ENHANCED AC VOLTAGE AND FREQUENCY CONTROL ON OFFSHORE MMC STATION FOR WIND FARM

For HVDC transmission systems based on modular multilevel converter (MMC) for connecting large offshore wind farms, the offshore AC voltage is regulated by the offshore MMC station and stable offshore AC voltage and frequency are important for the stable wind power generation and transmission. This paper proposes an enhanced AC voltage and frequency control strategy of the offshore MMC for wind farm integration, where an additional frequency loop is used to improve its AC voltage and frequency controllability. The proposed scheme considers the working principle of the phase locked loop (PLL) where the measured q-axis voltage drives the frequency output for the generated AC voltage. Thus, the output of the proposed frequency loop sets the q-axis voltage reference and feeds to the AC voltage loop to regulate the offshore AC frequency. Compared to conventional approach where no frequency loop is used and the converter simply produces the output as per the offline phase information, the proposed strategy can tightly control the AC voltage of the offshore network, which contributes to a stable transmission of the offshore wind energy. Simulation results in normal operation and during symmetrical fault confirm the feasibility of the proposed control scheme.


Introduction
Facing the more serious environment problems caused by traditional power generation, increased focus is moving to the renewable energy especially wind power.Due to higher average wind speed and lower turbulence, offshore wind energy has seen an increased growth in the past few years and the trend is likely to continue.However, the AC power system in offshore wind farms is likely to be weak because of its lower inertia and thus, frequency deviation and unstable offshore AC voltage frequently happen after fault occurrence [1], and wind farms experience poor stability with voltage oscillation or harmonic resonance [2].To transmit the wind power from long distance offshore wind farm, high voltage direct current (HVDC) transmission system based on modular multilevel converter (MMC) is likely to be used as there is no need for any compensation for reactive voltage drop under long distance transmission and its independent control on real and reactive power [3][4][5].In [6] and [7] MMC-HVDC was treated as a stable AC voltage which can connect to a weak grid without synchronous generators.Moreover, with MMC coordinated control, it also helps to prevent fault over-current and transient propagation.
Control of MMC which connected to power systems usually uses vector current control in the synchronous reference dq frame as the inner loop and uses phase locked loop (PLL) to tracking the frequency [8,9].By using the dq frame, the twoindependent currents on d-axis and q-axis have fast dynamic response and can create an independent control on the active and reactive power.However, when the connected power system is weak such as wind farms, there are several issues such as low frequency resonances and poor AC voltage tracking of the PLL.In order to reduce the impact caused by abrupt power changing, [10] proposed a virtual inertia control to provide dynamic frequency support which was verified under different power ratings.References [11] and [12] investigated a method that does not require a PLL synchronization and uses a power synchronization control.With this control technique, voltage source converter (VSC) works as a synchronous machine and has fast response with good dynamic performance when connecting with weak systems.However, in the event of an AC fault, MMC with power synchronization control cannot limit the over current and traditional vector current control has to be switched in [13].
This paper presents an enhanced frequency control of HVDC connected offshore wind farms.The dynamic response of the system will be investigated.Transient behaviour during offshore AC side network faults will be studied and system control is proposed to ensure post-fault fast system recovery.This paper organized as follows.Section 2 presents the layout of the MMC-HVDC system for connecting offshore wind farms.System control includes enhanced AC voltage and frequency control is described in Section 3. Case studies and simulation results are presented in Section 4. Finally, conclusions are drawn in Section 5. Fig. 1.System structure with lumped wind turbine cluster models.

System structure
The considered offshore wind farm connected with MMC-HVDC is schematically shown in Fig. 1, which consists of three lumped wind turbine models to represent different wind turbine clusters with a total power rating of 1000 MW.A symmetrical monopole HVDC system with rated DC voltage of ±400 kV is considered in this paper.AC circuit breakers (CB1, CB2, and CB3, Fig. 1) are equipped at each end of the cables for isolating the fault branch to enable continuous operation of the healthy wind turbines in the event of a fault in the offshore AC network.
The offshore station MMC1 controls the AC voltage and frequency of the offshore network while the onshore station MMC2 regulates the DC voltage of the HVDC link.The frontend WT converters operate on power control mode to transmit wind energy to the offshore grid.The WT DC voltage is regulated by the generator-side converters.

