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A review on voltage control methods using on-load tap changer transformers for networks with renewable energy sources
Renewable and Sustainable Energy Reviews 62 (2016) 1154–1161
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
A review on voltage control methods using on-load tap changer transformers for networks with renewable energy sources Charles R. Sarimuthu a,b,n, Vigna K. Ramachandaramurthy a, K.R. Agileswari a, Hazlie Mokhlis c a Power Quality Research Group, Department of Electrical Power Engineering, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia b School of Engineering, Taylor's University, Lakeside Campus, No.1, Jalan Taylor's, 47500 Subang Jaya, Selangor, Malaysia c Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 13 January 2015 Received in revised form 8 January 2016 Accepted 3 May 2016
Voltage control is an important method for regulating the feeder voltages in a distribution network. Various voltage control methods are used by distribution network operators (DNOs) in order to maintain the network voltages to be within an acceptable voltage level. Traditionally, on-load tap changer (OLTC) and automatic voltage control (AVC) relays are often employed in regulating the network voltages. However, the traditional voltage control techniques are no longer suitable when renewable energy (RE) sources are connected to the network because of the possibility of bidirectional power flows. The presence of reverse power flow will affect the feeder voltage profiles and influence the voltage control scheme practiced in the distribution system. This paper presents an overview on the various OLTC voltage control schemes which are used to control the voltage in distribution networks containing RE sources. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Power transformer On-load tap changer transformers Distribution systems Automatic voltage control relay
Contents 1. 2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 Traditional voltage control scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 2.1. Voltage control with load tap changer (LTC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 2.2. Voltage control with line drop compensation (LDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 2.3. Grading time (GT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 Enhanced voltage control AVC relay scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 3.1. Source drop compensation (SDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 3.2. Pre-emptive tap changer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 Voltage control scheme for OLTC in parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 4.1. Master-follower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 4.2. True circulating current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 4.3. Negative reactance compounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 4.4. Transformer automatic paralleling package (TAPP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 Modern voltage control scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 5.1. Enhanced transformer automatic paralleling package (Enhanced TAPP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 5.2. SuperTAPP nþ relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158 5.3. Intelligent AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158 5.3.1. ANN controller based AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158 5.3.2. Fuzzy logic controller based AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1159
n Corresponding author at: Power Quality Research Group, Department of Electrical Power Engineering, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia. Tel.: þ60 129353924; fax: þ60 3 56295477. E-mail addresses: [email protected] (C.R. Sarimuthu), [email protected] (V.K. Ramachandaramurthy).
http://dx.doi.org/10.1016/j.rser.2016.05.016 1364-0321/& 2016 Elsevier Ltd. All rights reserved.
C.R. Sarimuthu et al. / Renewable and Sustainable Energy Reviews 62 (2016) 1154–1161
5.3.3. State estimation based AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Concepts of OLTC control in distribution system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Electrical power distribution systems are normally operated at multiple voltage level. The voltage levels differ based on the amount of power generated by RE sources and load variations in the network. These different voltage levels are kept within acceptable limits by including an OLTC transformer where the substation secondary bus voltage is kept stable by adjusting the tap position [1]. Tap position adjustment is necessary to physically alter the ratios of the transformer for voltage regulation. The OLTC operate by changing the number of turns in one winding of the transformer to keep the transformer output voltage within predicted limits. The OLTCs are motorized mechanical switching arrangements that adjust the transformer turns ratio, typically in steps of 1.25% or 1.43%, whilst the transformers are in use and carrying a load [2]. The OLTC transformer is normally applied in the distribution networks to step down from 33 kV to 11 kV or 6.6 kV. Each OLTC transformer is linked to an AVC relay in order to increase or decrease the voltage by changing the tap position of transformer [3]. However, the operation of AVC relay can be affected by the possible existence of bidirectional power flows when RE sources are connected to the network. Innovation is required in OLTC voltage control scheme in order to support the current implementation of smart grid incorporating RE sources such as wind, solar and hydrogen [4]. Studies on the size, operating power factor mode [5,6], and location of RE sources in the network are needed in order to design an innovative OLTC voltage control scheme to cope with the increasing RE sources connected into the distribution network.
2. Traditional voltage control scheme Traditionally, the voltage control in distribution networks without RE sources is performed using OLTC, shunt capacitors, shunt reactors, static var compensation etc. This paper will focus on voltage control strategies involving OLTCs in distribution network connected with RE sources. Without RE sources connected in a distribution system, the power flow is assumed to be unidirectional and the set-values for the OLTCs are chosen according to the voltage drop along the feeder. Based on the set-values, the voltage at the secondary of the OLTC transformer is changed in order to prevent the voltage along the feeder from breaching the lower voltage limit and not exceeding the upper voltage limit. The change in voltage at the secondary of the OLTC transformer is achieved when the AVC relay initiates a signal to the tap changer system to change its tap position.
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normally consists of initial delay and inter tap delay. The initial delay ranges from 10 to 120 s and the inter tap delay for each step tap changer operation is from 5 to 60 s [8].
2.2. Voltage control with line drop compensation (LDC) In an OLTC operation, normally LTC is provided with LDC function to control the voltage at a remote point. Besides monitoring the transformer's terminal voltage, this function includes the measurement of the secondary current [9]. The measurement is used to simulate the voltage drop along the feeder impedance that exists between the transformer terminal and the load point [10]. In order to keep the correct voltage level at the load side, line resistance R and line reactance X is used to increase the regulated voltage at the transformer terminal. Voltage control at a nominal load point rather than at the transformer terminal is achieved using LDC.
2.3. Grading time (GT) AVC relay with LDC function operates in between different voltage levels in power supply networks. If a downstream tap changer is allowed to operate before an upstream tap changer, then the OLTCs might work against one another and become unstable. In order to correct this situation, the GT is introduced (Figs. 2 and 3). GT ensures that the initial time delay is longer for the downstream controllers compared to the upstream controllers. The different initial time delay for downstream and upstream controllers are required to ensure that upstream operations are given preference and carried out first. This time grading strategy requires the upstream transformer to finish its operation before the down-stream transformer restores the voltage level.
2.1. Voltage control with load tap changer (LTC) The basic arrangement of voltage control with LTC regulation is shown in Fig. 1. Since the voltage on a conventional distribution network (without RE source connection) decreases towards the end of feeder, the LTC shall then be set to ensure that the voltage at the feeder end is higher than the minimum allowed voltage and the sending-end voltage is lower than the maximum allowed voltage. The AVC relay determines whether to adjust the tap position or not in order to maintain the voltage level which is assumed to be equal to 1 p.u at the end of feeder [7]. An AVC relay
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Fig. 1. Basic LTC arrangement [7].
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complex and costly. This scheme incorporates the features of Source Drop Compensation (SDC) and Pre-emptive Tap Changer operation. 3.1. Source drop compensation (SDC)
Fig. 2. AVC relay scheme with LDC [11].
