Research on Improvement of Data Synchronization Method for Distribution Network Differential Protection Based on 5G

Abstract

To enhance the reliability of traditional current protection systems in the evolving landscape of future power grids, this paper proposes a differential protection data synchronization approach that leverages satellite and 5G network timing. This method addresses the risk of inadvertent operation due to 5G communication jitter. It employs the BeiDou satellite system for synchronizing the timing of 5G base stations and terminals. This synchronization extends to the 5G terminal and the protection device, thereby mitigating time deviations caused by air interface delays and reducing the 5G network’s end-to-end delay by approximately 4 ms on average. A simulation was conducted to assess the reliability and effectiveness of the proposed method under normal satellite signal conditions. The results indicated a significant improvement in protection accuracy, with the method reducing the protection activation time by approximately 5–7 ms compared to the 5G timing data synchronization method. This advancement offers technical support for the safe and stable operation of new power systems.

1. Introduction

In recent years, the increasing integration of distributed power sources, such as photovoltaic and wind power generation, into the distribution network has significantly impacted network protection. The complex topology and numerous nodes within the distribution network complicate short-circuit current calculation. The differential protection method, based on dual terminal quantity, offers a solution to these challenges. However, this method demands high-quality communication channels, and the installation of fiber optic communications is both complex and expensive. Additionally, the existing Synchronous Digital Hierarchy (SDH) communication network is strained by limited routing resources. This issue particularly affects new energy plants and load stations at the periphery of the SDH network, which suffer from considerable impact [1]. The time delay in the 4th generation mobile communication technology (4G) wireless private networks is notable [2], necessitating an evolution towards 5th generation mobile communication technology (5G) networking [3,4]. The International Telecommunication Union has identified eight key performance indicators for 5G [5], with high-speed, low latency, and large connectivity being its most significant features [6,7,8]. In scenarios requiring ultra-high reliability and low latency communication, 5G can achieve peak transmission rates of 20 Gbps and end-to-end delays theoretically below 10 ms. Reference [9] examines the communication channel requirements for differential protection and the performance indicators of 5G, confirming the feasibility of employing 5G as a differential protection channel in distribution networks. Although 5G communication has the potential to supplant optical fiber as a new differential protection communication channel, its synchronous timing capability introduces risks to data synchronization due to the wireless medium. Therefore, time synchronization is crucial.

Currently, numerous researchers are investigating the issue of 5G timing, focusing on methods to reduce end-to-end latency. Reference [10] outlines a high-precision time synchronization model for 5G, and describes the need for time synchronization in differential protection scenarios, but does not propose a solution to the delay jitter problem. Reference [11] discusses a high-precision time service for 5G, including time synchronization methods between base stations and terminals. This approach, however, necessitates calculating air interface delay, which involves extensive computation and is prone to errors. Reference [12] suggests a method for obtaining delay measurements without direct calculation, although this technique is limited to specific distribution network models and data from both ends. Reference [13] enhances the rapid response of differential protection in a 5G environment by optimizing its operational logic, but does not consider addressing the latency that exists in 5G itself. Further, references [14,15,16,17] introduce advanced algorithms such as improved dynamic time warping, particle swarm optimization, and correlation coefficient algorithms to reduce communication delays and jitter. These methods, however, must balance algorithmic accuracy with computational complexity. The principle of differential protection mandates simultaneous current sampling at both ends of a line and concurrent data sampling for differential calculation. To achieve data synchronization effectively, the initial step is to ensure uniform network time with minimal error.

