Voltage Sag and Swell Compensation with DVR Based on Asymmetrical Cascade Multicell Converter S. Masoud Barakati
Arash Khoshkbar Sadigh
Ehsan Mokhtarpour
ECE Department, University of Sistan and Bluchestan, Zahedan, Iran
[email protected]
EECS Department, University of Irvine, CA, 92617, USA
[email protected]
ECE Department, University of Tabriz, Tabriz, Iran
[email protected]
Abstract— This paper deals with a dynamic voltage restorer (DVR) as a solution to compensate the voltage sags and swells and to protect sensitive loads. In order to apply the DVR in the distribution systems with voltage in range of kilovolts, series converter as one of the important components of DVR should be implemented based on the multilevel converters which have the capability to handle voltage in the range of kilovolts and power of several megawatts. So, in this paper a configuration of DVR based on asymmetrical cascade multicell converter is proposed. The main property of this asymmetrical CM converter is increase in the number of output voltage levels with reduced number of switches. Also, the pre-sag compensation strategy and the proposed voltage sag/swell detection and DVR reference voltages determination methods based on synchronous reference frame (SRF) are adopted as the control system. The proposed DVR is simulated using PSCAD/EMTDC software and simulation results are presented to validate its effectiveness.
I.
INTRODUCTION
Due to increase the number of sensitive loads in the electrical power systems, the demand for high power quality and voltage stability has increased significantly. Most serious threats for sensitive equipments in present grids are voltage sags and swells [4]-[7]. These disturbances occur due to some events, e.g., short circuit in the grid, inrush currents involved with the starting of large machines, or switching operations in the grid [14].
The use of a dynamic voltage restorer (DVR), or a voltage disturbances compensator, is one of the most effective solutions for “restoring” the quality of voltage at its load-side terminals when the quality of voltage at its source-side terminals is disturbed [12]-[14]. A traditional DVR mainly consists of series and shunt converters connected back-to-back and a common dc capacitor used as an energy-storage element [11], [13], [15]. Numerous circuit topologies are available for the DVR since a widely used method is the two-level or multilevel converter. In comparison with the traditional two-level converters and by increasing the number of dc voltage sources (levels), the small voltage steps lead to the production of high power quality waveforms, lower harmonic components, lower voltage ratings of devices, lower switching losses, higher efficiency, and also reduction of dv/dt stresses on the load and gives the possibility of working with low speed semiconductors as well as implementation of DVR in the distribution systems with voltage in range of kilovolts [3]. The Neutral Point Clamped (NPC) converter, presented in the early 80’s is now a standard topology in industry on its 3-level version [9]. An alternative for the NPC converter are the multicell topologies. Different cells and ways to interconnect them generate many topologies which the most important ones are the
Cascaded Multicell (CM) and the Flying Capacitor Multicell (FCM) with its sub-topology Stacked Multicell (SM) converters [6], [7], [8]. The CM converters are classified into symmetrical and asymmetrical CM converters [16]. The main advantage of asymmetrical CM converter in comparison with the symmetrical CM converter is providing a large number of output levels without increasing the number of converters and input dc sources. In this paper, first the new configuration of asymmetrical CM converter which is proposed in [2] is described. The main advantage of this converter is reducing number of high-frequency switches by 50 percent. This progress is achieved by adding only 4 lowfrequency switches. However, it should be noted that the configuration of DVR based on this asymmetrical converter has not been proposed yet. Because of mentioned properties and in order to apply the DVR in the distribution systems with voltage in range of kilovolts, in this paper a novel configuration of DVR based on 15-level asymmetrical CM converter is proposed. Also, the pre-sag compensation strategy is applied to DVR to compensate the voltage sag/swell and methods based on synchronous reference frame (SRF), which are proposed by author in [6], [8]-[10], is used to detect voltage sag/swell and determine the three single-phase reference voltages of DVR which result in a good dynamic response time of the DVR. Simulation results are presented to validate the effectiveness and advantages of the novel configuration of DVR and its detection and determination methods. II.
source converters. It starts to inject three single-phase compensating voltages in series into the power line as soon as voltage sag or swell occurs. In this paper, asymmetrical CM converter, introduced in [3], based DVR is proposed to increase the number of output voltage levels and as a result, reduce the output voltage THD and make it possible to implement DVR in the distribution systems with voltage in range of kilovolts. A new 15-level asymmetrical CM converter is shown in Fig. 2. The main property of the new configuration of asymmetrical CM converter in comparison with the conventional asymmetrical CM converter is reduction in the number of required highfrequency switches by 50 percent while only 4 lowfrequency switches are added. As a result, the cost, size and the power loss of the new configuration of asymmetrical CM converter is decreased. The configuration of DVR based on configuration of 15-level asymmetrical CM converter is shown in Fig. 3.
