Power Control of a Nonpitchable PMSG-Based Marine Current Turbine at Overrated Current Speed With Flux-Weakening Strategy

TLDR: In this paper, the authors investigated power control strategies for a fixed-pitch direct-drive marine current turbine (MCT) when the marine current speed exceeds the rated value corresponding to the MCT nominal power.

Abstract: This paper deals with power control strategies for a fixed-pitch direct-drive marine current turbine (MCT) when the marine current speed exceeds the rated value corresponding to the MCT nominal power. At overrated marine current speed, the MCT control strategy is supposed to be changed from maximum power point tracking (MPPT) stage to constant power stage. In this paper, flux-weakening strategy is investigated to realize appropriate power control strategies at high marine current speeds. During flux-weakening operations, the generator can be controlled to produce nominal or overnominal power for a specific speed range (constant power range). These two power control modes are compared, and the constant power range is calculated. The relationship between the expected constant power range and generator parameter requirements [stator inductance, permanent magnet (PM) flux, and nominal power coefficient] is analyzed. A Torque-based control with a robust feedback flux-weakening strategy is then carried out in the simulation. The proposed control strategies are tested in both high tidal speed and swell wave cases; the results validate the analysis and show the feasibility of the proposed control method.

Figures

Fig. 1. General scheme of a PMSG-based direct drive MCT system.

Fig. 5. Generator operating characteristics (corresponding to the Appendix data).

Fig. 19. Rotor speed responses under swell effect.

Fig. 9. Generator power characteristics with CPSR = 2.4.

Fig. 3. The MCT power characteristic under different current speeds.

Fig. 2. MCT Cp curve.

Full text

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Power Control of a Nonpitchable PMSG-Based Marine
Current Turbine at Overrated Current Speed With
Flux-Weakening Strategy
Zibhin Zhou, Franck Scuiller, Jean-Frederic Charpentier, Mohamed
Benbouzid, Tianhao Tang
To cite this version:
Zibhin Zhou, Franck Scuiller, Jean-Frederic Charpentier, Mohamed Benbouzid, Tianhao Tang. Power
Control of a Nonpitchable PMSG-Based Marine Current Turbine at Overrated Current Speed With
Flux-Weakening Strategy. IEEE Journal of Oceanic Engineering, Institute of Electrical and Electronics
Engineers, 2014, pp.1-10. �10.1109/JOE.2014.2356936�. �hal-01086400�

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To cite this version :
Zibhin ZHOU, Franck SCUILLER, Jean-Frederic CHARPENTIER, Mohamed BENBOUZID,
Tianhao TANG - Power Control of a Nonpitchable PMSG-Based Marine Current Turbine at
Overrated Current Speed With Flux-Weakening Strategy - IEEE JOURNAL OF OCEANIC
ENGINEERING n°99, p.1-10 - 2014
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Power Control of a Non-Pitchable PMSG-Based
Marine Current Turbine at Over-rated Current
Speed with Flux-Weakening Strategy
Zhibin Zhou, Student Member, IEEE, Franck Scuiller, Member, IEEE, Jean Frédéric Charpentier, Member, IEEE,
Mohamed Benbouzid, Senior Member, IEEE, and Tianhao Tang, Senior Member, IEEE
Abstract—This paper deals with power control strategies for a fixed-pitch direct drive marine
current turbine (MCT) when the marine current velocity exceeds the rated value corresponding
to the MCT nominal power. At over-rated marine current speed, the MCT control strategy is
supposed to be changed from maximum power point tracking (MPPT) stage to constant power
stage. In this paper, flux-weakening strategy is investigated to realize appropriate power control
strategies at high marine current speeds. During flux-weakening operations, the generator can be
controlled to produce nominal or over-nominal power for a specific speed range (constant power
range). These two power control modes are compared and the constant power range is calculated
in this paper. The relationship between the expected constant power range and generator
parameters requirement (stator inductance, permanent magnet flux, nominal power coefficient) is
analyzed in this paper. A Torque-based control with a robust feedback flux-weakening strategy is
then carried out in the simulation. The proposed control strategies are tested in both high tidal
speed and swell wave cases; the results validate the analysis and show the feasibility of the
proposed control method.
Index Terms—Marine current turbine, fixed-pitch, PMSG, flux-weakening, high marine
current speed.
NOMENCLATURE
CAP = Constant active power;
CPSR = Constant power speed ratio;
MAP = Maximum active power;
MCT = Marine current turbine;
MPPT = Maximum power point tracking;
MTPA = Maximum torque per ampere;

PMSG = Permanent-magnet synchronous generator;
TSR = Tip speed ratio;
H
S
, T
p
= Swell significant height and peak period;
C
p
= Power coefficient;
f
B
= Friction coefficient;
i
d
, i
q
= Generator d- and q-axis currents;
I
max
= Maximum phase current magnitude;
J = Total system inertia;
K
T
= Generator torque constant;
L
d
, L
q
= Generator d- and q-axis inductances;
n
p
= Pole pair number;
P
N
= Turbine nominal power;
R = Turbine blade radius;
R
s
= Generator stator resistance;
T
e
, T
m
= Generator and turbine torque;
V = Marine current speed;
V
s
= Generator phase voltage magnitude;
V
dc
= DC-bus voltage;
V
max
= Maximum phase voltage magnitude;
v
d
, v
q
= Generator d- and q-axis voltages;
λ = Turbine tip speed ratio;
ρ = Sea water density;
φ = Generator power factor angle;
ψ
m
= Permanent magnet flux;
ω
e
, ω
eb
= Electrical angle and base speeds;
ω
m
= Mechanical angle speed;
ω
mN
= Rotor nominal angle speed.
I. INTRODUCTION
Due to high predictability of the marine tides and high power potential of marine tidal currents,
various marine current turbine (MCT) technologies have been developed to capture tidal current energy
in recent years [1-2]. However, difficulties of underwater accessibility make compact structure and low

