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Modular PMU Solutions for Evolving Applications

As the demands of synchrophasor applications become more diverse, phasor measurement units (PMUs) must adapt to meet the changing needs. Challenges such as accurate time distribution, hard-to-reach measurement locations, and inadequate communications networks can seriously hinder the deployment of a synchrophasor system. While SEL has been integrating PMU capability into protective relays for over a decade, we wanted to create a new PMU with the flexibility to satisfy the measurement requirements of even the most demanding synchrophasor applications. This issue of The Synchrophasor Report highlights the SEL-2240 Axion as a versatile PMU solution.

SEL Axion Overview

The Axion is a fully integrated, modular input/output (I/O) and control solution ideally suited for utility and industrial applications. It combines the communications, built-in security, and IEC 61131 logic engine of the SEL Real-Time Automation Controller (RTAC) family with a durable suite of I/O modules that provide high-speed, deterministic control performance over an EtherCAT network.


Fig. 1. The SEL Axion

The SEL Axion PMU Solution

The Axion chassis provides accurate measurements in remote locations as far as 5 km apart via the use of the EtherCAT network, which distributes submicrosecond time to each remote Axion chassis. One IRIG-B connection to the central Axion controller is all that’s needed for measurement system synchronization. Add 8 SEL-2245-4 AC Modules (4 PT and 4 CT inputs each) for local and remote synchrophasor measurement of up to 64 phasor quantities. This modular approach allows the Axion to fit a variety of synchrophasor applications while allowing for future expansion.

The Axion is an excellent PMU solution when accurate time synchronization must be distributed to difficult-to-reach locations, such as generator rooms in hydro generation sites and nuclear generation sites.

In the example in Figure 2, one Axion is installed at the substation near the top of the dam. It accepts an IRIG-B signal and distributes it over a fiber connection to a set of remote modules installed next to the generator deep within the dam.

Axion hydro application example

Fig. 2. Axion hydro application example.

The modularity of the Axion enables custom-tailored substation measurement solutions. It also means you can reduce commissioning time and equipment cost by adding only the required modules.

The example in Figure 3 shows the Axion as the single measurement and control solution for a substation with 4 PTs, 2 CTs, 8 breakers, and 3 generators. Only 4 PT/CT modules, 1 digital input module, and a pair of analog input and output modules (for generator control) are needed.

Axion PMU application example

Fig. 3. SEL Axion PMU application example in a substation with three generators.

As of November 2014, the Axion is compliant with IEEE C37.118-2011 measurement and communications standards as amended by IEEE C37.18.1a-2014, thereby taking the reigns as SEL’s most versatile and accurate PMU solution.

PMU Performance

To ensure compliance with the IEEE C37.118-2011 standard, the new server was subject to a set of rigorous tests, which covered Digital Signal Processing (DSP) accuracy, thermal compliance, and timestamp-to-packet transmit latency. The following table shows the various results of the performance testing.

Standard Compliance Over a Temperature Range (0° to 50° C)IEEE C37.118.1-2011 as ammended by IEEE C37.118.1a-2014
Performance ClassP and M
Reporting Rates1 to 60 message
Phasor QuantitiesMaximum of 64 phasors
Server Outputs1 to 2 servers with 64 phasors at 60 messages per second; 3 to 6 servers with 64 phasors at 30 messages per second

Simple and Powerful Configurations

Similar to most SEL relays, the Axion PMU capability merely needs to be turned on for all desired CT/PT modules. You can also scale phasor quantities for PT/CT angle compensation and create unique tag aliases for easy naming convention integration with existing phasor data repositories. You can even configure each IEEE C37.118 server with a unique set of tags via the RTAC’s Virtual Tag List. The basic steps for configuring the Axion PMU solution are shown below.

Axion PMU configuration

Fig. 4. Axion PMU configuration.


While the need for dependable power generation and delivery will remain a constant, supporting systems and technologies will continue to evolve. Power system monitoring solutions must be equally flexible. The SEL Axion as a modular PMU is a versatile and powerful solution to your present and future synchrophasor application needs.

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