Investigation of short-circuit analysis conducted at Kromco

Transcription

1 Investigation of short-circuit analysis conducted at Kromco by Bantubonke Matika and Alan Meru, Cape Peninsula University of Technology This article presents a short-circuit analysis conducted for the Kromco electrical power system using power system modelling, analysis and simulation software called DIgSilent PowerFactory. The main focus is determining the maximum short-circuit currents and fault levels at various locations on the network, and verifying the adequacy of power system components, to see if they can withstand rated prospective short-circuits. An efficient and reliable power system is a vital component of any industry especially manufacturing and packaging industries. Hence, a need arose to conduct short-circuit analysis in industrial power systems to evaluate the power delivery and utilisation operation efficiency [1]. Symmetrical three phase faults were investigated using the superposition method. Three operating conditions are analysed: 100%, 80% transformer full load, and when plant 2A is feeding via plant 1, which is normally open. Kromco is one of the large deciduous packing facilities in the Western Cape situated in Grabow, near Cape Town. Its dedication to delivering quality fruit to local and international markets makes Kromco a trusted brand. With its high tech cold stores and machinery for supplying consistent quality fruit it is vital that Kromco s electrical power system operates efficiently with minimal interruptions as each interruption may cause loss in production which can lead to high losses of revenue. Due to the modernisation programme of replacing old ring main units with new digitally controlled systems, a need arose for the study which was motivated to re-evaluate and verify the existing power system components and reliability. The system consists of an 11 kv feeder from Eskom as its source of electrical power. The supply voltage for the distribution system is then stepped down to 400 V, and 220 V as required by the loadings, from the main substation to ring main units feeding the transformers located at the packing shed, and plants 1, 2, 3 and 4. There are a total of eleven operating transformers in Kromco. These transformers range from 315 kva to 1000 kva. System models are constructed and simulations are conducted with certain assumptions being made in order to commence with the study. Loads are modelled as inductive loads at a power factor of 0,8 due to the fact that is was difficult to obtain actual load rated capacity and real time data. The IEC standard was used to obtain transformer X/R ratios. Research statement The main objective of the short-circuit analysis is to properly determine the shortcircuit currents and fault levels at various system locations using simulation software. This is to verify the adequacy of existing power system components as to whether they can withstand prospective shortcircuits. This was deemed necessary due to the fact that the current electrical system has never been revised and lacks proper documentation of load addition and system expansion since the establishment of the distribution network. Theoretical background Short-circuit analysis Electrical power systems in industrial plants are designed to supply loads in a safe, economical and reliable manner. However, even in a perfectly designed system, the occurrence of short-circuit currents are unavoidable [2]. According to IEC 60909, a short-circuit, or fault as its commonly known, is the accidental or unintentional conductive connection through a relatively low resistance or impedance between two or more points of a circuit which are normally at different potentials [3]. Fig. 1: Fault types. The duration of this fault can be at divided into three different areas: energize - December Page 49

2 Source IVI V Ideal voltage source 1 p.u. 0 Table 1: Source parameters. Transformer Z [%] Voltage X/R ratio kva Vector group T 1 5,11 11 kv/400 V 3, Dyn 11 T 2 5,11 11 kv/400 V 3, Dyn 11 T 3 5,12 11 kv/400 V 3, Dyn 11 T 4 4, kv/400 V 3, Dyn 11 Fig. 2: Symmetrical fault. T 5 4,91 11 kv/400 V 3, Dyn 11 T 6 4,66 11 kv/400 V 3, Dyn 11 T 7 4,29 11 kv/400 V 1,5 315 Dyn 11 T 8 5,16 11 kv/400 V 3, Dyn 11 T 9 5, kv/400 V 3, Dyn 11 T 10 5,01 11 kv/400 V 3, Dyn 11 T 11 4,8 11 kv/400 V 3, Dyn 11 Table 2: Transformer parameters. Cable R1 [Ω/km] X1 [Ω/km] Main sub to pack shed RMU (XLPE) 0, ,01848 Main sub to plant 1 RMU1 (XLPE) 0, ,04587 Main sub to plant 2 RMU2A (XLPE) 0, ,02688 Main sub to plant 3 RMU3 (XLPE) 0, ,00792 Main sub to plant 4 RMU4 (ABC) 0, ,09639 Pack shed RMU to plant 2 RMU2B (XLPE) 0, ,03475 Plant 1 RMU1 to plant 2 RMU2A (XLPE) 0, ,01529 Plant 2 RMU2B to offices TRF (XLPE) 0, ,03475 Table 3: Cable parameters. Load kva Voltage pf Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Load V 0,75 inductive Table 4: Load parameters. The sub-transient period, which occurs directly at the fault point and only lasts for a few cycles The transient period, which occurs for tens of cycles The steady-state period, which will last for a longer time, usually until there is a change in the system such as a line-fail or if a circuit breaker is opened. Fig. 1 shows the different states. Types of faults Fault currents may involve all three phases in a symmetrical manner, or may be asymmetrical where usually only one or two phases are involved. Faults may also be caused by either short-circuits to earth or between live conductors, or may be caused by broken conductors in one or more phases. Sometimes simultaneous faults may occur involving both shortcircuit and broken conductor faults (also known as open-circuit faults). Symmetrical three phase faults This study will be confined to symmetrical three phase faults as they are the most severe faults that can occur in a power system. These faults give the maximum short-circuit currents (SCC) and fault MVA. Hence, this worst-case result is then used as the basis to select the shortcircuit capabilities of switchgear from manufacturers tables. A three phase fault is a function of three equal fault impedances to the three phases as shown in Fig. 2. If = 0 then the fault is called a solid or bolted fault. These faults can be of two types: (a) phase to phase to phase to ground (PPPG) fault or (b) phase to phase to phase (PPP) fault. The system will remain balanced because all three phases are affected equally. Since the system is balanced we only need to know the positive sequence to analyse the fault. A single line diagram can be used, as energize - December Page 50