Fig. 1. Schematic diagram of DVR.
PROPOSED DVR
A typical DVR for voltage sag/swell compensation is shown in Fig. 1. When the supply-side voltage changes the DVR injects a series voltage to maintain the magnitude of the load voltage at its reference value. The DVR is essentially a voltage-source converter that produces an ac output voltage and injects it in series with the supply voltage. Note that the voltage injection also results in the supply or absorb of active and reactive power. Reactive power can be supplied without taking energy from the dc-side capacitor; however, an active power supply must involve the exchange of stored energy. The series converter consists of a three-phase voltage-source converter or three single-phase voltage-
Fig. 2. 15-Level asymmetrical cascade multicell converter.
III.
CONTROL STRATEGIES
A. Asymmetrical Cascade Multicell Converter Control Strategy The mentioned configuration of asymmetrical CM converter is controlled by level shifted sinusoidal pulse width modulation (LS-SPWM), as shown in Fig. 4, while the absolute of the reference signal is intersected with the level shifted triangle carriers. The low-frequency switch (Ja) is on for positive reference signal and is off for negative reference signal. All states of switches are illustrated in details in Table-I. B.
applied to DVR; the main reasons are its mentioned advantages, excellent performance particularly in the case of phase jumps in the grid voltage and the ability of compensation for any kind of voltage sags and swells. TABLE-I STATE OF SWITCHES IN 15-LEVEL ASY. CASCADE MULTICELL CONVERTER.
Output Voltage Level
Ja Positive Negative
′ , Vdvr ′ Vload
and
′ ) indicate variables after the I load
disturbance. The phasors prior to the disturbance are represented by Vgrid , Vload and I load . This compensation strategy leads to the lowest distortions at the load-side, because the amplitude and phase angle of the voltage at the load-side is not changed during the disturbance. For this strategy, a phase-locked loop (PLL) is synchronized with the load voltage. As soon as a disturbance occurs, the PLL is locked and therefore, the phase angle can be restored. Depending on the phase angle of the grid voltage during the disturbance, the DVR has to inject higher voltage amplitude to restore the correct voltage magnitude, because the phase jump of the grid has also to be compensated by the DVR; therefore, the system has to be designed for the highest possible voltage. This strategy is able to compensate any kind of voltage sag/swell including balanced or unbalanced voltage sag/swell with or without any phase-variations in each grid phase voltages. In this paper, the pre-sag compensation strategy is
1 0 ( S3 , S2 , S1 )
Voltage Sag/Swell Compensation Strategy
To avoid tripping of the load, the amplitude and phase angle of the load voltage has to be restored by the DVR. Different strategies can be used to achieve this goal. Three basic strategies are the pre-sag compensation [10], [14], in-phase compensation [4], [7], [10] and the energy-minimized compensation strategies [1], [4], [7]. The standard solution for compensating voltage disturbances is to restore the load exact voltage before the disturbance. Therefore, the amplitude and the phase angle of the voltage before the disturbance have to be exactly restored [10], [14]. The phasor diagram of the pre-sag compensation strategy is shown ′ , in Fig. 5. In this figure, the dashed quantities ( Vgrid
State of Switches
C.
0
(0,0,0)
1
(0,0,1)
2
(0,1,0)
3
(0,1,1)
4
(1,0,0)
5
(1,0,1)
6
(1,1,0)
7
(1,1,1)
Detection and determination methods
In this paper, the SRF, which is proposed in [6], [8][10], is used to detect the voltage sags/swells and determine the DVR reference injected voltage. At the beginning, the voltage sag/swell must be detected and then, the DVR reference injected voltage must be determined.
Fig. 3. Power circuit of the proposed DVR based on 15-level asymmetrical cascade multicell converter.
