In the April issue of The Synchrophasor Report, we discussed how synchrophasor measurements require a precise absolute time reference—one that provides better than 1 microsecond absolute accuracy. This time reference allows us to precisely measure voltage and current phasors across a wide area to better than 1 percent total vector error (TVE) as specified in the IEEE C37.118 Standard for Synchrophasors for Power Systems. Global Positioning System (GPS) satellite-synchronized clocks are the predominant means for providing precise time for phasor measurement unit (PMU) measurements. In this issue, we will examine ways to minimize the loss of the GPS time source.
As we increasingly rely on synchrophasors to measure, monitor, and control our power system, there is an increasing need to ensure the dependable availability of measurement data. Always having a precise clock signal for a timing reference is critical for synchrophasor measurements. If we lose the precise time reference, we can no longer accurately measure phase angles (magnitudes would still be valid, however), and thus, we would lose a significant portion of the measurement value that synchrophasors provide.
GPS satellite-synchronized clocks have enabled us to inexpensively distribute precise time to any substation or control center in the world. For these precise clocks to provide accurate time, however, they must continuously receive GPS satellite signals. It is reasonable to ask, “What might cause my GPS clock to temporarily lose its signal?”
Common causes of a lost GPS clock signal include the following:
Each of these situations can result in the loss of precise timing.
Improper installation is one of the most common causes of satellite clock signal loss. It is essential to mount the antenna with an unobstructed view of the sky. Installations where the antenna is even partially blocked by a building overhang or vegetation can cause the clock to intermittently lose satellite reception. As mentioned in our last article, a typical GPS receiver needs to be able to see at least four satellites initially to synchronize and lock to the GPS reference. The antenna needs a clear view of the sky to receive the satellite signals on the horizon.
Fig. 1. Improperly mounted GPS antennas can cause intermittent GPS reception.
Faulty cables or connections are another common cause of intermittent loss of clock signals. These failures are difficult to identify because the clock will operate fine most of the time but have short periods of signal loss to the PMU. To minimize this issue, use good quality cables, keep cable lengths to a minimum, and ensure all connections are tight. This is important for both the antenna cables and the IRIG-B cables that run from the clock to the PMUs.
Another consideration involves the proper connection of PMUs to the IRIG-B output signal from the clock. When using SEL clocks such as SEL-2401, SEL-2404, or SEL-2407® Satellite-Synchronized Clocks, no more than ten relays/PMUs should be connected in parallel to a single IRIG-B output. Additionally, it is important to properly terminate the IRIG-B connections using a 50 ohm load. This ensures that an accurate IRIG-B signal is seen by each PMU.
SEL relays allow users to check the status of the clock signal being received by the relay/PMU with relay word bits. Some examples of the relay word bits that indicate clock status include Time-Synchronized OK (TSOK), Timing Source IRIG (TIRIG), and Timing UPDated by High Accuracy IRIG (TUDPH).
To address short periods of GPS signal loss, some satellite-synchronized clocks offer a “holdover” capability, which allows the clock to maintain accurate time over a period of time when it is not receiving a GPS signal. The SEL-2401, SEL-2404, and SEL-2407 all offer holdover capability. During periods of lost GPS signal, the SEL clocks use their own internal reference clock, which maintains the required timing accuracy for approximately 18 seconds. This reduces the number of clock timing outages seen by the PMU and improves the availability of synchrophasor measurements.
Fig. 2. The SEL-2407 has “holdover” capability and can maintain ‹1 microsecond time accuracy for ~18 seconds.
Another approach that improves the availability of GPS clocks is to design redundancy into the clock system. A basic redundant clock system includes two different GPS clocks. One GPS clock provides the IRIG-B signal to the primary PMU, and a secondary GPS clock provides an IRIG-B signal to the backup PMU. The outputs of the two PMUs are then sent to a Phasor Data Concentrator (PDC) like the SEL-3373 Station Phasor Data Concentrator or the SEL-5073 synchroWAVe Phasor Data Concentrator. The PDC automatically switches between the two PMU inputs to provide an uninterrupted synchrophasor data stream (assuming that one of the PMUs and the clocks are providing accurate phasor measurements) for downstream use.
In 2010, SEL introduced the SEL ICON Integrated Communications Optical Network. The ICON is a SONET (Synchronous Optical Network) communications solution that incorporates Ethernet technology and other flexible drop interfaces to provide an integrated data and voice communications solution in a single platform. One of the innovative features incorporated into the ICON is the ability to maintain and distribute time over a wide-area network (WAN) with better than 1 microsecond accuracy so that very accurate relative time is maintained in the event of a GPS failure.
Fig. 3. The SEL ICON is shown configured in a SONET ring, providing accurate timing for each site.
Each ICON node includes a server module with an integrated GPS satellite-synchronized clock. This clock provides the high-accuracy timing needed for synchrophasors as well as the more stringent requirements of a Stratum 1 clock for SONET communications. When configured in a SONET ring, each individual node has a GPS clock. In SONET, one of these clocks is designated as the master (head end) time source and a secondary node is typically dedicated as a backup. If the GPS receiver at the primary location loses the GPS satellite signal, the system switches to the backup node and continues to maintain accurate time. This protects against localized GPS signal outages. Additionally, if there is a system-wide loss of the GPS signal (which is possible during a strong solar storm), the ICON system has a built-in time reference that maintains better than 1 microsecond relative time accuracy within the network of deployed ICONs. This provides an accurate system time reference for your power system during the GPS outage, thus allowing synchrophasor measurements to be made throughout your system. The system will slowly drift from GPS absolute time, but maintains accurate relative time. Once the GPS signal returns, the system returns to using GPS time.
Ensuring high availability of accurate time for synchrophasor measurements is critical because synchrophasor measurements are increasingly used to monitor, measure, and control the power system. There are several approaches that can be used to increase the availability of precise time. SEL can help you implement a solution using our GPS satellite-synchronized clocks, PMUs, and PDCs as well as integrate the ICON as the communications backbone with its ability to distribute accurate time over the entire network of ICONs. A complete system using the ICON provides a robust solution so your network never loses accurate time.
If you would like more information on how to increase the availability of precise and accurate time for your synchrophasor system or have any questions on how you can apply these solutions to your system, please fill in the form below to contact us.