Impact of Distributed Generation on Network Short Circuit Level Muhammad Aslam UQAILI1, Anwar Ali SAHITO1, Irfan Ahmed HALEPOTO2, Zubair Ahmed MEMON1, Sada Bakhash DARS3 1
Department of Electrical Engineering, Faculty of Electrical, Electronics and Computer Engineering, Mehran University of Engineering & Technology, Jamshoro, Sindh, Pakistan
[email protected] 2 Department of Electronics Engineering, Faculty of Electrical, Electronics and Computer Engineering, Mehran University of Engineering & Technology, Jamshoro, Sindh, Pakistan 3 Post graduate student, Institute of Information & Communication Technologies, Mehran University of Engineering & Technology, Jamshoro, Sindh, Pakistan Abstract—Distributed generation (DG) technology is spreading rapidly owing to advantages of clean environment, loss reduction and voltage improvement. Utilities in Pakistan are welcoming all generations to increase their supply capacity. Industries are installing DGs to meet their load requirements. In order to reduce energy bills, DGs are being used to supply spare power back to utility or even in off peak durations. DG interconnection changes electrical network characteristics of existing utility network. Protection problems may occur in the form of relay settings, islanding and increased short circuit currents. This paper investigates the effects of DG interconnection on short circuit currents. A feeder of SITE industrial area of Karachi is modeled and simulated for short circuit analysis. Analysis shows that DG interconnection to a radial feeder increases short circuit level at all nodes of the feeder. Keywords— Distributed generation; Short circuit current; Simulation; PSS SINCAL;
I. INTRODUCTION Distributed generation (DG) is generation directly connected to distribution system [1-5]. Conventionally, power system uses large fossil fuel or hydroelectric generating plants. These plants are located at the suitable places normally away from load centers. Generated power is transmitted over long distances using transmission lines. Extra high voltage is used for transmission system to reduce line losses and conductor size. Transmitted power is distributed in service areas by distribution system. Considerable power is lost in network components from generation to utilization [3-8]. Electricity sector in Pakistan is facing huge energy crises. Demand and supply gap is increasing day by day resulting in more than ten hours in a day. Insufficient generation, erroneously estimated demand modeling and load forecasting and high transmission and distribution (T&D) losses are major contributor to the scenario[13]. Transmission network is being operated at full capacity and even if generation is added to the system, the scenario will not change much. Various plans for generation and transmission network capacity enhancement are under development process but will take some time to integrate in system
DG can be considered as fast solution to increase power generation because of short construction times [4,7]. As DGs are connected to distribution system therefore there will be no need for enhancement in transmission network. In addition to that DG will not contribute much to energy losses. But DG will cause some protection problem such as safety, islanding and increased short circuit currents [7,9]. Due to various advantages offered by DG, extensive research around the world is being carried out to maximize its benefits and avoid any undesired situation. Duloo et.al (2014) highlighted different impacts of DG on distribution system including operation and control, change in short circuit capacity, stability and relay performance. They used IEEE 30 bus test system for analysis and concluded that DG reduce losses but may add to short circuit currents depending on the size and location of DG and system to which it is connected [1]. Gomez et. al (2013) investigated change in fault current for model power system. They concluded that relay settings need to be changed after DG is interconnected. They also suggested to carry out protection analysis for the system before DG interconnection [11]. Deng et. al. (2012) analyzed protection aspects of portion of feeder with DG interconnection and suggested to incorporate reverse power protection. They also concluded that DG interconnection will change the flow of fault currents and therefore relay settings must be analyzed before DG interconnection through simulation [5]. Zayandehroodi et. al. (2011) analyzed a four bus example system for fault current changes with DG interconnection. They concluded that relays may detect less or more current depending on the size and location of DG [8]. DG may increase protection problems including increase in short circuit level. Change in short circuit currents depends on network configuration, size and location of DG. This paper focuses on short circuit current contribution of DG. Power System Simulation Siemens Network Calculation (PSS SINCAL) software is used to simulate a real 11kV distribution feeder in SITE Industrial area of Karachi to analyze the effects of DG on short circuit level.
