Comparison of DFIG and Synchronous Machine for Storage ... · Mahdi Johar 1,*, Ahmad Radan 2, Mohammad Reza Miveh 3 and Sohrab Mirsaeidi 4 1, 2 Electrical Engineering Department of

Int. J. Pure Appl. Sci. Technol., 7(1) (2011), pp. 48-58

International Journal of Pure and Applied Sciences and Technology ISSN 2229 - 6107 Available online at www.ijopaasat.in

Research Paper

Comparison of DFIG and Synchronous Machine for Storage Hydro-Power Generation

Mahdi Johar1,*, Ahmad Radan 2, Mohammad Reza Miveh 3 and Sohrab Mirsaeidi 4

1, 2 Electrical Engineering Department of K.N. Toosi University of Technology 3, 4 Department of Electrical Engineering, Komijan Branch, Islamic Azad University, Komijan, Iran

* Corresponding author, e-mail: ([email protected])

(Received: 28-7-11; Accepted: 3-10-11)

Abstract: Among the alternative methods for energy storage , using the potential energy of water stored in different heights has been considered by human for ages. Storage pumping plants have been invented for this purpose and are considered as one of the most reliable and effective methods of storing the electrical energy. Factors such as having appropriate static and dynamic stability, high efficiency, suitable steady state and transient performance, the least cost of manufacturing, utilities and etc. can play a major role in choosing the type of machines for energy-making. Therefore, advantages and capabilities of variable speed storage -pumping plants, compared with conventional synchronous ones, have drawn into consideration in this paper. Different viewpoints such as structure and steady state behavior are focused for comparing the application of Doubly Fed Induction Generators (DFIG) and synchronous ones in storage hydro plants. Dynamic stimulations are also used to evaluate the performance of variable-speed versus synchronous fixed-speed generation units. Keywords: DFIG, Synchronous Machine, Power Generation, Simulation

1. Introduction: Main Developing quick responded energy storing systems is indispensable according to the expansion of power network and the problems of its stability control consecutively. One of the well-known energy storage systems in networks is Pumped storage power plants. Producing energy in pumped storage power plant is possible in two ways of fixed and adjustable speed. The usage of optimum operating points of system in recent times has made the production of electrical energy by adjustable speed systems more important. DFIG is one of the adjustable speed systems which has many advantages compared to

Int. J. Pure Appl. Sci. Technol., 7(1) (2011), 48-58. 49

other systems and became more considerable than synchronous generator in pumped storage power plant in late 1980’s. Many works have been done on variable speed pumped storage generators and numerous papers have been published on modeling [1,2], dynamic response[1,3,4,5] , control methods [6,7,8], electronic converters [9,10,11], and speed drive [2,12] of DFIG in pumped storage power plants but comparison between synchronous machine and DFIG in pumped storage power plants has never been performed up to now. Using DFIG systems comparing synchronous ones increases the usage efficiency and flexibility significantly. Nowadays using DFIG generators instead of synchronous ones is an efficient way to solve the problem of efficiency drop from pumped mode to generator mode due to significant reduction of frequency converter capacity. After this section, a history of variable speed pumped storage power plants, an evaluation of DFIG modeling, and finally a structural and steady state behavior comparison between the two machines are given respectively and the results are presented. 2. History of variable speed pump-storage power plants Unlike conventional pump-storage units, variable speed pump-storage units do not have a long history which is because of their dependency on high power transformers. Design of such systems was evaluated theoretically in 1970 for the first time. The first variable speed storage hydro-power generator was installed at Narude power plant, Japan with a capacity of 22 MVA in 1987 The motor-generator had a cylindrical rotor with a 3-phased wounded rotor. The 11 KV windings of its rotor had to tolerate a large amount of centrifugal force which was a lot more than what small motors had to use for pump-storage motor-generators. Today usage of rotating machines is conventional using advanced technology in rotor turns especially in materials and manufacturing. Table 1 shows installed samples of variable speed pump-storage generators around the world. 3. Principles and Modeling of DFIG: DFIG is the generator which is fed by both stator and rotor terminals. In other words dual feed induction generators are called DFIG. The DFIG configuration is shown in Figure 1 and consists of the stator directly connected to the grid while the rotor circuit with its variable voltage and frequency requires a back-to-back converter for the grid connection. The converter allows the active power flow in two directions: from the grid to the rotor in sub-synchronous operation and from the rotor to the grid in over-synchronous mode.

