412 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. LO, NO. 4, JULY 1995
A Flexible Active and Reactive Power
Control Strategy for a Variable Speed
Constant Frequency Generating System
Yifan Tang, Member, IEEE, and Longya Xu, Senior Member, IEEE
Abstract- Variable-speed constant-frequency genkrating sys-
tems are used in wind power, hydro power, aerospace, and
naval power generations to enhance efficiency and reduce friction.
In these applications, an attractive candidate is the slip power
recovery system comprising of doubly excited induction machine
or doubly excited brushless reluctance machine and PWM con-
verters with a dc link. In this paper, a flexible active and reactive
power control strategy is developed, such that the optimal torque-
speed profile of the turbine can be followed and overall reactive
power can be controlled, while the machine copper losses have
been minimized. At the same time, harmonics injected into the
power network has also been minimized. In this manner, the
system can function as both a high-efficient power generator and
a flexible reactive power compensator.
I. INTRODUCTION
ARIABLE-SPEED constant-frequency (VSCF) power V generating is desirable in many situations. A salient
example is the wind power generation, where turbine speed
should be able to vary according to the wind speed, such that
energy efficiency can Be improved with a reduced torsional
stress and windage friction on the wind mill blades. While in
variable speed, the system output voltage should be maintained
at a constant frequency to interface with the power system.
Other applications of VSCF include hydro power generation,
aerospace and naval power generation.
A promising VSCF generating concept is the slip power
recovery system composing of a doubly excited induction
machine and power converters. In recent years, renewed
interest has led to the progress of this concept. [l) proposed
a decoupled active and reactive power control strategy for
doubly excited induction machines. A cycloconverter was
used in the rotor circuit, resulting in control simplicity while
restricting control flexibility. The overall system power flow
problem was not studied, as only the output power from the
stator side is controlled. [2] studied the overall power flow of
a self-cascaded induction generator, for which a synchronous
condenser is needed to provide the necessary reactive power
for field excitation. Since only simple thyristor inverters were
employed in the system, the system’s ability to handle reactive
power is disabled. [3 ] studied power regeneration of a typical
Manuscript received February 14, 1994; revised March 13, 1995.
Y. Tang is with U.S. Electrical Motors, Emerson Motor Technology Center,
L. Xu is with the Department of Electrical Engineering, The Ohio State
IEEE Log Number 9412188.
St. Louis, MO 63136 USA.
University, Columbus, OH 43210 USA.
Fig. 1. Power flow of Slip Power Recovery System.
singly-excited adjustable speed drive by simple thyristor recti-
fier using an innovative dc reactor circuit. It is a good example
of cost-reduction for power regeneration of high-power drives.
For a generating system, however, this scheme would not be
a wise choice if reactive power control is required. [4] had
developed a stator field oriented control for the doubly-excited
induction generator, in which both the active and reactive
power of the stator are controlled by a PWM regulated current
in the rotor circuit. It had also been shown that the concept
in [4] is applicable to the doubly-excited brushless reluctance
machine [5].
Conventional slip power recovery systems had been ex-
tensively researched, such as in [6]-[8]. It appears that the
high-performance type, dual PWM power converter (Fig. 1),
slip power recovery VSCF system has not been fully studied,
such as regarding the control of both active and reactive
power of the overall system, stability problem as especially
associated with the dc link voltage and with the lack of
rotor damping circuit, control coordination between the two
converters, etc. In this paper, power control of VSCF slip
power recovery generating system are discussed. A closed-
loop control strategy is developed to coordinate the dual PWM
converters in the rotor circuit. Flexible and stable control
of overall active and reactive power is obtained, while the
machine copper losses are minimized. It is shown that with
the control strategy proposed, the VSCF system can actually
function both as a power generating system and as a reactive
power compensator. Application in wind power generation is
simulated to verify the proposed control strategy.
