Surge Protection; Head Pressure Reference Output; Vfd Start-Up Speed Control; Lead/Lag Control - Carrier AquaEdge 19XRV series Operation, Maintenance And Installation Manual

ACTIVE REGION as found on the SURGPREV screen is
based on how far into the surge prevention area that the load
point has moved. This is used to determine the size of the speed
boost to the VFD.
NOTE: If upon ramp-up, a chiller with VFD tends to go to full
speed before guide vanes open fully, it is an indication that the
lift at low load is excessive, and the operating point moved
directly into the surge prevention region. In this case, investi-
gate the ability of the condenser cooling means (e.g., cooling
tower) to provide cooling water in accordance with the design
load/entering condenser water temperature schedule.
A surge condition occurs when the lift becomes so high the
gas flow across the impeller reverses. This condition can even-
tually cause chiller damage. When enabled, the Surge Preven-
tion Algorithm will adjust either the inlet guide vane (IGV) po-
sition or compressor speed to maintain the compressor at a safe
distance from surge while maintaining machine efficiency. If
the surge condition degrades then the algorithm will move ag-
gressively away from surge. This condition can be identified
when the SURGE/HGBP ACTIVE? on the HEAT_EX display
screen displays a YES.
The PIC III monitors surge, which
Surge Protection —
results in a fluctuation on the compressor motor amperage.
Each time the fluctuation in amperage exceeds an operator-
specified limit (SURGE DELTA % AMPS) plus a load correc-
tion factor, both SURGE COUNTS are incremented by one and
the VFD will increase in speed provided that it is not already
operating at VFD MAXIMUM SPEED or that the VFD TAR-
GET SPEED is forced. If the VFD cannot increase in speed be-
cause the VFD is already at maximum speed of the target speed
is forced then the SURGE PROTECTION COUNTS are also
incremented by one. If more than 4 SURGE PROTECTION
COUNTS occur within an operator-specified time (SURGE
TIME PERIOD) and the ACTUAL VFD SPEED is greater than
90% then the PIC III declares an Excessive Compressor Surge
Alarm (238) and the chiller is shut down. If more than four
SURGE PROTECTION COUNTS occur within the SURGE
TIME PERIOD and the ACTUAL VFD SPEED is less than
90% then the chiller is shut down on a Excessive Compressor
Surge / Low Speed Alarm (236). Both SURGE COUNTS and
SURGE PROTECTION COUNTS are decreased by one if no
surges occur within the SURGE TIME PERIOD.
On chillers with VFDs, if a SURGE COUNT is registered
and the ACTUAL VFD SPEED is less than the VFD MAXI-
MUM SPEED, then TARGET VFD SPEED will be increased
by the amount configured in the VFD INCREASE STEP pa-
rameter. The VFD will not decrease in speed if SURGE
COUNTS is greater than zero.
The threshold at which a current fluctuation is interpreted as
a surge can be adjusted from the OPTIONS screen. The portion
of the surge threshold attributable to current fluctuations can be
changed by scrolling to the SURGE DELTA % AMPS parame-
ter and adjusting it with the INCREASE or DECREASE soft-
keys. The default setting is 10%. The SURGE TIME PERIOD
can be adjusted from the OPTIONS screen. Scroll to the
SURGE TIME PERIOD parameter and use the INCREASE or
DECREASE softkey to adjust the surge count time interval.
The default setting is 8 minutes.
SURGE PROTECTION COUNTS are displayed in the
COMPRESS screen. Both SURGE PROTECTION COUNTS
and SURGE COUNTS are displayed in the SURGPREV
screen.
Head Pressure Reference Output (See Fig. 29) —
The PIC III control outputs a 4 to 20 mA signal for the
configurable Delta P (CONDENSER PRESSURE minus
EVAPORATOR PRESSURE) reference curve shown in Fig. 29.
The DELTA P AT 100% (chiller at maximum load condition
default at 35 psi), DELTA P AT 0% (chiller at minimum load
condition default at 20 psi) and MINIMUM OUTPUT points
are configurable in the EQUIPMENT SERVICE-OPTIONS
table. When configuring this output ensure that minimum re-
quirements for oil pressure and proper condenser FLASC ori-
fice performance are maintained. The 4 to 20 mA output from
VFD TB1 terminals 17 and 18 may be useful as a reference
signal to control a tower bypass valve, tower speed control,
condenser pump speed control, etc. Note that it is up to the site
design engineering agent to integrate this analog output with
any external system device(s) to produce the desired effect.
Carrier does not make any claim that this output is directly us-
able to control any specific piece of equipment (that is, without
further control elements or signal conditioning), although it
may be.
The head pressure reference output will be on whenever the
condenser pump is operating. It may also be manually operated
in CONTROLS TEST. When the head pressure differential is
less than the value entered for DELTA P AT 0%, the output will
be maintained at 4 mA.
DELTA P
AT 100%
NON-ZERO
EXAMPLE OF
MINIMUM
REFERENCE
DELTA P
OUTPUT
DELTA P
AT 0%
0 mA 2 mA 4 mA
(0%)
Fig. 29 — Head Pressure Reference Output
VFD Start-Up Speed Control —
erating to a high VFD speed improves the ability of the com-
pressor to compensate for some start-up environments that ex-
ceed condenser water design conditions. The 19XRV chillers
initially accelerate to the VFD START SPEED as configured in
the SETUP2 screen, and then gradually slow the compressor, if
possible, while adjusting the guide vane position until a stable
operating point with improved chiller efficiency is attained.
Following a start command, the PIC III controls internally
set the TARGET VFD SPEED to the smaller of the VFD MAX-
IMUM SPEED or the VFD START SPEED. Provided that the
chiller has sufficient capacity, the VFD will continue to run at
the start-up speed during Ramp Loading until the chilled water
temperature falls within the CHILLED WATER DEADBAND
surrounding the set point. RAMP LOADING ACTIVE in the
SURGPREV screen will indicate YES during Ramp Loading.
The GUIDE VANE DELTA will be equal to zero when the
chilled water temperature is in the CHILLED WATER DEAD-
BAND. The VFD speed will then be ramped down at one half
of the VFD GAIN rate until, surge conditions are encountered,
the VFD MINIMUM SPEED is reached, the ACTUAL GUIDE
VANE POS reaches the GUIDE VANE TRAVEL LIMIT, or the
TARGET VFD SPEED is forced. VFD RAMPDOWN ACTIVE
in the SURGPREV screen will indicate YES during the ramp-
down process. The VFD speed will be regulated by standard
capacity control and surge prevention algorithms at the conclu-
sion of the rampdown process.
Lead/Lag Control —
matically starts and stops a lag or second chiller in a 2-chiller
water system. A third chiller can be added to the lead/lag
55
20 mA
(100%)
4 T0 20 mA OUTPUT
a19-1658
Immediately accel-
The lead/lag control system auto-