Paper accepted for presentation at 2003 IEEE Bologna Power Tech Conference, June 23th-26th, Bologna, Italy
Cost-Effective Power Quality Improvement for Industrial Plants Massimo Bongiorno, Student Member, IEEE, Ambra Sannino, Member, IEEE, and Luigi Dusonchet
Abstract - Disturbances in the voltage supply, especially voltage dips and short interruptions, can cause tripping of sensitive electronic equipment and cause production stops in many industries. This paper proposes a power-electronics based device, obtained by a combination of a Static Transfer Switch (STS) in series with a Static Series Compensator (SSC) for protection of large industrial distribution systems against voltage dips and short interruptions in a cost-effective way. The hybrid compensator is modeled and simulated with different types of dips using the simulation tool PSCAD/EMTDC. Design of the device and control strategies are discussed. An economic evaluation of the device is carried out. Index Terms - power quality, voltage sags (dips), power electronics, custom power.
I. INTRODUCTION Since some years, novel power electronic controllers, called Custom Power devices, have been introduced to improve the quality of power distribution in industrial plants [1]. Different types of industries have been reporting production stops due to short-duration voltage disturbances, like short interruptions and voltage dips. Among the most sensitive industries are paper mills [2], semiconductors facilities [3] and other industries with fully automated production, where the sensitivity of electronic equipment to voltage disturbances can cause problems to the whole facility. Loss of revenues due to power quality disturbances can be high and therefore solutions for mitigation are needed. The economics related to these solutions play an important role: the cost of the interruption must be reasonable compared to the loss due to production stops, leading to a payback time of one or two years. To mitigate the effect of voltage dips and short interruptions in the supply voltage, two devices are receiving particular attention:
• the Static Series Compensator (SSC) or Dynamic Voltage Restorer (DVR), shown to the right in Fig.1, usually designed to mitigate voltage dips with magnitude lower than 50% [4]. This is based on a Voltage Source Converter (VSC) that generates a compensation voltage, which is then injected in the distribution feeder by means of a series-injection transformer. Normally, an LC-filter between the VSC and the transformer is also present to remove high-order harmonic components from the converter output voltage. An energy storage device connected to the dc-link of the VSC provides the necessary active power for the compensation. • the Static Transfer Switch (STS), shown to the left in Fig.1, is able to limit the duration of interruptions and voltage dips to less than one half-cycle in most cases [5], by transferring the load from the affected line to a back-up feeder. This high speed of response is obtained by using two static switches, constituted each by two antiparallel thyristors, to perform the transfer of the load. Some installations already in service have proven to provide satisfactory plant protection [6][7]. However, the cost of power electronics components has not been decreasing as fast as anticipated and the cost of these devices is still a limit to their widespread introduction. Moreover, these devices present some limitations: the SSC is not suitable to compensate for interruptions of the supply voltage and the range of dips that it can mitigate depends on the size of the energy storage. On the other hand, the STS cannot mitigate dips that affect both feeders. STS alternative source SSC injection transformer load
vssc,abc(t) The work of Massimo Bongiorno was supported by the Foundation “Blanceflor Boncompagni-Ludovisi, neé Bildt”, Stockholm, Sweden. The work of Ambra Sannino was supported by a Marie Curie Fellowship of the European Community programme IHP under contract number HPMF-CT2000-00922. The work of Luigi Dusonchet was supported by MIUR, the Italian Board of Education and University Research. Massimo Bongiorno and Ambra Sannino are with the Department of Electric Power Engineering, Chalmers University of Technology, SE - 41296, Gothenburg, Sweden (e-mail:
[email protected],
[email protected]). Luigi Dusonchet is with the Department of Electric Engineering, University of Palermo, I-90128 Palermo, Italy (e-mail:
[email protected]).
0-7803-7967-5/03/$17.00 ©2003 IEEE
primary source
filter
vsts,abc(t)
Voltage Source Converter
energy storage
Fig.1. Hybrid compensator (single phase scheme) based on a combination of STS and SSC.
