Topological Scheme and Analysis of Operation Characteristics for Medium-Voltage DC Wind Turbine Photovoltaic Powered Off-Grid Hydrogen Production System

Abstract

Renewable energy has high volatility in the traditional off-grid AC hydrogen (H₂) production system, which leads to low reliability of the system operation. To address this issue, this paper designs the topology scheme of wind-photovoltaic generation powered off-grid H₂ production system. Firstly, a DC off-grid system topology scheme with the wind turbine (WT) and photovoltaic (PV) is connected to the medium voltage DC bus by two-stage conversion is proposed. The power fluctuation of WT and PV generation systems and the power-adjustable characteristics of electrolyzers are taken into consideration. Meanwhile, the scheme of distributed access of energy storage (ES) to the WT side and PV side to provide the voltage support for the system is proposed. Secondly, the operating characteristics of DC microgrids and AC microgrids under abnormal operating conditions, such as the fault of the source side, the fault of the load side, and communication interruption, are analyzed in this paper. Finally, the electromagnetic transient simulation model of the DC off-grid H₂ production system and the traditional AC off-grid H₂ production system is established. The effectiveness of the proposed topology scheme is verified by simulation of typical operating conditions.

1. Introduction

As an important secondary energy source, H₂ energy is a key energy source for building a clean, low-carbon, safe, and efficient modern energy system [1,2]. For sustainable development, new microgrids for direct preproduction of green H₂ from renewable energy sources (RESs) will become a new trend [3,4,5]. RESs have the characteristics of high randomness and volatility and need to rely on a large number of ES devices to provide power support for the systems. This cannot satisfy the goal of efficient and reliable operation of large-scale renewable energy hydrogen production (RESs-H₂) system. Therefore, new system topology schemes need to be explored to achieve economic, efficient, safe, and reliable operation of large-scale off-grid RES-H₂ systems.

Reasonable topology design is the key to the reliable operation of off-grid H₂ production systems with a high proportion of RES access. The existing research mainly focuses on AC off-grid topology. The RES devices, such as WT and PV are connected to AC buses through multi-stage power electronics converters to power the H₂ production equipment. The structure of the WT-powered off-grid H₂ generation system was proposed in [6], which utilizes H₂ load to smooth the power fluctuation of the WT. However, this method requires a high configuration of equipment for the charging and discharging of supercapacitors or lithium batteries. Supercapacitors need to have better ability for fast charging and discharging. The control strategy of the low voltage ride-through for the hydrogen production power supply is proposed in [7]. The topology of the microgrid coupled with H₂ production equipment at the AC bus is designed in [8]. This scheme has the problem of power fluctuation interactions between different AC sources. An AC off-grid WT-powered H₂ generation system topology scheme was designed in [9]. However, the H₂ production converter of this scheme uses a silicon-controlled rectifier (SCR), which has the disadvantage of low switching frequency and slow response speed. SCR unable to respond quickly to the voltage and current of the load under power fluctuation. Based on the studies [9], a three-phase hybrid rectifier topology was proposed to improve the efficiency and power quality of the system [10]. However, the different RESs are connected in parallel to the common AC bus, and the power fluctuations between different RES sub-grids affect each other. The topology of the PV-H₂ system was proposed in [11]. The above studies used an AC off-grid H₂ production system topology scheme, which has low system expandability and different power fluctuations between different RES sub-grids. Frequency oscillation and harmonics are easy to occur in an AC off-grid H₂ production system. The frequency fluctuation of the AC system will directly affect the reliable operation of the H₂ production system.

