Imperial College London

Dr Zohaib Akhtar

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

Laboratory Systems Senior Teaching Fellow



+44 (0)20 7594 6310z.akhtar Website CV




401Electrical EngineeringSouth Kensington Campus





Publication Type

9 results found

Akhtar Z, Opatovsky M, Chaudhuri B, Hui SYRet al., 2019, Comparison of point-of-load vs. mid feeder compensation in lv distribution networks with high penetration of solar photovoltaic generation and electric vehicle charging stations, IET Smart Grid, Vol: 2, Pages: 283-292, ISSN: 2515-2947

Increasing use of distributed generation (DG), mainly roof-top photovoltaic (PV) panels and electric vehicle (EV) charg-ing would cause over- and under-voltage problems generallyat the remote sections of the low voltage (LV) distribution feeders. Asthese voltage problems are sustained for a few hours, power electronic compensators (PECs) with input voltage control,i.e. electricsprings can not be used due to the unavailability of non-critical loads that can be subjected to non-rated voltages for long durationof time. However, PECs in output voltage control mode could be used to inject a controllable series voltage either somewhereon the feeder (mid-feeder compensation, MFC) or between thefeeder and each customer (point-of-load compensation, PoLC)both of which are effective in tackling the voltage problem without disrupting PV power output and EV charging. In this paper, acomparison between the MFC and PoLC option is presented in terms of their voltage control capability, required compensatorcapacity, network losses, PV throughput, and demand response capability. The criteria for selection of optimal location of thesecompensators is also discussed. Stochastic demand profile for different types of residential customers in the UK and a typicalEuropean LV network is used for the case study.

Journal article

Akhtar Z, Seyed AA, Kamyar M, 2018, Voltage control in LV networks using electric springs with coordination, 2018 IEEE Canadian Conference on Electrical & Computer Engineering (CCECE), Publisher: IEEE, ISSN: 2576-7046

The increasing use of distributed generation like rooftop solar panels and charging of large fleets of electric vehicles will result in over-and under-voltage problems in the low voltage (LV) distribution networks. Distributed electric springs have been proposed as an effective way of controlling these voltage problems. However, when multiple distributed electric springs are activated in a system, each electric spring tries to correct the local voltage problem. As a result, two groups of electric springs located in two different sections of the same radial network can be competing against each other at any given time. In the past, droop control has been suggested as a solution to avoid this conflict. This paper highlights the problem with simple drop control of electric springs in a radial distribution network and presents coordination between electric springs as an alternative. A comparison between the droop control and the coordinated droop control option is presented in terms of their voltage control capability, and required compensator capacity. It is established by means of a case study on a typical European LV network with stochastic demand profile for different types of residential customers.

Conference paper

Chakravorty D, Akhtar Z, Chaudhuri B, Hui SYRet al., 2016, Comparison of Primary Frequency Control Using Two Smart Load Types, IEEE Power and Energy Society General Meeting, Publisher: IEEE, ISSN: 1944-9933

Primary frequency control using smart loads withreactive only compensation (SLQ) has been shown in the past.In this paper, further improvement in frequency regulation isshown using smart loads with a back-to-back converter (SLBC)arrangement. This introduces additional flexibility and thereby,allows independent and wider control over active and reactivepower consumption of the smart load. The improvement infrequency regulation with SLBCs is compared against SLQsthrough two separate case studies on 4-generator, 2-area testsystem and also the 39-bus New-England test system. A futurescenario with reduced system inertia is considered for both casestudies. Unlike previous exercises involving smart loads, in thisstudy a detailed representation is used for both the multi-machinetransmission system and the distribution networks down to themedium voltage (MV) level where the smart loads (SLBC/SLQ)are connected. This avoids the inaccuracies associated with loadaggregation or use of system equivalents wherein the networkconstraints, spatial voltage variations etc. are not capturedproperly.

Conference paper

Luo X, Akhtar Z, Lee CK, chaudhuri B, Tan S-C, Hui SYRet al., 2016, Distributed voltage control with electric springs: Comparison with STATCOM, Power and Energy Society General Meeting (PESGM), 2016, ISSN: 1944-9933

Conference paper

Akhtar Z, Chaudhuri B, Hui SYR, 2016, Smart loads for voltage control in distribution networks, Power and Energy Society General Meeting (PESGM), 2016, ISSN: 1944-9933

Conference paper

Akhtar Z, Saqib MA, 2016, Microgrids formed by renewable energy integration into power grids pose electrical protection challenges, Renewable Energy, Vol: 99, Pages: 148-157, ISSN: 1879-0682