3 CS1 11 kv Main sub to plant 1 19, ,03824 Plant 1to T 2 (primary) 19, ,00418 T 2 (secondary) to 400 V busbar 19, ,6154 Plant V busbar 11 kv Main sub to plant , ,03633 Plant 1 to T 3 (primary) 19, ,00226 T 3 (secondary) to 400 V busbar 19, ,56223 Pack shed 400 V busbar 11 kv Main sub to pack shed 20, ,06490 Pack shed to T 1 (primary) 19, ,01945 T 1 (secondary) to 400 V busbar 19, ,03500 Pack shed to plant 2B cable 1, , kv Main sub to plant 3 19, ,04874 Plant 3 to T 4 (primary) 19, ,01403 T 4 (secondary) to 400 V busbar 19, , kv Main sub to plant 3 19, ,02280 Plant 3 to T 5 (primary) 18, ,99612 T 5 (secondary) to 4 00 V busbar 18, , kv Main sub to plant 2A 22, ,16926 Plant 2A to T 6 (primary) 21, ,13467 T 6 (secondary) to 400 V busbar 21, , kv Main sub to plant 2A 20, ,09611 Plant 2A to T 8 (primary) 20, ,06145 T 8 (secondary) to 400 V busbar 20, , kv Main sub to plant 4 19, ,02929 Plant 4 to T 10 (primary) 18, ,99612 T 10 (secondary) to 400 V busbar 18, , kv Main sub to plant 4 20, ,07050 Plant 4 to T 9 (primary) 19, ,03744 T 9 (secondary) to 400 V busbar 19, ,52953 Plant 2B 400 V busbar Pack shed cable to plant 2B 21, ,10768 Plant 2B to T 11 (primary) 20, ,09710 T 11 (secondary) to 400 V busbar 20, ,17038 Office transformer 400 V busbar Pack shed to plant 2B 7,0409 0,41489 Plant 2B to office transformer (primary) 7, ,37577 Office transformer (secondary) to 400 V busbar 7, ,33381 Table 5: 80% load capacity. Fig. 3: Equivalent single phase circuit. all three phases are equal but displaced by 120 [4]. where: I sc3 = Three-phase short-circuit V ph = Phase voltage [1] Z SC = Positive sequence impedance per phase Power system analysis tools Complete method The complete method is utilised to perform the short-circuit study. This method is commonly known as the superposition method and is summarised in Fig. 4. Modelling consideration The network consists of an ideal 11 kv voltage source with no internal impedance. Two winding transformer The two-winding transformer is modelled with a positive sequence network. Shortcircuit voltage Uk (%) and ratio X/R or copper losses. The magnetising circuit is not included due to the magnetising current being negligible [5]. Cables The positive sequence network for cables is modelled identically using the nominal π-method. The values for positive and negative is assumed to be equal. The sequence impedances consist of series resistance R in ( /km), series reactance X ( /km) [5]. DIgSilent neglects the capacitance in the positive and negative sequence networks when the nominal π-method is used. The positive sequence network of the simulation is illustrated in Fig. 5. Linear load The loads are modelled as fixed impedance loads. The loads contain static components which do not provide short-circuit contribution to a fault. Network parameters The parameters for the AC voltage source, energize - December Page 52

