RFL Electronics GARD 8000, Инструкция по эксплуатации

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Description of Operation
GARD 8000 Distance Relay RFL Electronics Inc.
9.15 BROKEN CONDUCTOR (PHASE UNBALANCE) UNIT (46)
The broken conductor unit detects imbalance between phase currents of the protected line. It compares
the negative sequence current with the positive sequence current, allowing for a more sensitive setting
as the unit operates on the ratio I2/I1. A broken conductor algorithm taking only I2 into account would
require a higher setting to account for higher imbalance at higher loads; operating on the ratio removes
this compromise.
The unit has settings for I2/I1 pick-up, time delay and minimum load current (I1) required. If load is
below the I1 setting, the unit is blocked. This again allows for a more sensitive setting, as imbalance
for very low load conditions can be proportionally larger.
The 46 settings are made on the web page for “Misc. Current Elements.”
The broken conductor unit is inhibited when a breaker pole is open.
9.16 THERMAL IMAGE UNIT
Overhead lines and cables are capable of carrying significant overload for some period of time. While
thermal image units are generally applied to transformers or generators, they do also have their place
for line protection. A simple overcurrent can be set with a time delay but cannot take into account the
effect of load prior to the overload event or repeated periods of overload shorter than the set time
delay. An overloaded line is normally overloaded for a reason (other essential lines have tripped) and a
trip for overload is undesirable if at all unavoidable. In order to provide as much restriction possible for
overload tripping and still protect the line for damaging amount of overload, the GARD 8000 includes
and overload protection with a thermal image. This unit provides both alarm and trip level settings.
The thermal image is selected for the type of object (line/cable) and uses the set heating and cooling
constants in its algorithm.
The thermal image unit is measuring the current and by use of a thermal equation estimates the
temperature of the object. When reaching set threshold, a trip and/or alarm signal is generated.
The algorithm is based on modeling the heating of a resistive element by the current applied. If,
following an overload of relatively short duration, the current drops below 0.5 A, a cooling constant is
applied to ‘reset’ the element.
The thermal image unit does not have a lower ‘start’ level, it is always asserted. The operating time
depends on level of circulating current. The formula used for this calculation is:
dt
d
I
θ
τ θ
⋅ + =
2
where
I is the applied current
τ
is the set time constant
τ
is defined as the time constant that will raise the temperature of the object from the initial
θ
0
to 63%
of the final temperature of
θ
∞
as illustrated in the following figure.
August 1, 2009 9-63 973.334.3100