SDC optimizes the time grading between voltage levels by determining the voltage at the regulation point by the source current and the feeder impedance between the upstream and down-stream transformer. The down-stream AVC relay uses this voltage when a voltage disturbance occurs. If the voltage of regulation point is outside its voltage dead-band, the down-stream AVC relay will wait until the voltage correction of upstream transformer is completed. However, when the voltage at the regulation point is within its dead-band, the upstream transformer operation is assumed to be complete and the down-stream AVC relay can proceed with the local voltage correction. This over-rides the grading time delays and minimizes the action of the communications based blocking scheme [13]. 3.2. Pre-emptive tap changer Pre-emptive tap changing strategy involves two types of time settings which are the Transient Time (TT) and GT. TT setting is the time that has to elapse before any voltage correction is permitted to take place and GT is the time in which a local tap changer delays the local voltage correction to enable upstream tap changers to complete their voltage correction. The AVC relay issues a tap change command to the OLTC after the transient time in the operation of Pre-Emptive tap changing scheme. The AVC relay then resets its delay time so that any subsequent tap changes occur after the grading time [13]. Therefore, it allows the local AVC relay to correct one tap but then grade with upstream OLTCs if any further tapchangers are required.
4. Voltage control scheme for OLTC in parallel
Fig. 3. Typical power supply network [12].
3. Enhanced voltage control AVC relay scheme The need for GT delay can be replaced by using a communications unit. If communications are available between the OLTC controllers at the different voltage levels, then a blocking signal can be issued to stop the operation of downstream transformer when the up-stream transformer starts its operation. The time delays between OLTCs at different voltage levels are minimized and any voltage correction is completed at the highest level since the blocking signal is removed when the up-stream transformer has done its correction. The system allows for OLTC to operate with optimum efficiency while maintaining voltage quality. Alternatively the Enhanced Voltage Control AVC Relay scheme provides autonomous tap-changer controls without the communications unit due to the fact that communication assisted voltage control schemes are
The features of SDC and pre-emptive tap changing strategy are used when OLTC transformers are operated in series. However, primary-substation transformers are often operated in parallel for higher security and reliability of supply. When transformers are operated in parallel, any difference in the positions of their tap changers will give rise to a circulating current, as shown in Fig. 4. In this condition, the AVC relay should perform to keep the voltage within allowable limits while minimizing the circulating current between parallel transformers. The master-follower method is used to avoid difference in tap changer position for transformers operated in parallel whereas the true circulating current method is used to minimize the circulating current if the tap positions for parallel transformers are different.
Fig. 4. Occurrence of a circulating current IC due to a difference in transformer tap positions [2].
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4.1. Master-follower In the master-follower method, all paralleled transformers are retained on the same tap position. In this method, one master transformer changes the tap position to a satisfactory voltage level and then other transformers follow the same actions as the master [14]. The master-follower scheme can be used with LDC and operates under varying power factor, reverse power flow and with presence of RE sources. The drawback of this method is that circulating current will flow between paralleled transformers using this method unless the transformers are equal with the same impedance, number of taps and incoming voltage. It is impossible to use this scheme across a network because connections between AVC relays are required. 4.2. True circulating current In true circulating current scheme, identical transformers as in the master-follower scheme are used. If the tap positions of the transformer are different, then circulating current will flow between the transformers. The circulating current is formulated as follows: ICIRC ¼ ðIT1 –IT2 Þ=2
ð1Þ
The biasing in opposite polarities is used to correct the OLTC to adjust the relay setting voltage. Therefore the circulating current is minimized [14]. This scheme is applicable with LDC and it performs well under varying power factor, reverse power flow and with integration of RE sources. The disadvantages of this scheme are that it is difficult to parallel transformers which are not in the same site and as the paralleled transformers must have similar impedance, incoming voltage and connections [2]. 4.3. Negative reactance compounding The negative reactance compounding (NRC) method helps to maintain similar tap positions for paralleled transformers by changing the polarity of reactance of LDC setting – XLDC [15]. The following formulas show the relationship between LDC settings and negative reactance compounding (NRC) setting: ZLDC ¼ RLDC þ jXLDC
ð2Þ
ZNRC ¼ RLDC –jXLDC
ð3Þ
The operating principle of NRC is illustrated in Fig. 5. Based on the phasor diagram shown in Fig.5, transformer T1 has much higher tap position compared to transformer T2. Due to the difference in tap position, a circulating current will flow between transformer T1 and transformer T2. The circulating current causes current IT1 to be shifted in clockwise direction and current IT2 in anti-clockwise direction. Both current IT1 and IT2 which passes through the ZNRC setting create a voltage drop IT ∙ ZNRC. The AVC relay uses this voltage drop to determine the proper tap position. Since the voltage at transformer T1 is higher than the target voltage therefore the AVC relay initiates a tap down operation. Similarly the AVC relay of transformer T2 initiates a tap up operation. The action stops with a similar tap position of both the parallel transformers when the circulating current is eliminated and target voltage is achieved.
Fig. 5. Operating principle of NRC [14].
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The advantage of NRC scheme is that it can operate with transformers at different positions in the networks and it does not need to be identical anymore due to the independent action of each transformers. However, the NRC fails to operate satisfactorily when the power factor changes from a set point. Integration of irregular RE sources into the network effects the NRC operation. Apart from that, negative value of XLDC setting could cause poor performance of LDC. An increased value of RLDC is needed in order to maintain the performance of LDC. 4.4. Transformer automatic paralleling package (TAPP) The transformer automatic paralleling package (TAPP) scheme is developed from the NRC scheme. The TAPP scheme reduces the amount of circulating current between transformer T1 and transformer T2. The reduction is achieved by dividing the measured current into circulating current and load transformer current. Circulating current in the TAPP scheme uses techniques based on the target power factor (pftarg) as shown in Fig. 6. Problems due to poor performance of LDC with NRC are eliminated in the TAPP scheme by using two separate circuits. The circuits are used for LDC and compounding purpose. However, the drawback of TAPP scheme is that the load power factor deviation will result in an error in the controlled voltage due to knowledge of the load current being considered as circulating current. The set power factor is the necessary factor to make the voltage control to be satisfactory.
5. Modern voltage control scheme The integration of RE sources into distribution network will affect the flow of power and the voltage profiles in the distribution system [16]. The integration of RE sources will result in an increased voltage at the point of connection. The steady-state voltage rise that occurs when connecting RE sources to distribution networks has been studied in [17]. This rise in voltage level affects the operation of AVC relay and causes voltage regulation problems since the AVC relay voltage reference is no longer proper for an effective operation of AVC relay. Due to this reason, improved voltage control scheme are the topic of on-going research to accommodate the presence of RE sources connected to distribution networks. 5.1. Enhanced transformer automatic paralleling package (Enhanced TAPP) Enhanced TAPP scheme is introduced to remove the drawback related to TAPP scheme [18]. The voltage profile of dynamic distribution networks can also be improved by Enhanced TAPP scheme. The true circulating current mode is applied to control voltage level and paralleled transformers on the same location, while TAPP mode is used to parallel transformers across the network. Principle of Enhanced TAPP scheme is illustrated in Fig. 7. Load current (IT1 þIT2) and the measured transformer current (ITN) is used to calculate the amount of circulating current in the True Circulating Current mode. The voltage detected by AVC relay
Fig. 6. Principle of TAPP scheme [14].
