1. Relay Protection Solution for Double-Circuit Lines on the Same Pole Cross-line faults may occur on double-circuit lines on the same pole. If both circuits are disconnected when a cross-line fault occurs, it will pose a serious threat to the safe and stable operation of the system. The ideal solution is to disconnect only the faulty phase when a cross-line fault occurs, and achieve phase-by-phase reclosing. Research on relay protection configuration schemes for cross-line faults on double-circuit lines on the same pole should focus on the phase selection tripping and reclosing issues during cross-line faults. This is of great significance in preventing both circuits from tripping when a cross-line fault occurs and maximizing the system stability after fault disconnection and reclosing. The main types of protection for cross-line faults on double-circuit lines on the same pole are as follows: Phase-Differential Current Protection: The protection device collects and converts the three-phase current of the line. While transmitting the three-phase current to the opposite side through a channel, it simultaneously receives signals from the opposite side. The data on both sides are strictly synchronized for differential comparison. The protection principle is simple, does not reflect load, is not affected by system oscillations and series compensation capacitors, and has inherent phase selection capability. When a cross-line fault occurs, all four protection sides can correctly and quickly select the phase for tripping. To ensure proper selection of the faulty phase during cross-line faults on double-circuit lines on the same pole, phase-differential current longitudinal protection is preferable. Phase-differential signal transmission distance longitudinal protection: This method uses a combined phase-comparison distance relay to measure the impedance of the fault loop. In complex cross-line faults, it can generally select the faulty phase; its accuracy depends on whether the measured impedance matches the impedance of the fault loop. Analysis shows that nearby faults (including cross-line faults) can correctly select the faulty phase (single-phase faults trip single-phase, two-phase faults trip both phases). As the fault point moves further to the end of the line, two-phase, two-conductor cross-line faults (e.g., IAIIBG) are all judged as phase-to-phase faults (IABIIABG). However, at this point, the fault is already within the range of distance protection stage II. Although the phase selection is incorrect (both circuits are judged as ABG faults), the protection stage II has a time delay, so it will not trip immediately. Once the opposite-side protection stage I correctly selects the faulty phase, both circuits will be single-phase faults. The phase-to-phase distance relay on this side will return, the grounding distance relay will begin measurement, and the grounding distance protection stage II will correctly select the faulty phase. In practice, this is achieved by first determining the ABG fault and then the AG (BG) fault, thus enabling continuous operation. When a cross-line fault occurs at the end of the line, the protection cannot correctly select the phase, but the opposite-side protection can. If both circuits have sufficient channels to transmit logic signals (blocking or enabling signals) by phase, rapid selection of the faulty phase across the entire line can be achieved. Selective phase tripping is achieved through continuous operation: If there is no channel to transmit the phase selection signal, the phase selection method, which only reflects the electrical quantities on one side of the line, cannot distinguish between single-circuit faults, cross-line faults, and double-circuit external faults at the end of the line. In this case, only the continuous operation of the distance relay of the distance protection can be used for phase selection tripping. The logic of phase selection and tripping by distance protection when longitudinal protection operates is as follows: a) If it is a single-circuit fault, phase selection is not difficult; b) If the phase selection result is a single-phase fault, the phase selection is definitely correct, and the faulty phase is immediately tripped; c) If both circuit protections operate and both select multi-phase faults, it may be a cross-line fault at the end of the line, so it cannot trip immediately. In this case, the longitudinal continuous operating circuit of the phase-to-phase distance relay can be used to wait for the other side to trip. If it is a two-phase, two-conductor cross-line fault, once the other side trips, this side changes to a single-phase fault, and then trips immediately. If it is a multi-phase fault at the end of the line, the longitudinal continuous operating circuit of the phase-to-phase distance relay trips three phases. 2. Longitudinal Protection Signal Transmission Methods There are two main types of synchronous data communication signal transmission methods for phase-separated current differential protection: dedicated optical fiber and PCM multiplexing. Distance longitudinal protection with phase-separated signal transmission has two signal transmission methods: PCM multiplexing and high-frequency carrier multiplexing channel. When using dedicated optical fiber, the protection remote transmission signal channel consists directly of a 2-core optical fiber and the optical interface of the protection device, with a channel delay of less than 1 ms. Its reliability relies on the direct optical cable between sites; when the cable breaks, the entire protection remote transmission signal is interrupted, with no alternative transmission route. In the fiber optic communication PCM multiplexing mode, the protection remote transmission signal is multiplexed on the 2 Mbit/s interface of the PCM device and transmitted via an SDH ring network. This does not occupy fiber cores, does not rely on a direct optical cable route between two sites, and is typically configured with a separate dedicated protection PCM device in engineering design, allowing multiplexing of multiple protection remote transmission signals. In point-to-point transmission, the channel