LOAD BEHAVIOUR DURING VOLTAGE DIPS: A VOLTAGE QUALITY STUDY IN LOW VOLTAGE DISTRIBUTION SYSTEM

Transcription

1 LOAD BEHAVIOUR DURING VOLTAGE DIPS: A VOLTAGE QUALITY STUDY IN LOW VOLTAGE DISTRIBUTION SYSTEM I. Rendroyoko R.E. Morrison Peter K.C. Wong* Department of Electrical & Computer Science Monash University, PO BOX 35, CLAYTON, VICTORIA 38 Phone: , Fax: Ignatius.rendroyoko@eng.monash.edu.au * United Energy Ltd., PO BOX 1185, Moorabbin, VIC 3189 Abstract Over the last ten years, many studies have been performed on voltage dip characteristics in industrial, commercial and residential systems. The characteristic of each voltage dip is unique and particular to each electrical system. This paper presents a study of load behaviour during voltage dips on a low voltage distribution system, consisting of light commercial and residential customers. The recorded voltage dip measurements were used to analyze the system voltage dip characteristic. The Power System Blockset (PSB) from MATLAB was used in modeling the distribution system components and simulating the voltage dips caused by faults. The influence of loads on voltage dips during faults, especially during the winter and summer season, is presented. It is concluded that the load may influence the voltage dip characterization and that load effects must be accounted for to achieve high modeling accuracy. 1. INTRODUCTION Voltage dips have become a major concern in power quality in the past decade. The cost of economical loses and inconveniences caused by voltage dips have triggered some studies and research activities. Many experts have tried to characterise voltage dips [1,2,3]. The existing standard on voltage dip characterises the voltage dips in terms of magnitude and duration. The characterisation of the standard is based on the assumption that faults will cause rectangular voltage dips. It is also assumed that the voltage drops to a certain low value immediately and when the fault is cleared, the voltage recovers back to normal immediately. The assumption of rectangular voltage dips, however, is not correct in a realistic system, which largely consists of rotating machines or motors. When a fault occurs, all the rotating machines in the system slow down and after the fault is cleared, the motors will accelerate to the normal condition. During acceleration, motor will draw high current from the system and thus prolong the voltage dip. This paper will discuss load behaviour on a system during and after a voltage dip. For the purpose of this paper, one sub-system in south-eastern Victoria was selected. This sub-system has significant differences in load characteristics between the summer and winter season. In the summer season, there is an increase of energy consumption, which is mostly due to the operation of air conditioners. Therefore, part of the load consists of electrical rotating machines. 2. SYSTEM COMPONENTS The distribution system under study is presented in fig. 1. A main 66kV bus bar supplies the 22kV distribution system trough 2 66/22kV 3-MVA transformers and the sub-system is supplied at 415 V from a 22/.415kV 4-kVA transformer. The transmission and distribution systems supply electric power to a south-east area of Melbourne in the geographical distribution area of United Energy Ltd. (UE). The sub-system supplies mostly commercial customers, and a few residential and light industrial customers.

