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What is the protection relay?

The protection relay is an intelligent device that protects and controls the electrical network. Companies sell these devices based on the number of protective functions the devices have. The protection relay can be parameterized and controlled from the control room or by a local human machine interface.

The verification of a development project in substation distribution automation
The verification of a development project in substation distribution automation

Types of Protection Relays

There are many types of protection relays. Relays have different functions and they are developed for different purposes. Usually protection relays measure network quantities and operates based on the measured values. It captures events and notifies from alarms.

Typical relay types are the following:

  • Generator protection relay,
  • Motor protection relay,
  • Transformer protection relay,
  • Voltage regulation relay,
  • Overcurrent relay,
  • Earth-fault relay,
  • Distance relay,
  • Line-differential relay and
  • Control relay.

Usually, only some of the functions are in use and others might not be needed. Many protection relays can share the most common protective functions e.g. many intelligent relays may have an overcurrent protection, but are designed into different applications.

The ABB line differential relay has the following protection and supervision functions:

  • Stabilized differential low stage,
  • Instantaneous differential high stage,
  • Over current stage for overload protection,
  • Over current stage for backup protection,
  • Directional and non-directional earth fault protection,
  • Auto-reclosing,
  • Current circuit supervision and
  • Protection communication supervision.

The transformer protection relay is shown in Figure 1. Every relay belonging to the same series has the same look. The protection relays and other intelligent relays are also known as Intelligent Electronic Devices (IEDs). The front of the IED has push buttons to control the IED locally.

Some of the IEDs are designed so that they can be pulled out from their casing easily. All hardware consists of circuit board modules that can be easily pulled out and changed. There are voltage, current, I/O, sensor and communication channels behind the IED depending on the standard configuration.

The standard configuration determines what kind of circuit board modules are required to meet the requirements of the application the IED is designed for.

Transformer protection relay
Figure 1 – Transformer protection relay


SCADA stands for “Supervisory Control And Data Acquisition”. MicroSCADA is a computer software developed by ABB and it is used for the management of the electrical network. It collects data from the IEDs and other intelligent devices e.g. to show in which state some circuit breaker is and which alarms are currently active.

SCADA has several different communication protocols to communicate with the different devices on the electrical network.


SYS600 is used to control and monitor a substation. There are displays which show the single line diagram of the specific substation. The single line diagram includes the switchgear and most of the devices that are connected to the electrical network at the substation. It also shows the most important measurements of the substation.

Usually these devices are disconnectors, circuit breakers, transformers, measuring units, fault indicators etc. Most important measurements are voltages, currents, powers and power factor from the primary and secondary side of the transformer.

It is a task of the system engineer to build the single line diagram and map all the process signals from the devices of the substation. The first task is to build the system configuration. The system configuration defines how the specific device communicates with SCADA. The system engineer has to map all the process signals from the application to SCADA. With this knowledge SCADA understands e.g. that the circuit breaker can be opened and closed by using the specified protocol and command.

The creation of the process display can start once the signals are mapped. An example of process display is shown in Figure 2. Once everything has been done the operator can use these displays to control one or many substation.

Different voltage levels are shown in different colours.

MicroSCADA SYS600 process display
Figure 2 – MicroSCADA SYS600 process display


DMS600 is a distribution management system that allows an operator to see all the electrical lines in the geographical view. SCADA operators have always both DMS600 and SYS600 window open on different screens. They can see from the geographical view which part of the network is functioning and which part of it is faulty.

DMS600 calculates automatically the most probable location of the fault by using the available information of the network.

Operators can monitor the voltage drop across the network. There are different reporting tools that calculate different values of the network. The quality of the network can be analyzed by using these values.

Operators can easily send a maintenance team to fix a specific fault in the network with the help of the geographical view and the available information of the fault location.

This view also helps to locate switches that must be opened before the maintenance can begin.

Title:Testing the project in substation distribution automation – Perttu Eskelä at VAASAN AMMATTIKORKEAKOULU UNIVERSITY OF APPLIED SCIENCES
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Testing the project in substation distribution automation
Testing the project in substation distribution automation

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  1. Kunle Ajayi
    Mar 19, 2023

    I love this portal honestly

  2. Dr B P Singh
    Jul 14, 2020

    Hi, good article n useful

  3. Ruhul Amin
    Jul 13, 2020

    This is helpful for me.

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