https://orcid.org/0009-0007-0458-9616
Abstract
In view of the possible voltage and current overshoot problems in the process of new energy grid connection, the dynamic characteristics analysis and control strategy of the network converter in the new medium voltage flexible interconnection project are proposed. Firstly, a synchronous control strategy based on voltage phase and amplitude compensation is proposed. Then, based on the characteristics of new AC-DC hybrid distribution network, the new energy medium voltage flexible interconnection network is analyzed. The simulation experiment reveals the dynamic response characteristics of the new energy grid converter under frequency interference and load change.
Introduction
With the growing global energy crisis and environmental problems, renewable energy, such as wind and solar energy, has been rapidly developed and widely used in power generation fields [1]. However, in the existing power system, most wind and photovoltaic power generation devices and their supporting energy storage systems mainly adopt the network control strategy and usually lack active support function, so they cannot provide inertial response and frequency/voltage control similar to the traditional synchronous generator [2, 3]. As the proportion of renewable energy in the power system increases, these problems lead to a decline in the transient stability and small signal stability of the grid, especially in weak grid environments with low short-circuit ratios, which may exacerbate the risk of system instability. In terms of frequency stability, the analysis of frequency disturbance events in the UK on 9 August 2019 shows that the replacement of large-scale new energy generators and the reduction of the system inertia level is one of the reasons for the large area of power failure. After the disturbance occurs, the system frequency change rate is too high, which triggers the off-grid of a large number of distributed power sources and further reduces the frequency of the system [4, 5]. In terms of voltage stability, the delivery terminal of large-scale centralized new energy system often faces the problems of insufficient local synchronous power supply and weak disturbance resistance ability. For example, the transmission end of Zhangxiong UHV project is in 5.4 million kW operation mode, and the short circuit ratio of multiple stations is only 0.91 to 1.13, which is a typical weak grid environment [6, 7]. In fact, after the removal of short circuit symmetrical fault, the converter controlled by the network may produce reactive excess, thus causing the transient overvoltage phenomenon, which constitutes a serious restriction on the transmission capacity of the project. In order to solve the above problems, the research frontier is turning to the use of structured network converter to achieve large-scale access to renewable energy. The main research directions include the transient response and stability of the network generator set, the fault crossing strategy and the influence on the low-frequency oscillation and secondary synchronous oscillation of the system after access. In the new energy medium voltage flexible interconnection project, the dielectric structure network inverter, as the key equipment, is the hot spot in the field of power electronics technology. Currently, researchers are working to gain a deep understanding of the dynamic response characteristics of frequency converters under different working conditions, including load changes, grid disturbances, and dynamic behavior in fault situations. Establishing accurate mathematical model and analyzing the transient and steady state characteristics of frequency converter can provide the theoretical basis for the design of control strategy.
In order to overcome the challenges caused by the intermittence and volatility of renewable energy generation, that is, large-scale access to the power grid may have a negative impact on the stability of the power system, the new energy medium voltage flexible interconnection system has been widely concerned and applied as an effective solution [8, 9]. The system significantly enhances the flexibility and reliability of the power grid through the application of advanced power electronics technology and control strategies. In recent years, with the progress of new energy power generation technology, more and more distributed power sources are connected to the power system. These distributed power sources usually use medium-voltage level access to reduce transmission losses and improve power quality.
As the key equipment connecting the new energy power generation unit and the power grid, the grid-grid converter plays an important role in maintaining the stable operation of the power system. However, the power grid frequency fluctuation, load fluctuation, and other factors may lead to the problems such as voltage and current mutation in the actual operation of the grid converter, which may threaten the stability and safety of the system [10, 11].
This study focuses on exploring the dynamic characteristics of network converter when connected, and proposing effective control strategies for possible problems. By comprehensively analyzing the dynamic characteristics of the structural network converter, we can more accurately understand its response behavior under different operating conditions and provide theoretical support for the design of more robust protection measures [12]. In addition, this study also proposes a synchronous control strategy based on voltage phase and amplitude compensation, as well as a series of protection measures for frequency disturbance, voltage overshoot and load change, aiming to improve the stability and reliability of the system and ensure the safe and efficient operation of the medium voltage flexible interconnection system of new energy [13].
