Abstract
The renewable microgrid (RMG) is a critical way to organize and utilize new energy. Its control and protection strategies during the fault are the core technologies to ensure the safe operation and stability of the system. The traditional protection principles are unsuitable for RMGs due to the flexibility of RMG operation, the complexity of RMG topology, and the variety of fault control strategies of inverter-interfaced distributed generators (IIDGs). The traditional fault component protection principle is affected by the low voltage ride-through (LVRT) control strategy and will fail in some scenarios. In order to make the fault component protection principle available in every scenario, a current-based fault control strategy is proposed. Specific fault characteristics are generated by the grid-feeding IIDGs during the fault so they can be equivalent to the open circuits, and the fault models in additional network can be simplified. By analyzing the fault characteristics, an RMG protection strategy based on the current-based fault control of IIDGs is presented. The fault directions of feeders can be distinguished and the fault feeder can be located accurately in both grid-connected and islanded RMGs. Then, the grid-feeding IIDGs can transit to LVRT mode smoothly. Thus, IIDGs are considered comprehensively in terms of coordinating with fault control and fault characteristic generation. Finally, the experimental results of the hardware platform prove the effectiveness of the proposed current-based fault control strategy, and the simulation results based on PSCAD/EMTDC verify the correctness of the protection strategy.
DUE to the primary energy depletion and increasing environmental pressures, renewable energy sources (RESs) are growing rapidly around the world [
The penetration of RESs in distribution system has led to the rapid development of RMGs and brought new challenges. Control systems of RMGs must be responsive to fluctuations and randomness of RESs. It is necessary to adjust the control strategy to recover the RMG from the effects of abnormalities and faults. DGs are connected to the RMG through inverters, which realizes the function of plug-and-play and the local consumption of new energy [
The IIDG system, as shown in
Fig. 1 Simplified model of IIDG system.
Opposed to the traditional power systems, RMGs comprised of IIDGs cannot provide large fault power and current like synchronous generators due to the limitation of semiconductors [
In addition to ameliorating the traditional overcurrent protections to adapt to RMGs, some scholars also pay attention to some new protection schemes based on intelligent algorithm or communication. An intelligent method for fault detection by using the undecimated wavelet transform is proposed in [
The fault control strategy for IIDGs has a decisive impact on the fault characteristics [
In this paper, a current-based fault control strategy of grid-feeding IIDGs and an RMG protection strategy are proposed. The output currents of grid-feeding IIDGs are maintained to simplify the positive-sequence additional network of the RMG. By analyzing the fault characteristics of the RMG, the phase difference between the positive-sequence current fault component (PSCFC) and the pre-fault bus voltage is used to distinguish the fault direction of the feeder, and the fault feeder can be located accurately in both grid-connected and islanded modes. Finally, the effectiveness of the proposed RMG protection strategy is verified by PSCAD/EMTDC. This strategy only needs modification to the control strategy of IIDGs without adding expensive hardware devices in the RMG.
The rest of this paper is structured as follows. The fault component protection principle and its limitation are presented in Section II. The current-based fault control strategy adapting to the fault component protection principle is explained in Section III. Section IV presents a new RMG protection strategy based on the current-based fault control strategy of IIDGs. Experimental results are presented in Section V. Discussion and comparison of the proposed control and protection strategy to other strategies are presented in Section VI. Section VII concludes the paper.
Based on the superposition principle, a current or voltage variable can be regarded as two components: a normal-running component and a fault component. The fault component can be calculated using superimposed networks [
(1) |
As (1) indicates, the fault component of current or voltage can be calculated by subtracting the normal-running (pre-fault) current or voltage from the fault (during-fault) current or voltage.
In discrete-time mode, (1) can be expressed as:
(2) |
where k is the sampling time; and N is the number of sampling points per period.
Under normal conditions, IIDGs only output positive-sequence power to RMGs. According to the instantaneous power theory, in the synchronous rotating reference frame, the average value of active power output Pout and reactive power output Qout of IIDGs can be expressed as:
(3) |
where is the d-axis component of the positive-sequence PC voltage; and and are the d-axis and q-axis components of current reference signal, respectively.
In terms of the German grid code, IIDGs need to output positive-sequence current and reactive power to support the RMG under fault conditions [
(4) |
where Iq is the reactive fault current; Un is the normal-running voltage; and Imax is the maximum output current of IIDGs.
