Comparative Study of Performance of Electric Spring and ... · Fig.4: Power inverter used as a reactive power controller ENERGY STORAGE The vector equation for the electric spring

Dr. H. V. Saikumar, Sharanki. R 37

Conference Proceeding of

6th International Conference on Innovative Research in Engineering Science and Management (ICIRESM-16) at The Institutions of Electroanic and Telecommunication Engineers (IETE), Lodhi Road, New Delhi, Delhi, India

on 9th October 2016 ISBN: 978-81-932712-8-5

Comparative Study of Performance of Electric

Spring and STATCOM

Dr. H. V. Saikumar Sharanki. R Professor Mtech. Power Systems

Department of Electrical and Electronics Department of Electrical and Electronics National Institute of Engineering, Mysore National Institute of Engineering, Mysore

[email protected] [email protected] Abstract- The Electric Spring is an emerging technology and

has been proposed recently as an effective means of distributed

voltage control. An Electric Spring (ES) a power electronics

device adaptive to fluctuating mains voltage. It can therefore be

distributed over the power grid (e.g. households, industrial sites

etc) to stabilize the main voltage. The idea is to regulate the

voltage across the critical (C) loads while allowing the non-

critical (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 the

performance of an Electric Spring and a STATCOM to control

the main voltage and hence the reactive power. For a given

range of supply voltage variation, the total reactive power

capacity required for each option to produce the desired

voltage regulation at the point of connection is compared. A

simple case study with an ES and STATCOM is presented first

to show that the ES and STATCOM require comparable

reactive power to achieve similar voltage regulation. The

performance of Electric Spring (ES) is further extended

through a case study on IEEE-14 bus test feeder system. In both

cases, it turns out that reactive power capacity required by ES

is less than that required by STATCOM.

I. INTRODUCTION

The control of voltage and reactive power is a major issue

in power system operation. This is because of the topological

differences between distribution and transmission systems.

Since the power system supplies power to a vast number of

loads and is feeding from many generating units, there is a

problem of maintaining voltages within required limits. As

load varies, the reactive power requirements of the

transmission system vary. Since the reactive power cannot

be transferred or transported over long distances, voltage

control has to be effected by using special devices installed

at various location of the system which possess difficulties

in keeping sufficient levels of voltage in the power system

network.

The proper selection and coordination of equipment for

controlling reactive power and voltage stability are among

the major challenges of power system engineering. These

challenges gave birth to some selected devices to

compensate reactive power. Voltage control in medium

voltage (MV) or low voltage (LV) distribution networks is

typically exercised through transformer tap-changers and/or

switched capacitors/reactors. Sometimes a Static

Compensator (STATCOM) is used for fast and precise

voltage regulation, especially for the sensitive/critical loads

[1].

The novel concept of Electric Spring (ES) has been

proposed for reactive power control and voltage regulation.

In electrical engineering terms, this electric spring is a

special form of reactive power controller. Here, the load is

divided into two type’s viz., a non-critical load and a critical

load. By connecting an electric spring in series with the non-

critical load, we can ensure that the voltage and power at the

critical load remain constant when the line voltage feeding

the load fluctuates [2]. Such an arrangement of load is

termed as “Smart load.”

At times of generation shortfall or network constraint, the

voltage of the Non-Critical (NC) loads is reduced while

regulating the voltages across the Critical (C) loads by

controlling the reactive power. One way to exercise this

control is to use the so called Electric Spring (ES) [1-2]. A

simple case study with an ES and STATCOM is presented

in this paper using MATLAB/SIMULINK to show that the

ES and STATCOM require comparable reactive power to

achieve similar voltage regulation. Further, the performance

of an Electric Spring (ES) is further substantiated through a

case study on the IEEE 14 bus test feeder system to show

that ES requires less reactive power capacity than

STATCOM.

II. PRINCIPLES OF ELECTRIC SPRING

The principle of ES is explained based on the principle of

mechanical spring described by Robert Hook in 1678. The

Hook’s law states that force of mechanical spring,

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𝑭 = −𝑘𝑥 (1)

Where, F is the force vector, k is the spring constant and x is

the displacement vector.

