EHV Systems Research Articles

Power System Fault Detection and Analysis Using Numerical Relay in Power grid Corporation Limited, Shoolagiri

The faults in a power system may be of various nature and they may affect the stability of the power system by causing outage of the line, there by overloading other connected lines and also causing cascading effect in the system. In order to access the nature of fault and analyze the impact of the same in a power system, fault analysis is required to be carried out as the same will be useful for the system operation and to take quick decisions in securing the system. In this paper we discuss the various types of faults in EHV system that may occur in line or substation. These faults are identified by Numerical relays which use IEDs [Intelligent Electronic Device]. The faults identified includes symmetrical and unsymmetrical faults in power system. Further in this project, we have taken into consideration the theoretical and practical study of the fault analysis through Numerical relays. In case of occurrence of Transient faults, Auto re-closure works with dead time of 1 sec. Current, voltage, Breaker status (open, close) are graphically represented by Disturbance Recorder. Events are recorded by Event logger. The above mentioned project for faults and analysis of faults in line or substation were carried out at Power grid Corporation Limited, 400 / 230kv Sub-Station Shoolagiri.

Comparison of electromagnetic fields emitted by typical overhead power line towers

The problem of electromagnetic fields produced by overhead power lines is still a widely discussed topic. These concerns itself are coming not only from the power line designers, but also from general public sector. Therefore, the topic can be separated into two different areas of interest: health concerns and technical problems. Conducted discussion and research regarding electromagnetic field’s health risks to general public led to creation of legislative time restrictions and absolute value limits for low-frequency electromagnetic fields. On the other hand, technical side of problem is mostly connected with electromagnetic induction in connection with surrounding objects around power lines. Still, precise calculations are needed for both of these areas of interest. The aim of this article is to carry an analysis comparing various electromagnetic fields emitted from basic types of power lines in 2D space. Comparison is made for the worst-case scenarios and for different sets of boundary conditions. The paper also shows the marginal difference of EMF generally emitted by EHV and HV systems. To achieve the most precise results, an integral–differential EMF calculation methodology is introduced and implemented into software interface. Final results are applicable in overhead lines design and planning process and are also suitable for future research of overhead power lines.

Investigation of the Interaction Between Substation Transients and Transformers in HV and EHV Applications

This paper summarizes the work performed by the Task Force -within Performance Characteristic Subcommittee of IEEE PES Transformers Committee- to investigate potential revisions to IEEE Std C57.142-2010. Although the scope of this document does not implicitly exclude HV and EHV systems, the existing document primarily includes examples and information on distribution voltage systems. A Task Force was formed to investigate the need to include information about power system transient events on HV systems more explicitly in this existing document. The Task Force was charged with the following tasks:-Establish system voltages encompassed by C57-142-2010 Guide. -Gather field data, reports, and literature on HV and EHV failures related to substation transients and transformer interaction. -Solicit input from the other technical committees on this subject. -Review IEC and CIGRE work in this area. -Recommend course of action on revision of the present Guide. -Recommend high level changes to the Guide. -Prepare final paper for the Subcommittee. In the course of this work several failure scenarios were identified. These are categorized and the failures are described in this paper. The final recommendations to the Subcommittee are summarized and recommendations are presented for the course of action on the revision/addition of HV and EHV material to the current C57.142 Guide.

Switching Surge Design of 1200kV UHVAC Transmission Line

With the increase in operating voltage of transmission system to transport large amounts of power to meet increased demand switching surges became governing factor in the insulation design of EHV and UHV systems. Insulation failure probability under switching surges that is switching surge flashover rate (SSFOR) is the main concern in the switching surge design of transmission lines. In this paper, SSFOR of 1200kV UHVAC transmission line under switching surges which are generated during energization of no-load transmission line is estimated.

Pollution Performance of RTV Coated Insulators

Theincrease in power demand necessitates the usage of EHV and UHV transmission system. The growth of industries causes higher pollution level in the ambient condition on insulators. This increases the pollution severity of the site which results in pollution accumulation on the surface of Insulators. This causes flashover across insulator. Flashover across polluted insulators poses a serious threat to the reliability of the system and leads to system outages. There are many remedial measures to minimize the flashover of a porcelain insulator under pollution conditions. One such method is the application of hydrophobic coatings such as Room Temperature Vulcanizing Silicone Rubber on the surface of ceramic insulators. Laboratory testing of coated insulators has been carried out based on the solid layer method and by the inclined plane tests at constant voltage to evaluate the RTV coatings withstands capability against tracking and erosion. The performance of the coatings was assessed by monitoring the leakage current on the insulator surfaces. The applied voltage and leakage current were monitored throughout the tests. In order to optimize and economize the usage of RTV coatings various tests were performed and results are analyzed. It was possible to conclude from the test results that one fourth length of RTV coating is sufficient to withstand the pollution severity.

