With the increased requirements for a more reliable, uninterrupted and continuous power supply, shorten the accident isolation time to milliseconds is required to support regional non-stop power services which makes severe challenges for the master station in centralized distribution automation.
Therefore, intelligent distributed feeder automation has become one of the trends in the development of the power distribution grid. Its characteristic lies in the distributed sinking of the processing logic of the original master station to the intelligent power distribution terminal. Through the 5G communication among the intelligent power distribution terminals and distributed master station, intelligent judgment, analysis, fault location, fault isolation and non-fault area power supply restoration operations can be realized.
In this way, the fault handling process can be fully automated, the fault scope can be restricted and fault handling time of the distribution network can be decreased from seconds to milliseconds.
As illustrated in the Figure 5.3.1-1, the distributed feeder automation system is mainly composed of distributed master station, distribution monitoring terminal and the communication system. The distribution master station is mainly used for information gathering and human-computer interaction, and the distributed terminal is used for feeder status information collection, judgment, fault location, isolation and power supply restoration. The implementation result will be reported to the distribution master station. The communication system is to provide the communication link among the distribution terminals.
The distribution master station is usually connected with the 5G system via wired or LAN which is out of 3GPP scope. Distribution master station manages multiple distributed terminals.
Each distributed terminal here is served by 5G UE to exchange the collected data with other distributed terminals. And from application aspect, the communication between distributed terminals is peer-to-peer. The connection between the distributed terminal and the 5G UE is out of 3GPP scope.
The 5G communication here should have high reliability. Therefore at least two communication links are usually deployed for hot standby or transmitting data synchronously between two distributed monitoring terminals. And it has the same condition between one distributed terminal and the distributed master station.
GOOSE protocol is a burst-based transmission application protocol used in Smart Grid. During the feeder system normal working phase, the heartbeat packet is periodic transmitted with 1s. When a fault occurs, it performs incremental periodic transmission with 2ms, 2ms, 4ms or 8ms time interval. After there is no sudden change in collected data, the heartbeat packet transmission of 1s is restored.
In general, the feeder distributed terminals can be deployed along the overhead line or integrated in one power distribution cabinet per one square kilometre. These two topologies will require different communication density.
Assumption of one CBD of a national sub provincial city, by calculating the load of various types of electricity consumption, it can be estimated the future power load in this area. Table 5.3.1-1 illustrates the typical estimated power load now and future in the area.
Generally, one single urban medium voltage distribution line should not be separated exceed 4 segments, and the load of each segment should not exceed 1MW. So, for one single circuit 10kV distribution line, the line load should not be higher than 4MW (1MW*4 = 4MW). In the medium level listed in the above table, the load density of the CBD area is about 20MW / km², the number of 10kV lines per square kilometre can be calculated as follows:
N = 20mV / 4MW = 5.
According to the distribution network reliability configuration principle, one backup is required and the N will be 6.
Considering the switches per segment and additional switches on the high voltage side, the number of required distribution terminal n can be calculated as follow:
Overhead line scenario: n = 6 * (5+4) = 54.
Power distribution cabinets scenario: n = 6 * (4 * 3 + 1) = 78.
In the future, when the power load density increases, or with differential protection utilized in the Feeder automation, more distribution terminals (e.g. more than 100) need to be considered.
Typically, the distributed Grid can be divided into urban and agricultural parts. In urban area, the power load is relatively concentrated, and the distributed grid working environment is better. But in the agricultural area, the power service range is very large, while the distributed grid has to face so many issues e.g. a large number of harmonic sources, three-phase unbalance, voltage flicker pollution.
Two 5G communication links are deployed for hot standby or transmitting data synchronously between UEs in distributed terminals and the network.
Data collection: The distributed terminal collects and reports related status information to the distribution master station or other distributed terminals in real time.
Fault detection and localization: The distributed terminal collects fault signal from itself and neighboring terminals. Then it executes data processing and fault location logic, and judges the fault whether an instantaneous or permanent fault.
When the distribution fault is an instantaneous fault, it will be skipped.
When the fault is a permanent fault, the distributed terminal will locate the feeder fault based on the signals of each power distribution terminal. The upstream power distribution terminal of the node will continue to send status information, while the downstream power distribution terminal will not report the fault signal. Therefore, in the corresponding fault node, only one switch should send out the fault signal. According to this feature, when a feeder fault occurs, every distributed terminal could judge its position relative to the fault location.
Fault isolation: When the fault node is determined, according to the preconfigured action order, all switches around the faulty node will be open to realize effective identification and isolation of the faulty area.
Fault restoration: For the restoration of power supply in non-fault area, it is necessary to clarify the number and wiring of the switches it tied to, e.g. when a fault occurs in the cabinet of the one-in-two-out ring main unit, if two outgoing lines have isolated power-loss area. The related distributed terminals will reconstruct the power distribution structure, and close the tie switch, to restore the power supply in the power-loss area.
Restoration confirmation: After the power supply is restored in the power-loss area, the system needs the distribution terminals report status information to determine whether the restoration is well done.
The 5G system shall be able to provide connection service wherever the distributed terminal is indoor, outdoor, low and medium altitude, or underground.
The 5G system shall be able to provide suitable APIs to enable application layer to monitor communication link status.
The 5G system shall be able to provide at least two back up communication links between every IoT device in a distributed terminal and the network.