Acdcac I

Synchronous Generator 30MVA, 6.6kV, H=2.5s

Grid connection

Doubly-Fed Induction Machine 7MVA, 6.6kV, 422.5MJ(max)

Resistive Load

13.5MW

Resistive Load

15MW

13.5MW

15MW

Fig. 2. Model system of an isolated power system Configuration of FESS

Fig. 3 shows a model configuration of FESS. The FESS consists of the adjustable speed generator, the flywheel mass for kinetic energy storage, and secondary excitation circuit for adjustable speed control [4]. The adjustable speed generator has basically the same construction as that of a wound rotor induction machine. The secondary excitation power is supplied from the terminal of FESS, and converted to DC power by the converter, then again converted to low frequency AC power by the inverter and supplied to the rotor. Thus, the rotor can rotate at asynchronous speed. The inverter controls active and reactive power output (PT and QT) of the generator, and the converter controls DC link voltage EDC and reactive power QL flowing into the secondary excitation circuit. These electronic power converters are modeled as 6 force-commutated power switches connected in a bridge configuration as shown in Fig. 4. A sinusoidal PWM operation is carried out and switching signals are generated by applying triangular carrier wave comparison. Conventional PI controllers are used for the inverter and the converter control as shown in Fig. 5 and 6 respectively. Parameters of the FESS generator are shown in Table II.

A method of frequency stabilization by using FESS

The main purpose of this study is to reduce the network frequency variation by using FESS. The configuration of the control system for the frequency stabilization is shown in Fig. 7. Reference of active power output of FESS, PT(ref), is determined according to the deviation of network frequency, which is detected by PLL at the terminal of FESS. When the frequency is decreased, FESS supplies active power to the network. When the frequency is increased, FESS absorbs active power from the network. These control schemes correspond to block (A) in Fig. 7. At the same time, PT(ref) is modified to prevent a shortage or an excess of the

Doubly-Fed Induction Machine (7MVA, 6.6kV, H=50.0)

Doubly-Fed Induction Machine (7MVA, 6.6kV, H=50.0)

*1 : Stored energy (J) is the ratedpower of the machine (W) x 0.02 (s).

Fig. 3. FESS circuit configuration

*1 : Stored energy (J) is the ratedpower of the machine (W) x 0.02 (s).

Fig. 3. FESS circuit configuration stored energy of FESS. In this study, the maximum and the minimum rotor speeds of FESS are specified 130% (1.3pu) and 70% (0.7pu) of the rated speed respectively. Considering these boundary speeds, the value of PT(ref) is modified to a lower (or a higher) value when the rotor speed is under (or over) 1.044pu, at which the stored energy becomes a half of the maximum storage energy. These control schemes correspond to block (B) in Fig. 7. Fig. 7 also includes a rule of FESS control to avoid operating under 0.7pu or over 1.3pu rotor speed as shown in Table I.

Fig. 4. Model of power converter

Frequency < 50

Frequency > 50

Wr_feSS

> 1.3

1

0

1.3 >

Wr_feSS

> 0.7

1

1

0.7 >

Wr_feSS

0

1

Fig. 5. Output power controller of FESS
Fig. 6. Excitation power controller of FESS

Wind farm model

The wind farm consists of an induction generator and a wind turbine. An aerodynamic characteristic of the turbine blade expressed by eqs.( 2) and ( 3) is adopted [5]. The captured power is expressed by eq.( 1). Since the induction generator is operated at almost constant speed (approx. 1.0 to 1.01 pu), the output power changes widely with respect to wind speed variations. Generally, a wind turbine is equipped with a pitch angle controller. The conventional pitch controller shown in Fig. 8, that maintains the output of the generator to be the rated power when the wind speed is over the rated speed, is also considered in this study. Parameters of the wind generator (IG) are shown in Table II.

A 1609

Active Power

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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