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Applicability study of single‐phase reclosing in tie line of photovoltaic power plant

2020; Institution of Engineering and Technology; Volume: 15; Issue: 6 Linguagem: Inglês

10.1049/gtd2.12075

1751-8695

Kehan Xu, Zhe Zhang, Qinghua Lai, Xianggen Yin, Wei Liu,

Power Systems Fault Detection

IET Generation, Transmission & DistributionVolume 15, Issue 6 p. 997-1012 ORIGINAL RESEARCH PAPEROpen Access Applicability study of single-phase reclosing in tie line of photovoltaic power plant Kehan Xu, Corresponding Author Kehan Xu d201880430@hust.edu.cn orcid.org/0000-0002-7655-6617 State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, China Correspondence Kehan Xu, State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, No. 1037 Luoyu Road, Wuhan, China. Email: d201880430@hust.edu.cnSearch for more papers by this authorZhe Zhang, Zhe Zhang State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, ChinaSearch for more papers by this authorQinghua Lai, Qinghua Lai State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, ChinaSearch for more papers by this authorXianggen Yin, Xianggen Yin State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, ChinaSearch for more papers by this authorLiu Wei, Liu Wei Power Dispatching Control Centre, Guangdong Power Grid Co., Ltd., Guangzhou, ChinaSearch for more papers by this author Kehan Xu, Corresponding Author Kehan Xu d201880430@hust.edu.cn orcid.org/0000-0002-7655-6617 State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, China Correspondence Kehan Xu, State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, No. 1037 Luoyu Road, Wuhan, China. Email: d201880430@hust.edu.cnSearch for more papers by this authorZhe Zhang, Zhe Zhang State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, ChinaSearch for more papers by this authorQinghua Lai, Qinghua Lai State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, ChinaSearch for more papers by this authorXianggen Yin, Xianggen Yin State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan, ChinaSearch for more papers by this authorLiu Wei, Liu Wei Power Dispatching Control Centre, Guangdong Power Grid Co., Ltd., Guangzhou, ChinaSearch for more papers by this author First published: 23 December 2020 https://doi.org/10.1049/gtd2.12075Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Overhead transmission line faults are mostly single-phase grounding faults, and to ensure the power supply reliability of photovoltaic power plants in the cases of tie line transient faults, a feasible solution is to employ the single-phase reclosing. However, for the single-phase reclosing of the tie line, photovoltaic power source may face the over-voltage problem during the non-full phase operation, which will seriously affect the success rate of the single-phase reclosing. This paper, according to different non-full phase operation control strategies of photovoltaic power source, establishes the corresponding sequence network analysis models, and proposes the theoretical calculation method of non-full phase operation voltage, which takes account of different neutral point grounding modes of step-up transformer and different load levels. Meanwhile, the closing impulse problem of the single-phase reclosing is discussed. The correctness of the theoretical method is verified by digital simulation results. On this basis, the applicable scope and application suggestions of the single-phase reclosing are given. The research conclusions can provide important guidance and reference for the application of single-phase reclosing technology in tie line of renewable energy power plant. 1 INTRODUCTION To solve the increasingly severe energy crisis and environmental problems, grid-connected photovoltaic power generation technology has developed rapidly, and photovoltaic power generation accounts for an increasing proportion of installed capacity in the power system [1]. Large-scale photovoltaic power source (PVS) is connected to the power grid via tie lines mainly composed of overhead lines [2], and the automatic reclosing methods used by tie lines can generally be divided into three-phase reclosing and single-phase reclosing. As for the three-phase reclosing method, compared with synchronous motors, PVS almost has no physical inertia and effective voltage control capability, which results in that it is very easy to disconnect from the power grid due to system frequency deviation and voltage deviation during the islanding operation [3, 4]. To ensure the high reclosing success rate of PVS in the case of tie line transient faults, and considering that