System parameters of a microgrid change in its two operating modes primarily due to output current limitation of PWM based inverters connected with renewable energy sources. The unavailability of an appropriate protection scheme, which must be compatible with both modes of a microgrid operation, is a major problem in the implementation of a microgrid. Two important properties of the microgrid components are peer-to-peer, and plug-and-play. It means that there is no component like a master controller which is critical for the operation of a system, and a distributed-generation unit can be installed at any location in a microgrid. These properties further complicate the protection of a microgrid. This paper reports the MATLAB/SIMULINK model of a microgrid along with the models of the conventional protection schemes and renewable energy distributed-generation resources. Malfunctioning in the conventional protection schemes in islanding mode is identified and models of newly proposed protection schemes are developed. Different types of faults are simulated in all the protection zones of the system and the system parameters are analysed to identify the possible fault detection methods. Based on the simulation results, a protection scheme is recommended that can meet the protection standards such as selectivity, co-ordination and reliability.

Journal article

Akhtar Z, Chaudhuri B, Hui S, 2015, Smart Loads for Voltage Control in Distribution Networks, IEEE Transactions on Smart Grid, Vol: 8, Pages: 937-946, ISSN: 1949-3061

This paper shows that the smart loads (SLs) couldbe effective in mitigating voltage problems caused by photovoltaic(PV) generation and electric vehicle (EV) chargingin low-voltage (LV) distribution networks. Limitations of thepreviously reported SL configuration with only series reactivecompensator (SLQ) (one converter) is highlighted in this paper.To overcome these limitations, an additional shunt converter isused in back-to-back (B2B) configuration to support the activepower exchanged by the series converter, which increases the flexibilityof the SL without requiring any energy storage. Simulationresults on a typical U.K. LV distribution network are presented tocompare the effectiveness of an SL with B2B converters (SLBCs)against an SLQ in tackling under- and over-voltage problemscaused by EV or PV. It is shown that SLBCs can regulate themain voltage more effectively than SLQs especially under overvoltagecondition. Although two converters are required for eachSLBC, it is shown that the apparent power capacity of eachconverter is required to be significantly less than that of anequivalent SLQ.

Journal article

Akhtar Z, Chaudhuri B, Hui SYR, 2015, Primary Frequency Control Contribution From Smart Loads Using Reactive Compensation, IEEE Transactions on Smart Grid, ISSN: 1949-3061

Frequency-dependent loads inherently contribute to primary frequency response. This paper describes additional contribution to primary frequency control based on voltage-dependent noncritical (NC) loads that can tolerate a wide variation of supply voltage. By using a series of reactive compensators to decouple the NC load from the mains to form a smart load (SL), the voltage, and hence the active power of the NC load, can be controlled to regulate the mains frequency. The scope of this paper focuses primarily on reactive compensators for which only the magnitude of the injected voltage could be controlled while maintaining the quadrature relationship between the current and voltage. New control guidelines are suggested. The effectiveness of the SLs in improving mains frequency regulation without considering frequency-dependent loads and with little relaxation in mains voltage tolerance is demonstrated in a case study on the IEEE 37 bus test distribution network. Sensitivity analysis is included to show the effectiveness and limitations of SLs for varying load power factors, proportion of SLs, and system strengths.

Journal article

Luo X, Akhtar Z, Lee CK, Chaudhuri B, Tan S-C, Hui SYRet al., 2015, Distributed voltage control with electric springs: comparison with STATCOM, IEEE Transactions on Smart Grid, Vol: 6, Pages: 209-219, ISSN: 1949-3053

The concept of electric spring (ES) has been proposed recently as an effective means of distributed voltage control. The idea is to regulate the voltage across the critical (C) loads while allowing the noncritical (NC) impedance-type loads (e.g., water heaters) to vary their power consumption and thus contribute to demand-side response. In this paper, a comparison is made between distributed voltage control using ES against the traditional single point control with STATic COMpensator (STATCOM). For a given range of supply voltage variation, the total reactive capacity required for each option to produce the desired voltage regulation at the point of connection is compared. A simple case study with a single ES and STATCOM is presented first to show that the ES and STATCOM require comparable reactive power to achieve similar voltage regulation. Comparison between a STATCOM and ES is further substantiated through similar case studies on the IEEE 13-bus test feeder system and also on a part of the distribution network in Sha Lo Wan Bay, Hong Kong. In both cases, it turns out that a group of ESs achieves better total voltage regulation than STATCOM with less overall reactive power capacity. Dependence of the ES capability on proportion of critical and NC load is also shown.

Journal article

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