The objective of protective relays and schemes, as we all know, is to protect electrical equipment such as transformers, lines, cables, bus bars, etc. during abnormal system conditions. Hence, protective relaying demands utmost attention and diligence. However, when dealing with relay protection the protection engineers inadvertently focus only on the relays. We talk about selecting appropriate relays, applying correct settings, checking out the wiring, ensuring correct interconnections and performing effective tests.
However, we must realize that relay protection does not exist on an island. In addition to protective relays, substations contain other critical equipment such as circuit breakers, current transformers (CTs), potential transformers (PTs), battery systems, transformers, etc. In most large utilities, there is often a barrier between relay department and substation equipment department. The substation equipment along with relays and relay protection schemes are tested at regular intervals by the respective departments.
Relay protection does not exist on an island. In addition to protective relays, substations contain other critical equipment such as circuit breakers, current transformers (CTs), potential transformers (PTs), battery systems, transformers, etc.
There is little realization regarding the critical dependency of relay protection on CTs, circuit breakers, etc. Hence, we should avoid isolating relay protection from CTs, PTs, breakers and batteries. These are intertwined. Figure 1 shown below illustrates the building blocks of the relay protection system: relays, current and voltage transformers, breakers, batteries and communication links. Failure in any one of these blocks will disrupt the relay protection scheme.
We should avoid isolating relay protection from CTs, PTs, breakers and batteries.
As mentioned above, utilities perform tests on CTs, PTs, the battery system and circuit breakers. However, the results of periodic maintenance testing typically are not shared with relay protection engineers. As an example, protection staff may be unaware of newly developed sluggishness in breaker opening time, or the age of a CT may have led to deterioration of the saturation characteristics level. The team responsible for maintaining relay settings should be kept updated with these test results since access to this knowledge can impact assumptions used to develop relay settings. In fact, the only time we notice an exchange of testing data of CTs, PTs and circuit breakers is in the event of relay mis-operation leading to forensic analysis.
A Holistic Approach to Relay Protection
The relay protection scheme is the nucleus with all necessary logic built in and the other elements support relay protection schemes. CTs and PTs provide current and voltage inputs to the relays. The relays in turn provide input to energize the trip circuit resulting in opening of the breaker to isolate the faulty section of the power system. Station batteries, another builidng block of relay protection, energize the trip circuit with a DC supply.
Understanding how each of these components contributes to proper operation of the relay protection scheme is essential to smooth operation of your protection system.
Figure 1 shows the building blocks of relay protection.
Relays
Engineers must select appropriate relays and provide appropriate settings. The relays/protection schemes need to be tested at the time of commissioning and subsequent maintenance must be performed at predefined intervals.
For relay testing, a software program that provides a comprehensive test environment flexible enough to accommodate technical and operational requirements, including for networks based on IEC 61850 standards is ideal. The right software can help standardize elements of the relay testing program, reduce complexity, provide consistency, and capture and store important maintenance information and test records. This automation enables increased efficiency, productivity, and accuracy.
Any shortcomings in selection, setting, wiring or testing can lead to mis-operation of the relay protection scheme during abnormal system conditions. In some schemes, there are communication links between the relays at the two ends of a transmission line. End-to- end testing must be performed to ensure correct operation of the protective relay scheme including transmitters and receivers.
Any shortcomings in selection, setting, wiring or testing can lead to mis-operation of the relay protection scheme during abnormal system conditions.
Furthermore, digital substations built around the IEC 61850 standard have different testing needs than conventional substations. Testing in digital stations involves extensive verification of communication between the relays. There is a need to test the entire system as an entity, and system simulators can provide a solution for effective testing IEC 61850-based protection devices and schemes.
Current Transformers (CTs)
The role of CTs is much simpler than the relays. The CTs simply provide current to the relays. The primary winding of the CTs sees the actual current in the circuit while its secondary winding provides reduced current levels based on the ratio of the CT. Under normal load current the CTs reproduce currents correctly.
However, during short circuit conditions when fault current is high, the CTs may not reproduce the current accurately. Under such conditions, the current provided to the relay is normally lower than what is expected. The degree of inaccuracy will depend on the magnitude of the short circuit current, X/R ratio, etc. This behavior of the CT is due to saturation of the CT core. CT saturation impacts the performance of simple overcurrent, distance and differential relays.
It is very important to perform the following test on CTs.
- CT excitation
- Current ratio
- Polarity
- Ratio and phase angle error
- Insulation resistance
- Winding resistance
- Burden check
Each utility determines the maintenance interval for testing based on its internal procedures.
Instruments designed specifically for testing CTs can help increase productivity and save time during commissioning, scheduled testing and testing performed as part of forensic analysis. The ideal test set can perform CT excitation, current-ratio, polarity, and phase angle tests, measure insulation and winding resistance of the CT’s secondary windings and measure the CT’s load burden. This can greatly increase effectiveness, productivity, and same time during the commissioning stage.
In digital substations that implement process bus, the analog values of CT outputs are fed into a merging unit. The merging units provide equivalent digitized samples that are fed into IEC 61850-ready intelligent electronic devices (IEDs). The IEDs are essentially microprocessor relays with significant built-in protection and communication logic. The IEC 61850-ready IEDs cannot accept analog signals from CTs and PTs, hence digitized samples must be provided. In this case, in addition to testing the CTs, the merging units must be tested as well.