Based on the above analysis, there is still a certain lack in the use of 5G itself to achieve timing, due to the existence of the time delay that leads to data asynchrony. The existing data synchronization method is not suitable for the differential protection of a distribution network under 5G communication, so it is necessary to study a data synchronization method that uses 5G timing and is suitable for a distribution network. In this paper, the satellite combined with a 5G network timing method was used to send the time information to the 5G base station and the terminal, then the 5G terminal and the protection device were synchronized to achieve time synchronization, then the sampled data were labelled with the time sequence number, and the data with the same time sequence number transmitted to the opposite side were considered to be synchronized. This ignored the time deviation introduced by the air interface delay to the base station and the terminal. A simulation model was built in Matlab/Simulink; after analysis, the method could shorten the protection action time by 5–7 ms, reduce the scope of power outage, and could effectively improve the protection false action.

2. The Combination of Differential Protection and 5G

2.1. Differential Protection under 5G Communication

Traditional differential protection predominantly relies on optical fiber for its communication channel. However, the integration of wind, solar, and other energy sources as distributed power sources into the distribution network results in a complex topology with numerous nodes, thereby significantly increasing the cost of laying optical fibers. The 5G communication network, characterized by its large connectivity, low latency, and high speed, has emerged as a viable alternative. In terms of speed, 5G’s peak bandwidth ranges between 10–20 Gbps, with a typical user experience speed of 50–100 Mbps. For low latency, 5G employs ultra-reliable low-latency communications (uRLLCs)-slicing customization technology, complemented by MEC mobile edge computing and user plane function (UPF) sinking [18,19]. Additionally, the use of D2D transmission mode facilitates direct device-to-device communication, shortening the propagation path. Furthermore, the 3GPP R16 standard incorporates time-sensitive networking (TSN), a time-sensitive Ethernet data transmission mechanism defined by the IEEE 802.1 working group, characterized by low latency and low jitter, into 5G. The R17 standard proposes an enhanced TSN architecture [20]. The seamless integration of TSN wired networks with 5G wireless networks [21,22] further reduces latency, achieving an end-to-end latency of approximately 15 ms in 5G networks. Regarding connectivity, 5G can support up to 1 million devices per square kilometer. The network-slicing technology of 5G divides a single physical network into multiple independent end-to-end logical networks [23], enhancing both security and reliability. The performance indicators of 5G communication align well with the requirements of differential protection communication channels, suggesting its potential to replace optical fiber as the information transmission medium for distribution network differential protection.

The communication architecture depicted in Figure 1 illustrates the process of ensuring protection at both ends of a network; where I_M and I_N denote the amount of power flow on both sides, respectively, and the vector sum of the two indicates that the fault occurs outside the zone f₂ if it is 0, and if it is a constant that is not 0, it indicates that the fault occurs inside the zone f₁. This is achieved by synchronously sampling current information through current transformers. The sampled data is then transferred to the 5G CPE terminal via Ethernet. From the CPE terminal, it is transmitted to the 5G base station. Through the core network’s interaction, local electricity flow information is relayed to the opposite side. The differential calculation results are utilized to ascertain whether a fault has occurred within the protected area, thereby determining the operational status of the protection system. A critical aspect of this process is the synchronization of current data sampled at both ends, which necessitates precise time synchronization at the moment of sampling.

2.2. Synchronization Method for 5G Timing

The 5G high-precision timing technology, as illustrated in Figure 2, comprises two primary components. Firstly, time synchronization is established using BDS/GPS satellite signals. This is achieved by acquiring timestamp information from the satellite receiver, which is then transmitted to the 5G time synchronization network. Such a process enables high-precision time synchronization between base stations with a time accuracy of 1.5 μs. Secondly, time information is disseminated to each CPE terminal via the air interface using broadcast frames from the base station. The CPE is tasked with calculating the air interface delay to adjust the time accurately. Subsequently, the corrected time is relayed to the protection device through wired timing.

