Design and analysis of relay protection system for AC DC hybrid system

. With the national new energy policy, AC DC integrated distribution network is more and more common, in the AC DC hybrid grid, the interaction between AC and DC side will lead to complicated fault characteristics, which further affect the relay protection system strategy, with typical system as the object of the AC DC relay protection strategy, fault characteristics aimed at different short dot simulation study are done, and verify the correctness and feasibility of the system configuration.


Introduction
With the continuous development of urban power grid, energy replacement, especially new energy power generation and energy storage devices in the form of DC.AC DC integrated distribution network is more and more common, in AC DC hybrid system, AC system depth coupling, when AC or DC system failure, the interaction between the two side will lead to fault function complexity, and then influence of relay protection system design, which makes the overall system relay protection design certain complexity [1][2].
In order to more clearly describe the relay protection design scheme of AC and DC distribution system, the paper designs a distribution network with AC and DC system, the system model and fault simulation are established to verify the correctness of the protection scheme.

Relay protection characteristics of AC DC hybrid system
Due to different electrical system, different voltage levels in AC DC hybrid system, the system has its own operating characteristics and affects each other, Therefore, it is quite different from the traditional single communication network, The short circuit fault of DC distribution network shows characteristics with large value, fast rise and rapid spread.Which make the design of its protection function more complex, Special relay protection device is used to protect the DC system, and realize the interconnection between the protection devices can greatly improve the ability and quality of the system protection; As the AC system invades into a DC failure, New electrical fault components are derived, Invading AC system through the converter, To changing the electrical characteristics of the AC system, then affect the AC system protection performance.

Design of AC DC hybrid network
With typical AC DC system as the object, the system consider from the grid incoming lead, the transformer down to rectifier device, rectifying output through DC switchgear to DC / DC converter and other DC load, transformer low voltage side configuration APF and grounding transformer, in addition to the system there are AC and DC, and different levels voltage, the system is as shown in Figure 1.According to the main wiring diagram 1, line protection and transformer protection devices shall be provided.The system protection configuration strategy is mainly according to the overall coordination of the protection device in case of different fault points listed in the main wiring.

Action analysis of each device during the F1 fault:
When the fault occurs on the DC load side, the first protection by the load side bus for rapid action.When the DC load fails, the AC load can still be powered through the power supply system.
Action analysis of each device when F2 fault: When fault F2 occurs at the output of the rectifier, the fault is first removed by the DC line protection device.At this time, the load side cannot take power, and the fault will not endanger the safety of the equipment on the load side.The APF device, grounding transformer and other devices can not perceive the fault current, so its protection will not act.
Action analysis of each device in case of F3 and F4 failure: When the fault F3 and F4 occur on both sides of the transformer, the regional fault belongs to the differential protection of the transformer.By the differential protection action of the transformer, the rectifier and DC -DC converter cannot perceive the fault current, so the protection does not act.
Action analysis of each device during the F5 fault: When the fault F5 fails, the incoming line protection device will jump into the switch, the line cabinet, the system power side fails, the system loses power, and the load cannot be powered through the power supply system.

AC protection configuration
The main protection functions of line protection, rectifier transformer protection and rectifier protection configuration are shown in Table 1.

DC protection configuration
Due to the particularity and complexity of DC system, the protection configuration of DC collection bus and its outgoing line in the DC switch measuring cabinet is emphasized, and the measuring point layout of DC system is shown in Figure 2. Differential protection is setting as the main protection, unbalanced protection, voltage and current protection as the backup protection, the bus differential configuration is as follows: Action Criteria:|IdP1+ IdP2+…+IdPn|> max(Idcb_set, k_set*Ires) Stalling current :Ires= max(IdP1,IdP2,…,IdPn) IdP1~ IdPn,for the positive electrode current of each branch of the bus line, the negative electrode difference formula are the same as above, and the electric current is IdN1~ IdNn,The positive and negative electrode currents in this system are respectively IdP1,IdP2,IdN1,IdN2,the schematic diagram of the test point is shown in Figure 2.
Restraint coefficient: k_set; Alarm section I: Idcb_set1,time-delay :T_set1; Section II trip trip:Idcb_set3,time-delay :T_set3.The DC partial protection function configuration is shown in Table 2: 4 Fault simulation of power distribution system PSCAD simulation tool is used to establish the simulation model in Figure 1.Some typical parameters in the simulation model are selected: distribution network equivalent internal resistance 10 Ω, grounding transformer grounding resistance 10 Ω, short circuit transition resistance 0.1 Ω, transformer is YD11 wiring mode, short circuit impedance 6%.The simulation waveform mainly reflects the voltage on both sides of the transformer, fault current, DC part stage and ground voltage under the fault state.Fault type Single phase failure, Alternative fault, three-phase fault.

AC portion
AC part of the fault mainly are carried out the simulation simulation from single phase, alternate and three-phase fault, due to the paper limit ,we cannot list all the fault points and fault type waveform and analysis diagram, this paper take F3 point AB fault as an example, the corresponding fault waveform and DC and load waveform is shown in figure 3 and 4.  It can be seen that when the fault phase voltage drops to zero, the non-fault phase voltage rises to the line voltage; the fault phase current increases significantly.Due to AC voltage imbalance, AC component is generated in the rectifier output; DC / DC converter output side is less affected.

DC portion
The DC part is mainly reflected in F1 and F2 fault points.Failure analysis mainly reflects monopolar fault (monopolar fault is divided into positive fault and negative fault) and interpole fault.
Take the F2 point positive electrode fault as an example, the fault condition is set as a metallic grounding fault, the fault duration is 0.1s, and the waveform diagram is shown in Figure 5.
The unipolar fault characteristics of the DC distribution network are directly related to the grounding mode of the system.When the connecting transformer adopts the valve side through the large resistance grounding mode, the unipole fault characteristics are similar to the AC distribution network, and the unipole grounding fault current is small, which can be operated for a period of time.Transcurrent equipment adopts fixed DC voltage strategy, that is, the voltage between positive and negative DC poles is unchanged; fault pole voltage is reduced, nonfault pole voltage is kept basically unchanged.The simulation results of positive pole fault and negative pole fault show that the fault incoming current does not exceed the rated current; the fault phase voltage is biased and the non-fault voltage remains unchanged.The DC / DC converter input side fault has no effect on the output side power supply.
The fault incoming current far exceeds the rated current; the fault phase voltage and non-fault voltage are reduced.At this time, the protection should be operated to remove the fault, and the input side fault of DC / DC converter will cause the power supply interruption on the output side of DC / DC converter, and the fault waveform diagram if the failure is not listed due to the space limit.
The simulation results of positive pole fault and negative pole fault show that the fault incoming current does not exceed the rated current; the fault phase voltage is biased and the non-fault voltage remains unchanged.The DC / DC converter input side fault has no effect on the output side power supply.

Conclusion
For typical AC DC hybrid system, the AC and DC part configuration related protection, and explains the specific protection function, analyzes the fault characteristics and protection action area, and on the basis of different fault point simulation, the paper gives several typical fault simulation and simulation analysis.

Fig. 1 .
Fig. 1.Main wiring diagram of the AC DC power system.

Fig. 2 .
Fig. 2. Measurement point and schematic diagram of the DC system.

Fig. 4 .
Fig. 4. AB phase fault DC and load wave form chart on F3.

Table 1 .
Protection configuration table of system AC section.

Table 2 .
The DC part protection configuration table.