Combining (3) and (4), it can be concluded that the IIDG connected to an RMG can be equivalent to a current source controlled by .
According to the IEEE 1547 standard, an RMG model is shown in
Fig. 2 RMG model.
DG2-DG4 represent the grid-feeding IIDGs like PVs or type-4 WTs, which adopt the LVRT codes and thus can be equivalent to current sources. DG1 represents a grid-forming IIDG like BESS or fuel cell, which can be used as the main power source in the islanded RMG to provide stable voltage and frequency for the bus. Thus, in the islanded mode, DG1 can be equivalent to a voltage source. However, in the grid-connected mode, DG1 should also adopt the LVRT code and thus can be equivalent to a current source.
Positive-sequence fault components exist under all types of RMG fault conditions [
Fig. 3 Equivalent additional network of positive-sequence fault component when a fault occurs at point F. (a) Islanded mode. (b) Grid-connected mode.
In
It is assumed that the positive direction of current is from bus to feeder, as shown in
According to the LVRT code, the grid-feeding IIDGs can be equivalent to voltage controlled current sources in LVRT mode. Taking DG3 as an example, the change of output current is shown in
Fig. 4 Output current of IIDG and phasors of PSCFC at Bus B. (a) Output current of IIDG. (b) Phasors of PSCFC.
Take the change range of PSCFC of feeder B3 as an example. With the increases of output reactive power of IIDG, the phase difference between fault component and pre-fault bus voltage uB,0 will change from 90°-180° to 0°-90°. At this time, the fault feeder cannot be located by the phase relationship between the PSCFC and the pre-fault bus voltage. Therefore, the fault component protection principle based on phase comparison cannot be applicable in some situations.
The traditional fault component protection principle is limited by the operation mode and the topology of RMGs, and the LVRT control strategy of IIDGs will also affect fault characteristics of RMGs.
The control strategy of IIDGs in an RMG affects the fault characteristics and the positive-sequence additional network model of the RMG, which leads to the failure of the fault component protection principle. According to the analysis in Section II, the grid-feeding IIDGs can be equivalent to current sources in the positive-sequence fault component network. By adjusting the current control strategy of grid-feeding IIDGs, the fault models can be simplified and the fault component protection principle can still be applicable. A proposed current-based control strategy of IIDG is shown in
Fig. 5 Proposed current-based fault control strategy of IIDG.
In
In the angle and amplitude extraction module, a notch filter and a sequence component extractor are combined to extract the positive-sequence electrical quantities , , and . In the reference generation module, a vector compression module is used to limit the reference value of the output current and power in equal proportion to ensure that the IIDG works in a safe range. Based on the current-based fault control module, with a delay module, the output current of IIDG can be maintained for a period of time. Considering the state-of-the-art power electronics control technology and the fault transient process, the delay time is 2 cycle in this paper. Thus, the PSCFCs of the grid-feeding IIDGs in (1) can be expressed as:
(5) |
where and are the positive-sequence current under fault and normal conditions, respectively.
Owing to the proposed current-based fault control strategy, the PSCFCs of IIDGs are zero during the fault. Therefore, the fault models shown in
Fig. 6 Additional network of positive-sequence fault component based on proposed current-based fault control strategy. (a) Islanded mode. (b) Grid-connected mode.
The PSCFC of the forward fault feeder at Bus K (K=A, B, C, D) can be expressed as:
(6) |
where is the PSCFC of the forward fault feeder; is the PSCFC of the reverse fault feeder; and n is the number of branch feeders contained at Bus K.
The PSCFC of the reverse fault feeder at Bus K can be expressed as:
(7) |
where is the equivalent positive-sequence impedance at the forward fault feeder side; and Zeq is the equivalent positive-sequence impedance at the other feeder side. and Zeq are mainly composed of positive-sequence impedances of feeders and loads, which means the impedance angles of and Zeq are nearly equal. Therefore, it can be observed from (7) that the phasors of PSCFC of forward fault feeders and reverse fault feeders are different, and the corresponding phasors are shown in
Fig. 7 Phasors of PSCFC at Bus K.
Based on the above analysis, it can be concluded that the feeders on the same bus have the following fault characteristics.
1) The equivalent impedances in (7) will only affect the amplitude of currents, but not the direction.
2) The phasor of PSCFC of forward fault feeders is almost opposite to that of reverse fault feeders.
