The Phenomenon; Recommendations - GE 269Plus Instruction Manual

METER OPTION
If a meter Communications Failure occurs, it may be necessary to press the RESET key to
remove the message if that alarm is assigned to a latching relay.
NOTE
On commissioning of a synchronous motor protected by a 269Plus and an MPM, correct wiring of the VTs and
CTs is crucial for accurate measurement and protection. Typically, commissioning and testing starts with the
motor unloaded. It is also typical to examine the power factor to verify the wiring and proper operation of the
relays, motor and associated equipment. Under such circumstances, the power factor measured by the MPM
appears to be swinging from a very low lagging value to a very low leading value with the field being constant.
This may mislead you to believe that wiring problems such as reversed CT or VT polarities or wrong connec-
tions exist. More often than not there is nothing wrong with the wiring. In order to understand why the displayed
power factor is swinging from lead to lag, it is important to understand how power factor is determined and why
power factor is not the best indication of proper operation and wiring when the motor is unloaded and the field
3
applied. Recommendations will be made for commissioning and checking for wiring problems.
a) THE PHENOMENON
By convention, an induction motor consumes watts and vars. This is shown in the 269Plus as positive watts
and positive vars. A synchronous motor can consume watts and vars or consume watts and generate vars.
This is shown in the 269Plus as positive watts, positive vars and positive watts, negative vars respectively (see
the diagram below).
Since the motor is unloaded, the real power or kW required to run the machine is at a minimum. The reactive
power or kvar is a function of the field and motor requirement, and is at a high value with the field applied. In
fact the motor will be running extremely overexcited. The apparent power or kVA is the vector sum of both kW
and kvar as seen below, and hence it has a high value with the field applied. The result is a significantly low
power factor with PF = kW / kVA (low value/high value). Because of these unrealistic motor conditions, and
because of digital technology of sampling waveforms, it is possible that the PF sign is detected is either leading
or lagging. This is clearly seen in Figure 3–8: POWER MEASUREMENT CONVENTIONS where around 270°,
the PF is very low and changes signs with the slightest movement around this angle in either direction.
b) RECOMMENDATIONS
By examining Figure 3–8: POWER MEASUREMENT CONVENTIONS on page 3–67, it is very obvious that the
only stable and reliable number that should be checked on commissioning of unloaded synchronous motors
with the field applied is the signed REACTIVE POWER or kvar. Under such circumstances the kvar number
should always be negative with a value significantly larger than that of the real power or kW. Glancing at the
kW number, it should be a very small value with possible fluctuations in the sign from positive to negative. By
examining the apparent power or kVA number, it should always be positive and also relatively large, almost
equal to the kvar number. Consequently, the PF number will be a very small value in the order of 0.02 to 0.2,
also with a possible unstable sign going from leading to lagging.
Once the kvar value is examined and found to be inconsistent with the observations above, it can be safely
assumed that there may be some switchgear wiring problems. However, it is important not to ignore the other
values, since a large kW value (regardless of its sign) is also an indication of wiring problems. Similarly, a large
value for the PF, regardless of its sign, is an indication of wiring problems.
3-
68
Courtesy of NationalSwitchgear.com
kW
kvar
Figure 3–9: POWER FACTOR PHASOR DIAGRAM
269Plus Motor Management Relay
kVA
3 SETUP AND USE
GE Power Management