APPLICATION OF A SUPERCONDUCTING MAGNETIC ENERGY STORAGE UNIT IN AN HVDC SYSTEM A. Abu-Siada*, S. Islam, W. W. L. Keerthipala, W. B. Lawrance Curtin University of Technology, Australia E-mail:
[email protected]
Abstract This paper discusses the use of a Superconducting Magnetic Energy Storage (SMES) unit in an HVDC system. The impact of converter station faults on the torsional forces induced in turbine-generator shafts with and without using a Superconducting Magnetic Energy Storage (SMES) unit will be explored. In this context, investigations have been conducted on a large turbine-generator unit connected to an HVDC system. Faults considered are fire-through, misfire, and a short circuit across the inverter station after steady state had been reached. The results of these investigations are studied using an electromagnetic transient program PSCAD/EMTDC and they are presented in the form of typical time responses as well as harmonic analysis.
1. INTRODUCTION Studies of the interaction between turbine-generator shaft systems and electrical network disturbances have shown that some electrical disturbances in the power system can cause more torsional stress on the turbine-generator shafts of the system than in the case of a three-phase fault at the generator terminals [1-2]. These disturbances include breaker operations associated with synchronizing, fault clearing and high-speed re-closing. The induced high stresses on the turbine-generator shaft reduce its expected lifetime and, in severe cases, may cause shaft damage. Besides the electrical network disturbances, turbinegenerator shaft torsional systems can interact with other power system stabilizers; static-var compensators, HVDC systems, high-speed governor controls and variable speed drive converters [3-5]. Attention has been focused on the effect of the HVDC controllers on the interaction of HVDC systems with turbine-generator shafts in most of the reported studies [6-7]. Fewer studies have investigated the impact of HVDC faults on turbine-generator shaft torsional torques. In all these investigations only DC line faults have been considered and no attempt has been made to consider the converter station faults [8]. This paper addresses the effect of HVDC converter station faults such as firethrough, misfire and a short circuit across the inverter side when steady state torque has been reached on the turbinegenerator shaft. A solution for this problem using an SMES unit is proposed. It has been observed that all other studies regarding the applications of an SMES unit in power systems have applied it only to AC systems. None of these studies have considered the application of SMES units in HVDC systems. In this paper, the EMTDC/PSCAD program is used to simulate the interaction between the HVDC system, SMES unit and the turbine-generator shafts [9]. The results of these investigations are presented in the form of typical time responses as well as harmonic analyses.
Fig. 1 Single line diagram of the system under study
1.1 System under study Figure 1 shows the system under study, which consists of a six pulse AC/DC converter station connected to an infinite bus and a synchronous machine at its terminals. In the system under investigation, a short transmission line is assumed to connect the AC/DC converter station to an infinite bus bar. Also, a local AC load (represented by a purely resistive load), and a group of 5th, 7th and 11th harmonic AC filters are connected to the AC bus of the converter station. The suggested location of the SMES unit is at the terminals of the synchronous machine to provide adequate damping for the turbine generator set. To carry out the investigations, a non-linear model of the system is developed to incorporate the interaction between the electric network and the torsionally oscillating shaft system of the turbine-generator under investigation. The system has been simulated such that all the non-linearities such as the exciter ceiling voltage limit and the current limit of the superconducting inductor have been included. The mathematical models of the system components can be found in more detail in [8]. However a brief description of the converter station of the DC system and the SMES unit will be provided in the next two sections.
2. Converter Station The converter station is modelled as a three-phase, 6-pulse GTO Graetz bridge as shown in Fig.2. A series impedance Z represents the converter transformer. In the normal operation of this 3- phase bridge, either two or three valves are conducting simultaneously [10]. Therefore, twelve different modes of operation exist per cycle as can be shown in Fig.3.
valve group that make up the twelve pulse valve group have a phase difference of 30 degrees which is utilized to cancel the AC side 5th and 7th harmonic currents and DC side 6th harmonic voltage, thus resulting in a significant saving in harmonic filters [11]. The current ISM passing through the superconducting inductor is unidirectional; however, the voltage VSM across the inductor terminals can be varied in a wide range between positive and negative values through the control of firing angles α1 and α2. In this way both active and reactive power of the power system can be modulated. If α1=α2 is selected, many problems will arise and the SMES unit will be less effective. Additional PID controllers have been suggested in other studies [12]. However, if unequal α-mode is selected, all problems associated with the equal α-mode can be eliminated. The necessity of incorporating an extra controller can also be eliminated [13].
