protocols · 7 min read

Power Profile IEC 61850-9-3 vs IEEE C37.238 compared

Power Profile IEC 61850-9-3 and IEEE C37.238 are the two PTP profiles for digital substations. What distinguishes them, and how do you choose the right grandmaster for your power grid?

Power grids place different demands on time synchronisation than data centres do. A differential protection on a high-voltage transformer must know on both sides that a fault current occurred at exactly the same moment, otherwise the protection trips either unfairly or too late. A synchrophasor measurement (PMU) loses its value if the timestamp drifts. For those scenarios two specialised PTP profiles exist: IEC 61850-9-3 (international, IEC stewardship) and IEEE C37.238 (originally North American, IEEE stewardship). This article compares them and gives a concrete selection guide for TSOs, DSOs and system integrators.

Why does standard PTP not suffice in substations?

A regular IEEE 1588 Default Profile grandmaster delivers sub-microsecond time to an Ethernet network. With that you go a long way in IT environments. In a substation three additional requirements apply that the Default Profile does not address explicitly.

Tight accuracy requirement for all relays. Banerjee & Matsakis (2023, section 14.2.1) on power grid management: *"the many geographically separated sources of electrical power must be kept in phase to a small fraction of their period (16 ms for 60 Hz, and half that for 120 Hz) despite variations in the strength of their sources and the load on the grid. Failure to align the phases will waste energy at best and bring down the grid at worst. For this purpose synchrophasors are a key element and their requirement is at the sub-microsecond level."* For fault localisation: *"timing at the 100-500 ns level is needed to identify the point of failure."* Redundancy without spanning tree convergence. Substations do not use classical Spanning Tree Protocol (STP). A fault-detection time of 50 ms is unacceptable in a substation. Instead: HSR (High-availability Seamless Redundancy) or PRP (Parallel Redundancy Protocol), duplicate packet injection over two independent rings or paths. PTP must operate on top of this. Interoperability between brands. A substation contains protection relays from ABB, Siemens, GE, Schneider, alongside the time server. If every manufacturer picks its own PTP profile variant, the system falls apart.

The two Power Profiles solve these three requirements, each with its own emphasis.

What is IEC 61850-9-3 Power Utility Profile?

IEC 61850-9-3 is a profile on top of IEEE 1588v2 specifically for IEC 61850 substations. The key aspects:

Accuracy requirement: ±1 µs end-to-end within the substation, often ±100 ns for protection applications.
Boundary clocks mandatory in every substation switch. No ad-hoc transparent-clock-only implementation.
HSR and PRP redundancy as the network layer beneath PTP. PTP must understand both rings.
Peer-to-peer delay measurement instead of end-to-end. Reason: every switch knows its direct neighbours and does not accumulate errors over multiple hops.
Multicast-only over Layer 2 (Ethernet). No UDP/IP encapsulation as in the Default Profile.
Management via SCADA interface: a Power Profile grandmaster often speaks IEC 61850 MMS or GOOSE for management.

The IEC 61850-9-3 stewardship lies with IEC TC 57 (Technical Committee Power System Control and Associated Communications). It is the de facto European choice for digital substations.

What is IEEE C37.238 Power Profile?

IEEE C37.238 (latest active revision 2017) emerged in parallel with IEC 61850-9-3, with the same goals but inside the IEEE ecosystem. The practical differences are small, and modern grandmasters support both at once.

Accuracy requirement: comparable to IEC 61850-9-3, ±1 µs end-to-end.
Boundary clocks required, peer-to-peer delay similar.
TLV extensions specific to power: the grandmaster transmits information about its own oscillator status and holdover condition in every PTP packet. That helps protection relays decide whether the time is reliable.
C37.238-2011 versus C37.238-2017: the 2017 revision improved interoperability with IEC 61850-9-3. Old C37.238-2011 deployments can give trouble; on upgrade always request the 2017 revision.

Comparison table

Aspect	IEC 61850-9-3	IEEE C37.238-2017
Stewardship	IEC TC 57	IEEE Power & Energy Society
Accuracy requirement	±1 µs end-to-end	±1 µs end-to-end
Redundancy requirement	HSR and PRP mandatory	HSR and PRP supported
TLV extensions	basic	extended (oscillator status, holdover info)
Geography	EU-dominant, worldwide in IEC countries	US-dominant, also IEEE-aligned utilities elsewhere
Interoperability	bidirectionally compatible with C37.238-2017	bidirectionally compatible with IEC 61850-9-3

In practice: modern Masterclock grandmasters such as the GMR series support both profiles at once. You configure per substation which profile is active, and the switches align accordingly.