3) The phase difference between the PSCFC of forward fault feeders and uK is less than 90°; the phase difference between the PSCFC of reverse fault feeders and uK is around 90°-180°.
4) Owing to the proposed current-based fault control strategy of IIDGs, the phase relationship in 3) is independent of the topology, parameters, and operation modes of RMGs.
Considering the small short-circuit current provided by IIDGs during RMG fault, the change of positive-sequence voltage of fault feeder is not obvious under high-impedance faults. Therefore, the amplitudes of the bus voltage fault components are selected as the starting criterion:
(8) |
where ,
According to the analysis in Section II, the fault feeder can be recognized by fault characteristics, because the amplitude is affected by the RMG operation modes and the types of faults. If two feeders have a similar amplitude of voltage, the fault feeder cannot be distinguished reliably. Hence, the phase difference between the PSCFC of the feeder and the pre-fault bus voltage can be selected as the fault characteristic and used to detect the fault feeder.
According to the analysis results of fault characteristics, the fault detection criterion for the forward fault feeder can be described as:
(9) |
For the reverse fault feeders, the fault detection criterion can be described as:
(10) |
By employing the proposed fault starting criterion and fault detection criterion, the fault direction of each feeder can be determined by (9) and (10). Based on the proposed current-based fault control strategy and fault detection criteria, the flow chart of the proposed RMG protection strategy under fault conditions is depicted in
Fig. 8 Flow chart of proposed RMG protection strategy under fault conditions.
In order to verify the effectiveness of the proposed current-based fault control strategy shown in
Fig. 9 Simplified model of IIDG utilized in hardware platform and PSCAD/EMTDC.
In order to evaluate the correctness of the proposed RMG protection strategy, an RMG shown in
A simplified power-electronics-based hardware platform with the controller TMS320F28335 is fabricated, as shown in Appendix A Fig. A1. This platform mainly comprises a grid-forming IIDG, a grid-feeding IIDG, and the loads. The hardware platform specifications are presented in
Fig. 10 Experimental result of hardware platform.
The grid-forming IIDG only outputs the positive-sequence voltage, thus its voltages are symmetrical and the phase-a voltage ua is shown in
Based on the PSCAD/EMTDC platform, in islanded mode, when s, a phase-phase fault occurs in the midpoint of feeder B2C1 and the transition resistance is 3 . IIDGs adopt the current-based fault control strategy. The voltage waveforms of the grid-forming IIDG (DG1) is shown in
Fig. 11 Voltage waveforms of grid-forming IIDG (DG1).
Fig. 12 Current waveforms of grid-feeding IIDG (DG2).
As can be seen from
The effectiveness of the proposed RMG protection strategy is verified in the following Cases 1-5. The RMG adopts ungrounded mode in Cases 1-4, thus the phase-phase and three-phase faults are verified. The grounded mode is adopted in Case 5, and a high-impedance single-phase-to-ground fault is verified.
Case 1: in grid-connected mode, when s, a three-phase fault occurs at the midpoint of feeder B2C1, and the transition resistance is 3 . The amplitude, phase, and phase difference of pre-fault and during-fault positive-sequence voltages of each bus and those of PSCFCs on the feeders are shown in Tables
As shown in the Tables
Case 2: in grid-connected mode, when s, a phase-phase fault occurs in the midpoint of feeder B2C1 and the transition resistance is 3 . The amplitude, phase, and phase difference of pre-fault and during-fault positive-sequence voltages of each bus and those of PSCFCs on the feeders in this case are shown in Tables
Compared with the Case 1, the amplitude of PSCFCs decreases, and the amplitude of the positive-sequence voltage of each bus slightly increases after the fault. B2 and C1 are detected as forward fault feeders as well. Therefore, the fault feeder can be located accurately through the proposed fault detection criterion.
Case 3: in islanded mode, when s, a three-phase fault occurs in the midpoint of B2C1 feeder and the transition resistance is 3 . The amplitude, phase, and phase difference of pre-fault and during-fault positive-sequence voltages of each bus and those of PSCFCs on the feeders in this case are shown in Tables
Compared with the Case 1, the RMG is disconnected to the utility grid. Since it lacks the fault current contribution of the utility grid, the amplitude of the positive-sequence voltage of each bus significantly decreases after the fault. It reflects that the fault severity of Case 3 is more serious than that of Case 1. But it does not affect the phase relationship between the PSCFCs and the pre-fault bus voltage. The fault feeder B2C1 can still be located accurately through the proposed fault detection criterion.
