Unbalanced Power Flow Analysis in a Micro Grid

Transcription

1 International Journal of Emerging Technology an Avance Engineering Unbalance Power Flow Analysis in a Micro Gri Thai Hau Vo 1, Mingyu Liao 2, Tianhui Liu 3, Anushree 4, Jayashri Ravishankar 5, Toan Phung 6, John Fletcher 7 School of Electrical Engineering an Telecommunications, The University of New South Wales, Syney, Australia Abstract This paper reviews ifferent approaches for unbalance loa flow analysis within a istribution network, especially in a microgri (MG). Moels of untranspose istribution lines an unbalance loas are consiere for each methoology, as these are common in a MG scenario. Various commercial software packages are also investigate an assesse in terms of solving the power flow of the MGs an to ecie on reliable tools for further stuies. Different case stuies are simulate to illustrate the results obtaine using sequence frame an phase frame power flow methoologies for a given MG. Keywors DIgSILENT, istribute generation, microgri, software tools, unbalance power flow I. INTRODUCTION The concept of microgri (MG) has been consiere by researchers more frequently in recent years, ue to its role in riving moern power systems to achieve clean energy generation [1]. MGs operate in two ifferent moes, namely interconnecte an islane moes. In the former moe, the Distribute Energy Sources (DERs) in the MG contribute to the majority of the eman within the MG an the ifference in supply an eman is mae available from the utility; while in the latter moe, DERs supply reliable an economical energy serving ifferent loas like resiential builings, commercial builings an inustrial parks [2]. In a MG scenario, balance power flow analysis may not be aequate ue to a number of unbalance aspects like single-phase loa, un-transpose conuctors or mix of balance an unbalance components [3]. While istribution networks are often configure as raial feeers which are fe from a substation, MG is escribe to be a complete network with various components like generators, loas, shunt evices an conuctors with raial or ring-bus layout. Similar to general istribution networks, MGs are often compose of single-phase meium/low voltage an unbalance consumer loas. It is obvious that balance system analysis tools use for transmission networks cannot be applie to this new type of electricity gri. Hence, an accurate an reliable tool for MG power flow analysis is esirable. In practice, power flow stuy is a numerical analysis utilizing approximate moels of a real physical system to examine normal steay state an contingency conitions in an electrical system. 536 Calculation methos are selecte base on characteristics an topologies of a given network or preferre reference frame. The power flow equations are non-linear equations an therefore iterative methos must be utilize in solving the equations. For unbalance system analysis, two methos of loa flow are in use, namely, the phase-frame approach an the sequence-frame approach. The sequence-components approach is base on symmetrical sets of balance phasors, positive, negative an zero sequences, wherein three unbalance phases are converte to three inepenent balance circuits [4]. This approach works better on general network topologies with synchronous generating units ue to reuction of size an the computational buren as compare to the phase-frame approach [5-7]. One main avantage of this approach arises from the fact that the three sequence networks have weak coupling leaing to the system state equations that can be solve using parallel programming [8]. A more common an intensively use approach is phaseframe base metho, which analyses each phase of unbalance system inepenently an then combine together to form the complete state parameters of phase A, B, C. For each topology of network, calculation metho shoul be appropriately selecte. Raial networks use the forwar-backwar sweep metho [9] while weakly meshe gris use the compensation metho [10]. System state equations can be solve using Newton-Raphson an Gauss an/or Gauss-Seiel methos. Ill-conitione cases nee to be carefully consiere in these approaches ue to the higher resistance/reactance (R/X) ratio of the lines [11]. In most istribution power flow calculations, the loas are often moele as a constant power, as voltage rop along a short istance is assume to be less significant. However, in reality, this is not vali because in istribution systems bus voltages are not controlle. Hence, the general loas are to be moele as constant power, constant current, constant impeance or a mix of these three types. It is to be note that for real-time monitoring, ispatching an/or planning an maintenance of MGs, two aspects are important, (i) a suitable tool to simulate the moel of MG as accurately as possible an (ii) a set of input ata for unbalance system computing. The focus of this paper is on the above two aspects. Section II of this paper offers a helpful review of existing power flow calculation methos an network component moels for MG.