With the development of new energy technology, frequency converter needs to be integrated with a variety of energy equipment. Researchers are committed to solving the compatibility problem between frequency converter and new energy equipment to ensure the overall operation of the system [14, 15]. At present, the researchers mainly focus on the dynamic response characteristics of frequency converter under different working conditions, including the influence of load change, power grid fluctuation, and other factors on the performance of frequency converter. By establishing the precise mathematical model, analyze the dynamic process of frequency converter to improve its adaptability and stability in new energy system. In order to improve the performance of the frequency converter, the researchers proposed various control strategies, such as vector control and direct torque control. These strategies are designed to achieve precise control of the inverter output frequency and voltage to accommodate the changing load requirements in new energy systems.
Medium-voltage flexible interconnection device and its working principle
Medium voltage flexible interconnection unit (flexible interconnection device, FID) is an advanced power electronics device that usually contains two or more voltage source converters (voltage source converter, VSC) connected to each other through their DC sides. Figure 1 shows a typical connection mode of the medium voltage flexible interconnection unit (FID-VSC) to the distribution network, including the AC feeder i. In view of the possible voltage and current overshoot problems in the process of new energy grid connection, the dynamic characteristics analysis and control strategy of the network converter in the new medium voltage flexible interconnection project are proposed. Firstly, a synchronous control strategy based on voltage phase and amplitude compensation is proposed. Then, based on the characteristics of new AC–DC hybrid distribution network, the new energy medium voltage flexible interconnection network is analyzed.
System structure.
The working principle of the medium voltage flexible interconnection device is that by adjusting the output voltage of the converter, it can effectively control the energy exchange between the distribution network and realize flexible power transmission and distribution [16]. This feature allows the FID–VSC to play a key role in improving grid stability and flexibility, especially when accessing distributed generation resources to the grid. Such devices can not only optimize the operating efficiency of the power system, but also enhance the overall controllability and reliability of the system.
In Fig. 1, the |${U}_{DC}$| side voltage representing the FID–VSC connected to the AC feeder i; |${E}_{\mathrm{iVSC}}$|and|${U}_{iVSC}$|, respectively, indicating the connection point voltage and the modulated voltage output by the converter. Furthermore, |${P}_{iV, SC\left(\mathrm{t}\right)}$| and |${Q}_{iV, SC(t)}$| represent the values of active power and reactive power exchanged between the converter and the distribution system at time point t, which can be calculated by specific formulas.
In the formula, |$\delta{E}_{\mathrm{iVSC}}{X}_L{E}_{\mathrm{iVSC}}{U}_{iVSC}$| represent the phase angle difference between and; and R and represent the resistance and resistance elements in the filter, which work on AC filter and limit short circuit current. Combined with Fig. 1 and Formula (1), fast and independent power control can be achieved in the active-reactive power (P-Q) quadrant by adjusting the phase angle (delta) and connection point voltage in the medium voltage flexible interconnection device (FID–VSC). In this way, the power balance and reactive power support between multiple stations and interconnected systems can be realized within its capacity limit. In addition, when an area or feeder in the interconnection system fails, the FID–VSC can implement current limit control to protect other interconnected stations from the fault and provide the necessary voltage and frequency support for the fault area.
The working principle of new energy construction network converter and its dynamic characteristics under different working conditions
Working principle of grid converter (grid-forming converter)
Grid-forming converter (grid converter, GFC) is one of the key equipments in the field of power electronics. Its main function is to connect renewable energy generation systems (such as solar photovoltaic panels, wind turbines, etc.) to the grid to achieve efficient transmission and distribution of energy. Its working principle involves the following core aspects:
Energy conversion: GFC uses the high-frequency switching function of power electronic switching devices (such as insulated gate bipolar transistor IGBT) to realize the conversion between direct current and alternating current. This conversion process utilizes pulse width modulation (PWM) technology to ensure the sinusoidal characteristics of the output current waveform [17].