most of overhead transmission line faults are single-phase grounding faults [5], a feasible solution is to employ the single-phase reclosing device on the tie line of PVS [6]. However, as for the single-phase reclosing, when a single-phase grounding fault occurring in the tie line is cleared by the single-phase trip of protection, PVS will be in the status of non-full phase operating. In this condition, affected by factors such as the control strategy of PVS, the neutral point grounding mode of transformer and so on, the disconnected phase voltage of PVS may be much larger than the setting value of over-voltage protection, leading to the disconnection of PVS from power grid and the single-phase reclosing failure. In addition, due to the limited ability of insulated gate bipolar transistor (IGBT) tubes in PVS inverter to withstand over-voltage and over-current [7], if the impulse current and voltage of the single-phase reclosing are too large, it will directly threaten the safe operation of PVS. Consequently, to ensure the success rate of reclosing and the operation safety of PVS, it is of practical significance to develop the research on non-full phase operation voltage characteristics and single-phase reclosing impulse problems of PVS. At present, most studies on the single-phase reclosing of the renewable energy power plant are about the adaptive reclosing [8, 9], whose key point is to correctly judge whether the line fault is transient or permanent, and then decide whether to reclose. Obviously the precondition for realising adaptive reclosing is that when the single-phase of tie line is disconnected, the renewable energy power plant cannot exit operation due to over-voltage or over-current. However, the existing fault analysis methods for the non-full phase operation (single-phase disconnection) of the power grid are mainly based on the assumption that power sources are all traditional synchronous generators [10-12], and are not applicable for fault conditions containing renewable power sources. Guo et al. [13] and Yang et al. [14], respectively, analysed the impact of photovoltaic power and wind turbine on the automatic reclosing of the tie line. It was believed that the frequency of photovoltaic power and wind turbine in the island state is prone to shift, which is the key factor that causes the three-phase reclosing failure. But the influence of renewable energy sources on the single-phase reclosing of the tie line was not considered. For the wind farm whose tie line is equipped with the single-phase reclosing, Xiong et al. [15] mainly studied the non-full phase operating voltage on the system side of the wind farm tie line. But considering that the fault current supplied by the wind farm is much smaller than that from the power grid, the wind farm side was treated as an open circuit in the theoretical analysis. Therefore, the analysis conclusion is only applicable to the system side of the tie line, lacking the evaluation of the non-full phase operation voltage on the wind farm side. According to a simulation study of the entire process from the fault occurrence to the action of the single-phase reclosing, Yuan et al. [16] demonstrated the feasibility of employing the single-phase reclosing on the high-voltage tie line of renewable energy power plants. The influence of the neutral point grounding mode of step-up transformer on the non-full phase operating voltage is not considered, and it is limited to qualitative analysis, lacking the support of the necessary theoretical analysis. Here, it is found that in some cases, there may be an over-voltage phenomenon of the renewable energy power plant during the non-full phase operation, which will lead to the failure of single-phase reclosing. According to different non-full phase operation control strategies of PVS and their corresponding analysis models of sequence networks, this paper proposes a theoretical calculation method of PVS non-full phase operation voltage, taking into account different neutral point grounding modes of the step-up transformer. Meanwhile, the closing impulse problem of single-phase reclosing is discussed. The correctness of the theoretical method is verified by digital simulation results. On this basis, the applicable scope and application recommendations of single-phase reclosing are further given. The research conclusions can provide important guidance and reference for the application of single-phase reclosing technology in tie line of renewable energy power plant. 