Table 1: variable speed pump-storage generators around the world [3]


Fig. 1: DFIG electric configuration [2]

DFIG is recommended in variable speed systems with a limited range of speed to 0

030± of synchronous speed. Depending on the poles of the machine the rotor speed is defined as follows:

1 2

1

,

.m

m R p

ω ω ωω

= ±= Ω

(1) Where, p1 and RΩ are the number of pole pairs and the mechanical rotor speed,

respectively. In comparison with fixed speed synchronous machines which are used in pumped storage units, DFIG has below advantages: • Possibility of operation in high efficiency especially in partial load (about 2%) • Capability of control and separation of active and reactive power • Capability of power factor control • Quicker system response • Reduction of the number of starts 4. DFIG Models: 4.1. DFIG Electrical Model: The model of the machine, in transient and steady states, is established in a general d-q rotating reference Frame Equivalent circuit and main equations describing a DFIG behavior are illustrated as follow:


Voltage equations:

' ' ' ' '

' ' ' ' '

' ' ' '

,

,

,

( ) ,

( ) ,

qs s qs ds q s

ds s ds qs d s

os s o s o s

qr r qr r dr qr

q r r d r r qr dr

or r o r or

dv r i

d td

v r id t

dv r i

d td

v r id td

v r id t

dv r i

d t

ω ψ ψ

ω ψ ψ

ψ

ω ω ψ ψ

ω ω ψ ψ

ψ

= + +

= + +

= +

= + − +

= − − +

= +

(2)

Where, λ is defined as follows:

' ''

'' '

'' '

0 000 0

0000

0 000

0 00 00

000 0

0 0 0 0

os osls

ds dsmls m

qs qsmls m

or orl r

m lr mdr dr

m lr mqr qr

iLiLL LiLL L

iL

L L L i

L L L i

λλλ

λλλ

+ + = + +

(3) Active and reactive equations:

3( ),

23 3

( ) ( )2 2

s ds qs qs ds

qs d s d s qs ds ds q s q s

P i i

Q s V i v i i i

ω λ λ

ω λ λ

= −

= − = +

(4) 4.2. DFIG Mechanical Model: The equation of Rotor rotation is obtained from equalizing inertial torque and accelerating torque.

( ) ( )

,

/ 1

2

mem m ech dam p

r bem m ech d am p

dJ T T T

d td

T T Td t H

ω

ω ω

= − −

= − − (5)

Where emT , mechT , dampT are electromagnetic torque, external mechanical torque and

damping torque respectively and H is defined as follows:


2

2b m

b

JH

S

ω= (6)

5. Structural Comparison of DFIG with conventional synchronous ones: Respect to dimensions and structures, a general comparison between mentioned machines are presented here (Figure 2).

Fig. 2: a general comparison between mentioned machines [13]

• Fixed speed synchronous machine has equal stator and rotor rotational speed of

fields and a DC excitation system which is located on the rotor. Power electronic equipment is located on the rotor side. The speed is a function of the number of poles and the frequency of the electrical system. The synchronous machine has a salient pole rotor as shown in Figure 3.

Fig. 3: salient pole rotor in synchronous machine[13]

• The design of DFIG is identical to wound rotor induction machine with the difference that

output power from DFIG can be increased from about 30MVA up to 50MVA by some modifications. DFIG has a cylindrical pole and a wounded rotor as shown in Figure 4. The speed is a function of the number of poles and the frequency of the electrical system. Power electronic equipment is located on the rotor side.


Fig. 4: round pole rotor in DFIG [13]

Stator of synchronous machine and DFIG are the same. The main difference between

synchronous machine and DFIG is the structure of their rotor as shown in figure 5.

Fig. 5:Comparison between rotors in synchronous machine and DFIG [13]

Excitation system of DFIG possesses high-voltage and large current AC field while excitation system of SG possesses low voltage and low current DC field. The rotor design of two mentioned machines are quite different and consists of some parts such as winding design, slip ring, retaining system, winding overhang, rotor rim, cooling system and retaining end winding. Between DFIG and salient pole machines with the same power and speed, DFIG possesses larger dimensions and inertia and smaller air gap. Since the flux must spread uniformly on rotor surface of DFIG its structure is uniform, cylindrical and laminated. A comparison of diameter, length, number of conductors and turns and volume of copper used in rotor winding with the same specification is done here. This specification is given as below:


Table 2: Machine specification

Type AW710L8X8RB

Rated output(KW ) 500

Frequency(cps) 50 cps

Rated voltage(V) 6600 V

Rated speed(rpm) 744 rpm

Power factor 0.8

Stator circuitry Star

Rotor circuitry delta

6. DFIG and synchronous machine common specifications: Designing values are 34000=ac , 67.0=avB , 955.0=wK and 8=P , D and L are as

follows: Table 3: Length and diameter in SM and DFIG

type Synchronous DFIG

D(m) 1.25 1.4

L(m) 0.73 0.63

As a result the volume of DFIG is 307.0 m more than the volume of synchronous machine which is true because this machine has a 3-phase rotor.

Table 4: Volume in SM and DFIG

type Synchronous DFIG

Volume(m3) 0.89 0

The differences in the number of conductors, the number of rotor turns and the volume of copper used in rotor windings are shown below:

Table 5: Number of conductors, Number of turns per phase and Volume of copper in SM and

DFIG

Type Synchronous DFIG

Number of conductors 232 480

Number of turns per phase 116 80

Volume of copper(cm3) 8346 15347

The number of rotor turns of a single phased synchronous machine is equal to 116 and the number of conductors is equal to 232 since each turn consists of two conductors. But in a DFIG machine the number of turns considering its 3-phased rotor is equal to 240 and the number of conductors is equal to 480. In addition, the surface area of rotor turns of a DFIG machine is 6 mm2 more than a synchronous one due to having a higher voltage


(because of using AC and DC) and working in 3-phased condition. This results show that the volume of used copper in rotor turns of a DFIG machine is 2101 cm3 more and consecutively its volume is 0.07 m2 more than a synchronous machine. Therefore in an equal specification a DFIG machine is slightly bigger than a synchronous one.

