The faults characteristics of the lines in AC microgrid are weakened due to the fault resistance, which may refuse protection action. To solve the problems caused by different types of the faults through fault resistance (FTFR, the faults where the fault point resistance is greater than zero) in AC microgrid, a novel FTFR protection scheme based on the active power of 0-frame component or
AC microgrid is defined as a small distribution system with Distributed Generation (DG) sources, like photovoltaics (PVs), wind turbines (WTs), and so forth, energy storage systems (ESS), and local loads [
However, the fault currents for grid-connected microgrid and islanded microgrid are significantly different, the bidirectional flow [
The above protection schemes perform well for the solid faults (the faults where the fault point resistance is zero) of AC microgrid; however, they are ineffective against the faults through fault resistance (FTFR, the faults where the fault point resistance is greater than zero). And now most studies are focused on the transmission network protection against fault resistance. In [
This paper proposes a novel backup protection scheme against fault resistance for AC microgrid based on the active power of 0-frame component or
The rest of this paper is summarized as follows. In Section
According to the definition of CERTS, Figure
Typical structure of AC microgrid.
The faults in power system can be divided into two categories according to the difference of the fault situation. One category is the solid faults, and the other category is FTFR. The characteristic of FTFR is the small fault current; therefore it cannot be detected easily [
To solve the problems caused by FTFR in AC microgrid, this paper proposes a novel backup protection scheme that can identify and isolate all types of FTFR (single-phase to ground fault, double-phase to ground fault, three-phase to ground fault, and phase to phase fault) effectively. Figure
Equivalent circuits for internal FTFR. (a) Single-phase to ground internal FTFR. (b) Double-phase to ground internal FTFR. (c) Three-phase to ground internal FTFR. (d) Phase-phase to ground internal FTFR.
The Park transform (
In order to utilize the self-characteristics of fault resistance, the calculation formula of the equivalent fault resistance in various cases is defined as
In AC microgrid, firstly the currents or voltages need to be resolved into direct current (DC) term, fundamental sinusoidal term, and multiple sinusoidal terms (harmonic wave) using Fourier transform; then fundamental sinusoidal terms of the measured currents or voltages are extracted, which are used in (
According to the 0-frame component or
As can be seen from Figure
In the aforementioned formulas,
As can be seen from Figure
In the aforementioned formulas,
As can be seen from Figure
The fault currents are shown as
In this situation, the
In the aforementioned formulas,
As can be seen from Figure
In the aforementioned formulas,
During external faults and normal operation,
According to (
As the above analysis,
In general, external faults are considered to set the protection threshold. During external faults,
In external faults cases,
Self-adaptive threshold setting is introduced in this paper to solve the problem that the reliability and the high sensitivity of the protection cannot be satisfied at the same time. Typically, the measured error of current transformers is 10% in engineering. Assuming external faults occurred near
In the aforementioned formulas,
Therefore, self-adaptive thresholds can be expressed as follows with reliability coefficient
The thresholds in the proposed protection are the variables, not the fixed values. Different fault types, fault resistance values, and fault positions are as unknown factors can affect
The values of
The logic diagram of the proposed protection scheme is shown in Figure
Logic diagram of the proposed protection scheme.
Formula (
The changes of
The existing protection schemes perform well for the solid faults in microgrid, and the performance for FTFR is poor. The protection scheme proposed in this paper is applied after the failure of the existing protection schemes suitable for solid faults, as backup protection to ensure the accurate removal of all fault lines in microgrid.
Since microgrid can operate in grid-connected mode and islanded mode, it is necessary to protect it in both modes of operation. This paper proposes the self-adaptive threshold setting to solve the problem that values of
Due to the error of current transformers, the values of
The test system is built using PSCAD/EMTDC in this paper. The structure of the microgrid system is shown in Figure
System parameters of AC microgrid.
Component | Data | Parameters | ||
---|---|---|---|---|
|
|
| ||
Cable ( |
3 km | 0.64 Ω/km | 0.34 mH/km | 0.1 uF/km |
Cable ( |
1 km | 0.64 Ω/km | 0.34 mH/km | 0.1 uF/km |
Cable ( |
2 km | 0.64 Ω/km | 0.34 mH/km | 0.1 uF/km |
Component | Data |
---|---|
PV 1 | 1 MVA ( |
PV 2 | 1 MVA ( |
WT1 | 1 MVA ( |
WT2 | 0.8 MVA ( |
ESS | 2.5 MVA ( |
Load 1, load 2, load 4 | 1 MVA ( |
Load 3 | 0.5 MVA ( |
|
10 kV |
Main transformer | 20 MVA |
The voltage and frequency of microgrid are maintained by the main system when it operates in grid-connected mode. PVs and WTs are taken in MPPT control method, and the output power of each is 0.6 MW, 0.8 MW, 0.75 MW, and 0.5 MW, the odd power is supplied by main system, and ESS is in charging state.