In this paper, a hybrid device that provides cost-effective mitigation of voltage dips and short interruptions will be proposed. The device will be tested via simulations with voltage dips due to faults in different points in the power system. Design of the device and control strategies will be discussed. Finally, an economic evaluation will be carried out. II. HYBRID COMPENSATOR
Moreover, it is further possible to reduce the size of the energy storage considering that its design also depends on the duration of the dip. B. Control system In Fig.3 the block scheme of the controller of the STS is shown. The detection of the voltage disturbance is based on Park’s transformation. When a fault in the system is detected, the control system provides to transfer the load from the preferred to the alternative source by using the transfer algorithm reported in [11]. For the control system of the SSC, the Double Vector Control (DVC) technique proposed in [10] has been used. As shown in the block-scheme of Fig.4, this controller uses the reference voltage of the filter capacitor (obtained subtracting the grid voltage from the desired load voltage) in order to regulate the load voltage. The inputs of the controller are:
The proposed device is obtained by a combination of an STS in series with an SSC, as shown in Fig.1. In this way, total protection can be obtained against both interruptions and voltage dips. The STS takes care of interruptions and voltage dips originated by faults in the distribution system, which are long and deep [8] and would deplete the energy storage of the SSC. The SSC will instead compensate for the voltage dips originated by faults in the transmission systems, which the STS cannot handle. Note, however, that dips originated in the transmission system are normally short and shallow [8]. Hence, the size of the energy storage of the SSC can be greatly reduced, with a consequent reduction of the cost of the device.
• the load current i ssc,abc (k) ;
A. Design of the system
• the converter output current i i,abc (k) ;
The system has been sized for medium voltage applications. Nothing changes as compared to the traditional design of the STS, because the load current is the same. The most important factor for the design of the proposed device is the magnitude and the duration of the transmission-related dips to be expected at the common point. Normally the SSC is designed to inject in the grid a maximum voltage of 50% of the rated voltage for 500 ms [6]. This should be enough to bring the voltage back to at least 90% of the rated value for the majority of the faults in the power system. Using the hybrid compensator, the requirement on the design of the SSC is to mitigate only dips due to faults in the transmission system that, as said before, are usually short and shallow. Assumed that generally these dips are not deeper than 70% and have a maximum duration of 100 ms [9], the size of the SSC can be reduced (Fig.2). Here, the SSC is designed for 30% voltage injection. The size of all components (series transformer, converter, filter and energy storage), which depends on the injected voltage, is reduced proportionally.
• the voltage across the filter capacitor v c,abc (k) .
voltage [%]
• the voltage downstream of the STS v ssc,abc (k) ;
The controlled variables are the converter output current and the capacitor voltage. This controller is also based on Park’s transformation and is constituted by two loops: a voltagecontrol loop, used to determine the reference current of the filter inductor, and a current-control loop, which gives the reference voltages for the PWM modulator. A sequence detection technique, which is not shown in the figure for clarity, has been implemented to allow for mitigation of unsymmetrical dips. vsts,abc(k)
(ab)
abc
vs (k)
angle detection
vs (k) +
transfer algorithm
-
firing pulses
(dq)
vref (k)
q(k)
Fig.3. Block-scheme of the control system of the STS.
angle detection
energy to be provided by the SSC
fault signal
(dq)
ab dq
ab
*(dq)
q(k)
vL (k) (dq)
(ab)
vssc,abc(k)
abc ab
vs (k)
ab dq
*(dq)
vs (k)
-
+
vc (k)
*(dq)
Voltage Controller
ii (k)
q(k)
100%
Vinj,max
issc,abc(k)
(ab)
abc ab
is (k)
abc ab
vc (k)
(dq)
ab dq
q(k)
70%
vc,abc(k)
ii,abc(k)
(ab)
(ab)
abc ab
ii (k)
ab dq
q(k) ab dq
is (k) (dq)
vc (k) *(dq)
ii (k)
(dq)
ii (k)
*(dq)
Current Controller
vc (k)
*(dq)
v (k)
100 ms
time [s]
Fig.2. Expected maximum voltage dip due to transmission system faults. Vinj,max is the maximum voltage that the SSC can inject in the grid.