Protection Scheme using RPI Concept in STATCOM Compensated EHV System

This research work present a protection scheme for fault detection and fault classification in EHV with static synchronous compensator (STATCOM) using Regulated Power index (RPI) concept. RPI per phase is defined as the ratio of the sum of sending end apparent power and receiving end apparent power to the apparent power at receiving end for that phase [1]. STATCOM is used for reactive power compensation and regulates the system voltage by absorbing and generating reactive power. As per the requirement of system parameter scheme is developed for the detection of fault, identification of faulty phase and tripping the faulty phase. This scheme will be validated by considering various fault test cases considering severe fault conditions with very low resistance, high fault resistance conditions and internal faults with static synchronous compensator (STATCOM) at sending end on the MATLAB model of transmission system at 220 KV level.

Estimation of Switching Surge Flashover Rate of a 1200 kV UHVAC Transmission Line due to Switching Overvoltages

The rapid increase in transmission voltages to fulfil higher demand of the transmitted power yields to put the switching surges as the governing factor in the insulation design process for EHV and UHV systems. Contemporary methods of insulation coordination utilise probabilistic approach and statistical methods particularly in the case of ultra-high voltage systems. This paper determines the statistical distributions of energization, re-energization, fault initiation and fault clearing overvoltages of the 1200 kV system modelled in PSCAD. Decisive figures like mean value, standard deviation and 2% statistical overvoltage values of switching overvoltage are estimated using randomness of the instants of closing with 526 operations for line energization and re-energization, and randomness of faulted phase, fault location and fault times with 3 faulted phases, 8 fault locations and 21 fault timings, 504 simulations for fault initiation and fault clearing. Ascertaining the investigations from the above simulations the stress on the towers is estimated. From the stress and insulation strength of the tower a systematic approach to calculate the switching surge flashover rate (SSFOR) is presented.

New development in relay protection for smart grid

This series of papers report on relay protection strategies that satisfy the demands of a strong smart grid. These strategies include ultra-high-speed transient-based fault discrimination, new co-ordination principles of main and back-up protection to suit the diversification of the power network, optimal co-ordination between relay protection and auto-reclosure to enhance robustness of the power network. There are also new development in protection early warning and tripping functions of protection based on wide area information. In this paper the principles, algorithms and techniques of single-ended, transient-based and ultra-high-speed protection for EHV transmission lines, buses, DC transmission lines and faulty line selection for non-solid earthed networks are presented. Tests show that the methods presented can determine fault characteristics with ultra-high-speed (5 ms) and that the new principles of fault discrimination can satisfy the demand of EHV systems within a smart grid.

Performance Measures of Tele-Protection System Based on Networked Microwave Radio Link

The purpose of paper is to compute reliability of Digital Tele-Protection System employing Digital Microwave Radio Link. In this paper, the whole Tele- Protection system is divided into six subsystems. The first subsystem is digital relaying equipment. Second subsystem is the tale-protection signaling equipment where the timing to sample line power currents is synchronized. Both relaying and Tele-Protection signaling equipments, are now assemble three-phase current data, sampled at 600 Hz for 50 Hz EHV systems and some control data into a 90 bit frame format at a transmission rate of 54 Kb/s. A frame consists of a frame synchronization field and some information fields including a cycle's redundancy check field. The third subsystem, Tele-Protection signed multiplexing equipment generates a super frame by multiplexing the seven, 54 Kb/s Tele-Protection signals into a 1.544 Mb/s stage using multi-point sampling technique to minimize the delay time variation and send them to the subsystem four, which is radio equipment, from here the data is transmitted with the help of the channel (subsystem 5) to the repeater station (subsystem 6). The subsystem six is used to remove the noise and to improve the power of the data. In this paper, we have taken standby digital relaying equipment and that can be online through an imperfect switching device. Also parallel redundant Tele-Protection signaling equipment is taken to improve system's overall performance.