As the first step of voltage sag/swell detection, the
line-neutral grid voltages are measured and transferred from abc coordinate system to SRF as follows: Vgrid , d cos(ωt) cos(ω t − 120) cos(ω t + 120) Vgrid , a 2 (1) Vgrid , q = sin(ω t) sin(ωt − 120) sin(ω t + 120) ⋅ Vgrid ,b 3 1 V 1 1 Vgrid ,0 grid , c 2 2 2
the d-component and q-component of grid voltages as follows: Vgrid =
1 2
2
(Vgrid , d ) + (Vgrid , q )
2
(2)
Then, the phase angle of phase a voltage in the presag state (healthy state) is stored as the reference angle as follows: Vgrid , d
θ ref = arctan
V grid , q
where, Vgrid , d
dc
and Vgrid , q
dc
dc dc
(3)
are dc values of d and q-
components of grid voltages in the SRF, respectively. In the case of single phase voltage sag/swell or unbalanced three phase voltage sag/swell, the zerocomponent of grid voltages in the SRF is not nil. Therefore, it can be used as a parameter to detect the voltage sag/swell. However, in the case of balanced three phase voltage sag/swell, the zero-component of grid voltages in the SRF is nil and only the rms value of balanced grid voltage in the sag/swell state changes. Because of that, it is necessary to use the rms value of line-neutral grid voltages ( Vgrid ) obtained from (2) as Fig. 4. Level shifted sinusoidal pulse width modulation, states of switches and output voltage for 15-level asymmetrical cascade multicell converter.
Fig. 5. Phasor diagram of the pre-sag compensation strategy.
another parameter to detect the voltage sag/swell. Finally, the following proposed algorithm based on two steps is performed iteratively to detect the voltage sag/swell: Step-1: The zero-component of grid voltages in the SRF which is obtained from (1) is analyzed. If it is not equal zero, it means that the unbalanced voltage sag/swell is occurred. So, the voltage sag/swell is detected and then, exit the algorithm. If the zerocomponent is equal to zero, go to step-2. The nil zerocomponent means that the grid voltages are balanced. So, the rms value of grid voltages must be analyzed as step-2. Step-2: The rms value of line-neutral grid voltages (
where, Vgrid ,a , Vgrid ,b , Vgrid ,c are the measured line-
Vgrid ) obtained from (2) is compared with the reference
neutral grid voltages of phases a, b and c, respectively and Vgrid , d , Vgrid , q , Vgrid , 0 are the d-component, q-
ref rms value of line-neutral grid voltages ( Vrms ). If they are
component and zero-component of grid voltages in the SRF, respectively. Also, the rms value (line-neutral) of balanced grid voltage ( Vgrid ) can be calculated using
not equal to each other, it means that the voltage sag/swell is occurred. So, the voltage sag/swell is detected and then, exit the algorithm. If there is no difference between them, go to step-1. Using the explained algorithm makes it possible to detect any
balanced or unbalanced voltage sag/swell. After the detection of voltage sag/swell, the reference rms value ref of line-neutral grid voltages ( Vrms ) and the obtained
reference angle ( θ ref ) are used to determine the values of reference grid voltages in the SRF as follows:
( )
(4)
( )
(5)
ref ref ref Vgrid , d = 2 ⋅Vrms ⋅ sin θ
ref ref ref Vgrid , q = 2 ⋅Vrms ⋅ cos θ
ref ref where, Vgrid , d and Vgrid , q are the reference d and q-
components of grid voltages in the SRF, respectively. Next, the differences between the dq0 values of lineneutral grid voltages and the dq0 values of reference line-neutral grid voltages are taken into account as dq0 values of DVR reference injected voltages as follows: ref Vdvr ,d
ref = Vgrid ,d
− Vgrid,d
(6)
ref Vdvr ,q
ref = V grid ,q
− V grid , q
(7)
ref Vdvr , 0 = −V grid , 0
(8)
all the tests, a resistive-inductive load is used. A. Unbalanced voltage sag without phase variation of grid voltage
In this case, the unbalanced voltage sag is applied to the system. Since the unbalanced voltage sag is occurred at the grid at t = 0.1 s , the grid voltages of phases a and b drop to 60% of their nominal values. As shown in Fig. 6(c), the used detection and determination methods are able to detect the voltage sag and determine the three single-phase reference voltages of DVR as fast as the DVR compensates the voltage sag. TABLE-II. PARAMETERS OF SIMULATED SYSTEM. System Parameters
Values
Nominal voltage (line to line), Vgrid (KV rms)
20
System frequency (Hz)
50
Smallest input dc voltage source, E (V) Switching frequency of cascade multicell converter (KHz) Inductance of output series filter L (mH)
500 0.05
Capacitance of output series filter Cf (µF)
30
f
10
Turn ratio of isolation series transformers
2.5
Resistance & inductance of load R (Ω), L (H)
50 ; 0.15
ref ref ref where, Vdvr , d , Vdvr, q and Vdvr, 0 are the reference d-
component, q-component and zero-component of DVR series injected voltages in the SRF, respectively. These values are transferred to abc coordinate system and then, three single-phase reference voltages of DVR are obtained as follows: ref ref Vdvr sin(ωt) 1 Vdvr , d cos(ωt) ref, a ref Vdvr ,b = cos(ωt − 120) sin(ωt − 120) 1 ⋅ Vdvr , q V ref cos(ωt + 120) sin(ωt + 120) 1 V ref dvr , 0 dvr ,c
(9)
ref ref ref where, Vdvr , Vdvr and Vdvr are the DVR reference ,a ,b ,c
injected voltages of phase a, b and phase c, respectively. IV.