3. 1 SMES Unit Controller Fig. 2 Bridge connected rectifier
In accordance with converter theory, the voltage Vsm in the DC side of the bridge is expressed by [12]: (1) Vsm = Vsmo (cos α 1 + cos α 2 ) where Vsmo is the ideal no-load maximum DC voltage of the 6-pulse bridges. The current and voltage of the superconducting inductor are related by t
I sm =
Fig. 3 Different modes of operation of the converter station during one cycle
1 Vsm dτ + I smo Lsm t∫o
(2)
where Ismo is the initial current of the inductor. For “charging” at the maximum rate, clearly Vsm should be held at its maximum positive rate. The inductor current Ism rises exponentially and magnetic energy Wsm is stored in the inductor. When the inductor current reaches its rated value Ismo, it is maintained constant by lowering the voltage across the inductor to zero. At any time during the charging period, the amount of energy stored in the magnetic field is expressed by:
W sm =
∫
B2 dV 2µ o
(3)
1 2 L sm I sm (4) 2 where B is the magnetic field strength in Tesla, V is the volume of the coil in m3, and µo is a constant that represents magnetic permeability of the material through which the magnetic field must pass. Equation 3 shows that the stored energy is related to the strength of the magnetic field in the coil. However equation 4 shows that the stored energy depends on the current in, and the inductance of the coil. The current Ism is an integral function of Vsm as given by equation (2). It can never reverse its direction, so it can only have positive values. At any time, the active and reactive power delivered or absorbed by the SMES unit is: Psm = Vsmo I sm (cos α 1 + cos α 2 ) (5) =
Fig. 4
Schematic diagram of the SMES unit
3. SMES Unit Figure 4 shows the main configuration of the SMES unit. The unit contains a superconducting inductor, 2-series six pulse AC/DC converters connected to the 3-phase AC power system via a Υ-∆ / Υ-Υ step down transformer. Consequently the AC voltages applied to each six pulse
Qsm = Vsmo I sm (sin α 1 + sin α 2 )
(6)
Because Vsm is a function of α1 and α2, its value can be varied in a wide range of positive and negative values via the control of α1 and α2. Ism is always unidirectional. Thus reversibility as well as magnitude control of power flow is achieved continuously and smoothly through the control of the firing angles α1 and α2. Equations (5) and (6) show that the SMES unit can operate in three modes: (i) The standby mode at firing angles equal to 90o, where the voltage across the SMES coil is equal to zero and the SMES coil current is at its rated value, consequently, there will be no energy transferred (ii) Charging mode at firing angles less than 90o, where power will be absorbed from the AC system (iii) Discharging mode at firing angles greater than 90o, where power will be injected in to the system. Adjusting the values of the firing angles can control the rates of charging and discharging. Using equations (5) and (6), the firing angles of the converter under four-quadrant operation can be calculated as [12-13]:
α 1 = cos −1 ( α 2 = cos −1 (
Psm 2 Psm2 + Q sm
Psm
) + cos −1 ( ) − cos −1 (
2 Psm2 + Q sm
2V smo I sm
)
(7 )
2 Psm2 + Q sm
) (8) 2 2V smo I sm Psm2 + Q sm In order to use the SMES unit as a torsional mode stabilizer, the active power Psm transferred by the converter is controlled continuously depending on the measured speed deviation of the turbine-generator rotor. The reactive power control is usually for the purpose of voltage stabilization. Then the reactive power Qsm transferred in the converter is controlled continuously depending on the measured voltage deviation of the generator bus terminal.
4. Simulation Results In all the cases studied in this paper, it is assumed that the turbine-generator is operating at steady-state delivering rated power to the infinite-bus system when the converter station and its group of filters are suddenly connected to the transformer high tension bus. To verify that condition the multi-mass system has been enabled at 0.1 sec. Attention has been given to the dynamic response of the generator shaft, i.e the electromagnetic torque (Te), the torque between high pressure and low-pressure turbines (Thl) and the torque between low-pressure turbine and generator (Tlg). Time domain waveforms due to specified faults without and with the use of the SMES controller have been compared.