Switching and Power Frequency Transients in EHV Systems A Review and Case study

Overvoltages caused by transients are important in a power system as they cause stresses on electrical equipment. Majority of power system failures are directly or indirectly related to transient problems rather than steady state operation. The insulation level of each element in a power system is governed by the transient voltages originating as a result of lightning, short circuits and switching actions. In contrast to lightning, switching overvoltages originates on the system and is inherently measured in terms of the system voltage. With the adoption of 400 kV voltages and above, it was clear that switching surges created in the system would determine the cost and strength of the major insulations to ground. This paper gives an insight to the electromagnetic transient phenomenon - switching and power frequency overvoltages and case study results of simulation for these overvoltages for a typical Indian 765 kV system.

Maximum undergrounding degree of HV subtransmission networks as dictated by unscheduled power flows

Meshed HV subtransmission networks are subject to undesirable parallel (unscheduled) active power flows from the EHV system. The phenomenon is worsened by conversion of HV overhead lines to underground cables (UGCs), which increases parallel flows through the HV network because of the attendant reduction in HV line reactance; the effect increase with network extension and are compounded by UGC charging power. The study deals with the assessment of the maximum undergrounding degree for a given subtransmission network. An analytical formulation of maximum HV line extension length as a function of EHV power flow and undergrounding degree is given; optimum UGC shunt compensation requirements are evaluated. Application to a typical 150 kV-50 Hz network is presented, showing a very good agreement with power flow simulation. In particular, the case study evidences that complete undergrounding of the HV network is possible if line length is under 50 km; the proposed shunt compensation effectively raises this limit to 70 km.

Study on AC Artificially Pollution Flashover Performance of EHV Class Insulator String of 32 Units

Study on pollution flashover characteristics of EHV system have great reference on engineering.In this paper,the relationship between 50% ac flashover voltage U50 of class insulator string of 32 units and ρESDD, ρNSDD was analyzed based on experimental results. It was shown that the relationship between U50 and ρESDD is the same as the one between U50 and ρNSDD, in which the U50 reduces with the increase of ρESDD by a power function, and the influences of ρESDD and ρNSDD on U50 are mutually independent. By using the least square method, the value of exponent of a and b were determined.

Switching Transient Mitigation By Controlled Switching: A Literature Survey

Energy is one of the major inputs for the economic development of any country. This is the reason electrical power system network is expanding day by day to serve the increasing energy demand of the nation. There are many types of equipments in power system.A circuit breaker is protective switching equipment and hence, it plays a very important role in the power system. To maintain the adequate reactive power for voltage control in EHV & UHV system, frequent switching of capacitors and reactors is required through circuit breaker.These are the few applications where switching transients can occur frequently.Apart from this unloaded transformer and transmission line energization also leads to inrush current and over voltages in the system. This condition results into electrical and mechanical stresses and sometimes may lead to equipment failure. Controlled switching has become an economical and technically viable solution to reduce switching transients. This paper presents a literature survey based on switching transients and controlled switching aspects.IEEE transactions,conferences, CIGRE reports and various catalogues of the controllers are referred for the literature survey. This survey is helpful for the researchers working in the area of controlled switching, circuit breaker manufacturers,controller manufactures and power utilities.

Progressive entropy based contingency grouping for deriving decision trees for multiple contingencies

This paper deals with decision tree based power system security assessment for multiple contingencies. In this paper, we propose a contingency grouping technique to produce good, but reduced number of decision trees for multiple contingencies. The grouping is based on the degree of overlap in class boundary regions or post-contingency performance measures of every contingency, as captured by the graphical index progressive entropy. The proposed method has been demonstrated on French EHV system for a voltage collapse problem in west France, Brittany region.

Contingency assessment under uncertainty for voltage collapse and its application in risk based contingency ranking

This paper proposes contingency assessment method that takes into account the nature of probability distribution of power system operating conditions to get realistic severity and risk estimations of contingencies. The uncertainty in operating conditions is captured by a mutually correlated non-parametric multivariate distribution, in addition to traditional standard distributions such as multivariate normal. While capturing these important features of operating conditions, the method also addresses reducing computational requirements by using linear sensitivities and machine learning techniques. The developed contingency assessment methods are applied on SEO region (Système Eléctrique Ouest, West France) of French EHV system to estimate severities and eventually rank the selected contingencies based on risk of voltage collapse.