The timing method described above allows protection devices to acquire unified time information. Following synchronous sampling, the sampled data is then transmitted to the opposite end for processing. However, the wireless medium between 5G base stations and CPE terminals remains a significant factor influencing time synchronization. Currently, 3GPP is working on integrating TSN with 5G communication to reduce latency. Despite this, challenges persist, such as the unpredictability of wireless channel changes [24]. The prevalent methods for cross-network time synchronization also encounter difficulties in measuring errors accurately and in the impact of air interface delay on timestamp precision. While the integration of these technologies has enhanced time precision, the issue of air delay persists. This could potentially affect data synchronization and lead to malfunctions in protection operations. Consequently, it is suggested to combine BeiDou satellite technology with 5G to obtain more precise time-scale information, thereby achieving more accurate data synchronization.

3. Consider Improving Time Synchronization by Combining Satellites with 5G Networks

3.1. Combination of Satellite and 5G

In alignment with national security, economic development, and social needs, China, by the end of 2020, had successfully launched 35 Beidou-3 satellites and established a Beidou satellite navigation system that integrates active and passive methods. This system offers a global timing accuracy of 20 ns. Each Beidou satellite is equipped with a spaceborne atomic clock, utilizing miniature rubidium and hydrogen atomic clocks, which confer the advantage of high-frequency stability [25]. This ensures that the Beidou time service system provides a highly accurate time source. As depicted in Figure 3, the time synchronization device and the carrier device typically employ the 1588v2 time synchronization interface protocol. For achieving high-precision timing between the carrier device and the application device, the 1PPS+TOD time synchronization interface protocol is utilized.

The 1588v2 protocol features an independent message mode, specifically designed for point-to-point path delay measurement. This measurement solely utilizes the PDelay_Req data frame series. This protocol represents an autonomous delay measurement process, independent of sync frames and the synchronization system establishment. Such an approach enhances measurement precision. By averaging multiple measurements, it is possible to obtain a more accurate path delay estimation.

As shown in Figure 4, the time delay between AB is

$$t_{AB}=T_2-T_1$$

(1)

The time delay between BA is

$$t_{BA}=T_4-T_3$$

(2)

Assuming that the average path delay time between ports AB is the same, the path delay time between ports AB is

$$t_{delay}=\frac{(t_{AB}+t_{BA})}{2}=\frac{T_4-T_1-(T_3-T_2)}{2}$$

(3)

The navigation satellite transmits and broadcasts a signal containing accurate standard time information. Receivers utilize this signal to calculate and process the clock difference between themselves and the satellite, employing the aforementioned method. This process allows for the correction of local time and the completion of timing. As depicted in Figure 5, regular users obtain precise time information through the 1PPS+TOD protocol.

The 1PPS (one pulse per second) signal conveys second pulse information, where its rising edge marks the commencement of a one-second interval. The rise time for 1PPS is specified to be less than 50 ns, and its bandwidth ranges from 20 to 200 ms. TOD (time of day) provides absolute time information, operating at a default baud rate of 9600 without parity checking. The start bit in the TOD signal is set to low, while the stop bit and idle frame are set to high. The idle frame encompasses 8 data bits. TOD information transmission begins 1 ms after the rising edge of the 1PPS signal and completes within 500 ms. This TOD message specifies the current time corresponding to the rising edge trigger of the 1PPS. TOD messages are transmitted at a frequency of once per second.

In light of the aforementioned methods, it is feasible to integrate the Beidou satellite with 5G communication technology. In this setup, 5G base stations and terminals act as carrier devices, while protection devices serve as application devices. As depicted in Figure 6, time information is initially received through the Beidou antenna and subsequently transmitted to the time server. This time information is then relayed to the 5G base station and CPE terminal via the 1588v2 protocol. Following this, the CPE terminal and protection device are synchronized using the 1PPS+TOD protocol. This approach ensures consistent time scales for protecting the sampled current data on both sides. Consequently, the uncertain air interface delay between 5G base stations and CPE terminals becomes negligible, thereby optimizing the timing aspects of relay protection.

This paper presents a method applied to differential protection in distribution networks, employing a specific process as depicted in Figure 7. The method encompasses the following steps:

Step 1: Accurate time information is transmitted to the 5G base stations and terminals via the Beidou satellite. This information is then synchronized with the protection device, ensuring temporal synchronization on both sides of the device.

