Distributing Timing Signals

Some substation IEDs include built-in GPS receivers to provide precise timing. More commonly, however, substations include numerous IEDs which all can benefit from synchronization. Therefore, utilities have generally opted for a satellite clock in the substation, which provides timing signals to the various IEDs. The following sections describe various methods that have been used for timing distribution, with varying levels of cost, complexity and performance.

IRIG-B
The Inter-Range Instrumentation Group of the Range Commanders' Council, a US military test range body, was faced several decades ago with a problem. Each test range had developed its own, unique time code. These time codes were recorded with test data on data tape recordings. The time codes were all incompatible, which made it difficult or impossible for the ranges to exchange data. Therefore, the IRIG group set about developing a common set of time codes. These have become known as the 'IRIG codes.'

These time codes have become widely used in both military and civilian applications. Particularly, the IRIG code B (generally abbreviated IRIG-B) has become widely accepted for time distribution in substations. This time code repeats each second, and has a total of 100 bits per second. Some of these are framing (sync) bits, some are assigned for time, and some are available for control functions (tables 1 and 2). The basic IRIG-B code has an 'ambiguity' of one year, since it does not contain year data.

IRIG-B code may be used in either logic-level (unmodulated) format, or as an amplitude-modulated signal with a 1 kHz carrier (figure 4). The modulated IRIG signal is particularly suitable for transmission over voice-band channels, including data channels on an instrumentation tape recorder. Because of the difficulty of accurately measuring the zero crossings, modulated IRIG-B code is generally capable of an accuracy exceeding one millisecond (one period of 1 kHz), but not usually better than ten microseconds. This level of accuracy is acceptable for some but not all substation applications. Modulated IRIG inputs to IEDs are generally transformer-isolated and provided with an automatic gain-control stage, so as to accept input signals with a wide range of amplitudes. A typical input level range is 0.1 to 10 volts peak to peak.

Fig. 4

The unmodulated IRIG-B code can deliver accuracy limited only by the slew rate of the digital signal, much better than one microsecond and, with care, in the range of a few nanoseconds. Unmodulated IRIG-B is normally distributed at a level of 5 volts (compatible with TTL or CMOS inputs). While some IEDs couple the IRIG-B signal directly to a logic gate, best practice has for some time used an optically-isolated input to break ground loops. Well-designed substation clocks can generally drive numerous inputs with either modulated or unmodulated IRIG-B signals, so a single clock can synchronize all the IEDs in a substation (figure 5).

Fig. 5

IEEE-1344 Extension to IRIG-B
The IRIG codes were originally developed for test range use, and a one-year ambiguity was not a limitation. Being a military code, times were always recorded in UTC (Coordinated Universal Time, military Zulu Time), so local offsets and summer time issues were also not a concern. Leap seconds (see below), which happen once or twice a year, or less, also were not a concern of the IRIG group. And the range officer would be required to guarantee that all recorders were synchronized properly before beginning a test.

Real-time operation, 24 hours per day, 365 days per year, year after year, imposes some additional requirements. The issues identified in the previous paragraph become real concerns. As part of the original Synchrophasor standard, IEEE Standard 1344-1995, an extension for the IRIG-B code was developed using the 'control bits' field to provide an additional 2 digits of year (subsequently adopted also by the IRIG standard), as well as local offset, time quality, and bits for leap second and summer time changeovers. Some IEDs support this extension, but many do not. You may find an occasional IED which requires the IEEE-1344 extension for proper operation. This may not be well documented in the product literature.

The IEEE-1344 extension also created a new 'modified Manchester' encoding scheme, which is a digital signal having zero average value, and suitable for transmission over an optical fiber. It encodes the same 100 bit/second data stream onto a 1 kpps square-wave carrier. This modulation method has also been accepted as part of the IRIG standard.

Signal levels and connection methods for the IRIG-B code, with or without the IEEE-1344 extensions, are identical. The only difference is the use of the control bits to provide the extra information required in continuous, real-time monitoring applications (table 1). This is a firmware feature, and does not affect the hardware interface.