SIMULATION RESULTS
Computer simulations are provided to verify the wellperformance of the proposed DVR configuration, as well as voltage sag/swell detection method and DVR reference voltage determination strategy. The parameters used in the simulation are given in Table-II. The system is simulated using PSCAD/EMTDC software. Following, the simulation results for two different voltage sags and one voltage swell are presented. For
B. Unbalanced voltage sag with phase variation of grid voltage In Fig. 7, the simulation results of the second case are depicted. In this case, the unbalanced voltage sag is occurred at the grid at t = 0.1 s while the grid voltage of phase a drops to 60% and the grid voltages of phases b and c drop to 70% of their nominal values. Additionally, a phase jump of -15 degrees is applied to the grid voltage of phase a. C. Unbalanced voltage swell with phase variation of grid voltage In Fig. 8, the simulation results of the third case are depicted. In this case, the unbalanced voltage swell is occurred at the grid at t = 0.1 s while the grid voltage of phase a rises to 130% and the grid voltages of phases b and c rise to 140% of their nominal values. Additionally, a phase jump of +15 degrees is applied to the grid voltage of phase a. As shown in Figs. 6-8, the proposed strategies are able to detect voltage sag/swell as well as determine the three single-phase reference voltages of DVR and compensate long-duration unbalanced sags
without delaying and stopping the suitable operation of proposed DVR. Considering the load voltage obtained from simulation results, it can be deduced that the compensation starts immediately when grid voltage sag/swell is detected; also, the DVR shows a good response time for detection of voltage sag/swell and compensating operation. Furthermore, due to application of pre-sag compensation strategy, the phase angle of the load voltage maintains equal to its value before the sag/swell where it is not possible with two other mentioned compensation strategies.
V.
CONCLUSION
Voltage sags/swells are major problems in power systems due to the increased integration of sensitive loads into them. DVR systems are able to compensate these voltage sags/swells. Because the multicell converters are very interesting for high-power/mediumvoltage applications, and also considerably improve the output voltage frequency spectrum, in this paper configuration of DVR based on asymmetrical CM converter has been proposed to improve the quality of DVR output voltage and to be used in the distribution systems with voltage in range of kilovolts. In the proposed configuration of DVR, number of required high-frequency switches is reduced by 50 percent. Therefore, the cost, size and power loss are decreased. Also, new methods based on the SRF have been used to detect the voltage sag/swell and determine the reference series injected voltage of DVR. As depicted in simulation results, the pre-sag compensation strategy and the proposed SRF based detection and determination methods show excellent performance and good dynamic response time.
Fig. 6. Voltage sag (without phase variation of grid voltage): (a) grid voltage; (b) DVR injected voltage; (c) load voltage. (All in Volt)
Fig. 7. Voltage sag (with phase variation of grid voltage): (a) grid voltage; (b) DVR injected voltage; (c) load voltage (All in Volt).
