2014 IEEE International Conference Power & Energy (PECON)
Coordination of Overcurrent Relays Protection Systems for Wind Power Plants Nima Rezaei Rezaei 1, 2, *; Mohammad Lutfi Othman 1, 2; Noor Izzri Abdul Wahab 1, 2; Hashim Hizam 1, 2 1 2
Department of Electrical & Electronic Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
Centre for Advanced Power and Energy Research (CAPER), Universiti Putra Malaysia, 43400 S erdang, Selangor, Malaysia *
Corresponding Author: Nima Rezaei, E mail:
[email protected]
implement simple protection schemes which leads to different levels of damages to power components in the plant. Moreover, most of the researches conducted regarding wind farm protection has been abundantly restricted to literatures and methodologies [3 - 5]. Some researchers have been studied the effect of fault on wind plants specially the generators and have investigated investigated the effectiveness of crowbars in protecting the wind turbine generators [6]. However an overall protection scheme has yet to come to solve the protection crisis in wind plants.
Abstract-Wind Abstract-Wind farms are one of the most indispensable types of sustainable energies which are progressively engaged in smart grids with tenacity of electrical power generation predominantly as a distribution generation system. Thus, rigorous protection of wind power plants is an immensely momentous aspect in electrical power protection engineering which must be contemplated thoroughly during designing the wind plants to afford a proper protection for power components in case of fault occurrence. The most commodious and common protection apparatus are overcurrent relays which are responsible for protecting power systems from impending faults. In order to employ a prosperous and proper protection for wind power plants, these relays must be set precisely and well coordinated with each other to clear the faults at the system in the shortest possible time. This paper indicates how the coordination of overcurrent relays can be effectively attained for wind power plants in order to protect the power constituents during fault incidence. Through this research Matlab/Simulink as a powerful simulation software have been applied to model a wind farm and achieve precise setting for coordination of overcurrent relays.
One of the most important studies of power quality and power system protection in wind plants is providing adequate and continual power to the loads, therefore in order to ensure having perpetual power from wind farms, wind plants must feed grids continually. One way of meeting this phenomena is applying a proper protection in the system that in case of fault, only the section of faulty feeder is disconnected from the system and the rest of healthy parts are kept connected to the system. By using overcurrent relays (OCRs) as a protection system and applying an accurate coordination in wind plants, not only in case of fault, the power components are protected from damages from excessive currents but also continual power flow is fed to the grid and superb power quality is provided by wind power power plants.
Keywords-Overcurrent Relay, Coordination of Overcurrent Relay, Wind Power Plant, Power System Protection
I.
I NTRODUCTION
The ever increasingly air pollution rate and the limitation of fossil fuel sources have led to comprehensive implementation of renewable energies specifically wind energy. Wind power plants have been vastly employed as the means of power generation in smart grids as a distribution generation (DG) system [1]. Undoubtedly, wind power has come to be mainstay of the energy systems in several countries and is regarded as a reliable and financially reasonable source of electricity. The contribution of wind energy to power generation has reached a considerable share even on the worldwide level. Among many countries that are investing hugely on wind power generation, generation, the top 10 leading nations in total power generation capacity are: China, USA, Germany, Spain, India, United Kingdom, Italy, France, Canada and Portugal [2].
This paper demonstrates how OCRs have been successfully used and properly coordinated in a wind power plant. The software which has been used is Matlab/Simulink which is known as one of the best simulation software for electrical engineers and researchers. All of the OCRs have been modelled and designed and the accurate settings have been selected to protect the wind wind plant. Section 2 of this paper, discusses about OCRs, their function, how they are set and coordinated to provide proper protection. Moreover IEC standards for setting the OCRs have also been represented. In section 3, the wind plant model studied in this paper has been illustrated and load flow during normal operation and during fault occurrence have been simulated as well. Section 4 has been dedicated to OCRs settings for the wind plant based on the results obtained in section 3. Beside that OCRs have been tested in order to assure their credibility and validity of relays function. At the end, Conclusion has been brought to summarize all of the materials discussed in the paper.
Progressively amplification of grids by wind farms have led to emergence of some significant electrical issues including security, protection, stability, reliability and power quality. Among Among these issues, protection aspect plays an enormous role which needs a serious attention by researchers. Although protection of wind farms is a crucial issue that needs a huge attention, wind power plants still
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II.
with each other, the relay op eration time and CTI must be taken into consideration. Af er the characteristics of these relays are designated, then t he coordination of OCRs can be properly undertaken.
