A power protection systems acts to help in the essential elimination of impairments from the power system with an aim of minimizing supply losses and resultant damages. The basic requirements for a complete power protection system include the availability of a sufficient number of circuit-breakers and current disconnecting components enough to cater for the probable responsibility occurrences. These circuit breakers and disconnecting components also require a means by which they can be controlled and be connected to eliminate the impaired power conditions.
Problem 1: Hardware
An important part of the protection systems is the relay. Relays open and close the protection systems with any detection of abnormalities or system impairment.
Electromechanical versus microprocessor-based relay
The differences in electrical signal quantities during the normal and impaired conditions enable relays to detect abnormal conditions in the power system. The relay measures the changes in entities such as the phase angle, the frequency, the current and the voltage. Different types of relays exist including, electromechanical and the microprocessor based relays. The electromechanical types of relays have certain differences with their microprocessor-based counterparts in terms of aspects such as; technology standard, operating principle, reliability and speed of response in the event of abnormality. The electromagnetic relays have a relatively higher reliability for function and also offer very low electric resistance (10 mille Ohms) compared to the second generation microprocessor based relays which are also have lower reliability in function and higher resistance. The electromechanical types of relays also have shortcomings compared to the microprocessor-based types. They are much too bulky and consume much space within the power protection systems, cannot carry out self-monitoring compared to the microprocessor based ones. They are slow in response to activity.
Architectural Representation of a Microprocessor Based Relay
A typical microprocessor-based relay is represented by the diagram below:
Standard components of a microprocessor-based relay
The microprocessor based relay consists of various main standard modules that make it distinct and operative. These include the Output Relays, the Microprocessor, the A/D subsystem, the Multiplexer and the communication cards. The isolation transformers, the anti-aliasing filters and the sample circuits are used to connect the line voltage and the current form the PTs and CTs. The multiplexer (labelled MUI) connects the AC inputs to the analog conversion system. The AC signals are instantaneously placed into the microprocessor memory through the AD converter system. The multiplexer applies every digital entity from the voltage values into the microprocessor. It reads information in sequences into the microprocessor. The isolated power sources have a sensing mechanism that can be used to detect the contacts status. The digital output subsystem on the other hand transmits the digit output relay to the power system. The Ram is used to temporarily store the raw data for analysis after a fault occurrence and other future needs. The Erasable RAM (ERAM) stores the relay program logic that ensures the time by time orderly execution of the program.
In the configuration of the hardware of a microprocessor based relay for the protection of distribution lines several parameters are put into consideration. The capability of the relay system to support certain aspects of the distribution line system is carefully examined to come up with an appropriate design for the microprocessor-based relay. The factors include the;
The system load in terms of the total distribution load and the voltage size of the transformer from the station supply. This is to ensure that the microprocessor based relay design can work efficiently with these kinds of values.
The voltage class of the distribution lines.
The transformers size and design
The capacity of the components of the MPBR to support the various requirements are determined and used to design an appropriate model. The sensitivity of the components to the very low voltage impairments is an important aspect to be considered here. The security and reliability of the components to work within the per-determined conditions is important to the design otherwise the relay would be rendered obsolete. The configuration mechanisms and technology considerations are made around several key decisions including; programmable logic, operation modes and selectable operating quantities, programmable time curves, programmable input and output contacts and user-definable display messages.
The MPBR receives both analog and digital input signals that include the electric currents and power system voltages. These input signals are limited using the low pass filters. The data input system processes the input signals through the input relay function to obtain a secondary data that is treated as the output. These output signals include data stored in the Random Access Memory (RAM) and the digital display at the detector and the computer readers. Apart from being read instantly, some of this output signal is stored temporarily for future inference.
One of the most important parts in the processing of primary signals is the AD converter. As the name suggests, the A/D converter receives analog signals form the auxiliary transformers and convert them to levels that are suitable for use within the relay system, that is, the A/D converter quantizes the analog input signals. The complexity of the analog to digital converter system can result into errors due to oversampling that is a result of low rate of conversion. However, these errors can be overcome by the design of high speed and accuracy A/D converters that also consume low power.