II. DISTRIBUTED GENERATION Small scale generation up to 50MW connected directly to distribution system is called distributed generation or embedded generation. DG normally use renewable energy sources such as wind turbines, solar photovoltaic cells, fuel cells, micro hydro and reciprocating engines. Major benefit of renewable energy as DG is less environmental pollution, which ultimately makes it possible to install a DG near or even within a city [5]. Fossil fuel generating plants emit greenhouse gases and nuclear power plants have great concern of hazardous nuclear waste. In addition to that oil prices are increasing rapidly making it difficult to keep electricity prices within acceptable limits. This ultimately results in excessive burden on country’s economy and valuable reserves are used to import oil. Current flowing through transmission and distribution conductors, cables and transformers cause I2R power losses. DG interconnection results in reduced current flow through existing transmission and distribution network and hence power losses are reduced. In addition to that DG installed near load centers will result in reduced voltage drops and thus less voltage regulation equipment will be required [4,9]. Like all the developed countries, DGs are making their way in Pakistan. Utility owned DGs are installed at different industrial areas of the cities to provide efficient and reliable supply to industrial consumers. In addition to these, consumers are also encouraged to install DG not only to meet their load demands but also to reduce their electricity bills by selling that electricity to utility. Number of small capacity DGs are connected to distribution network. Industrial consumers, large residential and commercial buildings, newly planned areas and utilities are installing DGs in all parts of the country. This will help to cope with the ever increasing energy demand. PV cells are most used DG technology in Pakistan and wind mills are just behind it. Fuel cells is another great option for DGs in Pakistan and can be used efficiently. In addition to these micro hydro turbines can be installed in canals and tubewells so that sufficient energy can be generated with almost zero running cost [11]. III. SHORT CIRCUIT LEVEL Fault in a power system is an unwanted condition that normally involves a short circuit between any phases to ground or between phases. Short circuit results in very high current flows and need to be interrupted before any considerable damage is done to system components. Circuit breakers are used to disconnect the faulty portion from rest of the system [12]. Relays and associated accessories are used to detect an abnormal condition and relays the signal to circuit breaker to open its contacts to disconnect the faulty portion. Symmetrical faults are considered most dangerous as all three phases are short circuited and maximum damage occurs to system components. These faults are analyzed on per phase basis. Unsymmetrical faults are abnormal conditions where all three phases lose their symmetry. Single line to ground, line to line and double line to ground faults are examples of unsymmetrical faults [7,11,12].
Circuit breakers have to carry the short circuit current for few cycles till circuit is disconnected. Circuit is to be interrupted during that short circuit current therefore a circuit breaker must be capable of breaking contacts while carrying that short circuit. Breaking capacity of circuit breaker is typically mentioned in MVA and is the product of maximum symmetrical fault current and system voltage. It is also called short circuit MVA (MVASC). MVASC =
ξ3*VL*If
(1)
106
Busbar capacity is also dependent on short circuit level. In a conventional radial distribution feeder, short circuit level decreases as we go from source (substation) along the feeder. When DG is interconnected to existing system, the situation no longer remains same. Fig. 1 shows one line diagram for a simple power system comprising of a generator, transformer and feeder. A DG is also shown connected to bus B. Following assumptions will make calculations simple without affecting accuracy for the calculations of short circuit level. • Generators are operating at rated voltage • Transformers are operating at nominal tapping • Shunt capacitance and series resistance are neglected
Fig. 1. One line diagram
Assume that a fault occurs at far end of feeder marked as F. For finding short circuit level base MVA and per unit method are used. Per unit quantities have advantage of simple calculations due to low value and give relative comparison of different components [3]. Select a common base of 10 MVA. Base kV will be 11kV for sections of generator and L.V side of the transformer. H.V side of the transformer, feeder and DG will have a base kV of 33kV. Per unit impedance of machines are specified on machine rating as base values. Per unit impedance can be transferred to new base values using MVA
kV
Znew =Zold * ൬ MVAbaseሺnewሻ ൰ * ൬kV baseሺoldሻ ൰ baseሺoldሻ
baseሺnewሻ
(2)
Impedance can be converted to per unit from its value in ohms by MVAbase
Zp.u = Zohms * ൬
kV2base
൰
(3)
All given impedances are converted to per unit on selected base MVA and kV values and are arranged in Table 1 given below. TABLE 1. Calculation of per unit impedances
Impedance
Eq.