2 DIFFERENT NON-FULL PHASE OPERATION CONTROL STRATEGIES OF PVS AND THEIR SEQUENCE NETWORK ANALYSIS MODELS PVS is a renewable energy source directly connected to the power grid through the inverter, and its output characteristics are closely related to the inverter control strategy. There are mainly two typical control strategies of PVS under the circumstance of symmetrical voltage. One is to employ the negative-sequence current suppression control method in the case of symmetrical voltage, in order to reduce the adverse impact of negative-sequence components on the power grid. The other is mainly used for the asymmetrical short circuit. Only when the positive-sequence voltage of the PVS port is lower than the threshold, such as 0.9 Vn, the negative-sequence current suppression method of the low-voltage ride-through (LVRT) control strategy is started. According to the above two typical control strategies of PVS, their corresponding sequence network analysis models are established in the following. 2.1 Negative-sequence current suppression control strategy is started During the non-full phase operation of single-phase reclosing, the three-phase voltage of the PVS port will be asymmetric. When the suppressing negative-sequence current control strategy using the positive-sequence voltage measurement is started [17, 18], phase-locked loop of the PVS inverter will separate the positive and negative-sequence components of port voltage, and generally directs the positive-sequence voltage of power grid to d-axis [19-21]. Meanwhile, the instruction value of the negative-sequence current is set to 0. Under the circumstance of this control strategy, the output current of PVS is shown in (1): i g d + = min P 0 U g + , I g max 2 − i g q + + 2 i g q + = 0 U g + > 0.9 1.5 ( 0.9 − U g + ) 0.9 ≥ U g + ≥ 0.2 I g max U g + < 0.2 i g d − = 0 i g q − = 0 , (1)where igd+ and igq+ represent the d-axis and q-axis positive-sequence components of the output current of PVS, respectively; igd- and igq- represent the d-axis and q-axis negative-sequence components of the output current of PVS, respectively; Ug+ represents the positive-sequence voltage amplitude of power grid; P0 represents the normal active power of PVS; Igmax represents the maximum allowable current of PVS. It can be seen from (1) that for the positive-sequence network, PVS can be equivalent to a voltage-controlled current source model whose amplitude and phase are both controlled by the positive-sequence voltage. For the negative-sequence network, since PVS does not output the negative-sequence current, the PVS side of tie line can be equivalent to an open circuit. 2.2 Negative-sequence current suppression control strategy is not started The non-full phase operation of the single-phase reclosing is different from the asymmetric short-circuit fault because it generally does not cause a large drop in the voltage of power grid. When the amplitude of the non-full phase operation voltage is greater than 0.9 p.u., the negative-sequence current suppression control strategy, which is mainly used for asymmetrical short-circuit fault, may be not started. In this condition, the positive-sequence output current characteristics of PVS are the same as in (1), which means that the positive-sequence network analysis model of PVS is still a voltage-controlled current source. But the negative-sequence output characteristics of PVS will change. According to controller equations of the voltage outer loop and current inner loop of PVS inverter, the negative-sequence network analysis model of PVS under the circumstance that the negative-sequence current suppression strategy is not started is established next. When the negative-sequence current suppression strategy is not started during the non-full phase operation, the output positive-sequence current and negative-sequence voltage, or the negative-sequence current and positive-sequence voltage of PVS will generate a double-frequency power [18], and their relation is shown in (2): P g 2 cos ( 2 ω 1 t + θ ) = P g 2 cos cos ( 2 ω 1 t ) + P g 2 sin s i n ( 2 ω 1 t ) Q g 2 cos ( 2 ω 1 t + θ ) = Q g 2 cos cos ( 2 ω 1 t ) + Q g 2 sin s i n ( 2 ω 1 t ) P g 2 cos P g 2 sin Q g 2 cos Q g 2 sin = u g d − u g q − u g d + u g q + u g q − − u g d − − u g q + u g d + u g q − − u g d − u g q + − u g d + − u g d − − u g q − u g d + u g q + i g d + i g q + i g d − i g q − , (2)where Pg2 and Qg2 represent the double-frequency active and reactive power, respectively; ugd+ and ugq+ represent the d-axis and q-axis positive-sequence components of PVS port voltage, respectively; ugd- and ugq- represent the d-axis and q-axis negative-sequence components of PVS port voltage, respectively; ω1 represents the fundamental frequency angular velocity. Since PVS generally directs