*(ab)
*
to the VSC
PWM
vabc (k)
abc
v (k)
ab
Fig.4. Block-scheme of the SSC controller.
dq ab
q(k)
0.5
a
0.4
III. PERFORMANCE ANALYSIS
b
0.2
voltage [pu]
The hybrid device has been tested with dips due to faults in different locations in the power system. The rated voltage is 11 kV. The load is resistive-inductive with rated power of 7.56 MVA and power factor 0.8. The simulations presented here have been carried out with the software PSCAD/EMTDC.
c
0.3
0.1
0
−0.1
A. Transmission system faults
−0.2
−0.3
−0.4
−0.5 0.11
0.12
0.13
0.14
0.15
0.16 0.17 time [s]
0.18
0.19
0.2
0.21
Fig.6. Reference voltages for the PWM modulator.
a b c
1
0.8
0.6
0.4
voltage [pu]
In Fig.5 the supply voltage is affected by an unbalanced voltage dip, which occurs after 128 ms and lasts for 80 ms. The magnitude of the lowest voltage is 67% (phase a) and the other two phases only drop by 15% and 10% (phase b and phase c respectively). This shallow dip, recorded at 11 kV with a power quality monitor, can have been caused by a transmission system fault. The actual dip recording was used as input in the simulation presented here. This fault is applied to both primary and alternative sources, and therefore the switching operation of the STS is not allowed. In this case, the SSC has to compensate for the load voltage by injecting the necessary voltage (with correct magnitude and phase) in the grid. In Fig.6, the reference voltages for the PWM modulator are reported. As shown, the reference voltages are never zero, even when there is no dip, because the control system compensates for the voltage drop due to the circulation of the load current on the filter reactor and on the series-injection transformer. The controller of the device works properly and the load voltages are perfectly compensated and equal to 1 pu, as shown in Fig.7.
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
0.11
b 0.8
c
0.13
0.14
0.15
0.16 0.17 time [s]
0.18
0.19
0.2
0.21
Fig.7. Load voltages.
a
0.6
B. Distribution system faults
0.4
The second set of results is obtained by simulating a dip due to a fault in the distribution system. In this case, the STS should operate to switch over the alternative source and transfer the load. Depending on the system configuration and on the dip type, the load transfer can take up to one period, due to low power factor of the load, high phase displacement between preferred and alternative source [11], high percentage of motor load in the power system [12][13]. The load will thus be affected by the dip for a duration equal to the necessary transfer time. In this case, the SSC can be used to inject voltage in the grid and partially cover the voltage dip due to the transfer delay. If the dip is due to a fault in the distribution system far from the location of the sensitive load, it should not be very deep and the SSC might be able to inject the necessary voltage to fully compensate for it. But if the dip is due to a fault in the local distribution system, its magnitude can be very low and
0.2
voltage [pu]
0.12
0
−0.2
−0.4
−0.6
−0.8
0.11
0.12
0.13
0.14
0.15
0.16 0.17 time [s]
0.18
0.19
0.2
0.21
Fig.5. Supply voltages during an unbalanced fault in the transmission system.
therefore an SSC with lower rating then 50% will not be able to restore the voltage to 1 pu. An example in which the dip magnitude is 30% and the SSC rating is 30% is shown in Fig.8.
1
c
b
0.8
a
0.6
voltage [%] voltage [pu]
0.4
100%
0.2 0 −0.2 −0.4 −0.6
60%
covered by the SSC
−0.8 −1 0.08
30%
0.085
0.09
0.095
0.1
0.105 time [s]
0.11
0.115
0.12
0.125
Fig.10. Load voltage during the load transfer by the STS.
C. Series compensation strategies
time [s]
transfer interval Fig.8. Voltage dip during load transfer by the STS.
