Protection adaptability analysis method for photovoltaic grid-connected generating system

Inverter power sources such as photovoltaic inverters and SVG have unique fault characteristics compared to traditional synchronous motors due to their unique power generation methods, circuit topology, and low-voltage traversal control strategies. The power grid faces various adverse effects due to the ongoing expansion of photovoltaic power station (PV power station) capacity. How to comprehensively analyze the impact of photovoltaic power stations above 1000 MW on power grid relay protection has become a problem. Therefore, a protection adaptability analysis method for photovoltaic grid-connected generating systems is proposed, which is suitable for engineering analysis before the connection of large-capacity PV power stations to ensure the safety of the power system. And it is found that current differential protection exhibits better adaptability than distance protection.


Introduction
Inverter power sources such as photovoltaic inverter and SVG have unique fault characteristics compared to traditional synchronous motors due to their unique power generation methods, circuit topology, and low-voltage traversal control strategies [1,2].After the large capacity PV power stations are connected, various adverse influences on the power system relay protection result in misoperation or rejection and threaten the power system.
The impact of new energy volatility is analyzed in [3] but mainly elaborates from a planning perspective.Furthermore, load level, fault voltage drop depth, and PI controller parameters are analyzed, but there needs to be a more theoretical solution method of protection [4].Wang et al. [5] analyzes protection adaptability but is limited to wind farms and 220 kV power grids.
This paper proposes a method that combines theoretical analysis and simulation analysis to solve the protection adaptability analysis problem for connecting large-capacity inverter power sources, especially in higher voltage levels power grids.Theoretical analysis includes the fault characteristics of PV power stations and the protection principle.Simulation analysis includes power grid modeling and protection criterion modeling.This method is more comprehensive, and the two analysis paths can support each other.

The PV power station fault characteristics
The photovoltaic inverter utilizes vector decoupling control, where P/Q decoupling establishes the outer control loop, and the inner control loop involves positive and negative/zero sequence currents.Typically, the negative sequence current command value is set to zero.When encountering faults, the photovoltaic inverter must possess the capability for low voltage ride-through to fulfill the reactive power demands of the power grid system.Additionally, the inverter continues to output active current, ensuring that the total current remains below the inverter's current limit.The PV power station fault current characteristics can be summarized as follows: • The transient process of photovoltaic inverter fault current is brief.The steady-state component of the fundamental wave of photovoltaic inverter only includes positive sequence components, excluding negative sequence and zero-sequence components.The amplitude and phase of the positive sequence current mainly depend on the degree of voltage drop at the connection point, the current limit of the inverter, the low voltage ride through (LVRT) operation control strategy, and the active power transmission level before the fault.• The fundamental amplitude of the positive sequence current is limited during the LVRT.The shallower the voltage drop and the lighter the load result is, the less reactive power is supported by the photovoltaic inverter and the smaller the fundamental amplitude value.On the contrary, the heavier the load and the deeper the voltage drop is, the greater the fundamental wave amplitude value is.• During LVRT, the fundamental phase angle of the positive sequence current exhibits significant fluctuations, ranging from -90° to 0°.When the system operates with zero reactive power, the fundamental phase angle remains at 0°.However, when the photovoltaic inverter is employed for reactive power support, a lighter load or a deeper voltage drop results in a more considerable fundamental phase angle, potentially reaching -90°.Conversely, a smaller voltage drop or a heavier load reduces the fundamental phase angle, with the minimum value being 0°.• During symmetrical faults, the output current frequency may shift when the fault point is far from the PV power station and the inverter is decoupled from the main grid based on synchronous generators.The mathematical model and control strategy of SVG is similar to the photovoltaic inverter, so its fault current response characteristics are similar, too.The difference is that the photovoltaic inverter mainly emits active power before the fault, while the SVG mainly emits reactive power.During the fault, the photovoltaic inverter outputs active power based on dynamic reactive power compensation, while SVG is still a pure reactive power supply.The fundamental phase angle of the positive sequence current remains unchanged during faults, and the fundamental phase angle is always 90° (slightly less than 90° due to the active power loss).Overall, PV power station has the following impacts on system fault analysis and relay protection: • The fault current fed out by the PV power station exhibits complex controlled and weakly fed characteristics.The current amplitude of the photovoltaic inverter and SVG and the phase angle change characteristics of the photovoltaic inverter exhibit nonlinear controlled characteristics.Therefore, the fault component specificity based on the synchronous generator equivalent model is no longer valid.• The fault current must be accurately calculated through iterative methods instead of node admittance matrix and boundary conditions.• In the setting calculation of protection, the maximum short-circuit current can be anticipated according to the inverter current limit.