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Fig. 7. Enhanced TAPP scheme in True Circulating Current mode [2].
Fig. 9. SuperTAPP n þ relay [19].
where IG is calculated as follows [19]: ETS ¼
Fig. 8. Enhanced TAPP scheme in TAPP mode [19].
of the transformer with higher tap position is increased using the voltage drop on the compounding setting (ICIRCZT).The same voltage drop is used to decrease the voltage detected by AVC relay of the transformer with lower tap position. Then the tap down action for the OLTC transformer on higher tap position and up action for OLTC transformer on lower tap position are operated until the measured voltage is within the bandwidth of the target voltage. This operation is necessary to achieve minimum circulating current. Comparison between the group load current and full load current is used to determine a suitable increase in LDC voltage as described by the following formula [14]: PT LDCVoltageBoost ¼ V LDC ¼ Z LDC þ
n ¼ 1 I Tn
I FL
ð4Þ
where n – number of transformers within the TAPP scheme IFL – full load current ITn – individual transformer current ZLDC – load drop compensation settings The circulating current in the TAPP mode is determined by total load current and target power factor as illustrated in Fig. 8. In the enhanced TAPP voltage control scheme, adjustment to the tap position is done for minimizing the circulating current. However, this adjustment to the tap position is unable to provide the effective voltage setting to control the voltage profile in the network with RE sources. 5.2. SuperTAPP n þ relay The SuperTAPP nþ relay [20] which is based on Enhanced TAPP scheme [19] is introduced to overcome the limitations faced by application of Enhanced TAPP scheme. The SuperTAPP n þ relay can estimate current output of the RE source IG by the additional current measurement IFG on the feeder 1 with RE source and ratio EST as illustrated in Fig. 9. The ratio EST represents the load share between feeders with RE sources to those without RE sources.
I TL ¼
loadonfeederswithgenerators I1 I FG ¼ ¼ loadonfeederswithoutgenerators I 2 I TL I FG n X
ð5Þ
I Tn ¼ I T1 þ I T2
ð6Þ
I G ¼ ðETS ∙ðI TL I FG ÞÞ I FG
ð7Þ
N¼1
The increase in voltage at the point of connection can be determined and the proper generator compensation bias, VG can be calculated as follows [19]: V G ¼ V GMAX ∙
IG I GMAX
ð8Þ
The above formula is applied to the AVC relay to determine the suitable voltage setting to control the voltage profile in the network with RE sources. 5.3. Intelligent AVC relay The increasing use of renewable energy resources due to its high efficiency and low environmental impact is the key factor for transforming the traditional power grid to smart grid in the future. The smart grid uses intelligent devices and a digital communication in power system to enhance the performance of distribution grids. Therefore, the OLTC voltage control method may need to be changed in the future as the control needs to be more flexible and smarter [21,22]. Fuzzy logic, artificial neural network and state estimation based AVC relay are the types of intelligent AVC relay that is available. 5.3.1. ANN controller based AVC relay S.K. Salman and I.M. Rida presented in [23] an attempt to design an AVC relay based on the application of Artificial Neural Network (ANN). In 1997, it has been reported that considerable attention has been given for application of ANNs to electrical power systems [24–26]. Various types of ANNs are described in literature [27,28]. The model developed in [23] shows that ANN based AVC relay are capable of operating similar to other types of AVC relay [23].The ANN-based AVC relay is trained using data file obtained from load flow studies under various operating condition. In [23], the ANN based AVC relay sends signal to change the tap-changer of OLTC transformer to retain the voltage within the
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allowable limits when the calculated AVC voltage exceeds the limits of 72% of reference voltage. The calculated results at every step are saved into an output file to obtain a particular trend for a specific operating condition. The drawback of this relay is that it performs well only after satisfactory training and testing [23]. 5.3.2. Fuzzy logic controller based AVC relay The integration of renewable energy resources has caused voltage regulation problems due to the interference with the performance of conventional AVC relay [6]. S.K. Salman and Z.G. Wan in 2007 [7] introduced fuzzy logic-based AVC relay to overcome this problem. Fuzzy logic systems are much preferred compared to other artificial intelligent systems because the control rules can be implemented using simple “IF-THEN” relations [29]. The other advantages of these systems are that they are simpler, faster and able to simplify design complexity and lessen the hardware cost [7].Fuzzy logic systems are beneficial because they allow the usage of fuzzy rules which are more expressive than crisp values [30]. Fig. 10 shows a fuzzy logic based AVC relay which consists of 3-input and 1-output. The inputs are low voltage side of OLTC transformer voltage (V), phase angle of the current through the OLTC transformer (PAI), the change of current (ΔI) and the output is the AVC relay voltage (VAVC) [7]. This fuzzy logic control system applies Mamdani's Fuzzy Inference System (FIS) [31] since it is more user friendly compared to the conventional Takagi-Sugeno method. The advantage of the fuzzy logic controller based-AVC relay is that its setting does not require re-adjustment as RE sources are connected into the network. However the relay setting might need to change if the relay is applied to different network. 5.3.3. State estimation based AVC relay Leite et.al., [32] developed a statistical distribution state estimator for application in the 11 kV voltage controller. The distribution state estimator is constructed using techniques which are drawn from transmission network state estimation and distribution state estimation research [33]. In an 11 kV network, state estimation method uses measurements at primary substations to estimate the voltage at each node of the network. In order to safeguard customer voltages within permitted levels, the voltage controller needs accurate data on the voltage at each network node [34]. The functional diagram showing the state estimation and control block which is the content of the voltage controller is illustrated in Fig. 11. The black arrows represent real-time signals whereas the white arrows represent offline data. The advantage of state estimation method is that it does not require electrical measuring equipment at every node of the network [35]. The inputs required for a distribution network state estimator are network topology and impedance data, information about customer loads and a few real-time measurements [36]. The drawback of state estimation based AVC relay is that all the above inputs are required for it to operate as desired. Tables 1 and 2 give a summary of the voltage control methods using on-load tap changer transformers. Both the tables provide description of the voltage control method involve in networks with
Fig. 10. Fuzzy logic based AVC relay [7].
Fig. 11. Voltage controller functional diagram [32].
and without RE sources. The tables also cover the relevant advantage and disadvantages of each voltage control method described.