An unbalanced voltage dip due to an unsymmetrical threephase fault in the distribution system has been simulated: the fault occurs at 100 ms and lasts for 250 ms. During the dip the voltage of phase a goes down to 0.35 pu, while the other two phases are affected by a drop of 0.1 pu. The power factor of the load has been set to 0.1. Due to the low power factor of the load, the STS takes 4.8 ms (at 50 Hz) to transfer the load to the alternative source, as shown by the voltages at the input of the SSC in Fig.9. The load voltage, shown in Fig.10, is still affected by a dip, but due to the presence of the series compensator it is partially mitigated. However, the load voltage is affected by an overvoltage when the transfer to the alternative source is completed.
c
It has been shown that, due to the reduced rating of the SSC, the device can be unable to restore the voltage of the load to 1 pu for a dip due to a fault in the distribution system, during the time necessary for the STS to complete the transfer. This is also shown by the phasor diagram in Fig.11. The phasor Vinj in Fig.11 represents the voltage injected with the controller currently implemented. The phasor Vload,pre indicates the load voltage in the pre-fault conditions, Vload,comp is the compensated load voltage, Vdip is the phasor of the supply voltage during the dip, Ψ is the phase-angle jump associated to the dip. There is the possibility of implementing different control strategies depending on the requirements of the load. If Vinj is the maximum voltage that the device can inject in the grid, each radius of the circle called “compensation region” in Fig.11 is suitable for compensation. However, none of these phasors allows achieving perfect compensation of the dip (1.0 pu with the same phase as the pre-fault voltage).
b
Vload,pre
0.8
Vdip
a
0.4
voltage [pu]
0.2
Vinj
compensation region
0 −0.2
Fig.11. Phasor diagram of the principle of the series compensation.
−0.4 −0.6 −0.8
0.08
Vload,comp
Ψ
0.6
0.085
0.09
0.095
0.1
0.105 time [s]
0.11
Fig.9. Input voltages to the SSC.
0.115
0.12
0.125
An alternative is to inject the necessary voltage to restore the phase of the load voltage, neglecting its magnitude. From Fig.12 it is clear that this kind of compensation is possible only if the compensation region intersects the direction of the load voltage in the pre-fault conditions. This compensation technique is called here Constant Phase Injection (CPI). This kind of injection technique is suitable for those loads that
require constant phase angle of the voltage for correct operation like, for example, thyristor rectifiers [14]. Vload,comp Ψ
Vdip
Vload,pre Vinj
compensation region Fig.12. Voltage injection using Constant Phase Injection.
If the load is sensitive only to the magnitude of the voltage, a solution is to inject a voltage that has the same phase of the dip voltage (Fig.13). The load voltage will thus be affected by a phase-angle jump equal to the phase-angle jump of the voltage dip, but its amplitude will be maximized (possibly up to 1 pu). This kind of compensation technique is called here Maximum Amplitude Injection (MAI).
Ψ
Vload,pre Vdip
Vload,comp
Vinj
compensation region
Fig.13. Voltage injection using Maximum Amplitude Injection.
Finally, if the circle with radius equal to the amplitude of the pre-fault load voltage intersects the compensation region, it is in principle possible to restore the load voltage to 1 pu and to reduce its phase angle jump. This control strategy, shown in Fig.14, is called here Amplitude Restoration Injection (ARI).
Vload,pre Ψ
Vload,comp Vdip Vinj
compensation region Fig.14. Voltage injection using Amplitude Restoration Injection.
It is clear from Fig.14 that this is possible when the voltage dip is shallow and the phase-angle jump associated to the event is big.
To limit the impact of the dips on the most sensitive loads, it can be important to identify and adopt the most suitable strategy for the requirements of the load.