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12] Fig. 8. Simulation results before and after the voltage swell: (a) grid voltage; (b) DVR injected voltage; (c) load voltage (All in Volt). [13]
REFERENCES [1]
[2]
[3]
[4]
H. K. Al-Hadidi, A. M. Gole and D. A. Jacobson, “Minimum power operation of cascade inverter based dynamic voltage restorer”, IEEE Trans. Power Delivery, vol. 23, no. 2, pp. 889–898, Apr. 2008. E. Babaei, “A cascade multilevel converter topology with reduced number of switches”, IEEE Trans. Power Electronics, vol. 23, no. 6, pp. 2657-2664, Nov. 2008. E. Babaei, S. H. Hosseini, G. B. Gharehpetian, M. Tarafdar Haquea and M. Sabahi, “Reduction of dc voltage sources and switches in asymmetrical multilevel converters using a novel topology”, Elsevier Journal of Electric Power Systems Research, vol. 77, no. 8, pp. 1073–1085, Jun. 2007. M. R. Banaei, S. H. Hosseini and G. B. Gharehpetian, “Inter-line dynamic voltage restorer control using a novel optimum energy
[14]
[15]
[16]
consumption strategy”, Elsevier Journal of Simulation Modeling Practice and Theory, vol. 14, no. 7, pp. 989–999, Oct. 2006. M. R. Banaei, S. H. Hosseini, S. Khanmohamadia and G. B. Gharehpetian, “Verification of a new energy control strategy for dynamic voltage restorer by simulation”, Elsevier Journal of Simulation Modeling Practice and Theory, vol. 14, no. 2, pp. 112– 125, Feb. 2006. S. H. Hosseini, A. Khoshkbar Sadigh and G. Gharehpetian, “Flying capacitor multicell converter based DVR with energy minimized compensation strategy”, in Proc. of 6th Int. Conf. ELECO, Nov. 2009, Bursa, Turkey, pp. 1–5. S. H. Hosseini, A. Khoshkbar Sadigh and A. Sharifi, “Estimation of flying capacitors voltages in multicell converters”, in Proc. 6th International Conference ECTI-CON, Pattaya, Thailand, May 2009, pp. 110–113. A. Khoshkbar Sadigh, E. Babaei, S. H. Hosseini and M. Farasat, “Dynamic voltage restorer based on stacked multicell converter”, in Proc. of ISIEA, Oct. 2009, Kuala Lumpur, Malaysia, pp. 1–6. A. Khoshkbar Sadigh, S. H. Hosseini, S. M. Barakati and G. Gharehpetian, “Flying capacitor multicell converter based dynamic voltage restorer”, in Proc. of 41st NAPS, Oct. 2009, Mississippi State, USA, pp. 1–6. A. Khoshkbar Sadigh, S. H. Hosseini, S. M. Barakati and G. Gharehpetian, “Stacked multicell converter based DVR with energy minimized compensation strategy”, in Proc. of 41st NAPS, Oct. 2009, Mississippi State, USA, pp. 1–6. C. S. Lam, M. C. Wong and Y. D. Han, “Voltage swell and overvoltage compensation with unidirectional power flow controlled dynamic voltage restorer”, IEEE Trans. Power Delivery, pp. 2513– 2521, Oct. 2008. J. Lamoree, L. Tang, C. DeWinkel, and P. Vinett, “Description of a micro-SMES system for protection of critical customer facilities”, IEEE Trans. Power Delivery, vol. 9, no. 2, pp. 984–991, Apr. 1994. S. A. Saleh, C. R. Moloney and M. A. Rahman, “Implementation of a dynamic voltage restorer system based on discrete wavelet transforms”, IEEE Trans. Power Delivery, vol. 23, no. 4, pp. 2366– 2375, Oct. 2008. P. R. Sánchez, E. Acha, J. E. O. Calderon, V. Feliu and A. G. Cerrada, “A Versatile control scheme for a dynamic voltage restorer for power-quality improvement”, IEEE Trans. Power Delivery, vol. 24, no. 1, pp. 277–284, Jan. 2009. B. Wang and G. Venkataramanan, “Dynamic voltage restorer utilizing a matrix converter and flywheel energy storage”, IEEE Trans. Industry Applications, vol. 45, no. 1, pp. 222–231, Jan./Feb. 2009. M. A. Pérez, P. Cortés and José Rodríguez, “Predictive control algorithm technique for multilevel asymmetric cascaded Hbridge inverters”, IEEE Trans. Industrial Electronics, vol. 55, no. 12, pp. 4354-4361, Dec. 2008.