4.1 Normal Operation Figure 5 illustrates the transient time responses of the AC/DC system during the normal operation of the HVDC converter station. It can be seen from the harmonic analysis of the electromagnetic waveform (shown in Fig. 6) that the turbine-generator electromagnetic torque contains harmonics of order 6k where k is an integer. There is also a unidirectional component. The unidirectional component is
due to the change in the system conditions (a step change in power: ∆P = PDC) and it subsequently varies with the turbine-generator natural frequency of oscillation. The power frequency (60 Hz) component is due to the DC offset in the turbine generator stator currents [2]. The sixth and twelfth harmonics (360 Hz and 720 Hz components respectively) of the turbine-generator electromagnetic torque are the result of the continuous switching process of the HVDC converter station. It can be observed from the harmonic analysis that the electromagnetic wave form contains also uncharacteristic harmonics such as 5th and 7th orders. These can be attributed to the unbalanced supply and the control equipment performs within finite tolerance limits. The turbine-generator torsional force at any shaft section is the summation of the shaft response to each of the individual components of the stimulating electromagnetic torque. It is well known that the turbine generator shaft system has a very low sensitivity to stimulating high frequency components (120 Hz and above [1-2]), the 360 Hz and the 720 Hz components of the turbine-generator electromagnetic torque make an insignificant contribution to the torsional torques induced in the turbine generator shafts. Therefore, it can be concluded that the turbinegenerator shaft system responds to the switching of an HVDC system similar to the case of a planned switching of a power ∆P. It is worthwhile to mention here that this is only true in the absence of all the converter control loops [1-2].
4.2 Fire-through Firethrough is the conduction of a valve having correct polarity for commutation before its programmed instant of conduction [3]. For such a fault, the firing delay angle of the faulted valve is reduced from its normal value to a smaller value or zero. Figure 7 illustrates the transient time responses of the AC/DC system during a sustained firethrough of valve T21 of the HVDC converter station. All the valves of the converter are operating with a firing delay angle α=57.3o except valve T21 which is conducting at α=0o. It can be seen from Figure 6 that such a fault introduces a significant increase in the harmonic content of the turbine-generator shaft torques, which can be confirmed by comparing the harmonic analysis of the electromagnetic torque shown in Figures 7 and 8. Figures 9 and 10 show the typical dynamic response of the system and the harmonic analysis of the electromagnetic torque under fire-through fault using the SMES unit respectively. These figures imply that, adding the SMES unit to the system will overcome the undesired harmonics and will smooth the dynamic response of the generator under a fire-through fault.
4.3 Misfire Misfire is the failure of a valve to take over conduction at the programmed instant although its voltage has the correct polarity [3]. Referring to Figure 2, if valve T21 fails to conduct at its programmed instant, commutation from valve T11 to valve T21 cannot take place, and valve T11 will
continue to conduct in series with valve T32. When valve T21 conducts, a DC short circuit is established through valves T11 and T12, making VDC-s equal to zero. When valve T31 conducts and takes over the conduction from valve T11, the short circuit is removed, but usually it reappears in every cycle if the cause of the misfire remains connected. Figure 11 illustrates the transient time responses of the AC/DC system during a sustained misfire of valve T21 of the HVDC converter station without using an SMES unit. It can be seen from this figure that such a fault increases significantly the magnitude of the sixth and twelfth harmonic components of the turbine-generator electromagnetic torque, which can be confirmed from the harmonic content of the electromagnetic torque shown in Figure 12. The effect of such large magnitudes on the torsional responses of the turbine-generator shaft system is clearly demonstrated in Figure 11, which shows a change in the pattern of the turbine-generator torques. Figure 13 shows the response under misfire fault using the SMES controller. The effect of adding an SMES unit to decrease the harmonic content is clearly shown in Figure 14.
4.4 DC Short circuit across inverter terminals This type of fault is assumed to occur at t=1 sec. In this case the voltage of the DC side VDC-s is assumed equal to zero. A short circuit across the inverter terminals leads to a very large over current on the DC side. Figure 15 illustrates the transient time responses of the AC/DC system during a short circuit across the inverter terminals without an SMES unit. The fault is assumed to occur at t=1sec, after the DC current has reached its steady-state value following the switching of the converter station. It can be seen from Fig.15 that the DC current reaches a high value, which is about five times the steady state value during the normal operation. Also the torsional forces induced in the (LPGEN) and (HP-LP) shafts will reach crest values of 2 p.u. and 0.96 p.u. respectively. These values are much higher than the corresponding ones during the normal operation. Figure 16 shows the system response to the same fault with the SMES unit connected to the generator bus. The waveforms in Fig. 16 imply that the system is completely stable and that all system eigne values have been moved to the left hand side of the s-plane.