the positive- and negative-sequence voltage of the power grid to the positive- and negative-sequence d-axis, respectively, the parameters ugq+ and ugq- in (2) are both 0. When the positive-sequence voltage during the non-full phase operation is high and the negative-sequence current suppression strategy is not started, it can be known from (1) that the d-axis component of the PVS output current will be much larger than the q-axis component. It can be approximated that PVS works in the operating state of unit power factor, and the parameter igq+ in (2) is 0. In this condition, the expression of the double-frequency active power in (2) can be simplified as shown in (3): P g 2 ( u g + i g − + u g − i g d + ) 2 − 2 u g + i g d + u g − ( i g − − i g d − ) u g + = u g d + u g − = u g d − i g − = i g d − 2 + i g q − 2 , (3)where ug+ and ug- represent the positive- and negative-sequence voltages of PVS port, respectively; ig- represents the negative-sequence current of PVS. Since the equivalent impedance of positive-sequence network is almost equal to the load impedance, it will be much greater than the equivalent impedance of zero-sequence network, which is mainly composed of the zero-sequence impedance of lines and transformers. And it is obvious that when the neutral point on the high-voltage side of step-up transformer is directly grounded, the negative-sequence voltage of the PVS port and tie line will be much smaller than the positive-sequence voltage. In this condition, the parameter ug- in (3) can be approximately regarded as 0. When the neutral point on the high-voltage side of step-up transformer is ungrounded, since the local load of PVS is mainly the active load, whose equivalent impedance can be considered as resistive and is much larger than the impedance of lines and transformers, the positive-sequence network equivalent impedance is almost the load impedance. The negative network equivalent impedance is nearly the parallel impedance of PVS negative-sequence equivalent impedance and load impedance. When the negative-sequence equivalent impedance of PVS is smaller than the load impedance, the parallel impedance of the two will be much smaller than the positive-sequence network equivalent impedance. In this condition, the positive-sequence current and voltage of the fault port of tie line are almost in phase, while the negative-sequence voltage and current of the fault port are in reversed phase, and the negative-sequence equivalent impedance of PVS is resistive due to the influence of external circuit characteristics. However, when the negative-sequence equivalent impedance of PVS is large, since the load impedance is resistive, the parallel impedance of the two and the equivalent impedance of the positive-sequence network are also resistive in series. And in this condition, the positive-sequence current and voltage of the fault port are also almost in phase, and the negative-sequence equivalent impedance of PVS is also resistive due to the influence of external circuit characteristics. Consequently, the value of ig- is almost equal to the value of igd- in (3), and (3) can be simplified as shown in (4): P g 2 = u g + i g − + u g − i g d + . (4) During the non-full phase operation of PVS, the double-frequency active power generated by PVS will result in the double-frequency fluctuation of the DC bus voltage, which will eventually affect the PVS port voltage through the inverter control link. To clearly explain the relationship between the double-frequency active power fluctuation and the PVS port voltage, the control block diagram of the PVS inverter is given next, as shown in Figure 1. In Figure 1, the subscript "*" represents the instruction value. FIGURE 1Open in figure viewerPowerPoint Inverter control block diagram in the case that the negative-sequence current suppression control strategy is not started It can be seen from Figure 1 that when the DC bus voltage has a double-frequency fluctuation due to the presence of the double-frequency active power, the output d-axis current instruction value will also generate a double-frequency component through the outer voltage control loop of inverter. However, the output q-axis current instruction value is not affected by the bus voltage fluctuation and has no double-frequency component. Since the actual output d-axis and q-axis currents are generally extracted through a notch filter link, their measured values are almost DC components. In this condition, according to the inner current control loop of the inverter, the PVS port d-axis voltage will also generate a double-frequency component, while the PVS port q-axis voltage has no