Power System Analysis: Power Flow Studies

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Thursday, 1 December 2011

Protection and Relay Co-ordination Studies

Overcurrent phase and earth fault relay coordination is necessary to achieve proper fault identification and fault clearance sequence. These relays must be able to distinguish between the normal operating currents including short time overcurrents that may appear due to certain equipment normal operation (example: Motor starting currents, Transformer inrush currents) and sustained overcurrent due to fault conditions. During fault conditions, these relays must operate quickly isolating the faulted section of the network only and allowing for continued operation of the healthy circuits. In the event of failure of primary relays meant for isolating the fault within its primary zone of protection, backup relays must operate after providing for sufficient time discrimination for the operation of primary relays. Hence, the operation of backup relays must be coordinated with those of the operation of the primary relays. The flexible settings of the relays (namely plug or tap setting, the time dial setting and possibly selection of suitable time-current operating characteristics), must be set to achieve the objectives stated in this section.

Once the relays are coordinated, the discrimination in the operation of primary and backup relays and their coordination with the maximum possible load currents will be plotted on the time current characteristics (TCC’s). The results of the relay coordination will be provided in the form of recommended settings and TCC’s. Relay coordination needs to be evaluated for maximum and minimum fault conditions and for various possible network configurations. Where a network has several levels of primary and backup relay levels, the source end relay operation can become quite delayed due to successive time discrimination at down stream load end coordination levels. In such cases it may be necessary to ensure isolation of fault at the earliest by possibly coordinating the source end relays with much faster dedicated equipment relays in the down stream (example differential protection of transformers).

Relay coordination may need to provide suitable instantaneous settings where it is possible that instantaneous settings can positively discriminate between the faults in the primary and subsequent zones. This is essential to ensure that the instantaneous settings will not act for faults outside its primary protective zone bypassing the needed discrimination between the primary relay to which it is a backup.

The following protection aspects will be covered in the study

Overcurrent Phase (51, 50) and Earth (51N, 50N) Fault Protection – Relay coordination with High set settings, and definite time releases. Coordination will also consider coordination with maximum load currents, motor starting current and motor starting time; transformer inrush currents, safe stalling time and current of motors, thermal withstand characteristics of the equipments.
Motor protection
Transformer protection
Unit Protection
Distance Protection

The complete protection system study of the plant will be performed considering the following criterias:

Accepted Standard Engineering practices in protection
Applicable IEEE standards
Information available in the relay application guides, and relay catalogues of the relay manufacturer. This criterion will be generally more used in determining relay settings in comparison to the other methods specified for equipment protections.

Evaluation of Load Shedding Schemes

The existing and proposed load shedding schemes will be evaluated using the transient stability simulation, with dynamic models of generators, controllers and loads. The following factors will be considered in evaluating the load shedding schemes and providing the recommendations

Governor/prime mover controllers will be modeled in simulation
Spinning reserves will be modeled appropriately in the governor/prime mover model
Frequency deviation, Rate of change of frequency deviation, Bus voltage variations as function of time will be considered
The power balance factor will also be considered in designing and evaluating the load shedding schemes.

Load shedding is essential only during under conditions with frequency still decreasing and never to be used when frequency is recovering or increasing. This factor will be considered appropriately while designing load shedding schemes.

Motor Starting Studies

The starting current of most AC motors is several times the normal full load current when starting them directly on line at full rated voltage. The starting torque varies directly as the square of the applied voltage. Excessive starting current causes a drop in terminal voltage which may result in the following:

Failure of motor starting due to low starting torques.
Unnecessary operation of under voltage relays.
Stalling of other running motors connected to the network.
Voltage dips at the power sources and consequent flicker in the lighting system.

Motor starting studies can help in the selection of best methods for motor starting, motor design, and system design thereby minimizing the impact of the motor starting.

