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9440300990 Rev J
Testing
DECS-250
Start and Stop Frequencies
The range of a Swept Sine test signal is determined by Start Frequency and Stop Frequency settings.
Frequency Step
The frequency of a Swept Sine test signal is incremented according to the sweep type used. For linear
sweeps, the test signal frequency is incremented by “step” every half-cycle of the system frequency. For
logarithmic sweeps, the test signal frequency is multiplied by 1.0 + step every half-cycle of the system
frequency.
Step Response Analysis
A standard technique for verifying overall system response is through step response measurements. This
involves exciting the local electromechanical oscillation modes through a fixed step change in the AVR
reference. Damping and frequency of oscillation can be measured directly from recordings of generator
speed and power for different operating conditions and settings. Normally this test is performed with
variations of the following:
•
Generator active and reactive power loading
•
Stabilizer gain
•
System configuration (e.g., lines out of service)
•
Stabilizer parameters (e.g., phase lead, frequency compensation)
As the stabilizer gain is increased, the damping should increase continuously while the natural frequency
of oscillation should remain relatively constant. Large changes in the frequency of oscillation, a lack of
improvement in damping, or the emergence of new modes of oscillation are all indications of problems
with the selected settings.
Step response testing is performed using the Step Response Analysis screen. This screen (
Figure 161
) is
accessed by clicking the Step Response button in the RTM Analysis window. The Step Response
Analysis screen consists of:
•
Metering fields: generator VA, total vars and PF, field voltage, and field current
•
An alarms window that displays any active alarms triggered by a step change
•
Control buttons to start and stop step response analysis and a button to close the screen
•
A checkbox to select triggering of a data record when a setpoint step change is performed
•
Tabs for controlling the application of step changes to the AVR, FCR, FVR, var, and PF setpoints.
Tab functions are described in the following paragraphs.
Note
If logging is in progress, another log cannot be triggered.
Response characteristics displayed on the Step Response Analysis screen are
not automatically updated when the DECS-250 operating mode is switched
externally. The screen must be manually updated by exiting and then
reopening the screen.
AVR, FCR, and FVR Tabs
The AVR, FCR, and FVR tabs are similar in their controls that enable the application of step changes to
their respective setpoints. AVR tab controls are illustrated in Figure 161. AVR, FCR, and FVR tab controls
operate as follows.
Step changes that increase or decrease the setpoint are applied by clicking the increment (up arrow) or
decrement (down arrow) button. Step-change setting fields (one for increase and one for decrease)
establish the percent change in the setpoint that occurs when the increment or decrement button is
clicked. A read-only setpoint field indicates the current setpoint and what the setpoint will be when a step
change occurs. A button is provided to return the setpoint to its original value. This original value is the
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