The SEL-2488 is Schweitzer Engineering Laboratories' satellite-synchronized network clock: it receives GPS and GLONASS and delivers IRIG-B, NTP and, with the corresponding option, PTP (IEEE 1588) to relays, recorders and network equipment. The SEL-2401 and SEL-2407 are simpler GPS clocks with IRIG-B outputs, for small cabinets and distribution substations.
What it is and where it is used
The SEL-2488 is the Satellite-Synchronized Network Clock from Schweitzer Engineering Laboratories (SEL). Together with the SEL-2401 and SEL-2407 it forms SEL’s clock family. With it, many medium-, high- and extra-high-voltage substations, and also industrial plants, give the same time to their relays, fault recorders, phasor measurement units and network equipment.
What it is for, in one sentence: so that every event in the substation carries a common time stamp. Without that, comparing an event report from one end of a line with the other end’s, or reconstructing a sequence of trips, becomes guesswork. For the general picture, see the guide to time synchronization in substations.
Variants: SEL-2488, SEL-2407, SEL-2401 and SEL-2404
| Model | Application | Main difference |
|---|---|---|
| SEL-2488 | Network clock for substations and control centers | GPS and GLONASS; IRIG-B, NTP and PTP (grandmaster option); eight configurable BNC outputs and four Ethernet ports |
| SEL-2407 | Substations with relays on IRIG-B | GPS; one modulated and six demodulated IRIG-B outputs; LED time display; optional fiber port |
| SEL-2401 | Recloser control cabinets and small substations | Compact GPS; one demodulated IRIG-B output; 9–30 Vdc supply |
| SEL-2404 | Legacy | No longer manufactured; SEL recommends the SEL-3401 with the 2401, 2407 or 2488 |
All are SEL GPS clocks, but they are not interchangeable: the outputs, the number of devices they can feed and the network protocols differ.
Main characteristics
According to SEL’s public information:
- Time sources: the SEL-2488 receives GPS and GLONASS. With the dual-constellation antenna, GLONASS is used to validate the GPS time and detect spoofing. It can also take time from a PTP source.
- Outputs: demodulated and modulated IRIG-B, pulse per second (1 PPS), timing pulses and, as an option, 10 MHz. The SEL-2401 and SEL-2407 deliver IRIG-B with IEEE 1344 extensions (year, leap second, daylight saving time).
- Network: the SEL-2488 serves NTP or PTP to four independent networks through its four Ethernet ports.
- Holdover: the SEL-2488 comes with a TCXO oscillator as standard and OCXO and DOCXO options, which keep time with greater or lesser accuracy when the satellite signal is lost.
- Environment: the SEL-2401 and SEL-2407 are designed to IEEE C37.90 and IEC 60255 to operate from –40 °C to +80 °C.
Communications and integration
- IRIG-B for relays and recorders that receive time over coaxial cable or fiber (with SEL-2812 or SEL-3405 transceivers). See IRIG-B and SNTP.
- NTP (NTPv4) for servers, HMIs, gateways and devices that accept time over the network.
- PTP IEEE 1588 as grandmaster with the default profiles, IEEE C37.238 (2011/2017) and IEC/IEEE 61850-9-3, which are the ones a digital substation with IEC 61850 process bus requires. We explain it in PTP IEEE 1588, power profile.
- Security: the SEL-2488 has role-based accounts, LDAP authentication, X.509 certificates, syslog logging and a table of accepted PTP masters.
- Configuration: through an HTTPS web interface on the SEL-2488; through DIP switches on the SEL-2407.
Data published by SEL: accuracy, ports and holdover
These values come from the SEL-2488 specifications page and should be checked against the data sheet for the version you have:
| Aspect | Published data |
|---|---|
| Accuracy relative to UTC | Demodulated IRIG-B and 1 PPS: ±40 ns average and ±100 ns peak; modulated IRIG-B: ±1 µs peak; NTP time stamps: under 100 µs typical; PTP time stamps: ±100 ns peak |
| Outputs | Up to 9 demodulated IRIG-B, PPS or kPPS; up to 4 modulated IRIG-B; Form A timer contact with 1 µs accuracy; Form C alarm contact |
| Ports | 1 front Ethernet management port; 4 rear Ethernet in pairs (10/100BASE-T, 100BASE-FX or 100BASE-LX10); 1 DB-9; 8 rear BNC |
| PTP profiles (option) | IEEE 1588-2008 default (UDP and IEEE 802.3), IEEE C37.238-2011 and 2017, IEC/IEEE 61850-9-3:2016 |
| Network redundancy | Up to two PRP (IEC 62439-3:2016) or active-backup interfaces; PTP operates as a dual-attached clock on the PRP ports |
| Holdover after 24 hours (typical) | Standard TCXO: 315 µs; optional OCXO: 5 µs; optional DOCXO: 2.5 µs |
| Power supply | 24–48 Vdc, or 125–250 Vdc/Vac |
| Operating temperature | –40 °C to +85 °C |
The required antenna is the SEL-9524B (GPS and GLONASS). SEL states that GPS and PTP are the time sources and that GLONASS is used for verification.