Step 2: Both sides of the protection device are set to sample current data at precisely the same time. The sampled data is tagged with a time sequence and subsequently transmitted to the 5G base station via the CPE terminal.

Step 3: A differential calculation is performed between the current data information received by one side through the 5G base station, which bears time serial numbers, and the current data information with matching time serial numbers sampled on the local side.

By integrating satellite and 5G timing methods, the air interface delay between 5G base stations and CPE terminals becomes negligible. This integration effectively circumvents the potential asynchrony in differential protection sampling, which is typically caused by the randomness and uncertainty of these delays. Such asynchrony can result in protection misoperation or failure to operate, thereby indirectly safeguarding the stability of the power grid.

In fact, when the method proposed in this paper is used, the problem of satellite signal loss due to the complexity of the environment also needs to be considered. At this time, it can be solved from two aspects; on the one hand, the BeiDou satellite and GPS can be used as a replacement for each other, i.e., when the BeiDou satellite signal cannot be received, the GPS can be used for timing, and vice versa. On the other hand, we can consider setting a self-timing circuit through the microcontroller [26]; when the satellite signal is lost, the accuracy of timing is still guaranteed.

3.2. Delay Analysis before and after Synchronous Improvement

The end-to-end delay distribution of differential protection in a 5G environment, as depicted in Figure 8, encompasses protection devices, CPE terminals, base stations, and core networks [27,28]. The components of this delay primarily include the delay between protection devices and CPE terminals, the air interface delay between base stations and CPE terminals, and the transmission delay within base stations [29]. Factors such as transmission interval, resource planning, mixed retransmission, terminal processing, and base station processing influence the air interface delay [30]. The target end-to-end latency index for 5G communication is set at 1 ms [31], yet the average latency remains in the range of tens of milliseconds. The air interface between the base station and the CPE terminal is subject to notable randomness and uncertainty, which are key contributors to latency and jitter. In China, strategies like optimized frame structures and edge computing technologies are predominantly employed to minimize the end-to-end delay. Within the realm of differential protection applications, numerous algorithms are utilized to reduce the entire protection action’s delay, aiming for reliable protection. Although current methods somewhat mitigate delay jitter, these issues are still present.

The implementation of an enhanced time synchronization method effectively addresses the issue of air interface delay between 5G base stations and CPE terminals. By leveraging satellite synchronization for time transmission to these base stations and terminals, the air interface delay can be reduced to zero milliseconds. This significant reduction in end-to-end delay consequently shortens the time required for protection actions.

4. Simulation Verification

In this article, a simulation model was established using Matlab/Simulink, as depicted in Figure 9. The model selected a segment of the 10 kV distribution network, spanning a total length of 7 km. It utilized standard parameters for overhead lines and power sources at both ends to verify the system’s reliability and speed performance. The initial phase angle for the left power supply was set at 0°, while the right power supply was set at 15°. The model included two short-circuit points: f1, representing an internal fault, and f2, denoting an external fault. The load capacity was established at 1MW. The simulation incorporated the 5G channel transmission as illustrated in Figure 2 and Figure 10, using the 5G toolbox, and it was configured based on the end-to-end delay analysis presented in Figure 8.

This paper compared the time limit of protection actions after sampling data from both ends using the 5G high-precision timing method against the improved method proposed herein. The example chosen was a three-phase short-circuit fault in the area, as depicted in Figure 11. In this figure, subfigures (a) and (b) represent the current waveforms obtained by the M-side and N-side protection devices at the same sampling time, respectively. The graph demonstrates that there was no deviation in the sampling time, thereby ensuring synchronous sampling. On both sides, the three-phase currents exhibited opposite directions and unequal amplitudes. The instantaneous current values were extracted and transmitted to the opposite end through the 5G network for differential calculation. This process ensured that the protection would operate correctly.