The potential energy stored in the mechanical spring is:

𝑃𝐸 =1

2𝑘𝑥2 (2)

Electric Spring is an electric device that can be used to

provide electric voltage support, store electric energy and

damp electric oscillation [3].

The basic relationship of ES is,

𝑞 = {𝐶𝑣𝑎 𝑖𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑒 𝑚𝑜𝑑𝑒−𝐶𝑣𝑎 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑖𝑣𝑒 𝑚𝑜𝑑𝑒

(3)

𝑞 = ∫ 𝑖𝑐 𝑑𝑡 (4)

where, q is the electric charge stored in a capacitor with

capacitance C, va is the electric potential difference across

the capacitor, and ic is the current flowing into the capacitor.

From equation (3) and (4) electric spring can be represented

as a current controlled voltage source.

A. Analogy of Electric Spring

Analogies of mechanical spring and ES under 3

conditions are represented in Fig.1. The neutral position of

ES represents a reference voltage at which the spring is

designed to maintain.

Fig.1: Analogy of Mechanical Spring and Electric Spring

Here, ES and 𝑍1 across main voltage are connected in series

to maintain 𝑉𝑠 to its nominal reference value 𝑉𝑆𝑟𝑒𝑓 in case of

neutral position. Similar to mechanical spring, ES can

provide voltage boosting and reduction functions.

The voltage boosting function is explained by compressing

the spring. In this case, to maintain 𝑉𝑆 = 𝑉𝑆𝑟𝑒𝑓 , ES voltage

increases in positive direction and output voltage of NC load

will be less than reference voltage. Similarly, voltage

reduction is explained by expanding the mechanical spring

as shown in Fig.1. In this case, to maintain 𝑉𝑆 = 𝑉𝑆𝑟𝑒𝑓, ES

voltage increases but in negative direction and the output

voltage of NC load will be greater than reference voltage.

The generation of ES voltage and the practical

implementation of ES and its characteristics are explained

in [3]. The operating limits of Electric Spring discussed in

[3-4]. The phasor diagram for both voltage boosting and

voltage reduction function is shown in Fig.2 [5]. The energy

storage capability of the electric spring can be seen from the

potential electric energy stored in the capacitor:

𝑃𝐸 =1

2𝐶𝑣𝑎

2 (5)

So the capacitor C serves as the energy storage element for

the electric spring.

Fig.2:

Phasor diagram for voltage boosting and voltage reduction

Since ES is connected in series with 𝑍1, it provides to

dissipate electric energy for damping purpose. The NC load

voltage 𝑉𝑜 and ES voltage 𝑉𝑎 can change in a special manner

such that the load power consumption of 𝑍1 will follow the

variation of renewable power generation. This unique

feature of ES offers a new solution in future power systems

with intermittent renewable energy source.

III. ELECTRIC SPRING AS REACTIVE POWER

CONTROLLERS

Electric springs are reactive power controllers with input

voltage control instead of the traditional output voltage

control used in series reactive power compensators [6]. The

inverter structure and functions of the ES are similar to the

reactive power compensators (RPC), but the concept and

operating principle are totally different.





The series RPC is used to regulate the (output) load

voltage/power; therefore the power generation has to follow

the load demand.

Conversely, an ES uses generation-side parameters such as

mains voltage as the control variable. Thus the non-critical

load voltage fluctuates and the ES would provide voltage

regulation and automatically shape the load power to follow

the power generated [7]. The requirement of installing an

electric spring with a non-critical load, as shown in Fig. 3, is

also different from that of conventional reactive power

compensators.

Fig. 3: Arrangement of an ES connected in series with a NC load

The electric spring comprises a power inverter with a dc bulk

capacitor on the dc side and an inductive-capacitive (LC)

filter on the ac side of the power inverter A full bridge

inverter is designed as shown in Fig.4. Voltage Source

Converter is implemented using MOSFETS. The four

freewheeling diodes of the power inverter behave like a

diode rectifier which rectifies the ac voltage into a dc one

𝑉𝑑𝑐 across the bulk capacitor. The pulse width-modulation

switching method is adopted in the power inverter to

generate a controllable ac voltage across the filter capacitor.

