Ground fault detection in substation DC system based on improved differential current method

DC system is the main electrical equipment and control signal system in the substation, and its stability and reliability are directly related to whether the power system can operate safely. This paper firstly introduces the common methods of insulation detection of DC power system in substation and its advantages and disadvantages, and proposes the differential current method based on unbalanced bridge, i.e., using the combination of unbalanced bridge and leakage current sensor to detect the insulation condition of DC system, which makes up for the defects of the traditional differential current method that can only detect the faults of simple branch circuits, and realizes the detection of multi-branch grounding under complex situations. Finally, the feasibility of this method is verified by building a simulation model through simulink.


1.Introduction
With the increase in the number of substations and the complexity of their structures in China, the stable operation of power systems has become more important.DC system, as an important part of substation, provides reliable power supply for various equipments, but its extensive power supply network is susceptible to environmental influences, and insulation degradation or grounding faults may occur.When the DC system is not able to find out the fault location quickly after one point of grounding, if another point of insulation degradation or grounding occurs, it may lead to power system failure or even accidents, resulting in large-scale power outages [1] .Among DC system faults, DC ground faults are currently one of the most common and difficult to investigate.Therefore, in-depth study of DC system to realize rapid determination of fault location is of great significance to the safe and stable operation of power system.Currently, the commonly used methods for finding ground faults in DC systems include unbalanced bridge and differential current method [2] .The unbalanced bridge method can only roughly determine whether there is a fault in the system, but can not accurately locate the fault point.The differential current method has limited ability to detect ground short circuit faults, especially high impedance ground faults, and if there are multiple faults or complex faults in the system, the differential current method may not be able to accurately detect all the faults [3] .This paper proposes branch circuit insulation detection based on the combination of unbalanced bridge and differential current method, and firstly analyzes the fundamentals of insulation problems in DC systems.Aiming at the limited detection ability of unbalanced bridge and differential current method for ground fault, the combination of unbalanced bridge and differential current method is proposed to determine the grounding polarity and calculate the grounding resistance, and take the three-branch system as an example to deduce the calculation method of insulation resistance, and finally build the simulation model through simulink to simulate the multi-branch negative polarity grounding test, and use the simulation data to verify the accuracy of the combination of unbalanced bridge and differential current method.

Pulling method
Pull circuit finding method is a commonly used method of finding ground faults in DC systems.It means that when a fault occurs, randomly select a circuit in the DC system to disconnect it, and then determine whether the ground fault still exists.If it still exists, it means that the DC circuit is not the circuit that triggers the grounding fault, and the normal power supply should be restored immediately, and the rest of the circuits should be operated in turn until the fault circuit is found [4] .Using this method, should ensure that the DC feeder power disappearance time as short as possible, find the principle of: first outdoor, after the indoor; first break the non-important circuits, after the break the important circuits; first break the newly commissioned equipment, after the break of the other equipment [5] .

Balanced bridge method
The principle of the balanced bridge method is to connect a balanced bridge between the DC bus bars and then measure the current flowing through the balanced bridge.When there is no abnormality in the system, the bridge is in equilibrium, the current flowing through the bridge is very small, and no alarm occurs in the system.When a ground fault occurs in the system, the balance of the bridge is broken, the current flowing through the bridge increases to the action value, and the system generates an alarm.

Figure 1. Resistor Balanced Bridge Schematic
Figure 1 is a schematic diagram of a resistive balanced bridge, R1, R2: equal value of the balanced resistance, XJJ is a signal relay, R+ is the positive insulation resistance, R-is the negative insulation resistance [6] .Signal injection method is when there is a ground fault to the DC system injected signals, through the branch circuit current sensor to collect each circuit current flow to determine the grounded branch circuit, usually can be divided into low-frequency signal injection method and dual-frequency signal injection method two.The simplified schematic diagram of the signal injection method is shown in Figure 2.

Differential current method detection
Differential current method, also known as leakage current method, is commonly used in the status of DC system branch ground faults, that is, the use of DC leakage current sensors to detect the leakage current of each branch, the principle is shown in Figure 3. System normal leakage current sensor in the current inflow is equal to the outflow, the difference current Id is zero, does not produce a branch grounding signal; when the branch occurs positive or negative grounding, the sensor current inflow and outflow difference Id is not zero, at the same time can be calculated for a single branch occurred grounding grounding resistance.

Figure 3. Schematic diagram of differential flow method
In practice, multiple DC leakage current sensors can be equipped at the same time to detect the grounding condition of multiple branches, and the most complicated multi-branch grounding is the condition that n branches are grounded positively and negatively, but the positive and negative grounding resistances can not be calculated with the existing method of differential current detection.