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Power Flow Studies

Power flow/Load flow calculations provide active & reactive power flows, bus voltage magnitude and their phase angle at all the buses for a specified power system and operating conditions. These values are typically subject to various factors like regulating capability of generators, synchronous condensers, static VAR compensator, HVDC controls, FACTS controllers, tap changing under-load transformers and specified net interchange between individual operating systems (utilities). Power flow information is essential for the continuous evaluation of the current performance of a power system and for analyzing the effectiveness of alternative plans for system expansion to meet increased load demand. These analyses require the calculation of numerous power flow cases for both normal, and emergency (contingency) operating conditions.

Applications of Power Flow Study and Analysis

Transmission expansion planning ,operation planning
Distribution expansion planning , operation planning
Industrial/Commercial distribution system planning, operational planning
Network interconnection, Grid interconnection studies
Evaluation of energy transactions between various stake holders
Energy audit to accurately determine network losses and estimate billing losses if any
Sizing of transformers, cables, overhead lines, transformer tap ranges, shunt capacitors, shunt reactors, reactive power management, FACTS devices, HVDC operation
System security assessment via static contingency studies
Decision making tool in operation planning and operation of the system in load dispatch center
Motor starting studies using load flow type analysis, where the starting impedance of the Induction motor is modeled as constant impedance model with starting impedance.
Evaluation of static voltage stability using load flow technique

The following general criteria of acceptability of design is used in power flow studies

Voltage Drop at all buses should be within +/- 5% of the nominal rating for all operating conditions considered
No over load conditions of any electrical circuits for all operating conditions considered
Reactive power generation/import/export to be within specified limits for all operating conditions considered
Ensuring quality power supply to all loads, under specified contingency conditions, as per design philosophy adopted.

The following study cases/ power flow outputs are generally considered in power flow studies

Extreme operating conditions of maximum and minimum loading conditions will be considered to check the adequacy of the network, even though some of these conditions may not exist during normal operation
Contingency conditions such as outage of lines, transformers and generators will be considered and network adequacy for power evacuation will be assessed
Operating solutions such as transformer taps, generator excitation, shunt reactive power compensations will be provided as needed.
Recommendations for strengthening and equipment upgradations will be provided to meet specific operating requirements.
Summary of load flow studies and concise reports in tabular formats and single line diagram formats will be provided, along with the summary of recommendations

Overview of Power System Analysis

An electric power system is a network of electrical components used to generate, transmit and use electric power. This could be the elaborate network that supplies power to a region’s home and industry through an electrical grid transmission system from generating plants located faraway or even a captive power plant/microgrid which generates and consumes power within the same premises itself. The planning, design, and operation commercial and industrial power systems require indepth engineering studies to evaluate existing and proposed system performance, reliability, safety, and economics. Due to high risk of human life and costly equipments associated with any power systems, properly conceived and conducted power system studies and simulations are the only cost-effective way to prevent surprises and to optimize equipment selection. These power system studies identify and help avoid potential deficiencies in the system before it goes into operation. In the case of existing systems, the studies help locate the cause of equipment failure and misoperation, and determine corrective measures for improving system performance.

In power engineering, an oneline diagram or single-line diagram (SLD) is a simplified notation for representing a three-phase power system. The one-line diagram has its largest application in power flow studies and in engineering design. Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols. It is a form of block diagram graphically depicting the paths for power flow between entities of the system. Instead of representing each of three phases with a separate line or terminal, only one conductor is represented. Elements on the diagram do not represent the physical size or location of the electrical equipment, but it is a common convention to organize the diagram with the same left-toright, top-to-bottom sequence as the switchgear or other apparatus represented. A typical SLD is illustrated below for reference:

Today, with the advent of digital computers, Power System Simulations are the basis of modern design of power systems for all industries. Typical studies most likely needed are load flow studies, ground mat design, harmonic measurements, cable ampacity studies, short-circuit studies, switching transient studies, coordination studies, and motor starting studies. The responsible engineer for system design must decide which studies are needed to ensure that the system will operate safely, economically, and efficiently over the expected life of the system.