Low Voltage Ride-Through Capability Enhancement

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The Journal of Engineering

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Low Voltage Ride-Through Capability Enhancement of GridConnected Permanent Magnet Synchronous Generator Driven Directly by Variable Speed Wind Turbine: A Review Mohammed H. Qais*, Hany. M. Hasanien†, Saad Alghuwainem* *Electrical Department, Faculty of Engineering, King Saud University, Riyadh 11421, Saudi Arabia (email: [email protected], [email protected]) † 2Electrical Power and Machines Department, Faculty of Engineering, Ain Shams University, Cairo 11517, Egypt (e-mail: [email protected])

Keywords: low voltage ride-through (LVRT), PMSG, wind turbine, variable speed.

Abstract The large power penetration of wind farms into the power grids requires new regulations by transmission system operators to keep them connected to the grid as long as possible. Grid disturbances such as voltage dips cause islanding of wind farms on the way to protect its apparatus from damage due to a high current flowing. The grid stability will suffer due to the islanding of large scale wind farms. Wind farms should keep on connecting to the grid during low voltages for a specific time (low voltage ride through (LVRT) capability) to support the grid stability restoring. LVRT capability of permanent magnet synchronous generator driven directly by a variable speed wind turbine (PMSG-VSWT) can be realized by modifying the control of grid side converter (GSC), machine side converter (MSC), pitch angle control, or using the existing Energy storage system (ESS). This paper presents a review of recent proposed improvements to enhance the LVRT capability of PMSG-VSWT. Many artificial intelligence and conventional controllers are used to enhance the control performance during voltage sags to keep the wind farms connected to the grid according to the recent grid codes.

1 Introduction Nowadays the grid-connected wind power plants (WPPs) deliver large electrical power into the grid, which play important role in the stability of grid. Transmission system operators (TSOs) established new regulations (grid codes) that regulate the behaviour of WPPs to ensure the stability and reliability of the grid. As stated by the published grid codes of different countries [1]–[3], WPPs should contribute in frequency and voltage control under normal situations, in addition to low voltage ride-through (LVRT) capability and reactive current supply during voltage sags [4]. Different grid codes existed for LVRT capability [5]–[7]. However, the combined contour of grid codes is presented in Figure 1. From this figure, wind farms must stay connected to the grid above the solid line.

Figure 1: Combined contour of LVRT capability Otherwise, under the solid line until 1.5 s, a short-time disconnection is possible followed by reconnection. If the voltage continues below than 40% of the rated voltage, wind farms are disconnected from the grid permanently. This paper presented a review of LVRT capability methods of WPPs based on the permanent magnet synchronous generator driven directly by variable speed wind turbine (DD-PMSGVSWT). This type of configuration (type 4) is connected to the grid through full scale frequency converters. The lower voltages of the grid, the lower power flow from WPPS then cause acceleration of wind turbine. The enhancement of LVRT capability is achieved by improving the control performance of grid side inverter (GSI), machine side converter (MSC), braking chopper (BC), energy storage system (ESS), and pitch angle as shown in Figure 2 [6], [8], [9].

Figure 2: DD-PMSG-VSWT configuration (type 4)

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2.4 Grid modelling

2 Wind turbine modelling 2.1 Output power of wind turbine The kinetic energy of the wind is captured by wind turbines and converted to mechanical power, then converted to electricity by the electrical generator. The captured power by a variable-speed wind VSWT turbine is modelled as Pm = 0.5ρ C p (λ , β )Av3 (1) 18.4

 151  C p (λ , β ) = 0.73  - 0.58β − 0.002 β 2.14 - 13.2  e λi  λi  1 1 0.03 = − λi λ + 0.02 β 1 + β 3

The grid voltage equations are expressed in the d-q frame as in [17], [20]  egd   igd   − Lg igq   vgd  d  igd    = Rg   + Lg   + ωg   +   (10) dt  igq   egq   igq   Lg igd   vgq  , where egd, egq, igd,, and igq are the d-q components of the grid voltages and currents. ωg is the rotational grid frequency, vgd, vgq are the d-q voltages at the GSI terminals, and Rg and Lg are the grid resistance and inductance.