Branch circuit insulation detection based on the combination of unbalanced bridge and differential current methods
The traditional differential current method is only applicable to the case of branch single-point grounding or multi-branch single-polarity grounding, and more complex grounding faults such as multi-point positive and negative grounding are not solved by the traditional differential current method.Based on the differential current method of the unbalanced bridge, by judging the polarity of the collected leakage current to distinguish whether it is positively grounded, negatively grounded, or ungrounded, and using the bus voltage Um and the value of the leakage current of each branch can be solved for the value of the branch grounding resistance .The improved differential current method is shown in Fig. 4, where K1 and K2 are the upper and lower bridge arm switches, Rz1+, Rz2+, ..., Rzn+ are the positive to ground resistors from branch 1 to branch n, and Rz1-, Rz2-, ..., Rzn-are the negative to ground resistors from branch 1 to branch n.

Figure 4. Branch Circuit Insulation Detection Schematic
As an example, the improved differential flow method is specifically pushed to the process by grounding all three branch circuits positively and negatively: First of all, K1 will be closed, K2 disconnected, the detection of the three branches of the leakage current 11 , 21 , 31 at the same time, but also to measure at this time the bus positive pole to ground voltage 1+ and the bus negative pole to ground voltage 1− , which can be from the 2: Then make K1 disconnected, K2 closed, detect the leakage current 12 , 22, 32 of the three branches at this time, and at this time, the bus positive pole to ground partial voltage 2+ and the bus negative pole to ground partial voltage 2− to obtain formula 2: This can be extrapolated to find Zl+ , Zl− : For general K branches, i.e., Zk+ , Zk− : From the above analysis and derivation process, it can be seen that the improved differential current method is used in the insulation detection of branches without distinguishing the type of ground fault, and one method is applicable to all types of insulation ground faults.The operation process is also simple and easy to realize, only need to cast two switching states, in different switching states, detect the leakage current of the corresponding branch, the bus positive voltage to ground, the bus negative voltage to ground, you can calculate the positive and negative resistance value of any branch to ground.6, and the DC side voltage is selected as 200 V. K1 and K2 are switches of positive and negative bridge arms, which conduct alternately to form an unbalanced bridge, K3 and K4 are used to simulate whether there is a ground fault in the bus, and K5 and K10 are used to simulate whether there is a ground fault in each branch.After determining the parameters, the different ground faults of the DC system can be simulated by setting the opening and closing states of K5 and K10.Table 1 shows the test data of multi-branch negative polarity ground detection simulated by the improved differential current method.0.9 20.3For the improved differential current method, a multi-branch negative polarity ground fault of a DC system is simulated by a simulation model, and the simulation results do not differ much from the simulated actual resistance.It shows that the design of the systematic method meets the actual requirements of DC system grounding faults, and proves that the improved differential current method really makes up for the shortcomings of the traditional differential current method.The improved differential current method can not only detect the faults in the most complicated situation of multi-branch grounding, but also calculate the grounding resistance value, and initially determine the fault grounding type through the grounding resistance value, which is more conducive to finding the grounding fault point.

Conclusion
This paper first analyzes the DC system grounding fault causes and hazard analysis, and then lists the commonly used grounding fault detection methods.The branch insulation detection based on unbalanced bridge and differential current method is proposed, and the simulation model is built through simulink to simulate the multi-branch negative polarity grounding test, and the simulation results show that the combination of unbalanced bridge and differential current method is able to calculate the grounding resistance value under the complex situation of multi-branch grounding, which makes up for the defects of the unbalanced bridge and the traditional differential current method used separately.

Figure 2 .
Figure 2. Signal Injection Method SchematicSignal injection method is when there is a ground fault to the DC system injected signals, through the branch circuit current sensor to collect each circuit current flow to determine the grounded branch circuit, usually can be divided into low-frequency signal injection method and dual-frequency signal injection method two.The simplified schematic diagram of the signal injection method is shown in Figure2.

Figure 5 .
Figure 5. Schematic diagram of insulation detection of three branch circuits

Figure 6 .
Figure 6.Improved Simulation Model for Insulation Detection by Differential Flow MethodThe simulation model is built as shown in Fig.6, and the DC side voltage is selected as 200 V. K1 and K2 are switches of positive and negative bridge arms, which conduct alternately to form an unbalanced bridge, K3 and K4 are used to simulate whether there is a ground fault in the bus, and K5 and K10 are used to simulate whether there is a ground fault in each branch.After determining the parameters, the different ground faults of the DC system can be simulated by setting the opening and closing states of K5 and K10.Table1shows the test data of multi-branch negative polarity ground detection simulated by the improved differential current method.Table1.Multi-branch negative polarity ground detection test.

Table 1 .
Multi-branch negative polarity ground detection test.