Filter processing: in order to reduce the harmonic composition in the output current and improve the power quality, GFC is usually equipped with an AC side filter. Filter design should consider system impedance matching to reduce loss and improve efficiency [18].
Control strategy: in order to ensure the stable operation of GFC and the power grid, an accurate control algorithm must be adopted [19].
Synchronization function: GFC synchronizes with the frequency and phase of the power grid through a built-in synchronization controller. This feature requires GFC to be able to accurately track grid frequency changes and maintain pace with the grid by adjusting its output characteristics [20].
Protection mechanism: in order to avoid equipment damage caused by overload or failure, GFC is usually equipped with overcurrent protection, overvoltage protection, and other functions. These protection mechanisms can quickly cut off the power supply in abnormal situations to ensure the safe and stable operation of the system [21].
Analysis of the dynamic characteristics of the structured network converter
The dynamic characteristics of the network converter under different working conditions have the important influence on its performance. The following is the dynamic characteristic analysis under several typical working conditions:
Power grid frequency disturbance working condition
·Frequency response: when the grid frequency is disturbed, GFC needs to be able to respond quickly to maintain the stability of output power. This means that the converter needs to have a good frequency tracking capability.
Among them, |${f}_{(t)}$| is the frequency after the disturbance, |${f}_0$| is the rated frequency of the power grid, and |$\triangle{f}_{(t)}$| the frequency deviation.
Power adjustment: in the case of frequency disturbance, GFC also needs to be able to quickly adjust the output power to maintain system stability. This involves the design of control strategies, such as using the virtual synchronous machine (virtual synchronous machine, VSM) technology to simulate the behavior of traditional synchronous generators.
Among，|${p}_{out}$|is the output power, |${p}_{ref}$| is the reference power, and |${k}_f$| is the frequency response coefficient.
Load change working condition
· Voltage stability: when the load changes, GFC needs to be able to quickly adapt to the load change to ensure the stability of the output voltage. This is usually achieved by optimizing the design of the control loop.
Among them, |${V}_{out}(t)$|is the output voltage, |${V}_{ref}$|is the reference voltage, and |$\triangle V(t)$|is the voltage deviation.
· Current response speed: GFC needs to be able to respond quickly to changes in current demand caused by load changes. This requires the design of a current control loop with a high bandwidth.
Among them,|${I}_{out}(t)$| is the output current, |${I}_{ref}$|is the reference current, and |$\triangle I(t)$| is the current deviation.
Failure working condition
In case of short circuit, GFC shall be able to quickly detect the occurrence of faults and take corresponding measures such as disconnecting from the power grid to avoid damage to the equipment. In addition, it can also include current-limiting control, over-voltage protection, and other functions.
The grid-connected flow converter realizes the self-synchronous grid-connected power output through droop control, which shows the controllable voltage source characteristics based on power synchronization. Specifically, |${I}_{GFM}$| is external characteristics are equivalent to a controlled voltage source, in Fig. 2, |${V}_{dq}$|is the equivalent voltage command for grid control, |${Z}_{Vc}$|is the voltage control equivalent series impedance, |${I}_{GFM}$| is the grid mode output current, |${V}_{dq}$|、|${Z}_{Vc}$|、|${I}_{GFM}$| are complex vectors. The main advantages of grid type control include: good stability in weak power grid conditions, strong voltage support ability, active support of power grid in transient process, and support independent operation mode. However, it has some disadvantages: slow power response, poor tracking performance at maximum power points, and high multimachine coupling strength, usually requiring large overload capacity design, which affects the economy.
Mesh converter structure.
Network-type converter is especially suitable for distributed microgrid scenarios with low proportion of synchronous generators, large system frequency fluctuation, or requiring independent operation. With the increasing proportion of new energy generation in the power system, the network construction technology still needs further research and development in the future.