2 International Journal of Emerging Technology an Avance Engineering A brief assessment of commercial tools ealing with unbalance three-phase systems is then escribe in section III. Case stuies are carrie out in section IV to prove that ifferent calculation approaches can achieve comparable results for a given MG moel. Finally, section V proposes the potential software caniates that offer flexibility in analyzing MG networks. II. SYSTEM MODELING There are four main components within a MG, namely, generators, istribution lines, transformers an loas. While generators an transformers can be consiere relatively balance uner normal operation, istribution lines an loas are the main causes for the unbalance of power flow. This section, therefore, focuses on these two components. A. Distribution Lines Base on the reference frame, the moel of istribution lines are represente ifferently. For phase-frame approach, with full mutual couplings between a line section, the wellknown amittance moel between noe (i) an noe (j), is shown in Fig. 1 [4]. In case of balance systems, the number of equations to be solve is 2N. This figure significantly increases to 6N nonlinear simultaneous equations for full mutual couplings in the system leaing to buren in computation. Another common approach is sequence frame, which is often applie to unbalance fault analysis using symmetrical components through which, any unbalance 3-phase system can be converte to three sets of symmetrical balance systems. The sequence frame approach with unbalance istribution section between two noes (i) an (j) is moele as in Fig. 2 [12]. V (i) 012 Bus i I 012 [Y 012 series] Y/2 shunt Y/2 shunt Fig. 2: Equivalent amittance of lines (sequence frame) Bus j Relation between voltage an current is now shown as, V (j) 012 V (i) A Y aa V (j) A [ ] [ ] [ ] [ ] [ ] V (i) B V (i) C Y ca Y bb Y ca Y ca Y cc X ca V (j) C Fig. 1: Equivalent amittance of lines (phase frame) Relation between voltage of noe (i), noe (j) an current via two noes for power flow calculation is, [ ] [ ] [ ] (1) For the general case of unbalance N-noe system, the mutual impeances are such that, an, (2) Each noe contains four variables (P, Q, V, θ), two of which are known an the remaining two are unknowns. V (j) B The given unbalance network can be solve inepenently using three balance sequence networks. The final result for a noe is then combine using the three components. It is obvious that for a given unbalance three-phase N-noe system, there are 12N variables in total (4N for each sequence), among which, 6N variables are known an 6N unknown, requiring a set of 6N nonlinear simultaneous equations to be solve. B. Loas The loas will consume active an reactive power at a nominal frequency. This power varies accoring to voltage an frequency at the noe to which the loas are connecte. With the assumption that the system frequency is far more stable than voltage, this paper will consier only the loa moel epening on voltage. Relation between power an voltage is shown as, where, [ ( ) ( ) ( ) ] (4a) [ ( ) ( ) ( ) ] (4b). (3) 537

3 Percentage Loa (%) International Journal of Emerging Technology an Avance Engineering At a given noe, there are ifferent types of loas connecte. The coefficients a, b an c efine proportions of constant impeance loa, constant current loa an constant power loa respectively at a noe. For constant power an constant current loas, the moel of passive source injection is use to escribe response of loa uner various voltages, while for constant impeance loa the moel of istribution lines with amittance matrix is use. Fig. 3 shows the response of ifferent types of loas to system voltage Percentage Voltage (%) Constant Z Constant current Constant MVA Fig. 3: Relation between power an voltage of loas III. TOOLS FOR UNBALANCED SYSTEM ANALYSIS The concept of unbalance power flow analysis is not new. A number of theoretical an practical works have been carrie out to eal with ifferent network topologies an operation moes base on ifferent reference frames. There is no oubt that each approach has its own strengths an rawbacks. For example, phase frame base approach oes not require any transformation from phase parameters or any simplifying assumptions but this metho nee to form a square Jacobian matrix