Securing Critical Infrastructures, Grenoble, October 2004
STATIC VAR COMPENSATOR FOR CERN’S
PROTON SYNCHROTRON PARTICLE ACCELERATOR
K. Kahle (*), D. Jovcic (**)
(*) CERN, 1211 Geneva 23, Switzerland; karsten.kahle@cern.ch
(**) University of Ulster, Newtownabbey, BT37 0QB, UK; d.jovcic@ulster.ac.uk
Introduction
CERN, the European Organization for
Nuclear Research, is an international organisation
with 20 Member States. Its objective is to provide for
collaboration among European States in the field of
high-energy particle physics research. CERN
designs, constructs and runs the necessary particle
accelerators and the associated experimental areas.
For the power system, particle accelerators
represent heavily pulsating electric loads with a
variable power factor, mainly caused by the twelvepulse thyristor power converters for main magnets.
Because of the large amplitudes and short rise times
of the pulsating power, rapid reactive power control
is necessary for voltage stabilisation and
compensation of varying reactive power. In addition,
strong filtering is required to eliminate the harmonics
generated by the power converters. For this purpose,
CERN is currently operating nine 18 kV Static Var
Compensators (SVC) with an installed total power of
more than 500 Mvar.
The Proton Synchrotron (PS) is the oldest and
most versatile of CERN's accelerators. The PS was
commissioned in 1959 and has been operating
continuously ever since. It has a diameter of
200 metres and reaches a final energy of 28 GeV. At
present, the PS complex can accelerate all stable and
electrically charged particles (electrons, protons),
their antiparticles (positrons, antiprotons), and
different kinds of heavy ions (oxygen, sulfur, lead),
which are then injected into the larger rings for
further acceleration.
The PS accelerator is continuously pulsating
with a cycle time of about 2 s. The electrical load
consists of twelve-pulse power converters supplying
the main magnets, and having a power swing from
zero to 45 MW and 65 Mvar once per cycle, and a
rise time of 600 ms.
In order to decouple this pulsing load from the
network and thus limit the disturbances to other
loads, a motor-generator set was installed in 1969.
The synchronous rotating machine (6MW) represents
a more or less stable load to the 18 kV CERN
network. An integrated large rotating mass serves as
a storage medium for the pulsing power of the PS
magnets and thus neatly resolves power quality and
voltage stability issues.
By now, the rotating machine has been in
service for more than 34 years. For this reason,
CERN has initiated an investigation of compensation
options based on an element from the family of
Flexible AC Transmission Systems (FACTS) [1].
The following study investigates the
possibility to directly connect the PS accelerator load
to CERN’s 18 kV network which is supplied from the
400 kV European grid. The direct connection of the
PS without a rotating machine would require the
installation of a Static Var Compensator for reactive
power compensation, voltage stabilization and
harmonic filtering. In such a case, the 400 kV
network would only supply the pulsating active
power pulses, while the reactive power would be
almost completely compensated by the SVC.
A stability analysis of the 400 kV network is
currently ongoing, investigating potential problems
associated with the supply of the pulsating active
power. Based on CERN’s experiences with the
existing Super Proton Synchrotron accelerator (SPS),
we do not expect difficulties. [2][3]
This paper presents the results of the studies
for a new Static Var Compensator +75/-10 Mvar for
the reactive power compensation, voltage
stabilisation and harmonic filtering of the PS
accelerator.
The PS electrical network
This study is concerned with the connection of
the PS accelerator to the 400 kV European grid via an
existing transformer 400/18 kV 90 MVA. This
transformer will be used exclusively for the PS
supply, because of power quality issues. The PS
accelerator and the new SVC will be connected to the
18 kV ME6 substation, as shown in Figure 1.
Securing Critical Infrastructures, Grenoble, October 2004
•
background harmonics coming from the 400 kV
network.
The total sum of pollution from all sources should be
below the specified limit.
It is found that 7 harmonic filters are necessary
to achieve the required harmonic performance.
Figure 2 shows the single line diagram for the filters.
The filter parameters are given in Table 1. The
magnitude of the filter impedance curve is shown in
Figure 3, whereas Figure 4 gives the maximum
harmonic level with SVC and PS in operation. The
Total Harmonic Distortion THD at the 18 kV ME6
substation will be 0.81 %.
Figure 1: Layout of the PS electrical network
Table 1: Harmonic filter design characteristics
Main SVC ratings
The periodic pulses of load reactive power have
maximum amplitude of 65 Mvar. Taking into
consideration some extra compensation margin to
stabilise the voltage in case of transformer tap
changer action, the required capacitive output of the
SVC is +75 Mvar. On the other hand, it is expected
to supply about 7 Mvar of inductive output during
periods of no-load. In the final configuration, an
SVC rating of +75/-10 Mvar is chosen. The SVC
consists of harmonic filters of +75 Mvar and a
Thyristor Controlled Reactor of -85 Mvar, as shown
in Figure 2.
