Why VI / EI IDMT char is used for LT distribution network ?

Power system are very complex network, it includes electrical power generation, transmission, distribution systems and its associated control, protective devices at each level.

Basic of Relay coordination

Relay setting to be calculated such that it shall provides us following advantage,

• Sensitivity : Sense the maximum and minimum fault of the system, It means calculated setting shall be operative for maximum as well as minimum fault of the system.
• Selectivity : Discrimination between faulty section and healthy section of the system. In simple word, protection setting shall isolate faulty section only.
• Stability : Protection remains stable under transient condition of network such as transformer charging, motor starting etc.

As a power system engineer our main objective is to set the relay such that, it will provides sensitivity under minimum as well as maximum short circuit fault, at the same time it shall provides clear cut discrimination between faulty section and healthy section of the system. Also calculated setting are such that it provides the immunity / stability under transient condition of the network (which is unavoidable) like TR charging, capacitor charging, Motor starting etc.

To achieve discrimination between two protective device time grading, current grading, combination of both current & time grading are used.

Following parameters are to be set in relay, which decides the operation of protective device,

• Relay pickup : Set higher than peak loading superimposing starting current of largest motor of network
• TMS : Calculated based on time discrimination required
• Selection of characteristics

Well, selection of relay characteristics is vital parameter during relay coordination, it depends on many factor such as relay location, transient condition of network etc.

The topic of relay coordination is very broad subject and can take considerable time to cover in depth. An important point which is to be taken care, while selecting IDMT char for LT network is described in this article.

So, Why VI/EI char for LT network ? How to select proper IDMT characteristics for LT network ?

VI Characteristics

VI is used where a substantial reduction of fault current. VI char is much steeper char as result operating time can be increase for reduction in fault current.

It means that characteristic take more time for operating for reduction of fault current value.

It can be understood from mathematical formula of the VI i.e.

Fig: Various IDMT Char

This equation states that, operating time of VI is around 1.5 s for 10 time of current than pickup, 13.5 sec for 2 time of current than pickup. Which means that reduction in current VI takes more time to operates, the same can be observed in above figure of IDMT char as well.

EI Characteristics

The operating time of EI characteristics is approximately inversely proportional to the square of current. Operating time of EI is around 0.80 s for 10 time of current than pickup, while it take more time for drastically reduction in current i.e. 26.6 sec for 2 time of current than pickup.

This makes it suitable for the protection of distribution feeder circuits in which the feeder is subjected to peak currents for few second only like starting of largest motor etc. At the same time EI characteristics is also closely match with Fuse characteristics.

Typical industrial LT distribution feeders are protected by relay or circuit breaker with release or fuse, depending on criticality of process, and equipment to be protected.

While selecting IDMT char for LT network following thing to be keep in mind.

• Upstream relay characteristics should well coordinated with downstream system protective device i.e. CB release or Fuse.
• It shall provide immunity / stability under transient condition such as starting of largest motor of LT network.

Condition-1 : Coordination between downstream protective device of network i.e. CB release or Fuse.

Most of LT network are protected by either circuit breaker with release or fuse.

The time required for melting the fuse element depends on the magnitude of current. The time current characteristic curve of fuses follows a I^2t char. It means current increase the time drops by square of current.

t α 1/I^2

Fuse interrupts much faster for low impedance fault which are generally having higher value of fault current, while it takes more time for high impedance fault which are generally having lower amount of fault current value.

Fig: Fuse char with IDMT char

Condition-2 : Stability under starting time of largest motor connected in LT network

This is the most appropriate reason to select VI/EI for LT distribution network.

IDMT char in relay shall be selected such that protection shall be remain stable or remain inoperative during transient condition of LT system such as starting of largest motor at peak loading, at the same time operating time of relay shall be remain as per time discrimination for actual fault in the network.

For the above stated reasons, we can say VI/EI characteristics is most prefer choice, as both char are taking more time for operating during lower value of fault current at the same time it operates faster for higher amount fault current which is similar to fuse char. Also, stability of protection can be achieved during motor starting at peak loading, as result nuisance tripping of protection can be avoided during starting of largest motor at peak loading.

Hence, In order to coordinate with downstream fuse, circuit breaker with release and to provides immunity / stability under transient condition like motor starting at peak loading VI/EI is suitable for protective device of LT network.

Any suggestion on the same are also welcomed.

Reference:

1. NPAG-2011 (Network Protection & Automation Gudie)
2. IEEE-242 - Buff book
3. Power system protection & switchgear by B. Oza