6. Concepts of OLTC control in distribution system Network control systems have been developed by the distribution system operators in order to maintain the voltage level at consumer's supply point to be within acceptable levels [37]. The network control systems are managed in real time [38]. These control systems consist of communication and control devices [39]. Network parameters such as OLTC tap settings are optimized by these control systems during network operation. OLTCs or management of reactive power flow is used in conventional distribution systems to control the voltage profile. The source of power in a distribution network is usually a substation, which may have OLTCs to supply the distribution network. Voltage profile in the network can be controlled by varying the tap setting on the OLTC. The placement and sizing of RE sources varies for different distribution network configurations [40]. In [41], a multi-objective problem is proposed for the placement and sizing of multiple RE sources in a distribution network to improve the transient stability index in addition to the losses and voltage profile. Maroosi et. al., [42], presented the potential of modern distribution systems to maximally utilize RE sources to reduce greenhouse gas emissions from the power system. Voltage control in distribution systems with RE source involves the use of coordinated voltage control through dispatch of RE source output, OLTCs and reactive power support [5]. The coordinated voltage management in distribution system is categorized as centralized or decentralized. Centralized distribution management system controls several distribution substations and it requires extensive communication networks in order to operate. In [14,32,43–45], it is seen that transformers, reactive power devices and RE sources are used to control the voltage in distribution systems. Transformers are used to vary the voltage directly. OLTCs at substations can raise or lower the voltage level in the distribution network. In networks where RE sources are used to regulate the voltage, large RE source greater than 30 percent of the feeder capacity could cause problem to the OLTC operation when the RE source is suddenly disconnected. This is due to the voltage which becomes too low to support the load and takes a minute or more to recover. The solution is using a control scheme that locks the OLTC at a preselected tap when the generator is operating and interconnected. One common strategy for line regulators is to take the regulator to the neutral tap when the reverse power is sensed by its control. In [46], it is seen that RE sources are considered as intermittent power generation sources. Independent System Operators (ISOs) execute security-constrained daily RE source generation scheduling which takes into account the intermittency of power generation by RE sources [47]. Izadbakhsh et.al., [48] presented shortterm resource scheduling of RE based micro grid which minimizes total operation cost and greenhouse gas emissions due to power
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Table 1 Summarized voltage control strategies of on-load tap changer transformers for networks without RE source connection. Without DG Voltage control method
Description
Load tap changer (LTC) [7]
Detects voltage at the end of the feeder and compares it with the Difficult to detect voltage at a remote point along the feeder.
Line Drop Compensation (LDC) [11]
Grading Time (GT) [11]
Source Drop Compensation (SDC) [13]
Pre-emptive tap changer [13]
Master –follower [14]
True circulating current [14]
Advantage(s)/disadvantage(s)
set value. If the voltage detected is not within allowable level then the tap position is adjusted to ensure the voltage at the feeder end is higher than the minimum allowed voltage and the sending-end voltage is lower than the maximum allowed voltage. Includes secondary current measurement to simulate the voltage drop along the feeder impedance that exists between transformer terminal and load. Coordinates the operation of upstream and downstream tap changer by applying time delay. The upstream tap changer is allowed to perform first before the downstream tap changer start to operate. Downstream tap changer is allowed to over-ride the grading time delay if the voltage at the regulation point is within its dead-band. Involves two types of time settings to allow local AVC relay to correct one tap but then grade it with upstream OLTC if any further tap-changers are required. Tap changer position for transformer operated in parallel are retained on the same tap position.
Can be used to control voltage at a nominal load point. Could potentially prolong the voltage discrepancy for the
Downstream tap changer is allowed operate irrespective of any upstream tap changer operations required.
Circulating current will flow between parallel transformers
Circulating current is minimized when biasing in opposite polarities is used to correct the OLTC for adjusting the relay setting voltage. Maintains similar tap position for paralleled transformers by changing the polarity of reactance of LDC setting.
duration of the time delay because downstream tap changer has to wait until upstream tap changer has completed its operation. Optimizes the time grading strategy.
Negative Reactance Compounding (NRC) [14]
Transformer automatic paralleling package (TAPP) [14]
Reduces the circulating current between paralleled transformers
by using numerical techniques based on the target power factor.
unless the transformers have the same impedance, number of taps and incoming voltage. Difficult to parallel transformers which are not in the same site and the paralleled transformers must have similar impedance, incoming voltage and connections. Can operate with transformers in different positions in the networks and it does not need to be identical anymore. Unable to operate satisfactorily when the power factor changes from a set point due to integration of irregular RE sources into the network. Error occurs in the controlled voltage due to load current being considered as part of circulating current when the load power factor deviates from the set power factor.
Table 2 Summarized voltage control strategies of on-load tap changer transformers for networks with RE source connection. With DG Voltage control method
Description
Advantage(s)/disadvantage(s)
Enhanced transformer automatic paralleling package (Enhanced TAPP) [19] SuperTAPP n þ relay [20]
This scheme is the combination of TAPP and circulating current Adjustment to the tap position is done for minimizing the method.
Additional current measurement on the feeder with DG and a
ANN controller based AVC relay [23]
Fuzzy logic controller based AVC relay [7]
ratio which represents the load share between feeders with DG to those without DG is used to estimate the DG output at remote point on the feeder. Data obtained from load flow studies under various operating condition is used to train the AVC relay. Fuzzy logic controller is used in calculating the voltage of AVC relay.
circulating current but unable to provide effective voltage setting to control the voltage profile in the network with DG. Able to eliminate the error from the LDC performance caused by DG.
Performs well only after satisfactory training and testing. Fuzzy logic AVC relay setting does not require re-adjustment when DG is connected into the network.
Relay setting might need to be changed if it is used in different network.
State estimation based AVC relay [32]
Network data, load data and a few real time measurements are Does not require electrical measuring equipment at each used to estimate the voltage at each node of the network.
generation by RE sources. The decision to connect or disconnect the RE sources to solve the multi-objective problem as above is done using artificial intelligent method [49]. Relationship between active / reactive power loss and generated power from RE sources are discussed in [50].Sudden connection or disconnection of smaller RE sources producing reactive power at a constant power factor can result in a relatively large voltage change that will persist until recognized by the utility voltage regulating system [51]. The solution for this includes faster tap-changing voltage regulators and requiring the load to be disconnected whenever the
node of the network.
Data accuracy affects the relay performance.
RE source is forced off due to a disturbance. Intelligent AVC relays which enables faster tap changes has certain drawbacks that restrict their implementation in real-time applications. In future, extensive research into improving these intelligence techniques is still required to ensure effective implementation of intelligent AVC relays in real-time applications. 7. Conclusion This paper has reviewed the existing OLTC voltage control scheme as well as the new voltage control techniques. The voltage
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control scheme for OLTC transformers connected both in series and parallel has been discussed in this paper. The Enhanced Automatic Paralleling Package and SuperTAPP nþ relay schemes are capable of improving voltage control for networks containing various levels of RE source. The main advantages of intelligent AVC relays are that their control performance is similar or even better than the conventional AVC relays. The intelligent AVC relays are much preferred since they reduce the hardware costs. Continuous innovation in OLTC voltage control schemes is necessary to cater for high penetration of RE sources in distribution network which causes considerable impact on voltage regulation of the existing OLTC voltage control system.