double-frequency component. According to (4) and Figure 1, the expressions of double-frequency components of the DC bus voltage, the output d, q-axis current instruction value and the PVS port d, q-axis voltage are shown in (5): U d c _ 2 r d = u g + i g − + u g − i g d + 2 C U d c ∗ ω 1 i g d + ∗ _ 2 r d = U d c _ 2 r d k p v 2 + k i v 2 4 i g q + ∗ _ 2 r d = 0 u g d + _ 2 r d = i g d + ∗ _ 2 r d k i v 2 + k i i 2 4 u g q + _ 2 r d = 0 , (5)where Udc_2rd, igd+*_2rd, igq+*_2rd, ugd+*_2rd and ugq+*_2rd represent the double-frequency components of the DC bus voltage, the d, q-axis current instruction values and the PVS port d, q-axis voltages, respectively; C represents the DC bus capacitance; Udc* represents the DC bus voltage instruction value; Kpv and Kiv represent the proportional and integral adjustment parameters of the outer voltage control loop, respectively; Kpi and Kii represent the proportional and integral adjustment parameters of the inner current control loop, respectively. When the double-frequency components of d, q-axis voltages is transformed from d, q coordinate system to abc coordinate system, the negative-sequence fundamental frequency voltage and the third harmonic voltage will be generated, and the expression of the negative-sequence fundamental frequency voltage is shown in (6): u g − = k u g + i g − 1 − k i g d + k = k p v 2 + k i v 2 4 k p i 2 + k i i 2 4 4 C U d c ∗ ω 1 . (6) According to (6), the equivalent impedance of PVS in the negative-sequence network is z g − = k u g + 1 − k i g d + , (7)where Zg- is the negative-sequence equivalent impedance of PVS. It can be seen from (7) that during the non-full phase operation, PVS, whose negative-sequence current suppression strategy is not started, can be equivalent to a non-constant impedance model in the negative-sequence network. And this impedance is related to the parameter k, the PVS port positive-sequence voltage and the output positive-sequence d-axis current. 3 THEORETICAL ANALYSIS METHOD AND CALCULATION MODEL OF NON-FULL PHASE OPERATING VOLTAGE Taking phase-A grounding fault of the tie line as an example, the schematic diagram of PVS non-full phase operation is shown in Figure 2. It should be noted that the main research object of this paper is the single-phase reclosing on the tie line of photovoltaic power plant. Generally speaking, the weaker the electrical connection between the photovoltaic power plant and the grid system, the more disadvantaged it is that the voltage and current of the photovoltaic power plant during the non-full phase operation are maintained at the rated value. Therefore, it is the most serious application scenario of the over-voltage problem that the photovoltaic power plant is interconnected with the grid system only through a tie line, and it is reasonable to focus on this scenario for analysis. FIGURE 2Open in figure viewerPowerPoint Non-full phase operation diagram of PVS As for the single-phase reclosing, on the one hand, it is necessary to deeply study the adverse impact of the non-full phase operating status on PVS. On the other hand, it is necessary to evaluate and analyse the possible closing impulse current and voltage of the single-phase reclosing. The above studies should comprehensively consider the effects of the neutral grounding mode of step-up transformer, the control strategy of PVS under unbalanced voltage, the local load level, etc. Among them, the influence of the neutral grounding mode of the step-up transformer dominates. The detailed analysis and description are as follows. 3.1 The neutral point on the high-voltage side of step-up transformer is directly grounded 3.1.1 The negative-sequence current suppression control strategy is started When phase A of the tie line is disconnected and the neutral point on the high-voltage side of step-up transformer is directly grounded, the positive-sequence, negative-sequence and zero-sequence networks are shown in Figure 3. Among them, since the negative-sequence current suppression control strategy is started, the PVS side of the negative-sequence network can be considered as open circuit. In Figure 3, subscripts 1, 2 and 0 represent the positive-sequence, negative-sequence and zero-sequence, respectively; I ̇ g1 and V ̇ g 1 represent the equivalent current source of PVS and the positive-sequence voltage of the PVS port, and their relation is shown in (1); E ̇ g ′ represents the equivalent voltage source of the power system; Zg represents the impedance of the package transformer of PVS; Z g ′ represents the equivalent impedance of the power system; ZT, ZLD and ZL represent the impedance of the step-up transformer, local