All the above studies are designed for AC off-grid systems, which have serious frequency fluctuation problems. At present, some scholars have conducted research on the design of small-scale RESs DC off-grid H₂ production system scheme. Compared with the AC H₂ production system topology scheme, the DC system does not need to consider reactive power, frequency, and phase tracking [12]. DC off-grid H₂ production system has lower loss and high system transmission efficiency. Studies [13] have constructed a DC WT-PV off-grid H₂ production system, and the DC bus voltage of system control only needs to consider voltage matching. A small-scale off-grid hydrogen production system was proposed in [14]. However, the research object is a 400 V low-voltage DC (LVDC) bus system; the system capacity is limited, and it is only suitable for small-scale off-grid H₂ production systems. Studies [15,16] designed the topology of a DC off-grid H₂ production system with centralized ES access. The centralized ES access method has serious unreliability problems. Once the ES system fails, the microgrid will lose power and voltage support, causing the whole system to be paralyzed and unable to operate stably. Studies [17] proposed a topological scheme of a PV-H₂ directly coupled system, which is simple and easy to implement. A strategy to optimize the operation of electrolyzers is proposed in the PV-H₂ system in [18]. In order to improve the utilization of H₂, a method of solid-gas coupling H₂ storage was proposed in [19]. However, the topology scheme does not consider the characteristics of day and night intermittent output power of the PV. The single RES leads to problems such as low H₂ production efficiency, unstable operation, and low reliability. Studies [20] put forward the topological structure of the WT-H₂ production system. This scheme can quickly adapt to the power fluctuations of WT by using the power-adjustable characteristics of the electrolyzer. A control based on Model Predictive control (MPC) for system frequency regulation of large-scale W-H₂ systems in [21]. However, the topology scheme is greatly affected by environmental factors and ignores the complementary characteristics of RESs. The topological scheme of WT and PV coupling to the DC bus was proposed in [22]. The power was supplied to H₂ production equipment through multi-level transformation. The scheme uses a centralized ES device, and the risk of stable operation of the system is high. The above schemes of off-grid H₂ production have initially achieved the goal of green H₂ production from RESs. However, the expandability of the system is low, and the scale is small. The centralized access of ES makes the system rely on the energy management system (EMS) for power flow distribution and power regulation. This method seriously reduces the reliability of power supply. Once the ES device fails, the system loses voltage and power support, which will lead to system paralysis.

RESs have the characteristic of uncertainty, electrolyzers have the characteristic of adjustability, and DC off-grid H₂ production systems have the complex dynamic characteristic of strong coupling. Under the influence of the above factors, it is difficult to design an efficient, safe, and reliable topological scheme of RESs for an off-grid H₂ production system. To solve these problems, a topology scheme of a medium voltage DC (MVDC) off-grid H₂ production system is designed in this paper. This paper designs a topology scheme of an MVDC RES off-grid H₂ generation system. Firstly, a structure is designed in which WT and PV are independently connected to the DC main grid under the support of their respective ES. Secondly, the control modes and operation characteristics of the DC off-grid H₂ system and the AC off-grid H₂ system are analyzed. Finally, the electromagnetic transient simulation models of the AC off-grid hydrogen system and the DC off-grid H₂ system are constructed. The hardware-in-the-loop (HIL) simulation model of the DC off-grid H₂ production system is established based on RT-LAB. The effectiveness of the proposed topology of the MVDC off-grid hydrogen generation system is verified.

2. Design of Topological Scheme of Medium Voltage DC WindPhotovoltaic-Powered Off-Grid Hydrogen Production System

2.1. Topological Scheme of Traditional AC Off-Grid Hydrogen Production System

The traditional AC off-grid topology scheme [7,8,9] is shown in Figure 1. The doubly fed induction generator (DFIG) is connected to the medium voltage AC (MVAC) bus via an AC/AC converter and a set-up transformer. The PV is connected to the MVAC bus by an DC/AC converter and a set-up transformer. The centralized access method is used for ES. The ES is connected to the MVAC bus through the DC/AC converter and a set-up transformer. Then, the MVAC is connected to the low-voltage AC (LVAC) bus through the step-down transformer. The H₂ production equipment is powered by a transformer and SCR. The AC off-grid topology scheme is simple in structure and more mature in technology. However, the traditional scheme has the following problems:

(1)

The capacity expansion of RESs and H₂ sub-grids is difficult.

(2)

The power supply reliability of the system is poor. Once a fault occurs in the ES, the system will have the risk of breakdown.

(3)

Low power quality. The AC system scheme adopts SCR, which produces large harmonics and reactive power.

(4)

The control method is more complicated. The cooperative control needs to consider the stability of voltage and frequency, and it relies on communication to balance the power of sources and loads.

Figure 1. Structure of traditional AC off-grid hydrogen production system.

Figure 1. Structure of traditional AC off-grid hydrogen production system.

