Research on Fault Current Controller of DC Microgrid

With the increasing consumption of fossil energy and increasingly severe environmental problems, methods to adjust the energy structure through the development and utilization of renewable energy sources such as wind power and photovoltaics are being studied and applied by more and more scientific research institutions and groups. However, a large number of distributed energy resources (DER) such as wind power and photovoltaics are random and volatile. If they are directly connected to the grid, it will have a significant impact on the peak regulation of the power grid and the safe operation of the system  , so the current grid connection method of DER is generally to integrate it into the main grid through microgrid to reduce the impact of distributed power sources on the main grid . Different from tracking the phase and frequency of the voltage in the AC microgrid, the DC microgrid generally improves the controllability and reliability through constant DC voltage control, reduces the voltage fluctuation between the DER and the load, and enables the load to be stabilized. Power voltage .​ Moreover, most distributed power sources are DC power sources, such as photovoltaics, energy storage, etc., and being connected to the DC grid can simplify the energy conversion process and improve efficiency. However, once an inter-pole fault occurs in a DC microgrid containing multiple distributed power sources, each distributed power source will inject short-circuit current into the fault point  . Due to the small coverage area of the microgrid, short power supply lines, and low line impedance, under the joint action of multiple distributed power sources, the DC microgrid short-circuit current rises quickly and has a large amplitude, which will cause serious impact on the system  . In order to ensure the safe operation of the system, simple and cheap overcurrent suppression methods should be considered as much as possible during the DC microgrid design stage to ensure the safe operation of the equipment.

At present, there are two main methods to suppress overcurrent in DC microgrids: (1) using fast disconnection technology to disconnect the fault or cut off the power supply before the fault current rises to a large value  , such as using solid-state circuit breakers  , hybrid circuit breakers  , or using fully controlled devices to replace the anti – parallel diodes in the commutator  ; (2) using current limiting technology to reduce the rise rate and amplitude of the fault current, such as various superconducting current limiters  , solid – state switch parallel resistors  HYPERLINK “javascript:;” or reactance current limiting, series current limiting reactors , etc. The above methods can achieve a certain current limiting effect, but still cannot achieve accurate control of the fault current size, making fault location, protection setting, and extreme difference coordination difficult. This paper takes a simplified DC microgrid based on a two-level voltage source converter (VSC) as an example, analyzes the characteristics of the fault current provided by the AC distribution network connected to the VSC when there is an inter-pole fault, proposes the idea of using a fault current controller based on a reverse voltage source, and gives the circuit structure and control strategy of the controller. The simulation results show that this current controller can accurately control the fault current provided by the power branch, and can automatically switch to normal operation mode after the fault disappears.

1. Basic principles of fault current controller

1.1 Working Principle

There are many types of DC power sources in the DC microgrid, some of which have voltage source characteristics, such as AC distribution network systems and energy storage systems based on voltage source converters, and some of which have current source characteristics, such as photovoltaic systems   HYPERLINK “javascript:;” . The fault current of the DC microgrid is mainly provided by the power supply end, among which the fault current provided by the power supply with voltage source characteristics is the main one. Considering that the fault current of most DC microgrids is small when a single-pole fault occurs, the damage to the equipment is small, and the equipment can continue to operate for a period of time after the fault occurs, the fault current controller designed in this article is only for inter-pole faults. Taking the AC distribution network system based on voltage source converters as an example, on the DC side, the AC distribution network can be equivalent to an adjustable DC voltage source, as shown in Figure 1.

Fig. 1 DC side equivalent circuit of voltage source converter

When an inter-pole fault occurs, the fault current provided by the AC distribution network can be expressed as follows:

i = u d /( Z + R f ) i=ud/(Z+Rf)

1

Where:

i i ——output current at DC side of voltage source converter (kA);

u d ud ——equivalent voltage of the AC distribution network on the DC side (kV);

Z —— Impedance between the fault point and the voltage source converter (Ω);

Rf Rf ——fault transition resistance (Ω) .

It can be seen that the fault current mainly depends on the size of the equivalent voltage u d ud .

The following relationship can be obtained from the principle of voltage source converter:

Ud = Uac / m Ud = Uac/ m

2

Where:

U d ——average voltage on the DC side of VSC (kV);

U ac ——VSC AC side voltage line voltage amplitude (kV);

m ——modulation ratio, with 0< m <1.

It can be seen that the regulation range of the DC side voltage of the voltage source converter is above the voltage amplitude of the AC distribution network  . Therefore, simply adjusting the DC side output voltage of the voltage source converter cannot effectively control the fault current.

If a reverse controllable voltage source is connected in series at the outlet of the voltage source converter, as shown in Figure 2, in normal operation, the output voltage of the controllable voltage source is controlled to be 0, which does not affect the normal operation of the microgrid; in the event of an inter-pole fault, the fault current provided by the AC distribution network becomes the following formula (3).

Fig. 2 Schematic diagram of fault current controller

i =( u d − u 1 )/( Z + R f ) i=(ud−u1)/(Z+Rf)

3

Where:

u 1 u1 ——Output voltage of the reverse voltage source (kV).

