One key focus of the synchrophasor community in 2013 is data quality. Regional coordination entities across the world are collecting synchrophasor data from utilities and recording metrics on data quality. This issue of the Synchrophasor Report focuses on tuning your phasor data concentrator (PDC) configuration to maximize high-quality data.
The phasor measurement units (PMUs) in synchrophasor systems generate time-stamped data on a synchronized schedule, but the various network delays between PMUs and PDCs practically guarantee that the PMU data packets will not arrive simultaneously at their destination. A core function of the PDC is to time-align synchrophasor data from multiple sources and serve these data to a client.
Time alignment is the process of collecting all available synchrophasor data packets with identical time stamps, packaging them into an aggregate packet, and serving them to a client. As you may have guessed, time alignment requires the PDC to wait for all packets to arrive before generating the aggregate packet. To accomplish this, the PDC uses a timer, often called the “wait timer,” which counts up to a user-defined maximum wait period. If one or more PMU packets are delayed beyond the maximum wait period, the PDC will generate and serve the aggregate packet, omitting the delayed data. Then the PDC will provide an indication that data from certain PMUs were never received.
Now that we’ve established the basics, let’s tackle some interesting questions.
There are two unique methods of time alignment defined by the IEEE C37.244 Guide for Phasor Data Concentrator Requirements for Power System Protection, Control, and Monitoring—absolute time alignment and relative time alignment.
Absolute time alignment means the PDC starts the wait timer on a Coordinated Universal Time (UTC)-based schedule. This method:
Relative time alignment means the PDC starts the wait timer based on an event, which is typically the receipt of data with a new UTC time stamp. This method:
Figure 1 shows an example of each type of time alignment. Notice in each case that all PMU data packets are received within the duration of the user-defined maximum wait period.
Fig. 1. Absolute and relative time alignment with no missed data.
SEL’s modern synchrophasor solutions employ relative time alignment for the sake of data completeness/quality. For latency critical applications, SEL suggests establishing a dedicated output from your PDC that includes only the critical data. The remainder of this report will focus exclusively on relative time alignment.
Figure 2 shows an example of relative time alignment in a case where one of the PMU data packets arrives “late.”
Fig. 2. Relative time alignment with missed data.
Note that the PMU 15 packet arrives after the PMU 2 packet’s arrival time and maximum wait period. As a result, the PDC waits no longer than the user-defined maximum wait period before closing the time alignment window and beginning the output processing.
Regardless of whether all PMU data were received within the maximum wait period, the PDC output configuration reserves space for all PMU data in the aggregate packet. This brings us to our next key question.
SEL PDCs indicate late data within the aggregate packet by zero-filling the data tags and setting the associated status bits for the late PMU to indicate poor-quality data. Figure 3 shows a partial sub-PMU data set within an aggregate data packet that has been zero-filled by SEL-5073 synchroWAVe PDC Software.
Fig. 3. SEL-5073 PDC "late" data handling.
A regional coordinator who receives this aggregate packet will flag the “Station 4” PMU with “invalid data” for the given time stamp. However, with correct PDC configuration, you can minimize late or invalid data in the aggregate packet so you can maximize your output data quality.
At this point, it may seem that the obvious answer is to simply increase the maximum wait period for the given output. While that is true, utilities are often faced with a latency budget. In other words, the utility must confirm that their synchrophasor data will be transmitted from the PMU to the regional authority within a maximum allowed time delay, as specified by the regional authority. A great way to tune the maximum wait period to a reasonable duration for high-quality data is by analyzing network statistics. Figure 4 shows how network statistics in the PDC Assistant Real Time page can be used to set a reasonable output wait period.
Fig. 4. PDC Assistant Real Time page aids in configuring the wait period.
The PDC in this example is not synchronized to UTC. The PDC system clock is offset from UTC by approximately 300 ms, which is shared among the calculated latencies of each input. Recall from Figures 1 and 2 that the differences in latency among the inputs were only relevant for relative time alignment. Therefore, the absence of a UTC synch does not affect this wait period tuning procedure.
Using the information from the Real Time page, the output maximum wait period setting can be tuned. The difference between the greatest maximum and smallest average is 355 ms. Therefore, a wait period of 400 ms should catch all packets, including the outliers, while a wait period of 150 ms should catch the majority of input packets.
Synchrophasor systems commonly employ multiple levels of PDCs, with rugged hardware PDCs typically located at each substation to send data on to a central PDC. The central PDC in turn sends the data further upstream and to local applications. When setting up such a system, special consideration should be taken when configuring the maximum wait periods of each PDC output. Figure 5 illustrates a possible scenario in a simplified synchrophasor system if all wait periods are unknowingly set to the same value of 100 ms.
The network latency of a set of PMU measurements is tracked with a common time stamp of 12:0:0:000. The red text indicates the packet latency accrued between devices; one PMU is offline for maintenance. Notice how just that one offline PMU can disrupt the data quality for an entire substation.
Fig. 5. Station PDCs and Central PDC maximum wait period settings are configured for 100 ms. For simplicity, PMU and PDC processing times (see Figs. 1 or 2) are assumed to be negligible.
When configuring PDCs in the system, worst-case scenarios like this should be considered. To avoid this data quality issue, maximum wait period settings should be configured across the system with increasing durations from the substation to the central PDC.
In the example above, configuring the station PDCs to a 75 ms maximum wait period and the central office to a 125 ms maximum wait period provides a simple solution to improve the overall data quality. This way, the central PDC would receive the aggregate packet from Substation B within the allotted time and only PMUd would reflect “poor-quality data” in the upstream applications.
The data quality of your synchrophasor system can be improved and more efficient with the right tools and knowledge.
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