To sum up, the network control based on current source characteristics has faster power response speed and better economy, while the network control based on voltage source characteristics has stronger power grid support ability and poor economy. The two control modes have some similarity in structure, that is, both contain power rings and current rings, so their advantages are complementary.
Synchronous control strategy based on voltage phase and amplitude compensation
Based on the phase analysis of the control ring, the improved DC voltage synchronization control strategy based on the phase compensator and the power feedforward is proposed. Both methods essentially provide the advanced phase for the DC voltage synchronous outer loop in the low-frequency range and increase the phase distance of 180°, so as to improve the extreme phase characteristics of the DC voltage synchronous outer loop.
Control strategy based on the voltage-phase compensator
The dynamic equation of the improved DC voltage synchronous outer ring can be expressed as follows:
where GLL (s) is the transfer function of the phase compensator. To ensure that the phase compensator does not run the system stably, its steady-state gain shall be set to 0. Thus, the specific expression for the GLL (s) is designed as:
where M is the proportional coefficient of the phase compensator, and T1 and T2 are the leading and lag time constants, respectively.
The angular frequency when the phase compensator provides the maximum lead phase compensation can be calculated by the following equation:
The maximum advance phase compensation provided by the phase compensator at ωcomp can be calculated by the following equation:
The parameter design of the phase compensator can be achieved in the following steps:
First, determine the frequency ωcross of the amplitude crossing 0 dB of the loop gain amplitude frequency characteristic and the phase φcross of the loop gain at the frequency point. The proportional coefficient m and the required minimum compensation phase are determined per ωcross and φcross. The calculation method is as follows:
Then, set the ωcross to the frequency when the phase compensator provides the maximum lead phase compensation. The maximum |${\varphi}_{comp}$|advance phase provided by the phase compensator must be greater than the minimum compensation phase |${\phi}_{\mathrm{min}}$|.
Finally, the advance time constant |${T}_1$|and lag time constant |${T}_2$| are calculated based on the selected frequency ωcross and phase |${\varphi}_{comp}$|, combined with the previously mentioned above.
Next, a mathematical model of is established in MATLAB and a structural network converter model based on DC synchronization is constructed in PLECS. The main parameters are shown in Table 1.
Main parameters of system
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| Dc capacitance Cd | 2.55 mF | Ac side voltage amplitude command V | 311 V |
| Filter inductance Lm | 100 pH | Grid voltage amplitude E | 311 V |
| Filtered inductance parasitism Resistance R | 0.10 | Ac side voltage fundamental Angular frequency w₁ | 314 rad/s |
| Filter capacitor C | 21 pF | Dc voltage-frequency ratio Control coefficient kw | 0.2244 |
| Line inductance L | 10 mH | Voltage loop PR controller Proportional gain K | 0 |
| Direct voltage reference value Vdm | 700 V | Voltage loop PR controller resonator gain K | 100 |
| Interiorinput power P | 20 kW | Current inner loop proportional control gain K | 2.5 |
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| Dc capacitance Cd | 2.55 mF | Ac side voltage amplitude command V | 311 V |
| Filter inductance Lm | 100 pH | Grid voltage amplitude E | 311 V |
| Filtered inductance parasitism Resistance R | 0.10 | Ac side voltage fundamental Angular frequency w₁ | 314 rad/s |
| Filter capacitor C | 21 pF | Dc voltage-frequency ratio Control coefficient kw | 0.2244 |
| Line inductance L | 10 mH | Voltage loop PR controller Proportional gain K | 0 |
| Direct voltage reference value Vdm | 700 V | Voltage loop PR controller resonator gain K | 100 |
| Interiorinput power P | 20 kW | Current inner loop proportional control gain K | 2.5 |
Main parameters of system
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| Dc capacitance Cd | 2.55 mF | Ac side voltage amplitude command V | 311 V |
| Filter inductance Lm | 100 pH | Grid voltage amplitude E | 311 V |