Control strategy of the offshore MMC station
The offshore MMC station works as a grid-forming converter to establish the offshore AC network frequency and voltage, as well as balancing the transmitted active power between WTs and the offshore network.With an outer AC voltage control loop, the inner current loop improves the system dynamics and enables fault current limiting capability of the MMC station during offshore AC faults.Fig. 2 shows the simplified circuit for the offshore MMC station and the AC network.

MMC AC voltage control a. Inner current loop
The inner current loop has been widely used for controlling VSC with the benefits of fast response and current limiting capability during external AC faults.For the converter circuit shown in Fig. 2, the MMC current loop dynamics in the dq reference frame where the d-axis is fixed to the MMC PCC voltage VPCC are expressed as where is the angular frequency of the offshore network; I1d and I1q are the MMC current in the dq reference frame, Vcd and Vcq are MMC output voltages in the dq frame.With proportional-integral (PI) regulators, the current control loop as illustrated in Fig. 3 is described as:

Outer AC voltage loop
The outer AC voltage loop sets the current references and its dynamics in the abc and dq reference frames are expressed as (3) and (4) respectively.
where i2 represents the current from the wind power collector and C represents the capacitance seen at the PCC point.The voltage controller in the dq reference frame which produces the dq current references is described as

Proposed frequency control
Stable offshore AC voltage and frequency are important for wind power generation and transmission.Due to the robustness and ease of implementation [5,13], the PLL is used widely in the control system to track the AC voltage angle and frequency.It measures the q-axis voltage Vpccq and drives the offshore frequency to obtain zero Vpccq, as illustrated in Fig. 4 and ( 6): The proposed frequency loop considers such operating principle of the PLL and sets the q-axis voltage reference V * pccq by frequency voltage droop to feed to the AC voltage loop and regulate the offshore AC frequency: When f < f * , the frequency loop outputs positive V * pccq and feeds to the AC voltage loop, as illustrated in Fig. 3.The feedback voltage Vpccq follows the reference V * pccq and is positive (Vpccq > 0).Thus, the measured frequency f increases, as depicted by ( 6), until being identical to reference (f = f * ).
Similarly, if f > f * , a negative voltage reference V * pccq is set by the proposed frequency loop and, under such conditions, the frequency detected by the PLL reduces due to the negative voltage (Vpccq < 0).With the proposed frequency control, the offshore frequency f tightly follows the reference (f = f * ) while the q-axis voltage Vpccq is well regulated at zero.
Compared to conventional approach where no frequency loop is used and the converter simply produces the output as per the offline phase information, the proposed strategy can tightly control the AC voltage of the offshore network, which contributes to a stable transmission of the offshore wind energy.

Fault current providing capability of the offshore MMC
In the event of an offshore AC fault, the offshore MMC station needs to limit the AC current whereas a certain fault current is also required for offshore AC protection purpose.For example, if the fault occurs on Cable 1, MMC1 initially tries to restore the AC voltage (due to the existence of the outer AC voltage loop) by reversing the power flow.This reaction leads to continuous increase of current I1 and soon the current will hit the current limitation.During the fault period, Id is limited in a lower level as no active power can be transmitted due to low AC voltage but Iq is increased to ensure sufficient fault current is provided for the protection relays connected to the wind turbine clusters.
The fault current providing capability of the offshore MMC station is achieved by properly setting the q-axis current reference Iqref, as shown in Fig. 3, the difference between Vpccd and V * pccd (V * pccd -Vpccd) is measured and compared to the preset threshold, e.g.0.4 p.u.During normal operation, V * pccd -Vpccd is less than the threshold and Iqref is set by the voltage control loop.Once the difference is over the threshold during the fault, an additional component Iq is added to the reference to rapidly increase the q-axis current.To provide fault current of 1.2 p.u., the gain kf between (V * pccd -Vpccd) and Iq is set at 3 (1.2/0.4=3).

