The Key to Relay Protection Success: Cooperation
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 system, transformers, etc. In most large utilities, there is often a barrier between the relay department and the substation equipment department. The substation equipment along with relays and relay protection schemes are tested at regular intervals by the respective departments.
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.
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 faulted section of the power system. Station batteries, another building block of relay protection, energize the trip circuit with a DC supply.
Figure 1: Building Blocks of Relay Protection Scheme
Understanding how each of these components contributes to proper operation of the relay protection scheme is essential to smooth operation of your protection system.
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, software such as Doble Protection Suite and Doble RTS™ are ideal. Protection Suite provides a comprehensive test environment flexible enough to accommodate your technical and operational requirements, including for networks based on IEC 61850 standards. RTS is a vendor-neutral application helps standardize elements of your relay testing program.
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 protective relay scheme including receivers and transmitters.
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. Doble offers the versatile F6150sv power system simulator, a solution for 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 it 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. Each utility determines the maintenance interval for testing based on its internal procedures.
- CT excitation
- Current ratio
- Ratio and phase angle error
- Insulation resistance
- Winding resistance
- Burden check
An instrument that can help teams increase productivity and save time during commissioning is the EZCT-2000C, designed specifically for CT testing.
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 and hence digitized samples have to be provided. In this case, in addition to testing the CTs, the merging units have to be tested as well.
There are some merging units that have a built 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 need to 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 in order 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.
Figure 4: Circuit Breaker
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, the 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 a 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. The Doble TDR9100 circuit breaker analyzer and the CT-800 S3 digital circuit breaker analyzer provide reliable testing solutions for circuit breakers.
Figure 4 shows a circuit breaker.
Check back soon for our next installment of this blog for details on how batteries, potential transformers/CCVTs and communication links impact relay protection schemes.
- Blog – Changing Objectives for Relay Protection Testing
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