4 CS2 11 kv Main sub to plant 1 19, ,04618 Plant 1to T 2 (primary) 19, ,00380 T 2 (secondary) to 400 V busbar 19, ,60438 Plant V busbar 11 kv Main sub to plant 1 19, ,04427 Plant 1 to T 3 (primary) 19, ,00188 T 3 (secondary) to 400 V busbar 19, ,55163 Pack shed 400 V Busbar 11 kv Main sub to pack shed 20, ,07585 Pack shed to T 1 (primary) 19, ,01929 T 1 (secondary) to 400 V busbar 19, ,03050 Pack shed to plant 2B cable 1, , kv Main sub to plant 3 20, ,05718 Fig. 4: Superposition calculation method flow diagram. Plant 3 to T 4 (primary) 19, ,01398 T 4 (secondary) to 400 V busbar 19, , kv Main sub to plant 3 19, ,03125 Plant 3 to T 5 (primary) 18, ,98801 T 5 (secondary) to 400 V busbar 18, ,17037 Fig. 5: Short-circuit nominal π model for a cable in DIgSilent. transformers, cables and loads are indicated in Tables 1, 2, 3 and 4. Simulation case study Three case studies were conducted. For all studies, firstly a load flow investigation was conducted to obtain pre-fault conditions. Secondly, a short-circuit study was performed using the superposition calculation method to attain maximum fault currents and fault levels. Case study 1 (CS1) For the first case study a symmetrical threephase fault on plant V busbar, and then plant 2A 400 V busbar was performed at 80% full load capacity. Case study 2 (CS2) For the second case study a symmetrical three-phase fault on plant V busbar, and then plant 2A 400 V busbar was performed 100% full load capacity. Case study 3 (CS3) A symmetrical three-phase fault was performed when plant 2 is fed via plant 1, which is normally open. Results Only the short-circuit power and phase 11 kv Main sub to plant 2A 22, ,17763 Plant 2A to T 6 (primary) 21, ,13457 T 6 (secondary) to 400 V busbar 21, , kv Main sub to plant 2A 21, ,1453 Plant 2A to T 8 (primary) 20, ,06135 T 8 (secondary) to 400 V busbar 20, , kv Main sub to plant 4 19, ,03659 Plant 4 to T 10 (primary) 18, ,99532 T 10 (secondary) to 400 V busbar 18, , kv Main sub to plant 4 20, ,07773 Plant 4 to T 9 (primary) 19, ,03661 T 9 (secondary) to 400 V busbar 19, ,50664 Plant 2B 400 V busbar Pack shed cable to plant 2B 21, ,10995 Plant 2B to T 11 (primary) 20, ,09683 T 11 (secondary) to 400 V busbar 20, ,16272 Office transformer 400 V busbar Pack shed cable to plant 2B 8,0896 0,42414 Plant 2B to office transformer (primary) 7, ,37558 Office transformer (secondary) to 400 V busbar 7, ,32832 Table 6: 100% load capacity. energize - December Page 53

5 cable between the main sub and plant 4 being buried in the ground does not comply with regulations. Buried cables must be mechanically protected, e.g. by being armoured or by being laid in a protective pipe, or otherwise protected by laying concrete slabs above the cable mm below ground level. ABC cables are not meant to be buried, although this is done in some instances. It is advisable to use Bucholtz relay protection on transformers rated at 1 MVA or higher. This type of protection can give early warning of faults developing within the transformer thus preventing catastrophic failure. The low voltage switchgear on the secondary of the transformers should be rated at, or higher then obtained values of fault level on the 400 V busbar. 11 kv Main sub to plant 1 21, ,12805 Plant 1 to T 1 (primary) 19, ,99974 T 1 (secondary) to 400 V busbar 19, ,49297 Fig. 6: Kromco's electrical network. Plant 1 to plant 2A 21, ,14196 Plant 2Ato T 6 (primary) 20, ,10020 T 6 (secondary) to 400 V busbar 20, ,25559 CS3 Table 7: Plant 2A fed via plant 1. Cable Size (mm 2 ) Current rating (A) 1 s short circuit rating XLPE ,58 XLPE ,01 XLPE ,6 ABC currents results were obtained for each case study. Table 5 indicates the results of the total short circuit power and the short circuit currents at 80% load capacity. Table 6 indicates the results of the total short-circuit power and the short-circuit currents at 100% load capacity. Table 8: Typical cable ratings. Table 7 indicates the results of the total short-circuit power and the short-circuit currents when plant 2A is fed via plant 1. Analysis of results Table 8 compares cables for short-circuit results. All cables will operate well within their rated short-circuit capacity. The ABC At 100% load capacity the transformers are heavily loaded from about 95,06 to 95,11% rated capacity which will operate the protection continuously if transformers are overloaded. Conclusion Fault analysis plays a vital role in a power system analysis as it is used to determine the appropriate protection scheme for a power system. The short circuit current based on the rated equipment values are used in properly determining the minimum protection requirement as the obtained fault current is the maximum fault current that may occur within the system. Thus, the obtained fault levels in this paper acts as prerequisite for protection coordination studies. Despite limitations of obtaining the necessary data PowerFactory simulations can be a guide line on loading of the network and short-circuit currents flowing in the network, for the system expansions. A thorough investigation can be conducted in future with cost implications for attaining time synchronous data loggers for measuring real time data of the different loading conditions of the plant. Therefore contingency plans can be drafted on a more efficient manner on the operation of the plant. References [1] MH Hairi, H Zainuddin, MHN Talib, A Khamis and JY Lichun: An investigation of short circuit analysis in Komag Sarawak operations factory, World Academy of Science, Engineering and Technology, [2] IEEE Violet Book: Recommended practice for calculating short-circuit currents in industrial and commercial power systems, [3] IEC : Short-circuit currents in threephase ac systems, [4] John J Grainger, William Stevenson. [5] DIgSILENT PowerFactory: DIgSILENT PowerFactory version 14v0E Technical Reference, DIgSILENT, Contact Bantubonke Matika, Metrorail, Tel , bmatika@metrorail.co.za energize - December Page 54