On-Site Engineering Test of Active Support Control for the PV Station and Wind Farm in the AC-DC Hybrid Power Grid under Extreme Fault Conditions

Abstract

Power systems have developed significantly because of the increasing share of renewable energy sources (RESs). Despite the advantages, they also bring inevitable challenges to power system stability, especially under extreme fault conditions. This paper presents a practical active support control strategy for RESs to support the power grid under extreme fault conditions. The proof process is taken in an AC-DC hybrid power grid integrated with large capacity of PV stations and wind farms. The on-site engineering test results reflect that RESs bring potential risks in the AC-DC hybrid power grid operation and validate the excellent engineering practical features of the proposed control strategy. In addition, test results also reveal predisposing factors of power system instability which are missing in the simulation and fault simulation device-based testing results. They prove the outstanding advantages of on-site engineering tests.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Power systems see more and more photovoltaic (PV) and wind generation integration. Within increasing renewable energy source (RES) penetration level, despite the advantages such as environmental friendly and sustainable development, it also brings problems to the utility grid [1–3]. Adjusting the power source structure brings an inevitable impact on the power system primary frequency response due to the conventional generators’ reduction and consequent loss of inertia [4]. Therefore, the provision of ancillary services is becoming an increasingly challenging task to this new generation power system operation.

The power source structure has developed significantly because of the increasing share of RESs in the power system, and the coupling interaction between RESs and the power grid is becoming significant which reduces delivery capability and power accommodation capacity of the power system [5]. In addition, within disturbance rejection ability and robust stability, low overload capacity, insufficient tolerance to voltage changes, and all these characteristics of power electronic equipment may also deteriorate the power system operation environment even further.

Most of the PV stations and wind farms do not participate in power grid control at the present stage. Some large-scale grid-connected PV stations or wind farms adjust output power according to the automatic generation control (AGC) and automatic voltage and reactive power control (AVC) system to regulate voltage and frequency as power system requirements [6, 7]. Consequently, the conventional control or operation mode is no longer efficient in the new generation power system [8, 9]. Zhang et al. [10] studied the control performance of AGC for wind power ramping based on deep reinforcement learning. Prasad and Padhy [11] proposed the synergistic frequency regulation control mechanism for DFIG wind turbines with optimal pitch dynamics. Complicated stochastic AGC modeling causes high computational burdens concurrently. Chen et al. [12] used the Itô-theory-based model to reduce the computational burden of optimization considering non-Gaussian wind power uncertainty. However, random communication delay and noise disturbance in the AGC/AVC control network usually cause the control system performance degradation or even system destabilization.

To address these challenges, the PV station and wind farm should provide active support to the power system in external faults and other transient processes. Using ESSs to add regulation capacity and improve the dynamic performance of AGC, particularly at the high RES penetration power systems, is a feasible solution [13–16]. Su et al. [17] proposed an adaptive robust sliding-mode control for energy storage system integrated PV and wind station to provide frequency and voltage control functionality for RESs. Wang et al. [18] used the supercapacitor in the large-scale hybrid wind-PV farm to improve stability in the multimachine power system. The large-scale grid-connected PV station or wind farm requires a large-capacity energy storage system which is not available at present. Therefore, it is more practical to use existing equipment such as the PV inverter and wind power generator and also conventional reactive power compensators such as static VAR compensator (SVC) or static VAR generator (SVG) to provide active support to the power grid. Karbouj et al. [19] proposed a self-adaptive voltage controller to enable solar PV power plant participation in voltage control ancillary service. STATCOM is used in large PV stations for fault-induced delayed voltage recovery alleviation in [20]. A coordinated damping optimization control strategy for wind power generators and their reactive power compensators is proposed in [21]. Wang et al. [22] analyzed the interval overvoltage risk caused by the impacts of load uncertainties and SVC.

Simulation and laboratory tests based on theoretical deduction are the most frequently used method in RES control strategy validation or power system. The performance of the proposed controller in [9] is demonstrated using simulation studies of an interconnected power system which are conducted within the DIgSILENT Power Factory platform. Case studies in [17] were developed based on MATLAB, while Varma and Mohan [20] presented the validation process by PSCAD/EMTDC. A three-phase four-wire hybrid simulation platform integrating the advantages of both digital simulation and physical simulation is developed in [23] which combines the physical simulation system and real-time digital simulator. Wang et al. [24] built laboratory platforms for experimental verification. Zimmerman et al. [25] presented the details of the network modeling and problem formulations used by MATPOWER. Reshikeshan et al. [26] verified the proposed autonomous voltage regulation scheme by power flow simulations on the EPRI Circuit 24 test feeder in an open-source distribution system simulation platform.

The large-scale PV station or wind farm is connected to the power grid with long electrical distance, and the reactive power control capability is relatively insufficient which makes voltage stability a challenging task for the power system, especially during large disturbances [27–30]. Laboratory tests only validate the operation performance of PV inverters or wind generators in the islanding mode, and some proof processes are taken in the microgrid. The power system is a very complicated, nonlinear, and strong coupling dynamic system, and experimental results based on islanding or grid-connecting setup are inadequate when it comes to a large-scale grid-connected PV station or wind farm.

On the contrary, the fault simulation device is the common option in RES on-site testing. However, because of the maximum voltage and current limitation, its capacity is also limited; consequently, it is almost impossible to simulate the power system. Fault simulation device is applied to PV inverter and wind generator onsite experiments; however, it is inadequate for large RESs station onsite engineering test, as it is impossible to simulate voltage waveform at grid connection point of RESs station by fault simulation device.

