Neutral Current Mitigation using Controlled Electric Springs Connected to Microgrids within Built

Environment

Krishnanand K.R., Syed Muhammad Farzan Hasani, Jayantika Soni, and Sanjib Kumar Panda Electrical and Computer Engineering Department

National University of Singapore, Singapore krishkr09@gmail.com, farzan@nus.edu.sg, jayantika.soni@nus.edu.sg, eleskp@nus.edu.sg

Abstract—Modern buildings require complex planning and efficient energy management to lead them towards the ideal of self-sustainable buildings. The penetration of intermittent renewables as local sources for the building is a major step towards that ideal, but such energy conversions intensify the issues of instantaneous demand management and voltage stabilization. Also, the loads in a building can have their own complex, fast-changing operation strategies which further elevate the difficulty of these tasks. Unbalanced voltages are direct consequences of this scenario, which cause high neutral current that adversely affects the components of microgrid. This paper presents a method to mitigate neutral current in three phase systems using recently introduced concept of “Electric Springs”. The electric springs consist of fast acting passive devices controlled through power-electronic switches, connected to the building microgrid based on load classifications of critical and non-critical. The proposed method would result in better equipment safety and longevity of the critical building loads.

I. INTRODUCTION

In a modern built environment, one of the primary goals is to achieve the maximum extent of self-sufficiency in the energy domain. This is possible through well-planned microgrids within the built environment which allow interfacing of fluctuating sources and loads; without compromising on voltage stability of the grid. The buildings which can facilitate the utilization of renewable energy conversions, like photovoltaic-based conversions, would be part of smart-grids of the future [1]. The options for deregulated energy trading act as incentive for higher inclusion of renewables in the local generation. Being intermittent, renewables would introduce source-side disturbances in the supply. The complex loads cause disturbances in power values at the loads also. One of the most commonly observed result of these disturbances is voltage imbalance among the phases in a 3-phase circuit. Even though the problem of neutral current mitigation has been studied over the years [2-4], mitigation of it in case of high penetration of renewables in a microgrid still requires attention. Neutral current, along with undesirable sequence

components of harmonics would heat up the devices in the microgrid and cause lesser lifespan of devices.

The very recently established idea of electric spring could be used as a solution to this problem. The term “electric spring” was first used in [5] as an analogy with the mechanical spring following Hooke’s Law. The suggested configuration for neutral current mitigation can be summarized as follows: the load is divided between a critical load and a non-critical load; the critical load is connected directly to the grid while the non-critical load is connected to the grid in series with the electric spring (which is essentially an inverter with a control system). The voltage across the non-critical load is then a result of the vector addition of the grid voltage with the inverter voltage. An appropriate control system can regulate the grid voltage by sacrificing some power from the non-critical load [5].

The concept of connecting an inverter in series with the load has also been mentioned in [6] as a means of interfacing renewable energy sources with microgrid. The vector addition of voltage is also explained here although there is no division between critical and non-critical load.

A general steady-state analysis of a series-connected inverter (or electric spring) is presented in [7] where its various modes of operation are discussed. It is shown how the electric spring can be used to adjust the power dissipation of the load as well as improve the power factor. Electric springs and their effects are further studied in [8] and [9]. These studies focus on the use of electric spring to stabilize the voltage of a weakly regulated grid. Each phase in a three phase system can be allotted independent single phase electric spring for flexibility in control.

The paper organization is as follows. In section II, the electrical configuration of the building’s microgrid is shown using its schematic. Section III contains the mathematical analysis for the proposed system. Simulation results are given in section IV and the final section presents the conclusions and future work.

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II. CIRCUIT CONFIGURATION

The proposed circuit configuration is shown in Fig 1. The critical and non-critical load division is also used in this case.

Figure 1. Proposed system configuration for three phase microgrid.

Fig.2. shows the electric spring component used for maintaining voltage. The difference is that the non-critical load is connected in series with the electric spring through a tap-changing transformer. This provides another controllable variable (tap ratio n) to extend the limits of compensation while not deteriorating the non-critical load voltage by a large extent. Although linear transformer is used in this simulation, a case can also be made for the use of a variable auto-transformer.

(a)

(b)

Figure 2. (a) Single line diagram of the system under study. The sources are inclusive of their source impedances. (b) Schematic of an electric

spring used for single phase connection.

