Overcurrent Protection Devices and their Time Current Curves
Understanding overcurrent protection device characteristics is very important in a protection coordination study. To start our discussion on common overcurrent protection devices, let us review the basic considerations of a coordination study. In our previous discussion on overcurrent protection and coordination study, the following considerations were presented:
- Short Circuit Currents
- Load Flow Currents
- Minimum Operating Criteria
- Delta-Wye Transformers
Overcurrent Protection Devices
Overcurrent protection devices can be categorized into three main types:
- Fuses
- Switching Devices
- Relays
Fuses
Fuses are essentially made up of a metal wire or strip that melts when excessive currents flow through. Being such, fuses operate on a continuous-ampere rating. Low-voltage power fuses can withstand 110% of their rating under controlled conditions. while medium- and high-voltage power fuses can withstand currents below 200% of their nominal rating. Low-level overcurrent takes a long time interval to melt the fuse while large overcurrent levels tend to melt fuses very quickly. A typical fuse time-current curve is shown below.
Fuses operate in a time-current band, between
- minimum melting time – the time when the metal strip starts to melt, and
- maximum clearing time – when the strip completely breaks and the arc fully extinguished.
- The difference between these is referred to as the arcing time.
Overcurrent coordination with fuses is a little tricky, especially for a remote backup fuse. The primary device which can be another fuse should clear the fault before the minimum melting time of the remote backup fuse. In other words, for fuse-to-fuse coordination, the maximum clearing time of the primary fuse (also referred to as the downstream fuse or the protecting fuse) should be lesser than the minimum melting time of the remote backup fuse (also referred to as the upstream fuse or the protected fuse). In most applications, the rating of the upstream fuse is approximately twice that of the downstream fuse.
The following are some of the advantages and disadvantages of fuses:
Advantages
- Simple
- Very fast
- Limits fault energy
- Little or no maintenance
Disadvantages
- Difficult coordination
- Limited sensitivity to earth faults
- Single phasing
- Fixed characteristic
- Need replacing following fault clearance
Switching Devices
Switching devices are another basic category for overcurrent protection devices. Miniature Circuit Breakers (MCBs), Molded Case Circuit Breakers (MCCB), Air Circuit Breakers (ACB) fall into this category and are usually used in low voltage applications.
Like fuses, switching devices detects and clears fault but do not need replacement after every fault clearance. The fault interruption is done using an integrated trip device. The trip action may be done mechanically using spring charge or compressed air to separate the contacts, or using the energy of the fault current to separate the contacts through thermal expansion or magnetic field.
Trip Device
The trip devices for low voltage circuit breakers are the following:
- Thermal Magnetic
- Electro-mechanical or Solid-State
Thermal-Magnetic Trip Device
Low voltage circuit breakers with a thermal-magnetic trip device allow for the discrimination between an overload from a fault. The thermal element acts as protection from overloading while the magnetic element is for protection from faults. This allows slow operation on overload and fast on fault. A typical time-current curve is shown in figure 6. Thermal Magnetic trip devices may be fixed or adjustable based on the ampere rating.
Electro-mechanical or Solid-State Trip Device
Electro-mechanical or Solid-State trip devices are more complex than Thermal Magnetic trip devices in that their trip characteristics can be divided into three sections. These are
- Long-Time Element – allows for protection on overloads.
- Short-Time Element – the intermediate protection between overloading and faults
- Instantaneous Element – allows for protection for faults.
Relays
Relays detect and isolate faults indirectly. Unlike fuses and switching devices, relays require CT and PT input to detect the fault, and a circuit breaker in order to isolate it. Relays have different functions and use currents, voltages, or their combination (impedance) to identify a fault. Basic overcurrent functions such an instantaneous overcurrent (50) and time-overcurrent (51) are usually common.
Instantaneous Overcurrent Relays (50)
These relays operate instantaneously when the current exceeds the pick-up value and reset with no intentional time delay. Most instantaneous overcurrent relays operate on minimum operating time.
Definite Time-Overcurrent Relay
These relays operate when the current exceeds the pick-up value after a set time delay. The time delay settings are adjustable and set following an overcurrent coordination study.
Inverse Definite Minimum Time (IDMT) Overcurrent Relay
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. However, like instantaneous overcurrent relays, IDMT overcurrent relays have a definite minimum operating time. Hence the name Inverse Definite Minimum Time.
ANSI and IEC Standard Curves
There are different characteristic curves available for Inverse Definite Minimum Time (IDMT) overcurrent relays.
ANSI Standard Curves
ANSI standard curves are described by the following general equation
where
Tt is the tripping time
TM is the time multiplier
I is the fault current
Ip is the pick-up current
A, B, p are constants
ANSI standard curves are provided with a disk emulating reset timer described by the following general equation
where
Rt is the reset time
D is a constant
Imin is the minimum operating current
The ANSI standard curve constants are defined in the table below.
| TCC Type | A | B | D | p |
| Extremely Inverse (EI) Very Inverse (VI) Inverse (I) Short Time Inverse (STI) Short Time Extremely Inverse (STEI) Long Time Extremely Inverse (LTEI) Long Time Very Inverse (LTVI) Long Time Inverse (LTI) | 6.407 2.855 0.0086 0.00172 1.281 64.07 28.55 0.086 | 0.025 0.0712 0.0185 0.0037 0.005 0.250 0.712 0.185 | 3.0 1.346 0.46 0.092 0.6 30.0 13.46 4.6 | 2.0 2.0 0.02 0.02 2.0 2.0 2.0 0.02 |
IEC Standard Curves
IEC standard curves are described by the following general equation
where
Tt is the tripping time
TM is the time multiplier
I is the fault current
Ip is the pick-up current
A, p are constants defined in the table below
| TCC Type | A | p |
| Extremely Inverse (EI) Very Inverse (VI) Inverse (I) Long Time Inverse (LTI) | 80 13.5 0.14 120 | 2.0 1.0 0.02 1.0 |
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.
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