Theory Of Operation; Output Protection - ABB 827E Product Manual

Theory of Operation

When the input voltage is greater than the minimum start-up voltage, the inverters will start-up and
convert DC input to AC output. Inverters are connected in parallel, providing power output that
increases with the addition of each inverter. A total of four fully loaded inverter shelves (24 inverters
total) may be placed in parallel. When the system is operating and within the preset parameters, the
green OK LED on each inverter will be illuminated.
When there is an inverter failure, a red FAIL lamp will illuminate on the front of the inverter and the
system will report a Minor alarm, both on the System Controller and the output relay. If two or more
inverters fail per shelf, the red FAIL lamp will illuminate on all affected inverters and the system will
report a Major alarm, both on the System Controller and the output relay.
The AC output voltage is monitored by the Distribution Monitor. In the event that the AC voltage
climbs to an undesirable range, this module will disconnect the AC output. This module is located
within the Distribution Disconnect Module that houses an AC output breaker. The breaker will trip if
the output current drawn exceeds its rating. The breaker is also provided for the user to manually
disconnect the system output from the load.
When the batteries voltage drops below -42.0Vdc for −48 Vdc input inverter systems (or +21.0Vdc for
+24 Vdc input inverter systems) inverters will turn off and an Inverter MN alarm will occur. When the
plant recovers (DC Input is restored), the Inverter MN, lamp will extinguish, MN Form C alarm will
retire, and the system will restart automatically and provide output. To learn more about any of
these features, refer to the Features Reference section. To learn about adjusting the settings of the
inverter or the system, refer to the Adjusting Setpoints section.

Output Protection

There are several levels of output current limit or output protection.
When gradually increasing the load, the inverter goes into the first protection mode at about 160%
of the nominal load. This protection mode decreases the inverter voltage and limits the output
current.
When short circuit or deep overload is applied to the inverter output, it goes in fast current
protection mode, which means that the output current is limited to 28A. In this mode the output
current is close to a square wave and the output voltage is usually very low.
The inverter stays in this mode for no longer than 5s. Within this time, the load fuse or circuit
breaker protecting the defective circuit should clear. If, after 5s, the short circuit remains, the
inverter will open its output relay.
When there is a system of N inverters, they do not lose synchronization during the short circuit
mode. After the short is cleared, the inverter outputs are synchronized and sharing again.
Figure 21 shows inverter Voltage on Ch2 and Inverter current on Ch1 while a short circuit is applied
with no distribution protection. The Inverter current limits at 28A for 5s, and then opens the output
relay.
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