Application of on load tap changer for voltage control.
Automatic on-load voltage regulation by using on-load tap changer (OLTC) is an effective and suitable method to deal with voltage changing in power systems (Hong and Wang, 1995; Yorino et al, 1997; Faiz and Siahkolah, 2011). Paper (Yulin et al., 2012) discusses that automatic on-load voltage regulation of transformer is an effective method to stabilize load voltage. Mechanic contact tap changing is not adequate for distributing transformer because of its high cost and low capability. Paper (Yulin et al., 2012) presents the structure and automatic on-load voltage regulating principle of distributing transformer, which employs solid state relay as non contact automatic on-load voltage regulating tap changer. The generation mechanism and limiting measure of circular current that is occurred in the process of changing tap joint. By theoretical analysis and experimental verification, it is concluded that it has different occurring process of circular current when voltage is regulated to higher or lower value. Paper (Capitanescu et al., 2009) reports on prospective tests of a system protection scheme against long-term voltage instability relying on a set of distributed controllers, each monitoring a transmission voltage, blocking tap changers and shedding loads in a zone. The emergency actions adjust in magnitude and location to the disturbance. Each controller acts in closed-loop, which guarantees robustness. The method is illustrated on a real-life model of the Western region of the RTE system. The choice of the controller settings is discussed in some detail and examples of performance are given, combining the above remedial action with capacitor switching and secondary voltage control. Paper (Yan et al., 2014) addresses the impacts of residential photovoltaic power fluctuation on on-load tap changer operation and a solution using DSTATCOM. The paper explains that recently penetration of Photovoltaic (PV) systems in Low Voltage (LV) distribution networks has drastically increased. The stochastic nature of PV output coupled with the characteristic of high R/X ratio of LV distribution network results in a direct correlation between PV power and voltage fluctuations. Currently distribution networks rely heavily on existing On Load Tap Changing (OLTC) transformers to mitigate voltage fluctuations. However, during the regulation process OLTC transformers can experience excessive tap changing with high PV penetration, which in turn results in an increased maintenance requirement and shorter lifetime. Without the knowledge of how an increase in PV penetration impacts the operation of an OLTC, a Distribution Network Operator (DNO) cannot effectively coordinate compensation. This paper examines the effect of PV penetration levels on OLTC transformers and voltage regulation. The investigation of tap changing frequencies and voltage profiles is done through quasi-static time- series simulations using a residential street network and one year real world data obtained from local DNOs and research institutes. Then, a mitigation strategy is developed using a Distribution Static Compensator (DSTATCOM) and interaction between OLTC and DSTATCOM is also analyzed. This will enable DNOs to choose an appropriate size of DSTATCOM based on economic and technical requirements of voltage regulation. Paper (Viawan et al., 2007) discusses voltage regulation on medium-voltage feeders with distributed generation (DG) using on-load tap changer (LTC) and line drop compensation (LDC). The analysis shows that LTC is robust against DG, whereas DG can affect the effectiveness of the voltage regulation provided by LDC. However, with proper coordination between DG and LDC, it is possible to ensure voltage regulation without unnecessarily restricting the integration of DG. It is shown that, while lowering the LTC setting can increase the DG integration limit, even higher increase can be obtained by activating the LDC feature, which is present in most LTCs, but often not used. LDC regulation is also compared with other alternatives such as using a DG unit with voltage control capability and installing a line voltage regulator.
This paper aims at showing the effects of on-load tap changer for voltage control as well as showing OLTC on the power system performance. A typical test system installed with OLTC is considered as case study and the performance of OLTC is investigated following changing input voltage.
1. On Load Tab Changer:
For many power transformer applications, a supply interruption during a tap change is unacceptable, and the transformer is often fitted with a more expensive and complex on-load tap- changing mechanism. On-load tap changers may be generally classified as either mechanical, electronically assisted, or fully electronic. The basic arrangements of tap windings are shown in Figure 1 (Dohnal, 2010).
