# Flywheel Energy Storage and Dump Load to Control the Active Power Excess in a Wind Diesel Power System

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## Abstract

**:**

## 1. Introduction

_{T}exceeds the load consumption P

_{L}, a situation that makes the WDPS unstable. The WTG power excess situation (P

_{T}> P

_{L}) is simulated in WD mode in three cases, namely DL and FESS-off (DL and FESS are turned off), only-DL (DL actuates but FESS is turned off) and only-FESS (FESS actuates but DL is turned off). In the DL and FESS-off simulation case, it is shown that to prevent the isolated power system collapse that the WTG power excess provokes, the solution is to trip the WTG circuit breaker (I

_{T}in Figure 1). In both the only-DL and only-FESS simulation cases, it is shown how the DL/FESS are commanded to consume controlled power, avoiding the WTG circuit breaker trip necessary in the DL and FESS-off case so that the WDPS absolute stability is increased. A previous paper [15] deals with simulations in WO mode of a system that comprises a WTG, a grid forming SM, load and a FESS. Ref [15] does not include a diesel engine, therefore this article’s isolated power system architecture is different. In a previous paper [8], the simulated WDPS includes a FESS, but no DL is considered. Furthermore, the WTG power levels in [8] are below the load consumption (P

_{T}< P

_{L}) in all the simulations, so [8] does not deal with the WTG power excess situation. In addition, [8] focuses on the control of the FESS power converter. Previous papers [5] and [7] use a battery ESS instead of a flywheel ESS, so the used power converter and ESS variables shown in the simulations are different. Additionally, [5] and [7] do not use the DL. The WD mode simulations in [7] do not consider the WTG power excess situation dealt in this paper. Ref. [5] deals with a high penetration WDPS and its main simulations are in WO mode.

## 2. Isolated WDPS Modelling

_{T}defines the DO/WD operation mode of the WDPS.

#### 2.1. The Diesel Generator Model.

_{D}and the SM converts the DE mechanical power into electrical power. The SM provides the isolated grid sinusoidal voltage waveform. By following the command of its automatic voltage regulator, the SM provides reactive power to the WDPS, which is necessary to keep the system voltage module V within the allowable limits. The relationship between the voltage waveform frequency f (Hz) and the shaft speed of the DE/SM ω (rad/s) is:

_{ref}– ω

_{d}in Figure 2, where ω

_{re}

_{f}is the DE reference speed and ω

_{d}the actual speed ω) so the DE speed control is isochronous, that is, in steady state the DE speed is rated one (therefore rated system frequency) provided that the electrical load is within 0–300 kW range. The actuator converts the output of the speed regulator into a proper signal to control a fuel valve. In this way, the incoming fuel rate to the DE is adjusted to control the DE produced mechanical power to the needed value to achieve the rated speed in the DE.

_{DG}is 1.75 s.

#### 2.2. The Wind Turbine Generator Model

_{T-MEC}with the wind speed (v_speed) and the WT shaft speed ω

_{t}. The output of the Wind Turbine block is the torque applied to the SCIG (T

_{m}= P

_{T-MEC}/ω

_{t})

_{T-MEC}is mainly a function of the cube of the wind speed [25]. The wind speed is quasi-random, so that the WT-SCIG behaves as an uncontrolled active power supplier. In spite of the previously commented disadvantages, the WT-SCIG fixed pitch constant speed type used in this article has remarkable features for the remote locations of WDPS, such as robust construction, simple maintenance and low cost. The used WT-SCIG type is more robust than [12,21,22,23,24] as it does not equip an electronic power converter and has less maintenance than [24], as the SCIG does not have slip rings.

_{t}– ω in per unit values) [25], the WTG-SCIG improves the system frequency by providing a damped response [26].

_{W}range between 2–6 s [27] for low–high power WTGs. The used WTG is a low power one, so that H

_{W}= 2 s is set for this parameter.