maintenance highly expected for MCTs. Several industrial MCT projects such as the OpenHydro (tested
by French utility company EDF), Hydro Beluga 9 (Alstom) and Voith Hydro turbine system adopt non-
pitchable turbine blades and use permanent magnet synchronous generators (PMSG). Two advantages
of these projects can be noticed: fixed blades enable simplifying the turbine system (avoiding pitch-
variable mechanism); and PMSGs enable realizing direct-drive systems (eliminating gearbox).
However, the MCT rated power would not be designed for the peak marine current speed due to the
fact that the peak current speed may appear only at large spring tides and corresponds only to a small
part of the statistical resource [3]. When the marine current speed is higher than the rated value, a fixed-
pitch MCT is unable to limit the extracted power via pitch control as in large wind turbines [4] and
some pitchable MCT turbines [5]. Therefore, an appropriate generator-side control strategy should be
applied to control and limit the MCT output power.
Based on the turbine power characteristic, one possible solution is to operate the turbine at over-
nominal speed during high marine current speed periods for reducing turbine power coefficient and the
harnessed power. However, over-nominal speed operation leads to high electromotive force (EMF) in
PM machines and may cause saturations in the generator regulators. Although various flux-weakening
control strategies have been studied to realize over-base speed operations for PM machines [6-15], the
flux-weakening operation mode for PMSG in renewable energy systems is still a new topic. The joint
operating characteristics of the MCT and the PMSG are focused in this paper. In a previous work [16],
the authors proposed to apply flux-weakening strategy for limiting the MCT power to its nominal power
at high current speeds. However, the maximal generator output power during the flux-weakening
operation and the impacts of generator parameters on the MCT operation range are not discussed. In this
paper, these issues will be focused. Figure 1 shows the general system scheme, and the generator-side
control will be focused in this paper.
In Section II, the turbine power characteristic and the generator model are presented. In Section III,
the flux-weakening operation and the generator parameters impacts on the MCT operational zone are
discussed. In Section IV, the proposed robust power control scheme is presented and the simulation
results of the two power control modes at high marine current speed are compared. And the conclusion
is then given in Section V.

Frequently asked questions

Q1. How much power can the MCT system produce at a high current speed?

At a high current speed of 4.0 m/s, the MCT system is still able to produce 1.77 MW under the MAP mode or 1.52 MW under the CAP mode. 

Q2. How can a horizontal-axis MCT be calculated?

The power harnessed by a horizontal-axis MCT can be calculated as2 31 ρ π 2 pP C R V (1)The turbine power coefficient Cp depends on the turbine blade structure and hydrodynamics. 

Q3. What is the typical control strategy for a non-salient PM machine?

The typical control strategy for a non-salient PM machine under the base speed is to set id = 0 for maximizing the torque per ampere ratio. 

Q4. What is the maximum power range for the generator?

At the flux weakening stage, there exists a constant power range during which the generator power can be either over the nominal power with iqmax or limited to the nominal power with iq given by (9). 

Q5. Why is the fluxweakening method chosen in this paper?

The fluxweakening method proposed in [13-14] is chosen in this paper for its robustness and high utilization of the DC-bus voltage. 

Q6. What is the effect of the flux-weakening strategy on the rotor speed?

When the current speed rises over the rated value, the flux-weakening strategy will be triggered and the rotor speed will rise over the nominal speed. 

Q7. What is the typical sea state in the winter?

In this paper, a medium-strong sea state with significant wave height Hs = 3 m and typical wave period Tp = 13.2 s is considered (it corresponds to typical sea state in the winter off the western coast of Europe). 

Q8. What are the two power control modes?

Two power control modes (the maximum active power mode and the constant active power mode) on the generator constant power range have been investigated. 

Q9. What is the power range of the generator?

Over the constant power range, the generator power will decrease rapidly below the nominal value with the increase of the rotor speed. 

Q10. What is the effect of flux weakening on the turbine power coefficient?

It should be noticed that the flux-weakening operation is on the decreasing slope of the Cp (λ > λopt) curve to reduce the turbine power coefficient during over-rated current speeds. 

Q11. What is the effect of swell on the turbine?

It should be noticed that under swell effect, the marine current speed is fluctuating and there is no steady-state for the turbine. 

Q12. Why can't the MCT system produce power at a current speed of 4.0 ?

the MCT system can not produce power at a current speed of 4.0 m/s due to small CPSR and low flux-weakening capability of the generator. 

Q13. what is the optimum power factor for a generator?

The compromise between high power factor and large constant power range should be noticed for appropriate generator parameter design. 

Q14. What is the Cp curve for a given turbine?

For a given turbine and based on the experimental results, the Cp curve can be approximated as a function of the tip speed ratio ( mλ= /R V ) and the pitch angle [17]. 

Q15. What is the difference between the MAP and CAP modes?

Figure 13 and 14 show that at over-rated marine current speeds, the CAP mode features lower generator torque to accelerate the turbine to a higher speed than the MAP mode.