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Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
A review on voltage control methods using on-load tap changer transformers for networks with renewable energy sources Charles R. Sarimuthu a,b,n, Vigna K. Ramachandaramurthy a, K.R. Agileswari a, Hazlie Mokhlis c a Power Quality Research Group, Department of Electrical Power Engineering, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia b School of Engineering, Taylor's University, Lakeside Campus, No.1, Jalan Taylor's, 47500 Subang Jaya, Selangor, Malaysia c Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 13 January 2015 Received in revised form 8 January 2016 Accepted 3 May 2016
Voltage control is an important method for regulating the feeder voltages in a distribution network. Various voltage control methods are used by distribution network operators (DNOs) in order to maintain the network voltages to be within an acceptable voltage level. Traditionally, on-load tap changer (OLTC) and automatic voltage control (AVC) relays are often employed in regulating the network voltages. However, the traditional voltage control techniques are no longer suitable when renewable energy (RE) sources are connected to the network because of the possibility of bidirectional power flows. The presence of reverse power flow will affect the feeder voltage profiles and influence the voltage control scheme practiced in the distribution system. This paper presents an overview on the various OLTC voltage control schemes which are used to control the voltage in distribution networks containing RE sources. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Power transformer On-load tap changer transformers Distribution systems Automatic voltage control relay
Contents 1. 2.
3.
4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 Traditional voltage control scheme. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 2.1. Voltage control with load tap changer (LTC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 2.2. Voltage control with line drop compensation (LDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 2.3. Grading time (GT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1155 Enhanced voltage control AVC relay scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 3.1. Source drop compensation (SDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 3.2. Pre-emptive tap changer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 Voltage control scheme for OLTC in parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156 4.1. Master-follower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 4.2. True circulating current. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 4.3. Negative reactance compounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 4.4. Transformer automatic paralleling package (TAPP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 Modern voltage control scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 5.1. Enhanced transformer automatic paralleling package (Enhanced TAPP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1157 5.2. SuperTAPP nþ relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158 5.3. Intelligent AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158 5.3.1. ANN controller based AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1158 5.3.2. Fuzzy logic controller based AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1159
n Corresponding author at: Power Quality Research Group, Department of Electrical Power Engineering, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia. Tel.: þ60 129353924; fax: þ60 3 56295477. E-mail addresses: [email protected] (C.R. Sarimuthu), [email protected] (V.K. Ramachandaramurthy).
http://dx.doi.org/10.1016/j.rser.2016.05.016 1364-0321/& 2016 Elsevier Ltd. All rights reserved.
C.R. Sarimuthu et al. / Renewable and Sustainable Energy Reviews 62 (2016) 1154–1161
5.3.3. State estimation based AVC relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Concepts of OLTC control in distribution system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Electrical power distribution systems are normally operated at multiple voltage level. The voltage levels differ based on the amount of power generated by RE sources and load variations in the network. These different voltage levels are kept within acceptable limits by including an OLTC transformer where the substation secondary bus voltage is kept stable by adjusting the tap position [1]. Tap position adjustment is necessary to physically alter the ratios of the transformer for voltage regulation. The OLTC operate by changing the number of turns in one winding of the transformer to keep the transformer output voltage within predicted limits. The OLTCs are motorized mechanical switching arrangements that adjust the transformer turns ratio, typically in steps of 1.25% or 1.43%, whilst the transformers are in use and carrying a load [2]. The OLTC transformer is normally applied in the distribution networks to step down from 33 kV to 11 kV or 6.6 kV. Each OLTC transformer is linked to an AVC relay in order to increase or decrease the voltage by changing the tap position of transformer [3]. However, the operation of AVC relay can be affected by the possible existence of bidirectional power flows when RE sources are connected to the network. Innovation is required in OLTC voltage control scheme in order to support the current implementation of smart grid incorporating RE sources such as wind, solar and hydrogen [4]. Studies on the size, operating power factor mode [5,6], and location of RE sources in the network are needed in order to design an innovative OLTC voltage control scheme to cope with the increasing RE sources connected into the distribution network.
2. Traditional voltage control scheme Traditionally, the voltage control in distribution networks without RE sources is performed using OLTC, shunt capacitors, shunt reactors, static var compensation etc. This paper will focus on voltage control strategies involving OLTCs in distribution network connected with RE sources. Without RE sources connected in a distribution system, the power flow is assumed to be unidirectional and the set-values for the OLTCs are chosen according to the voltage drop along the feeder. Based on the set-values, the voltage at the secondary of the OLTC transformer is changed in order to prevent the voltage along the feeder from breaching the lower voltage limit and not exceeding the upper voltage limit. The change in voltage at the secondary of the OLTC transformer is achieved when the AVC relay initiates a signal to the tap changer system to change its tap position.
1159 1159 1160 1161
normally consists of initial delay and inter tap delay. The initial delay ranges from 10 to 120 s and the inter tap delay for each step tap changer operation is from 5 to 60 s [8].
2.2. Voltage control with line drop compensation (LDC) In an OLTC operation, normally LTC is provided with LDC function to control the voltage at a remote point. Besides monitoring the transformer's terminal voltage, this function includes the measurement of the secondary current [9]. The measurement is used to simulate the voltage drop along the feeder impedance that exists between the transformer terminal and the load point [10]. In order to keep the correct voltage level at the load side, line resistance R and line reactance X is used to increase the regulated voltage at the transformer terminal. Voltage control at a nominal load point rather than at the transformer terminal is achieved using LDC.
2.3. Grading time (GT) AVC relay with LDC function operates in between different voltage levels in power supply networks. If a downstream tap changer is allowed to operate before an upstream tap changer, then the OLTCs might work against one another and become unstable. In order to correct this situation, the GT is introduced (Figs. 2 and 3). GT ensures that the initial time delay is longer for the downstream controllers compared to the upstream controllers. The different initial time delay for downstream and upstream controllers are required to ensure that upstream operations are given preference and carried out first. This time grading strategy requires the upstream transformer to finish its operation before the down-stream transformer restores the voltage level.
2.1. Voltage control with load tap changer (LTC) The basic arrangement of voltage control with LTC regulation is shown in Fig. 1. Since the voltage on a conventional distribution network (without RE source connection) decreases towards the end of feeder, the LTC shall then be set to ensure that the voltage at the feeder end is higher than the minimum allowed voltage and the sending-end voltage is lower than the maximum allowed voltage. The AVC relay determines whether to adjust the tap position or not in order to maintain the voltage level which is assumed to be equal to 1 p.u at the end of feeder [7]. An AVC relay
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Fig. 1. Basic LTC arrangement [7].
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complex and costly. This scheme incorporates the features of Source Drop Compensation (SDC) and Pre-emptive Tap Changer operation. 3.1. Source drop compensation (SDC)
Fig. 2. AVC relay scheme with LDC [11].
