Skip to main content

Digital Overcurrent Relay

Digital Overcurrent Relay

The overcurrent relay is the most commonly used for a type of protection relay protection of distribution systems. Digital relays are extensively used in the power industry operation. They operate when the circuit current exceeds the predetermined value. The microprocessor uses a multiplexer for sensing the fault currents in a number of circuits and sub-circuits. The microprocessor accepts signals in the voltage farms, the CT fault current derived is first converted into proportionate voltage signals, 'and then fed to the rectifier, multiplexer, analog to digital converter, and the microprocessor.

Block diagram of Digital Overcurrent Relay

The output of the rectified voltage signals is fed into the multiplexer. The microprocessor (microcomputer) then sends a command for switching ON the desired channel of the multiplexer in order to obtain the rectified voltage in the particular circuit.

Digital Overcurrent Relay


Since the microprocessor needs digital signals, the output of the multiplexer is fed into the analog to digital converter (A/D). Again, the microprocessor sends a signal to the A/D for starting the conversion and reads the end of the conversion signal to examine whether the conversion is over and compares the signal with the pre-determined pick-up value.

Benefits of Digital Overcurrent Relay 

  1. Lower cost and the ability to provide a full range of characteristics in one product.
  2. The required characteristics can be selected using the relay front panel
  3. Only one current input is required per relay.


Popular posts from this blog

Limitations of Terzaghi Theory

Limitations of Terzaghi Theory The value of the coefficient of consolidation has been assumed to be constant.  The distance d of the drainage path cannot be measured accurately in the field. The thickness of the deposit is generally variable, and an average value has to be estimated.  There is sometimes difficulty 1n locating the drainage face, sometimes thin previous seams that can act as good drainage face are missed in the boring operations. The equation is based on the assumption that the consolidation is one-dimensional. In the field, the consolidation is generally 3-dimensional. The lateral drainage may have a significant effect on the time rate of consolidation. The initial consolidation and secondary consolidation have been neglected. Sometimes these form an important part of the total consolidation. In actual practice, the pressure distribution may be far from linear or uniform. Read More Muller-Breslau principle

Price Guard Wire Method

Price Guard Wire Method Some form of  Price Guard Wire Method  is generally used to eliminate the errors caused by leakage currents over insulation. Fig. 3.14 illustrates the operation of This Method. In fig 3.14(a), a high resistance mounted on a piece of insulating material is measured by the ammeter voltmeter method. The micro-ammeter measures the sum of the current through the resistor (IR) and the current through the leakage path around the resistor. The measured value of resistance computed from the readings indicated on the voltmeter and the microammeter, will not be a true value but will be in error.   Figure 3.14 Application of  guard  circuit for measurement of high resistance In fig, 3.14 (b), the  guard  terminal has been added to the resistance terminal block. The  guard  terminal surrounds the resistance terminal entirely and is connected to the battery side of the micro-ammeter. The leakage current IL now bypasses t

Negative Booster

Negative booster A negative booster is employed to conform to the regulation that the potential difference between any two points of the rail return shall not exceed 7 V. Two boosters, positive and negative, are used which are mechanically coupled together and driven by a DC motor. The positive booster is connected to the trolley wire (near the generating station) and the negative booster (separately excited) is connected to the track rail.  The 'positive booster' adds voltage to the line while the 'negative booster lowers the potential of the point it is connected to. As we go along the trolley wire away from the generating station/sub-station, the potential drop increases, and the voltage of the trolley wire falls. Since the current returns via the track rail points away from the generating station acquire high potentials. This potential is brought down by the negative boost provided by the negative booster. When the load is sufficiently far away from the generating stati