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This article examines the Arbiter range of GPS Satellite-Controlled clocks. This type of clock is used to provide accurate frequency and time measurements for a variety of grid connected equipment from control centers to substation protective relaying equipment with vastly differing performance requirements. To understand my evaluation, you need to grasp three essential elements:
· The role of the SGN Scorecard · The purpose of GPS-controlled clocks in a Smart Grid · How Arbiter measures up
The role and importance of the SGN Scorecard
The SGN Scorecard was developed for a very important reason: most of today's products do not adhere to Smart Grid principles. They do not support the requirements envisioned by Smart Grid researchers such as EPRI, the California Energy Commission's Public Interest Energy Research program, the Modern Grid Initiative and DOE's GridWise program. Nor do they adhere to the mandates in the Energy Independence and Security Act of 2007.
In particular, several elements of the EPRI IntelliGrid Architecture are critical to implementing a Smart Grid: · Proven, Internet derived communication technologies · Service based architecture at the enterprise level · Self healing technology · Well defined interfaces and points of interoperability · Application of industry and international standards · Built in security and network management
Because of their failure to meet the basic principles, most of today's communication networks -- whether for automatic meter reading (AMR) or for advanced meter infrastructure (AMI) -- are inadequate. They may support "least common denominator" Smart Grid applications such as meter reading. However, they are not adequate for distribution automation, plug in hybrid electric vehicle management, microgrids, demand response and many other future applications.
SGN Warning: To avoid buying dead-end products, utilities must provide their vendors with a comprehensive and prioritized list of requirements covering current and future functionality. Then they must receive written assurances from vendors that their products meet those requirements.
The SGN Scorecard is a checklist that measures whether products meet minimum standards for a Smart Grid. We will use it as the benchmark for all Tech Talk reviews. You are invited to use it free of charge for your own evaluations. For a further explanation and a blank version you can copy freely, download the PDF version of the Scorecard.
The purpose of GPS-controlled clocks in a Smart Grid
GPS-controlled clocks are used to calibrate time and frequency sources throughout the grid. Without calibration, grid-connected devices can drift by a few minutes per month. GPS-controlled clocks work with end devices to null-out time and frequency errors. Timing sources in the grid span a large range of accuracy needs from a few microseconds to about one minute. Generally, a hierarchy of time sources is established with each layer providing less accuracy but to a larger number of devices (or through low-bandwidth channels). The accuracy classes fall into a few distinct groups:
· A few minutes : Local display of time, some security protocols which protect against replay attacks · A few seconds : time of use metering · 10’s of milliseconds : electrical disturbance recordings for power quality verification · A few milliseconds : grid control and substation protective equipment · 10’s of microseconds : wide-area system protection and remedial action schemes · A few microseconds : digital sample alignment for basic volt/ampere measurements
GPS-controlled clocks communicate with other grid-connected devices using 3 distinct mechanisms:
· Serial port : a low-accuracy channel mainly used for enterprise-level timing (inside data center) · Dedicated wires : high-accuracy connection to a limited number of devices · Network (Ethernet) port : standard time interface for IP-connected devices with optional clock authentication
Accurate and reliable time synchronization is critical to ensure that automatic control and system protection equipment operates correctly in a smart grid to allow optimal utilization of grid assets. When a system event does occur, it is important for later forensic analysis that all system events and data captured during the event are time stamped accurately so that the root cause of the event can be determined. The importance of time synchronization was one of the key findings in the investigation of the 2003 blackout. The lack of accurately time stamped data made it very difficult to piece together the sequence of events that led to the blackout.
More recently, GPS time synchronization is playing a key role in Phasor Measurement Systems – a technique that permits the real time visualization of instantaneous power flows, and the implementation of wide area protection, control, and voltage instability detection and prevention on the bulk power system – all key requirements for a smart grid.
How Arbiter measures up
Arbiter provides multiple ranges of clocks dependent upon accuracy needs from 100 nanoseconds to 1000 nanoseconds. For smart-grid uses, even the lowest accuracy device exceeds all present grid requirements. Arbiter has 3 types of time code output:
· Serial port : outputs ASCII time in a variety of formats either in polled or spontaneous mode · Dedicated wires : Modulated IRIG-B output (legacy), demodulated IRIG-B output, and 1 pulse-per-second output. The IRIG-B output code can optionally include IEEE 1344 year information. · Network (Ethernet) port : NTP/SNTP uses IETF RFC 2030 in polled and/or broadcast mode with optional authentication mode (DES or MD5 cryptographic checksums)
Where Arbiter and other still fall short
Like most competitors, Arbiter treats the serial and dedicated outputs as the primary sources of time and relegates the network port to low accuracy. Although wide-area Ethernet networks (WANS) are generally capable of only a few tens of milliseconds of accuracy, local area Ethernet networks (LANS) are capable of accuracy to a few microseconds with careful design. Arbiter also claims an NTP accuracy (as transmitted by the clock) of only a few milliseconds although the true accuracy is at least an order of magnitude better than this claim.
Arbiter Smart Grid Scorecard: 84 out of 100
Arbiter claims to provide a product which can be used at multiple levels of Smart Grid applications. Our Scorecard generally confirms this claim. Given these results, widespread adoption of Arbiter (or similar) clocks can have a positive effect on the power system.
EPRI IntelliGrid Architecture Web site
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