Electromechanical Relays and Numerical Relays: A Detailed Comparison

Many technologies that were popular a few years back, have started to disappear today. This is only because of the continuous change and innovation in technology. Due to the technological advancements, most of the electromechanical relays are being replaced by numerical relays. Read more as we cover in this blog the basic working of a relay along with a detailed explanation of electromechanical & numerical relays with a thoughtful comparison between them.

Importance of Protective Relaying

Protection is an art to arrange and synchronize the electrical equipment in such a manner that it provides the maximum reliability and dependability. It isolates the faults and abnormalities occurring in the system in such a way that the least part of the power system gets affected. So, to get a good return on the investment, a good relay protection scheme is required to be designed.

The system needs a relay protection scheme due to the following three important reasons:

• The complete system is not disturbed by any undesirable faulty conditions. When the fault occurs, it should interrupt the least affected area while unaffecting the remaining system.
• Due to protection, the system requires less maintenance and repair costs.
• People remain safe during unwanted conditions.

What is a Relay?

Definition: A device which detects the faulty conditions and initiates the circuit breaker operation to isolate the defective element from the rest of the system is known as a relay. A relay can detect many faults such as over current, over and under voltage, over and under frequency, short circuit, earth fault, etc.

Protective relays can be categorized as:

• Electromechanical relays
• Numerical relays

The following block diagram shows how a relay is installed in a distribution network.

As a relay is connected to the power system through a current transformer, so its designing or programming is to be done according to the parameters of the CT rather than the power system itself. The CT has a secondary voltage of 1A or 5A usually, so relays are also rated at either 1A or 5A.

A zone of protection is the portion of the power system which is secured by a particular protection device. For example, the figure below is showing protection zone of a differential relay. If any fault occurs in the region bounded by two CTs of differential scheme, there is significant effect on the CT ratios and the differential current (difference between secondary currents of both CTs) flowing through the relay becomes non zero. So, the relay issues trip signal and thus the equipment within the bounded region of CTs is always protected and any external fault i.e. outside the zone of protection goes undetected by the relay.

Protective Relay Settings and Parameters

Some parameters must be defined or set before the operation of a relay. They are as following:

Pick-up Current

It is the minimum amount of current at which the relay starts to operate. It is the sensing point of a relay. Pick-up current can be calculated as:

I_pick-up = Rated CT output × Current settings

Current Settings

These are used to adjust the pick-up current up to any desired value. It is usually achieved by the use of tapping on the relay coil. For example, the current flowing through a circuit is 80A, and CT connected to the circuit is of 100A/5A, then the current settings of CT will be at 80% so that it may correctly detect the 80A current as normal.

CT Ratio

In some cases, CT ratio is unknown, or its output is rated according to the relay. Then CT ratio can be selected according to the current flowing through the main circuit. For example, rated CT output is 1A and the current flowing through a circuit is 105A then possible standardized CT ratio can be 100A/1A having current settings of 105%.

Plug Setting Multiplier (PSM)

It tells us about the severity of fault and decisions regarding tripping can be made based on PSM. Its minimum value is 1 and can be calculated as:

PSM = (Fault current × 1/CT ratio) / I_pick-up

Time Setting Multiplier (TSM)

It gives the value of actual time of operation for a relay. It is a value that is used to set the tripping speed of the relay. Higher the value means increased time delay and for reducing time delay lesser will be the value of TSM. It is expressed in percentage.

Time of Operation:

It is the total amount of time taken by a system to make a trip. There are predefined graphs available for time vs PSM. For calculated PSM value, its corresponding time is selected from the graph and then the time of operation is evaluated i.e.

Time of Operation = TSM × Time from ‘time vs PSM’ curve

ELECTROMECHANICAL AND NUMERICAL RELAYS

What is Electromechanical Relay?

Definition: Electromechanical relay is a switch that is electromagnetically operated i.e. a small amount of current is used to induce magnetic field inside a magnetic core which is then used to operate a switch for controlling a large amount of current.

