Power System Analysis: December 2011

Thursday 1 December 2011

Harmonic Measurements, Analysis and Filter Design

Harmonics in power systems can result in undesirable influence such as Capacitor heating/failure, Telephone interference, Rotating equipment heating, Relay mis-operation, Transformer heating, Switchgear failure, Fuse blowing. The main sources of harmonics in power system are static power converters, arc furnaces, discharge lighting and any other load that requires non-sinusoidal current. In order to limit the harmonic current propagation in to the network, harmonic filters are placed close to the source of the harmonic currents. Harmonic filters provide low impedance paths to harmonic currents and thus prevent them from flowing into the power network. Harmonic analysis program computes indices such as total voltage harmonic distortion factor at system buses to evaluate the effect of the harmonic sources and to evaluate the effectiveness of the harmonic filters. Also, driving point impedance plots of the buses of interest are generated to identify whether series or parallel resonance phenomenon occurs at any harmonic frequency of interest.

Approach to Harmonic Analysis

In the first step, existing and functional networks harmonic current measurements are performed at selected points to identify the harmonic currents injected into the network by the harmonic sources. These measurements reflect harmonic currents for one operating configuration and the loads prevailing at the time of measurements only. These may not represent conservative estimates of harmonic currents available.
In the second step, the measurement information of the first step will be used along with design data of harmonics (where available) from non-linear loads generating harmonic currents. A computer network model will be prepared as per IEEE standards and the effect of various harmonic sources at various harmonic orders will be examined. Various harmonic distortion factors will be computed as outlined in relevant IEEE standards. The advantage of computer model and simulation is that it can take care of large number of operating configurations and conservative estimates of harmonic currents, which cannot be covered by field measurements. Field measurements of the first step can however be used to validate the computer model developed.
In the third step, harmonic driving point impedances of all buses of interest will be generated at various harmonic orders and plots of the driving point impedances will be generated with respect to a range of harmonic orders [orders 1 through 50]. These plots indicate series and parallel resonance conditions in network.
In the fourth step, analysis of results of the first 3 steps will be carried out and solutions needed to solve any harmonic related problems will be obtained. These solutions are verified by using the computer model developed. The problems that might arise could be excessive harmonic distortion factors beyond relevant IEEE specified standards, existence of resonance conditions close to an exciting harmonic frequency. Where these problems are encountered, solutions will be provided by introduction of harmonic filters and its design will be verified again by using the computer model developed. Recommendations include specifications on sizing of individual components of the harmonic filters.

Switching Over Voltage Study

Switching Over Voltage Study is based on computer transient simulation using the EMTP/ATP program. Switching under different loading conditions and load rejection at important 400 kV and 220 kV buses should be considered. The influence of surge arresters on the switching surges will also be investigated through conventional modeling. Simulation plots will be assessed and the worst switching conditions will be reported.
These studies are generally performed to assess the transients associated with:

Energization of overhead transmission lines and study of associated transients, surge arrester ratings, transient mitigation methods.
Energization of capacitor banks / reactors in industrial or public utility facilities.
Transients associated with various switching actions such as fault application and clearance.

Lightning Over Voltage Study
Lightning over voltage will be determined mainly by transient power system analysis [EMTP studies] , including statistical switching calculation taking account of the time difference between first and last pole to close or open.

Grounding Mat Design
The grounding study is strictly based on the procedure recommended in IEEE std.80 or IEC 479 for safety analysis.

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Insulation Co-ordination Studies

Insulation Coordination is the process of determining the proper insulation levels of various components in a power system as well as their arrangements. It is the selection of an insulation structure that will withstand voltage stresses to which the system or equipment will be subjected to, together with the proper surge arrester. The process is determined from the known characteristics of voltage surges and the characteristics of surge arresters.

