BE1-951
Protection and Control
4-25
Figure 4-13. Volts Per Hertz Element
VOLTAGE PROTECTION
BE1-951 voltage protection includes elements for overexcitation, phase undervoltage, phase overvoltage,
auxiliary over-voltage, and negative sequence Over voltage.
24 Volts per Hertz Overexcitation Protection
Figure 4-13 illustrates the inputs and outputs of the Volts Per Hertz element. Element operation is described
in the following paragraphs.
The volts per hertz element has two outputs: pickup
and trip. When monitored Volts Per Hertz increases
above the pickup setting, the pickup output becomes
true and the element starts timing toward a trip. The trip
output becomes true when the element timer times out.
The Block (BLK) input is used to disable protection. A
BESTlogic expression defines how the BLK input
functions. When this expression is true, the element is
disabled by forcing the outputs to logic 0 and resetting
the timer. This feature functions in a similar way to the torque control contact of an electromechanical relay.
The 24 element is enabled or disabled by the Mode input. Two modes are available, Selecting Mode 0
disables protection. Mode 1 enables the 24 element.
The pickup setting determines the volts per hertz pickup level. The measured volts per hertz is always
calculated as the measured voltage divided by the sensed system frequency. The measured phase depends
on the SG-VTP setting. The 24 function monitors VAB for both 3-wire and 4-wire connections. Thus, setting
is in VPP/Hz for VT connection = 3W, 4W, AB, BC, CA and VPN/Hz for VT connection = AN, BN, CN. For
more information, refer to Section 3,
Input and Output Functions. When the measured volts per hertz rises
above the pickup threshold, the pickup element becomes TRUE and integrating timer starts. If the volts per
hertz remains above the pickup threshold and the integration continues for the required time interval as
defined by the equations below and the set time dial, the trip output becomes TRUE.
If the target is enabled for the 24 element, the target reporting function will record a target when the trip
output is TRUE and the fault recording function trip logic expression is TRUE. See Section 6,
Reporting and
Alarm Functions, Fault Reporting functions for more information about target reporting.
The timer for the 24 element is an integrating timer with a variety of time dials and either an instantaneous
reset or a time delayed integrating reset. Equations
4-4 and 4-5 represent the trip time and reset time for
a constant volts per hertz level.
(
)
2
1
−
=
M
D
T
T
T
100
*
*
FST
E
D
T
T
R
R
=
Equation 4-4. Time to Trip
Equation 4-5. Time To Reset
Where:
T
T
= Time to trip
T
R
= Time to reset
D
T
= Trip Time Dial setting
D
R
= Reset Time Dial
M = Multiples of pickup = (measured V/Hz) divided by (V/Hz PU setting)
ET = Elapsed time
FST = Full scale trip time (T
T
)
ET/FST = Fraction of total travel toward trip that integration had progressed to (after a trip,
this value will be equal to one)
Figure 4-14 shows trip time for various time dials and multiples of pickup.
