By Ron Holm, Symmetricom
Every second of every day, thousands of events occur between generators, transmission lines and power substations scattered across thousands of miles. In full operation, a smart substation will generate and process data samples per second by the hundreds of thousands. Add in the terabytes of smart metering data generated and transmitted per day, and it becomes obvious that timing is essential to prevent chaos within data processing.
Timing has always had a smart grid role—with the usefulness of the time-stamped data being determined by the accuracy of the time-stamp, of course. The accuracy needed to support forensics has not been too challenging, traditionally. Protection relays and disturbance recorders are synchronized within one millisecond (1 ms) of a grid reference for post-event analysis. In the operational realm, SCADA needs are also met with data time-stamped inside 1 ms of the same reference.
So, the value of synchronization in the grid is well understood but, until recently, it was not critical technology.
As utilities strive to deliver more from their existing grids, there has been a significant evolution of the control and protection functions. Those advances depend on measurements that are more precisely synchronized and knowledge of time-stamp inaccuracy. New applications such as wide-area measurement systems and sample values now require microsecond accuracy. Synchronous sampling and time-stamping of sampled values is critical. Failure to do so across the substation can result in protection relays defaulting to the safe “tripped” state.
Timing is no longer optional, in other words. It is now an operational necessity, and an ad-hoc and unmanaged approach to timing cannot be sustained.
The challenge is distributing a one microsecond (1 µs) reference reliably and cost effectively within the substation. Engineering a parallel timing bus to every intelligent electronic device (IED) is not feasible, and 1 µs is beyond the reach of the LAN-based network time protocol. An IEEE Power System Relaying Committee group was tasked with defining a profile of the IEEE 1588 precise time protocol to deliver 1 µs in the substation; this is defined in the new C37.238-2011 standard. This solution overcomes the limitations of previous technologies: high precision, in-band distribution over the LAN and knowledge of the clock quality for IEDs and phasor measurement units. It is also foreseen that C37.238 will be referenced in the IEC 61850-9-2 standard to address the higher accuracy needs of sampled values, bringing IEEE 1588 into the global substation architecture standard.
The last consideration is the reliability of time distribution to the substation. GPS is a proven technology. It is not infallible, however, nor under the control of the utility community. By itself, it cannot meet the N-1 contingency needs for service affecting processes. The challenge of delivering an independent and economic time source to substations is met using the same IEEE 1588 protocol used within the substation. IEEE 1588 allows an autonomous clock reference to be distributed from a central location to substations, supporting the grid processes in the event the GPS service is compromised.
Precise time in the smart grid has come of age and plays a key role in time-sensitive control and protection applications. The IEEE 1588 protocol is a new toolbox item that supports the evolving need for accurate and reliable timing throughout the grid.
Ron Holm is Symmetricom’s product marketing manager responsible for the GPS time and frequency instrumentation product line. Ron has been involved in the time and frequency industry since 1990 and has held previous positions in engineering and engineering management.
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