When a fault occurs outside the protected area, the presence of time delay can lead to protection misoperation. Figure 12a,b illustrate the current sampling waveforms on both sides of the zone during such external faults. Analysis of these waveforms reveals that the three-phase short-circuit current values were unequal at the same time. Subsequent differential calculations yielded non-zero results, causing repeated tripping and closing of the protection system. As depicted in Figure 12c, the transition from 1 to 0 denoted a single protection action. Misoperations within the area can compromise the stability of the power system, result in economic losses, and may even pose threats to personal safety.

Upon implementing the method proposed in this article, the waveform of current sampling within the area during external faults is depicted in Figure 13a,b. Concurrently, the values of the three-phase short-circuit current were found to be equal, resulting in a differential calculation outcome of 0. This outcome signifies that the protection within the area was reliable and did not activate in response to an external fault.

When a fault occurred within the area, the sampling waveforms of various other types of fault currents could be analogized to those depicted in Figure 11. In the following analysis, a comparison will be made specifically on the protection action time after employing the method proposed in this article across multiple fault types. This comparison is illustrated in Figure 14.

Figure 14 illustrates the action time limit of the protection mechanism, comparing two timing methods and differential calculation through 5G communication transmission. The comparison demonstrates that the improved method significantly reduces the time limit of the protection action. According to caliper measurements shown in Figure 14a, during a single-phase ground fault in the area, the action time using the 5G timing data synchronization method was 26.894 ms, whereas using the satellite combined with 5G network timing data synchronization method, it was reduced to 22.045 ms, resulting in an overall reduction of 4.849 ms. Similarly, Figure 14b shows that for a two-phase short-circuit fault in the area, the action time using the 5G timing data synchronization method was 27.348 ms, while the satellite combined method brought it down to 20.379 ms, yielding a reduction of 6.969 ms. Lastly, as seen in Figure 14c, for a three-phase short-circuit fault in the area, the action time was 26.742 ms with the 5G timing method and 20.076 ms with the satellite combined method, leading to a reduction of 6.666 ms. These data collectively indicate that the improved synchronization method is more efficient and capable of achieving rapid differential protection in distribution networks.

Table 1. Comparison of protection action time limit of two methods under various faults.

		Single Phase Ground Fault	Two-Phase Short-Circuit Fault	Three-Phase Short-Circuit Fault
Data synchronization method for 5G timing	Action time limit/ms	26.894	27.348	26.742
Improved data synchronization method		22.045	20.379	20.076
Comparison time difference between the two/ms		4.849	6.969	6.666

presents a comparison of the protection action time limits for various faults, contrasting the two methods. This comparison reveals that the improved method can reduce the protection action time by approximately 5–7 ms. This reduction is tantamount to decreasing the average air interface delay between 5G base stations and CPE terminals. Consequently, this enhancement improves the speed of protection responses and minimizes the damage caused by power system faults.

5. Conclusions

The performance characteristics of 5G are highly conducive to meeting the communication channel requirements of distribution network differential protection. To tackle the challenge of data synchronization at both ends, time synchronization, namely unified sampling time, is essential. This paper introduces a dual-end data synchronization method that harnesses the combined capabilities of the Beidou satellite and 5G network for synchronous timing. This approach ensures simultaneous sampling and labeling with time sequence numbers. The method offers two key advantages:

(1) The integration of satellite and 5G network timing is a pivotal component in the convergence of 5G and satellite communications, aligning with the current evolution of 5G communication technology. The method proposed in this paper provides a potential direction for its integrated development.

(2) The fusion of satellite and 5G networks for synchronous timing achieves data synchronization at both ends of the protection mechanism, leading to a reduction in the protection action time by 5–7 ms. Furthermore, a comparison of the protection action time limits for various faults, as shown in Table 1, reveals that the proposed method is effective for multiple types of faults and can diminish the risk of power grid instability.