In the telecom networks of Europe, North America and Asia, ePRTC-based architectures are already in use to provide resilient synchronisation for LTE and 5G networks, where the requirements on time and phase are extremely high. These solutions are gradually finding their way into the electric-power industry as well — first of all at sites that use digital substations, PMU-based applications and other systems sensitive to the quality of synchronisation. In many countries, the task of becoming more resilient to disruptions of the global navigation satellite systems is no longer treated as theoretical but as a practical requirement on critical infrastructure.
One such architecture is built around two concepts: ePRTC and TDG.
What ePRTC is
ePRTC stands for enhanced Primary Reference Time Clock, that is, an enhanced primary reference source of time and frequency.
Unlike an ordinary GNSS clock, an ePRTC is not just a GPS/GLONASS receiver. It is an operator-class precise-time node, designed to synchronise time, phase and frequency in telecom and packet networks.
An ePRTC is normally synchronised from the global navigation satellite systems, but internally it has a very stable frequency source — for example, a caesium atomic standard or another highly stable oscillator. If satellite synchronisation is lost, the ePRTC enters precise-synchronisation holdover and continues to maintain its time scale on its own.
It is precisely thanks to this that such systems can keep very high accuracy for a long time even after losing synchronisation from the global navigation satellite systems.
Where the ePRTC is located
In this kind of architecture, the ePRTC is normally not installed at every substation. It is rather a reference time source for the network.
It can be placed in a hardened telecom hub, in a network operations centre, in a data centre or at another key point of the utility's infrastructure. From there, precise time is delivered over the terrestrial telecom network — for example, using PTP profiles common to the telecom industry.
In a simplified form it looks like this:
flowchart TB
subgraph TELECOM["Telecom domain"]
direction TB
HUB["Hardened telecom hub<br/><i>network centre / data centre</i>"]
EPRTC["<b>ePRTC</b><br/>Cs holdover<br/><i>PTP Grandmaster</i>"]
NET["Terrestrial transport network<br/><i>MPLS / fibre</i>"]
HUB --> EPRTC --> NET
end
subgraph SS["Digital substation"]
direction TB
TDG["<b>TDG</b><br/>Time Distribution Gateway<br/><i>quality control · source selection</i>"]
PTP["Substation network<br/><b>IEC 61850-9-3 PTP</b><br/>Power Utility Profile"]
DEV["IEDs"]
TDG --> PTP --> DEV
end
NET ==>|"precise time<br/>via the ground"| TDG
GNSS["GNSS<br/><i>GPS / GLONASS</i>"] -.->|"local reception"| TDG
style HUB fill:#F3F3F3,stroke:#888
style EPRTC fill:#E8F5E9,stroke:#43A047,color:#1B5E20
style NET fill:#EDE7F6,stroke:#7E57C2,color:#311B92
style TDG fill:#E3F2FD,stroke:#1E88E5,color:#0D47A1
style PTP fill:#EDE7F6,stroke:#7E57C2,color:#311B92
style DEV fill:#E0F2F1,stroke:#26A69A,color:#004D40
style GNSS fill:#FFF8E1,stroke:#F9A825,color:#E65100
In other words, precise time can reach the substation not only via GPS/GLONASS, but also «via the ground» — through the telecom infrastructure.
What TDG is
TDG stands for Time Distribution Gateway.
If the ePRTC is the reference time source for the network, the TDG is the local node on the substation that takes time from different sources, monitors its quality, selects the working source and feeds time to the substation devices in the required formats.