The contacts make up the most important constitution of the relay system. Contacts are relay system circuits that initiate or break signal transduction within the relay system. The contact output consists of the signal given when the contacts initiate circuit connection of secondary output signals. The basic functionality of a contact output system consists of a form factors A, B, and C, and poles. The contact output can be either make before break contact or break before make contact. The poles are switching circuits within the relay system and are composed of single, double and four Pole.
Problem 2: Phasors and Filters
Both time and phasor domain software packages are used in the analysis of the stability of power and power protection systems. However, the phasor-domain software is limited to some operations in the alternative transients program (ATP) that can be effectively done using the time-domain software. The time domain is used to predict the behavior of power systems and prevent inconveniences and avoid blackouts. Therefore, the time domain simulation package is used in case of the evaluation of both the stability of the power systems and transients; while the phasor-domain simulation packages can be used only in the stability evaluation, load flow, and short circuits.
Phasors are rotating vectors with a scaled line whose length is a representation of an alternating current quantity. Phasors have both direction and magnitude. Phasors are obtained when the input scaled voltage or a value of rotating vector is frozen at a point, for example 3600. Phasor definition for the window of a power system cycle is important since it helps in the difficulties associated with visualizing the angular variations for two or more forms of sinusoidal wavelengths.
Problem3: Relay Types
Capacitive voltage transformer (CVT) is basic equipment used in the protected system to send information about voltages to the relay. This is necessary to curb impairs in operation of distant protection of transmission lines. A generic CVT is made up of a tuning reactor, a Ferro-resonance suppression circuit, a step-down transformer and a capacitive voltage divider. The attenuation of the current overestimation by a CVT can be done using a bandpass differentiating filters or a mimic filter. In the protection of relay overcurrent function, the input signals are configured with the execution of protection algorithms called a protection pass. The execution of the protection pass occurs at a lesser rate per cycle. The protection pass is therefore an important consideration in the selection of the relay components.
The fast and precise estimation of a power system frequency is important. The estimation of a power systems frequency is applied in the areas of power system monitoring, protection and control. The accurate estimation is important for rapid-response activities like generator protection, control of renewable energy and load shedding. One of the methods of frequency estimation is the phasor based method. However, this method has some adverse impacts on the power system involving the slow convergence rate. This is solved by the application of Recursive least Square algorithm which implies a higher convergence rate. The least square estimation (LSE) method is implemented through the improvement of the channel information receiver samples and corresponding taps. This method has obvious flaws like time to time re-computing and low convergence rates.
The algorithm used in the estimation of frequency is phasor based, that calculates the variations in the phase angles using the transformed consecutive points of the discrete Fourier transform algorithm. This algorithm can be used to find out the exact solution amidst frequency variations and in the presence of harmonic and inter-harmonics. Its precision for a high range of frequency deviation is also remarkable. The curve below represent the frequency estimates for phasor based algorithm and improved phasor based algorithm.
Source:EEPZero crossing present one of the simplest ways of the frequency of the power system. In this method the intervals of time between the successive zero crossings of the signal of a power system is measured. Usually there are complications associated with the zero crossing method including the errors associated with harmonic distortions which are solved through the modification of this method. The modifications include the use of orthogonal, phase-locked loop and Kalman filtering approaches among others. Zero-crossing employs the DFT mechanism in the estimation of frequencies. The methods works based on the differences of amplitude gains of sine and cosine filters in the event of deviations of the normal frequency.
Phase comparators on the other hand help in the reproduction of two output signal of the same amplitude when they are fed by unknown signal and a reference. Either of the output signals is proportional to the sine of the phase angle difference between the reference and the unknown signal, while the other output is proportional to the cosine. The components of the phase comparator include a power divider, a 90 degrees hybrid, two balanced mixers and a phase-matching network.
Problem 4: Relay Types
Directional elements in a power protecting system respond to the over-production of currents for a specific direction of flow which the fault occurs. The directional overcurrent protection system becomes mal-operative whenever the current flows in an opposite direction. The directional elements do not directly measure the electric power but does help in the respond to the direction of flow of the power. When any of the direction components is connected backwards there results errors that compound to either reversed directionality or insufficient quantities of the currents. Such errors that result into faults can be solved through the configuring in the same direc...
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