2.2. Topological Scheme of DC Off-Grid Hydrogen Production System

In order to overcome the defects of traditional centralized ES access schemes this paper designs a DC off-grid H₂ production topology scheme with distributed access of WT, PV, and ES. As shown in Figure 2, the permanent magnet direct drive fans (PMSG) and PV are, respectively, supported by an AC/DC rectifier and a DC/DC converter. In addition, the local distributed LVDC sub-grid is formed under the support of their own ES devices. After the DC/DC converter boosts the voltage, each subnet is connected to the MVDC bus. The proton exchange membrane electrolyzer (PEM) and alkaline electrolyzer (ALK) are powered by the DC/DC hydrogen converter. The scheme has the following advantages:

(1)

The system has better expansibility. RESs can achieve plug and play without affecting system operation.

(2)

High reliability of system power supply. The WT sub-grid and PV sub-grid adopt the distributed access to the DC bus, which can ensure the normal operation of the remaining system when a single RES sub-grid fails.

(3)

ES adopts distributed access mode, with high system efficiency. ES adopts a distributed access way, only to provide voltage support for the system. There is no reactive power demand, and the capacity requirements of the ES converter are small.

Figure 2. Topology of RESs DC off-grid H₂ production system.

Figure 2. Topology of RESs DC off-grid H₂ production system.

3. Cooperative Control Strategy of Off-Grid Hydrogen Production System

Presently, the technology of DC off-grid H₂ production systems with large-scale RES access is in the initial stage of exploration. By analyzing the control strategy method and operation characteristics of the AC off-grid H₂ production system and the DC off-grid H₂ production system, the advantages of the topology scheme proposed in this paper are verified. Currently, most of the widely used AC off-grid H₂ production systems adopt the cooperative control strategy with communication. The DC off-grid H₂ production system adopts the load-follow-source dynamic control strategy without communication.

3.1. Cooperative Control Strategy of Traditional AC Off-Grid Hydrogen Production System

The cooperative control strategy relying on communication is used in the traditional WT- and PV-powered off-grid H₂ production system. As shown in Figure 3, the control strategy includes the constant power control of the PV converter and WT converter, the constant voltage control of the ES converter, and the power outer loop and current inner loop control strategy of the AC/DC H₂ production converter. The control strategy needs to obtain the output power of the WT, PV, and AC load in real time and calculate the reference value of the power distribution of the AC/DC converter of H₂ production.

$$P_{\mathrm{ALK}\_\mathrm{ref}}\left(t\right)=\eta k_{\mathrm{ALK}}\left(P_{\mathrm{pv}}\left(t\right)+P_{\mathrm{wt}}\left(t\right)-P_{\mathrm{AC}\_\mathrm{Load}}\left(t\right)\right)$$

(1)

$$P_{\mathrm{PEM}\_\mathrm{ref}}\left(t\right)=\eta k_{\mathrm{PEM}}\left(P_{\mathrm{pv}}\left(t\right)+P_{\mathrm{wt}}\left(t\right)-P_{\mathrm{AC}\_\mathrm{Load}}\left(t\right)\right)$$

(2)

where $P_{\mathrm{ALK}\_\mathrm{ref}}\left(t\right)$ and $P_{\mathrm{PEM}\_\mathrm{ref}}\left(t\right)$ are the reference value of the active power distribution, $P_{\mathrm{pv}}\left(t\right)$ and $P_{\mathrm{wt}}\left(t\right)$ are the active power of the PV and WT, and $k_{\mathrm{ALK}}$ and $k_{\mathrm{PEM}}$ are the droop coefficients. $P_{\mathrm{AC}\_\mathrm{Load}}\left(t\right)$ is the active power of the AC load, $\eta$ is the efficiency of the power electronic converter in the system.

The equation of the traditional AC/DC converter of the H₂ production is as follows:

$$\alpha =\pi -\left[\left(P_{\mathrm{dc}\_\mathrm{ref}}-P_{\mathrm{dc}}\right)\left(k_{\mathrm{pp}}+{\displaystyle \frac{k_{\mathrm{ip}}}{s}}\right)-i_{\mathrm{dc}}\right]\left(k_{\mathrm{pi}}+{\displaystyle \frac{k_{\mathrm{ii}}}{s}}\right)$$

(3)

where $k_{\mathrm{pp}}$ and $k_{\mathrm{ip}}$ are the proportions and integral coefficients of the power outer loop, $k_{\mathrm{pi}}$ and $k_{\mathrm{ii}}$ are the proportions and integral coefficients of the inner loop of the current, $P_{\mathrm{dc}}$ is the actual value of the H₂ production converter power, and $\alpha$ is the phase angle.

