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Multi-mode wide range subsynchronous resonance stabilization using superconducting magnetic energy storage unit

M. R. I. Sheikh, M. G. Rabbani, M. M. Ali

Proceedings of the ICEECE
December 22-24, 2003, Dhaka, Bangladesh
*Email: tamanna86131@yahoo.com 107
Controller
Y-Y/
Transformer
3-
AC
 Sy
ste
m
Bu
s
Co
nv
ert
er 
firi
ng
sig
nal
12-Pulse
bridge converter
Ind
uct
or
Ac
tiv
e p
ow
er
dem
and
 of
 M
ES
 Un
it
I d
System voltage
and frequency
Fig. 1: The schematic diagram of the scheme
Switching
Switched
Capacitor bank Sta
tic 
Va
r
Set
tin
g
sig
nal
signal
Application of Magnetic Energy Storage Unit for Power
System Load Frequency Control
M.R.I. Sheikh*,  M.G. Rabbani**, and  M.M. Ali***
*Assistant Professor **Associate Professor ***Professor
Department of Electrical and Electronic Engineering
Rajshahi University of Engineering and Technology, Rajshahi – 6204, Bangladesh
ABSTRACT
The fast acting energy storage devices can effectively damp electromechanical oscillations in a power system, because they
provide storage capacity in addition to the kinetic energy of the generator rotor, which can share the sudden changes in
power requirements. A simple and novel mode of control strategy for damping electromechanical oscillations through
control of converter firing angle over a wide range of 5-165 degree for Magnetic Energy Storage (MES) unit is proposed and
applied in the test network of a single area power system. Active power modulation is used under equal- mode of
operation. In this paper, a comprehensive control strategy to achieve the real power modulation is used for two-quadrant
operation of the converter and shows the effectiveness of the MES unit for the improvement of power system dynamics. It is
found that this mode of control greatly improves the overall performance of the P-f loops of a power system.
Keywords: Magnetic Energy Storage, Load Frequency Control, Active power modulation, Equal- mode.
1. INTRODUCTION
A sudden application of a load results in an instantaneous
mismatch between the demand and supply of electrical
power because the generating plants are unable to change
the inputs to the prime movers instantaneously. The
immediate energy requirement is met by the kinetic energy
of the generator rotor and speed falls. So system frequency
changes though it becomes normal after a short period due
to Automatic Generation Control (AGC). Again, sudden
load rejections give rise to similar problems. The
instantaneous surplus generation created by removal of load
is absorbed in the kinetic energy of the generator rotors and
the frequency changes. The damping of the system must be
such that the synchronous generators can return to their
steady state conditions after the disturbance [1]. Especially
when the load-end of the transmission line experiences
sudden load perturbations, the generators need continuous
control to suppress undesirable oscillations in the system.
Different types of energy storage systems such as pumped
hydro energy storage, compressed air energy storage has
been proposed. Except the battery energy storage all other
systems are inherently slow and battery storage system
being incompatible with the power rating and efficiencies
compared to the other systems. MES unit is very fast and
active and reactive power can be controlled by controlling
the firing angles of the two six-pulse converters [2]. The
schematic diagram of the proposed scheme is shown in
Fig.1.
2. APPLICATION OF MES UNIT IN LOAD
FREQUENCY CONTROL
Apart from the micro level smooth changes in load, small
random changes are superimposed on the load curve. This
sudden load changes due to switching or tripping in
industries and distribution buses create micro level time
mismatch in supply and demand of electrical power. As a
result, the system frequency tends to fluctuate. The problem
