Unit - 4
Monitoring & Control
a) Independent System Operator (ISO) to perform higher-level decision making
b) Transmission Owner ECC to do lower-level decision making
4. 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
5. 3 Components of ECC:
a) Energy Management System (EMS)
b) Supervisory Control and Data Acquisition (SCADA)
c) Communications that connect EMS & SCADA
Key Takeaway
This topic covers the overview of ECC functions and different types of control system and its components.
a) RTU
b) MTU
c) Communication between RTU & MTU
4. Objectives of SCADA:
a) To monitor physical parameters
b) Measure’s parameters
c) Acquires data from RTU
d) Data communication between MTU & RTU
e) Real-time data monitor & control
f) Transmission of commands to RTU
5. SCADA forms the backbone of many industries including energy, transportation, manufacturing, oil & gas, recycling etc.
Advantages of SCADA:
a) Decreases production waste and boosts overall efficiency by providing useful production insights to operators and management.
b) Information derived from a SCADA system can facilitate data-driven decisions and lead to increased output and better control of processes.
c) Allows instant notification and automated response to system alarms.
d) Reduction of substation design and construction & operating costs.
e) Development for information for non-SCADA operations.
Main Components of SCADA
Key Takeaway
This topic covers the parts, objectives & advantages of SCADA system to monitor & control real time data with transmission
Wide Area Measurement Systems (WAMS)
Phasor Measurement Units (PMUs)
Ψ(t) = ψm cos (t + φ)
where,
= signal frequency in radian per second
Φ = phase angle in radians
7. Phasor representation of this sinusoidal is given by:
A constant phasor representation of this sinusoidal means that the signal remains stationary at all times.
8. The positive phase angles are made in counter-clockwise direction.
Functional Blocks in a PMU
Key Takeaway
This topic offers accurate measurements of positive sequence currents and voltages at remote locations, using synchronized phasor measurements and used in monitoring, protecting & controlling applications in an advanced power system.
a) 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.
b) Although this appears to be an easy problem, we must realize the extensive unavailability of data and of large data error.
c) Data unavailability is due to two sources, namely some substations that have no SCADA and some RTUs having maintenance issues.
d) 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.
e) The measurement set obtained using state estimation allows statistical correlation and correction of the measurements and detects bad data.
f) The measurement set comprises of active and reactive line flows, voltage measurements and bus injections.
g) 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.
h) 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
a) To obtain the vector of bus injections and check for large metering errors
b) The vector of bus injections is then used for security analysis by a Newton-Raphson on-line load flow and to determine closed-loop corrective control for some line outages.
c) Bad data detection is based on the value of the sum of the squared residuals, i.e., the performance index, J.
d) After two back-to-back failures of the J-test, a process is taken up to identify bad data based on the estimation cycle.
e) If this also fails, a logic procedure is initiated to determine network model errors.
Key Takeaway
The state estimator is a program that first receives the SCADA measurement information and then uses statistics to obtain most accurate estimate of the actual state of the system. State estimation results in a power flow model that can be used for assessment of security.
a) A Power system security assessment consists of the assessment of operating network to identify all possible failures in the system, their consequences and remedial measures.
b) The power system is said to be operating under two sets of constraints: load constraints and operating constraints.
c) Load constraints ensure that the load demand must be met by the system.
d) Operating constraints force minimum and maximum operating limits on system variables and are associated with both stability and steady state limitations.
e) Load constraints can be expressed in the form of load flow equations while operating constraints can be expressed in the form of inequalities such as bus voltage, generator real, equipment loading, phase angle differences, reactive powers etc.
f) The operating conditions of the power network can then be categorized into three operating states:
Key Takeaway
This topic assess Power system security assessment consists of the assessment of operating network to identify all possible failures in the system, their consequences and remedial measures
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.
A system is said to be in an emergency state when operating constraints are not satisfied completely. Two types of emergency states exist: First is when only steady state operating constraints are being violated (for example: an equipment-loading limit is exceeded or the voltage at a bus is below a given level). This is known as steady state emergency. The second is when a stability operating constraint is not satisfied and as a result, the system is unable to maintain stability. It is called dynamic emergency.
A system is said to be on restorative state when the load constraints are not completely satisfied, leading to either a partial or a total system shutdown. When the extreme emergency (extremis state) comes into effect, extreme disturbance occurs and may lead to shutting down of the major parts of the power system. Control action should be powerful, such as the shedding of the load, to prevent system failure.
Key Takeaway
This topic covers the normal, alert, emergency & extremis conditions under disturbance.
A contingency is an unexpected failure of a transmission line, transformer, or generator due to the occurrence of a fault, or short-circuiting in one of these components.
When such a failure occurs, the protection systems sense the failure and remove the component along with the failure, from the system. However, with one component less, the overall system becomes weak, and undesirable effects begin to occur.
A Contingency analysis technique is used to predict the effect of such equipment failures and outages, and helps to take necessary actions to keep the power system reliable and secure. The contingency analysis is divided into three stages:
Key Takeaway
A contingency is an unexpected failure of a transmission line, transformer, or generator due to the occurrence of a fault, or short-circuiting in one of these components
Preventive Control:
In the event that a system administrator induces from the working information that a framework is in an alert state, at that point he makes preventive control moves to take the system back to normal state. Assuming the state is normal, a framework administrator may wish to do some minor changes in genuine and responsive planning (from a monetary point of view), if such adaptability exists. Nonetheless, if any such change can't deliver the framework to secure state from alert state, at that point the operator needs to attempt to direct it into the safe state by genuine or receptive power rescheduling (Preventive Control). Be that as it may, rescheduling is simply done to improve security and may bring about greater expense if less expensive generators are approached to "back down" their created power while costlier ones are increased. Hence, regardless of whether preventive control is to be done, it ought to be done in a way which will limit any expense increment while at the same time guaranteeing security.
Emergency Control:
In the event that a system operator deduces from the working information that a framework is in an alert state, at that point he makes preventive control moves to take the system back to a normal state. In any case, it is conceivable that the system operator can't act on schedule before an unexpected failure really happens because of significant expense of preventive control or because of deficient reserve margins. In such circumstances, a system in an alert state may course into an emergency and an all-out power outage if no control moves are made. Crisis control measures can be utilized to recover such circumstances. Since equipment can withstand a brief time thermal overload, there is a little window of time wherein some manual crisis measures can be executed. For other crisis circumstances, time might be excessively short and predesigned programmed crisis measures are important. Some crisis control moves that can be made are:
a) Generation control (lessening or expanding according to prerequisite)
b) Load shedding or Generation Tripping
c) Re-routing of power flows and voltage control
Key Takeaway
This topic helps in understanding Preventive & Emergency Control where normalized condition needs to be attained in proper functioning of a system.
References:
1. Power system Analysis-by John J Grainger William D Stevenson, TMC Companies, 4th edition.
2. The Power System Analysis and Design by B.R. Gupta, Wheeler Publishing
3. Power System Analysis by Hadi Saadat – TMH Edition.
4. Modern Power System Analysis by I.J. Nagaraj and D.P.