2 subsystem could be due to air conditioners. In summer, the sub-system will have more rotating machines Subsystem Max Load KVAR1 KW1 KVA1 KVAR2 KW2 KVA Load Figure 1 One-line diagram of the sub-system under study The circuit parameters of the distribution system under study are shown on table 1. Circuit Voltage Postv Seq-%on 1MVA Zero Seq-%on 1MVA From To No kv Type R1 X1 B1 R X B Source NW66kV 66 Generator/Source 2.5% 6.99% 2.11% 17.95% NW66kV NW22kV 1 66/22 Trfr - 2/3MVA 1.98% 51.36% 7.92% 2.54% NW66kV NW22kV 2 66/22 Trfr - 2/3MVA 1.93% 5.96% 7.72% 2.38% NW22kV WHORSE-SVLE 1 22 O/H- 19/3.25AAC kV/433V 4kVA 1 22/.433 Trfr - 4KVA 4.% 4.% Cap Banks NW22kV Mvar Cap Banks NW22kV Mvar Table 1 Sub-system's Circuit Parameters For monitoring and measurement purposes, UE has installed power quality monitoring equipment at the 22kV bus and 415V bus. Thus, every fault occurring in the system will be recorded. 3. SUMMER AND WINTER LOADS The subsystem under study has a specific characteristic in load trend. To some extent, the amount of load is different between summer and winter seasons. Usually, in summer, the system has more loads than in winter. The load variation between summer and winter is shown in fig. 2. Since, the subsystem consists of residential, commercial and light industrial customers, the difference of load between summer and winter in this 5 1 : Winter 1 load 2 : Summer load Days Figure 2 Summer and Winter Max Load Variation 4. VOLTAGE RECOVERY Faults in the distribution system might cause voltage dips. The location of fault, type of fault, fault clearing time and the electrical system configuration will also affect the voltage dip [3]. A Voltage dip is normally characterised by a magnitude and duration, however, another researcher also mentioned phase angle jump and post fault dip as a further important characteristics [4]. A voltage dip occurring in a system that has resistive loads, will have rectangular shaped dips. When the fault occurs, the voltage directly reduces to a particular value, and when the fault is cleared, the voltage recovers back to its original level immediately [1]. This does not happen when parts of the load consist of rotating machines such as induction motors or air conditioner motors. One of the results of the voltage dip recording is shown in figure 3. Figure 3 shows 415V bus voltage due to a fault of 2ms on a 22kV distribution system. The fault which has occurred on the 22kV system is a single phase to ground fault. However, it is seen in the low voltage side as a two phase to ground fault because of transformer vector connection (delta/star). The voltage dip was recorded at 2:51:34, on 1 December 2. When the fault occurred, the bus voltage dip did not follow step change but instead decreased to a certain point and then decaying to a lower rapidly voltage

3 levels during the short circuit period. After the fault is cleared, the voltage did not directly recover to its level before fault. The voltage need a longer time to recover back and this could be caused by air conditioning motor loads. 5.1 Balanced Fault During a 3Ph-G fault in the 22kV system, the 415V bus bar voltage also drops in magnitude. Some of the simulation results are shown below: V Figure 3 Voltage dips in the subsystem 5. SIMULATION OF POST FAULT LOAD BEHAVIOUR Figure 4 Voltage dip for a 3Ph-G fault Load Current during I3 In order to evaluate and analyse the rotating machine's influence to the sub-system, a simulation model of the sub-system has been developed. The simulation uses the power system blockset tools within the MATLAB package. For simulation purposes, a SLGF and a 3Ph-G fault are simulated at both 22kV and 415V. In the model, the subsystem mimics the real subsystem shown in figure 1. There are six feeders supplying electricity to consumers consisting of resistive loads and rotating motor load models. The air conditioning motor load is 27% of the load in the subsystem. The typical air conditioning motor parameters used in the simulation are given below: Phase 1 Ph Motor 3Ph Motor Rated Capacity 1 Hp 8 HP Power Supply 24 V 415 V Frequency 5 Hz 5 Hz Rs (stator resistance) 1.2 Xs (stator reactance) Rr (rotor resistance).9.4 Xr (rotor reactance) Xm (magnetizing reactance) J (rotor inertia) Table 2 Air conditioning motor parameters Figure 5 Load current during 3Ph-G fault Figure 4 and 5 show the 415V-bus voltage and load current during a 3Ph-G fault in the 22kV system. At the 3Ph-G fault, the bus voltage will be suppressed until the fault is cleared. The characteristics of the airconditioning motor load affects the voltage drop and voltage recovery. The subsystem voltage may swing for a few cycles before returning to normal. These swings lengthen the duration of the recovery process [3]. 5.2 Unbalanced Fault Most of the faults on a medium voltage system are single-phase to ground faults [5]. Single-phase faults