Assuming the faults located in the middle of the
Simulation results of various faults in grid-connected mode. (a) Single-phase internal FTFR. (b) Phase-phase internal FTFR. (c) Double-phase internal FTFR. (d) Phase-phase external FTFR.
Figures
For external faults, phase to phase external FTFR (fault resistance is 1 Ω) occurring near
Table
Active power values and the protection threshold setting values of various types of FTFR in different internal fault positions under grid-connected mode.
Grid-connected mode | Types of fault | Fault position (MW) | ||||
---|---|---|---|---|---|---|
|
25% | 50% | 75% |
|
||
|
ABG (5 Ω, 5 Ω) | 7.52 (0.91) | 2.02 (0.80) | 0.97 (0.67) | 1.83 (0.65) | 7.36 (0.60) |
ABG (5 Ω, 6 Ω) | 6.80 (1.03) | 1.28 (0.59) | 0.27 (0.15) | 1.14 (0.14) | 6.22 (0.12) | |
AB (5 Ω) | 1.06 (0.22) | 0.89 (0.20) | 0.51 (0.17) | 0.86 (0.14) | 1.05 (0.13) | |
AG (5 Ω) | 4.00 (0.75) | 0.78 (0.17) | 0.28 (0.12) | 0.70 (0.11) | 3.86 (0.11) | |
ABCG (4 Ω, 5 Ω, 6 Ω) | 6.34 (1.08) | 1.67 (0.42) | 0.23 (0.11) | 1.45 (0.11) | 5.93 (0.10) | |
|
||||||
|
ABCG (5 Ω, 5 Ω, 5 Ω) | 17.1 (2.84) | 6.12 (1.51) | 2.05 (1.0) | 5.78 (0.09) | 16.3 (0.09) |
The results in Table
A sensitivity study has been conducted. Figure
Relationship between
The voltage and frequency of microgrid are maintained by ESS when it operates in islanded mode. PVs and WTs are also taken in MPPT control method, and the output power of each is 0.7 MW, 0.9 MW, 0.5 MW, and 0.6 MW, and the odd power is supplied by ESS. Assuming the faults located in the middle of the
The value of
Various faults in islanded mode. (a) Three-phase internal FTFR (fault position and fault resistance value of each phase are the same). (b) Three-phase external solid fault (fault position of each phase is the same).
The values of
Figure
Table
Active power values and the protection threshold setting values of various types of FTFR in different internal fault positions under islanded mode.
Islanded mode | Types of fault | Fault position (MW) | ||||
---|---|---|---|---|---|---|
|
25% | 50% | 75% |
|
||
|
ABG (5 Ω, 5 Ω) | 1.05 (0.13) | 0.26 (0.10) | 0.12 (0.08) | 0.22 (0.08) | 0.97 (0.07) |
ABG (5 Ω, 6 Ω) | 0.93 (0.10) | 0.17 (0.08) | 0.04 (0.02) | 0.16 (0.018) | 0.89 (0.016) | |
AB (5 Ω) | 0.30 (0.07) | 0.16 (0.04) | 0.11 (0.03) | 0.14 (0.03) | 0.26 (0.02) | |
AG (5 Ω) | 0.51 (0.10) | 0.10 (0.03) | 0.04 (0.02) | 0.09 (0.019) | 0.50 (0.018) | |
ABCG (4 Ω, 5 Ω, 6 Ω) | 0.91 (0.15) | 0.30 (0.08) | 0.03 (0.014) | 0.26 (0.014) | 0.83 (0.013) | |
|
||||||
|
ABCG (5 Ω, 5 Ω, 5 Ω) | 0.97 (0.16) | 0.36 (0.09) | 0.13 (0.058) | 0.32 (0.055) | 0.96 (0.05) |
The above simulation results show that the proposed protection scheme is able to identify internal FTFR correctly in both grid-connected mode and islanded mode, and it also can isolate the internal faults with high reliability and high sensitivity.
By using the fault characteristics of the fault resistance in the AC microgrid, this paper proposes a novel protection scheme based on the active power of 0-frame component or It can identify and isolate the internal FTFR validly without using the phase selection device. It was almost not affected by the operation modes of AC microgrid. It only needs the measured impedances and the 0-frame component or
This paper defines the current flow from bus to line as the positive direction, assuming the lines current flow from
Assuming the lines current flow from
Assuming the current transformers at
The corresponding conclusion can be achieved by the similar procedure when external faults take place near
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work is supported by the State Key Program of National Natural Science of China (61433004, 61627809, and 61621004), the Fundamental Research Funds for the Central Universities (N140404003, N130104001), and IAPI Fundamental Research Funds (2013ZCX14).