| Filtered inductance parasitism Resistance R | 0.10 | Ac side voltage fundamental Angular frequency w₁ | 314 rad/s |
| Filter capacitor C | 21 pF | Dc voltage-frequency ratio Control coefficient kw | 0.2244 |
| Line inductance L | 10 mH | Voltage loop PR controller Proportional gain K | 0 |
| Direct voltage reference value Vdm | 700 V | Voltage loop PR controller resonator gain K | 100 |
| Interiorinput power P | 20 kW | Current inner loop proportional control gain K | 2.5 |
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| Dc capacitance Cd | 2.55 mF | Ac side voltage amplitude command V | 311 V |
| Filter inductance Lm | 100 pH | Grid voltage amplitude E | 311 V |
| Filtered inductance parasitism Resistance R | 0.10 | Ac side voltage fundamental Angular frequency w₁ | 314 rad/s |
| Filter capacitor C | 21 pF | Dc voltage-frequency ratio Control coefficient kw | 0.2244 |
| Line inductance L | 10 mH | Voltage loop PR controller Proportional gain K | 0 |
| Direct voltage reference value Vdm | 700 V | Voltage loop PR controller resonator gain K | 100 |
| Interiorinput power P | 20 kW | Current inner loop proportional control gain K | 2.5 |
Effect of different compensation phases on the system loop gain. Based on the inverter parameters in Table 1, the available ωcross = 74.9 rad/s and |${\varphi}_{cross}$|= − 3.66 rad were calculated. The ωcross was set as the maximum advance phase frequency, and the maximum advance compensation phase was 40°, 50°, and 60° were selected for comparative analysis. The parameters of the different phase compensators are obtained by calculation as shown in Table 2.
Parameter design of phase compensator
| |${\varphi}_{cross}$| | m | T₁ | T₂ |
|---|---|---|---|
| 40 | 0.4667 | 0.0286 | 0.00623 |
| 50 | 0.3636 | 0.0367 | 0.00485 |
| 60 | 0.2680 | 0.0498 | 0.00358 |
| |${\varphi}_{cross}$| | m | T₁ | T₂ |
|---|---|---|---|
| 40 | 0.4667 | 0.0286 | 0.00623 |
| 50 | 0.3636 | 0.0367 | 0.00485 |
| 60 | 0.2680 | 0.0498 | 0.00358 |
Parameter design of phase compensator
| |${\varphi}_{cross}$| | m | T₁ | T₂ |
|---|---|---|---|
| 40 | 0.4667 | 0.0286 | 0.00623 |
| 50 | 0.3636 | 0.0367 | 0.00485 |
| 60 | 0.2680 | 0.0498 | 0.00358 |
| |${\varphi}_{cross}$| | m | T₁ | T₂ |
|---|---|---|---|
| 40 | 0.4667 | 0.0286 | 0.00623 |
| 50 | 0.3636 | 0.0367 | 0.00485 |
| 60 | 0.2680 | 0.0498 | 0.00358 |
The effect of the different compensation phases on the system loop gain is shown in Fig. 3. As can be seen from the figure, the amplitude of the loop gain remains basically the same, but the frequency point of the phase crossing-180°moves to a higher frequency, away from the high gain zone in the low-frequency range, thus preventing the loop gain from phase through-180°in the frequency range whose amplitude exceeds 1, which violates the stability criterion and may cause low-frequency oscillations. Moreover, increasing the compensated phase can increase the amplitude margin at the phase crossing-180° frequency point, which has a stronger inhibition effect on low-frequency oscillations and helps to improve the damping performance of the system.
Effect of compensation phase on loop gain.
A control strategy based on power, amplitude compensation feedforward
Control strategy of power amplitude compensation feedforward:
The dynamic equation of the improved DC voltage synchronous outer loop can be expressed as:
where kpw is the feedforward coefficient of the active power. When the power feedforward control is added, the dynamic characteristics of the DC voltage synchronous outer loop change to:
Here |${G}_{\delta p, pw}(s)$| represents the small signal dynamic relationship between the power input and output difference and the work angle. Compared to the original equation, the modified equation adds a first-order term to the molecule, which is provided by the active power feedforward control and can provide certain advance phase compensation for the DC voltage synchronous outer loop. We can see from the modified equation that the larger the first-order term coefficient on the molecule, the more advanced phase is provided in the low-frequency range. Therefore, the provided advance phase can be adjusted by adjusting the active-power feedforward coefficient |${k}_{pw}$|. However, the amplitude of |${G}_{\delta p, pw}(s)$| can also be affected by changing the |${k}_{pw}s$|. Therefore, kdcw is required to ensure that the amplitude of |${G}_{\delta p, pw}(s)$| is stable in the low-frequency range.
