Type - 2 Coordination

Introduction

Motors are the backbone of any industry, and their use is also rapidly increasing in commercial establishments. Protection of motor, hence, becomes important to keep these processes functioning safely and without any interruption. The main purpose of motor protection system is to prevent excessive temperature built up in the windings because of over current or short-circuit current. Following are the reasons for over-current.

• Overloading
• Single Phasing
• Voltage Imbalance

Studies show that about 40% of the motor failures are due to electrical faults like overload, single phasing & short circuit. Hence it is extremely important to select effective motor protection devices to safeguard motors against any of the above faults, that will make motor windings to exceed safe working temperature. More importantly, the protection devices should be coordinated.

Protection Devices

Thermal overload relay should protect the motor against single phasing and overloading or locked rotor condition. At the same time, it should permit starting of the motor. In other words, it should withstand starting current for a duration equal to the starting time of the motor.

Short circuit protection devices (SCPD) are generally fast in clearing short circuit (S.C.) faults, they do take ?nite time to operate. By the time SCPD interrupts short circuit current, certain amount of fault energy passes through the protected circuit. All the downstream devices and cables in the protected circuit are subjected to stresses corresponding to this energy. Generally, switch disconnector fuse (SDF), moulded case circuit breakers (MCCBs) or motor protection circuit breakers (MPCBs) are used as SCPD in case of motor protection.

Single line diagram (Fig 1) of typical Direct On Line (DOL) motor starter feeder can be referred to for better understanding.

IEC/IS/EN Standards require thermal overload relays and SCPD are coordinated to ensure that they operate satisfactorily under all load and fault conditions. Following two aspects need to be considered to achieve proper co-ordination:

• Discrimination between thermal overload relay and SCPD
• Adequacy of short circuit protection

?So, a question comes in mind - what is coordination?

Co-ordination of Thermal Overload Relay & SCPD

Co-ordination means matching the characteristics of a SCPD and a downstream equipment to ensure that the let-through energy and peak cut-off current do not rise above the levels that the feeder can withstand. Improper co-ordination can lead to:

• High electro-dynamic force
• High thermal stress leads to excessive heat (I2t let-through).
• Nuisance tripping/operation of SCPD under small overloads leading to reduced life of SCPD.
• Nuisance tripping of SCPD during motor starting.
• Nuisance tripping of SCPD during transient conditions like open transition star delta starter starting.

Standards specify motor protection requirements for selection of switching & protection device for motor feeders. Since there are more than one switching & protection device, it is necessary for the devices to co-ordinate among each other for a motor feeder. This is to be done keeping in mind the capabilities of individual devices. Such a coordinated selection will ?rstly ensure safety to the user & secondly, provide the expected performance & life of the feeder devices. Selection of components involves co-ordination of characteristics of various devices i.e. of the overload relay & of short circuit protection device of the motor feeder. As per the standard two types of co-ordination are permissible, Type “1” and Type “2”.

Type “1” co-ordination requires that under short-circuit conditions, the contactor or the starter shall cause no danger to persons or installation. The motor feeder may not be suitable for further service without repair and replacement of the parts.

Type “2” co-ordination requires that under short-circuit conditions, the contactor or the starter shall cause no danger to persons or installation and shall remain suitable for further use. However, contact welding is recognized. Also, the time-current characteristics of the overload protection device should not change.

Thus, in other words Type-2 means safety, low down time and continued protection.

Recommended combination needs to be proven through short-circuit tests at

• Prospective current “r”
• Conditional short-circuit current “q”

Test at Prospective current “r” is done to verify the performance under fault conditions practically possible at the motor feeder end. These faults are normally associated with the motor and the associated feeder. Prospective current “r” is speci?ed according to the rated operational current (Ie AC-3) of the feeder. If the motor feeder is not speci?ed according to utilization category AC-3, the prospective current “r” shall correspond to the highest rated operational current for any utilization category claimed by the manufacturer.

Did You Know: Test at Conditional short-circuit current Iq is carried out to verify the performance under system level faults. Iq is declared by manufacturer. This is the maximum fault current that the feeder can withstand. Generally, the declared value of Iq is 50 kA.

Trends in the motor feeder protection

• Fused protection with S-D-F
• Fuse less protection with MCCB and MPCB

S-D-F, which incorporates H.R.C. fuses, is the most e?cient and popular in the industry. S-D-F, like conventional fuse-switch units, is capable of switching and protecting electrical circuits.

In addition, these are also suitable for isolating downstream equipment. Typical motor starter feeder with SDF and coordination can be understood from Fig 2A and 2B

MCCB earlier was primarily used for protection of distribution circuits. However, with the development of current limiting MCCBs, it has become possible to employ MCCBs in motor feeders also. With the availability of various accessories, MCCB as SCPD offers several advantages. Motor starter feeder with MCCB and coordination can be understood from Fig 3A and 3B.

MPCB can be used in two ways. It can be used for switching of a motor. This is very cost effective. However, it has limited electrical life as compared to that of a contactor. Moreover, a separate undervoltage protection is required. Alternately, MPCB can be used along with a contactor. Since, MPCB combines thermal as well as short circuit protection, it will trip and interrupt even small overloads (which otherwise could be interrupted by a contactor).

Motor starter feeder with MPCB and coordination can be understood from Fig 4A and 4B.

Did You Know: L&T Electrical and Automation offers comprehensive portfolio of MPCBs (rocker and rotary type) up to 63A with Icu up to 100kA.

Summary

Effective motor protection should protect motor and the associated feeder against any overcurrent including short circuit current. More and more users demand Type '2' co-ordination because it helps to ensure a safe working environment. In the view of down times and maintenance costs, though Type '2' co-ordination has higher initial costs, in the long term it proves to be economical. Manufacturer having all the products in its product portfolio is better placed to recommend the combinations for proper Type '2' co-ordination.

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