3.2. Fault Ride-Through Control Strategy of Traditional AC Off-Grid Hydrogen Production System

When the AC off-grid H₂ production system is short-circuited or one of the RESs is faulty, it is difficult for the system to recover to a stable state. Therefore, it is necessary to do low voltage ride-through (LVRT) control for WT and PV. The Crowbar control strategy and DC side unloading circuit are adopted to realize the LVRT control of the WT. As shown in Figure 4, the generalized second-order generalized integrator (SOGI) and phase-locked loop (PLL) structure is used to realize the LVRT control of the PV. The cooperative control method of an AC off-grid system is more complicated than the control method of a DC off-grid system. When the communication is interrupted or delayed, once the system is disturbed, the voltage and frequency in the system will fluctuate, and the system will not be able to respond quickly. When the above problems occur in the system, it is easy to cause the system to split. SOGI-PLL has the following advantages:

(1)

Simple to implement. SOGI-PLL can obtain accurate phase locking when the grid voltage is sinusoidal and unbalanced.

(2)

SOGI-PLL produces a sine wave with a phase difference of 90 degrees, independent of frequency.

(3)

The generated orthogonal system is filtered without delay by the same structure due to its resonance at the fundamental frequency.

Figure 4. Control block diagram of SOGI-PLL.

Figure 4. Control block diagram of SOGI-PLL.

SOGI-PLL is to add SOGI link to PLL, which has better attenuation ability for high-order harmonic components. As shown in Figure 4, $u_i$ is the input voltage, k is the damping coefficient, and ω’ is the resonant frequency. $u_0$ and $q{u}_0$ are the output signal of orthogonal voltage after SOGI processing. The transfer function of SOGI-PLL is in [23,24]:

$$G(s)={\displaystyle \frac{u^{\prime }(s)}{u(s)}}={\displaystyle \frac{k{\omega }^{\prime }s}{s^2+k{\omega }^{\prime }s+{{\omega }^{\prime }}^2}}$$

(4)

$$q^{\prime }G(s)={\displaystyle \frac{q^{\prime }{u}^{\prime }(s)}{u(s)}}={\displaystyle \frac{k{{\omega }^{\prime }}^2}{s^2+k{\omega }^{\prime }s+{{\omega }^{\prime }}^2}}$$

(5)

$u_{\alpha }^{+}$ and $u_{\beta }^{+}$ can be expressed as:

$$u_{\alpha }^{+}=u_{\alpha }^{\prime }-q^{\prime }{u}_{\beta }^{\prime }={\displaystyle \sum _{\lambda =1,p}{G}_{\alpha \lambda }}\left(s_{\lambda }\right)u_{\lambda }$$

(6)

$$u_{\beta }^{+}=u_{\beta }^{\prime }+q^{\prime }{u}_{\alpha }^{\prime }={\displaystyle \sum _{\lambda =1,p}{G}_{\beta \lambda }}\left(s_{\lambda }\right)u_{\lambda }$$

(7)

where $s_{\lambda }=\mathrm{j}2\pi f_{\lambda }$, $G_{\alpha \lambda }\left(s_{\lambda }\right)=G\left(s_{\lambda }\right)+\mathrm{j}{q}^{\prime }G\left(s_{\lambda }\right)$, $G_{\beta \lambda }\left(s_{\lambda }\right)=q^{\prime }G\left(s_{\lambda }\right)-\mathrm{j}G\left(s_{\lambda }\right)$, $v_{\lambda }$ is the input voltage signal at frequency $f_{\lambda }$, and $u_{\alpha }^{+}$ and $u_{\beta }^{+}$ are positive sequence components of $u_{\alpha }$ and $u_{\beta }$.

The expressions of $u_d^{+}$ and $u_q^{+}$ can be obtained after the αβ/dq transformation in [25,26,27,28].

$$u_d^{+}=G_{\alpha 1}\left(s_1\right)u_1+G_{\alpha p}\left(s_p-s_1\right)u_p$$

(8)

$$u_q^{+}=-\Delta \theta \left(G_{\alpha 1}\left(s_1\right)u_1+G_{\alpha p}\left(s_p-s_1\right)u_p\right)$$

(9)

where $\Delta \theta$ is the phase difference between the synchronous coordinate system of the control ring and the synchronous coordinate system determined by the fundamental frequency voltage of the Point of common coupling. $u_p$ is the Fourier coefficient of the disturbance voltage.

