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Frequency Control by Aluminum Smelter Load Response in an Isolated Wind Power System: a Study for an Industrial Case Hao Jiang, Student Member, IEEE, Jin Lin, Member, IEEE, Yong-hua Song, Fellow, IEEE, Xiao-min Li and Jian-xun Dong geography distribution, this solution is attractive for China’s industry. This is because most wind resources in China are located in the western provinces. And coincidently, abundant of aluminum ore and coal, as well as aluminum smelter loads, are also located in the similar regions. The wind resources can hence geographically match with aluminum and coal resources. However, from the technical aspect, the wind solution becomes technically difficult. It is because power systems which supply for smelter loads are mostly isolated from interconnected power grid. Owners of smelter loads intend to be isolated since aluminum ores are usually far away from load center and grid companies have to charge high prices in order to compensate for investments on long transmission systems. Considering the electricity cost takes up to 50% of the entire cost of aluminum production, owners prefer to construct coalfired power plants to self-supply for smelter loads locally to pursue for higher profit return in aluminum market. In an isolated system, frequency stability is the most critical Index Terms—Demand response, wind power, frequency technical issue. And the issue is becoming even worse if high stability, isolated power system, self-saturable reactor percentages of wind power are then integrated. Stochastic wind power outputs have great impacts on operations of I. INTRODUCTION LUMINUM is one of the most commonly-used metals in isolated systems. That results in significant over/under the world. As introduced in [1], the aluminum production frequency deviations, which might cause tripping of generators is relying on the process of electricity smelting to transform and critical loads. Thus, wind power integrations in an oxides of aluminum into metal aluminum. This smelting isolated system for aluminum smelter are hence becoming process, however, consumes giant amount of electricity energy. difficult if frequency issue cannot be suitably mitigated. In order to address the issue, this paper proposes a demandTaking China as an example, China produces 40% of primary side frequency control scheme, which uses self-saturable aluminum of the world and the aluminum industry consumes reactors to control power consumptions of aluminum smelter around 8% electricity energy of China annually. Due to such loads responding to frequency deviations of isolated systems. high energy consumption, aluminum industry has been We expect to enable smelter loads participate within frequency categorized into an energy-intensive industry by China’s regulations of an actual industrial power system to prevent government [2]. And how to reduce the fossil consumption during smelting is of significantly meaningful for China’s over/under frequency tripping during system transient. The energy section in order to fulfill the ambitious energy problem of that is formulated in Section II. This demand-side response scheme proposed in Section III has never been reduction target in 2016 [3]. One solution is to supply for part of smelter loads by wind studied by existing published literatures before, but it is still power for aluminum production. From the aspect of energy designed based on previous research works such as the smelter load modeling and flexibility studies in [5] and [6]; studies of This work was supported by National High-Technology Research and harmonics due to smelter loads in [7] and [8]; passive filters design studies in [9] and [10]; and some fault diagnosis studies Development Program ("863" Program) of China (SS2012AA050218). Hao Jiang, Jin Lin and Yong-hua Song are from State Key Laboratory of in [11]. A numerical case study in Section IV demonstrates the Power Systems, Department of Electrical Engineering, Tsinghua University, effectiveness of the proposed control scheme applied in an Beijing 100084, P.R. China. (e-mail:
[email protected]). industrial case system with considering wind power outputs. Xiao-min Li and Jian-xun Dong are from China Power Investment Abstract-- Reducing fossil consumptions of aluminum smelting productions, which consumes around 8% electricity annually in China, is of significantly meaningful for China’s energy section. One possible solution is by integrating wind power to supply for part of aluminum smelter loads. Based on this background, the paper studies an actual industrial case which is an isolated power system with as high as 30% wind penetration level integrated for aluminum production. After the integration of wind power, frequency stability issues become critical in such a system. This paper hence proposes a demand-side frequency control scheme, which uses self-saturable reactors to control load power of aluminum smelters responding to frequency deviations of this system. The scheme is designed based on the relationship between the input DC voltage of smelting loads and the commutation voltage drop which can be adjusted by self-saturable reactors. A numerical case study demonstrates the effectiveness of the proposed control scheme under the scenario considering with wind power outputs.
