Toshiba GR-200 Series Instruction Manual page 40

Inverse time and definite time delay characteristic
2.1.2
The OC element monitors the currents; when one of the currents is beyond the threshold of
operation, the OC element will operate within a period that is defined by the time
characteristic feature. The user can choose the OC operate time delay characteristics, either
dependent (inverse) time characteristic delay (i.e., IDMT) or independent (definite) time
characteristic delay (i.e., DT). The operate time of the IDMT depends upon the level of the
energizing current, and the operate time in the DT is a constant.
For the reset-time-delay in the IDMT, either inverse (dependent)-time-reset or definite
(independent)-time-reset is characterized by setting.
The user can select which characteristic will be applied to each stage of the OC element
by setting a scheme switch. We shall see the scheme switch later.
Operation principle
(i)
Dependent time characteristic (inverse time delay)
The inverse time OC element has an inverse time feature for both operation and
resetting; for resetting, both definite time and dependent time resetting are available.
With inverse time operation, provided that the fault current has the magnitude
required enabling the determination of the location of the fault and provided that the
fluctuation in the impedance of the power source does not affect the correct
determination, an inverse time OC function provides an effective protection. With
regard to the inverse time characteristic, for example, the function can provide a fast
trip for close-in faults, but will enable a delayed trip for remote faults. The characteristic
curve of the inverse time element can be selected from among 10 characteristic curves†
in conformance with IEC, IEEE, US, and UK standards.
†Note: A user-defined 'Original' curve can also be applied in addition to the 10
predefined characteristics provided.
With dependent time resetting selected, if during the resetting period the
energizing current exceeds the threshold current of the OC element, then the OC
element will pick-up. The OC element will reset later in a period determined by the
characteristics of dependent time resetting. In other words, the user can choose the
behavior of the OC element for either faults that are repetitive intermittent or faults
that occur in rapid succession. The characteristic for dependent time resetting is
selectable from five dependent time characteristics in conformance with IEEE and US
standards.
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6F2T0200 (0.15)
GRE200 (1,2)