III. MATHEMATICAL ANALYSIS

The goal of balancing a 3-phase system is achieved if equal impedance is seen by the source for each phase. Therefore, this analysis will focus on the impact of electric

spring on the overall phase impedance. A point to be noted here is that it will be assumed that the grid voltage is already regulated (there are no constraints on power generation which automatically adjusts to change in perceived load). It will be shown later how the system can be extended to include voltage regulation in case of fluctuating power generation as one of its functions.

The phase impedance seen by the grid in case of an inactive electric spring (and a transformer tap ratio of 1) is simply a parallel combination of the 2 types of load impedances. Subscript NC corresponds to non-critical and C corresponds to critical loads respectively. Subscript ES corresponds to electric spring.

CNC

CNCCNCeq ZZ

ZZZZZ

||1

The following voltage divider equation has to be true for a working electric spring. Using nodal analysis,

ESNC

ES

ES VV

nZ

Z

Z

2

The effective impedance of the electric spring is then

12

ES

NCES

VV

n

ZZ

The equivalent impedance of the branch containing the electric spring is given by

VVn

Z

n

ZZ

ES

NCNCES

1

122

The equivalent impedance of the whole phase can then be calculated

CES

NC

CNCeq

ZV

VnZ

ZZZ

12

2

A factor γ can be defined such that

V

Vn ES12

The equivalent impedance can then be represented as

CNC

CNCeq ZZ

ZZZ

2

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Comparing equation (7) to equation (1), we can see there is a difference only in the denominator, which is illustrated in the following phasor diagram. The dotted lines represent a few possible changes this factor can make to the second term of the denominator, which corresponds to the ability of the electric spring to provide both active and reactive compensation.

Figure 3. Phasor diagram illustrating the difference in denominator of the

equivalent impedance of the system without and with electric spring.

The value of n should be set such that the magnitude of voltage across the critical load is equal to the magnitude of phase voltage. This can be represented by the equation

VV

nZ

Z

n

Z

nNC

ES

NC

2

2

This simplifies to

VV

nES

1

1

If we consider the loads to be purely resistive in nature (or alternately, consider n to be a phasor quantity), we get an interesting set of equations that can also be applied in the general case. The first requirement can be represented by the equation

Zn

ZZ NC

ES 2

The second requirement is equation (8) (without the absolute signs, considering the above assumptions). Solving the two equations simultaneously for the value of n gives the following result

)( dgeneralizeZ

Zor

Z

Zn NCNC

The effect of taking the magnitude will only be a phase difference in the non-critical load voltage. The required electric spring impedance can then be calculated using equation (10) and the required ES voltage is calculated using

equation (2). The application of these equations is illustrated in Fig. 4 and 5.

Figure 4. Block diagram showing calculation of n and VES from voltage

and current measured at the load.

Figure 5. Control system schematic for generating the calculated Electric

Spring (VES) voltage.

The only unknown variable in Fig. 4 is Zreq. This is the desired impedance that all 3 phases should portray so that system is balanced. A reasonable value can be the arithmetic mean of the three (existing) phase impedances of the unbalanced system, corrected to get the desired power factor. This is the method used in the following simulation results. This required impedance can also be multiplied by a factor that is controlled by the difference in grid voltage from its nominal value, resulting in the additional functionality of voltage regulation.

IV. SIMULATION RESULTS

Simulations have been performed for simultaneous variations in the phases due to imbalance of demands in phases. The simulations are performed in MATLAB/Simulink environment in a modular manner. The simulation follows per unit values with load as the base. Results from a sample simulation are shown below. Figures 6-8 illustrates the effective operation of the proposed method. Step changes (up and down) are induced in phase a, while ramp changes with different gradients are imposed on phase b. Phase c is kept undisturbed. A proportional-integral (PI) controller is used for damping the disturbance. During unbalanced situation, the neutral current is quite high normally, but when the controlled electric spring is introduced, then the neutral current magnitude damps out very soon.

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Figure 6. Variations in the voltages of phases cause unbalanced situations which give rise to neutral current. This causes undesirable heating and

corresponding losses. Sensitive equipment could get damaged by high neutral currents.

Figure 7. The peak amplitudes of the voltages of each phase are shown, the deviation from the nominal value causes unbalance in the three phase system and hence neutral current. The durations within which the system changes occur are shown within dotted rectangles. The rest of the durations are steady-

state. Variations in unbalances cause different neutral currents at different points in time.