This controllable ac voltage is the output voltage of the

electric spring.

Fig.4: Power inverter used as a reactive power controller

The vector equation for the electric spring is,

𝑣𝑜 = 𝑣𝑠 − 𝑣𝑎 (6)

the power balance equation can be expressed as,

𝑃𝑖𝑛 =𝑣𝑠

2−𝑣𝑎2

𝑅1+

𝑣𝑠2

𝑅2 (7)

𝑃𝑖𝑛 = 𝑃1 + 𝑃2 (8)

Where, 𝐑𝟏 and 𝐏𝟏 are the resistance and power consumption

of the NC load respectively; and 𝐑𝟐 and 𝐏𝟐 are the

resistance and power consumption of the C load

respectively; 𝐯𝐬 and 𝐯𝐚 are the root-mean-square values of

the ac mains and electric spring voltage respectively.

Examples for NC loads include electric heaters,

refrigerators and lighting systems. Critical loads refer to

electric loads that require a well-regulated mains voltage,

such as life-supporting medical equipment and computer

controlled equipment. Equation (8) indicates that, if the

electric spring can regulate 𝑣𝑠 , 𝑃2 should be constant and

𝑃1 should follow the time-varying profile of the intermittent

power generation as shown in Fig. 5.

Fig. 5: Power profiles of NC load P1 and C load P2 when the mains voltage

vs is regulated by the reactive power controller. Let the rated power of the non-critical load when the electric

spring is not activated (i.e.,𝑣𝑎 = 0 ) be:

𝑃1𝑚𝑎𝑥 =

𝑣𝑠2

𝑅1 (9)

Let the actual power of the non-critical load power under

the control of electric spring (i.e., 𝑣𝑎 ≥ 0𝑉) be:

𝑃1𝑒𝑠 =

𝑣𝑠2−𝑣𝑎

2

𝑅1 (10)

From Equation (9) and (10), the relationship between the

rated power of NC load for without and with ES is given by,

𝑃1𝑚𝑎𝑥 ≥ 𝑃1

𝑒𝑠 (11)

Where 𝑃1

𝑚𝑎𝑥 and 𝑃1𝑒𝑠 are the rated power of NC load when

ES is not activated and ES activated respectively.

III. ELECTRIC SPRINGS FOR REDUCTION OF

ENERGY STORAGE

Now consider a general power grid consisting of an AC

generator, a renewable power source, energy storage

(battery banks), a set of non-critical loads and a set of





critical loads as shown in Fig. 6. The power flow diagram is

shown in Fig. 7 in which the power from the energy storage

can be positive or negative depending on whether the storage

device is discharging or charging [6-7].

Fig. 6: Schematic of a power grid.

The power balance equation of the power grid in Fig. 7 can

be expressed as:

𝑃𝐺 + 𝑃𝑅 + 𝑃𝑆 = 𝑃1 + 𝑃2 (12)

Fig. 7: Power flow diagram.

PS is positive when the battery is discharging and negative

when it is charging. Re-arranging Equation (12) with the

storage power as the subject of the equation,

𝑃𝑆 = −𝑃𝐺 − 𝑃𝑅 + 𝑃1 + 𝑃2 (13)

Where, PG is the power generated by the ac generator, PR is

the renewable power, and PS is the power from the energy

storage.

Without the electric spring, the energy storage requirement

for duration of T is:

𝐸𝑆 = ∫ 𝑃𝑆𝑇

0 𝑑𝑡 = − ∫ 𝑃𝐺

𝑇

0 𝑑𝑡 − ∫ 𝑃𝑅

𝑇

0 𝑑𝑡 +

∫ 𝑃1𝑚𝑎𝑥𝑇

0 𝑑𝑡 + ∫ 𝑃2

𝑇

0 𝑑𝑡 (14)

With the electric spring, the energy storage requirement for

the same duration is:

𝐸𝑆𝑒𝑠 = ∫ 𝑃𝑆

𝑇

0 𝑑𝑡 = − ∫ 𝑃𝐺

𝑇

0 𝑑𝑡 − ∫ 𝑃𝑅

𝑇

0 𝑑𝑡 +

∫ 𝑃1𝑒𝑠𝑇

0 𝑑𝑡 + ∫ 𝑃2

𝑇

0 𝑑𝑡 (15)

The difference of the energy storage requirements with and

without the electric spring can be obtained by subtracting

Equation (14) from (15), resulting in:

𝐸𝑆 − 𝐸𝑆𝑒𝑠 = ∫ 𝑃1

𝑚𝑎𝑥𝑇

0 𝑑𝑡 − ∫ 𝑃1

𝑒𝑠𝑇

0 𝑑𝑡 (16)

In view of Equation (9),

∫ 𝑃1𝑚𝑎𝑥𝑇

0 𝑑𝑡 ≥ ∫ 𝑃1

𝑒𝑠𝑇

0 𝑑𝑡 (17)

Therefore,

𝐸𝑆 ≥ 𝐸𝑆𝑒𝑠 (18)

Equation (18) shows that the use of the electric spring can

theoretically reduce the energy storage requirements in the

power grid.

V. PRACTICAL EVALUATION OF TEST SYSTEM

WITH ELECTRIC SPRING AND STATCOM

A. Test system

In order to compare the voltage regulation performance

of a single ES against that of a STATCOM, a simple test

system as shown in Fig. 8 has been considered [5-7]. The

experimental setup for evaluating the response of the ES

under a weakly regulated power grid is shown in Fig.8. The

details of system and Electric Spring specifications are

explained in [5-7].

Fig. 8: Simulation set-up with an intermittent source and an equivalent

power grid

B. Simulink model and results

The base system is modeled in MATLAB/SIMULINK

without controller, with ES and STATCOM. Fig. 9 shows

the Simulink model with Electric spring.

Two sets of experiments are carried out with different levels

of active or reactive power fluctuation generated by the

renewable energy source simulator.





Fig. 9: Simulink model of the test system using Electric Spring

(i) Voltage Suppression mode

The voltage across the critical load is increased above the

nominal value (216 V) by reducing the reactive power

absorption of the renewable energy source. At t=1 s, the

reactive power absorption by the intermittent renewable

source is reduced from 1.7 KVAr down to 1.2 KVAr.

Without any voltage control, the load voltage increases from

the nominal value of 216 V up to 235 V as shown by Fig.

10(a). Both ES and STATCOM are able to restore the

voltage across the critical load back to the nominal value as

shown in Figs. 10(b) and 10(c). The ES achieves this by

injecting about 166 V in series with the non-critical load

voltage across which drops to about 50 V as shown in Fig.

11 and Fig. 12 respectively. In order to suppress the voltage,

both ES and STATCOM absorb reactive power (indicated

by positive sign of Q) from the system. As shown in Fig.

13(a), Electric spring absorbs reactive power of about 165

Var more than the STATCOM. It is observed that the

reactive power consumed by ES to restore the critical load

voltage to normal value is higher than the reactive power

consumed by STATCOM to achieve the same voltage. This

is because the improper proportion of critical and non-

critical loads, as explained in [1-5].

(a) (b) (c)

Fig. 10: Critical load voltage (a) Without controller, (b) with ES and (c) with STATCOM

Fig. 11: Electric Spring Voltage Fig.12: Non-critical load voltage

(a) (b)

Fig 13: Reactive Power exchange (a) with Electric Spring and (b) with

STATCOM

(ii) Voltage Support mode

At t=1.0 s, the reactive power absorption by the

intermittent renewable energy source increased from

500VAr to 700VAr. Without any voltage control, the load

voltage is seen to drop from the nominal value of 216 V to

slightly below 120 V as shown in Fig. 14(a).

As before, both STATCOM and ES are able to

restore the voltage across the critical load back to the

nominal value as shown in Figs. 14(b) and 14(c). The ES

achieves this by injecting about 119 V in series with the

non-critical load the voltage across which drops to about 97

V as shown in Fig. 15 and Fig. 16 respectively. In order to

suppress the voltage, both ES and STATCOM inject

reactive power (as indicated by negative sign of Q) into the

system.