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Corporation, Beijing 100140, P.R. China.
978-1-4799-1303-9/13/$31.00 ©2013 IEEE
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II. PROBLEM FORMULATION A. The features of an Isolated Industrial Power System The study case of this paper is from an actual power system which is located in Inner Mongolian invested and owned by China Power Investment Corporation. The single line diagram of this system is shown in Fig. 1 and its main features are described as follows.
Fig. 1. Main structure of the isolated industrial power grid
1) Synchronous generators. There are 8 coal-fired synchronous generators in total. The total generation capacity is 1800MW (G3, G4: 100MW, G5, G6: 150MW, G7, G8: 300MW, G9, G10: 350MW). 2) Loads. The main load of this system is aluminum smelter loads ASL1, ASL2, ASL3, 1400MW in total. 3) Wind power. Two 400MW wind farms WF1 and WF2 with DFIG (Type 3) are connected to the system. B. Aluminum Smelter Configuration Before the discussion on frequency issues, we must study the configuration of the aluminum smelter loads in advanced which are the dominant loads of this system. Taking the aluminum smelter load ASL1 as example, it can be equivalently represented by Fig. 2, in which a 6-branchparallel rectifier is used to generate DC voltage output U L to supply for aluminum smelter. In Fig. 2, each branch consists of two full-bridge rectifiers with self-saturable reactors S1~S6 deployed between the grid and diode bridges. By selecting
paralleled branches, namely one series of smelter load, also must be tripped in together. This is because if only one branch is tripped, power flow of the tripped branch would transfer to the other paralleled branches. Thus, power consumptions of smelter loads are not reduced and almost have no contribution for power balance rebuilding. Therefore, one series of smelter load must be tripped entirely if there is significant power imbalance existing in the system. C. Frequency Stability Issue due to Wind Power Integration If without wind power integration, system operators usually dispatch power outputs of the generators trying to match with power inputs of smelter loads. For example, power outputs of G7~G10 are usually set around 300MW so that ASL1 load, which consumes at around 350MW, can be tripped correspondingly to prevent from under-frequency accident during transient if one generator of G7~G10 is tripped. However, if high percentages of wind power are integrated, power outputs of synchronous generators have to be adjusted according to stochastic wind power outputs. During high wind season, the synchronous generators must reduce their outputs; in contrast, they raise their outputs during low wind season. The adjustment significantly increases the technical difficulties of load-shedding during transient. One example is as shown in Fig. 3, which illustrates frequency transient of the isolated power system while the power outputs from WF1 and WF2 are at 300 MW. From the solid line in Fig. 3, after G7 at 200 MW accidently trips, the system frequency rapidly falls to 48.5Hz around if without any smelter load tripping action for power balances. This results in under-frequency tripping of other generators and the entire system then collapses. But if smelter load ASL1 is tripped to maintain frequency stability, system frequency then rushes up to as high as 51Hz from the dashed line in Fig. 3. This results in over-frequency protection of other generators. Therefore, there is no suitable scheme now after the integration of wind power. We have to develop other methods which are able to address the frequency issues during transient in a “smoother” manner.
proper phase shift ( ±12.5 , ±7.5 , ±2.5 in this case) for each paralleled branch, the pulse magnitude of DC output U L can o
o
be significantly suppressed to ensure better power quality for aluminum production [7].