5. Conclusions In this paper, studies have been performed on a onemachine infinite bus system to investigate the effect of converter station faults on the torsional torques induced in turbine-generator shafts with and without using of the SMES unit. Faulted considered are; Firethrough, Misfire, and SC across the inverter when steady state has reached. The results of the simulation yield the following conclusions for the system under study: 1. Turbine-generator shaft torque responses due to switching in a DC line are similar to that due to a planned switching of a power ∆P. This is due to the fact that the impacts of the high frequency component (360 Hz) of the
turbine-generator electromagnetic torque resulting from the continuous switching process of the HVDC converter station is almost negligible. 2. Firethrough and misfire in the HVDC converter station result in a slight increase in the harmonic levels of the torques induced in the turbine-generator shafts compared with those during the normal operation of the HVDC converter station. With the SMES unit, the distortion of the torsional forces can be overcome and hence the harmonic content reduced. 3. The shaft systems of turbine-generators are subjected to high torques during a short circuit across the inverter terminals. However, using an SMES unit will reduce these high values to almost the normal steady state values.
REFERENCES [1] A. Abu Siada, et. al.,Simulation Study of Converter Station Faults on Generator Shaft Torsional Torques in HV System, The 5th International Power Conference, Singapore, 2001, 134-139 [2] A. M. EL-Serafi and S.O. Faried, Effect of HVDC Converter Station Faults on Turbine-Generator Shaft Torsional Torques, IEEE SM Trans. on Power Systems, 12(2), 1996, 875-881 [3] J. Arrillaga, High Voltage Direct Current Transmission (Peter Peregrinus Ltd, London, 1983) [4] K. R. Padiyar, HVDC Power Transmission Systems (Wiley Eastern Limited, New Delhi, 1992) [5] Kimbark, E. W., Direct Current Transmission (Vol. 1 Wiley Interscience , New York,1971) [6] J.J Vithayathil, Digital Dynamic Simulation, Proc. of National Workshop on HVDC Transmission, I.I.T., Kanpur, January 1987 [7] R.M. Mathur and Sharaf A. M., Harmonics on the DC Side in HVDC Conversion, IEEE Trans. PAS-96 (5), 1977, 1631-1683 [8] A. Abu Siada, Effect of HVDC Harmonics on Synchronous Machine Transient and Dynamic Behaviour (M.Sc. Thesis, Ain Shams University, Egypt 1998) [9] Manitoba HVDC Research Centre, PSCAD/EMTDC Software Manual (version 2, Canada, 1994) [10] A.M. El-Serafi and S.A. shehata, Digital Simulation of an AC/DC System in Direct Phase Quantities, IEEE Transactions on Power Apparatus and Systems, PAS-95, 1976, 731-742 [11] Ned Mohan, Tore Undeland and W. Robbins, Power Electronics, Converters, Applications and Design (2nd edition, John Willy & Sons, 1995) [12] M.G.Rabbani, J.B. Devotta and S. Elangovan, Application of simultaneous active and reactive power modulation of SMES unit under unequal α-mode for power system stabilization, IEEE Transaction on Power System, 14(2), 1999, 547-552 [13] A. Abu Siada, W W L Keerthipala and W B. Lawrance, Application of a Superconducting Magnetic Energy Storage unit to improve the Stability Performance of Power Systems, IEEE CCECE02 Proceedings, Canada, 2002, 201-206
Fig. 8 Frequency spectra of electromagnetic torque during fire-through without SMES unit
Fig. 5 Transient time responses of the system during normal operation without SMES unit
Fig. 6 Frequency spectra of electromagnetic torque during normal operation without SMES unit
Fig. 9 Transient time responses of the system during sustained firethrough with SMES unit
Fig. 10 Frequency spectra of electromagnetic torque during fire-through with SMES unit
Fig. 12 Frequency spectra of electromagnetic torque during misfire without SMES unit
Fig. 7 Transient time responses of the system during sustained fire-through without SMES unit
Fig. 11 Transient time responses of the system during sustained misfire without SMES unit
Fig. 15 Transient time responses of the system during a SC on the inverter side at t=1 sec without SMES unit
Fig. 13 Transient time responses of the system during sustained misfire with SMES unit
Fig. 16 Transient time responses of the system during a SC on the inverter side at t=1 sec with SMES unit Fig. 14 Frequency spectra of electromagnetic torque during Misfire with SMES unit