Simulation results
The proposed control strategy of the offshore MMC station is tested in Matlab/Simulink environment using the model shown in Fig. 1, where the generator-side WT converters are represented by DC voltage sources for simplicity.Averagevalue model and detailed switch model are adopted for MMC stations and the front-end WT converters respectively.The offshore wind system has three lumped wind turbine models rated at 500 MW, 450 MW, and 50 MW respectively, and connected to the collector buses through 10 km, 5 km and 3 km cables respectively.The detailed parameters of the offshore MMC-HVDC system and the WT converter are listed in Table 1 and Table 2   At 0.5 s, all the three lumped wind turbines are connected into the transmission system and start to generate power from 0.6 s, 0.65 s, and 0.7 s, respectively.The active power transferred to the MMC increases from 0.6 s to 0.9 s, as displayed in Fig. 6.
The voltage amplitude at PCC slightly deviates from the reference and fast restores after the power reaches the rated value, Figs.7 (a) and (b).As shown in Figs.7 (c) and (d), the AC currents of the offshore MMC station increase with the increase of the power and the offshore frequency is tightly regulated around the reference during the start-up, benefitting from the proposed frequency control.

Steady state operation
After t=1 s, the power generated by the three wind turbines reaches to the rated value.The offshore MMC operates in steady state and transmits rated power to the onshore side.The offshore voltage current and frequency are well controlled around the references, as shown in Fig. 8.

Offshore AC fault ride through operation
The performance of the proposed control is also assessed during an offshore AC fault.At t=1.5 s, a symmetrical offshore fault is applied at the Cable 1 as shown in Fig. 1 and the offshore AC voltage rapidly drops to around zero.As the voltage on PCC cannot follow the reference, MMC1 reverses the power flow to try to restore the voltage, leading to saturation of the current loop.Thus, the offshore MMC provides fault currents with maximum current capability which can be used for fault detection, as shown in Fig. 9 and Fig. 10  (c).After fault isolation, the AC voltage of the offshore network gradually restores to the rated value by MMC1 as shown in Fig. 10 (a) and (b).The offshore frequency is well regulated and fast follows the reference after the fault isolation, Fig. 10 (d).During such serious offshore AC fault, the whole system is still well controlled by the proposed AC voltage control scheme and does experience significant overcurrents.

Wind farm side operation
Figs. 11 (a)-(c) display the currents of the three wind turbines during the whole studied period including start up, power increase, fault and post-fault recovery.As can be seen, after the fault occurs at 1.5 s, the output currents of VSC1, VSC2, and VSC3 of the three lumped turbines rapidly increased and hit the current limit.As shown in Fig. 11 (d), CB1 experiences significant overcurrent because the fault currents provided by the offshore MMC and wind turbines VSC2 and VSC3 all flow through circuit breaker CB1.This enables fault detection and the breaker CB1 is thus opened at around t=1.55 s to isolate the faulty branch.During the fault period from 1.5 s to 1.55 s, all three wind turbines limit the fault currents and once the fault at Cable 1 is clear, currents on Cable 2 and Cable 3 are quickly recovered to their rated values.In the simulation and for illustration purpose, WT1 (VSC1) remains operational (operating at current limit) although Cable 1 has been disconnected by CB1 after 1.55 s.

Conclusion
The enhanced AC voltage and frequency control strategy of the offshore MMC for wind farm integration is proposed in this paper.An additional frequency loop is used in the proposed control and its output sets the q-axis voltage reference and feeds to the AC voltage loop to regulate the offshore AC frequency.Compared to conventional approach without frequency loop, the proposed strategy can better control the AC voltage of the offshore network and contribute to a stable transmission of the offshore wind energy.Simulation results confirm the feasibility of the proposed control in normal operation and during offshore AC fault.

Fig. 2 .
Fig. 2. Simplified circuit for the offshore MMC station and the AC network.

Fig. 3 .
Fig. 3. Voltage and frequency control strategy of the offshore MMC station.

Fig. 5 .
Fig. 5. Simulation waveforms during start-up to build the offshore AC voltage.

Fig. 7
Fig. 7 Simulation waveforms at wind energy increasing stage.

Fig. 11 .
Fig. 11.Current waveforms of wind turbines and circuit breaker during offshore fault.
respectively.After the stabilization of the HVDC DC link voltage regulated by the onshore station MMC2, the offshore station MMC1 is enabled at 0.05 s to build the offshore AC voltage.The AC voltage amplitude of the offshore grid during start-up with the conventional and proposed frequency control methods are compared in Fig.5 (a).With the proposed control, the d-axis voltage Vd follows the reference more tightly than that with the conventional control with fixed frequency.The three-phase AC voltages and currents at the PCC, as well as the frequency response with the proposed control are displayed in Figs.5 (b)-(d) respectively.