IV. ECONOMIC EVALUATION The proposed hybrid device is likely to cost more of each of two devices separately, but will also be more effective. Assume for example that for a facility, 62% of process disruptions are due to short interruptions and voltage dips caused by faults on the distribution system and the rest are due to dips caused by faults on the transmission system [9]. Based on the number of dips per year and the effectiveness of the two solutions (STS and SSC), one could decide to install one of the two devices. This may depend very much on the cost of the process disruption. Assume that the cost of the STS for 10 MVA load is 600 000 USD [15], including losses and maintenance calculated on the expected lifetime of the equipment. The cost of the SSC, according to [15], is 300 USD/kVA, when it is sized for 50% voltage injection and 500 ms dip duration. For the whole facility rated 10 MVA, the total cost of the second solution would thus amount to 3 000 000 USD (again including losses and maintenance calculated on the expected lifetime of the equipment). Solution 1: Assume that the STS can save the plant from shutdown in 60 % of the total power quality events during one year. If the cost of the STS is CSTS, the cost of a production interruption is Cint and their number nint, and the pay-back time for the investment is denoted as Tpayback,
0.6 ⋅ Cint ⋅ nint ⋅ Tpayback = CSTS With CSTS=600 000 USD and Cint =100 000 $
nint ⋅Tpayback =
600000 = 10 0.6 × 100000
i.e. with 10 interruptions a year the investment would pay back within one year or, which is the same, if a payback time of e.g. two years is accepted, the balance is reached for 5 interruptions a year in average. Solution 2: assume the SSC to be able to compensate for 75 % of the power quality events causing process disruption during one year. It is in fact reasonable to assume that a SSC with 50 % voltage injection capability would be able to compensate not only for the transmission-related dips, but also for part of the distribution-related ones. If the cost of the SSC is CSSC, then
0.75 ⋅ Cint ⋅ nint ⋅ Tpayback = CSSC Assuming CSSC=3 000 000 USD, Cint =100 000 $ yields
nint ⋅Tpayback =
3000000 = 40 0.75 × 100000
i.e. with 40 interruptions a year the investment would pay back within one year. On the other hand, if a payback time of e.g.
two years is accepted, the balance is reached for 20 interruptions a year in average. With 10 interruptions a year, the payback time is 1 year for the STS and 4 for the SSC. Solution 3: Assume that by reducing the voltage injection of the SSC down to 30 % and by combining it with the STS, 100 % coverage of the critical power quality events for the plant is reached. Moreover, assume that the cost of the SSC varies proportionally with the voltage injection: this is reasonable because when reducing the maximum injected voltage we reduce the size of both the converter and the injection transformer. With the same symbols used before:
Cint ⋅ nint ⋅ Tpayback = CSTS +
30 CSSC 50
and with the same values used before we have
nint ⋅Tpayback =
600000 + 1800000 = 24 100000
Moreover, if the SSC compensates for short dips, the size of the storage can also be reduced. Assume that the duration is reduced down to 100 ms and that the cost also reduces proportionally, i.e.
Cint ⋅ nint ⋅ Tpayback = CSTS +
30 100 CSSC 50 500
With the same values as before
nint ⋅Tpayback =
600000 + 360000 = 9 .6 100000
which now becomes the most economical solution. However, one has to keep in mind that the most economical solution must be found on a case-by-case base, depending on the cost of the process disruption and the statistical distribution of events that the load can be subjected to. V. CONCLUSIONS This paper has proposed a hybrid compensator for protection of large industrial distribution systems against voltage dips and short interruptions. The device is based on the combination of a Static Series Compensator (STS) in series with a Static Series Compensator (SSC). This configuration takes advantage of the different operating characteristics of the two devices, and results in total coverage of critical power quality events. Design and control strategies for the device have been discussed. It has been demonstrated that, depending on the statistical distribution of events leading to disruption of the process and on the cost of plant outages, this solution can prove more cost-effective than the application of just one of the two devices. VI. REFERENCES [1] [2]