of 6N x 6N to solve equations for an N-noe system. This will lea to a computation buren as this matrix nees to be upate in every iteration [4]. In the sequence frame with 2N x 2N matrix, each sequence can significantly reuce the size of the Jacobian matrix an it can be beneficial as parallel computation technique is use. However, this approach requires a transform step from phase parameters to sequence parameters an then converts the results back from sequence to phase values; this process may accumulate significant calculation errors. Both phase frame an sequence frame approaches use ifferent approximation moels for the same physical evices within an electric system. In general, both methos must give the same answer for state variables (P, Q, V, θ) of a given noe. This has been proven by several researchers on phase frame base an sequence frame base approaches [6, 8, 13]. However, when these approaches are applie for the analysis of MGs, there arises an aitional ifficulty, as MGs are inherently unbalance ue to changing loas an generations. This leas to the ifficulties in power flow calculation for real-time MG operation, monitoring, protection coorination, expansion plan or maintenance. Presence of nonlinear loas, active sources or smart evices makes the power flow analysis in a MG even more complex. Therefore, the nee of a powerful an reliable tool to manage MG analysis is obvious. Balance system power flow analysis is quite common in commercial software tools; however, not all software tools can hanle unbalance systems. This paper gives an overview of three well-known software packages with frienly interactive interfaces that can eal with unbalance MG operation, namely, PSCAD, DIgSILENT Power Factory an PSS/Aept. Power System Simulator Avance Distribution Engineering Prouctivity Tool (PSS/ADEPT) is a istribution system analysis program evelope by Power Technologies, Inc. (PTI) for planning, esigning, an analyzing istribution systems. Although another software package istribution (PSS SINCAL) has been prouce by PTI, it is still a popular software package for istribution operator all over the worl [14]. PSS/Aept has a frienly graphical user interface for engineers to a, eit or remove all electric evice moels on workspace. From the point of view of operation engineers, it is a helpful tool for power flow analysis, short-circuit analysis, motor starting analysis, capacitor optimization, tie open point optimization, an preictive reliability analysis. However, if further analytical consieration is require, such as fully couple impeance matrix in three phase system, it may not be a promising caniate. PSCAD is a simulation software for the esign an verification of power systems. PSCAD is also known as PSCAD /EMTDC because EMTDC is the simulation engine, which is now the integral part of PSCAD graphical user interface. PSCAD is a powerful analyzing tool for simulating time omain instantaneous responses in both electrical an control systems with visual online controllers for user to ajust the parameters while running [15]. DIgSILENT Power Factory is a transmission an istribution network analysis tool with the capability to simulate wie range of power electronic evices, renewable energy plants an fully couple conuctor moels [16]. It is a helpful combination of easy-to-use software for engineers (i.e., PSS/ADEPT) an a novel tool for researcher (i.e., PSCAD). 538

4 Feature International Journal of Emerging Technology an Avance Engineering TABLE I DISTRIBUTION SOFTWARE COMPARISON Balance/Unbalance Power Flow One/Three Phase Line One/Three Phase Loa Moel Full Coupling Mutual Impeance Unbalance Loa Moel Power Electronics Element Fault Analysis Transient start analysis DC Loa Flow Small signal analysis (eigenvalues) Static an ynamic voltage stability VAR/STATCOM Win/Solar Panel Harmonics, flicker an resonance analysis Tie Open Point Optimization Capacitor Placement Optimization Protection an coorination PSCA DigSil PSS/A D ent ept Yes Yes - Yes Yes - Yes Yes Limite Limite Yes - Yes Yes - Yes Yes Yes Limite - Yes Limite - Limite Yes Yes Limite Yes Yes Summary of these istribution software packages is liste in Table 1 highlighting the main functions for unbalance power flow. It is to be note that these software packages use phase