Calculation 11 10 Generator Eq. 2 j0.10* ൬ ൰ * ൬ ൰ XG 11 10 11 12 Transformer Eq. 2 ൰ * ൬ ൰ j0.06* ൬ XT 11 10 10 33 DG Eq. 2 j0.05* ൬ ൰ * ൬ ൰ XDG 5 33 10 Feeder Eq. 3 j11* ൬ 2 ൰ XL 33 Reactance diagram for the system before interconnection are shown in Fig. 2(a) and (b).
Result j0.10p.u j0.05p.u
TABLE3. Short circuit capacity comparison at different buses
Bus
Without With Difference Percentage DG DG Change F 40 62.5 22.5 56% A 100 166.67 66.67 66.67% B 66.67 166.67 100 150% Table 3 indicates an increase on short circuit level of all the buses. This is a great concern for protection system design including circuit breaker capacity and relay settings. IV. SIMULATION FOR SHORT CIRCUIT CAPACITY
j0.10p.u j0.10 p.u and after DG
Fig. 2. Reactance diagram (a) without DG (b) with DG
Calculate the impedance to the fault point using Thevenin’s theorem (Zth). Then calculate short circuit level by MVAbase MVASC = (4) Z th
Short circuit current can then be calculated, as MVASC *106 ISC = (5) 3 ξ3*kVbase *10 Table 2 shows Zth, MVASC and ISC with and without DG interconnection. Increase in short circuit levels and percentage change are also given in table 2. TABLE 2. Short circuit calculations
Without With Difference Percentage DG DG Change Zth (p.u) 0.25 0.16 - 0.09 -36% MVASC 40 62.5 22.5 56% ISC (A) 700 1093.5 393.5 56% Table 2 clearly shows that short circuit MVA is increased at fault point when DG is interconnected at point B. Impedance to the fault is decreased and is indicated by negative sign in difference and percentage change for impedance. Short circuit MVA can also be calculated at different locations using the procedure explained above. Table 3 showsthe short circuit level comparison at different buses before and after DG interconnection.
Real systems are long and complex. Hence, calculation of short circuit level will require a lot of time. Therefore, computer programming is used to calculate short circuit level for the large systems. General programs for such calculations are written and required data input calculates the desired quantities. Simulation is a graphical method of such computer program developed for ease of utility. PSS SINCAL® is a user friendly graphical simulation software. Built in programs for short circuit calculation, allow user to analyze large system with ease. Develop model of the system under study just like a one line diagram and insert data required to simulate and observe the results. In this paper, we have selected an 11 kV feeder and collected real data for analysis. The network is used to supply electricity to industrial consumers in SITE area of Karachi. It starts from 11kV busbar at SITE substation and supplies mostly industrial loads. There are 27 nodes/buses excluding the substation busbar. Seven 11/0.4kV distribution transformers are energized from the feeder. Total capacity of these seven transformers is 4750kVA. In addition to these transformers five industrial loads are directly supplied through 11kV. Total connected load on the feeder is 6MVA. 11 kV cables are used to interconnect different buses. Buses on main stream of feeders are named N1 to N6. Bus B1 refers to substation in branch network. T1 to T7 represent high voltage terminals of distribution system and nodes L1 to L7 represent low voltage terminals. Five loads supplied directly by 11kV are represented by L01 to L05. Network is modeled on PSS SINCAL and short circuit levels are observed for different buses. Later 3MVA synchronous machine DG has been interconnected to the feeder at node N11. The network with DG has been simulated for short circuit analysis and is shown in Fig. 3. Short circuit MVA for each node has been depicted in the simulation diagram. The feeder specifications for its line length, loading condition and transformer positions and capacities have been shown in the simulation platform. Results obtained for the two cases and their comparison are arranged in Table 4. Short circuit level is maximum at 11kV substation busbar and decreases on main stream from N1 to N6. This decreases on 11 kV nodes on the main stream illustrates the behavior of a radial distribution feeder. Reason for this trend is eq.2 as impedance of lines from source add up and increased Zth reduces short circuit level. Similar pattern is observed for the network before and after DG interconnection. Fig. 4 shows
graphical comparison of short circuit MVA A of main feeder buses (from N1 to N6) graphically. Trend iss also visible from the graph. Short circuit level at all nodes havee been increased. Maximum change of 4.05% in short circuit leevel is observed at node N4 which is point of DG interconneection. Percentage increase decreases from N4 to N6 on the dow wnstream of feeder. Same is the case with upstream from N4 to suubstation. Percentage increase in short circuit levells for 0.4kV nodes (N4, N12, N16, N18 and N21) is small whenn compared to rest of 11kV nodes. It is due to impedance of the transformer, which limits the flow of short circuit current.
