit, Regulate voltage, Synchronization of feeders, Load Shedding etc

b) Level 2: Area Load Dispatch Center – Transmission Network – receives and processes information for related control measures

c) Level 3: State Load Dispatch Center – Transmission System – system wide load monitoring & control followed by corrective actions

d) Level 4: Regional Control Center – Interconnected Power Systems – monitor & control inter-state & inter-regional power regulation

3 Components of ECC:

a) Energy Management System (EMS)

b) Supervisory Control and Data Acquisition (SCADA)

c) Communications that connect EMS & SCADA

Q14) Which disturbances are handled by Power System Control?

A14) The objective of power system control is to maintained continuous electric supply of acceptable quality by taking suitable measures against the various disturbances that occur in the system.

These disturbances can be classified into two major heads:

Small Scale Disturbances: Small scale disturbances comprise slowly varying small magnitude changes occurring in the active and reactive demands of the system. The small-scale disturbances can be overcome by regulating controls using governors and exciter.

Large Scale Disturbances: The large-scale disturbances can only be overcome by proper planning and adopting emergency switching control. Large scale disturbances are sudden large magnitude changes in system operating conditions such as faults on transmission network, tripping of a large generating unit or sudden connection or removal of large blocks of demand.

Q15) What do you understand by contingency analysis?

A15) Contingency analyses are important tasks for the safe operation of electrical energy network. Potential harmful disturbances that occur during the steady state operation of a power system are known as contingencies. Contingency analysis is carried out by using repeated load flow solutions for each of a list of potential component failures. This process has to be executed for all the possible contingencies, and repeated every time when the system load or structure changes significantly. Conventional methods are tedious and time-consuming process, which is not desirable for real time applications.

Power Systems are operated so that overload do not occur either in real-time or under any statistically likely contingency. This is often called maintaining security. Steady state power system insecurity such as transmission lines being overloaded causes transmission elements cascade outages which may lead to complete blackout. The power system operator must know the system state at any instant. The contingency analysis is used to predict which contingencies make system violations and rank the contingencies according to their relative severity. Contingency Analysis is useful both in the network design stages and for programmed maintenance or network expansion works to detect network weaknesses. The weaknesses can be strengthened by transmission capacity increase, transformers rating increase besides circuit breakers ratings increase.

Q16) Explain on-line security analysis?

A16) There are three basic elements of on-line security analysis and control, namely, monitoring, assessment and control. They are tied together in the following framework:

Step 1: Security Monitoring: Using real-time system measurements, identify whether the system is in the normal state or not. If the system is in an emergency state, go to step 4). If load has been lost, go to step 5).

Step 2: Security Assessment: If the system is in the normal state, determine whether the system is secure or insecure with respect to a set of next contingencies.

Step 3: Security Enhancement: If insecure, i.e., there is at least one contingency which can cause an emergency, determine what action should be taken to make the system secure through preventive actions.

Step 4: Emergency Control: Execute proper corrective action to bring the system back to the normal state following a contingency which causes the system to enter an emergency state. This is sometimes called remedial action.

Step 5: Restorative Control: Restore service to system loads.

Q17) Write a short note on Wide Area Measurement Systems?

A17)

This technology is known to offer accurate measurements of positive sequence currents and voltages at remote locations, using synchronized phasor measurements.

Phasor Measurement Units (PMUs) are the instruments that perform this function.

WAMS allows us to obtain a detailed data on steady-state of the power system in operating mode, which may arise due to various power system failures.

A WAMS process comprises of 3 interconnected sub-processes, namely data acquisition, data transmission and data processing.

Measurement and communication systems, combined with energy management systems perform these sub-processes.

WAMS acquires system data from conventional resources and transmits it to the control centre through a communication system which then processes it. On extracting adequate information from system data, decisions are taken on the operation of power system.

At times, WAMS may also command certain actions that are performed by the system actuators in remote locations.

WAMS makes efficient use of data and helps to achieve a more secure plan for the flow of electrical energy.

Q18) Discuss the importance of state estimation.

A18)

The primary function of state estimator is to determine the operating conditions of the system, i.e., the bus voltages, load levels, and generation levels.

Although this appears to be an easy problem, we must realize the extensive unavailability of data and of large data error.

Data unavailability is due to two sources, namely some substations that have no SCADA and some RTUs having maintenance issues.

All analog measurement devices contain some measurement error, which typically, is small for a single device, but the use of thousands of devices, each having small error can result in significant inaccuracy in the overall system analysis.

The measurement set obtained using state estimation allows statistical correlation and correction of the measurements and detects bad data.

The measurement set comprises of active and reactive line flows, voltage measurements and bus injections.

State estimation runs every minute using the last set of measurements which are scanned every second. Furthermore, it starts whenever there is a network change.

Weighted Least Square (WLS) is the conventional state estimator technique to find the best state vector to fit a scattered data set, which is obtained due to the imperfect measurements of voltages and currents. The measurement equation is given below:

z = h(x) + e where,

z = Measurement vector formed by voltage magnitude, real and reactive power flows and power injections

h(x) = The non-linear function relating the error - free measurements to the system states.

x = State Vector

e = Noise in measurements

Q19) Differentiate between normal & alert state of a power system.

A19) A system is said to be in the normal state when both, load and operating constraints are taken care of. It can be assumed that in the normal state, the power system is in a quasi-steady state condition. At any given time, the intersection of load constraints and operating constraints defines the space of all feasible normal operating states, in which the power system may be operated.

A normal operating point can be termed as either secure or insecure with respect to an arbitrary set of disturbed data. A secure system can undergo any contingency in the next-contingency set without entering an emergency condition. On the other hand, if there is at least one contingency in the next-contingency set which can cause an emergency, the normal system is called insecure or in an alert state.

Q20) Write a short note on preventive & emergency control for a power system failure.

A20) Power system security is more and more in conflict with economic and environmental requirements. Security control aims at making decisions in different time horizons so as to prevent the system from undesired situations, and in particular to avoid large catastrophic outages. Traditionally, security control has been divided in two main categories: preventive and emergency control.

In preventive security control, the objective is to prepare the system when it is still in normal operation, so as to make it able to face future (uncertain) events in a satisfactory way. In emergency control, the disturbing events have already occurred, and thus the objective becomes to control the dynamics of the system in such a way that consequences are minimized.

Types of control actions: generation rescheduling, network switching reactive compensation, sometimes load curtailment for preventive control; direct or indirect load shedding, generation shedding, shunt capacitor or reactor switching, network splitting for emergency control.

Uncertainty: in preventive control, the state of the system is well known but disturbances are uncertain; in emergency control, the disturbance is certain, but the state of the system is often only partially known; in both cases, dynamic behavior is uncertain.

Open versus closed loop: preventive control is generally of the open loop feed-forward type; emergency control may be closed loop, and hence more robust with respect to uncertainties. In the past, many utilities have relied on preventive control in order to maintain system security at an acceptable level. In other words, while there are many emergency control schemes installed in reality, the objective has been to prevent these schemes as much as possible from operating, by imposing rather high objectives to preventive security control.

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