Practical compatibility: how does it work together?

A typical digital substation contains:

1 or 2 PTP grandmasters with GNSS disciplining and OCXO or Rubidium holdover. Two for redundancy.
PTP-aware substation switches with boundary clock functionality, HSR and PRP rings configured.
Protection IEDs (Intelligent Electronic Devices, often ABB REF or REL, Siemens SIPROTEC, GE Multilin) that receive PTP time.
PMUs (Phasor Measurement Units) that report synchrophasors to a PDC.
Merging Units for the process bus that generate GOOSE signals with PTP timestamped sampled values.

The grandmaster must speak the profile that the IEDs expect. A mismatch between "grandmaster says IEC 61850-9-3" and "IED expects Default Profile" leads to non-locked relays, a protection system that formally functions but without guaranteed sync.

Banerjee & Matsakis (2023, section 7.4) name the risks explicitly: even with identical profile claims, brands can be incompatible. Therefore: factory acceptance test (FAT) with all components in the same chain, not just the time equipment in isolation.

What holdover requirements apply in power-utility?

GNSS outage is not an academic concern. Banerjee & Matsakis (2023, section 14.2.3) on telecom, directly transferable to utilities: *"a GNSS disciplined clock capable of maintaining time to 1 microsecond over a 24-h GNSS outage"* is the baseline requirement. For substations that means:

OCXO as minimum oscillator. A regular TCXO drifts within hours outside ±1 µs.
Rubidium for substations without or with disturbed GNSS reception (bunker locations, urban canyon, jamming-risk areas).
Anti-jamming and anti-spoofing in the GNSS receiver. Modern grandmasters offer detection and switchover to alternative GNSS constellations (see GNSS disciplining).

The combination GNSS disciplined + OCXO gives sub-µs holdover for at least 24 hours, covering most practical GNSS outage scenarios.

What to buy as a TSO or DSO?

Three recommendations, in order of importance:

1. Specify both profiles at once (IEC 61850-9-3 and IEEE C37.238). That keeps your substation future-compatible with IEDs from different vendors. 2. OCXO as minimum, Rubidium for critical sites. The extra investment in Rubidium is a fraction of the total substation build cost and pays back at every GNSS event. 3. Request a FAT with your own IEDs. Do not rely on the datasheet alone. A correct vendor does this without argument.

Frequently asked questions

Should I choose IEC 61850-9-3 or IEEE C37.238?

In Europe IEC 61850-9-3 is the dominant choice; in North America historically IEEE C37.238. In practice: pick a grandmaster that supports both profiles at once (such as the Masterclock GMR series), and your substation stays flexible for IEDs from different vendors.

Does a Power Profile grandmaster also work in a regular data centre?

Yes. The profiles are supersets of IEEE 1588 with additional functionality. A grandmaster that supports IEC 61850-9-3 can also speak Default Profile or Enterprise Profile for IT deployments. Backwards-compatible.

How big is the accuracy difference between Default Profile and Power Profile?

Theoretically identical (both based on IEEE 1588, both sub-µs capable). In practice it differs because of mandatory boundary clocks in Power Profile, peer-to-peer delay, and HSR and PRP redundancy. The Power Profile guarantees end-to-end ±1 µs in a substation environment, the Default Profile only guarantees that under ideal circumstances.

What is a synchrophasor and why is timing so critical for it?

A Phasor Measurement Unit (PMU) measures the magnitude and phase of voltage and current at a network node. By synchronising PMUs across an entire grid with sub-µs accuracy, a grid operator can see phase differences in real time and detect instability. A drift in timing introduces an error in the measured phase angle and makes the PMU data unusable.

What happens during a GNSS outage in a substation?

The grandmaster switches to holdover via its internal oscillator (OCXO or Rubidium). With OCXO timing stays within ±1 µs for at least 24 hours. With Rubidium for weeks. During holdover the grandmaster communicates its status in PTP TLVs so IEDs know that the source is running on internal reference. For sites with chronic GNSS outage (bunkers, urban canyon, jamming risk) Rubidium is recommended.