OVERCURRENT R ELAY
OCRs have the same basic I/O signal o eration as other types of relays. In these relays, if the inc ming current is higher than the preset current value, the rel ay will send out an output signal to the circuit breaker (C ) to disconnect the circuit in order to protect the power c mponents from the result of current excess. There are thre e main types of OCRs used in power systems, which are: definite current relay, definite time relay and inverse time relay. The most common type is inverse time relay whic has an inverse curve characteristic. This curve defines the operation of the relay which functions in a faster time as the current increases. These types of relays are usuall y included with an instantaneous unit which causes the elay to operate instantaneously when the current reach s a high limit magnitude thus eliminating the damage to the power components.
Coordination of OCRs b sically means that the closest relay to the fault location, which is referred to as the primary relay, must first trip the CB, and in case the relay does not trip or malfunctions , the other relay closest to the primary relay, which is called the backup relay, must trip. This coordination is extreme ly crucial and is conducted in order to decrease the expand d power loss and avert power quality compromise. The coordination phenomenon is depicted in Fig 1. In thi figure, OCR1 as primary protection must trip to he fault. In case of any malfunction, OCR2 as back p protection should trip. Also if OCR2 does not operate, OCR3 as the second backup protection must trip and disc nnect the feeder.
Inverse time OCRs based on their s nsitivity to the current and time can have several charact ristics which is reliant on the application. These OCRs typ es, according to IEC standard are depicted in Table 1. Belo . Table 1. Different Characteristic of OCRs Based on IEC Standards Type of OCR Normally Inverse Very Inverse
Op ration time
T
.TSM I . I
T
Extremely Inverse
T
Long Time Inverse
T
.TSM
I I
Fig 1.The Concept of OCRs Coordination
TSM I I
III.
TSM I I
SIMULATION R ESULTS FOR OCR S COORDINATION IN A WIND PLANT
Matlab/Simulink as a po erful software has been used to model the wind plant, rel ays, set the relay settings and coordinate them well with each other. A typical wind power plant has been modelled in this paper and based on the load flow, OCRs usi g IEC standard has been designed, set and coordinate .
In power systems, all of these OCRs ust be properly coordinated with each other in order to p otect the power elements from the currents. To do so, the vital settings of OCRs, which are the Plug Setting Multi lier (PSM) and the Time Setting Multiplier (TSM), must be set suitably. PSM is varied in the range of 50% to 200 and in steps of 25% [7]. This setting is only used for inver se current relays which detect phase to phase fault. For the r elays that detect phase to ground fault, the PSM is quite different. It is varied in in the range of 10% to 40% in ste ps of 10%, or in the range of 20% to 80% in steps of 20% . The point that should be taken into consideration is tha the more Plug Setting (PS) the relay has, the higher c rrent the relay requires to trip. TSM ranges from 0 to 1 in steps of 0.1. However, sometimes it varies in steps of 0.05. The maximum TSM is 1 and the minimum is .05. In order to coordinate OCRs with each other, there i a time interval between a primary relay and a backup rel y operation and this is called the Coordination Time Inte val (CTI). This time interval is in the range of 0.3 and 0.5 seconds for conventional relays, while for numerical r elays it is set at 0.2 seconds, which means they operate fas ter compared to conventional relays [8]. So in order to c oordinate relays
The wind power plant m odelled in this paper, consists of 3 wind turbines that eac h of them produce 2.5 MW power. Their voltage and fr quency are 575V and 60 Hz respectively. Transformers corresponding to each wind turbine has voltage ratio f 575V/25KV in star delta configuration where the st ar side is earthed. The last Transformer corresponding t the grid has the voltage ratio of 25KV/110KV and delta s ar configuration where star is earthed. The transmission li es have 20 Km length each. The wind power plant mode is illustrated in Fig 2. In this figure, since the protection rea is the main scope of this paper, the breakers have be en highlighted as Red colour named by CB1, CB2 … CB and the corresponding relays to each breakers, are highlig ted as green colour shown by R1, R2 … R8.