Efficient Database Generation for Decision Tree Based Power System Security Assessment

Decision tree based planning tools provide operators with the most important system attributes that guide them in deciding as to what situation requires operator action. Key to this approach is the manner in which different operating conditions are sampled to form a database for training. This paper develops an efficient sampling strategy that maximizes database information content while minimizing computing requirements. The approach involves two stages: stage-I to find the high information content region in the multidimensional operating parameter state space and stage-II to bias the sampling towards that region using importance sampling. The proposed approach is applied for deriving operating rules against voltage stability issues on the Brittany region of the French EHV system. The results show that the decision trees produced by the proposed efficient sampling approach have significantly improved classification performance and offer economic benefits compared to conventional sampling strategies, all at greatly reduced computational requirements.

Determination of Flicker Transfer Coefficients Near Large Synchronous Generators

Large disturbing loads such as electric arc furnaces (EAF) create fluctuations in source voltage at the point of common coupling (PCC). These voltage fluctuations then propagate throughout the power system with varying degrees of attenuation. The amount of attenuation is, in general, a function of system impedance and load composition, and can be characterized by what is known as a flicker transfer coefficient. The frequency response of synchronous generators near 60 Hz has a significant effect on flicker propagation, particularly in networked HV or EHV systems. To date, information available in the literature has not adequately described this phenomenon from a theoretical perspective or shown how to take this affect into consideration when computing flicker transfer coefficients. The following paper addresses these issues by: 1) providing a formal description of a synchronous generator model that can be used to determine flicker transfer coefficients when traditional modeling methods may not be appropriate and 2) providing a novel way of calculating flicker transfer coefficients using the described machine model. Synchronized flicker measurements, made at an EAF installation and nearby generating facility in the Southern Company service area, were used to validate the results of the proposed method.

Evaluating station post insulator performance from electric field calculations

This paper illustrates the use of electric field calculations for evaluating the performance of porcelain post insulators in stations for AC system voltages up to 1200 kV. The electric field was calculated using a 3-dimensional commercially available computer package that is based on the Boundary Element Analysis method. A comparison has been made of the effectiveness of several commonly used methods for improving contamination performance, namely, adding leakage distance (taller posts, ribbed sheds and creepage extenders) and coating the insulator with hydrophobic materials. It is shown that for station post insulators used in EHV and UHV systems (greater than 240 kV), coating the insulators with hydrophobic materials or replacing them with composite insulators offers superior protection to flashovers initiated under wet and contaminated conditions than provided by methods that simply increase the leakage distance. For lower voltages (less than 240 kV), all methods considered are effective.

Fuzzy Expert Approach with Curtailed Controllers for Improved Reactive Power Dispatch

This paper addresses the problem of curtailing the number of control actions using fuzzy expert approach for voltage/reactive power dispatch. It presents an approach using fuzzy set theory for reactive power control with the purpose of improving the voltage profile of a power system. To minimize the voltage deviations from pre-desired values of all the load buses, using the sensitivities with respect to reactive power control variables form the basis of the proposed Fuzzy Logic Control (FLC). Control variables considered are switchable VAR compensators, On Load Tap Changing (OLTC) transformers and generator excitations. Voltage deviations and controlling variables are translated into fuzzy set notations to formulate the relation between voltage deviations and controlling ability of controlling devices. The developed fuzzy system is tested on a few simulated practical Indian power systems and modified IEEE-30 bus system. The performance of the fuzzy system is compared with conventional optimization technique and results obtained are encouraging. Results obtained for a modified IEEE-30 bus test system and a 205-node equivalent EHV system a part of Indian southern grid are presented for illustration purposes. The proposed fuzzy-expert technique is found suitable for on-line applications in energy control centre as the solution is obtained fast with significant speedups with few number of controllers.

Optimum Reactive Power Dispatch and Identification of Critical On-load Tap Changing (OLTC) Transformers

The voltage stability problem has become a major concern in planning and operation of today's power system as a result of stressed conditions. It is known that the following disturbances, on-load tap changers (OLTCs), act to restore the load bus voltage and thus restore the load power to near pre-disturbance level. But the operation of OLTCs has a significant influence on voltage stability under some operating conditions. This article discusses the effect of OLTCs with different objectives of reactive power dispatch and identify critical OLTCs based on voltage stability criteria. Results obtained on an equivalent system of 24-node EHV system are presented.