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Mesta's inverter uses a unique pulse-width-modulation (PWM) design that results in a sine wave output
while being able to power non-linear loads without derating. The unit delivers 300% peak transient
overload current (with or without the utility present) for any commonly occurring transient load
conditions (such as load equipment being powered up). The unique current-limit used by Mesta even
allows the UPS output to be short-circuited. Under a short-circuit or other heavy overload condition, the
switch connecting the UPS output to the utility is turned off, and the inverter is current-limited to 300%
of the UPS rated output for a specified period of time. If the overload persists beyond this period of
time, the UPS electronically shuts down until reset by an operator via the front panel keypad/LCD
display. During all such overloads no damage is sustained by the UPS or to any fuses or breakers in
the UPS. The UPS will sustain overloads up to 150% rated load for 1 minute and overloads exceeding
150% rated for .34 seconds (10 and 15 KVA systems) or .17 seconds (20 through 30 KVA).
System Block Diagram
- Figure 8 on the next page is a block diagram demonstrating the basic
operation of the Mesta UPS system.
The heart of the system is the master control, which possesses a 16 bit, 16 MHz microcontroller. It
continually monitors all of the important parameters such as input and output currents and voltages,
temperatures, and input and output requirements. It instantly commands the inverter through the gate
drives to generate a high quality sine wave output to the critical load.
The three phase inverter bridge consists of isolated power IGBT (insulated gate bipolar transistor)
devices capable of operating at a high carrier frequency with minimal switching and "ON" losses. The
voltage interface PC board receives the input, output and battery voltages and properly interfaces them
with the master control.
The keypad and LCD, as well as the remote communication blocks, constitute and provide the means
by which the user can communicate with the UPS. The LCD screen consists of an 8 line by 40
character display, making it possible for various instructions and diagnostic comments. The remote
communications block consists of an isolated RS232 communications protocol for either a voltage or
current loop interface. A number of status relay contacts are also provided.
The gate drives are optically isolated devices with both positive and negative supplies that properly
drive the IGBT power devices on and off as commanded by the master. The LC filter reduces the high
frequency harmonics produced by the inverter to the system transformer, and keeps them to a minimal
component of the output voltage.
Surge suppression circuitry on the AC line input provides for input surge/spike protection as per IEEE
C62.41-1980 (formerly IEEE 587) to protect the UPS and the load from being damaged by high voltage
spikes that may occur on the utility feed. The high speed solid state switch is the block containing three
IGBT modules along with proper protection which, when gated "ON", permits the utility to coexist with
the inverter output.
The three phase system transformer has three isolated windings: the inverter, the AC input and AC
output. Both the inverter and AC input windings are connected as a Delta while the output is connected
in a Wye (Y) configuration.
The Transfer Switch permits the user to completely bypass the UPS. When the switch is in the "On
Bypass" position, the load is driven directly by the utility; thus, the operator can shut the UPS down to
perform maintenance. In order for the Transfer switch to go to or to leave the “On Bypass" position, the
master controller must be consulted by pressing the transfer request button and following the
instructions on the display. In fact, the Transfer Switch can not be turned unless the master control
assures proper phase lock and voltage equalization between the inverter output and the utility.