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Fig 2. Simulink Model for Wind Power Plant Fig 4. Load Flow through C B7 during Normal Operation
In wind power plants, since the win is not always stable and is fluctuating all the time, ther fore the current generated by the wind turbines is also vary ing according to the wind velocity. The minimum adequate wind speed for wind turbines to produce electricity is 5 ps however the maximum wind speed that wind turbines can tolerate is 25mps. If the wind velocity exceeds that v alue, then it will damage the wind turbine generators and s ometimes cause fire in case of long duration of high wind s eed. In order to protect the wind turbines from high wind speed in this paper, a protective block is located to trip he wind turbine as soon as the wind speed exceeds 25. Wi nd speed in this paper is selected to be varying in range of 5 to 25mps. The wind plant currents characteristics at each CB is depicted in Fig 3 to 6 at normal operation.
200
150 ) A ( t n e100 r r u C
50
0 0
10
20
30 ime (S)
40
50
60
Fig 5. Load Flow through C B2 during Normal Operation 200
150
In order to set the relays and coordinat them properly, the exact value of current and short circuit current flowing through each CB should be derived. Fi 7. to Fig 10. Depicts the characteristic of current in A per unit at each CB before, during and after fault. In this simulation, the total simulation time is 60s. A three phas e fault has been imposed to each breaker at time 30 lasting or 5s.
) A ( t n e100 r r u C
50
0 0
10
20
30 ime (S)
40
50
60
Fig 6. Load Flow through C B1 during Normal Operation
120 100
350 ) A ( t n e r r u C
80
300
60
250
) A ( t 200 n e r r 150 u C
40 20 0 0
100
10
20
30 Time (S)
40
50
60
50 0 0
Fig 3. Load Flow through CB8 during Norm l Operation
20
30 ime (S)
40
50
Fig 7. Load Flow thr ugh CB8 during Fault
500 400 ) A ( 300 t n e r r u200 C
100 0 0
10
10
20
30 Time (S)
40
50
60
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As an example, when there is fault near CB8, relay 8 must detect the fault and send the proper tripping signal to the CB8 to disconnect the system until the fault is cleared. As it is clear in the pictures, relay8 trips at time 30.1141 and the CB8 has disconnected the feeder exactly at 30.1141 which shows the relay and CB are working well.
1500
) 1000 A ( t n e r r u C 500
0 0
10
20
30 Time (S)
40
50
The other scenario that must be taken into consideration is that in case relay 8 has not tripped and malfunctioned, the closest relay to relay 8 which is relay7 must trip after a specific delay time which is known as CTI. In Fig 15. This phenomena is shown. Since the CTI is set to be as 0.3s, then as it is expected, relay7 must trip and command the CB7 to disconnect the feeder at time 30.5055. This concept is repeated for the rest of the relays as well.
60
Fig 8. Load Flow through CB7 during Fault 2000
1500 ) A ( t n 1000 e r r u C
This procedures have been tested for all of the faults at each CB and the results of relay settings, have been compiled in Table 2. In this table all of the current measurements are in Amper unit. Ipickup and Ipickup relay refers to the minimum magnitude of current that the relay trips before and after the Current Transformer (CT) respectively. The fourth column represents the CT ratio at each relay. PS, PSM and TSM corresponds to the relay settings that describes how each relay has been set and behaves in case of fault. The last column illustrates T that is the amount of delay time that the relay trips. One thing that should be taken into consideration is that since all of the 3 wind turbine feeders have the same current characteristics, therefore relay settings for relays1, 3 and 5 are the same. Also the relay setting for relays2, 4 and 6 are the same as each other too.
500
0 0
10
20
30 Time (S)
40
50
60
Fig 9. Load Flow through CB2 during Fault 2000
1500 ) A ( t n e1000 r r u C
500
0 0
10
20
30 Time (S)
40
50
60
Through the simulation results it is resulted that relays have been set accurately and are well coordinated with each other in order to protect the wind power plant. All of the relays settings have been conducted using IEC standards and according to section 2 of this paper regarding OCRs settings, all of the TSM has been set by standardization of 0.05 which means the value of each TSM has been rounded to higher value with value of 0.05. Thus OCRs can be considered as one of the best and most successful technique of protection for wind farms.