Therefore, the on-site engineering tests are necessary in the PV station and wind farm active support control study. The power system with large-scale RES and power electric device-based projects such as high-voltage direct current (HVDC) have much more possibility of voltage and frequency instability.

This paper proposes a practical active support control for the PV station and wind farm to support the power grid under extreme fault conditions. The excellent engineering practical features of the proposed control strategy are important since active support capability is an obligation for the PV station and wind farm in the future, and upgrading the RES with the large-capacity energy storage system is uneconomical, plus for some PV stations or wind farms, there is no space for ESSs. In addition, their control capability is verified through on-site engineering test in an AC-DC hybrid power grid integrated with large capacity of the PV station and wind farm. The on-site test includes three categories, and each has fifteen grounding faults at different sites.

2. Proposed Active Support Control for the PV Station and Wind Farm

In this section, the construction of the proposed active support control strategy for the PV station and wind farm is presented. Figure 1 gives the topology of the PV station. The PV inverters are connected to the power grid through a 10 kV/35 kV transformer; then, it is integrated to a 110 kV collection substation through a 35 kV/110 kV transformer with a long transmission line.

[figure omitted; refer to PDF]

The topology of the wind farm is presented in Figure 2. The wind power generators are connected to the power grid through a 0.69 kV/35 kV transformer; then, it is integrated to a 110 kV collection substation through a 35 kV/110 kV transformer with a long transmission line.

[figure omitted; refer to PDF]

Figures 10 and 11 present output voltage and current waveforms of the centralized PV inverter in PV station 8. And its output active and reactive power is given in Figure 12.

[figure omitted; refer to PDF]

Figures 13 and 14 present output voltage and current waveforms of the series-connected PV inverter in PV station 8. And its output active and reactive power is given in Figure 15.

[figure omitted; refer to PDF]

Figures 16 and 17 present voltage and current waveforms at one point of common coupling (PCC) in PV station 8. The power flow value of this PCC is given in Figure 18.

[figure omitted; refer to PDF]

Figures 19 and 20 present voltage and current waveforms at another point of common coupling (PCC) in PV station 8. The power flow value of this PCC is given in Figure 21.

[figure omitted; refer to PDF]

Table 5 gives the voltage value at the high voltage side of main transformer $\#1$ in 750 kV and 330 kV substations; the voltage value on transmission lines is given in Table 6. $u_{\text{before}}$ is the voltage before the fault, and $u_{\text{after}}$ is the voltage when the fault occurs.

Table 5

Voltage value at 750 kV substations.

Station	$u_{\text{before}}$ (kV)	$u_{\text{after}}$ (kV)	Voltage drop (%)
Station 2	446.505	344.051	22.95
Station 3	447.42	277.349	38.01
Station 4	439.684	291.556	33.69
Station 6	438.422	354.295	19.19
Station 8	201.137	137.614	31.58
Station 11	195.322	133.143	31.23
Station 19	199.053	37.915	86.61
Station 22	200.141	39.249	86.17

Table 6

Transmission line voltage value.

Station	Line	$u_{\text{before}}$ (kV)	$u_{\text{after}}$ (kV)	Voltage drop (%)
Station 2	2-6	446.963	344.375	22.95
	2-1	446.532	344.078	22.94
	2-4	446.855	344.092	23.00
	2-35	203.175	167.189	17.71
	2-36	202.523	166.642	17.72
	2-37	203.029	167.096	17.70
	2-38	203.018	167.079	17.70
Station 3	3-6	445.532	276.217	38.00
	3-4	445.727	276.327	38.01
	3-14	200.112	11.951	91.35
	3-19	200.541	11.195	94.42
Station 4	4-1	437.51	289.99	33.72
	4-2	438.806	290.989	33.69
	4-3	437.28	290.908	33.47
	4-5	439.117	291.03	33.72
	4-8	200.377	137.554	31.35
	4-11	201.233	138.107	31.37
	4-26	200.716	137.75	31.37
	4-15	200.663	137.738	31.36
	4-18	200.734	137.75	31.38
Station 6	6-3	437.09	353.268	19.18
	6-4	437.34	353.407	19.19
	6-45	436.66	352.894	19.18
	6-46	436.993	353.185	19.18
Station 8	8-4	201.166	138.347	31.23
Station 11	4-11	193.102	132.145	31.33
Station 20	3-20	200.203	30.123	84.95
Station 19	3-19	199.938	37.820	86.63
Station 22	3-22	200.986	36.296	87.22

5. Conclusions

The dynamic performance of PV inverters in PV station 8 during artificial grounding faults verifies the proposed active support control strategy, which means RES has the capability of disturbance rejection under extreme fault conditions, so it is practical for the RES to regulate voltage and frequency. There are several results seen only during on-site engineering tests:

(1) Through the on-site tests, it is clear that numbers of PV inverters are disconnected when a short circuit fault occurs. However, it did not occur in the fault simulation device experiments.

(2) Blocking and disconnection of the SVC and SVG are found in both PV stations and substations. Therefore, parameter modification or equipment upgrading is the reasonable solution for them.

(3) The test results reflect potential risks in the power grid operation and reveal predisposing factors of power system instability which is not shown in simulation experiments.

(4) Electromechanical simulation and electromagnetic simulation are practical methods in the power system and RES study, but they cannot fully reflect the electromagnetic characteristics during extreme conditions such as grounding faults and transient voltage sag.

(5) The outstanding benefits of on-site engineering tests are proved.

Acknowledgments

This work was supported by the Qinghai Science and Technology Program (Grant no. 2018-GX-A6) and State Grid Jibei Electric Power Company Zhangjiakou Chongli District Power Supply Branch Technology Project.

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