Basler DECS-250 - 10 Protection; Voltage Protection; Overexcitation (Volts per Hertz)

9440300990 10-1
DECS-250 Protection
10 • Protection
The DECS-250 offers protection relating to generator voltage, frequency, power, field parameters,
rotating exciter diodes, power input failure, and generator-to-bus synchronism. Configurable protection
elements supplement this protection with additional, user-defined system parameters that have multiple
pickup thresholds per parameter. Most protection functions have two groups of settings labeled Primary
and Secondary. Two setting groups enable independent protection coordination which is selectable in
BESTlogic™Plus.
Voltage Protection
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Voltage
HMI Navigation Path: Settings, Protection, Voltage Protection
Voltage protection includes overexcitation, generator undervoltage, generator overvoltage, and loss of
sensing voltage.
Overexcitation (Volts per Hertz)
Volts per hertz protection is annunciated if the ratio of the per-unit voltage to the per-unit frequency
(volts/hertz) exceeds one of the Volts per Hertz Pickup Level settings for a definite amount of time. If the
Volts per Hertz Pickup level is exceeded, timing will continue until the volts per hertz ratio drops below the
dropout ratio (95%). Volts per hertz protection also guards against other potentially damaging system
conditions such as a change in system voltage and reduced frequency conditions that can exceed the
system’s excitation capability.
Several volts per hertz settings enable the DECS-250 to provide flexible generator and generator step-up
transformer overexcitation protection. An inverse square timing characteristic is provided through the
Inverse Time Pickup Setpoint and Time Dial settings. These settings enable the DECS-250 to
approximate the heating characteristic of the generator and generator step-up transformer during
overexcitation. A linear reset characteristic is provided through the Reset Dial setting. Volts per hertz
protection can be enabled and disabled without altering the pickup and time delay settings.
Two sets of fixed-time, overexcitation pickup settings are available through the Definite Time Pickup #1,
#2 and Definite Time Delay #1, #2 settings.
The following equations represent the trip time and reset time for a constant V/Hz level. Volts per hertz
characteristic curves are illustrated in Figure 10-1 and Figure 10-2.
n
NOMINAL
MEASURED
T
T
HzV
HzV
D
T








−
=
1
/
/
100××=
FST
E
DT
T
RR
Equation 10-1. Trip Time Equation 10-2. Reset Time
Where:
T
T
= time to trip
T
R
= time to reset
D
T
= time dial trip
D
R
= time dial, reset
E
T
= elapsed time
n = curve exponent (0.5, 1, 2)
FST = full scale trip time (T
T
)
E
T
/FST = fraction of total travel toward trip that integration had progressed to. (After a trip, this value
will be equal to 1.)