As the RESs in the system adopt the way of distributed access to the AC bus, the frequency and voltage of the system will fluctuate greatly when the three-phase short-circuit fault occurs on the PV side of the AC system and is removed. At this time, the voltage will drop sharply and generate a large current. The WT needs to increase the cooperative control of the Crowbar circuit and Chopper circuit to achieve the LVRT of the WT in Figure 5.

3.3. Analysis of Communication Interruption in Traditional AC Off-Grid Hydrogen Production System

The AC off-grid H₂ production system adopts a cooperative control strategy with communication. When the system communication is interrupted, the output power of the RESs signal received by the H₂ production load is the signal of the moment before the communication is interrupted. The actual output power of RESs is greater than or less than the current output power of RESs, which will lead to uneven power distribution between the source and load of the AC system. When the output power of RESs in the system exceeds the power received by the H₂ load after communication interruption, the ES is charged. If the output power of RESs in the system is less than the power received by the H₂ load after communication interruption, the ES is discharged. When the ES charge and discharge exceed the maximum and minimum limits of the state of charge (SOC), the system will split.

When the system communication is interrupted, the power of the load is in [25]:

$$P_{\mathrm{ALK}}\left(t\right)=\eta k_{\mathrm{ALK}}\left(P_{\mathrm{pv}}\left(\tau \right)+P_{\mathrm{wt}}\left(\tau \right)-P_{\mathrm{AC}\_\mathrm{Load}}\left(\tau \right)\right)$$

(10)

$$P_{\mathrm{PEM}}\left(t\right)=\eta k_{\mathrm{PEM}}\left(P_{\mathrm{pv}}\left(\tau \right)+P_{\mathrm{wt}}\left(\tau \right)-P_{\mathrm{AC}\_\mathrm{Load}}\left(\tau \right)\right)$$

(11)

where $\tau$ indicates the moment when the system communication is interrupted.

After communication interruption, the actual output power of RESs is:

$$P_{\mathrm{new}}\left(t\right)=P_{\mathrm{pv}}\left(t\right)+P_{\mathrm{wt}}\left(t\right)$$

(12)

The reference output power of RESs received by the H₂ load terminal after communication interruption is as follows:

$$P_{\mathrm{new}}^{\prime }\left(t\right)=P_{\mathrm{pv}}\left(\tau \right)+P_{\mathrm{wt}}\left(\tau \right)$$

(13)

After communication interruption, the charge and discharge capacity of the energy storage system is as follows:

$$\left\{\begin{array}{l}E(t)=E(t-1)-P_{\mathrm{BESS}\_\mathrm{d}}\Delta t/{\eta }_{\mathrm{d}},P_{\mathrm{new}}\left(t\right)<P_{\mathrm{new}}\left(\tau \right)\hfill \\ E(t)=E(t-1)-{\eta }_{\mathrm{c}}{P}_{\mathrm{BESS}\_\mathrm{c}}\Delta t,P_{\mathrm{new}}\left(t\right)>P_{\mathrm{new}}\left(\tau \right)\hfill \end{array}\right.$$

(14)

where $E(t)$ is the capacity of ES at time t, $P_{\mathrm{BESS}\_\mathrm{c}}$ and $P_{\mathrm{BESS}\_\mathrm{d}}$ are the charging and discharging power of ES, respectively, ${\eta }_{\mathrm{c}}$ and ${\eta }_{\mathrm{d}}$ are the charge and discharge efficiency of ES, respectively.

At this time, the power supply efficiency in the system is:

$${\eta }_{\mathrm{s}}={\displaystyle \frac{P_{\mathrm{ALK}}\left(t\right)+P_{\mathrm{PEM}}\left(t\right)+P_{\mathrm{AC}\_\mathrm{Load}}\left(t\right)+P_{\mathrm{BESS}}\left(t\right)}{P_{\mathrm{new}}\left(t\right)}}\times 100\%$$

(15)

where $P_{\mathrm{BESS}}\left(t\right)$ is the charge and discharge power of ES, ${\eta }_{\mathrm{s}}$ is the efficiency of the system.

