In this issue of The Synchrophasor Report, Anaisha Jaykumar, SEL associate product engineer, describes PMU capabilities of the SEL-735 Power Quality and Revenue Meter.
Emerging technologies will always open the door for new and exciting ways to solve problems. SEL recently added phasor measurement unit (PMU) capabilities to the SEL-735, allowing customers to collect near-real-time synchrophasor measurements from every SEL-735 Meter across the power system.
When used as a PMU, the meter provides the following synchrophasor data: frequency, phase angle, positive sequence measurements, four user-defined analog quantities, and 14 digital status bits. Synchrophasors are filtered at the fundamental frequency (nominally 50 or 60 Hz) to remove the influence of harmonics and distortion. The SEL-735 publishes and transmits data in the protocol defined by IEEE C37.118.2, which allows PMU concentrators to collect data efficiently from several PMUs.
Instrument transformers and secondary wiring may introduce phase-shift error in the measured signal. Compensation settings in the meter allow users to correct for this steady-state error. In a system with synchrophasors from different types of devices, some may be connected to the same CT and PT. If devices show a slightly shifted phase angle, these settings allow measurements to match a common reference PMU.
Fig. 1. Waveform at meter terminals may have a phase shift.
The following table includes a summary of the SEL-735 Meter’s synchrophasor performance class in compliance with the IEEE C37.118-2011 standard.
|Synchrophasor Performance Class Summary|
|Standard Compliance||IEEE C37.118-2011|
1 to 60 messages/second for 60 Hz systems
|Phasor Quantities||Phase and positive-sequence components|
|Digital Status Bits||16|
|Optional Analogs||Four user-defined quantities, including rms values|
|Server Outputs||1–2 sessions at 60 (or 50) messages/second|
When compared to IEEE C37.118-2005, the 2011 version has several differences, some of which are outlined here.
Performance Classes: IEEE C37.118.1-2011 introduced two performance classes—P class and M class. The SEL-735 provides P class synchrophasors, making it best-suited for applications requiring a faster response and less filtering.
Specified Latency: Latency is defined as the time interval from when an event occurs on the power system to the time that it is reported in data. The 2005 standard did not have any latency requirements. The SEL-735 meets the latency requirements for IEEE C37.118-2011 P class synchrophasors.
Dynamic Performance: IEEE C37.118.1-2011 introduced dynamic performance requirements, while the 2005 standard only specified performance when the power system was in a steady state. The details of this difference and the impact on installed units is beyond the scope of this article. The SEL-735 complies with all the P class performance requirements (dynamic and steady-state) of the standard.
Rate of Change of Frequency (ROCOF): IEEE C37.118.1-2011 introduced requirements on the responsiveness of ROCOF beyond what the 2005 standard required. The effect of this change is to make ROCOF more sensitive to noise, so care should be taken before applying the ROCOF value to a control scheme.
For feeder monitoring applications, the SEL-735 can be integrated with an SEL RTAC. Use the RTAC to gather synchrophasor data from the SEL-735 and process the data with custom logic. One application using this topology allows the RTAC to use the high-speed real-time stream of data, along with other measurements, to detect a falling conductor. The system then sends the trip command to the breaker to de-energize the conductor before it comes in contact with the ground.1
As new challenges arise in the changing arena of distribution monitoring and control, the SEL-735 perseveres as an extremely flexible and capable power quality and revenue meter solution.
1Application patented by San Diego Gas & Electric Company