51.5
With ASL1 tripping
51
50.5
f (Hz)
o
50
Without ASL1 tripping
49.5
49
48.5 0
S1
S2
S4
S3
S5
S6
2
4
6
8
10
t (s)
12
14
16
18
20
Fig. 3. System frequency with and without ASL1 tripping
III. FREQUENCY CONTROL BY ALUMINUM SMELTER LOAD RESPONSE E
UL
RL
Fig. 2. Electrical diagram of aluminum loads ASL1 in Fig. 1
Notice that the six branches must be paralleled in normal state for better power quality. During system transient, the six-
This section proposes a “smoother” control strategy for addressing the frequency stability issue as explained in Section II by using self-saturable reactors S1~S6 of the aluminum smelters. The reason for selecting smelter load responding to
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frequency deviation other than wind power plants is that frequency regulation capabilities from wind power [12] are varied with their wind speed inputs. And wind power plants can only provide temporal active power support but cannot permanently maintain increased power outputs to fill the gap between generation and consumption [13]. A. The Simplified Model Representation of Smelter Loads The full model of aluminum smelter loads is as shown in Fig. 2 which is a bit too complicated to be used. We need to have a simplified model in order to derive a practical control scheme. According to [14], if harmonics and DC pulse factors are neglected, the smelter load can be represented by a simplified model as shown in Fig. 4 in which six paralleled branches are equivalent to one individual full bridge with six diodes D1~D6 and with six self-saturable reactance SR1~SR6 deployed before the diode at each arm. The DC side components remain as the same as shown in Fig. 2 consisting of the EMF (electromotive force) E and the resistance load RL . The simplified model representation of smelter loads is applied in the next sections for further studies.
reactor value L of SR1, which is proportional to the slope of the B − H curve in Fig. 5, is at the largest state. The operation point of SR1 moves from the operation point N to I circularly for every voltage cycle, the period of which is 20ms (1/50Hz) or 16.67ms (1/60Hz) in terms of different grid frequency. The analysis from the aspect of magnetism is thorough but much too complicated for control practice. Considering the equivalent reactor value L is changed circularly between the operation point N and I, this value L can hence be simplified from the equivalent circuit point of view [16]. According to [17], the reactor value L can be calculated by the following equation:
L = K1e
K2 Ic
I c K3
(1)
where K1, K2, K3 are constants, Ic is the magnitude of DC current in the control winding CW1. So from the circuit point of view, the self-saturable reactor can be represented as a controlled reactance and its reactance value is controlled by the magnitude of DC current I C in the control winding CW1.
B. The Feature of Self-saturable reactors The power output of smelter load can be adjusted by changing the DC voltage U L over the resistant RL which can
D1
D3
D5
SR1
SR3
SR5
+
IL E
be assumed as constant during transient. However, the main component for rectifying is the full bridge which is diodebased as shown in Fig. 4. The diode-based rectifier lacks the capability to adjust the DC output U L . Therefore, the only
UL
RL
controllable component for DC voltage U L adjustment is selfsaturable reactor SR1~SR6 in the smelter load. Taking SR1 as an example, its detailed model is represented in Fig. 5, in which there are two windings wounded on the same iron core IC1: the control winding CW1 connected with a controllable DC source and the power winding PW1 connected between the grid node A and the diode D1. From the view of magnetism, there are two operating points (namely point N and point I on the B − H curve as shown in Fig. 7) of the self-saturable reactor SR1 with different states of the diode D1. 1) Point N. If the diode D1 is forward biased, D1 is turned on so there is current I P in the power winding PW1. Since the
−
D4
D6
D2
SR4
SR6
SR2
−