4 often result from lightning, wind, tree-branch contact or insulator failure. The behaviour of the sub-system under study during an unbalanced fault is more complicated than during a three-phase to ground balanced fault. The results of this process are shown in figure 8 and 9 below V V Figure 6 Voltage dip for a 1Ph-G fault Load Current during I3 Figure 8 Positive, negative and zero sequence voltage for the single phase to ground fault shown in fig I Figure 7 Load Current during 1Ph-G fault The busbar voltage during a single phase to ground fault at 22kV line is shown in fig. 6. The 22/.415kV transformer connection makes the voltage dip seen as a 2Ph-G fault at the 415V busbar voltage. Single phase to ground faults give less severe problems to motor loads than 3Ph-G faults. However, voltage recovery after the fault is still affected. Using symmetrical components, the characteristics of the voltage waveform can be clearly seen by dividing them into positive, negative and zero sequence voltage components Figure 9 Positive, negative and zero sequence current for the single phase to ground fault shown in fig. 7 After fault initiation, the positive-sequence voltage decreases while the negative-sequence voltage increases. In fig. 9, the positive-sequence current drops after fault initiation and suddenly jumps to almost three times than normal load current before its slowly decays to a steady level during the fault. This phenomenon is caused by the air conditioning motor characteristics. When the fault occurs, the motors slow down causing a decrease in positivesequence impedance. This decrease in positivesequence impedance is the cause of the increase in positive-sequence current and the decrease in positive-

5 sequence voltage. The effect is probably due to speed reduction of the motor. 6. CONCLUSIONS It has been reported that voltage dips which occur on a system with no rotating machines result in a rectangular profile dip. The voltage directly drops to a particular level during fault. After the fault is cleared, the voltage returns to the level present before the fault occurred to 1983 and at Staffordshire University from 1983 to He joined Monash University, Australia in 1997 as director of The Centre for Electrical Power Engineering. Peter KC Wong received his BSc in Electrical Engineering from the University of Hong Kong in He is currently the Protection & Planning Manager, United Energy, where he is responsible for protection strategy and long-term system planning of the electricity and natural gas distribution networks. He is a member of IEE and IEAust. A different phenomenon can be found in a system with rotating machine loads. When a fault occurs, the voltage does not directly drop to its minimum level but it decays until reaching a steady condition before the fault is cleared. At that time, the voltage does not directly return, but recovers slowly until reaching its original level. From these two results, it can be concluded that load influences the voltage dip characteristization. 7. REFERENCES 1. Math H.J Bollen, "Understanding Power Quality Problems: Voltage Sags and Interruptions", IEEE Press, New York, Math H.J Bollen, The Influence of Motor Reacceleration on Voltage sags, IEEE Trans. on Ind. Applicat., Vol. 31, No. 4, July/August J.C Das, The effects of momentary voltage dips on the operation of induction and synchronous motors, IEEE Trans. Ind. Applicat., vol.26, pp , Lidong Zhang; Math H.J. Bollen, Characteristic of (Sags) in Power Systems, 8 th International Conference on Harmonics and Quality of Power ICHQP 98, 1998, pp McGranaghan, Mark F., Mueller, David R., Samotyj Marek J., Voltage Sags in Industrial Systems, IEEE Trans. on Ind. Applicat., Vol. 29, No. 2, March/April Shaffer, John W., Air Conditioner Response to Transmission Faults, IEEE Transactions on Power Systems, Vol.12, No.2, May 1997 Ignatius Rendroyoko, a student member of IEEE, was born in Indonesia in 197. He graduated from the Institute Technology of Bandung, Indonesia in 1994 and served as an electrical engineer in PLN since He is currently working towards a M.Eng.Sc. degree at Monash University, Australia. Professor RE Morrison was born in Stoke on Trent, United Kingdom in He received his BSc degree and PhD degree from University of Staffordshire University, UK in 1973 and 1981 respectively. Professor Morrison worked for ALSTOM (UK) from