SDC optimizes the time grading between voltage levels by determining the voltage at the regulation point by the source current and the feeder impedance between the upstream and down-stream transformer. The down-stream AVC relay uses this voltage when a voltage disturbance occurs. If the voltage of regulation point is outside its voltage dead-band, the down-stream AVC relay will wait until the voltage correction of upstream transformer is completed. However, when the voltage at the regulation point is within its dead-band, the upstream transformer operation is assumed to be complete and the down-stream AVC relay can proceed with the local voltage correction. This over-rides the grading time delays and minimizes the action of the communications based blocking scheme [13]. 3.2. Pre-emptive tap changer Pre-emptive tap changing strategy involves two types of time settings which are the Transient Time (TT) and GT. TT setting is the time that has to elapse before any voltage correction is permitted to take place and GT is the time in which a local tap changer delays the local voltage correction to enable upstream tap changers to complete their voltage correction. The AVC relay issues a tap change command to the OLTC after the transient time in the operation of Pre-Emptive tap changing scheme. The AVC relay then resets its delay time so that any subsequent tap changes occur after the grading time [13]. Therefore, it allows the local AVC relay to correct one tap but then grade with upstream OLTCs if any further tapchangers are required.
4. Voltage control scheme for OLTC in parallel
Fig. 3. Typical power supply network [12].
3. Enhanced voltage control AVC relay scheme The need for GT delay can be replaced by using a communications unit. If communications are available between the OLTC controllers at the different voltage levels, then a blocking signal can be issued to stop the operation of downstream transformer when the up-stream transformer starts its operation. The time delays between OLTCs at different voltage levels are minimized and any voltage correction is completed at the highest level since the blocking signal is removed when the up-stream transformer has done its correction. The system allows for OLTC to operate with optimum efficiency while maintaining voltage quality. Alternatively the Enhanced Voltage Control AVC Relay scheme provides autonomous tap-changer controls without the communications unit due to the fact that communication assisted voltage control schemes are
The features of SDC and pre-emptive tap changing strategy are used when OLTC transformers are operated in series. However, primary-substation transformers are often operated in parallel for higher security and reliability of supply. When transformers are operated in parallel, any difference in the positions of their tap changers will give rise to a circulating current, as shown in Fig. 4. In this condition, the AVC relay should perform to keep the voltage within allowable limits while minimizing the circulating current between parallel transformers. The master-follower method is used to avoid difference in tap changer position for transformers operated in parallel whereas the true circulating current method is used to minimize the circulating current if the tap positions for parallel transformers are different.
Fig. 4. Occurrence of a circulating current IC due to a difference in transformer tap positions [2].
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4.1. Master-follower In the master-follower method, all paralleled transformers are retained on the same tap position. In this method, one master transformer changes the tap position to a satisfactory voltage level and then other transformers follow the same actions as the master [14]. The master-follower scheme can be used with LDC and operates under varying power factor, reverse power flow and with presence of RE sources. The drawback of this method is that circulating current will flow between paralleled transformers using this method unless the transformers are equal with the same impedance, number of taps and incoming voltage. It is impossible to use this scheme across a network because connections between AVC relays are required. 4.2. True circulating current In true circulating current scheme, identical transformers as in the master-follower scheme are used. If the tap positions of the transformer are different, then circulating current will flow between the transformers. The circulating current is formulated as follows: ICIRC ¼ ðIT1 –IT2 Þ=2
ð1Þ
The biasing in opposite polarities is used to correct the OLTC to adjust the relay setting voltage. Therefore the circulating current is minimized [14]. This scheme is applicable with LDC and it performs well under varying power factor, reverse power flow and with integration of RE sources. The disadvantages of this scheme are that it is difficult to parallel transformers which are not in the same site and as the paralleled transformers must have similar impedance, incoming voltage and connections [2]. 4.3. Negative reactance compounding The negative reactance compounding (NRC) method helps to maintain similar tap positions for paralleled transformers by changing the polarity of reactance of LDC setting – XLDC [15]. The following formulas show the relationship between LDC settings and negative reactance compounding (NRC) setting: ZLDC ¼ RLDC þ jXLDC
ð2Þ
ZNRC ¼ RLDC –jXLDC
ð3Þ
The operating principle of NRC is illustrated in Fig. 5. Based on the phasor diagram shown in Fig.5, transformer T1 has much higher tap position compared to transformer T2. Due to the difference in tap position, a circulating current will flow between transformer T1 and transformer T2. The circulating current causes current IT1 to be shifted in clockwise direction and current IT2 in anti-clockwise direction. Both current IT1 and IT2 which passes through the ZNRC setting create a voltage drop IT ∙ ZNRC. The AVC relay uses this voltage drop to determine the proper tap position. Since the voltage at transformer T1 is higher than the target voltage therefore the AVC relay initiates a tap down operation. Similarly the AVC relay of transformer T2 initiates a tap up operation. The action stops with a similar tap position of both the parallel transformers when the circulating current is eliminated and target voltage is achieved.
Fig. 5. Operating principle of NRC [14].
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The advantage of NRC scheme is that it can operate with transformers at different positions in the networks and it does not need to be identical anymore due to the independent action of each transformers. However, the NRC fails to operate satisfactorily when the power factor changes from a set point. Integration of irregular RE sources into the network effects the NRC operation. Apart from that, negative value of XLDC setting could cause poor performance of LDC. An increased value of RLDC is needed in order to maintain the performance of LDC. 4.4. Transformer automatic paralleling package (TAPP) The transformer automatic paralleling package (TAPP) scheme is developed from the NRC scheme. The TAPP scheme reduces the amount of circulating current between transformer T1 and transformer T2. The reduction is achieved by dividing the measured current into circulating current and load transformer current. Circulating current in the TAPP scheme uses techniques based on the target power factor (pftarg) as shown in Fig. 6. Problems due to poor performance of LDC with NRC are eliminated in the TAPP scheme by using two separate circuits. The circuits are used for LDC and compounding purpose. However, the drawback of TAPP scheme is that the load power factor deviation will result in an error in the controlled voltage due to knowledge of the load current being considered as circulating current. The set power factor is the necessary factor to make the voltage control to be satisfactory.
5. Modern voltage control scheme The integration of RE sources into distribution network will affect the flow of power and the voltage profiles in the distribution system [16]. The integration of RE sources will result in an increased voltage at the point of connection. The steady-state voltage rise that occurs when connecting RE sources to distribution networks has been studied in [17]. This rise in voltage level affects the operation of AVC relay and causes voltage regulation problems since the AVC relay voltage reference is no longer proper for an effective operation of AVC relay. Due to this reason, improved voltage control scheme are the topic of on-going research to accommodate the presence of RE sources connected to distribution networks. 5.1. Enhanced transformer automatic paralleling package (Enhanced TAPP) Enhanced TAPP scheme is introduced to remove the drawback related to TAPP scheme [18]. The voltage profile of dynamic distribution networks can also be improved by Enhanced TAPP scheme. The true circulating current mode is applied to control voltage level and paralleled transformers on the same location, while TAPP mode is used to parallel transformers across the network. Principle of Enhanced TAPP scheme is illustrated in Fig. 7. Load current (IT1 þIT2) and the measured transformer current (ITN) is used to calculate the amount of circulating current in the True Circulating Current mode. The voltage detected by AVC relay
Fig. 6. Principle of TAPP scheme [14].