#### 2.3. The Dump Load Model

_{T}-P

_{L}) to regulate system frequency [23,28]. The Figure 2 DL consists of eight three-phase resistors connected in series with solid-state switches, which connect/disconnect the resistors at the zero crossing in order to prevent harmonic injection. The values of the resistors follow a binary progression, with values R, R/2, R/2

^{2}…R/2

^{7}. If P

_{0}is the rated power of the resistor of value R (P

_{0}= V

_{n}

^{2}/R, where V

_{n}is the rated system voltage), the eight resistor’s rated powers are P

_{0}, 2·P

_{0}, 2

^{2}·P

_{0}, …, 2

^{7}·P

_{0}. Being I

_{J}the state closed(1)/opened(0) of the three phase switch associated with the resistor of power 2

^{J}·P

_{0}, the power consumed by the DL P

_{D}if the system voltage is the rated value, can be expressed as:

_{D}= (I

_{0}+ I

_{1}·2

^{1}+ … + I

_{7}·2

^{7})·P

_{0}

_{D}can be varied discretely in steps of P

_{0}from 0 to 255· P

_{0}. P

_{0}was chosen to be 1.4 kW, so the DL rated power P

_{D-NOM}is 357 kW (1.4·255). The DL control block of Figure 2 determines which resistors have to be connected to consume the assigned P

_{D-REF}. The details of the DL implementation in Simulink can be found in [18].

#### 2.4. The Flywheel Energy StorageSystem Model

_{r}

_{-min}to a maximum ω

_{r-}

_{max}rotating speed, so the maximum available kinetic energy, Ec-max is:

_{r}. The rotor flux is:

_{r}= L

_{m}i

_{mr}

_{mr}and L

_{m}are the magnetizing current and inductance respectively. The relation between i

_{mr}and the stator direct current i

_{sd}is given by:

_{r}is the rotor time constant and T

_{r}= L

_{r}/R

_{r}, where R

_{r}and L

_{r}are the rotor resistance and inductance respectively. Therefore, i

_{sd}produces the rotor flux Ψ

_{r}, but with slow dynamics due to the high value of T

_{r}. The ASM electromagnetic torque T

_{el}is proportional to the product of the rotor flux and the stator quadrature current i

_{sq}[31]:

_{mr}= i

_{sd}= constant) at its optimal value and the required T

_{el}is obtained by setting the corresponding quadrature current i

_{sq}according to Equation (6). If all the ASM losses are neglected (stator and rotor resistance and stator iron losses), the exchanged ASM active power can be approximated to the product of the electromagnetic torque T

_{el}and the ASM rotor speed ω

_{r}. Also neglecting the FESS double power converter losses, the exchanged FESS active power P

_{S}is approximately:

_{S}≈ T

_{el}ω

_{r}

_{r}speed constant when it is compared to the electric dynamics. Hence, the FESS exchanged power is controlled by setting the needed i

_{sq}.

_{S-NOM}) during 2 min and with the flywheel operating speed range ω

_{r-min}–ω

_{r-max}within 1500–3300 rpm [8], so Equation (3) gives 380 kg·m

^{2}for the flywheel moment of inertia I. The selected ASM is a standard 50 Hz one, with a single pole pair and 300 kW of rated power [8]. The model of the 300 kW ASM-FESS uses the block included in the SimPowerSystems blockset for Simulink [16] and its electrical model is a fourth-order one. The flywheel 380 kg m

^{2}inertia is included in the inertia parameter of the FESS-ASM SimPowerSystems model to simulate the flywheel.

## 3. The WTG Power Excess Situation and the DL and FESS-off Case Simulation

_{T}> P

_{L}). If there is no ESS and no DL in the WDPS to consume additional power, the DE should consume the active excess power to balance active powers in the system and thus to control system frequency. However, the DE speed governor cannot command the DE to consume power. In the extreme case of no fuel injection into the DE cylinders, the DE power will be negative (P

_{DE}< 0) and will consist of losses from compression in the cylinders, shaft, etc., but the DE losses are not controllable. Therefore, if the WTG power excess surpasses the DE losses, the system frequency will rise without control. This frequency increasing is better observed by analyzing the equation which relates the active powers of the DE (P

_{DE}), WTG (P

_{T}) and load (P

_{L}) with the DG speed (ω)/frequency (f):

_{DE}and P

_{T}) are positive when produced and the consumer load power P

_{L}positive when consumed, J is the DG moment of inertia and the losses are neglected. Since the left side of Equation (8) is positive in the WTG power excess case and J and ω are positive, this implies dω/dt > 0 and a continuous uncontrolled frequency increase.

