EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH European Laboratory for Particle Physics CERN – TS DEPARTMENT
CERN-TS-2004-009 (EL)
EDMS Nr. 588386
Static Var Compensator for CERN’s Proton Synchrotron Particle Accelerator Karsten Kahle 1 Dragan Jovcic 2
Abstract The following paper summarises the design studies for a new 85Mvar 18kV Static Var Compensator (SVC) for CERN’s Proton Synchrotron (PS) accelerator. Currently, the PS is supplied using a rotating motor-generator set in order to decouple the pulsating load from the electrical network. A study is undertaken to investigate the replacement of this rotating machine by an SVC. The proposed solution comprises a 85Mvar Thyristor Controlled Reactor and seven harmonic filters with a total power of 75Mvar. The paper gives a detailed description of the project background, system design and control strategy. Finally, the results of the computer studies are presented, showing the expected dynamic performance of the SVC.
1 2
CERN – Geneva, Switzerland University of Ulster Newtownabbey, UK
Paper presented at the 2nd International Conference on Critical Infrastructures, Grenoble (France), 25-27 October 2004
Geneva, Switzerland December 2004
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;
[email protected] (**) University of Ulster, Newtownabbey, BT37 0QB, UK;
[email protected]
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
