Maximum Torque Angle
The word Maximum Torque Angle has its
roots in use of electromagnetic relays. In previous section direction sensing
explained by considering generic devices DA and DC.
Historically these devices were electromagnetic devices; where torque get
developed as a result of tow magnetic fluxes developed by tow electrical
quantities displaced by certain angle. Depending upon the direction of torque
produced direction of power (current) can be decided.
When two magnetic fluxes are derived from
two different electrical quantities the torque produced by these fluxes will
depend upon phasor relationship of these two quantities. Obviously for energy
meter and Watt Meter; for correct functioning of the devices; torque will be
maximum when applied voltage and current are in phase; while for VAR meter for
capacitor it would be maximum if current leads voltage by 900 and
for VAR meter for reactor it would be maximum if current lags voltage by 900.
This angle of applied current with
respect to applied voltage at which maximum torque get produced is called as
Maximum Torque Angle.
However situation explained above
entirely changes in case of a device which can discriminate direction of
current in case of fault (Directional Overcurrent Relay) due to following two
reasons
1) The voltage of faulty phase decreases drastically
2) Angle between voltage and current of faulty phase is nearly 900
Hence
direction decision making in case of fault becomes very difficult. How to
overcome these difficulties depends upon type of fault. There are mainly two
types of fault in electrical system a) Line-Line fault and b) Line-Ground
fault. Direction decision making philosophy and thus requirement of MTA
(Maximum Torque Angle) in case of each type of fault is entirely different and
we will discuss it in next section.
NOTE:
Now a days use of numerical relays become very common; where direction decision
making done numerically; by sampling voltage and currents many times during
each cycle and by using proper algorithm. However use of the word Maximum
Torque Angle carried forward as it is by most of the relay manufacturer. Some
relay manufacturer uses RCA (Relay Characteristic Angle) instead of MTA due to
this technological change.
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