INVERSE DEFINITE MINIMUM TIME LAG (IDMTL) RELAYS

INVERSE DEFINITE MINIMUM TIME LAG (IDMTL) RELAYS

Historically this type of relay characteristic has been produced using electro-magnetic relays, and many such units still exist in power systems. A metal disc is pivoted so as to be free to rotate between the poles of two electromagnets each energized by the current being monitored. The torque produced by the interaction of fluxes and eddy currents induced in the disc is a function of the current. The disc speed is proportional to the torque. As operating time is inversely proportional to speed, operating time is inversely proportional to a function of current. The disc is free to rotate against the restraining or resetting torque of a control spring. Contacts are attached to the disc spindle and under  pre-set  current  levels  operate  to  trip,  via  the  appropriate  circuitry,  the required circuit breaker. The theoretical characteristic as defined in IEC 60255-3 is based on the formula:

 

where,

 t=theoretical operating time

G=value of applied current

G_b=basic value of current setting 

K and a=constants

With K = 0.14 and a = 0.02 the ‘normal’ inverse curve is obtained as shown in figure. This characteristic is held in the memory of modern microprocessor controlled solid state relays. Electronic comparator circuits are used to measure the source current and initiate tripping depending upon the relay set-tings. In comparison with grading by time settings alone the IDMTL relay characteristic is such that it still allows grading to be achieved with reduced operating times for relays located close to the power source.

 

This type of relay has two possible adjustments:

1. The current setting by means of tap ‘plugs ‘on electromagnetic relays or ‘DIP’ switches on solid state relays for values between 50% and 200% in 25% steps (the plug setting multiplier or PSM) for overcurrent relays and between 10% and 40% or 20–80% in 10% or 20% steps for earth fault relays. The 100% PSM corresponds to the normal current rating of the relay which may be 5, 1 or 0.5 amps to suit the CTs employed. Thus on a 100% tap a 5A relay is stable under power circuit full load conditions with up to 5A flowing in the CT secondary and relay input circuit. From figure it can be seen that the relay will  operate  in  approximately  30s  for  overloads  in  the  primary  circuit  of 1.3x full load or with 5x1.3= 6.5A in the relay input circuit.

2. The operating time at a given current PSM. This is achieved by a continuously adjustable time multiplier torsion head wheel on an electromagnetic relay and potentiometer or DIP switches on solid state relays. The time setting may be varied between 0.05 and 1.0s (the time multiplier setting or TMS).

 

The actual pick-up level is best obtained on site by secondary current injection. Operation of IDMTL relays at currents greater than 20x PSM is not covered by the standards and ideally the protection engineer tries to use CT ratios and relay settings which avoid operation in this region. This is because the capability and characteristic of the CT used to drive the relay under heavy fault conditions may be far from linear. In addition, at the larger values of current the thermal rating of the relay must be considered. Some solid state relays operate to the normal  IEC  60255-3  characteristic  up  to  20xPSM  and  then  follow  a  definite-time characteristic above this current level.