Types of Electromechanical Relays

According to application and device structure, electromechanical relays can further be classified as:

General-purpose Relay

This type of relay has mechanical contacts which are operated by a magnetic coil. When the relay coil gets energized by supplying coil terminals, a magnetic field is generated inside the electromagnet which then pulls the armature down with the support of a spring and thus the contacts of supply from the main load is broken. This is how a general-purpose relay works. It can be more easily understood from the figure below:

Troubleshooting of general-purpose relays is simple and they are generally inexpensive. For this reason, they are applicable for commercial and industrial use where cost and handling are high priorities. Most of these relays consist of plug-in features which makes fast replacement and easy troubleshooting.

Heavy-duty Relay

This type of electromechanical relay has convertible contacts (that can be placed either in the position of normally open or normally closed by changing terminal bolts and shifting component by 180^⸰). These contacts are operated by a coil. Concept of convertible contacts can be observed in the figures below:

Heavy-duty relays are used for machine control and other industrial applications. These relays are usually long-lasting and allow easy access for maintenance.

Reed Relay

Such types of relays consist of a switch placed inside a solenoid. Contacts of the switch are enclosed in glass or ceramic duct to protect against corrosion. These contacts are magnetic which are under the impact of the solenoid’s field. Reed contacts have fast switching and they draw a little amount of power from the control circuit however, they require regular maintenance.

Reed relays are used for several radiofrequency and microwave switching applications. They can be used for circuits that are operated at very low currents. Contacts of reed relay are shown in the figure below:

What is Numerical (Microprocessor) Relay?

Definition: Numerical relays are those in which electrical (analog) quantities are sampled and changed into numerical or digital data, that is treated arithmetically and then a tripping decision is issued accordingly.

How does a Numerical Relay work?

As the whole working of this relay is based on microprocessor’s programming, it can also be called a programmable relay. The basic working of numerical relays can be explained by the following block diagram:

Example: SEPAM relay (manufactured by Schneider)

SEPAM is a numerical/microprocessor relay which has the following main functions:

Protection

• Overcurrent and ground fault protection.

• Detection of phase unbalance.

• RMS thermal protection that accounts for external operating temperature and ventilation operating rates.

• Rate of change of frequency protection.

Setting and operating software

The SFT2841 PC software tool gives access to all the SEPAM relay functions, with the convenience of a Windows environment.

Communication Protocols

A protocol is a method used for transmitting information over serial lines between electronic devices. Using the following communication protocols, SEPAM relays can be linked to an administration communication network i.e. S-LAN:

Modbus TCP/IP:

TCP is for Transmission Control Protocol and IP is for Internet Protocol. These two protocols are combinedly used as internet protocol. When information is needed to transfer via these protocols, data is first sent to TCP where additional information is attached and then it is handed over to IP. Data is put into packets by IP and is finally transmitted.

Modbus RTU: 

The serial interface used for communication in this protocol is RS-485 and almost every SCADA, OPC server, HMI, or any other data collection software application supports it. That’s why it is very easy to incorporate Modbus compatible equipment into new monitoring and control applications.

Monitoring System

A software named as ‘StruxureWare’ is used by Schneider for power monitoring. This software enables tracking of real-time power conditions with features such as analysis of power quality and reliability and has a quick response to alarms to avoid critical situations.

Diagnosis

For enhanced operation, three kinds of diagnostic data exist:

• Network and Machine Diagnosis: Trip current, records of disturbances, unbalance ratio.
• Switchgear Diagnosis: Aggregate breaking current, time of operation.
• Diagnosis of the Protection Unit and Additional Modules: Constant self-testing, inspection.

SEPAM relays can cope with equipment from a central remote observing system as all compulsory data is accessible through the communication ports:

Data that can be read: all quantities, settings of protection, alarms, etc.

