Relay Coordination: Importance In Power Systems

In all interconnected power systems, relay protection is the primary instrument. These systems isolate fIn all connected power systems, a relay protection device is a primary instrument. These systems separate fault locations with a healthier network. You can detect a fault by monitoring several changes. These are voltage dip, current changes, frequency, temperature, etc. However, only proper relay coordination ensures fast and reliable relay operation.

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What is relay coordination?

When the protective relay trips in a sequence in a power system to protect them, it is called relay coordination. The relay coordination should remove the faulty part with minimal relay and circuit breaker operation.

Here is an example. 

Suppose you work on distribution and transmission networks. It has four substations: A, B, C, and D. These substations have a series connection, and their features help in an operating sequence. Thus, they are coordinated or selective. Substation A is for the generation. On the other hand, substations B, C, and D form a distribution network. Now, if a fault current occurs in substation D, the substation D relay must operate. If an adjacent relay operates, it will shut down the whole system, an avoidable trip. You should provide optimum coordination between all four substations. 

relay cubicle with components and wires

Image: relay cubicle with components and wires

Characteristics of relay Coordination

InIn a transmission network, the protective relay coordination helps to get sensitivity. Besides, it helps in quick fault clearance. Thus, industries strive for an optimal relay setting.


A relay system must operate correctly. Also, it must not work incorrectly. The degree of certainty in these situations is reliability. In short, how sound the protection system is, decides its reliability.


A protection system must isolate the minor portion of the fault condition, making it more selective, limiting service gaps, and ensuring continuous service.

Speed of Operation: 

A protection system must be fast. It should isolate the faulty areas in minimum time. It leads to less thermal stress. Also, it minimizes the short circuit damage to equipment.


A power system protection relay should be simple. You can find and correct faults quickly in a simple system.


A sensitive protection system detects minor fault conditions in no time. Sensitivity is essential in high impedance faults.


An efficient protection system will require a considerable cost. However, the best would be to achieve maximum protection in budget.

The objective of power system protection

Now let’s learn the need for relay protection in power systems. 

Prevent human injury

A relay offers proper protection, and this is important to avoid any injury to humans. Make sure that you configure security relays correctly. Choose circuit breakers and fuses in the correct sizes and functionality.

Limit damage to the equipment

Equipment in any power system is quite expensive, and you must avoid damage to equipment due to relay issues because the cost of repair and replacement is high. A protection system design offers overcurrent protection, and as a result, it minimizes the cost. 

Ensures continuity of service

A brief shutdown of the system can result in huge losses. Luckily, a reliable relay protection system offers continuous service.

Two engineers working on relay protection system

Image: Two engineers working on relay protection system

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What is a Protection Coordination Study?

In this, you study the protection coordination scheme and circuit breaker settings. You also learn the coordination of faulty current protection devices in the electrical systems. Here, you mainly focus on obtaining protection with selectivity, and it includes the determination of fault clearing time. Additionally, you determine the possible operating scenarios that arise due to any changes or additions to the system.

In industrial power systems, the protective relays are mainly in digital form. It is a computer-based system that has software algorithms to detect electrical faults. It helps in the calculation of relay settings. You can also create one-line diagrams with it. You can also call them microprocessor protective relays. Along with protection, they also offer communication and self-test functions. 

Why is it Important to Conduct Protection Coordination Studies?

Electrical systems use protective devices for equipment protection. These devices are fuses or circuit breakers, and the equipment can be cables, transformers, or motors. Generally, when a short circuit occurs in a machine, it affects only that device. However, if a device’s fault affects the entire system, it is an issue.

You can understand this with an example. Suppose a fault condition arises in an electrical load. This electrical load is far from the main distribution panel. However, the fault affects the main breaker and leads to its tripping. As a result, there is a complete blackout. Proper protection in the relay does not affect adjacent equipment. 

On the contrary, the circuit breaker nearest to the fault must trip. Sometimes, these types of defects occur in electrical systems, and it is because of the non-coordination of protective equipment.  Sometimes faulty designs of the equipment also cause these issues. And here, the study of protection coordination helps the professionals to understand the problems in a better way.

With protection study and circuit analysis, you can avoid any useless tripping. So you must conduct a relay coordination study.

  • Firstly, when a new system is in its planning stage
  • Secondly, when you modify an existing system or install new loads
  • Finally, when you replace equipment with a higher-rated one.

Image: Relay protection system

Purpose of the Protection Coordination Study

The protection coordination study has several purposes. These are:

  • Better and reliable system
  • Reduces cost of repairment
  • Better protection of equipment
  • Increased efficiency in operating
  • Prevents downtime
  • Identifies underrated equipment and prevents damage
  • Identifies overload equipment and prevents the breakdown

Steps to Study Protection Equipment Coordination

During coordination study, you analyze the characteristics curves of circuit breakers and fuses. After that, you compare them on a log plot. If there is mis-coordination, overlapping curves indicate them.

Planning and data collection

Before the coordination study, you need to collect some data. Follow these steps for data collection and planning.

  • First, create a single line diagram of the electrical system, and it should have equipment rating details.
  • Collect short circuit data and load flow data. Calculate the highest and lowest value of faulty current in phase and earth fault. These values should be for each relay location.
  • Get data regarding setting ranges and time-current curves. Also, collect data for protection devices characteristics and transformer connections.
  • Note down the voltage setting value of all power transformers. Also, get details about winding connections.
  • Find out switching conditions in ordinary and in an emergency.
  • Collect impedance data of transformers and interconnected networks. Also, find out conductor sizes and their type and installation method.

You can use this database to store device characteristics for future use.

Coordinating time intervals:

When plot coordination curves, you must maintain a time interval between different protective devices, which ensures selectivity. In these intervals, you take into account several factors. For example, relay overtravel, breaker interrupting time, tolerance in relay features.

You may set a closing time interval of 0.4 seconds for inverse electromagnetic relays. For solid-state relays, you may reduce it to 0.3 seconds. 

Check the difference between their allowable I2t. The upstream device should have a higher good I2t than the downstream device.  You can set a closing time interval of 0.2 seconds when coordinating the electromagnetic relay with the fuse. For solid-state relays, you can even reduce this time to 0.1 seconds.

Recent trends:

Technological advancements occur every day. You cannot rely on conventional methods of relay coordination. Thus, efforts are made to add innovation in protective relaying. Earlier, it was impossible to detect a bearing failure through electromagnetic devices. Nowadays, neural network techniques create a current spectrum of bearing frequencies. These frequencies include both conditions, i.e., normal motor state and motor with the load. Changes in frequencies help to detect any failure.

Similarly, you can notice advancements in fault localization systems and high impedance fault detection. Further, there are multi-functional microprocessor relays, and these offer various functions apart from protective ones. Metering data, data logging, and remote communication are some of them.


Without relay coordination, electrical systems can incur heavy losses. Thus, it is essential to coordinate a Without relay coordination; electrical systems can incur heavy losses. Therefore, you should check the coordination of relays correctly. If you need any help, Cloom is here to help you. Contact us with your queries, and our professionals will readily solve them.

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