3.4. Cooperative Control Strategy of DC Off-Grid Hydrogen Production System

In order to solve the problem that the control strategy of the traditional AC off-grid H₂ production system depends on communication, which leads to the low stability of the system. A cooperative control strategy without communication is proposed in this paper. As shown in Figure 6, the load-follow-sources dynamic cooperative control strategy without communication is used in this paper. The ES converter adopts the voltage-current double closed-loop control strategy to stabilize the voltage of the LVDC bus. The constant power control is used by the WT sub-grid and PV sub-grid. The boost DC/DC converter uses a voltage rise control strategy. The voltage rise control strategy superimposes the power fluctuations of the WT and PV sub-grid to the voltage control loop of the main DC bus. The strategy of constant input DC voltage and constant output DC current control is adopted by the converter of H₂ production. The fluctuation of DC bus voltage caused by the change of RESs output power acts on the inner current loop controlled by the double closed loop of the H₂ production converter through the outer loop of the voltage. The control strategy realizes the goal of adaptive adjustment of load following the output power of RESs. The control strategy of a DC system is simpler than that of an AC system, and it does not need to add the fault ride-through control strategy. The voltage rise control strategy is adopted in the DC/DC boost converter in the system. When the system is operating within the boundary range, the energy storage is not charged and discharged, which maximizes the capacity configuration of the energy storage. The use of this control strategy improves the economy of system operation.

If the renewable energy generation exceeds the absorption capacity of the H₂ production load, the hydrogen production load operates at maximum power. If the SOC of the battery is less than SOC_max, the surplus power generation is used to charge the ES. The charging power is not more than 0.5 C until the ES pack SOC is restored to SOC_max. If the available power at the source side is higher than the rated capacity of the H₂ production device or the ES rechargeable capacity is insufficient. The system should limit the output power of the RESs or remove part of the RESs. The EMS flow chart of the DC off-grid H₂ production system is shown in Figure 7.

4. Simulation Analysis

4.1. Simulation Settings

In this paper, the simulation models of traditional AC off-grid H₂ production systems and DC off-grid H₂ production systems are built based on Matlab/Simulink to compare and analyze the system topology. Through the simulation analysis of the typical working conditions of the system and the influence of communication faults on the H₂ production system, the superiority of the topology of the DC off-grid H₂ production system proposed in this paper is verified. The simulation parameters of each device in the system are shown in

Table 1. Simulation parameter of RESs off-grid H₂ production system.

Equipment	Parameter	Value
PV	open-circuit voltage	50.84 V
	short-circuit current	13.99 A
	rated power	5 MW
WT	rated speed	680 r/min
	rated voltage	2100 V
	rated power	5 MW
BESS	rated power	1.25 MW
	rated voltage	1500 V
	rated capacity	2.5 MWh
ALK	rated power	5 MW
	rated voltage	700 V
PEM	rated power	1 MW
	rated voltage	300 V
AC Loads	R	0.08533 Ω
	L	2.0372 × 10−4 H

. The AC system mainly consists of three DFIG generators, one PV generator, three SCR devices, two ALK electrolyzers, three PEM electrolyzers, and AC loads. The DC system mainly consists of three PMSG generators, one PV generator, four DC/DC boost converters, two ALK electrolyzers, three PEM electrolyzers, and AC loads.

4.2. Comparison and Verification of Simulation Cases

In order to verify the superiority of the proposed DC off-grid H₂ production system topology scheme over the traditional AC off-grid H₂ production system, the following working conditions are designed for simulation verification.

Case 1: At 10–15 s, the output power of a single WT increases from 0.8 MW to 3.25 MW, and the PV output power increases from 0 MW to 4.4 MW.

Case 2: On the basis of stable operation in case 1, at 20 s, the PV fault is simulated. The AC circuit breaker removes the faulty branch within 100 ms, and the DC circuit breaker removes the faulty branch for 10 ms.

Case 3: On the basis of stable operation under case 1, at 20 s, a short circuit fault occurred on the AC side in front of the phase-controlled rectifier device. Then, the AC circuit breaker quickly instants at 100 ms. In the DC system, a DC positive and negative short circuit occurs at the input terminals of the hydrogen converter in the 1# electrolyzer. Then, the circuit breaker of the H₂ generation distribution branch is quickly disconnected and quickly recovered within 10 ms.

