Induction type directional overcurrent relay

An induction type directional overcurrent relay operates only when the current exceeds its setting and flows in a chosen direction (e.g. towards a fault in the protected line). It has two elements in one case: a directional (power) element and a non-directional induction overcurrent element.

        CT                       PT
        │                        │
   ┌────┴─────────────┐   ┌──────┴──────┐
   │ Directional elem │   │ voltage coil│
   │ (wattmetric disc)│◄──┤             │
   │  current coil    │   └─────────────┘
   │  contacts 1-2 ───┼──┐
   └──────────────────┘  │ (in series with
   ┌──────────────────┐  │  lower coil of O/C)
   │ O/C element      │◄─┘
   │ upper coil (CT)  │
   │ lower coil       │
   │ disc + spring    │──► trip contacts
   └──────────────────┘

Construction

1. Directional element: works like an induction wattmeter. The upper magnet has a voltage coil (fed from the PT); the lower magnet has a current coil (fed from the CT). An aluminium disc or cup lies between them. Torque T ∝ V·I·cos(θ − τ), where τ is the maximum torque angle.
2. Overcurrent element: a wattmeter-type induction disc relay. The upper magnet carries the main current coil (with plug setting tappings) and a secondary winding; the secondary winding feeds the lower magnet coil through the contacts of the directional element.
3. A control spring, a brake (permanent) magnet for damping, and a disc spindle carrying the moving contact; the time is set by moving the backstop (TMS).

Principle of operation

• Current in the non-tripping (reverse) direction: the directional torque acts in the reverse direction, so its contacts stay open. The lower coil of the O/C element is then open-circuited and the O/C disc has no torque, so there is no trip even if the current is high.
• Fault with power flow in the tripping direction: the directional disc closes its contacts within a few cycles; the lower coil gets current and the O/C disc develops torque. If the current exceeds pick-up, the disc rotates and closes the trip contacts after a time decided by the current magnitude and the TMS.

IDMT characteristic and how it is obtained

IDMT (Inverse Definite Minimum Time): the operating time is inversely proportional to current at low multiples of setting, and tends to a definite minimum time at high multiples (PSM about 10–20).

 time│\
     │ \
     │  \__
     │     \____
     │          ‾‾‾‾‾‾‾‾  definite minimum
     └─────────────────── PSM
       2   5   10   20

How it is obtained in the induction relay:

• Driving torque ∝ I², so at low currents the disc speed rises with current and the time falls (inverse part).
• The electromagnet core is designed to saturate at higher currents (about 10× setting). Beyond saturation the flux and torque hardly rise, so the time stays nearly constant: the definite minimum part.
• The brake magnet gives a damping torque ∝ speed, so the disc runs at a steady speed set by the driving torque.
• Standard IDMT (3-s, normal inverse) curve: t = 0.14 × TMS / (PSM^0.02 − 1), which gives about 3 s at PSM 10 with TMS = 1.