of minimizing the deviation of frequency from normal value
under such circumstances is known as the load frequency
control (LFC) problem.
To be effective in load frequency control application, the
energy storage system should be fast acting i.e. the time lag
in switching from receiving (charging) mode to delivering
(discharging) mode should be very small.
ISBN: 984-31-1528-7
Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
108
Switching
signal
Y-Y/
Transformer
Fig. 2: Schematic diagram of the MES   unit
Id
Ed
Inductor12-pulse bridge
converter
For damping the swing caused by small load perturbations
the storage units for LFC application need to have only a
small quantity of stored energy, though its power rating has
to be high, since the stored energy has to be delivered
within a short span of time. However, due to high cost of
superconductor technology, one can consider the use of
non-superconducting of lossy magnetic energy storage
(LMES) inductors for the same purpose. Such systems
would be economical maintenance free, long lasting and as
reliable as ordinary power transformers.
A MES system seems to best meet the above requirements.
The power flow into an energy storage unit can be reversed,
by reversing the d.c voltage applied to the inductor within a
few cycles. A 12-pulse bridge converter with an appropriate
control of the firing angles can be adopted for the purpose.
Thus, these fast acting energy storage devices can be made
to share the sudden load requirement with the generator
rotors, by continuously controlled the power flow in or out
of the inductor depending on the frequency error signals.
3. ANALYSIS OF THE MAGNETIC ENERGY
STORAGE UNIT [3,4,6,7]
The MES inductor converter unit for improvement in power
system LFC application essentially consists of a D.C.
inductor, an ac/dc converter and a step down Y-Y/
transformer. The inductor should be wound with low
resistance, large cross-section copper conductors. The
converter is of the 12-pulse cascaded bridge type shown in
Fig.2, connected to the inductor in the D.C. side and to the
three-phase power system bus through the transformer in the
ac side.
Control of the firing angles of the converter enables the
D.C. voltage applied (Ed) to the inductor to be varied
through a wide range of positive and negative values as
shown in Fig. 3. Gate turn off thyristors (GTO) allow us to
design such type of converter [5]. When charging the
magnet, a positive D.C. voltage is applied to the inductor.
The current in the inductor rises exponentially or linearly
and the magnetic energy is stored. When the current reaches
the rated value, the applied voltage is brought down to low
value, sufficient to overcome the voltage drop due to
inductor resistance. When the extra energy is required in the
power system, a negative D.C. voltage is applied to the
inductor by controlling the firing angles of the converter.
The losses in the MES unit would consist of the transformer
losses, the converter losses, and the resistive loss in the
inductor coil. The inductor loss can be kept at an acceptable
level by proper design of the winding.
If the transformer and converter losses are neglected, the
bridge voltage is given by
Ed= 2 Ed0cos - 2 Id Rc (1)
where
 is the firing angle
Ed is the D.C. voltage applied to the inductor
Id is the current through the inductor
Rc is the equivalent commutating resistance and
Ed0 is the maximum open circuit bridge voltage of
each 6-pulse bridge at =0.
0 20 40 60 80 100 120 140 160 180-6
-4
-2
0
2
4
6
Ed
,In
duc
tor
 Vo
ltag
e (
kV
)
Alpha (degree)
Id=2.0 kA,Ed0=2.5 kV
Rc=0.00 Ohm
Rc=0.05 Ohm
Rc=0.10 Ohm
Fig. 3: Effect of Inductor Voltage with the variation of
firing angle (alpha)
When the inductor is charged initially, the current build up,
expressed, as a function of time with Ed held constant, is
given as