Filter
F2
F3
F5
F7
F11
F13
HF
Tuning
f [Hz]
100
150
250
347.5
547.5
647.5
947
Type
C
C
LC
LC
LC
LC
HP
Damping
3.8
4.45
80
80
80
80
9.8
Rated power
[Mvar]
10.8
8.3
9.5
8.5
11.2
8.9
17.8
Figure 3: Magnitude impedance diagram for
75 Mvar filters
THD=0.81%
Figure 2: Simplified single-line diagram of the SVC
Harmonic filter design
Based on the previous experiences with existing
particle accelerators at CERN, the Total Harmonic
Distortion at the 18kV bus THD(U18 kV) shall remain
below 1 % during the entire PS pulse cycle. The
following sources of harmonic distortion are
identified:
•
harmonics generated by the PS power converters
•
harmonics generated by the Thyristor Controlled
Reactor (TCR) of the SVC
Figure 4: Harmonic level at 18 kV substation ME6,
with the SVC and PS in operation
Modelling of the PS load
The PS accelerator load consists of two twelvepulse line-commutated converters that supply DC
power to the accelerator magnets. The model of the
main electrical circuit is shown in Figure 1.
Securing Critical Infrastructures, Grenoble, October 2004
α ps
The PS control system consists of two fast DC
voltage feedback control loops, one for each pole. At
the outer control level, there is a DC current control
loop which supplies reference to the fast controls.
The purpose of the slow control loop is to keep the
firing angle within the operating range and to prevent
the commutation failure in the inversion operating
mode. The input control signal for PS is the DC
voltage reference, which has the shape of square
pulses for the cycle duration. These reference pulses
are pre-calculated in the technical control room on
the basis of the power cycle demand.
The system model is developed in
PSCAD/EMTDC [4]. Initially, the control circuit
parameters were not known; thus they had to be
selected to match the measured power curves. The
simulation responses show excellent matching
against the power measurements, see Figure 7.
SVC model and controls
d(Qload)/dt
Qload
Pload
Vacmref
+
Vacm
1
ω n = 20 Hz
KQdf
KQf
+
KPf
+
+
kpsvc
kisvc*1/s
feedforward
signal
+
+
+
linearisation of
non-linear
susceptance
Btcr
α
+
Btcr =
2π − 2α − sin 2π − 2α
Btcr 0π
18kV AC voltage controller
Figure 5: SVC control system
Figure 6 outlines the principle of the estimation
of PS active and reactive power, which is further
discussed below.
P=F1(Idc,αps)
αps
Idc
Q=F2(Idc,αps)
P
Q
dQ/dt
Figure 6: Estimation of PS active and reactive power
The basic converter theory equations for the PS
load are given below [5]:
•
AC active and reactive power:
Pps = Eac I ac cos Φ, Q ps = Eac I ac sin Φ,
(1)
where Eac is the secondary AC voltage which is
assumed constant Eac=const, presuming good AC
voltage control. The unknowns are converter current
Iac and the phase angle Φ which are calculated using
DC side variables.
•
The SVC control system consists of a PI AC
voltage feedback controller and a direct
compensation of disturbance as shown in Figure 5.
The direct disturbance compensation improves
transient responses. It includes three signals: PS
reactive power (Qload), PS reactive power
differential (dQload/dt) and PS active power (Pload).
These load power signals could be obtained by
measuring the variables on AC side but normally it is
difficult to measure AC signals in a wide bandwidth.
To measure AC variables vector transformations or
Phase locked Loop (PLL) is employed, which
introduce harmonic noise and phase lag. A faster
measurement is achieved if the PS converter
variables are measured on the DC side, and the AC
power variables are then calculated. In this way the
measurement of the disturbance signals are closer to
the origin of disturbance which is the converter DC
voltage. The controller gains are given in the
appendix.
PS load
measurements
Idc
AC current as the function of DC current:
I ac = B
I dc 6
π
(2)
where B=4, the number of 6-pulse converters
and Idc is the DC current in the PS magnets.
•
Phase angle as the function of DC variables:
⎛3 3
⎞
cos Φ = cos α ps − Rdc I dc / ⎜⎜
Eac ⎟⎟
⎝ π
⎠
(3)
where αps is the PS converter firing angle and
Rdc is the total resistance on DC side.
By replacing (2) and (3) in (1) we obtain active and
reactive power as a function of DC variables. Note
that the estimation algorithm assumes that the AC
voltage is constant.
Since the equations (1)…(3) are only valid for steady
state operation, the actual coefficients need to be
adjusted using response matching to enable accurate
estimation during transients. Because of the wide
bandwidth of the Q measurement, it is also possible
to calculate the derivative dQload/dt. Simulation
against measured data confirms that the above
method achieves good accuracy.