frame approach for unbalance power flow analysis, as the computation buren can be overcome by using powerful computers nowaays. In summary, from network operators points of view, PSS/Aept seems to be the suitable caniate with the capability to moel hunres of noes easily to perform loa flow analysis or check contingencies of ifferent conitions. PSCAD, on the other han, has the strength of focusing on each component s response not only uring steay states but also uring transient states. Thus, it might be the choice for electrical machine esigners or researchers working on etaile level of electricity gri. However, DIgSILENT Power Factory is a promising alternative thanks to a novel combination between network analysis an component analysis packages. It contains array of pre-efine components, such as generators, transformers, cables or switching evices from ifferent well-known manufacturers an has programming language support for users to evelop their own moels. 539 With wie range of built-in moules, DIgSILENT is able to conuct various stuies from power flow, shortcircuit, harmonics, flicker, stability, moal analysis an protection coorination. For unbalance power flow stuies, this software seems to be an effective potential tool an hence this software is use in this paper. The next section emonstrates the unbalance power flow analysis using phase frame approach with the chosen software DIgSILENT Power Factory. The results are then compare to sequence frame approach for valiation. IV. CASE STUDIES AND SIMULATION In orer to compare the convergence an accuracy of phase frame base an sequence frame base methoologies an to verify the valiity of line moels in Fig.1 an Fig. 2, the five-bus stuy system of Fig. 4 is selecte an simulation is performe. A. System Description Fig. 4: Schematic of five-bus system The five-bus system is a three-phase, four-wire, multigroune network that inclues untranspose lines an balance/unbalance loas. This system is erive from an original transmission system [6] with the changes in voltage levels an base MVA [13]. It can be consiere as a MG operating in islane moe. The system is supplie by two DER units. G1 represents a three-phase synchronous generator that also serves as the system slack bus. G2 is another three-phase synchronous generator, which is moele as a PV noe that keeps voltage at 1.05 p.u. at bus 5. This system was analyze using sequence frame approach by other researchers an simulation was performe with another commercial software package [13]. Thus, it becomes an ieal test case to compare ifferent approaches base on ifferent tools.

5 International Journal of Emerging Technology an Avance Engineering Using the voltage level 13.8kV an 100kVA as base, system parameters are given in Tables 2 to 5. From a technical point of view, G1 cannot be large enough to be the slack bus; however, for power flow analysis, at least one slack bus must be well efine. Hence, it is assume that G1 can keep the voltage an angle of bus 1 constant. An unbalance constant-power, star-connecte loa is connecte at bus 3. Table II Series Impeance of Single Circuit 1-2 an 2-3 (Ω) a b c a i i i b i i c i Table III Shunt Amittance of Single Circuit 1-2 an 2-3 (µs) a b c a i i i b i i c i B. Case Stuies Two case stuies are performe with generator G2 set at ifferent moes, either voltage regulate (PV moe) or fixe power injection (PQ moe). In the former moe, it is assume that G2 is equippe with voltage regulator an active power controller while in the latter moe G2 is a regular generator with specific active an reactive output. The simulation is performe with DIgSILENT using phase frame metho. The results obtaine are compare with sequence base approach [6, 13]. For the first case, the voltage controller of the DER G2 unit regulates voltage of noe 5 at1.05 p.u. For the secon case, the DER G2 injects 0.6 p.u. active power an 0.2 p.u. reactive power at bus 5. In these simulations, transformer parameters an installe capacity, an reactive power of G2 play a vital role in regulating the voltage profile at noes. The comparison is shown in Fig. 5 an Fig. 6. Table IV Series Impeance of Double Circuit 1-3 (Ω) a b c a b c a i i i i i i b i i i i i c i i i i a i i i b i i c i Table V Shunt Amittance of Double Circuit 1-3 (µs) a b c a b c a i i i i i i b i i i i i c i i i i a i i i b i i c i 540