It can be seen from this that by controlling the output voltage u 1 u1 of the controllable voltage source during a fault , the size of the fault current can be arbitrarily controlled.

1.2 Basic Structure

From the analysis in Section 1.1, it can be seen that in normal operation, the output voltage of the controllable voltage source u 1 should be close to 0 to reduce the impact of the series voltage source on the normal operation of the DC microgrid; in the event of a fault, the output voltage of the controllable voltage source u 1 should be appropriately increased. When u 1 u1 = u d ud , the fault current provided by the AC power grid to the fault point drops to 0. Therefore, the output voltage of the controllable voltage source u 1 u1 should be in the continuously adjustable range of 0 to u ( u < u d ud ). Considering the bidirectional power flow characteristics of the microgrid, the controllable voltage source u 1 u1 should also have the ability to flow current in both directions.

Fig. 3 Structure diagram of fault current controller

Figure 3 , Eb is the terminal voltage of the energy storage element. When the DC microgrid operates normally, the switch tube S1 is normally open and S2 is normally closed. The output voltage of the controllable voltage source u1u1 is approximately the conduction voltage drop of the switch tube, which has little effect on normal operation. The inductor L can also play a role in reducing the DC current ripple. When an inter-pole fault occurs in the DC microgrid, the conduction duty cycle of the switch tube S1 and the switch tube S2 can be controlled to adjust the output voltage u1u1 , thereby arbitrarily controlling the fault current.

The AC distribution system based on the voltage source converter connected to the DC microgrid, after the fault current controller shown in Figure 3 is connected in series, its circuit structure is shown in Figure 4. As shown in Figure 4 , the DC side of the VSC satisfies the following relationship:

Fig. 4 VSC circuit structure of series fault current controller

U2 = Ud − U1 U2 = Ud− U1

4

Where:

U 2 U2 ——DC bus voltage (kV);

Ud Ud ——average value of DC side output voltage (kV) ;

U 1 U1 ——Average value of fault current controller output voltage (kV).

2. Analysis of the impact of fault current controller on microgrid

2.1 Analysis of the control range of fault current

From the above analysis, it can be seen that the control principle of fault current is achieved by controlling the output voltage. Therefore, in order to achieve the control effect of fault current, it should be ensured that both VSC and fault current controller are in a controllable state, that is, the power electronic devices in VSC and fault current controller should not be in a state where the overcurrent protection stops sending trigger pulses. Considering that the overcurrent protection of power electronic devices is generally set to 2 to 3 pu, the control target of fault current should not be greater than 2 pu.

If the control target of the fault current is too small, such as less than 1 pu, the fault current will be too small and the overcurrent protection will not be able to diagnose the fault. Therefore, the minimum control target of the fault current should be considered in conjunction with fault diagnosis and protection setting.

2.2 Impact Analysis

The use of fault current controller should give full play to its effectiveness while minimizing the negative impact on the system. Therefore, for the application scenario of the fault current controller shown in Figure 4 , the DC bus voltage U 2 can be controlled to always be the rated voltage of the DC microgrid system during normal operation, so that the DC microgrid side cannot feel the fault current controller during normal operation. existence. In addition, it can be seen from equation (4) that the microgrid DC bus voltage is established after the AC grid voltage undergoes two-stage voltage regulation ( U d , U 1 ). Therefore, when the AC side grid disturbance causes fluctuations in DC voltage U d , the fluctuation of microgrid DC bus voltage U 2 will be significantly reduced after the secondary adjustment of the fault current controller .

Due to the additional addition of devices, additional losses will inevitably occur, which will increase the total system loss.

3. Simulation analysis

The simulation analysis in this article is mainly used to verify the effectiveness of the fault current controller . The simulation model can be established using the simplified microgrid structure shown in Figure 5. The microgrid is mainly composed of an AC distribution network, a VSC, a fault current controller and a load. The detailed circuits of the VSC and the fault current controller are shown in Figure 4. In the simulation, the DC bus inter-pole voltage is set to 0.75 kV, and R 1 and R 2 are both 10 Ω. The pulse triggering of the VSC and the fault current controller is started at 0.1 s, and the inter-pole fault f 1 occurs at 0.3 s. Among them, the VSC converter adopts constant DC voltage control. During normal operation, the fault current controller adopts constant DC side bus voltage control, and adopts constant current control under fault conditions. The control target is 2 times the rated operating current, that is, 0.3 kA.

Fig. 5 Simplified DC microgrid structure diagram for simulation

3.1 Fault current limiting effect

According to the simulation structure and parameter settings of FIG5 , the waveform of the fault current of the DC microgrid before and after the installation of the fault current controller can be obtained, as shown in FIG6 .

Fig. 6 Comparison diagram of fault current control effect

When the fault current controller is not added, the f1 inter-pole fault occurs at 0.3 s , and the fault current reaches a maximum value of 2.563 kA at about 18.8 ms. At this time, all the insulated-gate bipolar transistors (IGBT) of the VSC have triggered the overcurrent protection and stopped sending the trigger pulse. With the addition of the fault current controller, the fault current reaches a peak value of 0.513 kA about 0.7 ms after the fault, and then with the regulation of the fault current controller, the fault current is finally controlled to 0.3 kA, which is consistent with the control target.

Comparing the fault current waveforms before and after the installation of the fault current controller, it can be seen that the effect of installing the fault current controller on limiting the fault current is as follows:

1) Greatly reduce the peak value and steady-state value of the fault current. In this example, the peak value of the fault current decreased by 80% and the steady-state value decreased by 85%.

2) Greatly shorten the time that a large fault current flows through the system. In this example, the duration of the first peak of the fault current does not exceed 2 ms, which will not cause unexpected tripping of the circuit breaker or cause serious impact on the equipment in the system.

3) The steady-state value of the fault current is adjustable and controllable within the controllable range of the equipment.

3.2 DC bus voltage

According to the simulation structure and parameter settings of FIG5 , the VSC DC side output voltage waveform and the microgrid DC bus voltage waveform after the fault current controller is installed can be obtained, as shown in FIG7 .

Fig. 7 Voltage waveform diagram of VSC DC side and DC bus

Figure 7 shows the waveform of the VSC DC output voltage and the DC bus voltage of the microgrid after the fault current controller is installed. From time 0, the VSC DC port capacitor begins to charge, and the VSC DC output voltage gradually rises until it reaches the uncontrolled rectifier output voltage value. The VSC pulse trigger is started at time 0.1 s, and the VSC output voltage stabilizes to the control target value after a short adjustment. At time 0.3 s, an inter-pole short circuit occurs, and the VSC output end is affected by the fault and produces a short-term voltage fluctuation, which then stabilizes.

Since the output voltage of the fault current controller is required to be close to 0 during normal operation, the output capacitor of the fault current controller should not be charged to a higher voltage before the fault current controller is started, otherwise the discharge of the port capacitor will generate a large discharge current when the fault current controller is started. As shown in Figure 3, the capacitor is discharged through the inductor and IGBT device S2 , causing a large impact current to flow through S2, which may cause S2 to be damaged. To prevent this phenomenon, the simulation assumes that the switch K is disconnected before the fault current controller is started. Therefore, before the start trigger pulse, the DC bus voltage is 0. After the start trigger pulse, the fault current controller is in voltage control mode, and the voltage is further adjusted based on the VSC output voltage, so that the DC bus voltage stabilizes to the rated voltage of 0.75 kV faster than the VSC output voltage. After the inter-pole fault occurs, the fault current controller switches to current control mode after fault diagnosis, the output voltage of the fault current controller rises rapidly, and the DC bus voltage drops rapidly to achieve effective control of the fault current.

It can be seen that the peak current of the fault current is closely related to the fault diagnosis speed. The fault current controller has the fault control function only after it diagnoses the fault and switches the control mode.

3.3 Current flowing through the device

Figures 8 and 9 show the current flowing through the IGBT devices in the VSC and fault current controller. Among them, the adjustment speed of the fault current is related to the control parameters of the fault current controller, etc. The comparison of the effects of fault current adjustment speed is shown in Figure 6. During rapid adjustment, the fault current can quickly stabilize to the control target 0.3 kA. However, it can be seen from Figure 8 and Figure 9 that during rapid adjustment, the current flowing through the IGBT device in the fault current controller in the early stage of current adjustment is significantly greater than that in the steady state. of current. It can be seen that the fast-adjusting fault current controller needs to leave a larger margin when selecting the device, otherwise the current control range of the fast-adjusting fault current controller should be reduced accordingly. Comparing Figure 8 and Figure 9 , it can be seen that the adjustment speed of the fault current has little impact on the device current of the VSC. And after the fault occurs, the current flowing through the VSC device does not increase exponentially, and the device overcurrent protection will not be triggered.

Fig. 8 Current flowing on IGBT device in VSC and fault current controller (Fault current regulation speed is slow)

Fig. 9 Current flowing on IGBT device in VSC and fault current controller (Fast regulation of fault current)

4. Conclusion

In view of the characteristics of low impedance and large inter-pole fault current in DC microgrid, this paper proposes a method for accurately controlling fault current based on controllable reverse voltage source. Firstly, its working principle and basic structure are analyzed. Secondly, the influence of the fault current controller based on reverse voltage source on the normal operation of microgrid is analyzed. Finally, through simulation verification, the fault current control range of the fault current controller is discussed. The simulation shows that the fault current controller proposed in this paper can greatly reduce the fault current and realize precise control, so that the system is in a controllable state before and after the fault without locking protection. In steady-state operation, the fault current controller can also assist VSC to further stabilize the DC bus voltage. In order to cooperate with the normal operation of the relay protection device and avoid VSC triggering overcurrent protection lockout, it is recommended that the fault current control range be set between 1 and 2 pu.

 

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