11. CONTROL STRATEGY DEVELOPMENT
A schematic slip power recovery system is shown in Fig. 1 ,
with the reference directions of active and reactive power
indicated. As a generating system, obviously in most situations
0885-8993/95$04.00 0 1995 IEEE
TANG AND XU: FLEXIBLE ACTIVE AND REACTIVE POWER CONTROL STRATEGY 413
Ps < 0. When the machine in variable speed operates
below synchronous speed, slip power P, > 0; when the
machine operates above synchronous speed, P, < 0. Note
that doubly-excited generators are inherently capable of super-
synchronous speed operation. To ensure sub-synchronous and
super-synchronous speed range operation, the requirement lies
in the configuration of the power converter.
For most of the configurations with cycloconverters, natu-
rally or line commutated converters and low-frequency forced
commutated thyristor converters, harmonic distortion, and
poor power factor are the major shortcomings, along with
limited control flexibility. To realize advanced control, such as
the field oriented control of the variable speed generator, and to
achieve overall active and reactive power control and harmonic
reduction, the dual PWM converter structure with a dc link is
an attractive candidate. As a result, new control issues arise
and of most importance is the coordination between the two
PWM converters. For convenience, in the following analysis
the two converters are termed rotor side converter and line
side converter, respectively.
The PWM converter has seldom been applied in slip power
recovery systems, which have traditionally been used in
high power, narrow speed range applications without high-
performance requirement. The PWM converters inevitably
bring higher switching losses, and the losses increase at higher
switching frequencies and power levels. However, on the one
hand the power converters handles only the slip power, and
their rating is only a portion of the machine rating depending
on the speed range, and on the other hand the rapid technology
advance in solid-state power devices is making higher power
switching feasible at an ever reducing cost.
A. Field Oriented Control Through Rotor Side Converter
In the VSCF generating system, control schemes for the
doubly-excited induction machine are expected to achieve the
following objectives: 1) The induction generator is required to
track a prescribed torque-speed curve, for maximum power
capturing; 2 ) The stator output voltage frequency must be
constant; 3) Flexible reactive power control is. achievable.
These control objectives must be achieved with adequate
stability of the system which also includes the power converter
and the dc link.
The stator field orientation control is based on the stator field
dq model, where the reference frame rotates synchronously
with respect to the stator flux (linkage), with the d-axis of the
reference frame instantaneously overlaps the axis of the stator
flux. In short, w = we and A,, = 0. In such a reference frame,
the machine dynamical equations can be written as [4]
d A d s
v d s = rsids + - d t
Since the d-axis of the reference frame is the instant axis
of the stator winding flux, the phase angle of the stator
line voltage is generally not a constant in the reference
frame, although its frequency and magnitude are constants
constrained by the power system. The electromagnetic torque
and stator active power can be derived as
In the doubly excited machine, the level of the stator flux
remains approximately unchanged, restricted by the constant
magnitude and frequency of the line voltage. Therefore, as
can be observed from (6), the torque control can be achieved
by controlling the rotor current component orthogonal to the
stator winding flux. Then from (7), stator active power is
subsequently controlled.
The reactive power at the terminal of the stator winding
can be derived as
or, from (1) and (2), with the stator flux remaining unchanged
As ( 3 ) indicates, i d s is controllable by id,, with Ads un-
changed. Therefore, the d-axis component of the rotor current,
i d , can be controlled to regulate the stator reactive power.
As a result, the control of stator active power P, via i,, and
the control of stator reactive power Q, via zdr are essentially
decoupled, and a decoupler is not necessary to implement field
orientation control for the slip power recovery system. The flux
control i s generally unnecessary since it maintains a constant
level, while the control of reactive power becomes possible.
B. Minimization of Machine Copper Losses
It is well known that the slip power recovery configura-
tion has improved energy efficiency. Field oriented control
described above enhances this improvement by permitting
variable speed operation with reactive power control. There
is, however, still another improvement possible. By controlling
the reactive power circulation of the system, the copper losses
can be minimized. This is analyzed in this subsection.
The machine overall copper losses can be expressed as
3 . 3 .
Pcu = T(z:, + &)rS + 5 ( z : ~ + i;,)r,. (10)
By using (1) to (4), (10) can be derived as
In (1 I), i,, has been used to control torque or active power,
and Ads remains approximately unchanged as described above,
then the amount of machine copper losses is a function of id,.