  tL
R-
L
d
d
L
e1R
EI (2)
where L and RL are the inductance and the resistance of
inductor respectively.
Once the current reaches its rated value Id0 it is held
constant by reducing the voltage to a value Ed0 enough to
overcome the resistive drop. In this case
Ed0 = Id0. RL (3)
As this value of Ed0 is very small, the firing angle will be
nearly 900. At any instant of time the amount of energy
stored in the inductor is given by
 
t
t dd0d 0
dPWW (4)
where, 2d0d0 LI2
1W  is the initial energy in the inductor.
Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
109
Vt1 VL0
EIi
Pt+jQt
Xt
Pd+jQdMES Unit
PL+jQLLoad
Fig. 4: Single line diagram for test network
Once the rated current in the inductor is reached, the unit is
ready to be coupled with the power system application. The
frequency deviation f of the power system is sensed and
fed to the MES unit as the error signal. Ed is then
continuously controlled depending on this signal.
When there is a sudden increase in load in the power
system, the frequency falls and a negative voltage,
expressed by equation
Ed = K0 f (5)
is impressed on the inductor. The converter bridges
maintain a unidirectional current flow and as the circuit is
inductive the current does not change instantaneously. In
this mode of operation, a positive converter voltage
produces positive power, which means charging the coil,
and a negative converter voltage produces a negative power
and discharges the inductor. When the frequency dip in the
power system causes a negative voltage to be applied to the
inductor, power flows from the inductor into the power
system, sharing the sudden load requirement.
The reverse process takes place when there is a sudden load
rejection in the power system. The frequency increase
causes a positive voltage to be impressed on the inductor
and the MES unit absorbs the excess power from the power
system.
In actual practice the inductor current should not be allowed
to reach zero to prevent the possibility of discontinuous
conduction in the presence of the large disturbances. To
avoid such problems, the lower limit of the inductor current
is set at 30% of Id0 [2]. Thus, if the lower current is chosen
at 0.3Id0, the upper inductor current based on the equal
energy absorption/discharge criterion is set at 1.38Id0.
As the inductor has a finite inductance and hence a finite
amount of energy stored in it, the current in the inductor
falls as energy is withdrawn from the coil. This deviation in
the inductor current is expressed as
L.sR
EI
L
d
d 
 (6)
Prior to the load disturbance, let the magnitudes of voltage
and current are Ed0 and Id0 (nominal values). Initial power
flow into the coil is
Pd0 = Ed0. Id0 (7)
In response to the load disturbance the incremental change
of power flow into the coil can be expressed as
Pd = Id0. Ed+ Id0. RL. Id+ Ed .Id (8)
Following a sudden increase in load in the power system,
the incremental power expressed by equation (8) is
discharged into the power system by the energy storage unit
to share with the generator rotor, the extra load demand.
4. THE REAL POWER (Pd) CONTROL STRATEGYFOR ON-LINE OPERATION OF MES UNIT
Figure 4 shows a typical configuration of a single area
power system equipped with a MES unit. Application of a
sudden load results in load-voltage and frequency
deviations. Following these variations, the MES unit
responds to improve the dynamic performance of the system
subjected to its limitation.
The proposed controller as shown in Fig. 5 of the MES unit
performs its task by receiving the measured values of area
control error (ACE), f and inductor current deviation, Id.
The control procedure described in detail can be
summarized as follows:
At first the required inductor voltage, Ed and the converter
power, Pd are calculated by using the equation did0d IKfKE  (9)
and, Pd = (Ed0+Ed) (Id0+Id) (10)
where K0 and  Kid are the gains corresponding  to the
frequency variation (f) and the  inductor current variation
(Id) respectively. The nominal parameters of the power
system and the MES unit are given in Appendix-I.
5. INDUCTOR CURRENT DEVIATION FEEDBACK
It is desirable to restore the inductor current to its rated
value quickly after a system disturbance so that it can
respond to the next load disturbance. This can be achieved
by employing negative inductor current feedback in the
MES control loop. Figures 6 and 7 shows the system
performance using this control scheme. It can be observed
that the deviation of Id is less in both the cases and the
inductor current returns to the rated value quickly. When
the power system demands the extra power during the first
few seconds following the disturbance, power is supplied by
Fig. 5: Superconducting magnetic energy storage unit
control system
dc
0
sT1
K
 dc
id
sT1
K