To investigate the effect of the advance phase regulation introduced by active power feedforward control on system low-frequency stability, we designed three sets of synchronous outer loop control parameters as shown in Table 3. Under different parameter settings, the dynamic characteristics of the synchronous outer loop of the system |${G}_{\delta p, pw}(s)$| as follows: it can be seen that in all three sets of parameter setting, the amplitude of |${G}_{\delta p, pw}(s)$| is basically unchanged, and the phase is ahead of-180°, and with the increase of kpw, the degree of |${G}_{\delta p, pw}(s)$| also increased, effectively improve the extreme phase characteristics of DC voltage synchronous outer loop.
Parameters of synchronization outer loop control based on active power feedforward
| Group | kw | k |
|---|---|---|
| 1 | 0.090 | 0.005 |
| 2 | 0.045 | 0.010 |
| 3 | 0.009 | 0.050 |
| Group | kw | k |
|---|---|---|
| 1 | 0.090 | 0.005 |
| 2 | 0.045 | 0.010 |
| 3 | 0.009 | 0.050 |
Parameters of synchronization outer loop control based on active power feedforward
| Group | kw | k |
|---|---|---|
| 1 | 0.090 | 0.005 |
| 2 | 0.045 | 0.010 |
| 3 | 0.009 | 0.050 |
| Group | kw | k |
|---|---|---|
| 1 | 0.090 | 0.005 |
| 2 | 0.045 | 0.010 |
| 3 | 0.009 | 0.050 |
Triggers current limiting control
When the power grid fails, in order to prevent the power device damage due to overcurrent, the output current of the grid converter is usually limited to a safe range by the current limiting algorithm, usually 1.1 to 1.5 times the rating (pu). In general, the work angle curve of the network converter is determined by the current limiting algorithm, which is very different from the case of synchronous generator, as shown in Fig. 4. There are two types of widely used current limiting algorithms: one is the internal current loop of the converter; the other is the current limiting by introducing virtual impedance. From the perspective of transient stability, although the fault current amplitude of the grid converter is limited to a fixed level, its phase depends on the specific flow-limiting algorithm adopted, and there may be large differences. The relevant literature further quantifies the effect of the fault current phase of the grid converter on its transient stability, and proposes the improvement of the performance of the central control terminal and the mutation disturbance of the voltage phase across the grid.
Power angle curve of the netted converter after starting current limiting.
Control technology of network configuration converter during failure
When the short circuit fault occurs, the impedance between the grid converter and the short circuit point will decrease instantly, because of its voltage source characteristics, it will lead to the short circuit current to increase significantly. In order to prevent the switching device from being damaged due to excessive current, the traditional control method includes using virtual impedance or current limit to limit the short circuit current output of the converter, or even directly switching to the grid control mode. These methods actually limit the capacity of the grid converter to support the grid during failure.
From the hardware point of view, this problem can be solved by increasing the overcurrent bearing capacity of the switching devices and related components of the network side converter. This method has been applied in some grid-type energy storage systems and SVG devices, and has been successfully implemented in pilot projects. Based on the conventional structure of the new energy delivery system and the parameters of the lines and transformers, the overcurrent capacity is usually set at 2.5 to 3 times the rated value. This setting ensures that the converter can maintain a short circuit fault at the high voltage side of the 220 kV transformer without the current limit. However, it is difficult to completely avoid the risk of overflow in the event of a failure near the converter.
