Design and Simulation of Excitation Control System

The Eighth International Conference on Electronic Measurement and Instruments

ICEMI'2007

Design and Simulation of Excitation Control System for AC-Excited Generator
Liu Yanping Du Taihang Zhang Wenling Yang Yanjie
(He bei University of Technolog y ,Tianjin 300130 China) Abstract: In this paper, the excitation control system of ACEG(AC Excitation generator) is studied. The mathematical model for ACEG based on Park equation and the mathematics model of cyclo-converter is given. And with these mathematics models, the performance of ACEG is simulated. Second, the control strategy of the excitation system is discussed. At last, a new practical scheme of excitation control system is given and some simulation results are presented. Keywords: AC-Excited generator, Excitation control, Simulation ,Vector Control cyclo-converter

excitation system was discussed. At last, a new practical scheme of excitation control system was given and some simulation result was presented.

2

Mathematic Model

(1)Mathematic model for ACEG based on Park equation. The mathematics model of ACEG was derived from the basis Park equation; it is the fundamental to make computer simulation and to construct ACEG's vector control system. By 3-2 coordinate transformation, the variables of stator and rotator under 3-phase ABC coordinate are changed to variables of 2-phase d-q coordinate. At 2-phase d-q coordinate, it is easy to analysis the electromagnetic relations between each variables. The d and q axis components can be obtained as follows:
ud uq
T

1

Introduction

With the rapid development of electric power systems, the stability of the power system is having more and more concern in recent years. There are many methods to make the power system operate safely and steadily. A new attractive method is using AC-Excited Generator (ACEG). ACEG is a doubly-fed, new type generator where both stator winding and rotor winding take part in the electromechanical energy conversion. ACEG is also called Variable speed Constant frequency Generator or Asynchronous Synchronous Generator. The rotor winding is 3-phase with AC excitation current. Under the control of excitation system, the active power and reactive power of ACEG can be adjusted separately and conveniently, and the steady operation range can be expanded. ACEG have been used in pumped-storage power plant at some country. In this paper, three aspects will be discussed. First, the mathematics model of ACEG & the mathematics model of cyclo-converter are given. Second, the control strategy of the

C uA

uB

uC

T

The transformation matrix C is written as: cos cos( 120 o ) cos( 120 o ) 2 C sin sin( 120 o ) sin( 120 O ) , 3 0.5 0.5 0.5 Where is the displacement angle between the stator A-phase magnetic axis and the d axis, and express as follows:
1

dt

0

0 is the starting included angle between phase A of stator magnetic axis and the d axis, We can get the d-q mathematics model of ACEG.

U B I CI Where,
U I u ds ids u qs iqs idr u dr iqr u qr
T T

1-4244-1135-1/07/\$25.00 2007 IEEE.
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The Eighth International Conference on Electronic Measurement and Instruments

ICEMI'2007

Ls B 0 Lm 0 Rs L 1 s 0 2 Lm B 1 CI

0 Ls 0 Lm
1

Lm 0 Lr 0 Ls Rs L 2 m 0

0 Lm 0 Lr 0 L 1 m Rr 2 Lr
1

Lm Lr

C

0
2

We adopt cosine intersection point to control voltage-type cyclo-converter. The 3-phase AC/AC converter has 3-phase commutating bridges. Every bridge has 6 group thyristors. Define 3-phase on-off matrix S: S 11 S 12 S 13 S 14 S 15 S 16 S (i, j ) S 21 S 22 S 23 S 24 S 25 S 26 S 31 S 32 S 33 S 34 S 35 S 36 Sij(i=1,2,3 j=1,2,3 ,6). If S1j=1,one of two inverted-parallel thyristors is on, if S1j=0, both of the thristorys are not on. S2j, S3j are B, C phase state. Elements of matrix S are get by cosine intersection point method, which is through compare-son line voltage of secondary winding with reference voltage. Output voltage of cyclo-converter Uo:
3 3