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Fig. 7. Enhanced TAPP scheme in True Circulating Current mode [2].
Fig. 9. SuperTAPP n þ relay [19].
where IG is calculated as follows [19]: ETS ¼
Fig. 8. Enhanced TAPP scheme in TAPP mode [19].
of the transformer with higher tap position is increased using the voltage drop on the compounding setting (ICIRCZT).The same voltage drop is used to decrease the voltage detected by AVC relay of the transformer with lower tap position. Then the tap down action for the OLTC transformer on higher tap position and up action for OLTC transformer on lower tap position are operated until the measured voltage is within the bandwidth of the target voltage. This operation is necessary to achieve minimum circulating current. Comparison between the group load current and full load current is used to determine a suitable increase in LDC voltage as described by the following formula [14]: PT LDCVoltageBoost ¼ V LDC ¼ Z LDC þ
n ¼ 1 I Tn
I FL
ð4Þ
where n – number of transformers within the TAPP scheme IFL – full load current ITn – individual transformer current ZLDC – load drop compensation settings The circulating current in the TAPP mode is determined by total load current and target power factor as illustrated in Fig. 8. In the enhanced TAPP voltage control scheme, adjustment to the tap position is done for minimizing the circulating current. However, this adjustment to the tap position is unable to provide the effective voltage setting to control the voltage profile in the network with RE sources. 5.2. SuperTAPP n þ relay The SuperTAPP nþ relay [20] which is based on Enhanced TAPP scheme [19] is introduced to overcome the limitations faced by application of Enhanced TAPP scheme. The SuperTAPP n þ relay can estimate current output of the RE source IG by the additional current measurement IFG on the feeder 1 with RE source and ratio EST as illustrated in Fig. 9. The ratio EST represents the load share between feeders with RE sources to those without RE sources.
I TL ¼
loadonfeederswithgenerators I1 I FG ¼ ¼ loadonfeederswithoutgenerators I 2 I TL I FG n X
ð5Þ
I Tn ¼ I T1 þ I T2
ð6Þ
I G ¼ ðETS ∙ðI TL I FG ÞÞ I FG
ð7Þ
N¼1
The increase in voltage at the point of connection can be determined and the proper generator compensation bias, VG can be calculated as follows [19]: V G ¼ V GMAX ∙
IG I GMAX
ð8Þ
The above formula is applied to the AVC relay to determine the suitable voltage setting to control the voltage profile in the network with RE sources. 5.3. Intelligent AVC relay The increasing use of renewable energy resources due to its high efficiency and low environmental impact is the key factor for transforming the traditional power grid to smart grid in the future. The smart grid uses intelligent devices and a digital communication in power system to enhance the performance of distribution grids. Therefore, the OLTC voltage control method may need to be changed in the future as the control needs to be more flexible and smarter [21,22]. Fuzzy logic, artificial neural network and state estimation based AVC relay are the types of intelligent AVC relay that is available. 5.3.1. ANN controller based AVC relay S.K. Salman and I.M. Rida presented in [23] an attempt to design an AVC relay based on the application of Artificial Neural Network (ANN). In 1997, it has been reported that considerable attention has been given for application of ANNs to electrical power systems [24–26]. Various types of ANNs are described in literature [27,28]. The model developed in [23] shows that ANN based AVC relay are capable of operating similar to other types of AVC relay [23].The ANN-based AVC relay is trained using data file obtained from load flow studies under various operating condition. In [23], the ANN based AVC relay sends signal to change the tap-changer of OLTC transformer to retain the voltage within the
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allowable limits when the calculated AVC voltage exceeds the limits of 72% of reference voltage. The calculated results at every step are saved into an output file to obtain a particular trend for a specific operating condition. The drawback of this relay is that it performs well only after satisfactory training and testing [23]. 5.3.2. Fuzzy logic controller based AVC relay The integration of renewable energy resources has caused voltage regulation problems due to the interference with the performance of conventional AVC relay [6]. S.K. Salman and Z.G. Wan in 2007 [7] introduced fuzzy logic-based AVC relay to overcome this problem. Fuzzy logic systems are much preferred compared to other artificial intelligent systems because the control rules can be implemented using simple “IF-THEN” relations [29]. The other advantages of these systems are that they are simpler, faster and able to simplify design complexity and lessen the hardware cost [7].Fuzzy logic systems are beneficial because they allow the usage of fuzzy rules which are more expressive than crisp values [30]. Fig. 10 shows a fuzzy logic based AVC relay which consists of 3-input and 1-output. The inputs are low voltage side of OLTC transformer voltage (V), phase angle of the current through the OLTC transformer (PAI), the change of current (ΔI) and the output is the AVC relay voltage (VAVC) [7]. This fuzzy logic control system applies Mamdani's Fuzzy Inference System (FIS) [31] since it is more user friendly compared to the conventional Takagi-Sugeno method. The advantage of the fuzzy logic controller based-AVC relay is that its setting does not require re-adjustment as RE sources are connected into the network. However the relay setting might need to change if the relay is applied to different network. 5.3.3. State estimation based AVC relay Leite et.al., [32] developed a statistical distribution state estimator for application in the 11 kV voltage controller. The distribution state estimator is constructed using techniques which are drawn from transmission network state estimation and distribution state estimation research [33]. In an 11 kV network, state estimation method uses measurements at primary substations to estimate the voltage at each node of the network. In order to safeguard customer voltages within permitted levels, the voltage controller needs accurate data on the voltage at each network node [34]. The functional diagram showing the state estimation and control block which is the content of the voltage controller is illustrated in Fig. 11. The black arrows represent real-time signals whereas the white arrows represent offline data. The advantage of state estimation method is that it does not require electrical measuring equipment at every node of the network [35]. The inputs required for a distribution network state estimator are network topology and impedance data, information about customer loads and a few real-time measurements [36]. The drawback of state estimation based AVC relay is that all the above inputs are required for it to operate as desired. Tables 1 and 2 give a summary of the voltage control methods using on-load tap changer transformers. Both the tables provide description of the voltage control method involve in networks with
Fig. 10. Fuzzy logic based AVC relay [7].
Fig. 11. Voltage controller functional diagram [32].
and without RE sources. The tables also cover the relevant advantage and disadvantages of each voltage control method described.
