Ametek California Instruments MX15 Series User Manual page 166

9.9.5 Option–SNK: Current Sink (AC or DC)
9.9.6 General
This section describes the SNK option available for the MX Series Power Source.
9.9.7 General Description
The –SNK or current sink option enables the MX power source to sink current from the unit
under test. This mode of operation is particularly useful when testing grid-tied products that feed
energy back onto the grid. The ability of the MX to simulate the grid provides unique
opportunities to test the EUT for immunity to commonly occurring line anomalies like voltage
and/or frequency fluctuations. Typical examples of these types of EUT's are solar and/or wind
power inverters.
The SNK option requires special amplifiers that have a different control loop from the standard
MX amplifiers. This different control loop ensures greater stability under regenerative load
conditions. Due to this modified characteristic however, the upper frequency limit of an MX
configured with the –SNK option is reduced to 500Hz from the standard 819Hz.
Regenerative Mode of operation The MX will automatically operate in regenerative mode when
the measured current and power is negative, indicating energy is being fed back into the MX
amplifiers. In this mode of operation, the current limit mode will behave differently than it does
under normal load conditions.
When the absolute value of the current exceeds the programmable current limit set point
(current limit is set in the REGENERATE CONTROL screens), the output voltage of the MX will
be increased gradually in an attempt to reduce the amount of current being fed back. Note that
there is no other way for the MX to limit the current as the current is not generated by the MX
itself but rather by the load (inverter). Consequently, normal current limit operation does not
apply in this mode of operation. The voltage will continue to be raised until the user set over
voltage trip point is reached. This trip level can be set in the REGENERATE CONTROL screen
located under the APPLICATIONS SETUP 2 screen. At this point, and after the delay set by the
"DELAY F" parameter is reached, the AC frequency will be shifted by the amount set in the
dFREQ parameter field. The dFREQ is irrelevant for the DC SNC operation. A consideration in
the AC SNC mode is the fact that most AC inverters will shut down when detecting a sudden
change in frequency. If the frequency shift (dFREQ) is set to zero however, the output voltage
will be dropped to the under voltage limit setting (UNDER VOLT) set in the REGENERATE
CONTROL screen instead of the frequency shift. At this point, the EUT should shut down due to
an under voltage condition. Finally, the output replay is opened after the user set delay expires.
This will happen regardless of the EUT state.
For the DC SNC mode the MX must be set to the DC Voltage mode and voltage range that
accepts the maximum desired set voltage. The OL Mode must be set to constant current (CC)
To prepare the MX for regenerative mode, The Regenerative state must be set to ON. The
dFREQ is irrelevant for DC operation.
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