(2)

3 LVRT capability methods of PMSG-VSWT (3)

Rωm (4) v where Pm is the mechanical power of the wind turbine, ρ is the air density, Cp is the power coefficient, A (= πR2) is the swept area, ωm is the mechanical speed, R is the radius of the turbine blades, and v is the wind speed. Power coefficient Cp (λ, β) is obtained from Equation (2) [6–9]. λ and β are the tip speed ratio (TSR) and pitch angle, respectively [14].

λ=

2.2 Drive train modelling For DD-PMSG-VSWT, the drive train model is considered a single-mass shaft model because the PMSG is connected to grid through the full-scale power converter [11–13]. The drive train system is modelled as d ωm j + Bωm = Tm − Te (5) dt p (6) ωe = ω m 2 where Tm is the mechanical torque, j is the total inertia of the turbine and PMSG, B is the damping coefficient, Te is the electromagnetic torque of the PMSG, ωe is the electrical speed, and p is the number of poles. 2.3 PMSG modelling The stator voltages of the PMSG are modeled in a d-q frame using the Park transformation as [14, 12, 15]  − Lq isq   vsd   isd  d  Ld isd  (7)    = − Rs   −   + ωe   vsq   isq  dt  Lq isq   Ld isd + ψ f  3 p (8) Te = (ψ f isq − ( Ld − Lq )isd isq ) 22 where vsd, vsq, isd, and isq are stator voltages and currents in the d-q frame, Ld and Lq are d-q inductance, Rs is the stator resistance, ωe is the electrical speed, and ψf is the flux linkage formed by the permanent magnet. Ld and Lq are nearly the same, so the electromagnetic torque Te is 3 p (9) Te = ψ f isq 22

3.1 Pitch angle control (PAC) Voltage dips in a power system cause an increase in wind turbine speed (over-speed). PAC is mainly used to extract the maximum power by controlling the blade pitch angle. Therefore, PAC can be used to reduce the rotational speed of a wind turbine by reducing the amount of wind incident to the blades, which then reduces the wind power converted into mechanical power as shown in Figure 3. The time response of PAC is limited because of mechanical issues; it is preferred that this method be combined with another controller to enhance the LVRT of PMSG-VSWT. In [21]–[23], the combined control of PAC and braking choppers (BCs) to enhance the LVRT capability of PMSG-VSWT was investigated. 3.2 Braking chopper (BC) The chopper circuit consists of a resistor and an IGBT switch as shown in Figure 4. During voltage sags, the DC link voltage rises over the rated value due to excess energy. The chopper circuit protects the DC link capacitor from damage by short circuiting the DC link by braking resistor. The braking resistor disperses the excess energy throughout voltage sags. In [21]–[25], the BC is used for LVRT of PMSG-VSWT combined with other approaches such as PAC and control of DC-DC converter of battery energy storage system. Ding and Zhu [26] proposed equivalent model of PMSG-VSWT, which used BCs and the coordinated control of active and reactive powers by GSI.


Figure 3: Pitch angle control

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Figure 4: Braking chopper control 3.3 DC-DC converter control of ESSs The most common ESSs used in WPPs are battery (BESS), flow battery, flywheel (FESS), electric double-layer capacitor (EDLC), and superconducting magnetic energy storage (SMES) [8]. ESS is located in the DC link of the back-to-back converter of a PMSG as shown in Figure 5. Yao et al. [27] presented an FESS with an enhanced control strategy using a proportional-integral-resonant (PIR) controller to suppress the DC link voltage oscillations during an unbalanced grid fault. Nguyen and Lee [25] proposed hybrid control of ESS and BC for LVRT capability enhancement and using GSI as STATCOM to inject reactive power for assisting voltage recovery. In [28], SCESS was used for LVRT combined with power smoothing. Zang, et al. [29] proposed an integrated control strategy based on the characteristics of PMSG-VSWT with the existing SMES. Hasanien and Muyeen [30] applied a PSO algorithm to optimize tuning of the PI controller that are used in SMES control for enhancing the LVRT capability of the wind farm. SMES and FESS are commonly used to enhance the stability of grid connected WPPs [31][32][33]. 3.4 MSC control method The control of the PMSG side is flux oriented, where the active power is controlled by the q-current and the machine flux (reactive power) is controlled by the d-current as shown in Figure 6. The DC voltage can be controlled instead of active power flow [34]. Yassin et al. [35] used an MSC controller to control the DC link voltage using an interval type-2 FLC considering the nonlinear relationship between DC link voltage and PMSG rotor speed. Jeong et al. [36] designed MSC control using a sliding-mode controller (SMC) to regulate the DC link voltage by storing the excess energy in the rotor inertia during voltage dips. Storing excess energy in rotor inertia for improving LVRT capability of PMSG is used in [37][38]. Kim et al. [39] used feedback linearization theory in MSC control for controlling the DC link voltage for LVRT capability.