Case 4: On the basis of stable operation in case 1, at 20 s, the faulty branch is excised after a fault occurs in the simulated ES of the AC system. The DC system simulated the ES of the PV side, which failed and was removed.

Case 5: On the basis of stable operation in case 1, a communication interruption of the AC system is simulated at 12 s.

As shown in Figure 8, simulation results of case 1. During 10–15 s, the output power of a single WT increases from 0.8 MW to 3.25 MW, and the output power of the PV increases from 0 MW to 4.4 MW. As shown in Figure 8a, the AC bus voltage is stable at 35 kV. The DC bus voltage is stable at 20 kV in Figure 8b.

As shown in Figure 9a, a short circuit fault occurs on the PV side of the AC system at 20 s, and the faulty branch is quickly removed after 100 ms. Due to the overshoot of short circuit current in the system at the moment of short circuit, the output power of the WT and ES rises rapidly. Under the control of LVRT in the system, the system recovers stable operation. However, the system voltage fluctuation at the moment of short circuit is large, which exceeds the specified fluctuation range of microgrid operation. As it can be seen from Figure 9b, after the fault source branch of the DC system is removed, the remaining system can resume normal operation without LVRT control, and the system fluctuation is small.

As shown in Figure 10a, at 20 s, a short circuit fault occurs on the power supply side of the AC microgrid for H₂ production, and the voltage and power severely of the system oscillate seriously, making it difficult to restore stable operation. As it can be seen from Figure 10b, after a short circuit fault occurs in the DC microgrid, the breaker will act quickly within 10 ms, and the system can be restored to normal operation.

As shown in Figure 11, the AC off-grid H₂ production system uses centralized ES access. When the ES fails and is removed, the system loses voltage and reactive power support, and serious oscillations and unstable operation occur. On the other hand, the DC system adopts a distributed ES access method. After the failure of one of the ES, the other remaining ES can continue to provide voltage support to ensure the safe and stable operation of the system. The simulation results verify the effectiveness of the distributed ES access scheme proposed in this paper.

As shown in Figure 12, a communication interruption occurred at 12 s. The output power of RESs continues to rise in the system after the communication interruption. The reference power signal received by the H₂ converter side is the power at 12 s. At this time, the ES is charged. When the SOC of ES reaches the maximum limit, the output power of the RESs in the system is greater than the output power of the H₂ production load. At this time, the AC bus voltage rises rapidly, and the system is split.

In summary, WT and PV use parallel access to the AC bus in the AC off-grid H₂ production system; the power fluctuations between different RESs affect each other. The method of connecting the DC bus is used by the DC off-grid H₂ production system to avoid the mutual influence between different RESs. Centralized ES is used by AC off-grid H₂ production systems. When the ES fails and is cut out, the system loses voltage, and reactive power support causes the system to separate. The DC off-grid H₂ production system adopts a distributed ES access method to avoid the problem of system splitting caused by ES failure. Because the AC system considers the reactive power demand of the 35 kV main grid. The AC system is equipped with a larger capacity of ES converter. The ES capacity of the AC off-grid H₂ production system is 15 MWh. The DC off-grid H₂ production system does not need to consider the reactive power demand, and the total ES capacity in the system is 10 MWh. The control strategy of the AC off-grid H₂ production system depends on communication and is so complicated. The control strategy of the DC off-grid H₂ production system is without communication and is easy to implement. The comparison results of the AC off-grid H₂ production system and the DC off-grid H₂ production system are shown in

Table 2. Topology analysis of the AC and DC systems.

Parameters	AC System	DC System
Expandability	difficult to expand	extensibility is greater
Response speed	slow	fast
Method of control	complex	easy to implement

.

5. HIL Simulation Verification

5.1. HIL Simulation Settings

In this paper, the HIL experiment platform of the system is built based on RT-LAB + RCP in Figure 13. The main circuit model was built in RT-LAB OP5700, and the control strategy was built in RCP M1070. RT-LAB and RCP exchange information via Ethernet. The system includes six PMSGs, two PVs, eight ES devices, five ALK electrolyzers, and three PEM electrolyzers and power electronic converters.

