~ 18 ~
UBZ-304
NOVATEK-ELECTRO
Figure 3.1
–
Principle of protection with definite time delay
Protection with dependent time delay corresponds to the standards IEC 60255-3 and BS 142.
In corresponds to the set-point
“Rated Inom” (
motor rated
current);
T (“Imax delay”
parameter is time constant of the protection
operation) corresponds to time delay of tripping for 10*In.
For very large currents the protection has a feature with
definite time delay:
Figure 3.2
–
Principle of protection with dependent time delay
Appendix
А
prov
ides curves for the time constant of the protection to equal 1 second (“Imax delay” parameter).
When setting the different value of the time constant, the response time of the protection is changed proportional to
the time constant (for example, when “Imax
delay” = 10 seconds, operating time of protection at the same ratio of
currents will increase 10 times).
3.4.3 Ground fault protection:
–
It is enabled when ground-fault current reaches the tripping threshold
(“I earth tresh”
parameter);
–
the motor switches off if the ground-fault current is more than specified for the time
T (“I earth delay”
parameter).
3.4.4 Negative-sequence current protection (imbalance)
Negative-sequence current protection (imbalance) is enabled when a component of the negative sequence is
more than the set-
point (“I2 rev tresh” parameter) and stops the motor when time of this excess is more than
specified value (“I2 rev delay” parameter”).
If the analysis of tripping cause is enabled
(“A
-
s I2 prot”=”On”),
then in case of protection tripping due to
exceeding of negative sequence current not because of line voltages imbalance (in this case the motor problems
are assumed), ARS after tripping will not occur (regardless of the value of “I2 rev protec” parameter
).
The coefficient of negative voltage (current) sequence is characteristic of unbalance of three-phase voltage
(current). Approximately the coefficient of negative voltage sequence is determined by the formula:
K
2Ui
=
2(1)
1(1)
100
i
i
U
U
,
Where:
U
2(1)
i
—
RMS value of negative voltage sequence of fundamental frequency of three-phase voltage system in
i
-
observation, V;
U
l(l)
i
.
—
RMS value of positive voltage sequence of fundamental frequency in
i
- observation, V.
U
2(1)
i
is calculated by the approximate formula:
U
2(1)
i
=0.62(
U
нб
(1)
i
)
–
U
нм
(1)
i
),
where -
U
нб(1)
i
,
U
нм(1)
i
-
maximum and minimum RMS values of the three phase-to-phase voltage of the fundamental
frequency in
i
- observation, V.
The coefficient of negative current sequence
K
2Ii
is calculated similarly.
If currents imbalance is caused not by voltage imbalance, then motor fault is determined. To determine the
cause of currents imbalance it is necessary to calculate the ratio of the coefficient of negative current sequence to
the coefficient of negative voltage sequence (
K
2Ii
/ K
2Ui
). And if the ratio is more than the value of
“A
-
s I2 coef”
parameter, then UBZ considers that the motor has malfunction.
3.4.5 Minimum phase current protection:
–
it is enabled when the currents of all three phases drops lower than the set-point
(“Imin tresh”
parameter) and
stops the motor when this drop time is more than the specified one
(“Imin delay”
parameter);
–
it is not active when the load current is less than 10% In (when decrease of the current is due to motor
shutdown, not due to decrease of its load);
–
It has its own definite time delay of ARS
(“AR time Imin”
parameter).
3.4.6 Delayed start and rotor blocking
The principle of delayed start protection and rotor inter-blocking is given in Fig.3.3.
3.4.6.1
Delayed start