How to read those numbers in practice
- Accuracy versus need. A relay that receives demodulated IRIG-B rarely needs better than a millisecond to correlate events. Phasor measurement units and sampled values do require microseconds. For example, the 1 % total vector error set by the synchrophasor standard is equivalent, at 60 Hz, to about 26 µs of time error; that is why the time reference is sized from the most demanding application.
- Holdover and maintenance. If the antenna fails or there is interference, the time drifts at the rate of the oscillator. With 315 µs after 24 hours the standard clock is fine for events and common protection, but not for phasors over several days; the OCXO and DOCXO options exist for that case. More criteria in GNSS clock, holdover and redundancy.
- A single clock is a single point of failure. In critical substations two clocks, or a clock with PTP backup, are used; the choice of inputs and outputs is defined in the design, not in the device.
- Antenna and cabling. The antenna needs a clear view of the sky, lightning protection and a cable length compatible with its attenuation; the delay of the coax or fiber to each device is a compensation that must be documented.
- Modulated or demodulated output. Each IED accepts one form of IRIG-B; connecting the wrong one is a classic cause of devices being “out of time”. What the relay manufacturer requires comes first.
When time is lost or drifts
The usual symptoms are loss-of-synchronization alarms on several relays at once, time stamps that jump after a time change, or events from the two ends of a line that do not line up. The origin is almost always the satellite source, the time distribution network or the time zone setting of each IED. See loss of time synchronization (PTP or IRIG-B) and PTP, NTP or IRIG-B. Once the time is reliable, analyzing COMTRADE files from two ends stops being a conjecture. For redundancy of the clock’s own network connection, see PRP and HSR.
Frequently asked technical questions
- Manual and data sheet: they are in SEL’s public documentation. What they do not say is which output suits each device in your substation; that comes from the communications architecture.
- Relays show different times: almost always an IRIG-B format issue (with or without IEEE 1344 extensions), a time zone offset applied twice, or excessive load on one output.
- PTP does not synchronize: the profiles must match on the clock, the switches and the IEDs. A switch that does not take part in PTP degrades accuracy.
- Synchrophasors: phasor measurement units need an accurate and stable reference; see PMU and PDC synchrophasor network.
How we help
At Tiza Group we configure time synchronization for substations and industrial plants as part of control and communications engineering: we define which device receives time over IRIG-B, NTP or PTP, configure the clock and verify that each relay is on time. We configure IEDs, switches and gateways remotely, worldwide, through remote configuration of SEL relays and communications, with a backup before and after each change. See the scope under control, protection and communications configuration and, if your project uses IEC 61850, IEC 61850 engineering. To request a quote, write to us through the contact page.
Related guides
- Time synchronization in substations
- IRIG-B and SNTP
- PTP IEEE 1588, power profile
- Substation Ethernet switch
- Event reports and sequence of events
- PMU and PDC synchrophasor network
- Meinberg LANTIME, Tekron TCG and Arbiter 1088B GPS clocks
- Loss of time synchronization
References
- IEEE C37.238, PTP profile for power system applications.
- IEC/IEEE 61850-9-3, PTP profile for power utility automation.
- IEEE 1344 / IEEE C37.118, IRIG-B extensions and synchrophasors.
- Schweitzer Engineering Laboratories, product pages for the SEL-2488, SEL-2407, SEL-2401 and SEL-2404.
Frequently asked questions
What is the difference between the SEL-2488 and the SEL-2407?
The SEL-2407 is a GPS clock with IRIG-B outputs (one modulated and six demodulated) and an LED time display. The SEL-2488 is a network clock: in addition to IRIG-B it delivers NTP over four Ethernet ports and, with the grandmaster option, PTP with the power profiles IEEE C37.238 and IEC/IEEE 61850-9-3. Which one fits depends on which devices need to be synchronized and over what medium.
Where is the SEL-2488 manual?
SEL publishes the SEL-2488 data sheet and instruction manual in the documentation section of its product page. Configuration is done through an HTTPS web interface; the manual explains the options, but which output, format and profile to use is defined by the substation architecture.
Is the SEL-2404 still manufactured?
No. SEL states that the SEL-2404 is no longer in production and that it continues to support it; as a replacement it recommends the SEL-3401 together with the SEL-2401, SEL-2407 or SEL-2488.