Rr

Change into standard state equation,
I B 1U

In above equations, subscript s represents values of stator. Subscript represents values of rotor. Electro-magnetism torque equation: Tem 1.5PL m (i qs i dr i ds i qr ) Rotor motion equation: d 2 J Tem dt where, 2=s 1, s is slip ratio. (2)Mathematics Model for cyclo-converter Cyclo-converter also called AC/AC converter has not DC part, its conversion efficiency is higher than AC/DC/AC converter. Because of the good output waveform high reliability and large capacity, the cyclo-converter is the best choice for ACEG excitation system, which need 3-phase low frequency AC input. This paper gives the 6-pulse cyclo-converter mathematics model, which is widely used in practice. Figure 1 shows the scheme diagram of one phase of the 3-phase 6 pulse cyclo-converter. Before get the model of cyclo-converter, we give some hypothesis: 1) Cyclo-converter works under the mode of no-circulation. 2) Ignore dead time of positive and negative half-wave, and current of load is continuous. Every thyristor is ideal switch component, resistance under on state is zero, under off-state is infinite.

Uo

k 1

S (i,2k 1) U k
3

k 1

S [i, F (k )] U k
3

S (i,2k 1)
k 1 k 1

S[i, F (k )]

Where i=1,2,3. Uk(k=1,2,3) respond to Ua, Ub, Uc, which is transient value of output voltage.
F (k )

(k 1)(k 2) 2 (k 2)(k 3) 6 (k 3)(k 1)

According to the interface of ACEG and cyclo-converter, we adopt method of direct projection. The port voltage, which is calculated from cyclo-converter mathematics model, can be projected to d-q mathematics model directly, through park transformation. Like this, the interface of ACEG and cyclo-converter is simplified.

3

Excitation Strategy of ACEG

Uas Ubs Ucs

Fig.1 one phase of the 3-phase 6 pulse cyclo-converter

To control the active power and reactive power of ACEG through excitation system separately, vector control method is employed [1]. Through the stator magnetism oriented vector control (which is shown in Fig.2), the electromagnetism relationship between power and stator current can become very simple, because ACEG's stator working frequency is equal to industrial frequency always and stator's resistance can be ignored, the phase angle between stator's whole flux and voltage is 90o.
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The Eighth International Conference on Electronic Measurement and Instruments

ICEMI'2007
1

i dr

(U s (
1

L s i ds

R s i qs ) /(

1

Lm )

d
s

i qr

L s i qs

R s i ds ) /( L m )

q

ar as I1 U1

Stator magnetism oriented vector control system can realize generator and prime mover uncoupling thoroughly, and make ACEG have good steady state stability and dynamical performances.

4

Fig.2 ACEG vector diagram

We can deviate: ACEG stator active power is direct proportion to the q axial part of stator current, so is reactive power and d axial part of stator current. Thus, power control is current control, rotator current control corresponding to stator current, in fact. ACEG's vector control system equation as follows. According upper analysis, active and reactive equation: P U qs I qs U s I qs Q U qs I ds U s I ds We can get the relation of rotator current and stator current from ACEG's basic equations. Voltage equations: u ds p ds R s i ds 1 qs u qs p qs R s i qs 1 ds u dr p dr R r i dr dr 2 u qr p qr R r i qr 2 dr Flux equations: L s i ds L m i dr ds L s i qs L m i qr qs L m i ds L r i dr dr L m i qs L r i qr qr
2

A Practical Scheme of Excitation Control System and Simulation

Basing on ACEG's mathematics model and cyclo-converter mathematics model[3], which have been given, we have gone on ACEG system simulation. (1) cyclo-converter simulation We adopt the mathematics model mentioned in section 2 to simulate the operation of cyclo-converter. Fig.3 , Fig.4 and Fig.5 show simulation results.
/x100v

Fig. 3 input line voltage wave

s

1

Where s is slip ratio. Use the direction of stator voltage as q axis, so vector control constraint equation is: u ds 0 u qs U s Stator current equation: i ds I s sin i qs I s cos Is is amplitude of stator current, is angle of power factor. From upper equations, we can get the relation equation of stator current and rotator current.
Fig.4 cycloconverter output voltage

Adopt first Euler, to one load, we calculate current of phase a. Simulation parameter for Fig.3 Fig5.: Input phase voltage Ui=220V, input frequency f1=50Hz, output phase voltage Uo=300V, output
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The Eighth International Conference on Electronic Measurement and Instruments

