Hello, learners. Welcome to the Introduction to Numerical Protection. At the end of this topic, you should be able to explain numerical relays and explain working and hardware architecture of numerical relays. An electrical analog quantity is sampled and transformed to numerical or digital data before issuing a trip decision by numerical relays. Relays are used to prevent electrical networks from being overloaded by unexpected fault currents. Numerical relays are the most popular for their versatility. Multiple parameters can be monitored by a single numerical relay, including current, voltage, frequency, onset time, offset time and more. Multiple faults such as overcurrent, overfluxing, and varied current can all be monitored and analyzed by the same relay. The generalized numerical relay is made up of the following hardware modules and functions: isolation and analog signal scaling, analog anti-aliasing filtering, analog-to-digital conversion, algorithm for phasor estimation, trip logic and the relay algorithm. Let us take a look at some of the numerical relay processes one by one. Let us start with signal conditioning. Signal conditioning is the process of modifying an analog signal to make it more suitable for processing. The following are some examples of how to condition analog signals: analog input isolation and scaling, conversion of current-to-voltage, and filtering. Now let us have a look at analog input isolation. The relay is electrically isolated from the power system by using auxiliary transformers which receive analog signals from the transducers and reduce their levels to make them suitable for use in the relays. MOV is used to protect circuits against excessive transient voltages. Now let us have a look at current-to-voltage converter. A current-to-voltage converter is a circuit that takes current as an input and produces voltage as an output. Current given to the inverting terminal of an operational amplifier based current-to-voltage converter results in the generation of an output voltage as shown in the figure. The anti-aliasing filter is a low-pass filter and is necessary at the input of an analog-to-digital converter, ADC, to remove frequencies above half the sampling rate. The same filter can be used as a reconstruction filter at the output of a digital-to-analog converter, DAC. Now let us have a look at the process of analog-to-digital conversion. The analog interface makes the signal compatible with the processor. The analog interface, which includes sample and hold circuits, analog multiplexers, and analog-to-digital converters receive the signal conditioner's outputs. During the ADC conversion period, a sample and hold circuit is used to acquire samples of the time-varying analog signal and keep the instantaneous sampled values constant. Figure shows a typical digital relay analog input stage in which the multiplexer is controlled by the microprocessor and the S/H amplifiers are all selected into either the sample or hold modes at the same time. Analog-to-digital converters transform the continuous time signal's instantaneous sampled values to corresponding numerical values and output the numbers as binary outputs that represent the analog signal at the sampling points. As a result of the AD converter's quantization, analog electrical signals are represented by discrete values of samples taken at specific times. The signals in the form of discrete numbers are processed using numerical methods by a relaying algorithm. CT's current input to the relay is scaled down from 5 to 1A and transformed into volts. To protect the high-frequency part from mixing with the fundamental frequency element, these scaled signals are filtered by a low-pass filter. The filtered signals are combined together and amplified using an analog multiplexer. The ADC converts digital data from a sample of the multiplexed analog stream. Now let us discuss the numerical relay working and hardware architecture. Numerical relay can be called a small computer because they have both similar hardware architecture with minor variances. Their architecture may appear to be perplexing, but we can categorize as follows: input module, CPU, memory, analog-to-digital converter and multiplexer, output module, communication or digital input module. Let us take a look at each one individually now. We will start with the input module. With the current transformer and potential transformer, high-powered analog signals are stepped down. A low-pass filter is used to send it to the numerical relay. The low-pass filter is used to remove noisy signals in the system that are caused by corona or induction from a neighboring high-voltage line. Let us discuss about the CPU now. The central processing unit, CPU, is the system's brain. It handles all data protection algorithms, as well as digital inputs and filtering. Now let us talk about memory. Memory can be divided into two sections: RAM and ROM. The input data to the relay is stored in random access memory, RAM, which is also where the data is processed during compilation. The storage unit of the relay is read-only memory, ROM. It keeps track of the software required, as well as other information about the incident or disruption. The storage unit is an essential component since it aids in the analysis and troubleshooting of each event that occurs when a fault occurs. Now let us talk about output module. When the CPU issues a trip command, the digital contacts in the output module are activated. Pulses are created as a response signal in these digital contacts. Depending on the relay's application, the reaction time can be adjusted. Now let us talk about communication or digital input module. A relay like a computer includes serial and parallel ports for interfacing with the substation's control and communication systems. To extend the tripping command, add auxiliary relays to the digital output connections. Now let us have a look at the general hardware outline flow of a numerical protection relay. Relaying voltages of 110 volts or 50 volts and currents at five amps or one amp are first passed through isolation transformers. Since analog-to-digital conversion is usually performed on voltages, the current signals are converted to representative voltage signals by passing the current through a known resistance value. All the signals are then filtered using very simple analog anti-aliasing filters. Under microprocessor control, an analog multiplexer is used to pick the appropriate signal into the ADC in a sequential manner. Because the ADC has a fixed set conversion time, usually 25 microseconds, the incoming signal must be held for the duration of the conversion. This is accomplished using the sample and hold amplifier. The signals can now be controlled by the CPU after being transformed by the ADC. It is not uncommon to see many microprocessors employed to execute the relay algorithm. The relaying program will be stored in the read-only memory, ROM, while the sampled amounts and intermediate products in the relaying process will be stored in the random access memory, RAM. The electrically erasable programmable read-only memory stores the relay settings, that is EPROM. A relay algorithm, which is part of the program, processes the collected data. Using signal processing techniques, the algorithm calculates the magnitudes and angles of voltage and current phasors. In some cases, the frequency of the system is also measured. These data are also used to determine other values such as impedances. The digital technique of input quantities is carried out by numerical relays. A communication port on the relay can be used to get at the processed data. Figure 1 shows a configuration for remote communication via a telephone line-star. Figure 2 illustrates a telephone line-ring setup for remote communication. For a star arrangement, the relays can be linked together in an optical loop or with a fiber optic hub. Tx transmit and Rx receive optical ports are provided for the relays through a telephone line-ring arrangement. Other functions of modern numerical relays include monitoring circuit breaker failure, loss of load, and conductor broken trip circuit. Circuit breaker condition monitoring which includes programmable digital outputs and inputs for various logic circuits designed for blocking in general. Let us have a look at bay monitoring and control. By combining numerical relays with other IEDs and communication devices, an integrated system can be created for supervisory management of all bay equipment, such as transformers, bus sections, feeders, lines, and capacitors such as CB fail status. Let us summarize all that you have learned. The numerical relays, working and hardware architecture of numerical relays.