5.2. HIL Simulation Results

The following four cases are set to verify the reliability of the topology scheme proposed in this paper:

(1)

Case 1: Before 4 s, the output power of each WT is 1.6 MW, and the output power of each PV is 0.5 MW. During the 4–14 s period, the simulated wind speed increased, and the output power of each WT increased from 1.6 MW to 3.5 MW. The output power of PV is unchanged.

(2)

Case 2. Before 8 s, the output power of each WT is 1.6 MW, and the output power of each PV is 0.5 MW. During 8–18 s, the simulated light intensity increased, and the output power of the PV increased from 0.1 MW to 4 MW. The output power of the WT is unchanged.

(3)

Case 3. During the 4–14 s period, the output power of each WT is 1.7 MW, and the output power of PV is increased from 0.1 MW to 4 MW. At 25 s, the circuit breaker on the simulated PV side trips and exits from operation, and then the system gradually transitions to the new operating state.

(4)

Case 4. The output power of each WT is 1.6 MW, and the output power of each PV is 0.5 MW. At the time of 25 s, a short circuit between the positive and negative DC electrodes occurred at the input end of the H₂ production power supply of the 1# electrolyzer. The circuit breaker quickly acted to remove the faulty branch, and then the system gradually transited to the new operating state.

As shown in Figure 14 and Figure 15, HIL simulation results of RESs DC off-grid H₂ production systems with increased wind speed and light intensity are, respectively, simulated. During the 4–14 s period, the power output of the WT gradually increased to 3.5 MW, and the output power of the PV was 0.5 MW in Figure 14a. During the 8–18 s period, the power output of the PV gradually increased to 4 MW, and the output power of the WT was 1.6 MW in Figure 15a. As shown in Figure 14b and Figure 15b, under the voltage rise control of the ES, the power output of ES is 0. The DC bus voltage is stable at 20 kV.

As shown in Figure 16, the PV is simulated to be faulty and excised. In Figure 16b, the energy storage power output sum is 0 under the control of voltage rise. The DC bus voltage is stable at 20 kV. When the system is at 25 s, the simulated PV fails, and the fault source is removed. As can be seen from Figure 16a, after the fault source is removed, the remaining system is reconnected and runs stably. The efficiency and security of the proposed topology scheme are verified.

The output power of PV is 0.5 MW, and the output of the WT is 1.6 MW in Figure 17a. As shown in Figure 17b, the power output of ES is 0. At 25 s, a short circuit between positive and negative DC electrodes occurred at the input end of the H₂ production converter of one of the ALK electrolyzers. The circuit breaker of the H₂ generation distribution branch quickly cuts out the fault, and then the system gradually transitions to the new operating state. It is verified that the cooperative control strategy used in this paper can run stably after the short circuit fault of H₂ production equipment recovers quickly.

In summary, the HIL simulation model of the RESs MVDC off-grid H₂ production system established in this paper verifies the topology scheme designed in this paper has a good dynamic response capability. The system can still achieve stable operation under extreme conditions such as fluctuation of power output of PV and WT. When some of the sources or loads fail and are taken out of operation, the system can still be reconnected and run stably.

6. Conclusions

In this paper, a topology scheme of a DC off-grid wind-solar complementary H₂ production system is designed considering the operation characteristics of RESs equipment and electrolyzers. This scheme can ensure the long-term, efficient, and reliable operation of the DC off-grid H₂ production system. The main conclusions drawn from the analysis of the system operation characteristics are as follows:

(1)

In this paper, a model of a DC off-grid wind-solar complementary H₂ production system is established. A distributed access 20 kV DC bus of the PMSG sub-grid and PV sub-grid is proposed in this paper. This scheme effectively avoids the mutual influence between different RES sub-grids. After the fault removal of one of the RES sub-grids, the remaining system can run normally, which improves the reliability of system operation.

(2)

A distributed access method of ES is proposed in this paper, which only provides voltage support for DC bus. This approach reduces the capacity allocation of ES. At the same time, each ES subsystem can share the DC transformer and 20 kV DC circuit breaker with the WT sub-grid and PV sub-grid. This method improves the economy of the system.

(3)

This paper presents a new topology scheme of an off-grid H₂ production system with distributed RES access. This scheme solves the problems of high harmonics, high reactive power, and low response speed in traditional AC microgrids.