There are some merging units that have built-in in optical CTs and do not require input from conventional inductive CTs. These merging units require special testing tools and techniques.
Figures 2 and 3 show bushing and standalone CT.
Circuit Breakers
Circuit breakers must trip (open) when called upon by the protective relaying. If the dedicated breaker does not open, back up protection will cause other related breakers to open to isolate the faulty equipment. However, the fault clearance by backup relaying will cause an outage of a larger part of the system; although this is not desirable, it is necessary. For this reason, it is essential for breakers to be maintained and tested appropriately to perform this function.
In addition to a breaker failing to open, we can also have issues if it takes longer than necessary to open the contacts. In protective schemes, breaker opening time is taken into consideration when setting a relay scheme. If the actual contact opening time exceeds the time that was used in relay settings, we can cause unnecessary outage of a larger part of the system.
For example, when we set up a breaker failure scheme, we use the breaker opening time in the calculations. If the actual time turns out to be greater, we will trip several breakers as part of a back-up scheme. This is an undesirable situation leading to the tripping of the entire bus. Similarly, when coordinating two overcurrent relays, we will run into miscoordination if breaker takes much longer to open. Hence, testing breakers is very critical to ensure that they are operational within published parameters including the contact opening times.
To ensure the integrity of circuit breakers we need to run the following tests:
- Breaker timing
- Static contact resistance
- Dynamic contact resistance
- Trip and Close coil currents
- Minimum pickup voltage
- Travel time
- Power factor test
In addition, there are some specific tests conducted on SF6, vacuum and air blast breakers. A state-of-art circuit breaker analyzer can provide efficient and accurate performance measures for all types of circuit breakers. Digital circuit breaker analyzers can perform dynamic resistance tests and measure main contact and resistor contact time, stroke, velocity, over-travel, bounce-back, and contact wipes.
Figure 4 shows a circuit breaker.
Batteries
The battery system plays a vital role in completing the sequence of tripping circuit breakers when called upon by the protective relaying scheme. The trip coil that is responsible for tripping the circuit breakers is energized when DC voltage is provided to it and the protective relays provide the permissive. If relays provide the permissive but no DC power is available, the trip coil will not energize, and the breaker will not trip. Batteries are a crucial link in the chain of events and must be tested at regular intervals to ensure integrity.
Figure 5 shows a battery bank.
To ensure the battery system is operational and will comply within specified parameters, the following tests are recommended:
- Impedance
- Intercell resistance
- Voltage (Battery float and cell float)
- Specific gravity
- Current
- Temperature
- Discharge testing
- Infrared scanning
- Capacity testing
Potential Transformers/CCVTs
The impact of potential transformers both inductive and capacitive on relay performance is important. However, PTs/CCVTs do not undergo the type of duty that CTs undergo and the performance of CTs has a much greater impact than those of PTs/CCVTs. During short circuit faults, the high current imposes extreme pressure on CTs. On the other hand, the voltage during short circuit faults is depressed and hence the PTs do not experience any stress and continue performing as usual.
Figure 4 shows potential transformer.
The tests recommended are:
- Voltage ratio
- Polarity
- Ratio and phase angle error
- Insulation resistance
- Winding resistance
- Burden check
- Power factor Test
These tests to be conducted and related maintenance interval are listed in the maintenance procedures. In general, the maintenance interval for PTs/CCVTs are much longer than those for CTs.
Communication Links
Communication is a critical link in several protection schemes. In a high-speed communication-assisted protection schemes there is communication between the two substations connected by a transmission line. The relaying at each end depends on information received from the other end to operate correctly. The medium used for communication can be fiber optic, power line carrier, microwave, etc. with transmitters and receivers installed at each end. To test such relay schemes utilities, conduct end-to-end testing to ensure the scheme works appropriately and this testing includes both the transmitters and receivers.
In a digital substation designed per IEC 61850 standard, the IEDs communicate with each other via GOOSE (Generic Object-Oriented Substation Event) and Sampled Value (SV) protocols. GOOSE/SV is a Layer 2 protocol, i.e. messages are transported over Ethernet. In a conventional station communication between relays is accomplished via point-to-point wiring. Hence, communication assumes a much bigger role in a digital substation and the relay protection scheme relies heavily on proper communication. Initially, IEC 61850 standard the messaging between IEDs was limited to IEDs within the station. However, now messaging between IEDs located at different stations is implemented via R-GOOSE. Testing such protection schemes involves extensive involvement of debugging the communication network.
Conclusion:
Relay Protection is a multidimensional scheme comprising of relays, breakers, CTs/PTs/CCVTs, batteries, and communication links. Effective relay protection is achieved only if all intertwined components work together and perform within acceptable level of accuracy and performance.
Ed has 45 years of experience in the various disciples of electrical power systems.
He worked at GE in Schenectady, NY for 10 years in the Consulting group performing various types of studies including harmonics, filter design, equipment sizing for power plants, load flow, short circuit and stability studies.
He was also employed by ABB, KEMA and SEL for varying amounts of time.
He has Masters in Electrical Engineering from Texas A&M.
This article was originally published in the September 2026 issue of the Innovations In Insulation Systems magazine.
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