The following standards are used by consultants, while performing the insulation coordination:

Insulation Co-ordination, Part 1: Definitions, principles and rules IEC 71-1, standard.
Insulation Co-ordination, Part 2: Application guide IEC 71-2, standard.
IEEE Guide for the Application of Insulation Coordination. IEEE Std 1313-2-1999

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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Transient Stability Analysis

The recovery of a power system subjected to a severe large disturbance is of interest to system planners and operators. Typically the system must be designed and operated in such a way that a specified number of credible contingencies do not result in failure of quality and continuity of power supply to the loads. This calls for accurate calculation of the system dynamic behavior, which includes the elector-mechanical, dynamic characteristics of the rotating machines, generator controls, static var compensator, loads, protective systems and other controls. Transient stability analysis can be used for dynamic analysis over time periods from few seconds to few minutes depending on the time constants of the dynamic phenomenon modeled.

The dynamic performance of the system with respect to the disturbance listed below, but not limited to, shall be studied for the following cases:

3 phase fault
Unbalanced fault
Fault clearance
Loss of transformers
Loss of lines
Loss of loads
Loss of generators
Load shedding
Any other relevant contingency

Report and Recommendations from Transient Stability studies

Plots of dynamic response of generator rotor angles, frequency, power outputs, voltages, excitation system outputs and governor prime mover outputs
Plots of Lines, transformer flows, bus voltages, bus frequencies
Plots of the system variables that are of interest from protection point of view like frequencies, fault current seen from over-current relays,etc
Recommendation related to protection and control, operating strategy, Load shedding schemes, control settings of equipments ( for example: power system stabilizer, relay settings etc), based on various study cases considered

Contingency Studies

Contingency Studies

Maintaining power system security is one of the most important requirements for any utility and this includes planning for various contingency situations like outage of transmission lines; loss/recovery of generators and loads; changing/setting of switched shunts; injection groups; as well as opening/closing of buses and equipments. Contingency evaluation is carried out by using static as well as dynamic analytical tools such as load flow analysis and transient stability analysis. Real time control and monitoring solutions in energy control centers or energy management systems or load dispatch centers usually use an algorithm called contingency ranking algorithm to shortlist credible contingencies for real time evaluation and control of power systems. Often contingency ranking algorithm will use some approximate and fast load flow type algorithms from a list of contingencies and rank them in the decreasing order of severity. This ordered or ranked list will normally be considered for a detailed contingency evaluation to assess system security. Often contingency ranking algorithm will use some approximate and fast load flow type algorithms from a list of contingencies and rank them in the decreasing order of severity. This ordered or ranked list will be considered for a detailed contingency evaluation to assess system security.

Short Circuit Studies

Short Circuit Studies is critical for the safe, economical and efficient operation of all electrical power systems. In case of a short circuit, excessive electric current will flow through the system and could have disastrous consequences including personnel injury, damaged electrical equipment as well as costly downtime. Short circuit calculations provide currents and voltages on a power system during fault conditions. This information is required to design an adequate protective relaying system and to determine interrupting requirements for circuit breakers at each switching location. Fault conditions can be balanced or un-balanced shunt faults or series (open conductor) faults. Often information about contributions to a fault from rotating machines such synchronous machines, large motors would be required as a function of time to determine making and breaking requirements.

Recommendations and guidelines are given in the IEEE Violet Book (IEEE STD 551-2006) as well as North American ANSI C37.5, ANSI C37.010, ANSI C37.13 and International IEC-60909 guidelines. It also supports conventional short-circuit studies without reference to any particular standards.

The results of these studies are provided in tabular form giving details of making and breaking fault currents requirements at each breaker locations (bus locations). Further, plots of peak fault current, dc component of fault current, symmetrical RMS component of fault currents will be provided with respect to time. These calculations will be performed for all types of shunt faults such as 3 phase faults, single line to ground fault, line to line faults and double line to ground faults.

Apart from determining fault levels at various buses, short circuit studies are useful in determining post fault bus voltages in the entire system, post fault network currents in the entire system, and negative and zero sequence currents in various electrical network elements. These calculations generally provide most of the information needed for protection system design, protection setting calculations and relay coordination. These studies may be carried out for various operating scenarios of the plant with existing earthing system and the performance of the plant can be reviewed and remedies can be suggested.The results of these studies will be provided in tabular form and also on single line diagram of the plant. These calculations will be performed for all types of shunt faults.

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.

Thursday 1 December 2011

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