Case 4: in islanded mode, when s, a phase-phase fault occurs in the midpoint of B2C1 feeder and the transition resistance is 3 . The amplitude, phase, and phase difference of pre-fault and during-fault positive-sequence voltages of each bus and those of PSCFCs on the feeders in this case are shown in Tables
Compared with the Case 3, the amplitude of the PSCFCs is lower and the amplitude of the positive-sequence voltage of each bus is higher. It reflects that the fault severity of Case 4 is relatively lighter than that of Case 3. The fault feeder B2C1 can still be located accurately.
Case 5: in islanded and grounded mode, when s, a high-impedance single-phase-to-ground fault occurs in the midpoint of B2C1 feeder and the transition resistance is 100 . The amplitude, phase, and phase difference of pre-fault and during-fault positive-sequence voltages of each bus and those of PSCFCs on the feeders in this case are shown in Tables
As shown in the
Cases 1-5 present the simulation results of different grounded modes, fault types, and transition resistances. The negative-sequence components exist in Cases 1-4, while the zero-sequence components exist in Case 5.
These two components only affect the fault starting criterion (8) but are independent of the proposed fault detection criteria (9) and (10).
Through the above simulation analysis, IIDGs using the current-based fault control strategy can generate specific fault characteristics when different types of faults occur in the RMG, which is consistent with the theoretical analysis in Section III. The RMG fault protection strategy proposed in Section IV can locate the fault feeder in both islanded and grid-connected RMGs accurately.
This paper proposes an RMG fault protection strategy coordinating with a current-based fault control strategy. With the proposed current-based fault control strategy, IIDGs generate the specific fault characteristics and then adopt the LVRT strategy according to grid code.
By improving the protection scheme of the traditional distribution network, the effective protection techniques on microgrid are mainly sorted into five categories, which are overcurrent protection [
Although an RMG contains multiple types of RESs, they can all be equivalent to a voltage- or current-controlled source when different control strategies are applied. The main difficulty of RMG fault protection is the influence of IIDGs on fault characteristics. Most researches only focus on the identification of fault characteristics in RMGs but ignore the improvement of the fault control strategy of IIDGs. Although some researchers [
The proposed fault protection strategy has the following advantages.
1) By adjusting the fault control strategy of IIDGs, the equivalent fault models of IIDGs can be changed, and the equivalent additional network of positive-sequence fault components can be simplified. In this way, without the complicated parameters or fault calculation, the fault feeder of every fault type in the RMG with different topologies and operation modes can be located.
2) With IIDGs adopting the current-based fault control strategy, the fault feeder can be accurately located during the delay cycle. Furthermore, IIDGs can track the LVRT reference after the delay cycle. Without modifying the control strategy of IIDGs complicatedly and adding expensive hardware protection devices, the fault component protection principle can still be applicable owing to the proposed fault control strategy. This fault control strategy takes both protection and voltage support of RMGs into account and realizes the coordination design of fault control of IIDGs and RMG protection strategy.
3) The calculation of PSCFCs only needs the pre-fault and during-fault current quantities. Therefore, the proposed current-based fault control strategy and fault detection criteria can be applicable in complex scenarios.
4) The proposed current-based fault control strategy is based on the low inertia, fast response, and high controllability characteristics of DGs, thus it can be widely utilized in PV, battery, type-4 WT generator, and other low-inertia IIDG systems.
5) Owing to the delay in the control strategy, IIDGs can maintain the specific fault characteristics for some periods. Therefore, the proposed fault detection criteria do not need high-speed communication network and are not affected by the performance of PLL.
A current-based fault control strategy and a new RMG protection strategy based on fault component protection principle are proposed in this paper. Owing to the fault control strategy, specific fault characteristics are generated by IIDGs to ensure that the fault component protection principle is still applicable in the RMG, and based on this, the fault feeder can be located accurately. Then, IIDGs can transit to the LVRT mode smoothly. According to the analysis of fault characteristics, a new RMG fault detection criterion is proposed, which uses the phase difference between the PSCFC and the pre-fault bus voltage to locate the fault feeder. The proposed fault detection criteria are suitable for both grid-connected and islanded RMGs. The experimental and simulation results validate the correctness of the current-based fault control strategy and the effectiveness of the RMG fault protection strategy.
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