Combining the above two control strategies, a more comprehensive control scheme during the failure of the network converter can be formed, as shown in Fig. 5. By appropriately expanding the hardware capacity of the converter and combining with the virtual impedance control, the grid converter can reach a connection point voltage of 0.2p.u.–1.3p.u, maintaining the network construction capability during failure in a wide range.
A control scheme during the failure of a network converter.
Simulation and experimental validation
Simulation and experiment are used to verify the accuracy of the signal stability analysis and the effectiveness of the proposed optimization control method.
Simulation validation
The simulation model of grid-connected converter system based on DC voltage synchronization is constructed using PLECS software, and its control topology is shown in Fig. 1, related, and the parameters are detailed in Table 1. First, Fig. 6a compares the output AC voltage frequency changes between consideration and without voltage ring control. The system is initially in a stable state without the voltage loop control at 5 s. It is observed that the AC voltage frequency output by the inverter begins to produce low-frequency oscillation, and the low-frequency amplitude increases with time, eventually leading to the instability of the system frequency. This indicates that the introduction of voltage rings reduces the stability of the system in the low-frequency band, making the system more likely to produce low-frequency oscillations. When there is no appropriate phase compensation measures, the system with voltage ring control is difficult to maintain stable operation. Therefore, the effects of the two proposed improved DC voltage synchronization control on the system stability is next explored.
Simulation waveform.
Figure 6b shows the dynamic response of the output AC voltage frequency when switching from the no-voltage ring to the voltage ring control under different phase compensation amounts. It can be seen that as the compensation phase increases, the oscillation dynamic time after the control switch decreases. Figure 6c shows the dynamic response of the output AC voltage frequency when the system controls the switching under different synchronous outer loop control parameters based on the power feedforward. As the advance phase provided by the power feedforward increases, the damping effect of the system on the low-frequency oscillations increases, and both the oscillation dynamic time and amplitude decrease. The results of Fig. 6b and c are compared with Fig. 6a, confirming that the proposed method can effectively improve the stability of the system in the low-frequency range.
In addition, the influence of DC voltage synchronous outer loop control parameters on the system stability is also studied. Figure 6d shows the effect of changing the ‘DC voltage-frequency’ proportional control factor |${K}_{dcw}$| on the low-frequency oscillation of the output AC voltage frequency. In the simulation, reducing the compensation phase of the phase compensator at 5 s results in low-frequency oscillation of the output AC voltage frequency. Then, reducing the |${K}_{dcw}$| at 6.5 s effectively suppressed the low-frequency oscillation, proving that reducing |${K}_{dcw}$| helps to improve the low-frequency stability of the system. Figure 6e shows the effect of the resonator gain K of the voltage loop proportional integral controller on the low-frequency oscillation of the output AC voltage frequency. Similarly, reducing the phase compensator phase at 5 s causes low-frequency oscillations; while increasing |${K}_{r1}$| at 6.5 s gradually suppressed the oscillation, indicating that increasing |${K}_{r1}$| can improve the dynamic response of the voltage loop, thereby improving the low-frequency stability of the system. It is worth noting that only changing |${K}_{dcw}$| and |${K}_{r1}$| cannot fundamentally change the low-frequency oscillation characteristics of the system. These methods can further improve the stability of the system only under the premise of applying the proposed control strategy to ensure the stable operation of the system.
Interpretation
First, the waveforms of DC voltage and current at the grid side are compared with and without voltage loop at time |${t}_0$|. After the control mode is changed from the control mode without voltage loop to the control mode with voltage loop, the DC voltage and the current waveform on the grid side both appear at low-frequency oscillation, which indicates that the introduction of voltage loop has an effect on the low-frequency stability of the system.
Figure 7a shows the effect of using a phase compensator to suppress low-frequency oscillations at time t. After switching to voltage loop control, DC voltage and grid side current begin to oscillate at low frequency. Subsequently, at time |${t}_{p1}$|, by increasing 30. The amplitude of oscillation is somewhat reduced, but not completely eliminated. Then, at time |${t}_{p2}$|, the phase compensation is further increased to 40°, and the low-frequency oscillation of the DC voltage and the grid side current is successfully suppressed.
