How to calculate relay settings for IEC 61850-9-2 Sampled Values Channel Latency

Network latency is a measurement of delay in a system. Latency accounts for processing delays, network queuing delays, and propagation delays in the channel. A simple Ethernet-based remote data acquisition system using IEC 61850 Sampled Values (SV) technologies is shown in the figure below. A Merging Unit (MU) is connected to a network through its communications processor interface (CP) and a protection relay connected to the same network.

This remote data acquisition scheme introduces delays and directly impacts the performance of the protection relay. The SV Network Delay (SVND) is the sum of the MU processing delay and the process bus network delay.

IEC 61850-5 standard defines the time requirements for latency in protection systems as the transfer time, TT6 transfer time is desirable (3ms latency) but only accounts for the process bus network delay. To define the overall latency, the MU processing delay also needs to be considered. Therefore, latency for an IEC 61850-9-2 data acquisition can be formulated as:

SV_Latency = MAX(SVND) + (N + 1) * TS

Where:

1. MAX(SVND) is the maximum SV network delay for all subscribed streams.
2. N is the number of lost packets that the DSS relay is able to ride through by interpolating missing packets. *(N + 1) is used as a sampled safety factor.
3. Ts is the sampled period (4.8 kHz for 60Hz, and 4.0 kHz for 50Hz using 9-2LE version).

Figure below shows two measured SVND over time from two different MU manufacturers, MU-A and MU-B

The maximum network delay recorded was 662.5 μs from MU-A and 702.75 μs from MU-B. Therefore, if the measurement is taken from the reception device, the MAX(SVND) can be considered as the overall process bus network delay (processing delay plus propagation delay).?

The N is a safety factor introduced by the subscriber interpolation capabilities, and it is used to ride through missing sample situations and packet losses. Therefore, it is essential to balance N with the protection system time performance; for example, N = 3 causes an additional 4-sample delay (N + 1). The MU profile characteristic defines ts; for IEC 61850-9-2 LE at 60 Hz, the sampling rate is 4.8 kHz. Therefore, Ts is 208.3 μs. The table below summarizes the overall SV latency for using MU-A and MU-B. Latency doubles in case of IEC 61869-9:2016 compliant 4.8 kHz stream which packs two data points into one SV frame, for a publishing rate of 2.4 kHz.

The SV network latency can be used as a relay setting that defines the maximum latency acceptable by the protection system. Therefore, in a system that uses MU-A and MU-B, the protection relay latency setting should be set to a value greater than 1.536 ms to accommodate the overall network delay and ride through four consecutive packet losses.

Other measurements like jitter and bandwidth must also be logged into the SV digital system performance analysis account to better design a digital substation. If you want to learn more about those metrics, take a look at the reference below or add a comment, and I will be happy to discuss.

Reference:

A. Shrestha, M. Silveira, J. Yellajosula S. Mutha - Understanding the Impacts of Time Synchronization and Network Issues on Protection in Digital Secondary Systems - Presented at the PAC World Global Conference 2021 Virtual Format August 31–September 1, 2021