1.4.1
Synchrophasor Estimation
Presently, synchrophasors are mainly used for wide area monitoring. Increasingly, synchrophasors are being considered for protection applications. Protection applications require shorter response times and lower overshoot/undershoot values during large disturbances. The IEEE Standard C37.118.1-2011 [14] does not specify performance requirements during large amounts of step changes. Currently, there is a lack of synchrophasor estimation algorithm which satisfies all the steady state and dynamic performance criteria of the IEEE Standard C37.118.1-2011 [14] and which is also suitable for the protection applications. The motivation of this work is to present a synchrophasor estimation algorithm which satisfies all the criteria of IEEE Standard C37.118.1-2011 [14] and which is also suitable for protection applications.
1.4.2
Transient Synchrophasor Detection
Grid operators take control actions depending on the outcomes of the synchrophasor applications. Faults and switching operations often cause step changes in voltage and current waveforms. A fault/switching transient synchrophasor is computed over a window of pre and post fault/switching samples. Many synchrophasor applications are not designed [21] to use fault/switching transient synchrophasors as inputs. In these cases, applications give erroneous results [21] if they use fault/switching transient synchrophasors. So, there is a need to detect fault/switching transient synchrophasors. In [21] it is written that some ‘technique should be used to flag an unusable phasor’. The methods of [12][22] can be used for fault/switching transient synchrophasor detection. However, these algorithms have some limitations when applied for fault/switching transient synchrophasor detection. The detection (fault/switching transient synchrophasor) performance of [12] deteriorates in presence of harmonics and decaying DC. Similarly, harmonics and decaying DC components should be removed before applying algorithm [22]. Currently, there is a need of a robust algorithm which can detect fault/switching transient synchrophasors which performs satisfactorily in presence of harmonics and decaying DC.
1.4.3
Synchrophasor Communication
PMUs are often installed in substations which are geographically far away from the control center PDC. Expensive and dedicated communication networks are needed to bring synchrophasors from far locations. Network bandwidth requirement increases linearly with the number of installed PMUs (Figure 1.4 [23]). According to [24], 10 to 50 thousands PMUs will be deployed across North American grid in future. Power utilities need to make very large investments to build/upgrade communication networks to meet the large bandwidth need. Utilities would find it difficult to arrange large investments to meet the increased bandwidth demand. Bandwidth requirement of WAMS also increases with the increasing synchrophasor reporting rates. In WAMS, synchrophasor reporting rate is mainly limited by the available bandwidth [25]. PMU data links with higher reporting rates are economically infeasible for many power utilities. Currently, most of the utilities find it difficult to install PMUs with higher reporting rates. New system monitoring and control applications can only be developed once the synchrophasors are available at higher rates [26].
Figure 1.4: Bandwidth requirement for reporting rate 60 frames/s [23]
In addition, there are other issues with WAMS communication. PMUs may generate bad synchrophasors in many practical situations. Single or multiple missing data often pose challenges to the WAMS applications.
In conventional approach, synchrophasor reporting rate should be equal or greater than the Nyquist rate to avoid aliasing. In C37.118.1-2011 [14], 0.1-5 Hz frequencies have been considered for synchrophasor domain oscillations. Synchrophasors which are reported below 10 frames/s rate are exempted from satisfying the accuracy requirements during system dynamics [14]. Lower synchrophasors reporting rates cannot be used in the grid dynamics monitoring applications due to possible violation of the Nyquist theory.
1.4.4
Using Transient Synchrophasors in Applications
During faults, magnitudes and phase angles of power system parameters go through step changes [21]. As a result, there are some transient state synchrophasors which are computed over windows consisting of pre-fault and fault samples. Currently these transient synchrophasors are discarded and generally not used in the wide area monitoring/control applications [21]. In many situations, the computational window of the synchrophasor estimation filter can be greater than the fault clearing time of the circuit breaker. In this case, there will not be enough fault samples within the measurement window to compute a synchrophasor corresponding to the fault samples only. So, synchrophasor computed over fault samples will not be available for use. As an example, assume, a PMU is reporting synchrophasors at 30 frames/s rate for a transmission line. The fault clearing time is about 3 fundamental cycles. The length of the synchrophasor estimation filter is 14.9 fundamental cycles (as per Table C.1 of [14]). A fault occurs on the transmission line and gets cleared by the breaker within 3 fundamental cycles. In this case, fault samples corresponding to 3 fundamental cycles are only available. However, the estimation filter needs fault samples spanning 14.9 fundamental cycles to compute a synchrophasor corresponding to the fault condition. In this case, fault transient synchrophasors computed over fault transients are only available for use. Synchrophasor applications may get affected in such situations. As an example, fault location on transmission line will be affected when the filter length is greater than the fault clearing time. Accurate fault location helps to speed up restoration and reduce outage time. The existing synchrophasor based fault location algorithms (examples [27][28][29][30][31]) always assume the availability of fault synchrophasors (synchrophasors computed over fault samples only). These fault location algorithms are
not designed to use the fault transient synchrophasors in the fault location applications. Currently, there is a lack of algorithm which enables use of fault transient synchrophasors in the WAMS applications.