_{G}in Figure 1) and shut down the DE by cutting the fuel supply to the DE cylinders. Among these alarms is the overspeed alarm, which avoids the uncontrolled acceleration of the DG and therefore excessive centrifugal forces that could damage the rotating parts of the DG. The overspeed alarm is activated when the DG speed is greater than the overspeed setpoint, which is normally 1.1 the DG rated speed. Additionally, all DG have some SM-CB trip alarms, among them the reverse power alarm. This alarm is activated when the DG is connected in parallel with other generators and the DG output power is negative (the SM is behaving as a motor) during a certain time interval. During a DG reverse power activation, the system frequency is supported in the rated value by the other supplying generators.

_{T}> P

_{L}), the DG output power is negative and at the same time the system frequency increases, so that this situation could be confused with an overspeed alarm if the overspeed set point is reached and therefore the protection control will shut down the DG. The WTG power excess case could also be confused with the reverse power alarm if the conditions to active this trip alarm occur. In both cases, the SM-CB will be open and there will not be supply for the consumer load. However, to prevent the discontinuation in the power supply in the WTG power excess case, the solution is to open the WTG CB (I

_{T}in Figure 1) and allow the DG to continue supplying the loads.

## 4. The WTG Power Excess Situation in the Only-DL/Only-FESS Cases

#### 4.1. The DL and FESS Control

_{DE}= P

_{L}+ P

_{DL}− P

_{T}

_{DL}is the DL consumed power and using for P

_{DL}the same consumer load sign criteria. As the DL rated power (357 kW) is greater than the WTG rated power (275 kW), Equation (9) indicates that it is possible, by controlling the DL consumed power, to obtain a positive P

_{DE}in steady state, even with null load (P

_{L}= 0). In this article, the DL dumped power follows the P

_{INV}output shown in Figure 6 (P

_{D-REF}= P

_{INV}). The Figure 6 control is an integral one, and its aim is to keep the DG active power (P

_{SM}) in the reference range 15–21 kW (5–7% P

_{DG-NOM}) in steady state when a WTG active power excess exists. The control output P

_{INV}increases when the DG power P

_{SM}is less than 15 kW and decreases its value when P

_{SM}is greater than 21 kW. In Figure 6, the integral control is limited to the DL power limits [0, P

_{D-NOM}]. The integral constant K

_{I}of Figure 6 was tuned taking into account the used DL discrete nature. To follow P

_{INV}, the DL control must order the DL resistors to switch on and off, and this switching produces system voltage variations, as the following simulations graphics show. Therefore, excessive switching must be avoided.

_{DL}is substituted by the power exchanged by the FESS P

_{S}. As the FESS rated power is 150 kW, the FESS cannot guarantee a positive P

_{DE}in steady state if the WTG power excess surpasses 150 kW. In the only-FESS case, the reference power P

_{S-REF}to be consumed by the FESS is given by the following equation:

_{f}as input (e

_{f}= f − f

_{NOM}, where f/f

_{NOM}are the current/rated WDPS frequency) is added to the formerly explained term P

_{INV}. K

_{P}and K

_{D}are the PD proportional and derivate constants. The integral control limits of P

_{INV}are [0, P

_{S-NOM}] in this only-FESS case. The PD regulator supports frequency regulation, improving the transients of the WDPS and it is compatible with the PID regulator inside the DE speed governor. K

_{P}and K

_{D}were adjusted to moderate the system frequency over/under shooting. Equation (10) makes use of the fast-acting power electronic converter with PWM unlike the DL, which uses the zero-crossing connection of the resistors and this results in slower actuation. Moreover, the DL power is in discrete steps and it may lead to excessive voltage variations as explained previously.

