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Content for  TR 22.867  Word version:  18.2.0

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5.4  Use case of line current differential protection in power distribution gridWord‑p. 24

5.4.1  DescriptionWord‑p. 24

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.
Copy of original 3GPP image for 3GPP TS 22.867, Fig. 5.4.1-1: Line Current Differential Protection by two protection relays (Relay_a and Relay_b), deployed in two substations
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.
Option 1:
Per existing protection relay algorithm implementation, channel-based alignment presumes the delay in each communication direction to be (nearly) half of the RTT. If 5G system provides this condition, existing relay algorithm can be reused. According to IEEE C37.243 Guide, 2ms can be required as the max communication channel latency asymmetry between the two protection relays. Below are some additional details how protection relays performs channel-based alignment:
Relay_a attaches a timestamp to the transmitted measurement data, Relay_b receives the timestamp from Relay_a and re-attaches the same timestamp to the next out-going data packet towards Relay_a. By receiving the original timestamp in return packet, Relay_a determines the RRT by subtracting the present local time with the returned timestamp. Halving the RTT, Relay_a obtains the amount of time shift/alignment it shall apply to the current samples received from Relay_b. Therefore, it is required that the communication channel from Relay_a to Relay_b incurs near-equal latency as the channel from Relay_b to Relay_a. Following this approach, excessive communication channel asymmetry between Relay_a and Relay_b will lead to misalignment of currents (such as the I_b'_Tx and I_a'_Rx at Relay_b in Figure 5.4.1-1), manifesting in phase shift. This will result in increase or decrease of the apparent differential current, causing blocking of the protection or in the worst case a false trip and further negatively impact Smart Grid availability and reliability.
Option 2:
Alternatively, instead of requiring the communication channels (from Relay_a to Relay_b, and from Relay_b to Relay_a) to be highly symmetrical regarding latency, a different approach could be proposed as a new 5G service to improve protection relay design by the Smart Grid OEMs. To achieve the same goal as for Relay_a to know how much it needs to time shift the received current samples from Relay_b to align with its local current, it is sufficient if the 5G system could provide such a protection relay with the latency of the relevant communication channel (latency from Relay_b to Relay_a for Relay_a, and latency from Relay_a to Relay_b for Relay_b) with good confidence/precision. This provided latency value could either be estimated or assigned by the 5G system. In this way, the channel latency information is directly provided to relays by the 5G system, a protection relay does not need to carry out its own estimation. This could open new possibilities for the protection relay manufacturers to design new and possibly simpler time-alignment algorithms.
Option 3:
Using the existing IEEE 1588 time master of the NG-RAN. In this case, the complexity could be the use of the IEEE 1588 power/utility profile (a.k.a. IEC 61850-9-3 [7]) instead of using the telecom profile.

5.4.2  Pre-conditionsWord‑p. 26

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.

5.4.3  Service FlowsWord‑p. 26

Step 1.
Relay_a samples local current values I_a', stores them locally and sends them to Relay_b periodically. Timestamp is attached to the sampled values to help Relay_b match the data correctly.
Step 2.
Relay_b samples local current values I_b', stores them locally and sends them to Relay_a periodically. Timestamp is attached to the sampled values to help Relay_a match the data correctly.
Step 3.
Relay_a receives samples from Relay_b within the latency required by IEC 61850-90-1. Depending on the applied voltage levels, the max allowed latency is between 5ms and 10ms. Relay_a stores the received samples in a local buffer.
Relay_b receives samples from Relay_a within the latency required by IEC 61850-90-1. Depending on the applied voltage levels, the max allowed latency is between 5ms and 10ms. Relay_b stores the received samples in a local buffer.
Step 4.
Inside both Relay_a and Relay_b, a microprocessor decides that all the relevant data for a same instant in time are collected. The Relay then aligns these data and uses the algorithm to calculate the differential current for this time instant.
Step 5.
Differential current calculated at both Relay_a and Relay_b stays in the restraining region (below threshold). None of the relays trips. The system continues to function in normal condition.
Step 6.
(Example incident) Suddenly, a strong wind blows down a tree branch, which during the fall with its additional weight brings down the overhead distribution line close to the ground. The voltage of the power line causes an electric discharge with objects on the ground, causing spark leakage. This discharge causes current from both substations to flow with increased magnitude into the power line.
Step 7.
Since both the relays are still measuring the current and sends the sampled values to each other. Relay_a detects from a very instant in time, the differential current exceeds the threshold. Relay_a triggers a trip signal to the connected circuit breaker.
Step 8.
Circuit breaker opens, stops current from flowing into the power line to cause more serious damage.

5.4.4  Post-conditionsWord‑p. 27

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

5.4.5  Existing features partly or fully covering the use case functionalityWord‑p. 27

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.

5.4.6  Potential New Requirements needed to support the use caseWord‑p. 27

The 5G system shall support an end-to-end latency of less than 5 ms or 10 ms, depending on the applied voltage level. Here the end-to-end latency is between two UEs including two wireless links.
The 5G system shall support communication channel symmetry in terms of latency (latency from UE1 to UE2, and latency from UE2 to UE1) between the two relays, with the max asymmetry < 2 ms.
The 5G system shall provide a UE with communication channel latency from the remote UE, with an accuracy of the provided latency < 1 ms.
The 5G system shall provide the protection relay with timing information with the comparable precision as GNSS-based precision. The IEEE 1588 time master in NG-RAN should provide protection relays with IEC 61850-9-3 based power/utility profile

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