N. Hingorani, “Introducing Custom Power,” IEEE Spectrum, vol.32, no.6, June 1995, pp.41-48. A. Campbell, R. McHattie, “Backfilling the sinewave. A dynamic voltage restorer case study,” Power Engineering Journal, vol.13, no.3,
[3] [4] [5] [6] [7]
[8] [9]
[10] [11] [12] [13] [14] [15]
June 1999, pp.153-158. T. Davis, G.E. Beam, C.J. Melhorn, “Voltage sags: their impact on the utility and industrial customers,” IEEE Trans. on Industry Applications, vol.34, no.3, May-June 1998, pp.549-558. E.R. Collins Jr., S.W. Middlekauff, “System and customer impact: considerations for series custom power devices,” IEEE Trans. on Power Delivery, vol.13, no.1, January 1998, pp.278-282. J.W. Schwartzenberg, R.W. De Doncker, “15 kV medium voltage static transfer switch,” in Proc. of IEEE Industry Applications Society Annual Meeting 1995, vol.3, pp.2515–2520. N. Woodley, L. Morgan, A. Sundaram, “Experience with an inverterbased dynamic voltage restorer,” IEEE Trans. on Power Delivery, vol.14, no.3, July 1999, pp.1181-1184. Bill Carter, “A Static Transfer Switch (STS) Application to Enhance Power Quality at an Automobile Components Plant,” Panel Session on “Application of Static Transfer Switches for Enhanced Power Quality,” IEEE Power Engineering Society Winter Meeting 1998. M.H.J. Bollen, Understanding power quality problems: voltage sags and interruptions, New York, IEEE Press, 1999. CIGRE WG 36.07, “Power Quality Indices and Objectives for MV, HV and EHV systems”, to be presented at the 17th International Conference on Electricity Distribution, CIRED 2003, Barcelona, Spain, 12-15 May, 2003. H. Awad, J. Svensson, “Double Vector Control for Series Connected Voltage Source Converters,” in Proc. of IEEE Power Engineering Society Winter Meeting 2002, vol.2, pp. 707-712. A. Sannino, “Static Transfer Switch: analysis of switching conditions and actual transfer time”, in Proc. of IEEE Power Engineering Society Winter Meeting 2001, vol.1, pp.120-125. H. Mokhtari, S.B. Dewan, M.R. Iravani, “Effect of regenerative loads on a static transfer switch performance,” IEEE Trans. on Power Delivery, vol.16, no 4, Oct. 2001, pp. 619-624. A. Sannino, “Power quality improvement in an industrial plant with motor load by installing a static transfer switch,” in Proc. of IEEE Industry Applications Society Annual Meeting 2001, vol.2, pp.782-788. N.S. Tunaboylu, E.R. Collins, Jr., S.W. Middlekauff, R.L. Morgan, “Ride-through issues for DC motor drives during voltage sags,” in Proc. of IEEE Southeastcon ‘95, Visualize the Future, 1995, pp.52-58. M. McGranaghan, B. Roettger, “Economic evaluation of power quality,” IEEE Power Engineering Review, vol.22, no 2, Feb.2002, pp.8-12.
VII. BIOGRAPHIES Massimo Bongiorno (S’ 02) received the M.Sc. degree from the University of Palermo, Italy, in April 2002. From September to December 2001 he was a Guest Researcher at the Department of Electric Power Engineering of Chalmers University of Technology, Gothenburg, Sweden, where he is currently a PhD student since September 2002. His interests include applications of power electronic in power systems and power quality. Ambra Sannino (S’ 99, M’ 01) received the M. Sc. and Ph.D. degrees from the University of Palermo, Italy in April 1997 and February 2001, respectively. She has been working as a trainee at ABB Corporate Research Center, Heidelberg, Germany from April to September 1998. From August 1999 to September 2000 she was a guest researcher at the Department of Electric Power Engineering of Chalmers University of Technology, Gothenburg, Sweden, where she is currently working as Assistant Professor. Her interests include applications of power electronics in power systems and power quality. Luigi Dusonchet received the Doctor’ s degree in Electrical Engineering from the University of Palermo, Italy, in 1975. Since 1978 to 1990 he has been Associate Professor and now he is Full Professor of Industrial Electrical Systems at the Faculty of Engineering of the University of Palermo. His main research interests are in the following fields: simulation of electrical power system; transmission over long distances; mixed three-phase/six-phase power system analysis; optimization methods in electrical distribution system’ s design and operation; distribution automation; applications of power electronics in power systems and power quality.
