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Got Latency?

One common concern with real-time synchrophasor systems is “how real-time is the data?” Or stated another way: “What is the latency between the point of measurement (at the phasor measurement unit) and the point of application?” In this issue of The Synchrophasor Report, we will look at the sources of latency and some new tools that can be used to measure latency. Knowing the latency at various points in the system is useful for a wide variety of system applications, such as configuring and commissioning the system. Also, understanding the communications system latency over time can provide a deeper understanding of how the system operates during periods of heavy and light communications traffic.

System Architecture

In a typical synchrophasor system, there may be tens, hundreds, or even more than a thousand phasor measurement units (PMUs) spread across distribution feeders, generation sites, transmission substations, etc. Synchrophasor systems are often called wide-area measurement systems because the PMUs may geographically span thousands of miles. Systems generally use a hierarchical architecture: PMUs send data to local PDCs, which in turn send data to centralized PDCs located in utility control centers. Utilities often send data on to neighboring utilities and/or to independent system operators (ISOs) to provide measurement information for an entire regional interconnection. With the system architecture shown in Fig. 1, PMU data will travel over many communications links, through many communications devices, and through many PDCs before arriving at the final destination.

Typical synchrophasor system architecture.
Fig. 1. Typical synchrophasor system architecture.

Sources Of Latency

As a PMU sends data from the point of measurement to the point of use, latency is introduced along the path. Sources of this latency include the PMU itself, PDCs, data encryption devices, communications devices, and communications links.

PMU

The time latency through a PMU occurs as a result of sampling, filtering, and processing of synchrophasor data. For an SEL PMU, the latency is typically 15 to 55 ms, depending on the synchrophasor message rate and filtering used.

PMU to PDC Communications

Depending on the location of the PMU, the communications link between the PMU and PDC can be a wired connection, a serial or Ethernet cable, or a wireless link (such as serial or Ethernet radio). The latency across the communications link is directly proportional to the length of a wire or the distance between radios. In general, latency of a wired communications link tends to be less than that of wireless links, although the SEL-3031 Serial Radio Transceiver adds as little as ~5 ms of latency. In our experience, wired communications between the PMU and the PDC can vary from a ~1 ms for a short serial cable to >100 ms for some wireless Ethernet radios. In some remote locations, PMU data have been sent via microwave or even satellite radios, which can add significant latency (>500 ms).

Communications Devices/Communications Format Transitions

In moving data from the substation to a centralized control center, PMU data may transition across different communications interfaces. Data may start out as serial or Ethernet and be multiplexed onto a T1, SONET, or MPLS network for transport. Additionally, data are often encrypted and go through security gateways or firewalls that help securely transport data to the desired locations. Latency across these communications devices can vary greatly depending on the number of devices, distance traveled, and type of security devices used. In systems that SEL has commissioned, we have seen latency that ranges from as low as a few milliseconds to greater than 100 ms.

PDC

The latency time through a PDC is essentially the processing time measured from the point when all of the PMU input data have arrived at the PDC to the time when the PDC outputs the concentrated data. PDC latency will vary depending on whether the PDC is software- or hardware-based. Software-based PDCs, like the SEL-5073 synchroWAVe PDC Software, operate on Microsoft Windows operating systems (OS). Since Windows is a non-real-time OS, latency can vary from approximately 10 ms to more than 100 ms depending on the burden of the CPU and other software applications running on the computer. Hardware-based PDCs, such as the SEL-3373 Station PDC, add less latency because they use a real-time operating system. The SEL-3373 Station PDC can add less than 10 ms of latency.

Measuring Latency

SEL has added two new latency measurement calculations to the SEL-3373 Station PDC and the SEL-5073 synchroWAVe PDC to help set up and maintain synchrophasor systems. The network latency measurement calculation measures the time difference between the PMU time stamp and when the PDC receives the PMU data. The interpacket latency measurement calculation measures the time between received data packets. Both measurement calculations provide instantaneous, maximum, and average measurement values and are very easy to configure, as shown in Fig. 2.

Configuring latency calculations using the PDC Assistant for the SEL-3373 or SEL-5073 PDC.
Fig. 2. Configuring latency calculations using the PDC Assistant for the SEL-3373 or SEL-5073 PDC.

All latency values are treated as analog quantities and, therefore, can be archived and sent in the IEEE C37.118 data frame to applications such as the SEL-5078-2 synchroWAVe Central Visualization and Analysis Software. Latency measurements can be made at each SEL-3373 or SEL-5073 PDC throughout the system, allowing the user to identify where latency is being introduced. With the trended displays, users can see the current latency as well as how the latency varies over time. While the displays shown in Figs. 3 and 4 are only over a few minutes, the trended displays can be used to see latency over 24 hours, an entire week, or longer time period, thus letting the user correlate the changes in latency with other events or activities that share the communications system.

SYNCHROWAVE Central displays instantaneous (white) and average (blue) network latency for a single PMU. Units are in thousands of microseconds (~36 ms).Fig. 3. synchroWAVe Central displays instantaneous (white) and average (blue) network latency for a single PMU. Units are in thousands of microseconds (~36 ms).

SYNCHROWAVE Central displays the average network latency of several PMUs on the upper half of the screen and average interpacket latency on the lower half of the screen.
Fig. 4. synchroWAVe Central displays the average network latency of several PMUs on the upper half of the screen and average interpacket latency on the lower half of the screen.

Conclusion

Understanding latency in your system is critical to optimizing operations. The new latency measurement calculations included in the latest release of the SEL-3373 PDC, SEL-5073 PDC, and SEL-5078-2 synchroWAVe Central Visualization and Analysis Software allow detailed latency measurement analysis of the entire synchrophasor system. Understanding latency is important when optimally configuring PDCs and when synchrophasor data are used in wide-area control schemes.

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