Measurement Computing WavePort 312P rev.1.0 - Manual (Page 67)

PowerVista/312 User’s Manual Cycle-by-Cycle Capture 8-3
This method of display however can be confusing in terms of what data has actually been collected and stored.
When the user sees the last event within the window, or feels data acquisition is complete, that data may not
actually be captured into the database. Remember, immediately after an auto snap, the view will contain a mix
of old, already snapped data, and newly captured data. Thus, it is recommended that when the user determines
that the viewed data in the Result Window is complete, wait for one more auto snap to the database to occur,
and then terminate the acquisition.
Note: When collecting data with Auto Snap to Database, and collecting is thought to be complete, wait for
one additional auto snap to the database to occur to guarantee that all data has truly been archived in
the database.
8.3.2 Power Grid Dynamic Response
One of the major uses for Cycle-by-Cycle acquisition is the capture of staged dynamic events on the power grid.
EasyPower Measure has been used for monitoring both voltage and power to determine dynamic oscillations
as well sudden voltage excursions after switching events. In almost all cases when such analysis is performed,
there is a significant amount of post processing desired. The Copy Delimited feature allows quick access to all
digitized data for import into any application that uses Windows Standard Clipboard functionality.
There is no specialty setup needed for dynamic measurements. Simply use the unit to monitor demand
quantities on any feeder, motor, generator, transformer or transmission line. The only significant concern would
be regarding the digitizing sensitivity. If dynamic voltage excursions on the power grid are very small, then
some preliminary analysis should be performed to guarantee results are worth measuring.
EasyPower Measure can be purchased with either a 12 bit or a 16 bit converter. To illustrate the sensitivity
issue, lets assume a typical substation potential transformer (PT) with a 120 V RMS secondary, and that we are
using a 12 bit converter. This input voltage is 169.7 V peak on the voltage wave. For this condition, a voltage
input range setting of 250 V Peak should be selected in the System Configuration to maximize accuracy.
With a 250 V peak input range, the best sensitivity we can achieve is 250 / ( 2 12 / 2 ) = 0.122 volts, remembering
that our signal is bipolar, and that only half of the digitizing dynamic range capability is available. At 69 kV, on
the other side of the PT, the theoretical sensitivity is (assuming 69kV : 120 V) 70.2 V or 0.1% of 69 kV. Thus,
if the voltage excursions on the 69 kV are 1% or 702 V, we can expect only 10 decimation intervals to represent
the excursion from 0 up to 1% or 702 V.
With a 16 bit converter, sensitivity is of course greatly enhanced. For the above example at 69 kV, the
theoretical sensitivity for a 16 bit converter is 4.4 V or 0.006%. This gives us 160 decimation intervals from 0
to 1% or 702 V.
8.3.3 Static Var Compensator Response
Static Var Compensators (SVC) have historically been used in two major modes, Open Loop Control and
Closed Loop Control. The response of an SVC in closed loop mode is slow, in the .1 to 2 second range, since
the unit is being used to control voltage and possibly stability in a utility network. In open loop mode however,
an SVC is being used to respond within cycles to suddenly changing load conditions. The most common
application of the open loop control mode is on an Arc Furnace. For such an application, the SVC has been
installed for the purpose of controlling voltage, and thus reducing flicker.
For closed loop mode SVC applications, Cycle-by-Cycle acquisition can be used as described in the Power Grid
Dynamic Response application note.
For open loop mode SVC applications, Cycle-by-Cycle acquisition can be used to determine the response of the
SVC for sudden load changes. Of critical importance in these applications is the response time of the SVC
(typically below 3 cycles), and the amount of compensation applied. If the SVC is improperly tuned or has
questionable response, a software modification to the SVC control may be necessary to improve response.