sLR
1
L  Id
Id
Ed
Ed
Pd
f

+ -
+
+
Id0
Ed0
+
Ed
+ +
+
Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
110
sensing f signal. The (Kid.Id) term becomes effective only
when the inductor current has deviated by a considerable
amount. Consequently, the reduction in total energy
discharge from the MES unit does not bring any appreciable
deterioration in the frequency and power deviations of the
power system.
6. SIMULATION RESULTS
Non-linear dynamic equations are used in the solution
process. They are solved using 4th order R-K method.
Two case studies were conducted on the system: case 1
corresponding to sudden load change of (0.001+j0.001) p.u.
and case 2 corresponding to (0.006+j0.006) p.u.
The frequency and rotor angle deviations of the power
system with and without MES unit for the above cases are
shown in Fig. 6 and 7 respectively. It is evident that with the
addition of MES unit the system improves the dynamic
performance more rapidly, also in the proposed mode of
control the system stabilizes within 5 sec. This ensures the
effective use of its power modulation. In both cases, the P-
modulation by the MES unit reduces the oscillation in the
frequency.
Significant improvements in the first overshoot and settling
time are also clearly observed. The performance of the
AVR shows that it is fast enough to pull back the voltage
following a sudden application of load. The coupling effect
between the P-f and Q-V loops is the main cause for the
oscillations. In both cases, the inductor current deviation is
much less and the effective use of active and reactive power
modulation is ensured.
7. CONCLUSIONS
This paper presents a new mode of controlling the MES unit
for improving the transient performance of single area
power system. The control logic suggested for this purpose
takes the area control error as its input and uses inductor
current deviation feedback to generate required control
signals for 12-pulse converter. Two-quadrant operation
makes it possible to handle different type of load change. It
has been shown in the present work that MES unit with
suitable control can effectively reduce the frequency and
power oscillations following a sudden small load
perturbations. This method of improving the load frequency
control of power system has the advantage that it does not
require governor or any other part of the power system to
perform any sophisticated control action. The MES unit
responds quickly to any type of load change and the system
stabilizes within a few secs.
-5
0
5 x 10
-3
Frequency Deviations (Hz)
Without MES Unit
-5
0
5 x 10
-3
Frequency Deviations (Hz)
With MES Unit
-0.04
-0.02
0 Rotor Angle Deviations(rad)
-0.02
-0.01
0 Rotor Angle Deviations(rad)
-1
0
1     P-Modulation of MES Unit (MW)
-1
0
1     P-Modulation of MES Unit (MW)
0 5 10 15-1
0
1     Inductor Current Deviations(KA)
Time in sec
0 5 10 15-0.2
0
0.2      Inductor Current Deviations(KA)
Time in sec
Fig. 6: Response of the power system with and without MES unit [case 1]
Proceedings of the ICEECE, December 22-24, 2003, Dhaka, Bangladesh
111
-0.02
0
0.02 Frequency Deviations (Hz)
Without MES Unit
-0.02
0
0.02 Frequency Deviations (Hz)
With MES Unit
-0.2
-0.1
0 Rotor Angle Deviations(rad)
-0.2
-0.1
0 Rotor Angle Deviations(rad)
-1
0
1 P-Modulation of MES Unit (MW)
-5
0
5 P-Modulation of MES Unit (MW)
0 5 10 15-1
0
1 Inductor Current Deviations(KA)
Time in sec
0 5 10 15-1
0
1 Inductor Current Deviations(KA)
Time in sec
Fig. 7: Response of the power system with and without MES unit [case 2]
REFERENCES:
[1] P.M. Anderson and A. A. Fouad, Power System
Control   and   Stability, Iowa State University press,
Ames, Iowa, 1977.
[2] S. Banerjee, J. K.Chatterjee, and S. C. Tripathy,
Application of Magnetic Energy Storage Unit as
Continuous Var Controller, IEEE Trans. on Energy
Conversion, 5 (1990) 39-45.
[3] H. J. Boening and J. F. Hauer, Commissioning Test of
the Bonneville 30 MJ Super-Conductive Magnetic
Energy Storage Unit, IEEE Trans. on Power Appar.
Syst., Vol. no. 2, PAS - 104 February 1985, pp. 302-
312.
[4] T.Ise, Y. Murakami and K.Tsuji,”Simultaneous Active
and Reactive Power Control of Superconducting
Magnetic Energy Storage Using GTO Converter”,
IEEE transactions on power delivery, Vol. PWRD-1,
No. 1, January 1986.
[5] S. Bandopadhyay,’ Lossy Magnetic Energy Storage
Systems for Improvement of Power System Stability’, a
Ph.D. thesis, center for energy studies, IIT, New Delhi,
1987.
[6] Dewan and Straughen,’ Power Semi Conductor
Circuits’, a book, willy-interscience publication, 1975.
[7] Y. Mitani ,  K. Tsuji, and  Y.  Murakami ,  Application
of  Superconducting  Magnetic Energy Storage to
Improve  Power System Dynamic Performance, IEEE
Trans. Power Syst., 3(1988),  1418-1425.
APPENDIX - I [2]
System Parameters
Area capacity PR = 2000 MW, Base MVA=2000.
Nominal loading =1000 MW at 0.9 p.f. Lagging
Nominal load-end voltage = 1.0 00 p.u.
f 0 = 50 Hz, X d = 1.0 p.u., X d = 0.25 p.u., Xt = 0.3 p.u., H
= 5.0 s, R = 2.4 Hz / p.u. MW, K0= 150.0 Hz / p.u. MW, Kr
= 0.5, Tp = 20.0 s, TT = 0.3 s , TG =  0.08 s, Tr =  10.0 s, KI
= 0.8, T=0.06s, Td0=5.9s.
MES Unit
Smax = 10 MVA, L = 3.0 H, Id0 = 2 kA, Ed0 = 2.5 kV, RC =
0.02, RL=0.05  and Tdc=0.026 seconds


Application of magnetic energy storage unit for power system load frequency control

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Multi-mode wide range subsynchronous resonance stabilization using superconducting magnetic energy storage unit

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