delay is less than 1 ms; when transmitted through an SDH fiber optic communication network, as long as the number of sites is controlled to within 8, the total channel delay can still be controlled to within 15 ms. When a fiber optic communication circuit fails in one direction, information can be continuously transmitted to the other side via an SDH self-healing ring. From a statistical perspective, the probability of fiber optic circuit switching and power line failure occurring simultaneously is extremely small. Its disadvantages are that when the corresponding site's SDH optical transceiver or dedicated protection PCM device fails, the protection remote transmission signal is interrupted; if the protection remote transmission signal is transmitted via fiber optic branch circuit, the channel does not have SDH self-healing switching protection function. Carrier communication is a point-to-point communication method, with mature and reliable technology, which can meet the requirements of remote transmission protection signals. Although there is only one channel, a multi-channel FSK multi-command signal system is generally used to improve the phase selection performance of the protection: five frequencies can be switched within one channel, fG is the monitoring frequency, fA, fB, and fC are the hopping frequencies for phases A, B, and C respectively, and f3 is the hopping frequency for three-phase tripping. The monitoring frequency is transmitted normally and disappears after a fault. If any of fA, fB, or fC is received, it indicates that a certain phase tripping is allowed; if f3 is received, it indicates that three-phase tripping is allowed. With this multi-command signal system, when there is a cross-line fault at the end, as long as the opposite side selects to trip a single phase fault and sends the hopping frequency for that phase, the local side receives the signal and selects that phase for its own protection, and immediately trips that phase without relying on continuous operation. Due to the limitation on the number of frequency hopping, in the event of a two-phase fault, regardless of which two phases are faulty, the f3 frequency hopping is transmitted. After receiving this frequency hopping and verifying that the fault is also determined to be multi-phase on this side, the three-phase trip is immediately initiated. 3. Case Study of Protection Configuration for Double-Circuit Lines on the Same Pole Substation M is a 500 kV substation, and substation K is a 220 kV substation. The double-circuit 220 kV line between substations M and K is constructed using a single-circuit installation. Both circuits are equipped with one set of high-frequency blocking distance protection and one set of high-frequency phase difference protection. In 2003, when this double-circuit line was integrated into the newly built 220 kV substation (N substation), the 220 kV line outgoing corridor was adjusted due to urban planning constraints. The M-N and N-K double-circuit lines were both re-installed as double-circuit lines on the same pole, thus requiring a redesign of the line protection. In the power communication network design, a 32-core OPGW optical cable was installed synchronously with the line between substations N and K, forming a section of the main optical cable line of the provincial power fiber optic communication ring network. Substations M and N are two main nodes on the same city's SDH fiber optic communication network. Based on the requirements for dual protection systems with different principles for 220 kV lines and the need for rapid fault clearing, and considering the low reliability and security of opening dual high-frequency protection channels on a double-circuit line on the same pole, it is necessary to consider configuring fiber optic protection channels. The design adopts a protection configuration scheme for the double-circuit line on the same pole using phase-separated current differential protection and phase-separated signal transmission permissive distance longitudinal protection. Two schemes are available based on different signal transmission methods: 1. Dedicated fiber optic phase-separated current differential protection + PCM multiplexing distance longitudinal protection scheme; 2. Dedicated fiber optic phase-separated current differential protection + carrier multiplexing distance longitudinal protection scheme. Both schemes can meet the requirement of dual protection using different protection principles, and in the event of a fault within the protection zone, the protection on both sides of the line can quickly clear multi-phase faults. Source: http://tede.cn Since substations N and K are located in two different regional bureaus and not within the same SDH fiber optic communication network, if Scheme 1 is adopted, the dedicated fiber optic channel and PCM channel can only be transmitted through the OPGW fiber optic cable route from substation N to substation K. This means that all four sets of longitudinal protection for the N-K double-circuit line would use only one fiber optic cable route, resulting in low safety and reliability. Therefore, Scheme 2 is adopted for the protection configuration of the N-K double-circuit line in this case. Considering factors such as ease of construction and commissioning and simplified operation, Scheme 2 is also adopted for the protection configuration of the M-N double-circuit line. The actual configuration uses RCS-931B phase-differential current protection and RCS-902C distance longitudinal protection to form a dual main protection system. The RCS931B phase-differential current protection uses a dedicated fiber optic channel to transmit protection commands, while the RCS-902C uses a carrier channel to transmit permissive protection commands in phases. The protection configuration is shown in Figure 1. This scheme uses different routes to transmit protection signals for the two main protection systems, improving safety and reliability. When using a dedicated fiber optic channel, each protection device is equipped with two optical fibers as the channel. Configure 6-core optical fibers in an N+1 backup configuration (2 cores for each line, 2 cores as spares), and output the signal through the optical interface built into the protection device. Simultaneously, change the existing phase-to-ground coupling, single-frequency operating high-frequency protection carrier channel to a phase-to-phase coupling, dual-frequency operating protection multiplexing carrier channel. After configuring protection signal multiplexing and transmission interface equipment, multiple relay protection commands can be multiplexed on the voice transmission carrier channel. 