flowchart LR
A["ePRTC<br/><i>via the telecom network</i>"]
B["Local GNSS<br/><i>GPS / GLONASS</i>"]
C["Existing time-<br/>distribution systems"]
D["Neighbouring TDG<br/><i>reserve channel</i>"]
TDG{{"<b>TDG</b><br/>Time Distribution<br/>Gateway"}}
A --> TDG
B --> TDG
C --> TDG
D --> TDG
TDG --> O1["<b>IEC 61850-9-3 PTP</b><br/><i>Power Utility Profile</i>"]
TDG --> O2["<b>IEEE 1588</b><br/><i>Power Profile</i>"]
style A fill:#E8F5E9,stroke:#43A047,color:#1B5E20
style B fill:#FFF8E1,stroke:#F9A825,color:#E65100
style C fill:#F3F3F3,stroke:#888
style D fill:#EDE7F6,stroke:#7E57C2,color:#311B92
style TDG fill:#E3F2FD,stroke:#1E88E5,color:#0D47A1
style O1 fill:#E0F2F1,stroke:#26A69A,color:#004D40
style O2 fill:#E0F2F1,stroke:#26A69A,color:#004D40
This is an important point: a TDG is not just a PTP switch and not just «one more clock». It plays the role of a gateway between the telecom time domain and the technological network of the substation.
In the telecom network one PTP profile may be in use, while in the digital substation a different one applies — for example, the IEC 61850-9-3 / Power Utility Profile. The TDG provides the coupling between these domains, the isolation, the redundancy and the delivery of time in the formats understood by IEDs, merging units, process-bus I/O devices, PMUs, protection relays and event recorders.
Why this matters for a digital substation
For a conventional substation, losing precise time most often meant problems with time tags, SOE or oscillography. That is unpleasant, but not always critical for the core protection function.
For a digital substation the situation changes. With Sampled Values, phasor measurements, distributed measurements and inter-substation functions, time becomes part of the technological process.
A precise and trusted time base is needed for:
- synchronisation of merging units, process-bus I/O devices and protection terminals;
- correct operation of Sampled-Values-based protection schemes (in particular, schemes built on physically separated protection sets);
- PMUs and synchronised-measurement applications;
- event recording and post-fault analysis;
- correlation of data captured at different sites.
If all these functions rely solely on a local receiver of the global navigation satellite systems, jamming or spoofing of the satellite signal becomes a problem.
Why two GPS/GLONASS receivers may not be enough
At first glance the issue looks easy to solve: deploy two GNSS receivers, two antennas, separate cable routes and treat the time sources as redundant.
That does improve reliability, but it does not eliminate the main vulnerability: both sources still depend on the same physical nature of the signal — a satellite radio channel.
If the site experiences GNSS jamming, strong interference or spoofing, both receivers may lose a trusted time reference at the same time.
A heterogeneous architecture looks more resilient:
flowchart TB
SKY["☀ GNSS — «from the sky»<br/><i>GPS / GLONASS<br/>radio channel</i>"]
GND["🌐 ePRTC — «via the ground»<br/><i>terrestrial telecom network<br/>fibre / MPLS</i>"]
TDG{{"<b>TDG</b><br/>quality control<br/>main source selection<br/>holdover"}}
SKY --> TDG
GND --> TDG
TDG ==> PTP["<b>IEC 61850-9-3 PTP</b>"]
PTP ==> DEV["Digital-substation devices<br/><i>IEDs · MUs · digital instrument transformers · PMUs · protection</i>"]
NOTE["Jamming, interference or spoofing<br/>cannot take out both sources<br/>at once — their nature differs"]
TDG -.-> NOTE
style SKY fill:#FFF8E1,stroke:#F9A825,color:#E65100
style GND fill:#E8F5E9,stroke:#43A047,color:#1B5E20
style TDG fill:#E3F2FD,stroke:#1E88E5,color:#0D47A1
style PTP fill:#EDE7F6,stroke:#7E57C2,color:#311B92
style DEV fill:#E0F2F1,stroke:#26A69A,color:#004D40
style NOTE fill:#FAFAFA,stroke:#AAA,color:#555
One time source comes «from the sky», the other «via the ground». This is no longer just redundancy of clocks, but redundancy of the very nature of the time source.
The key idea
GNSS remains a convenient and important source of precise time. But for a digital substation it is dangerous to treat the global navigation satellite systems as the only «source of truth» for time.
An ePRTC- and TDG-based architecture offers a different approach: precise time stops being a function of a separate GPS receiver on the substation and becomes part of the utility's overall technological infrastructure.
And it is possibly in this direction that typical solutions for digital substations will evolve: from local GNSS clocks — towards a resilient, distributed and monitored precise-time infrastructure.