Numerical relays

Digital relay or numerical relay is the requirement of the modern-day power industry.

What is a numerical relay?

It is based on digital devices e.g. microprocessors, microcontrollers, digital signal processors, etc.

The numerical relay is a type of relay that acquires the value of the measuring quantities in digital form, analyzes and processes it, checks the system for fault, and makes trip decisions.

The design and operation of both the electromechanical and static relay are different from this type of relay.

The numerical relay monitors the analog signals (current and voltage) through the current transformer and voltage transformer. These signals are sampled and conditioned at particular instants of time and then converted into a numeric form for displaying and recording.

Read more: How does a current transformer make it possible to measure high voltage current?

Then the DAS (data acquisition system) acquires the sample of ac quantities in digital form and processes the data numerically using an algorithm to calculate fault and make a trip decision.

If there is a fault in the protective zone, a trip signal is issued to the circuit breaker.

How does numerical relay work?

At the power system, current and voltage levels are so high. Therefore, the current and voltage levels are reduced by the current and voltage transformer.

The outputs of CT and VT are served to the signal conditioner. Signal conditioner is a crucial part of DAS.

In the signal conditioner, the auxiliary transformer isolates the relay from the power system and reduces the levels of signals to make them suitable for use in the relays.

The I/V converter converts current signals into proportional voltage signals to make them accessible to the A/D converter. Because the A/D converter only accepts only voltage signals.

The low pass filter or anti-aliasing filter prevents the aliasing of the signal.

The outputs of the signal conditioner are applied to the analog interface.

The analog interface sample the reduced level signals and convert their analog levels to equivalent numbers that are stored in memory. Then a relaying algorithm processed the discrete numbers.

The quantities are compared with the predetermined setting to decide whether the power system is experiencing faults or not.

If there is any fault in the power system, the relay sends a trip signal to the circuit breakers for isolating the faulted zone for the power system

Data acquisition system

A data acquisition system or in short DAS is a system that performs data acquisition.

Data acquisition is the process of sampling and conversion of analog quantities into digital numeric values that can be manipulated.

For the numerical relay, the DAS acquires samples of AC quantities and converts them into digital values for further processing.

Below are the few components of a data acquisition system:

Signal conditioner

DAS hardware sometimes has a hard time reading analog signals, so the analog signals must be conditioned to make them compatible.

The signal conditioner converts analog input signals into a form that can be converted to digital numeric form. This makes a signal conditioner an important component of the data acquisition system and numerical relay.


Mechanism of signals, where components of the high-frequency signal appear to be the component of low-frequency signal is known as aliasing.

This creates a problem in the conversion of analog signals to sequences of numbers because it requires a sampling rate that should be selected in such a way that higher frequency components appear as components of interest.

To tackle this problem the analog signals are applied to a low pass filter that removes most of the high-frequency components and then their output is further processed.


Sampling is the process of converting a continuous-time signal into a discrete-time signal.

Analog interface

The analog interface makes the output signal of the signal conditioner compatible for processing.

The output of the signal conditioner is applied to the analog interface, which reduces the level signal and converts their analog levels to equivalent numbers that are stored in memory for processing.

The analog interface consists of a few components:

  1. Sample and hold circuit
  2. Analog to digital converter
  3. Analog multiplexer

Numerical relay types

  1. Over-current/ earth fault relay: Overcurrent relay operates when the current in any circuit exceeds a certain predetermined value.
  2. Differential relay: This relay is based on the current comparison. It trips when the phase difference of 2 or more identical electrical quantities exceeds the specified value.
  3. Directional relay: This relay trips when the fault drives power to flow in opposite to specified direction.
  4. Under/ over voltage relay: This relay trips when the voltage in an electric network drop below or rise above a predetermined value.
  5. Distance relay: This relay measures the impedance or some components of line independence at the relay location.

Why do we need numerical relay protection?

The tremendous growth in size and complexities of the structure of the modern-day power system requires a fast, accurate, and very reliable protection scheme against faults.

To protect major equipment and provide stability to the power system, a reliable, fast, multifunctional numerical protection scheme is used that consists of the numerical relay.

Read more: Modern-day electrical power system

The conventional relays (electromechanical and static relay) are good in their place. These types of relays had no significant drawbacks in their protective function.

But the additional features that are provided by the numerical type relay are what make them effective for the power industry.  

Advantages and disadvantages of digital relay


  • It is compact
  • Flexibility: this relay is programmable. It presents flexibility as compared to electromechanical relay.
  • Multiple functions: one of the biggest advantages of this relay is, it can perform various functions. That includes adaptive logic, self-testing, communication with other relays, etc.
  • Reliable in operations
  • Economical: numerical relay performs many functions at a low cost compared to its other counterparts (electromechanical and static relay).


Digital relay hasn’t achieved perfection yet, so they also suffer from a few disadvantages. Its disadvantages are following:

  • Short life cycle: rapid changes in microprocessors increases functionality but make the previous generation numerical systems out of date.
  • Difficult to test and set: this type of relay can perform many functions, therefore it has countless settings. And the relay is tested by using special testing techniques, so a proper procedure is a must to follow to ensure appropriate settings.
  • Susceptible to transients: numerical-based relay uses microprocessors, therefore they are more susceptible to incorrect operations due to transients.

Quick summary

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