TABLE 4: Short circcuit MVA comparison
Node
MVASC Without With DG DG
Change In MVASC
%age Change
Substation
1100.00
11166.00
16.00
1.45%
N1
471.00
485..13
14.13
3.00%
T1
466.82
480..72
13.90
2.98%
L1
15.73
15.775
0.01
0.07%
N2
431.06
444..97
13.91
3.23%
T2
427.52
441..22
13.70
3.20%
L2
15.70
15.771
0.01
0.08%
B1
397.10
409..06
11.96
3.01%
T3
394.08
405..87
11.79
2.99%
L3
5.34
5.335
0.00
0.03%
LO1
388.32
399..80
11.47
2.95%
N3
356.65
370..13
13.48
3.78%
T4
354.19
367..50
13.31
3.76%
L4
10.54
10.554
0.01
0.07%
LO2
349.50
362..48
12.98
3.71%
LO3
349.50
362..48
12.98
3.71%
N4
328.38
341..68
13.31
4.05%
T5
326.28
339..43
13.15
4.03%
L5
15.56
15.557
0.02
0.10%
N5
225.22
231..75
6.52
2.90%
T6
224.22
230..69
6.47
2.88%
L6
15.29
15.331
0.02
0.10%
LO4
222.30
228..66
6.36
2.86%
LO5
222.30
228..66
6.36
2.86%
N6
213.95
219..86
5.91
2.76%
T7
213.04
218..90
5.86
2.75%
L7
19.91
19.994
0.03
0.14%
Short Circuit MVA M Comparison 500
MVA
450 400 350 300 250 200 0
2 Before DG G
4
6
8
After DG
Fig. 4. Graphical comparison off short circuit level of main feeder Fig. 3. Simulation diagram for network with w DG
V. CONCLUSION Despite of various advantages offered by DG, there is a great protection concern associated with it. Simulation results confirm that short circuit levels increases with the interconnection of DGs in the existing systems. This necessities enhancement of circuit breaker capacity for safe and reliable operation of the system. This variation in short circuit level depends upon location, type and size of the DG. Existing network impedance parameters will also affect the increase in short circuit level. Therefore, short circuit analysis of each case is different and short circuit levels must be analyzed through simulation before any DG is interconnected to utility network. If DGs are interconnected without analysis, results may be dangerous and may lead to long shutdowns and expensive maintenance or replacement of system components. In addition to the short circuit level, relay settings may also be changed based on the new analysis. If not done, results may be either nuisance tripping or relay blinding. Detailed analysis for over current relay setting on a radial distribution feeder with DG interconnection is left for future work. REFERENCES [1] Dulau, Lucian Ioan, Mihail Abrudean, and Dorin Bica., "Effects of Distributed Generation on Electric Power Systems." Procedia Technology 12 (2014): pp. 681-686. [2] Mijalili, M. M., A. R. Sedighi, and M. R. Haghifam. "A novel method for DG allocation with considering its positive and negative impacts." In Electrical Power Distribution Networks (EPDC), 2013 18th Conference on, pp. 1-5. IEEE, 2013. [3] Gomez, Juan C., Jorge Vaschetti, Carlos Coyos, and C. Ibarlucea., "Distributed Generation: Impact on Protections and Power Quality." Latin America Transactions, IEEE (Revista IEEE America Latina) 11, no. 1 (2013): 460-465. [4] Javadian, S. A. M., M-R. Haghifam, M. Fotuhi Firoozabad, and S. M. T. Bathaee. "Analysis of protection system’s risk in distribution networks with DG." International Journal of Electrical Power & Energy Systems 44, no. 1 (2013): 688-695.
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