Fig. 4. Topology of three-phase full-bridge rectifier and structure of selfsaturable reactors
Rc
Uc
IC1
E
Ic CW1
+
D1
PW1
IP
IL
UL
RL
− Fig. 5. Detailed model of self-saturable reactor SR1
current I P is much higher than the DC current I C in control winding SR1. The iron core IC1 is then deeply magnetized and the self-saturable reactor SR1 is working at the point N. At the operation point N, the equivalent reactor value L of SR1, which is proportional to the slope of the B − H curve in Fig. 6, is at the smallest state. 2) Point I. If the diode D1 is backward biased, D1 is turned off and there is no current in the power winding SR1. The iron core IC1 can be demagnetized by controlling the DC current I C in the controlling winding CW1. The self-saturable reactor SR1 is hence working at a different operation point I, which is decided by the magnitude of I C . At this point, the equivalent
Fig. 6. Typical curve of an iron core used in self-saturable reactor
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C. Active Power Control by Self-Saturable Reactors The feature described in Section III.B that the equivalent reactor value L can be controlled by the current I c is thus applicable for controlling the DC output voltage U L . Qualitatively, it is because different reactor value can change the current commutation angle of full-bridge rectifier. Fig. 7 demonstrates the influence by illustrating the DC output voltage U L varying with reactor values. The magnitude of U L is reduced due to the delayed current commutation caused by the reactor. UL
Fig. 7. Output DC voltage of rectifier influenced by reactor
Quantitatively, Equation (2) shows the relationship between U L and reactor value L [18]:
U L = 1.35U L − L −
1.35U L − L − E π RL +1 3ω LSR
(2)
IV. NUMERICAL CASE STUDY This section presents two numerical simulation results to demonstrate the system transients after the synchronous generator G7 accidently trips off from the system under the scenarios while wind power outputs from WF1 and WF2 are at 300MW. Also we compare with the performances of grid frequency regulation with and without the participation of smelter load ASL1~ASL3 by the proposed control scheme via self-saturable reactors in this section. Fig. 9 presents the grid frequency f and the sum of active power consumption PASL=PASL1+PASL2+PASL3 of the smelter load ASL1~ASL3 without deployed with the frequency controller proposed in Fig. 8. After G7 trips, the grid frequency f rapidly dips down to around 48.5Hz as shown in Fig. 9 (a) since the power consumption PASL reduces 6.4% off from 1400MW to only 1310MW. This passive reduction is due to the voltage variation during the system transient due to the trip of G7 other than the active power reduction responding to the frequency variation. As mentioned in Section II, even though the grid frequency f rises to 49.4Hz at last, the frequency nadir at around 48.5Hz still might result in underfrequency tripping of other generators and then causes the collapse of the entire system. 50
where U L − L is the line-line voltage of the grid side, LSR is the
by aluminum production can be calculated with (3): U −E ⋅U L P= L RL
49.6
f (Hz)
reactor value of self-saturable reactors, ω is grid angular frequency, E is magnitude of EMF. As mentioned in Section III.A, the smelter loads can be simply represented as a resistance RL and an EMF E , so the power consumption P
49.8
49.4 49.2 49 48.8 48.6 0
(3)
2
4
6
8
10
t (s)
12
14
16
18
20
14
16
18
20
(a) System frequency 14.1
with the frequency reference f ref and the measurement f . Then the frequency error is then passed through a PI controller. Then the controller adjusts the reactance value of selfsaturable reactor so the active power consumption by aluminum smelters is changed. It should be noticed that the PI controller has a limiter within itself to prevent large change to the reactance value because the DC output voltage must be in a proper range. f ref +
Δf −
f
I Lref +
+
−
IL
ΔI L
I Cref + +
ΔI C
IC
ΔU L = f ( I c )
ΔU L
+
−
UL
+
1.35U L − L
−
1 R
IL
UL
×
E
Fig. 8. Block diagram of proposed isolated system and its control strategy