ICEMI'2007

frequency fo=10Hz, load parameter, reluctance R=3.184 induce L=0.3107H. Simulation result:

uqr udr Tem

Fig.6 (b) voltage response Stator output power adjustion with constant cos , Fig.5 load current wave
idr iqr iqs ids

(2)ACEG's dynamic performances simulation. We use d-q axis mathematics model to simulate ACEG's dynamic performances. Simulation parameter: RS=3.74 Rr=3.184 LS=0.3042H Lr=0.3107H Lm=0.292H, rated stator voltage U1=220V rated power P1=1500W rated slip ratio s=0.05 rated frequency f1=50Hz rated power factor cos =0.9 rotating inertia J=0.00125kgm2 pair of poles Np=2, sampling cycle 100 s, calculate with forth Rungekutta. Simulation result:
idr iqr iqs ids

Fig.7 (a) current response

2

Tem

Fig.6 (a) current response

Fig.7 (b) slip and torque response Slip range from 0.04 to 0.05 with constant P1, cos , U1

From simulation results, we can see: ACEG with vector control technology can realize generator's rapid response and instruction track of active power, power factor, rotation rate, etc.
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The Eighth International Conference on Electronic Measurement and Instruments

ICEMI'2007

So, system has good dynamic performances. (3)ACEG's system simulation We adopt method of direct projection to simulate ACEG's dynamic and static performances. Some of simulation results were given as follows:

CT

Sub control computer V Transducer I Transducer AC/AC convert Pulse amplifier Excitation transformer

Fig.10 excitation control system diagram

Fig.8 stator current wave

Fig.9 roator current wave

Simulation parameter: RS=3.74 Rr=3.184 LS=0.3042H Lr=0.3107H Lm=0.292H, rated frequency f1=50Hz rotating inertia J=0.00125kgm2 pair of poles Np=2, sampling cycle 100us, rotator input voltage amplitude 30V, input frequency 5Hz calculate with forth Rungekutta. At last, we give the whole digital excitation control system diagram for ACEG . ACEG excitation control system mainly include PT, CT, measurement adapter, main control computer, sub-computer, pulse amplification part, cyclo-converter, keyboard and indicator, digital input/output, voltage transducer, current transducer, excitation transformer. Fig. 10 shows it main components.
2-453

Main control computer is consisted of PC104 serials industrial computer that is usually used for industry control. It was used to sample data, vector control calculation, trigger angle calculation, sub-computer manage etc. Sub-computer is composed of DSP TMS320F2812 mainly. It communicates with main computer through CAN bus. It is used to receive main computer's data and product trigger pulse to control cyclo-converter. Measurement adapter is used to pre-treat measure signals of generator's stator voltage, stator current, and rotator voltage, rotator current, rate of rotation and frequency, so computer can use them directly. For stator voltage and stator current measurement, we use normal PT, CT. To measure excitation voltage and current, hall device is adopted. Employ photoelectric coding mask to measure rate of rotation. The purpose of digital input/output is used to isolated transfer the command from center control room or local control unit, and output state signals and alarm/fault signals from excitation system to upper system. The 3-phase AC/AC converter has 3-phase commutat-ing bridges. Every bridge has 6 group thyristors. One group is composed of two inverted-parallel thyristors. It is used to convert 3-phase 50Hz AC to 3-phase low frequency AC whose frequency and amplitude can be adjusted. Excitation transformer supplies 3-phase AC to 3-phase cycloconveryter. It composes of 3-phase transformers, every 3-phase transformer supplies one phase of cycloconverter. Excitation transformer

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The Eighth International Conference on Electronic Measurement and Instruments

ICEMI'2007

can be connected with factory power supply system, when ACEG runs under separate-excitation mode, or connected with ACEG's stator under shunt-excitation mode, here ACEG need the excitation start device. Keyboard and indicator are used to display, modify and store parameters of excitation system.

Author Biography
Liu Yanping: was born in 1966, she received master degrees from Hebei university of technology, Tianjin, China in 2000.She is working as a professor at Hebei university of technology now. Her research interests include measurement and control technology, EDA application technology, and DSP application research.

5 Conclusions
Excitation control system is an important part to ACEG. This design employing whole digital circuit, using computer's strong calculation function and developed using C language, can realize the real-time control and improve the dynamic performances of the ACEG. Reference
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p772-779,Dec. 2005

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