Development of IDMT Relay Curves
Inverse Definite Minimum Time (IDMT) overcurrent relays operate when current exceeds the pick-up value and with an operating time that varies inversely the magnitude of the current. This means that the operating time decreases with increasing current magnitude. However, like instantaneous overcurrent relays, IDMT relay curves have a definite minimum operating time. Hence the name Inverse Definite Minimum Time.
In our previous article in overcurrent coordination, we have discussed the different overcurrent protection devices and their time-current curves (TCC). We presented, in particular, the various types of relay TCCs e.g. instantaneous, definite time, and inverse definite minimum time overcurrent relays. We also presented the concept of Coordination Time Intervals (CTIs) and how they are used in the coordination of overcurrent devices. In this article, we will take a step back and present how the concept of time-current grading otherwise known as coordination for IDMT relays came to where it is now in its application in the power systems industry.
Overcurrent Coordination Methods
Discrimination by Current
Before the concept of IDMT relay curves, instantaneous overcurrent protection was very common. These relays operate instantaneously when the current exceeds the pick-up value and reset with no intentional time delay. Coordination is done based on the fact that short-circuit currents generally are higher in magnitude the closer they are to the source.
By setting the pick-up value of each relay equal to the short-circuit current at the farthest end of its protected zone, ideally, coordination can be achieved.
However, for faults near the zone boundaries, the discrimination of the faulted section can be very difficult. Consider for example the figure below.
At zone boundaries, the short-circuit current magnitudes are very close. With voltage variations, current transformer and relay measurement errors, and DC offset (affects instantaneous elements, why?), upstream relays may overreach to the next zone.
To account for these tolerances, a margin of safety is recommended. A pick-up setting of 120% to 130% of the short-circuit current at the farthest end of the protected zone is usually sufficient in most applications.
With the application of the safety margin, there will be no protection for end zone faults. This is illustrated in the following figure.
Because of this, reliability may be compromised in favor of selectivity.
Another downside of ‘discrimination by current’ is the difficulty of application because of the dynamic nature of power systems, e.g. operational reconfigurations due to line, substation, and/or generating plant maintenance, projects, etc. Source impedances vary constantly affecting the network fault levels.
Consider the example shown in figure 3. Ipu1 is set to 1.2 to 1.3 times the short-circuit current, IF1, for a source impedance, ZS. Now, if we let Z’S > ZS (loss of generation), the fault level at the farthest end of zone 1 will decrease to I’F1. This will result in the underreaching of the protection element as shown in figure 4.
On the other hand, in cases where Z’S < ZS, e.g. parallel operation of transformers, etc., the fault level at the farthest end of zone 1 will increase to I’F1. This will result in the overreaching of the protection element as shown in figure 5.
Discrimination by Time
In order to address the limitations of ‘discrimination by current’, overcurrent protection using definite-time overcurrent relays was introduced. These relays operate when the current exceeds the pick-up value after a set time delay. Take note that the operating time of these relays is independent of the short-circuit current magnitude. The pickup setting is based on the maximum expected load and short-time overloads (in contrast to the maximum short-circuit current set for instantaneous relays). Coordination is done based on configured time delays i.e. the relay on the farthest end has the lowest time delay walking towards the source with progressively higher delay. The relay closest to the source ends up with the highest time delay. Consider for example the figure below.
The good thing about ‘discrimination by time’ is that it is completely independent of the short-circuit current magnitude. Concerns such as voltage variations, current transformer and relay measurement errors, DC offset, and source impedance variations are eliminated. However, the single most detrimental to ‘discrimination by time’ is the time-delay itself in that the relay closest to the source which actually experiences the highest short-circuit current magnitude has the longest fault clearance time. The disadvantage is even more pronounced with multiple devices in series.
Discrimination by Time and Current
The ‘discrimination by time and current’ is usually associated with Inverse Definite Minimum Time (IDMT) relays. These relays operate when the current exceeds the pick-up value and with an operating time that varies inversely the magnitude of the current. This means that the operating time decreases with increasing current magnitude.
The application of IDMT relays is better understood with the use of time current curves (TCC). A TCC plot is a graphical representation of the operating characteristics of overcurrent protection devices at different magnitudes of fault currents. It is a two-dimension plot with the current at the x-axis and the time at the y-axis with both axes are in logarithmic scale.
By plotting the IDMT relay curves, one can estimate the relay operating time for a specific value of short-circuit current. Consider for example the figure below.
It can be seen that at any value of current, relay R3 will always operate first while relay R1 will operate last. Unlike instantaneous overcurrent relays, IDMT relays operate for a wide range of short-circuit currents above pickup and unlike definite time relays, IDMT relays operate in a wide range of time depending on the magnitude of short-circuit current.
IDMT Relay Curves
There are three major types of IDMT relays curves. These are the following:
- Inverse (I)
- Very Inverse (VI)
- Extremely Inverse (EI)
Inverse (I) Type
The operating time of the ‘I’ type IDMT relay curve does not vary much with the current magnitude. This is commonly used in systems where the short-circuit current does not vary much with distance and variations to the source impedance.
Very Inverse (VI) Type
Recommended in systems with a substantial reduction of short-circuit current with distance. The VI type IDMT relay curve has faster operating times for higher currents and slower operating time for lower currents.
Extremely Inverse (EI) Type
The ‘EI’ type IDMT relay curve operates at very fast at high short-circuit currents and very slow at low short-circuit currents. This type of IDMT curve is most suited for coordination with fuses.
Ideally, the same type should be used throughout the system. However, the selection of the IDMT relay curve is often based on preferences or standardization. In other words, the application is more of an art than a science.
References
ETAP Enterprise Solution for Electrical Power Systems Online Help
Blackburn, J. (2014). Protective Relaying Principles and Application, 4th ed. Boca Raton, FL: CRC Press.
G. Pradeep Kumar (2006), Power System Protection, notes on Power System Protection Training, Visayan Electric Company, Cebu City, Philippines.
This is very informative.