414 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 10, NO. 4, JULY 1995
It can be shown that for P,, to achieve the minimum, it is
necessary that
(12)
However, in the field oriented control, as already discussed,
i d s controls the stator reactive power. Therefore, it is sufficient
to conclude that the stator reactive power flow determines the
level of copper losses. From (9) and (12), the optimal stator
reactive power flow can be shown as
L s T r x d s
i d s =
T, L& + rr L:.
(13)
3 P L . J J $ , & - - - w e ' - 2 2 rsL& + r,.L:'
C. Control of Line Side Converter
Through field oriented control of the rotor side converter,
the optimal torque-speed profile can be tracked and the stator
output reactive power can be separately controlled. The dc link
capacitor provides dc voltage to the machine side converter
and any attempt to store active power in the capacitor would
raise its voltage level, and vice versa. Thus to ensure stability
of the dc link, the power flow of the converters should attempt
to meet the following control objective:
Pl - Pwl = Pr + Pw2
where Pwl and Pw2 are the losses of the line side converter and
the rotor side converter, respectively, with the same direction
Fig. *.
Since vds 0, uqs ?lm, pl and Ql can be controlled
by iql and &, respectively. In the Same reference frame as
determined by the machine stator flux, i , ~ and idl are also field
oriented currents, produced by the line side current regulated
(14) PWM converter.
111. IMPLEMENTATION AND SIMULATION
of the power flow as indicated in Fig. 1.
,
The dc link dynamical equation can be written as
dV1 . c- .dt = a 1 - i 2
in which Vd is the dc bus voltage and c is the capacitance.
The dc fink currents i l and "2 as indicated in Fig. 1 can be
derived as
From (15) through (17), as long as (14) is satisfied, the
dc link voltage maintains stable, though small ripples may
exist due to the instantaneous inequality between i l and i 2
and a small variation may occur during transients as a result
of energy transferring. As can be seen from Fig. 1, another
result of (14) is that the overall generated active power equals
to the electromagnetic power minus the system losses, i.e.,
Reactive power flow constitutes another control objective:
where &* is the overall reactive power command required by
the power network.
In the stator flux dq reference frame,
A. Implementation Scheme of Closed Loop System
Based on the control strategy discussed above, Fig. 2 shows
an implementation of the overall control system, which enables
the slip power recovery system to function as both a VSCF
generating system (or a high performance variable speed drive
system) and a reactive power compensator. Individual control
of the rotor side converter and of the line side converter and
related feedback between the two converters are shown. For
speed control purpose, a speed control loop generates the
torque command.
A current-regulated pulse-width-modulation (CRPWM)
voltage source converter provides the field oriented currents
iqr and i d r to the rotor circuit, controlling the electromagnetic
torque and the stator reactive power, respectively. The torque
command is given by the turbine optimal torque-speed profile
and the reactive power command is calculated to minimize
the machine copper losses. Overall active power generated is
directly related to the torque, as indicated by (6) and (18).
Another CRPWM voltage source converter is used to inter-
face with the power network. In the same dq reference frame
as determined by the machine stator flux, its currents i,l and
i d l are also field oriented, controlling Pl and Q1, respectively.
As discussed earlier, Pl is controlled through i,l to stabilize
the dc bus voltage and & 1 is controlled through i d l to meet
the overall reactive power command. One way to maintain a
stable dc bus voltage is to satisfy (14) to (17). Another way
is to directly use the dc bus voltage feedback and the ideal
reference to generate i,l by PID regulation.
Depending upon different performance requirements and
engineering considerations such as cost, simplicity and re-
liability, the implementation structure shown in Fig. 2 can
TANG AND XU: FLEXIBLE ACTIVE AND REACTIVE POWER CONTROL STRATEGY 475
TABLE I
PARAMETERS OF SIMUIATED MACHINE
[I Hrated) I 50hv 1
time (sec) time (sec)
(a) (b)
wind-mill tque ___.-------
___.---
0.5 1 1.5 2 2.5 3 3.5 4
time (sec)
(C)
(aHc) Turbine speed tracking. Fig. 3.
be simplified to a varying degree. As an example, the sta-
tor current measurement can be eliminated if
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