P
(MW)
13.8
ASL
controlling windings so that the smelter load can participate within the frequency regulation responding to the primary grid frequency regulation. Fig. 8 shows the block diagram of the proposed control scheme applied for the isolated power. In Fig. 8, the frequency deviation Δf is calculated by comparing
14 13.9
P
D. Frequency Control Scheme by Self-saturable Reactor From the explanations in Section III.A-C, the active power of smelter load can be controlled by the current I c in
13.7 13.6 13.5 13.4 13.3 13.2 13.1 0
2
4
6
8
10
t (s)
12
(b) Active power consumption PASL Fig. 9. System frequency f and active power consumption of PASL without self-saturable reactors control scheme
By comparing with the simulation results in Fig. 9, Fig. 10 presents the grid frequency f and the sum of the active power consumption PASL with considering the deployment of the proposed frequency controller. As shown in Fig. 10 (a), the nadir of the grid frequency f is lifted to around 49.8 Hz comparing with the corresponding simulation in Fig. 9 (a). This is because the power consumption PASL actively reduces 11.4% off from 1400MW to 1240MW. Notice that the power reduction is 70MW higher than the case shown in Fig. 10 (b). Therefore, it demonstrates that the deployed frequency controllers in ASL1~ASL3 are activated after detecting the dipping of grid frequency. And the isolated system with wind power integrated can survive after the tripping of G7 since the
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frequency nadir at 49.8Hz is absolutely acceptable for this system. 50.05
[4] [5]
f (Hz)
50
[6]
49.95
49.9
[7]
49.85
49.8 0
2
4
6
8
10
t (s)
12
14
16
18
20
(a) System frequency
[8]
14.4 14.2 14
PASL (MW)
13.8
[9]
13.6 13.4 13.2
[10]
13 12.8 12.6 12.4 0
2
4
6
8
10
t (s)
12
14
16
18
20
(b) Active power consumption PASL Fig. 10. System frequency and active power consumption of ASL1 with selfsaturable reactors control scheme
[11]
[12]
V. CONCLUSIONS This paper proposes a demand-side frequency control scheme for addressing the frequency stability issue in an isolated industrial power system. This scheme is by controlling self-saturable reactors to reduce the DC voltage as well as the active power of smelting loads. We present the numerical cases under the scenario while wind power outputs are at 300MW. Though this case is simple, it is still sufficient to demonstrate the under-frequency tripping risk of the studied system and the effectiveness of the proposed controller against frequency deviations. In future works, we will pay more efforts for simulating several different scenarios involving more different wind power outputs and system fault types. The proposed controller is mainly for the primary frequency regulation. But, we also notice that OLTC transformers of smelter loads have similar capabilities of participating within frequency regulations. A study of coordinated frequency control schemes has also been under planning by the authors which will include both primary and secondary frequency regulation by self-saturable reactors together with OLTC transformers. VI. REFERENCES [1] [2]
[3]
B. Voller, “Early aluminum production in the Pacific Northwest [History],” IEEE Ind. Appl. Mag., vol. 16, no. 3, pp. 10–12, Jun. 2010. National Development and Reform Commission, “The Notice on the Tariff Concession Cancellation of the Aluminum High-EnergyConsuming Industries”, National Develop. and Reform Commission: Beijing, China. [Online]. Available: http://www.sdpc.gov.cn/zcfb/ zcfbtz/2007tongzhi/t20080222_193192.htm Joanna Lewis, “Energy and Climate Goals of China's 12th Five-Year Plan,” Center for climate and energy solutions, Arlington, VA. [Online]. Available: http://www.c2es.org/docUploads/energy-climate-goals-chinatwelfth-five-year-plan.pdf
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[18]