Linear arrangement (Figure 1-a), is generally used on power transformers with moderate regulating ranges up to a maximum of 20 %. The tapped turns are added in series with the main winding and changes the transformer ratio. The rated position can be any one of the tap positions. with a reversing change-over selector (Figure 1-b) the tap winding is added to or subtracted from the main winding so that the regulating range can be doubled or the number of taps reduced. during this operation, the tap winding is disconnected from the main winding. The greatest copper losses occur, however, in the position with the minimum number of effective turns. This reversing operation is realized using a change-over selector which is part of the tap selector or of the selector switch (arcing tap switch). The rated position is normally the mid position or neutral position. The double reversing change-over selector (Figure 1-c) avoids the disconnection of tap winding during the change-over operation. In phase-shifting transformers (PsT) this apparatus is called the advance-retard switch (Ars). Using a coarse change-over selector (Figure 1-d) the tap winding is connected either to the plus or minus tapping of the coarse winding. during coarse selector operation, the tap winding is disconnected from the main winding. In this case, the copper losses are lowest in the position of the lowest effective number of turns. This advantage, however, places higher demands on insulation material and requires a larger number of windings. The multiple coarse change-over selector (Figure 1-e) enables multiplication of the regulating range. It is mainly used for industrial process transformers (rectifier/furnace transformers). The coarse change-over selector is also part of the OLTC. which of these basic winding arrangements is used in each individual case depends on the system and operating requirements. These arrangements are applicable to two winding transformers as well as to autotransformers and to phase-shifting transformers (PsT). where the tap winding and therefore the OLTC is inserted in the windings (high-voltage or low voltage side) depends on the transformer design and customer specifications.
2. Test System:
Figure 2 shows the test system. A 25 kV distribution network consisting of three 30-km distribution feeders connected in parallel supplies power to a 36 MW/10 Mvar load (0.964 PF lagging) from a 120 kV, 1000 MVA system and a 120kV/25 kV OLTC regulating transformer. Reactive power compensation is provided at load bus by a 15 Mvar capacitor bank. The OLTC transformer model implement a three-phase regulating transformer rated 47 MVA, 120 kV/25 kV, Wye/Delta, with the OLTC connected on the high voltage side (120 kV). The OLTC transformer is used to regulate system voltage at 25 kV buses B2. Voltage regulation is performed by varying the transformer turn ratio. This is obtained by connecting on each phase, a tapped winding (regulation winding) in series with each 120/sqrt(3) kV winding. Nine (9) OLTC switches allow selection of 8 different taps (tap positions 1 to 8, plus tap 0 which provides nominal 120kV/25 kV ratio). A reversing switch included in the OLTC allows reversing connections of the regulation winding so that it is connected either additive (positive tap positions) or subtractive (negative tap positions). For a fixed 25 kV secondary voltage, each tap provides a voltage correction of 0.01875 pu or 1.875% of nominal 120 kV voltage. Therefore, a total of 17 tap positions, including tap 0, allow a voltage variation from 0.85 pu (102 kV) to 1.15 pu (138 kV) by steps of 0.01875 pu (2.25 kV).
3. Simulation Results:
In order to show the OLTC performance, the input voltage is changed as shown in Figure 3. It is clear that voltage at bus 1 has three changes at seconds 10, 50 and 100 respectively. The tap position following these changes is shown in Figure 4. The tap position follows the input voltage pattern and tracks the references values. Figure 5 shows the voltage at bus 2. It is clear that voltage at this bus is under control and tracks a reference value. This controllability has been obtained by using OLTC. Active and reactive powers at bus 1 are also depicted in Figures 6 and 7 respectively. The simulation results show that all parameters are affected by tap changing. It is also clear that OLTC can successfully control the voltage at bus 2 and permitted rage of voltage is satisfied.
This paper presented the effects of on load tap changing on the power system performance. A typical test system installed with OLTC was considered as case study and the performance of OLTC was investigated following changing input voltage. The simulation results demonstrated that OLTC is a suitable component to control voltage following changing input voltage.
Received 25 January 2014
Received in revised form 12
Accepted 14 April 2014
Available online 25 June 2014
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Mojtaba Shirvani, Mostafa Abdollahi, Ali Akbar Dusti, Iman Baghbani
Department of Electrical Engineering, College of Engineering, Boroujen Branch, Islamic Azad University, Boroujen, Iran
Corresponding Author: Mojtaba Shirvani, Islamic Azad University, Department of Electrical Engineering, Boroujen, Iran. Tel: 933824223812; E-mail: email@example.com
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|Author:||Shirvani, Mojtaba; Abdollahi, Mostafa; Dusti, Ali Akbar; Baghbani, Iman|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Jun 1, 2014|
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