Many sample scenarios are evaluated and verified for successful mitigation of the neutral current. Accommodating multiple algorithms and prioritized strategies in the controller could make the electric springs follow multiple objectives through role-switching.

V. CONCLUSION

A novel method for reducing three phase neutral current in building microgrids using the recent technology of electric spring is proposed. The system configuration, simulations and mathematical analyses have been performed to establish

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(a) Phase a voltage and its peak fundamental amplitude

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-202

So

urc

e V

olta

ge

s (i

n p.

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(b) Phase b voltage and its peak fundamental amplitude

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Time (in seconds)

(c) Phase c voltage and its peak fundamental amplitude

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3

4(a) Peak source voltage of phase a

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3

4(b) Peak source voltage of phase b

Am

plitu

de

of

volta

ge

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 22

3

4(c) Peak source voltage of phase c

Time (in seconds)

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Figure 8. Quantification of neutral current with and without electric spring – from the plots it is clear that the neutral current without the electric spring has

much more magnitude and duration than when electric spring is used. The use of electric springs protect critical loads from high neutral current and from subsequent heating up of the devices.

the operation of electric springs for neutral current elimination. The results show that this configuration along with its control can provide significant reduction in neutral current and hence would have higher efficiency in the building energy. Since the electric spring functionality can be decided by the control algorithm used, multiple functionalities, like power control and power-quality maintenance, can be assigned to the same physical device. The future work involves validating the operation of electric-spring-inclusive building microgrid using real-time hardware-in-loop technology and building hardware prototypes of electric springs. Subsequently, optimal control strategies could be developed for maximum energy efficiency.

ACKNOWLEDGMENT

This research is funded by the Republic of Singapore’s National Research Foundation through a grant to the Berkeley Education Alliance for Research in Singapore (BEARS) for the Singapore-Berkeley Building Efficiency and Sustainability in the Tropics (SinBerBEST) Program. BEARS has been established by the University of California, Berkeley as a center for intellectual excellence in research and education in Singapore.

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[3] T. Zaveri, B. R. Bhalja and N. Zaveri, "Load compensation using DSTATCOM in three-phase, three-wire distribution system under various source voltage and delta connected load conditions," Electrical Power and Energy Systems, vol. 41, pp. 34-43, 2012.

[4] S. Dasgupta, I. V. Prasanna, S. K. Sahoo and S. K. Panda, "A novel four-leg three-phase inverter control strategy to reduce the Data Center thermal losses: Elimination of neutral current," in IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society, Montreal, 2012

[5] S. Y. Hui, C. K. Lee and F. F. Wu, "Electric Springs—A New Smart Grid Technology," IEEE Transactions on Smart Grid, vol. 3, no. 3, pp. 1552-1561, 2012.

[6] S. Dasgupta, S. K. Sahoo, S. K. Panda and G. A. J. Amaratunga, "Single-Phase Inverter-Control Techniques for Interfacing Renewable Energy Sources With Microgrid—Part II: Series-Connected Inverter Topology to Mitigate Voltage-Related Problems Along With Active Power Flow Control," IEEE Transactions on Power Electronics, vol. 26, no. 3, pp. 732-746, 2011.

[7] S.-C. Tan, C. K. Lee and S. Y. Hui, "General Steady-State Analysis and Control Principle of Electric Springs With Active and Reactive Power Compensations," IEEE Transactions on Power Electronics, vol. 28, no. 8, pp. 3958-3969, 2013.

[8] C. K. Lee and S. Y. Hui, "Reduction of Energy Storage Requirements in Future Smart Grid Using Electric Springs," IEEE Transactions on Smart Grid, 2013.

[9] C. K. Lee, N. R. Chaudhuri, B. Chaudhuri and S. Y. Hui, "Droop Control of Distributed Electric Springs for Stabilizing Future Power Grid," IEEE Transactions on Smart Grids, 2013.

0.4 0.6 0.8 1 1.2 1.4 1.6-0.4

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(a) Neutral current signals with and without electric spring

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0

0.2

0.4

Pe

ak

Am

plitu

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(in

p.u

.)

Time (in seconds)

(b) Peak fundamental amplitudes of neutral currents with and without electric spring

Without Electric Spring With Electric Spring

Without Electric Spring With Electric Spring

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