Statistics of primary aluminum production [Online]. Available: http://www.world-aluminium.org/statistics/primary-aluminiumproduction/. A. Molina, A. Gabaldon, F. Faura, and J. A. Fuentes, “New approaches to model electric demand in aluminium smelter industry,” in ThirtySixth IAS Annual Meeting 2001, vol. 2, pp. 1426–1431. A. M. Garcia, M. Kessler, M. C. Bueso, J. A. Fuentes, E. G. Lazaro, and F. Faura, “Modeling aluminum smelter plants using sliced inverse regression with a view towards load flexibility,” IEEE Trans. Power Syst., vol. 26, no. 1, pp. 282–293, Feb. 2011. J. Jiang, B. H. Zhang, Z. G. Hao, Y. Y. Yuan, Z. Q. Bo, and A. Klimek, “Study on factors affecting the harmonics of large-scale rectification device in power grid,” in 45th International Universities Power Engineering Conference (UPEC) 2010, pp. 1–5. S. Perera, V. J. Gosbell, D. Mannix, and N. Gersch, “Investigation into the harmonic behavior of multipulse converter systems in an aluminium smelter,” in Proceedings Australasian Universities Power Engineering Conference AUPEC 2000, pp. 178–184. B. Badrzadeh, K. S. Smith, and R. C. Wilson, “Designing Passive Harmonic Filters for an Aluminum Smelting Plant,” IEEE Trans. Ind. Appl., vol. 47, no. 2, pp. 973–983, Apr. 2011. M. Ermis, A. Acik, B. Gultekin, A. Terciyanli, E. Nalcaci, I. Cadirci, N. Ozay, C. Ermis, M. Gokmen, and H. Kiziltan, “Power quality solutions for 12-pulse smelter converters in ETI aluminum works,” IEEE Trans. Ind. Appl., vol. 40, no. 6, pp. 1644–1655, Dec. 2004. A. M. R. Amaral and A. J. M. Cardoso, “State condition estimation of aluminum electrolytic capacitors used on the primary side of ATX power supplies,” in 35th Annual Conference of IEEE Industrial Electronics, 2009, pp. 442–447. S. Yuan-zhang, L. Jin, and Z. Zhao-sui, “Frequency-security based operations in an industry power grid: Technical issues considering wind power integration,” in 2012 IEEE Power and Energy Society General Meeting, 2012, pp. 1 –8. I.D. Margaris, S.A. Papathanassiou, et al. “Frequency Control in Autonomous Power Systems With High Wind Power Penetration,” , IEEE Trans. Sustain. Energy, vol.3, no.2, pp.189-199, April 2012 A. P. Agalgaonkar, K. M. Muttaqi, and S. Perera, “Response analysis of saturable reactors and tap changer in an aluminium smelting plant,” in International Conf. on Power Syst. 2009, pp. 1–6. Shenyang Design Institute of Aluminum and Magnesium, “Electrical design of silicon rectifier station,” (in Chinese) Beijing: Metallurgy and Industry Press, 1983, p. 8. W. H. Hayt, J. E. Kemmerly, and S. M. Durbin, Engineering circuit analysis. New York: McGraw-Hill, 1986. S. B. Abbott, D. A. Robinson, S. Perera, F. A. Darmann, C. J. Hawley, and T. P. Beales, “Simulation of HTS saturable core-type FCLs for MV distribution systems,” IEEE Trans. Power Del., vol. 21, no. 2, pp. 1013–1018, Apr. 2006. N. Mohan, T.M. Undeland and W.P. Robbins, “Power electronics: converters, applications, and design,” Beijing: Higher Education Press, 2010, pp. 106-108.
VII. BIOGRAPHIES Hao Jiang (S’ 12) was born in Tangshan China, 1987. He received the B.S. degree in Department of Electrical Engineering, Tsinghua University, China, in 2010. He is now a Ph. D. candidate in Department of Electrical Engineering, Tsinghua University, China. His research interests include wind power control and integration. Yong-hua Song (F’ 08) was born in January 1964. He received his BEng and PhD from Chengdu University of Science and Technology, and China Electric Power Research Institute in 1984 and 1989 respectively. He was a Postdoctoral Fellow at Tsinghua University from June 1989 to March 1991. He then held various positions at Bristol University, Bath University and John Moores University from 1991 to 1996. In January 1997, he was appointed Professor of Power Systems at Brunel University where he was Pro-Vice Chancellor for Graduate Studies from August 2004. In 2004, he was elected Fellow of the Royal Academy of Engineering (UK). In 2008, he was elected Fellow of the Institute of Electrical and Electronics Engineers (USA). He returned to Tsinghua University in February 2009 as a Professor at the Department of Electrical Engineering. His research areas include Smart Grid, electricity economics, and operation and control of power systems.