load and tie line, respectively. FIGURE 3Open in figure viewerPowerPoint Three-sequence network diagram in the case that the neutral point of step-up transformer is directly grounded and the negative-sequence current suppression strategy is started: (a) positive-sequence network diagram, (b) positive-sequence simplified network diagram, (c) negative-sequence network diagram and (d) zero-sequence network diagram In Figure 3, since the load impedance is generally much larger than the impedances of lines, transformers, and power system, the positive-, negative- and zero-sequence equivalent impedances of the fault port can be approximated as shown in (8): Z 1 Σ = Z L D 1 Z 2 Σ = Z L D 2 Z 0 Σ = Z T 0 + Z L 0 + Z ′ g 0 . (8) When phase A of the tie line is disconnected, the boundary condition of the fault location is shown in (9): I ̇ F a = 0 Δ V ̇ F b = Δ V ̇ F c = 0 . (9) According to (9), the composite-sequence network in the case of phase-A disconnection can be obtained as shown in Figure 4(a). FIGURE 4Open in figure viewerPowerPoint Composite-sequence network diagrams of fault phase: (a) composite-sequence diagram and (b) simplified composite-sequence diagram According to (8), since the negative-sequence equivalent impedance is almost the load impedance, which is generally much larger than the zero-sequence equivalent impedance, the composite-sequence network shown in Figure 4(a) can be simplified to the equivalent circuit shown in Figure 4(b). In Figure 4, V ̇ f represents the open-circuit voltage of the fault port, and its value is the difference value between the PVS equivalent voltage source (ZLD I ̇ g1) and the power system equivalent potential source ( E ̇ ′ g) in Figure 3(b). For three-sequence voltages of the tie line, it can be seen from Figure 4(b) that since Z1∑ is approximately equal to the load impedance and much larger than Z0∑, the positive-sequence voltage of fault port (Δ V ̇ fa1) is much smaller than the voltage drop on the load impedance. And combined with Figure 3(b), it can be seen that the positive-sequence voltage of the tie line ( V ̇ L1) is similar to E ̇ ′ g, which means that the effective value of V ̇ L1 is always approximately equal to 1 p.u. Since both negative-sequence voltage ( V ̇ L2) and zero-sequence voltage ( V ̇ L0) of the tie line are lower than Δ V ̇ fa1 and much smaller than V ̇ L1, the phase voltage of the fault phase is also approximately equal to E ̇ ′ g, whose value is almost the rated voltage value. For the output current of PVS, since the negative-sequence current suppression control strategy is started, the output current contains only the positive-sequence component. Meanwhile, since the effective value of V ̇ L1 is approximately equal to 1 p.u., it can be seen from (1) that the output current of PVS is only related to its output active power, whose value is 1 p.u. in the case that PVS outputs rated power. Define the ratio of the local load capacity to the output capacity of PVS as the load level. For three-sequence currents of the tie line, when the load level is between 0% and 100%, PVS delivers power to the grid, and since Z2∑ is much larger than Z0∑, the amplitudes of the positive and zero-sequence current of fault phase are basically equal, and their phases are opposite. Meanwhile, as the load level rises, the non-fault phase current will decrease continuously to 0. When the load level is greater than 100%, PVS will absorb power from the grid, and since Z2∑ is no longer much larger than Z0∑, as the load level rises, the positive-sequence current of the tie line continues to increase, and the non-fault phase current will also increase from 0. 3.1.2 The negative-sequence current suppression control strategy is not started When the negative-sequence current suppression control strategy of PVS is not started during the non-full phase operation, it can be seen from (7) that PVS can be equivalent to a non-constant impedance model. In this condition, the negative-sequence network is shown in Figure 5, and both positive-sequence and zero-sequence networks are the same as in Figure 3. FIGURE 5Open in figure viewerPowerPoint Negative-sequence network diagram in the case that the negative-sequence current suppression strategy is not started It can be seen from Figure 5 that when the negative-sequence current suppression strategy of PVS is not started, since the load impedance is in parallel with the PVS equivalent impedance, Z2∑ is smaller than that in the circumstance of starting the negative-sequence current suppression strategy (as shown in Figure 3(c)). In this condition, the parallel equivalent impedance of the negative-sequence