On-Load Tap-Changer
On-Load Tap-Changer (OLTC) is a device for changing the tapping connections of a winding suitable for operation while transformer is energised and on-load condition to regulate voltage. The downstream bus voltages of the transformer can be regulated by optimal selection of tap position. The number of service tap positions of OLTC shall be considered to keep voltage within permissible range as per statutory regulatory requirements. At normal operating scenario, tap position of OLTC set at nominal value. Optimal tap position of OLTC, maintains the voltage within statutory limit, reduces the power loss and enhances energy savings. As shown in Figure 1, OLTC with 7 steps from (-)10 per cent to (+)5 per cent variations in steps of 2.5 per cent is considered. Under normal operating scenario, OLTC tap position is fixed at nominal tap position ‘3’. Voltage variation at HV side with respect to each tap position is shown in Figure 1.
Electrical control system (ECS) is a collection of software tools along with communication layer designed to monitor and control the distribution network efficiently and reliably in real-time. Improving the reliability and power quality of service in terms of reducing outages, minimising outage time, maintaining acceptable frequency and voltage levels are the key deliverables of ECS. ECS serves as a decision tool to assist the operating personnel by monitoring and control of the electric distribution system. ECS communicates with each electrical equipment’s of the distribution network through IEC 61850 communication protocol. Therefore, status of each electrical equipment (including metering) is made available in real-time at human machine interface (HMI) of ECS control room. ECS continuously monitors the state of the distribution system and issues command to control switching of respective electrical equipment based on the volt-var optimisation. Based on volt-var optimisation process carried out at ECS control center, DNO issues scheduling command to OLTC to increase or decrease the tap position.
Distribution system optimal power flow model in GAMS
• a) Objective function
The objective function is given in equation (1) to minimise cost of energy loss subject to technical constraints as specified in equations (2)-(9).
Minimis 
• b) Optimal power flow (OPF) constraints
• Real and reactive power balance should be maintained as,
• Permissible voltage variation, voltage angle limit and power flow limit in each feeder and is represented by equations (6)-(8).
• Tap position of OLTC
Simulation Results with OLTC
In this case study, impact of OLTC tap position on the voltage profile, power loss reduction and energy cost savings is investigated for various load models on IEEE 33-bus 12.66 kV radial distribution system. Peak load demand on IEEE 33-bus test system is 3715+j2300 kVA. In Table 1, summary of simulation results of bus voltage and real power loss are given. It is noticed that voltage regulation is approximately 10 per cent which is violating the statutory requirement of 5 per cent. Therefore, OLTC action is enabled automatically from ECS. Impact of OLTC action on voltage profile and power loss for each load model is highlighted in Table 1. The simulation results clearly illustrate that OLTC tap position is to be set at ‘1’ to maintain voltage within the permissible range. To keep the voltage within the acceptable limits in reconfigured network, OLTC tap position is set at ‘2’ which is one position lower than without reconfiguration case. From the simulations results, it is observed that with OLTC action ensures voltage stabilisation in distribution systems without an interruption of power supply.
Volt-var management based on assumption of constant power load model is not effective. The decisions and simulation results with constant power load model is not technically and economically feasible if employed on the actual test system connected with voltage dependent load models. In the competitive electricity markets, the assumption of a constant power load model for system studies is no longer valid to quantify benefits to DNO for reactive power management.
During Volt-Var optimisation, OLTC tap position is optimised as shown in Figure 3 to regulate voltage within the permissible range of 0.95 p.u to 1.05 p.u. Figures 4-5, show real and reactive power loss optimisation with OLTC. Moreover, minimum voltage of the distribution system is depicted in Figure 6. Finally, savings in cost of energy loss is displayed in Figure 7.
Conclusions
This article investigates effect of voltage dependent load models on optimal reactive power management and reconfiguration in distribution systems. A multi-objective optimisation model developed in GAMS to determine optimal tap position in distribution system to enhance energy savings, reduce power loss and minimise voltage regulation. With volt-var optimisation OLTC tap position is determined hourly to prevent low voltage and high energy loss in the system. It is observed that load models have considerable impact on reactive power requirement and reconfiguration of the distribution system. Total power loss is reduced and voltage throughout the distribution system is maintained within permissible range with OLTC.
Prof Ashwani Kumar
Department of Electrical Engineering
NIT Kurukshetra.
V.V.S.N Murty
Engineers India limited