Fig 10. Load Flow through CB1 during Fault
As it can be seen from the simulation, at time 30, when a three phase fault is imposed to the system, current is increased abundantly and voltage dips drastically which can damage the power systems and compromise the power quality. Therefore a proper protection must be employed to prevent this catastrophe. In this paper OCRs as the best protection relay in wind power plants have been implemented and the results in the next section have affirmed its prosperity, effectiveness and accuracy. IV.
Table 2. OCRs Settings for the Wind Power Plant
R ESULTS AND DISCUSSION
After getting the required data for setting the relays, including exact value of load current and short circuit current at each CB, OCRs can then be modelled, set and coordinated. In order to get the best results with purpose of relays coordination, the exact value of short circuit current located near each CB should be extracted and based on the maximum load current, relays can be set.
Relay R1 R2 R3 R4 R5 R6 R7 R8
The results below demonstrates that relays have been successfully set and are well coordinated with each other. CTI has been opted as to be 0.3s and normal inverse relay has been chosen in this simulation. Fig 11. To Fig 14. Illustrates the relays behaviour at each fault occurred from time 30 to 35. In these figures, 1 means the relay is in normal condition and has not tripped, and 0 means the relay has tripped due to the fault current. Fig 16. To Fig 19. Depicts the CBs operation corresponding the each relays.
I pickup 75 75 75 75 75 75 187.5 37.5
I pickup relay 3.75 3.75 3.75 3.75 3.75 3.75 6.25 3.75
CT 100:5 100:5 100:5 100:5 100:5 100:5 150:5 50:5
PS 75% 75% 75% 75% 75% 75% 125% 75%
PSM 45.27 13.51 45.27 13.51 45.27 13.51 3.91 19.59
TSM 0.65 0.30 0.65 0.30 0.65 0.30 0.1 0.05
34
35
T 1.1484 0.8055 1.1484 0.8055 1.1484 0.8055 0.5055 0.1141
2 1.5 Tripping at 30.1141
) 1 A ( t n 0.5 e r r u C 0
-0.5 -1 29
30
31
32 33 Time (S)
Fig 11. Relay8 Tripping during Fault
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1500
1.5 Tripping at 30.5055
) 1 A ( t n e 0.5 r r u C 0
) 1000 A ( t n e r r u C 500
-0.5 -1 29
30
31
32 33 Time (S)
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0 0
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Fig 12. Relay7 Tripping during Fault
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30 Time (S)
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Fig 17. CB7 Operation during Fault
2
2000
1.5 1500 ) 1 A ( t n 0.5 e r r u C 0
Tripping at 30.8055
) A ( t n 1000 e r r u C
500 -0.5 -1 29
30
31
32 33 Time (S)
34
35
0 0
36
Fig 13. Relay2 Tripping during Fault
10
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30 Time (S)
40
Fig 18. CB2 Operation during Fault
2
2000
1.5 1500
Tripping at 31.1484
) 1 A ( t n e 0.5 r r u C 0
) A ( t n e1000 r r u C
500 -0.5 -1 29
30
31
32 33 Time (S)
34
35
0 0
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Fig 14. Relay1 Tripping during Fault
V.
R8 R7 R2 R1
1.5
-0.5 31
32 33 Time (S)
34
30 Time (S)
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CONCLUSION
In this paper, a comprehensive protection for wind power plants has been successfully implemented by using OCRs. Three phase fault has been imposed at each CB and the settings for each relay has been conducted. Moreover all of the relays have been modelled based on IEC standards in order to provide proper protection for the system, prevent the damage from fault current to the power components, provide perpetual power to the grid and contribute to superb power quality. The results have shown that OCRs can be successfully employed for wind power plants and has proved to be effective, accurate, and be considered as the best method for protection.
) 1 A ( t n 0.5 e r r u C 0
30
20
Fig 19. CB1 Operation during Fault
2
-1 29
10
36
Fig 15. Operation of Relay 7, 2 and 1 in Case Relay 8 malfunctions 350 300 250
Acknowledgement
100
The authors wish to thank the Universiti Putra Malaysia for the research grant “Geran Putra IPB”, project no. GP–IPB/2013/9412101 and vote no. 9412101 that funds this work.
) A ( t 200 n e r r 150 u C
50 0 0
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30 Time (S)
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References
Fig 16. CB8 Operation during Fault
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2014 IEEE International Conference Power & Energy (PECON)
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