Main experimental waveforms.
Figure 7b shows the results of using active power feedforward control to suppress low-frequency oscillations, similarly, at time |${t}_0$|.
After switching to voltage loop control, the DC voltage and grid side current oscillate at low frequency. Then, at the moment, by enabling active power feedforward control, the oscillations of DC voltage and grid side current are quickly suppressed. These results verify the effectiveness of these two control strategies in suppressing low-frequency oscillations in the system.
In addition, the proportional control coefficient K of ‘DC voltage-frequency’ is also studied |${K}_{dcw}$|.The shadow of the low-frequency oscillation of the system. After the phase compensation is reduced, the DC voltage and the grid side current appear low-frequency oscillation. Then, by reducing |${K}_{dcw}$| at the time dew, the low-frequency oscillations of DC voltage and current at the network side are successfully suppressed, which proves that reducing |${K}_{dcw}$| can improve the low-frequency stability of the system. Finally, the effect of the resonator gain K of the voltage loop PR controller on the low-frequency oscillation of the system is discussed. At time t. After the phase compensation is reduced, the DC voltage and the grid side current will again oscillate at low frequency. Next, when |${K}_{r1}$| is increased at time |${t}_{kr}$|, the low-frequency oscillations of DC voltage and net-side current are effectively suppressed, which verifies that increasing |${K}_{r1}$| helps to improve the low-frequency stability of the system.
Conclusion and outlook
In this study, the structure of network converter in new energy medium voltage flexible interconnection project is discussed. First, we verify the basic working principle of the structural grid converter and its dynamic characteristics under complex power grid conditions through theoretical analysis and simulation. Subsequently, a synchronous control strategy based on voltage phase and amplitude compensation is proposed. In addition, combined with the characteristics of new energy AC–DC hybrid distribution network, a series of protection strategies are put forward, including frequency disturbance response, voltage overshoot control, load change response, and fault detection and protection strategies. Through these strategies, the efficient synchronization of the structured network converter and the power grid is realized, thus to ensure the stable operation of the system under various working conditions. The results show that the proposed protective control strategy can quickly and accurately detect and isolate faults, improving the reliability and immunity of the system. This study provides theoretical basis and technical support for the practical application of new energy medium voltage flexible interconnection engineering. The results show that the proposed protective control strategy can quickly and accurately detect and isolate faults, improving the reliability and immunity of the system. This study provides theoretical basis and technical support for the practical application of new energy medium voltage flexible interconnection engineering.
Author contributions
Jiaohui Shang (Conceptualization [equal], Data curation [equal], Formal Analysis [equal], Investigation [equal], Resources [equal], Software [equal], Supervision [equal], Visualization [equal], Writing—original draft [equal]), Mingqiang Guo (Conceptualization [equal], Funding acquisition [equal], Investigation [equal], Project administration [equal], Software [equal], Validation [equal], Writing—original draft [equal], Writing—review & editing [equal]), Haitao Niu (Conceptualization [equal], Formal Analysis [equal], Funding acquisition [equal], Methodology [equal], Project administration [equal], Resources [equal], Writing—original draft [equal]), Jian Yang (Conceptualization [equal], Data curation [equal], Formal Analysis [equal], Funding acquisition [equal], Investigation [equal], Software [equal], Supervision [equal], Validation [equal]), Shijin Xin (Data curation [equal], Formal Analysis [equal], Funding acquisition [equal], Investigation [equal], Writing—original draft [equal], Writing—review & editing [equal]), and Qing Xu (Data curation [equal], Formal Analysis [equal], Funding acquisition [equal], Methodology [equal], Project administration [equal], Validation [equal], Visualization [equal]).
Conflict of interest
None declared.
Funding
State Grid Gansu Electric Power Company 2024 Science and Technology Project: Research on Key Technologies and Typical Configuration of Flexible Interconnection of Medium voltage Distribution Network (No. : B72703241203).