SVC dynamic performance
The results of the PSCAD/EMTDC computer
simulations are presented in Figures 7-10. The
simulations are based on the minimum possible
network short circuit level since this gives largest AC
voltage deviations.
Securing Critical Infrastructures, Grenoble, October 2004
PS cycle, +75/-10 Mvar SVC
Active Power [MW], Reactive Power [MVAr]
P [MW]
P-PSCAD
Q [MVAr]
Q-PSCAD
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
2
2.1 2.2
2
2.1 2.2
Time [sec]
Figure 7: Active and reactive power during a load cycle
PS cycle, +75/-10 Mvar SVC
Active Power [MW], Reactive Power [MVAr]
Q-PSCAD
Q load + SVC
Q TCR
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Time [sec]
Figure 8: Reactive power balance during a load cycle
Securing Critical Infrastructures, Grenoble, October 2004
PS cycle, +75/-10 Mvar SVC
Vref
V18 RMS
1.05
1.045
1.04
1.035
AC Voltage [pu]
1.03
1.025
1.02
1.015
1.01
1.005
1
0.995
0.99
0.985
0.98
0.2
0.3 0.4
0.5 0.6
0.7 0.8
0.9
1
1.1 1.2
1.3 1.4
1.5 1.6
1.7 1.8
1.9
2
2.1 2.2
Time [sec]
Figure 9: 18kV bus voltage during a load cycle
PS cycle, +75/-10 Mvar SVC
TCR angle
2.4
2.25
2.1
1.95
1.8
1.65
1.5
1.35
1.2
1.05
0.9
0.75
0.6
0.45
0.3
0.15
0
170
160
150
140
130
120
110
100
90
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Time [sec]
Figure 10: Firing angle and TCR current during a load cycle
2
2.1 2.2
TCR angle [deg]
losses [MW], current [kA]
ITCRrms
Securing Critical Infrastructures, Grenoble, October 2004
Appendix
Figure 7 shows the matching of measured and
simulated active and reactive power of the load pulse.
In Figure 8, the Qload+svc curve shows that there
will be some reactive power exchange with the
network in order to compensate for voltage variations
caused by the PS active power. This exchange is
occurring since the SVC is supplying additional
reactive power to compensate for the voltage drop
caused by the active power flow in the network.
In Figure 9 we observe good quality of 18 kV
bus voltage control. However, because of the large
and steep power change at 1.45 s there is a sharp AC
voltage peak of 4.5 %, and smaller dip of 1.5 % (6 %
peak-to-peak) where the particles have already left
the accelerator. Similar peaks will occur for all
sudden power changes in PS acceleration cycles.
Figure 10 shows the TCR firing angle, which
confirms that the operating range is within the design
limits [90.5 deg< α <170 deg], and some margin is
allowed for tap changer action. The initial value of
the TCR angle is about 94 deg enabling just adequate
margin for control.
Conclusions
CERN’s Proton Synchrotron particle accelerator
demands very short and steep pulses of active and
reactive power that have a negative impact on the
power quality of the network. A Static Var
Compensator, consisting of a TCR (-85 Mvar) and
seven harmonic filters (+75 Mvar in total), is
proposed for reactive power compensation, voltage
stabilisation and harmonic filtering.
The SVC control system consists of a PI AC
voltage feedback controller and direct feedforward
signals of PS reactive power, PS reactive power
differential and PS active power which are
estimated based on DC side measurements.
The simulations of the PS accelerator cycle
together with the compensator shows that good AC
voltage control can be achieved during the load
pulse. However, there will be a transient AC voltage
disturbance of 6 % peak-to-peak at the end of the
pulse. This disturbance should not be critical for
particle accelerator operation, as the particles have
already left the PS before this occurs. The
PSCAD/EMTDC
simulations
also
illustrate
satisfactory operation of the internal SVC control
variables.
The SVC is expected to keep the Total
Harmonic distortion THD(U18 kV) below 1 % during
the entire load cycle.
Gain
Kpsvc
Tisvc
kP
kQ
kQdiff
value
1.3
0.0046
0.1
0.5
0.005
SVC controller gains
References
[1] N.G. Hingorani, L. Gyugyi: “Understanding
FACTS: Concepts and Technology of Flexible AC
Transmission Systems,” IEEE Press, 2000
[2] O. Bayard: “The supply of the 148 MW pulsed
power to the CERN SPS”, IEEE Transactions on
Nuclear Science, vol. NS-26, No.3, June 1979
[3] K. Kahle, J. Pedersen, T. Larsson, M. de Oliveira:
“The new 150 Mvar, 18 kV Static Var Compensator
at CERN: Background, Design and Commissioning”,
CIRED 2003
[4]
Manitoba
HVDC
research
Center
“PSCAD/EMTDC users manual” Winnipeg 2003
[5] P. Kundur: “Power System
Control”, McGraw Hill Inc. 1994
Stability and