Figure 6: MSC control modelling 3.5 GSI control method GSI control is oriented using the grid voltage phase angle. The d-axis current is responsible for controlling the active power by controlling the DC link voltage. The reactive power is controlled by controlling the q‐axis current. If the reactive power is not addressed, therefore, the q‐axis current reference is retained at zero. The basic control model of the GSI is shown in Figure 7. Hasanien and Muyeen [40]–[42] presented an optimum design for cascaded control using statistical methods such as the response surface method and Taguchi method integrated with optimization tools such as affine projection and genetic algorithms. Cascaded control, which uses PI controllers, is utilized in the frequency converter of a PMSG-VSWT. Alizadeh and Kojori [43] developed a virtually adaptive PI controller by adding a wavelet neural network in series with the PI controller in each closed control loop. Beddar et al. [44] proposed a fuzzy fractional-order PI+I controller that implements the FLC in parallel with fractionalorder and conventional PI controllers. The initial optimal parameters are obtained using the PSO algorithm, and an experimental test was carried out. In [45], SMC was used for a GSI controller, which improved the GSI performance compared with the conventional vector control methods. Gui et al. [46] used nonlinear control for GSI based on the portcontrolled Hamiltonian system. Nasiri and Mohammadi [47] proposed a controller for GSI to limit the peak current by controlling the generated active power directly. In [48], the GSI controller was modified on the basis of modulation voltage closed-loop control. Valenciaga and Fernandez [49] designed a multi-variable controller to regulate the active and reactive powers delivered to the grid. The controller was designed over second-order sliding-mode methods by a twostage cascade structure.

Figure 5: Control of DC-DC converter of EDLC


Figure 7: GSI control modelling


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Methods Pitch angle control

Main advantages • Decreases the mechanical stress due to acceleration during faults.

Braking chopper

• Protects the DC capacitor from overvoltage by dissipating the extra energy during grid fault. • ESSs store the extra power during grid faults. • The control of charge and discharge responds rapidly. • Stores the excess energy in the rotor inertia which can be used for system stability restoration.

DC-DC converter control of ESS MSC control

GSI control

• Supports the grid with reactive power to recover the voltage. • Controls the DC voltage during the fault then decreases the power dissipated by braking resistors.

Main limitations • Slow response. • Nonlinear relationship between pitch angle and rotor speed. • Wasting the power. • Larger dissipated power, larger heat emission. • Additional cost. • BESS capacity. • Transfers the stress from the electrical side into the mechanical side of the PMSG. • Fault symmetry

[3]

This paper focused on the recent improvements of low voltage ride-through (LVRT) capability of grid-connected PMSGs driven directly by a variable-speed wind turbine. LVRT capability with reactive power support are the grid code requests for grid-connected wind power plants. The LVRT capability of PMSG-VSWT is achieved by many methods: pitch angle control, braking choppers (BCs), energy storage system (ESS) control, machine side converter (MSC) control, and grid side inverter (GSI) control. The advantages and disadvantages of each method is summarized in Table 1. Recent studies have focused on the enhancements of GSI control to improve LVRT capability because it controls the DC link voltage and support the grid with reactive power during fault. Fuzzy logic control and evolutionary algorithms are used to enhance the control performance of GSI, MSC, PAC, and ESSs for maintaining the LVRT profile.

References

[2]

• BC is coordinated with PAC or ESS. • ESS is mainly used for power smoothing, so it can be used for LVRT in coordination with another control method. • MSC control can be coordinated with PAC to decrease the mechanical stress.

• Modifying the control of GSI is a competitive method for LVRT capability improvement. • Fuzzy logic and artificial intelligence are used in GSI control either for gain parameters tuning of PI controllers or can be used instead of PI controllers. Table 1: Comparison of LVRT capability enhancement methods for grid-connected PMSG-VSWT

Conclusion

[1]

Notes • Should be coordinated with another faster control such as BC.

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