6. Concepts of OLTC control in distribution system Network control systems have been developed by the distribution system operators in order to maintain the voltage level at consumer's supply point to be within acceptable levels [37]. The network control systems are managed in real time [38]. These control systems consist of communication and control devices [39]. Network parameters such as OLTC tap settings are optimized by these control systems during network operation. OLTCs or management of reactive power flow is used in conventional distribution systems to control the voltage profile. The source of power in a distribution network is usually a substation, which may have OLTCs to supply the distribution network. Voltage profile in the network can be controlled by varying the tap setting on the OLTC. The placement and sizing of RE sources varies for different distribution network configurations [40]. In [41], a multi-objective problem is proposed for the placement and sizing of multiple RE sources in a distribution network to improve the transient stability index in addition to the losses and voltage profile. Maroosi et. al., [42], presented the potential of modern distribution systems to maximally utilize RE sources to reduce greenhouse gas emissions from the power system. Voltage control in distribution systems with RE source involves the use of coordinated voltage control through dispatch of RE source output, OLTCs and reactive power support [5]. The coordinated voltage management in distribution system is categorized as centralized or decentralized. Centralized distribution management system controls several distribution substations and it requires extensive communication networks in order to operate. In [14,32,43–45], it is seen that transformers, reactive power devices and RE sources are used to control the voltage in distribution systems. Transformers are used to vary the voltage directly. OLTCs at substations can raise or lower the voltage level in the distribution network. In networks where RE sources are used to regulate the voltage, large RE source greater than 30 percent of the feeder capacity could cause problem to the OLTC operation when the RE source is suddenly disconnected. This is due to the voltage which becomes too low to support the load and takes a minute or more to recover. The solution is using a control scheme that locks the OLTC at a preselected tap when the generator is operating and interconnected. One common strategy for line regulators is to take the regulator to the neutral tap when the reverse power is sensed by its control. In [46], it is seen that RE sources are considered as intermittent power generation sources. Independent System Operators (ISOs) execute security-constrained daily RE source generation scheduling which takes into account the intermittency of power generation by RE sources [47]. Izadbakhsh et.al., [48] presented shortterm resource scheduling of RE based micro grid which minimizes total operation cost and greenhouse gas emissions due to power
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Table 1 Summarized voltage control strategies of on-load tap changer transformers for networks without RE source connection. Without DG Voltage control method
Description
Load tap changer (LTC) [7]
Detects voltage at the end of the feeder and compares it with the Difficult to detect voltage at a remote point along the feeder.
Line Drop Compensation (LDC) [11]
Grading Time (GT) [11]
Source Drop Compensation (SDC) [13]
Pre-emptive tap changer [13]
Master –follower [14]
True circulating current [14]
Advantage(s)/disadvantage(s)
set value. If the voltage detected is not within allowable level then the tap position is adjusted to ensure the voltage at the feeder end is higher than the minimum allowed voltage and the sending-end voltage is lower than the maximum allowed voltage. Includes secondary current measurement to simulate the voltage drop along the feeder impedance that exists between transformer terminal and load. Coordinates the operation of upstream and downstream tap changer by applying time delay. The upstream tap changer is allowed to perform first before the downstream tap changer start to operate. Downstream tap changer is allowed to over-ride the grading time delay if the voltage at the regulation point is within its dead-band. Involves two types of time settings to allow local AVC relay to correct one tap but then grade it with upstream OLTC if any further tap-changers are required. Tap changer position for transformer operated in parallel are retained on the same tap position.
Can be used to control voltage at a nominal load point. Could potentially prolong the voltage discrepancy for the
Downstream tap changer is allowed operate irrespective of any upstream tap changer operations required.
Circulating current will flow between parallel transformers
Circulating current is minimized when biasing in opposite polarities is used to correct the OLTC for adjusting the relay setting voltage. Maintains similar tap position for paralleled transformers by changing the polarity of reactance of LDC setting.
duration of the time delay because downstream tap changer has to wait until upstream tap changer has completed its operation. Optimizes the time grading strategy.
Negative Reactance Compounding (NRC) [14]
Transformer automatic paralleling package (TAPP) [14]
Reduces the circulating current between paralleled transformers
by using numerical techniques based on the target power factor.
unless the transformers have the same impedance, number of taps and incoming voltage. Difficult to parallel transformers which are not in the same site and the paralleled transformers must have similar impedance, incoming voltage and connections. Can operate with transformers in different positions in the networks and it does not need to be identical anymore. Unable to operate satisfactorily when the power factor changes from a set point due to integration of irregular RE sources into the network. Error occurs in the controlled voltage due to load current being considered as part of circulating current when the load power factor deviates from the set power factor.
Table 2 Summarized voltage control strategies of on-load tap changer transformers for networks with RE source connection. With DG Voltage control method
Description
Advantage(s)/disadvantage(s)
Enhanced transformer automatic paralleling package (Enhanced TAPP) [19] SuperTAPP n þ relay [20]
This scheme is the combination of TAPP and circulating current Adjustment to the tap position is done for minimizing the method.
Additional current measurement on the feeder with DG and a
ANN controller based AVC relay [23]
Fuzzy logic controller based AVC relay [7]
ratio which represents the load share between feeders with DG to those without DG is used to estimate the DG output at remote point on the feeder. Data obtained from load flow studies under various operating condition is used to train the AVC relay. Fuzzy logic controller is used in calculating the voltage of AVC relay.
circulating current but unable to provide effective voltage setting to control the voltage profile in the network with DG. Able to eliminate the error from the LDC performance caused by DG.
Performs well only after satisfactory training and testing. Fuzzy logic AVC relay setting does not require re-adjustment when DG is connected into the network.
Relay setting might need to be changed if it is used in different network.
State estimation based AVC relay [32]
Network data, load data and a few real time measurements are Does not require electrical measuring equipment at each used to estimate the voltage at each node of the network.
generation by RE sources. The decision to connect or disconnect the RE sources to solve the multi-objective problem as above is done using artificial intelligent method [49]. Relationship between active / reactive power loss and generated power from RE sources are discussed in [50].Sudden connection or disconnection of smaller RE sources producing reactive power at a constant power factor can result in a relatively large voltage change that will persist until recognized by the utility voltage regulating system [51]. The solution for this includes faster tap-changing voltage regulators and requiring the load to be disconnected whenever the
node of the network.
Data accuracy affects the relay performance.
RE source is forced off due to a disturbance. Intelligent AVC relays which enables faster tap changes has certain drawbacks that restrict their implementation in real-time applications. In future, extensive research into improving these intelligence techniques is still required to ensure effective implementation of intelligent AVC relays in real-time applications. 7. Conclusion This paper has reviewed the existing OLTC voltage control scheme as well as the new voltage control techniques. The voltage
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control scheme for OLTC transformers connected both in series and parallel has been discussed in this paper. The Enhanced Automatic Paralleling Package and SuperTAPP nþ relay schemes are capable of improving voltage control for networks containing various levels of RE source. The main advantages of intelligent AVC relays are that their control performance is similar or even better than the conventional AVC relays. The intelligent AVC relays are much preferred since they reduce the hardware costs. Continuous innovation in OLTC voltage control schemes is necessary to cater for high penetration of RE sources in distribution network which causes considerable impact on voltage regulation of the existing OLTC voltage control system.
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