7. Comparison of DFIG and synchronous machine steady state operations: For evaluating steady state behavior of a DFIG and a synchronous machine different working points in usage area are chosen in our simulation which delivers one below conditions:

Equal stator output power and power ratio It should be mentioned that transmitted power of a DFIG to network is equal to

sum of the stator output power and rotor input power, while transmitted power of a synchronous machine to network is equal to the stator output power.

7.1. Equal stator output power and power ratio: In this paragraph at first the stator output power and the power ratio of DFIG machine are assumed to be equal to those of synchronous machine and then the stator and the rotor currents will be compared in order to study the steady state behavior of them according to the outcome of simulations and calculations and after that the input power of the rotor will be evaluated. It should be noted that only the results of the calculations are presented here. Pre-defined values of the stator output power and the power ratio are as 1.6 MW and 0.8, respectively.

According to these values, the results of simulations and calculations of DFIG and synchronous machine are as follows:

(a) (b)

Fig. 6: Stator current in equal stator output power and power ratio (a) synchronous machine (b) DFIG.

Table (6) stator current in equal stator output power and power ratio

Is calculation simulation

synchronous 1673.2 1658.8

DFIG 1673.2 1658.8

As it is shown in Figure 6 for equal stator output and power ratio, stator current of DFIG and synchronous machine in steady state can be assumed equal with a very good accuracy.


(a) (b)

Fig.7: Rotor current in equal stator output power and power ratio (a) Synchronous machine (b) DFIG.

Table 7: The results of simulation and calculation of rotor current in equal stator output power and power ratio

Ir,If calculation simulation

synchronous 5235.5 5233

DFIG 2063.5 2092.3

As it is shown above, in an equivalent condition rotor effective current in DFIG machine is

less comparing with synchronous machine which is because of the smaller gap in DFIG

machine and consecutively less maximum flux density on teeth comparing with synchronous

machine. It should be mentioned that in table (3) excitation current of synchronous machine

in rotor reference is DC and rotor current of DFIG machine is AC which is transmitted to

stator (effective value is presented). Considering the transformation ratio the difference in

rotor current will be about 500A.

(a) (b)

Fig. 8: The results of simulation and calculation of rotor active power in equal stator output

power and power ratio (a) Synchronous machine (b) DFIG machine.

Table 8: rotor active power in equal stator output power and power ratio

Ir,If calculation simulation

synchronous -4319 -4321

DFIG 4.424e4 4.434e4

As it is shown in figure (8), whereas rotor circuit in DFIG machine produces some additional

power and transmits it to transmission network according to the results of the simulation it

produces 44.24 KW of extra active power while 4.319 KW of active power is wasted in

synchronous machine.


(a) (b)

Fig. 9: Active power transmitted to network in equal stator output power and power ratio (a) Synchronous machine (b) DFIG machine.

Table 9: Active power transmitted to network in equal stator output power and power ratio

Qg calculation simulation

synchronous 1.2e6 1.199e6

DFIG 1.322e6 1.325e6

(a) (b)

Fig. 10: transmission of reactive power to network in equal stator output power and power

ratio (a) Synchronous machine (b) DFIG machine.

Table 10: Reactive power transmitted to network in equal stator output power and power

ratio

Qg calculation simulation

synchronous 1.2e6 1.199e6

DFIG 1.322e6 1.325e6

If we relinquish wasted power in rotor of synchronous machine due to its small amount the transmitted active power to network (Pg) will be equal to stator active output power (Ps) while in DFIG machine transmitted active power to network (Pg) will be equal to the sum of stator and rotor active power (Ps + Pr). If so for equal stator output power, DFIG machine can definitely transmit more active and also reactive power even with less rotor current in comparison with synchronous machine. So we can manufacture DFIG machine in smaller dimension and less cost which leads to condone larger size of DFIG machine in comparison with synchronous machine in equal condition that concluded in design paragraph.

8. Conclusion: Application of variable speed generation using DFIG in pump storage hydro power plants is studied in this paper. Comparing to conventional applications with fixed-speed synchronous


machine having the same stator, the paper concludes that, in viewpoint of structure, the DFIG machine should have a slightly bigger volume because of 3 phased rotor with AC currents. Regarding the steady state performances of both machines, the results show that DFIG can deliver more active and reactive power to network with less rotor current because of the extra power delivered by the rotor. . DFIG machine can therefore be manufactured in smaller dimension with less cost for the same needed power. The ability of faster response to network demand for active and reactive powers in case of DFIG is also a remarkable advantage which can be utilized for enhancing the network stability under perturbations.

References

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Comparison of DFIG and Synchronous Machine for Storage ... · Mahdi Johar 1,*, Ahmad Radan 2, Mohammad Reza Miveh 3 and Sohrab Mirsaeidi 4 1, 2 Electrical Engineering Department of

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