Content for  TR 22.867  Word version:  18.2.0

Line current differential protection (defined as 87L in IEEE C37.2-2008 [3]) has been widely used in electrical transmission systems to protect High-Voltage (HV) transmission lines. As a proven protection mechanism, it is also deployed for power distribution networks to protect (Medium-Voltage) MV distribution lines where applicable. The popularity of line current differential protection comes from the fast protection mechanism, reliability and the absolute selectivity of protected zones. Therefore, for Low-Voltage (LV) and MV power lines (both underground and overhead), current differential protection could be deployed easily with cellular technology without having to lay dedicated communication cables, either in refurbishment or new distribution substation construction projects. The mechanism of line current differential protection follows the Kirchhoff's current law, which is that the sum of currents at a junction of a circuit equals to zero. As illustrated in Figure 5.4.1-1, two protection relays deployed at two substations form the protection zone, within which the power line is protected from incidents such as short circuit. Each protection relay continuously measures its local current and transmits it towards the other. Each protection relay compares the locally measured current and the current received from the remote relay to calculate the differential current at a specific instant of time. Figure 5.4.1-1 shows two communication channels (illustrated as dashed arrow boxes) between the two protection relays. Here in this contribution the "communication channel" refers to the channel used for transferring the phase segregated current value data between the two protection relays. The current phasors from the two protection relays, deployed geographically apart from each other, should be aligned in time for the current differential algorithm to execute correctly. For Relay_a, at a given moment the local current is I_a'_Tx, and the time-aligned remote current from Relay_b is I_b'_Rx. Using them as input, the protection algorithm in Relay_a derives the differential current. The same mechanism functions in Relay_b. Whenever the differential current exceeds the threshold values as determined by the relay restraint characteristics, the relay will send a trip command to the circuit breaker (XCBR) to open the circuit, thus protecting the power line from being burnt down and any secondary damages a fireball blaze on the power line can cause.

The protection function of the protection relay depends on three things:

1. Sampling, buffering and transferring local current.
2. Receiving the sampled current values from the remote protection relay.
3. Time synchronization of the two protection relays - performing time-alignment of the locally buffered current samples with received remote samples.

In terms of sampling, a protection relay needs to sample the local current periodically, and transfers sampled data within a pre-defined time period T. In other words, the communication latency should not exceed T. Max of T is specified in IEC 61850-90-1 [4] to be between 5ms and 10ms, which infers the latency requirement for this use case. Secondly, once the buffered samples pertinent to the same instant in time are available, the relay must align them in time. As a relay needs to perform correct alignment of local and received data before calculating the differential current, the relay needs to know well enough when the remote relay transmits a specific data packet. Current clock synchronization is realized by relays attaching timestamps to measurement samples before transmission. A modern relay with an Ethernet interface normally needs to resort to IEEE 1588 Precision Time Protocol [5] for synchronization, since the relay assumes the Ethernet network to be non-deterministic. Regarding time alignment of local and received remote data, two methods exist, namely the method to use external time source such as GNSS, or "channel-based" alignment method. Due to various reasons in some smaller substations a GNSS receiver is not available. Even for a substation installed with a GNSS receiver, relays shall fall back to channel-based alignment for time synchronization, should GNSS become unavailable. For this reason, support of the "channel-based alignment" is the focus of the proposed use case. Different from GNSS-based alignment that is not adversely affected by communication channel asymmetry, the channel-based alignment is critically dependent on channel symmetry - near equal latency in transmission and reception directions between two protection relays respectively. Currently in the Smart Grid automation market, the max communication channel asymmetry is dependent on the chosen type of differential protection relay and is vendor-specific. For instance, an old-fashioned TDM-based differential protection relay is more sensitive to asymmetry than a modern type differential protection relay with an Ethernet interface. The latter can deal with asymmetry till 3ms, above which the relay will enter blocking mode. According to the IEEE C37.243 Guide [6], the asymmetry in terms of communication channel latency is around 2ms. From here on, focus is on how to satisfy channel-based alignment requirements using services from 5G system.

Typically in a distribution grid, a MV power line transmits electricity between two substations. Two protection relays are installed at both ends of the power line. Relay_a continuously samples and measures the local current I_a' and sends it to Relay_b, so does Relay_b.

The abnormal condition of the power line in the protected zone is duly isolated from the electrical grid.

The communication mechanism is partly covered by existing 5G functionalities. Support of IEEE 1588 PTP is an existing feature. Additional traffic from running IEEE 1588 PTP is around 0.004 Mbit/s. In TR 22.804 there is attempt to touch upon the similar case, where the clock synchronization accuracy ≤ 10 μs, and latency requirement is 15 ms.

[PR.5.4-001] [PR.5.4-002.option1] [PR.5.4-002.option2] [PR.5.4-002.option3]