Page 225 - Decision Making Applications in Modern Power Systems

Role of microphasor measurement unit Chapter | 7 189

Nyquist criteria. The filters are designed such that all the analog signals
receive same phase shift and attenuation. This helps in maintaining both con-
stant phase angle as well as magnitude difference between various signals.
The signals are then sampled at a suitable sampling frequency in the A/D
(Md3) converter. The sampling frequency varies from application to applica-
tion. The CT/PT module, low-pass filter module, and the A/D module consti-
tute the data collection module of µPMU.
The CPU module incorporates a microprocessor that estimates voltage
and current phasors from the signals obtained from A/D module. Depending
on the application, apart from voltage and current phasors, several other
parameters can be estimated. Most commonly, frequency and rate of change
of frequency (ROCOF) is estimated for most of the applications. With the
help of the GPS module (Md5), the parameters are time stamped with refer-
ence to universal coordinated time (UTC). The GPS system is shown in
Fig. 7.6. It consists of several satellites arranged in six orbital planes having
an angular difference of 60 degrees. The GPS satellites provide the GPS
time (different from Greenwich Mean Time). The GPS receivers correct the
GPS time and provide UTC time required by µPMU. Hence, the output of a
µPMU essentially consists of time-stamped voltage and current phasor (i.e.,
phase angle and magnitude), frequency, and ROCOF.
The communication interface module (Md6) consists of suitable modems
required to transfer the time-stamped µPMU outputs to higher level in the
power system hierarchy. Communication of data is an important aspect of
µPMU. Two factors are taken into account while considering the communi-
cation channel of µPMU for a particular application—channel capacity and
latency. Channel capacity is the amount of data (in kilobits or megabits) that

Earth

FIGURE 7.6 GPS satellite system.