#### 4.2. The Simulation Tests for the Only-DL and Only-FESS Cases

#### 4.3. The Flywheel Variables

_{sd}and quadrature i

_{sq}stator currents of the ASM, relative to the 360 A ASM base current and the flywheel shaft speed, relative to the 3000 r.p.m ASM base speed are shown for the FESS test in Figure 11. The direct current creates the ASM magnetic flux, and its sign is always positive. The quadrature current sets the ASM torque and its sign is positive/negative for motor/brake torque (sign criteria usually employed with servos), increasing/decreasing the flywheel speed, and with the FESS consuming/supplying power from/to the isolated grid. There is no PWM current ripple in quadrature and direct currents, since the FESS power converters use average models for faster simulation. The ASM (flywheel) speed indicates how the FESS changes its stored energy, as FESS SOC is proportional to the square of the flywheel speed. Figure 11 left scale is for currents pu and right scale is for speed pu.

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## List of Abbreviations

ASM: asynchronous machine |

CB: Circuit Breaker |

DE: Diesel Engine |

ESS: Energy storage system; BESS: battery ESS |

SCIG: squirrel cage Induction Generator |

SM: synchronous machine |

WDPS operation modes: Diesel Only (DO), Wind Diesel (WD), Wind Only (WO) |

WT: Wind Turbine |

## Appendix A. Wind Diesel Power System (WDPS) Parameters

WDPS rated frequency and voltage = 60 Hz and 440 V |

Diesel Generator (DG) rated power and inertia constant = 300 kVA and 1.75 s. |

Wind Turbine Generator (WTG) rated power and inertia constant = 275 kW and 2 s. |

Dump Load (DL) rated power = 357 kW |

DL integral control K_{I} = 9 s^{−1} |

Flywheel Energy Storage System (FESS); LS-FESS: low speed FESS |

FESS rated power, P_{S-MOM} = 150 kW |

FESS maximum available energy = 18,000 kJ |

FESS Direct Current (DC) -link voltage = 800 V |

FESS capacitor bank capacitance = 4.7 mF |

FESS inductance (L) filter = 2.5 mH |

FESS Proportional and Derivative (PD) constants: k_{P} = 151 kW/Hz, K_{D} = 7 kWs/Hz |

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**Figure 1.**Layout of the Wind Diesel Power System (WDPS) with Dump Load (DL) and Flywheel Energy Storage.

**Figure 5.**Wind turbine generator (WTG), diesel generator (DG) and load active powers (kW) for the DL and FESS-off case.

Event/Case | DL and FESS-off | Only DL | Only FESS | |
---|---|---|---|---|

wind step | %Δf | 0, +8 | −0.1, +0.83 | −0.11, +0.42 |

%Δv | −12, +23 | −1.41, +1.16 | −0.56, +0.42 | |

WTG CB trip | In t = 2.935 s | Not necessary | Not necessary | |

load step | %Δf | No apply | −0.57, +0.14 | −0.37, +0.02 |

%Δv | −2.46, +1.57 | −1.26, 1.1 |

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## Share and Cite

**MDPI and ACS Style**

Sebastián, R.; Peña-Alzola, R.
Flywheel Energy Storage and Dump Load to Control the Active Power Excess in a Wind Diesel Power System. *Energies* **2020**, *13*, 2029.
https://doi.org/10.3390/en13082029

**AMA Style**

Sebastián R, Peña-Alzola R.
Flywheel Energy Storage and Dump Load to Control the Active Power Excess in a Wind Diesel Power System. *Energies*. 2020; 13(8):2029.
https://doi.org/10.3390/en13082029

**Chicago/Turabian Style**

Sebastián, Rafael, and Rafael Peña-Alzola.
2020. "Flywheel Energy Storage and Dump Load to Control the Active Power Excess in a Wind Diesel Power System" *Energies* 13, no. 8: 2029.
https://doi.org/10.3390/en13082029