Theoretical analysis of the influence of PV power stations on protection
When the power supply capacity of PV power stations is small, quick switching of PV power station units during faults or limiting the connection capacity is generally used to reduce its impact on grid protection [6,7].The configuration plan, implementation principle, and setting method of grid protection are the same.Nonetheless, as the power supply capacity of PV power stations continues to grow, it exerts various detrimental impacts on the power grid's protective measures.It is necessary to conduct systematic evaluation and analysis and propose effective measures to ensure the power grid [8].
The transmission lines of PV power stations are analyzed as an example.

Distance protection
The composition of distance protection mainly includes phase selection elements, directional elements, and impedance measurement elements.The principles and algorithms used by different manufacturers also vary widely, which needs to be analyzed separately.The PV side distance protection for the outgoing lines of PV power stations should be analyzed, especially when a forward fault occurs, including operation correctness and the ability to withstand transition resistance.For distance protection on the power grid side, more attention should be paid to the protection performance when a reverse fault occurs in the backward direction, such as whether there will be misoperation outside the protection zone.
Overall, the criterion based on negative sequence current probably needed to be revised.The phase difference between the measurement current of impedance components and the current of the fault point may be large, which affects the accuracy of impedance measurement.The ability of distance protection to withstand transition resistance is insufficient.When a three-phase short circuit occurs, the inverter PLL is unstable, which may cause misoperation outside the protection zone.

Phase-segregated current differential protection
The inverter current limit and control strategies impact the sensitivity of protection during an internal fault, depending on the type of fault.The sensitivity of differential protection decreases during an internal fault when the photovoltaic inverter supplies a significant proportion of current or when more active power is generated.However, this sensitivity generally does not fall below the level observed when only a single power source is present.In cases where the short-circuit current of the power system significantly exceeds that of the PV power station, the influence on the sensitivity of phase-segregated current differential protection is relatively minor, and it does not fail the differential protection system.

Zero-sequence current differential protection
When a single-phase grounding short-circuit occurs, the zero-sequence differential current increases.Meanwhile, photovoltaic inverters generally do not output zero-sequence current.So, the zero-sequence current phases are not affected by the photovoltaic inverter.Therefore, the PV power stations generally do not lead to a decrease in the sensitivity of zero-sequence current differential protection but improve the protection sensitivity.

Simulation analysis of the impact of PV power stations on protection
Theoretical analysis explores the potential degradation in protection performance and guides simulation.Simulation analysis can verify the rejection or misoperation of the protection and prove theoretical analysis.An equivalent modeling method for expanding the capacity of a single photovoltaic unit is proposed.A detailed PV station model is established in PSCAD 4.6 simulation software and the equivalent model.Both simulation models are shown in Figure 1.The detailed simulation model includes four 5 MW photovoltaic units connected to the 35 kV bus through their respective box transformers and power collecting lines.The equivalent model expands the capacity of the single-unit model and is directly connected to the 35 kV bus.Under full load conditions, when the three-phase short circuits and BC interphase short circuits occur, the measured current waveform of the two models is shown in Figure 2. The fault time is 0.6 seconds.The black and red represent the three-phase current of the detailed model and equivalent model.It can be found that the detailed simulation model and equivalent model provide a good fit of current during faults, which indicates that the equivalent modeling method has great accuracy and meets the fault simulation requirements.The influence of box transformers and power-collecting lines is relatively small.This method can further reduce computation and improve simulation speed by modeling PV power stations.In addition, the power grid and nearby generators should be established, and the relevant protection principles should be programmed.The simulation results should be input into the protection We mainly conducted simulation analysis on two sets of distance protection of Section I and differential protection, and the main situations of incorrect actions are as follows.The simulation results show that when faults occur in K2, including A phase grounded fault, BC phase short circuit, and BC phase grounded fault, rejection action of distance protection will occur due to the failure of negative sequence directional components.Ground distance protection can withstand greater transition resistance than phase distance protection.For the quadrilateral criterion, when the transition resistance of K3 is larger than 1 Ω, phase distance protection is possible to refuse action.But for polarization criteria, the situation will be better.In some extremely special cases, actions beyond the protection range, such as a fault in K4, may also occur.
It is found that the simulation and theoretical analysis of protective action are consistent, especially when there is no synchronous generator near the PV power station.The current differential protection can all run correctly, and the sensitivity change can also be tolerated.Because only distance protection operates faster than the current differential protection of Section I, and the distance protection is prone to incorrect actions, it can be considered to exit the distance protection of Section I.

Conclusion
Through theoretical analysis of the PV power stations fault characteristics and the principle of relay protection, it is possible to quickly identify faults that can cause rejection or misoperation of protection.Furthermore, simulation analysis is used to reproduce and obtain accurate parameters of the fault situation.Combining theoretical and simulation analysis can achieve protection adaptability analysis and further approach setting verification.

Figure 1 .
Figure 1.The simulation model of photovoltaic units.

Figure 2 .
Figure 2. The simulation result of two models.

Figure 3 .
Figure 3.The simulation model of the photovoltaic grid-connected generating system.