4. Interface Design between Protection Device and Carrier Unit Generally, permissive protection commands for faults on double-circuit lines on the same pole have two interface methods: one is a dual-command input/output audio interface method, with single-phase trip command 1P and three-phase trip command 3P input and output, but fault clearance relies on sequential actions. As shown in Figure 1, in a double-circuit shared-pole line, if faults occur on lines L1 and L2, the N-side of line L1 selects phase A as the fault and issues a single-trip permission command. The N-side of line L2 selects phase B as the fault and issues a single-trip permission command. However, the M-side protection determines both phases A and B as faults and issues a triple-trip permission signal. The triple-trip permission condition is selecting two or more phases as the fault on this side and receiving a triple-trip permission signal. The single-trip operation condition is selecting a single phase as the fault on this side and receiving any trip permission signal. Therefore, the M-side protection does not meet the tripping conditions, and the phase selection element returns to its original position. After the N-side protection trips phase A of line L1 and phase B of line L2 respectively, the single-phase selection element of the M-side protection operates, successively disconnecting the faulty phases. Another method uses a four-command audio interface, i.e., selecting A-trip, B-trip, C-trip, and triple-trip, allowing simultaneous tripping on both sides. On the M side of line L1, if two-phase tripping is selected and a tripping permission command for phase A is received, the tripping mode is set to phase A tripping. On the N side of L1, if phase A tripping is selected and a tripping permission command for all three phases is received, the tripping mode is also phase A tripping. This method allows for simultaneous tripping on both sides. The communication design in this scheme uses an ETL carrier unit, and in conjunction with longitudinal protection, it uses an NSD550 audio interface. The NSD550 is an embedded protection interface installed on the ETL carrier unit. It uses the pilot signal from the ETL carrier unit as the monitoring frequency. When a protection command arrives, the protection interface converts it into a communication transmission frequency signal and controls the transmitter to cut off the voice channel and boost the transmission level to full power to send the command signal. On the other side of the line, the receiver determines that the monitoring signal has disappeared and detects a valid frequency hopping signal, and the corresponding output terminal activates. If both monitoring and frequency hopping signals are received simultaneously, or if neither is received, the receiver issues an alarm. The NSD550 can transmit command signals in two ways according to the requirements of the protection device: two permissive trip commands and two direct trip commands (two uncoded + two coded), suitable for redundant protection of double-circuit lines or backup protection of single-circuit lines; or three permissive trip commands and one direct trip command (three uncoded + one coded), suitable for phase-by-phase protection of single-circuit lines. In either way, the coded direct trip command takes precedence over the uncoded permissive trip command. The RCS-902C distance longitudinal protection device outputs three single-phase permissive trip signals (TA, TB, TC) to the carrier protection interface device, but does not have a separate three-trip command output/input interface. The NSD550 uses a four-command (3+1) audio interface. Therefore, in the design, the NSD550 protection interface device is equipped with two multi-functional remote protection interface boards (G4AI). The interface circuit between the G4AI board and the RCS-902C protection device is shown in Figure 2. The encoding function and internal logic of the NSD550 protection interface device are used for programming to realize four command inputs/outputs. The three-phase permissive trip signals TA, TB, and TC output by the protection device are connected to the A, B, and C command input terminals of the two G4AI boards, respectively. The A, B, and C command output terminals of the first and second G4AI boards are connected in parallel and connected to the three-phase permissive trip signal input terminals TA, TB, and TC of the protection device. The A, B, and C command outputs of the first G4AI board are set to TA, TB, and TC command outputs, while the A, B, and C command outputs of the second G4AI board are all set to TD (three-phase trip) command outputs. Thus, after circuit improvement and corresponding software adjustments, the priority relationship of each command transmitted by the NSD550 device in 3+1 operation mode is optimized as shown in Table 1. 5. Conclusion: The two sets of longitudinal protection for the line should preferably use different transmission channels. For double-circuit lines on the same pole, in order to selectively clear cross-line faults, it is advisable to set up one set of phase-separated current differential protection and another set of phase-separated signal transmission distance longitudinal protection. The two sets of protection should use different routes. With the increasing application of fiber optic communication in power grids, fiber optic channels should be prioritized when conditions permit: dedicated fiber optic cable + PCM multiplexed channel. If a dual-circuit line has four sets of longitudinal protection but only one fiber optic cable route, a dedicated fiber optic cable + carrier multiplexed channel scheme is recommended.