6 International Journal of Emerging Technology an Avance Engineering Vpu Vpu Fig. 5: Case 1 Results (a) Sequence frame, (b) Phase frame Noe 1 Noe2 Noe 3 Noe 4 Noe 5 Noe 1 Noe2 Noe 3 Noe 4 Noe 5 Fig 6: Case 2 Results (a) Sequence frame, (b) Phase frame As can be seen from Fig. 5 an Fig. 6, the maximum error in phase voltage is less than 2% for case 1 an oes not excee 5% for case 2. There are some minor ifferences between sequence frame an phase frame in the above case stuies ue to the slack bus reference moels. In the sequence frame base approach, slack bus is assume as a limite capacity while the phase frame consiers noe 4 a real slack bus. Other important factors that may iffer the results are G2 reactive power limits an the transformer parameters with ifferent configurations. Acting as a PQ noe, G2 will inject a fixe capacity to a given noe leaing to its voltage value, mainly ecie by the amount of reactive power injecte. While in the PV generating moe, G2 only has capability to keep the voltage of connecte noe as close to pre-efine value as it can. Once the reactive power limits are reache, G2 no longer can regulate the voltage at the connecte noe. Overall, this proves that phase frame base metho gives nearly ientical results as sequence frame base metho for the ifferent case stuies consiere here. V. CONCLUSION Phase A (a) Phase A (b) Phase B (a) Phase B (b) Phase C (a) Phase C (b) Phase A (a) Phase A (b) Phase B (a) Phase B (b) Phase C (a) Phase C (b) Although phase-frame an sequence frame can give the same results for unbalance power flow in networks, phase-frame base methoology is more commonly aopte by software venors. 541 Case stuies suggest that DIgSILENT Power Factory is a sufficient an reliable software in terms of ealing with unbalance power flow within MGs. Therefore, it can be conclue that DIgSILENT Power factory is a promising caniate for further work on istribution networks in general an smart micro gris, in particular. REFERENCES [1 ] G. Tambling, P. Laver, M. Oliphant, an N. Stevens Renewable Opportunities: A Review of the Operation of the Renewable Energy (Electricity) Act Australian Greenhouse Office. [2 ] M. Agrawal an A. Mittal Micro gri technological activities across the globe: A review. Int. J. Res. Rev. Applie Sci, vol. 7, pp , [3 ] S. Khushalani, J. M. Solanki, an N. N. Schulz, "Development of three-phase unbalance power flow using PV an PQ moels for istribute generation an stuy of the impact of DG moels". IEEE Transactions on Power Systems, vol. 22, pp , [4 ] J. Das Power System Analysis: Short-Circuit Loa Flow an Harmonics. CRC Press. [5 ] M. Abel-Akher, K. M. Nor, an A. H. A. Rashi, "Improve Three- Phase Power-Flow Methos Using Sequence Components". IEEE Transactions on Power Systems, vol. 20, pp , [6 ] K. L. Lo an C. Zhang, "Decompose three-phase power flow solution using the sequence component frame". Generation, Transmission an Distribution, IEE Proceeings C, vol. 140, pp , [7 ] M. Abel-Akher, K. M. Nor, an A. H. Abul-Rashi, "Development of unbalance three-phase istribution power flow analysis using sequence an phase components". Power System Conference, 2008, pp [8 ] M. Kamh an R. Iravani, "A unifie three-phase power-flow analysis moel for electronically-couple istribute energy resources". Power an Energy Society General Meeting, 2011, pp [9 ] W. H. Kersting Distribution system moeling an analysis. CRC press. [10 ] S.-Q. Hu an S.-M. Li. Unbalance Loa Flow for Weakly Meshe Distribution Systems with Distribute Generation. in Electrical an Control Engineering (ICECE), 2010, pp [11 ] S. Khushalani an N. Schulz, "Unbalance Distribution Power Flow with Distribute Generation". in Transmission an Distribution Conference an Exhibition, 2005/2006 IEEE PES, 2006, pp [12 ] M. Z. Kamh Component Moeling an Three-Phase Power- Flow Analysis for Active Distribution Systems. University of Toronto. [13 ] M. Z. Kamh an R. Iravani, "Unbalance Moel an Power-Flow Analysis of Microgris an Active Distribution Systems". IEEE Transactions on Power Delivery, vol. 25, pp , [14 ] (2008). PSS/Sincal Guie [Prouct Brochure]. Available: [15 ] PSCAD : A comprehensive esign an analysis environment. Available: [16 ] (2012). DigSilent: Power Factory. Available: