Owner`s manual

4.8.2.1 Static Switch Driver
On the right-hand side panel of the “receiver” cabinet is the “Static Switch” gate driver printed circuit board. This
board serves two functions. One, it has a 120/240VAC, 50/50Hz bias transformer mounted to it which provides the
auxiliary 48VDC for operating the “Controller” and “LCD” “Display”. In addition, the transformer has two low voltage
AC windings, which are used to make two isolated DC voltages, that are used to turn ON the “Static Switch “ SCRs.
This transformer is connected to the “Utility” AC input. Selection of the 120VAC or 240VAC connection is by means
of a jumper plug. The printed circuit board has two small transformers, which are used to measure the AC input
and AC output voltage. Selection of the input voltage range, 120/240VAC, is accomplished by the same jumper plug
as used to select the operating mode for the bias transformer. Each transformer has two secondary windings, one
for each “Controller” of a redundant system.
When the “Static Switch” SCR module is turned ON, a connection between the “Utility” AC input and the inverter
output. Under normal conditions, the SCR module is ON at all times and commanded OFF by the “processor”. Thus
a “fail safe” mode of operation is provided. When the power to the load is from the “inverter” system, the “Controller”
will turn ON an optical coupler on the “Static Switch” gate driver board. When these photo couplers are turned ON,
the gate drive current will be turned OFF, thus the SCR module will disconnect the “Utility” AC input.
Two isolated, low voltage, DC supplies were created, one for each SCR in the dual SCR module. The voltages will
range between 5 to 10 VDC, depending upon the “Utility” input voltage. “P- channel” FETs and resistors are
connected from this DC supplies to the gates of the SCR module. The FETs are connected so that they are ON.
That is, its “source” is connected to the positive supply terminal, the FETs “gate” to the negative supply through a
resistor. The FETs “drain” connects to the SCR gate through a resistor. Photo couplers are connected between the
“source” and “gate” of each FET. When the photo couplers are ON, the FET gates are shorted out, thus the FETs
are OFF. With no gate drive current, the SCR modules are OFF.
4.8.3 Current Sensing
AC current transformers are used to measure the system output current and also to measure “back-feed” current.
They are “Square-D” devices with a 650:1 turns ratio. There are four current transformers in a system that has a
dual “Controller”. These current transformers are located on the wire going to TB1-1 (output terminal) and TB1-2,
(input terminal).
4.8.3.1 Output Sensing
The “burden” resistors for the AC output current transformers are located on the “back plane” printed circuit board.
This back plane is used to interconnect the “processor” PCBs, AC raceway, and “Alarm” relay PCB. A “receiver”
cabinet, which can accommodate six power modules, will have six parallel 14.3 ohm resistors for each AC output
current transformer. A cabinet, which can accommodate five power modules, will have hive parallel resistors. A
system with four power modules will have four parallel resistors, etc. The number of parallel resistors depends upon
the number of power module positions available in the rack, not the number of modules to be installed. A system
which has two “:Controller” boards (redundant mode) will have two sets of burden resistors. Selection of these
resistors will be via a 12 position “DIP” switch, located on the “back plane”. This switch is factory set and should
not be changed by the customer. A one module system, switch position 1 and 12 will be turned ON, for a two module
system, switch positions 1, 2, 11, and 12 will be ON, for a three module system, switch positions 1, 2, 3, 10, 11, and
12 will be ON. All switches will be set ON for a six module system.
4.8.3.2 Back-Feed Current Sensor
Back-feed current transformer is installed on the AC input wires. This device is necessary to determine if the “Static
Switch” is functioning properly. If a SCR in the “Static Switch” should fail in the shorted mode, power from the inverter
could be applied to the AC input terminals. This would present a safety hazard for service personnel working on the
“utility” wiring. CSA and UL require back-feed protection. This is accomplished by the use of an AC current transformer
that will sense any current flowing through the SCR when it is OFF. If current is detected, the inverter will be shut down.
3.5 to 21 kVA N+1 Inverter
Theory of Operationpage 4 — 14