Data that can be written: breaking remote-control instructions of device etc.

Types of Numerical Relay Based on Protection Scheme

Based on the protection schemes incorporated in power systems, numerical relays are divided into the following categories:

• Over-current / Earth fault relay
• Differential relay
• Directional relay
• Under / Over-voltage relay
• Distance relay

Advantages of Numerical Relays over Electromechanical Relays

Electromechanical relays hold characteristics that are entirely different from numerical relays. They are discussed below:

Size:

Electromechanical relays are larger while numerical relays have a compact size. Electromechanical relays contain a lot of components to perform a function. But numerical relays are based on programming within a microprocessor, so less quantity of hardware is required for it.

Flexibility:

Electromechanical relays are inflexible as a relay can provide only single protection. For any additional type of protection, there is a need to design another relay. While in numerical relays there is the flexibility to provide different types of protection through a single relay i.e. required functions can simply be activated by installing software within the microprocessor.

Reliability:

The reliability of electromechanical relays is very low due to the chances of wear and tear within its mechanical parts. But numerical relays have higher reliability as the likelihood of failure is very small as compared to electromechanical relays.

Measuring Method:

Electrical quantities to be measured are converted into mechanical force, torque, etc. in electromechanical relays. In numerical relays for measuring electrical/analog quantities, they are digitalized by using an analog to digital conversion mechanism and different algorithm techniques.

Storing of Data:

Electromechanical relays have no feature for data storage. In comparison, numerical relays can accumulate a large amount of data like fault records i.e. their types, timings, nature, occurring duration, etc. We can use this data to analyze the performance and reliability of the system.

Parameter Setting:

Plug setting and dial setting is done by mechanical means in electromechanical relays. But in numerical relays, there is a keypad or computer system available for setting values.

Operational Speed:

Electromechanical relays have very low operational speed as mechanical parts take some time to initiate and execute an operation. While numerical relays are very quick to operate because of installed programming which responses instantaneously.

Periodic Service:

Electromechanical relays are needed to be periodically removed, cleaned up, adjusted, etc. Numerical relays require no periodic service, adjustment, etc.

SCADA System:

Electromechanical relays can’t be used for SCADA or substation automation system cause of no programmable feature in relays. Comparatively, numerical relays are compatible to work on substation automation or SCADA systems as these relays are entirely programmable.

Communication:

Electromechanical relays have no kind of communication, but numerical relays are communicable with all standard protocols. Multiple relays can be linked to each other via communication links.

Visual Indication:

Flags, targets, etc. are used for a visual indication in electromechanical relays. While numerical relays use LCD or LEDs for the indication.

Capital & Maintenance Cost:

Electromechanical relays hold high capital and maintenance costs as they contain a lot of components that demand frequent maintenance. Due to a combination of various functions within a single unit, the capital cost of numerical relays is comparatively small and there is low repairing cost because of less maintenance required.

Self-Diagnosis & Auto Resetting:

Electromechanical relays cannot distinguish when a normal condition has been achieved after a fault, so it has no feature of auto-resetting. But numerical relays can detect normal conditions after every fault, hence auto-resetting exists in such relays.

Burden:

Electromechanical relay exhibits high burden over instrument transformers i.e. CT & PT while microprocessor-based/numerical relays have a very low burden on the instrument transformers.

Formerly, electromechanical relays were used everywhere but after that, most of the power systems switched to numerical relays. There are several advantages of using numerical relays over electromechanical relays. Protection systems have been modified and have become more reliable due to the use of numerical relays. More than that, a digitalized system is cost-effective, long lasting, and user friendly. Conclusively, it can be said that numerical relays are advanced enough to compete with modern technology.

Get AllumiaX Protective Relaying Services for Your Facility

Learn more from Power System Experts at AllumiaX. Engineers at AllumiaX are capable of providing high quality protective device coordination study including Protective Relay Settings and Coordination Services. Contact us today!

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  About The Author

  Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.