Teleprotection and 87L over IP/MPLS: what changes

Short answer

When migrating from SDH to a packet network, teleprotection and 87L line current differential lose the fixed, symmetrical delay of TDM circuits. Delay variation appears and, above all, asymmetry between the go and return paths, which in a channel-synchronized 87L turns into false differential current. It is solved with deterministic, bidirectional paths along the same route, strict priority, properly sized circuit emulation, or external time synchronization.

Why the migration changes teleprotection

For decades, teleprotection and line differential traveled over direct fiber or SDH multiplexers. SDH is a time-division multiplexing (TDM) network: each service has a fixed time slot, the delay is constant and, on a normal route, equal in both directions.

Utilities are replacing SDH with MPLS-based packet networks, which carry SCADA, voice, video, corporate data and teleprotection over the same infrastructure. In a packet network the delay is no longer fixed: it depends on queues, route and load. For SCADA that does not matter. For protection, it does.

What teleprotection requires

IEC 60834-1 assesses a teleprotection system by three parameters that compete with each other:

Security: not receiving false commands.
Dependability: not losing genuine commands.
Transmission time: the command must arrive in time.

A packet network affects all three: packet loss, errors and, above all, variable delay. The teleprotection equipment and teleprotection schemes guides explain what is sent and why timing matters.

Latency, delay variation and asymmetry

Latency: the total end-to-end delay. It adds up fiber propagation, the processing in each node, queuing and, with circuit emulation, packetization and the buffer.
Packet delay variation: how much the delay changes from one packet to the next. The receiver absorbs it with a buffer, which in turn adds delay.
Asymmetry: the difference between the go and return delays. It appears when each direction takes a different route, for example after a protection switchover that moves only one of the two directions.

Why asymmetry is critical for 87L

Line current differential 87L compares the currents at both ends at the same instant. To align the samples, many relays measure the round-trip time of the message and assume each direction takes half. If go and return take different times, the alignment error is half the difference. At 60 Hz, each millisecond of alignment error equals 21.6 degrees (18 degrees at 50 Hz). That angle error shows up as differential current proportional to the load and can cause a trip with no fault.

The alternatives are to guarantee symmetry through the network, or to synchronize the relays with an external time reference (GNSS or PTP) at each end, which removes the dependence on symmetry. The field symptom is described in 87L trip on channel failure.

MPLS-TP versus IP/MPLS

Aspect	MPLS-TP	IP/MPLS
Path configuration	Static, from a management system	Dynamic, with routing and signaling protocols
Go and return	Bidirectional paths along the same route	Each direction can take a different route unless pinned
Supervision	Built into the path (OAM)	Available, with more options
Behavior	Similar to SDH: deterministic	Flexible; needs traffic engineering to be deterministic
Typical use	Utility transport networks	Carrier and corporate networks

Neither is good or bad in itself. What protection requires is a deterministic path, with go and return along the same route and a protection switchover that moves both directions together.

Circuit emulation

Existing relays and teleprotection equipment usually have TDM interfaces: IEEE C37.94, G.703 or contacts. To carry them over a packet network, the edge device packetizes the bit stream and the device at the other end rebuilds it at the same rate. This is called circuit emulation.

What needs attention:

Packet size: small packets give less packetization delay, but more header overhead.
Jitter buffer: it must absorb the delay variation without adding more delay than needed, and be the same in both directions.
Clock: the rebuilding needs the same frequency at both ends, recovered from the network or from a common reference.
Strict priority for protection traffic over everything else.

What to check when moving an 87L onto a packet network

Is the 87L synchronized by external time (GNSS or PTP) at both ends?
- YesCheck the total latency and the availability of time at each end
- No, it aligns through the channelDoes the network guarantee go and return along the same route, also after a switchover?
  - YesSupervise asymmetry in the relay and set delay alarms
  - NoConfigure bidirectional paths along the same route, or move to external time synchronization

What the standards say

IEC 60834-1 sets the performance of command-type teleprotection: security, dependability and transmission time.
IEC TR 61850-90-12, the IEC 61850 wide area network engineering guideline (Edition 2, 2020), covers communication between substations and with the control center: topology, redundancy, latency and quality of service, traffic management, synchronization and security.
IEEE C37.94 defines the optical interface between the relay and the multiplexer.
MPLS-TP is an MPLS profile developed by the IETF with ITU-T for transport networks; its requirements (RFC 5654) include bidirectional paths.

Common mistakes

Migrating 87L without measuring asymmetry after every possible switchover.
Main and backup teleprotection over the same packet network and the same fiber.
Misconfigured priority, which puts protection in the same queue as video.
Different buffers in each direction in circuit emulation.
Testing only under normal conditions, without switching the network during the test.

Teleprotection engineering with Tiza Group

Moving teleprotection onto packets is decided with numbers: delay, asymmetry and behavior on each switchover. Our team specifies the network requirements for protection as part of our communications engineering service, remotely, and reviews the channel, alarm and synchronization settings of 87L relays through remote relay configuration, worldwide, at high and extra-high voltage. End-to-end testing is done with our end-to-end tests, in Colombia, or elsewhere on request. Write to us.

References

IEC 60834-1: teleprotection equipment of power systems, performance and testing, Part 1: command systems.
IEC TR 61850-90-12:2020, Wide area network engineering guidelines.
IEEE C37.94-2017: N × 64 kbit/s optical interface between teleprotection and multiplexer equipment.
IETF RFC 5654: requirements of MPLS-TP.
ITU-T G.707: SDH (background).

Frequently asked questions

What is the difference between MPLS-TP and IP/MPLS for teleprotection?

MPLS-TP is an MPLS profile for transport networks: statically configured, bidirectional paths with both directions along the same route and built-in supervision, which resembles SDH behavior. IP/MPLS uses a dynamic control plane that can recompute routes; it can serve teleprotection if explicit, symmetrical paths are set and switchover is controlled.

Why does asymmetry affect 87L and not so much a distance scheme?

Because 87L compares currents sampled at both ends and must align them in time. If it does so by measuring the round-trip time and assuming each direction takes half, any difference between directions becomes an angle error. A distance scheme with teleprotection only sends a command: what matters is the total delay, not how it is split between go and return.

Can an IEEE C37.94 interface be carried over a packet network?

Yes, with circuit emulation: the edge device packetizes the N × 64 kbit/s stream and the device at the other end rebuilds it. The cost is additional delay from packetization and from the jitter buffer, which must be sized and supervised.