(a) (b) (c)

Fig. 14: Critical load voltage (a) Without controller, (b) with ES and (c)

with STATCOM

Comparing Fig. 17(a) and Fig. 17(b), it is observed that ES

inject about 150 VAr less than the STATCOM. It is observed

that the reactive power consumed by ES to restore the critical

load voltage to normal value is lesser than the reactive power

consumed by STATCOM to achieve the same voltage. This

is due to the fact that an increase in ES voltage will result in

a reduction of non-critical load voltage which causes a

decrease in reactive power than an equivalent STATCOM to

restore the system voltage.

Fig.

15: Electric Spring Voltage Fig. 16: Non-critical load voltage

(a) (b)

Fig 17: Reactive Power exchange (a) with Electric Spring and (b) with STATCOM

VI. CASE STUDY: IEEE 14-BUS TEST FEEDER SYSTEM

A. Test System

After comparing the performance of a single ES against a

STATCOM, the focus is to compare the performance of

electric spring against that of STATCOM in IEEE test feeder

system.

To investigate this, the IEEE 14-bus test feeder system

shown in Fig.18 is considered [10].

Fig 18: Standard IEEE 14-bus system

It consists of five Generator buses, three of which are

synchronous compensators used only for reactive power

support. There are 11 loads in the system totaling 259 MW

and 81.3 Mvar. The modeling is done in

MATLAB/SIMULINK software tool. Usually bus no.1 is

considered as slack bus. Synchronous generators are

connected at bus 1 and 2 whereas synchronous

compensators are connected at bus 3, 6 and 8 as shown in

Fig. 18. Since synchronous compensators are used for

reactive power support, consider the synchronous

compensator at bus 6. This bus 6 is directly connected to the

loads at bus 5, 11, 12 and 13 respectively. Modify the IEEE

14-bus system by connecting Electric Spring (ES) and a

STATCOM to sensitive loads at bus 12 and 13 respectively.

A distribution transformer is connected between bus 6 and

bus 12, 13 as shown in Fig. 19 to step down the source

voltage. Each load assumed to have an equal proportion

between critical and non-critical load. All other circuit

parameters are exactly the same as feeder is set up to study

unbalanced operation.

B. Simulation Results and discussion

(i) Voltage Suppression Mode

The performance of Electric Spring connected at bus 12

has been compared with that of a STATCOM, which is

connected at bus 13. A 5% step increase in the source

voltage at bus 6 is simulated. Without Electric Springs and

STATCOM it is observed that the load voltage at bus 12 and

13 (voltage across the critical load) increases from 400 V to

415V as shown in the fig 20(a). Both ES and STATCOM

are able to restore the voltage across the critical load back

to the nominal value as shown in Figs. 20(b) and 20(c)

respectively.





The ES achieve this by injecting about 170 V in series with

the non-critical load voltage across which drops to about 225

V as shown in Fig. 21 and Fig.22 respectively. In order to

suppress the voltage, both ES and STATCOM absorb

reactive power (as indicated by positive sign of Q) from the

system. Comparing Fig. 23(a) and Fig. 23(b), it is found that

𝑄𝑉𝐴𝑟 of Electric Spring (ES) is 530 KVAr and 𝑄𝑉𝐴𝑟 of

STATCOM is about 2100KVAr. Thus it is observed that the

reactive power capacity required for the ES is 3 to 4 times

less than that required for STATCOM.

Fig 19: Modeling of IEEE 14-bus system with ES and STATCOM at bus

12 and 13 respectively

(a) (b) (c)


with STATCOM

Fig. 21: Electric Spring Voltage Fig. 22: Non-critical load voltage

(a) (b)


(ii) Voltage support mode

A 5% step reduction in the source voltage at bus 6 is

simulated. Without Electric Springs and STATCOM it is

observed that the load voltage at bus 12 and 13 (voltage

across critical load) decreases from 400 V to 380V as shown

in the fig 24(a). Both STATCOM and ESs are able to restore

the voltage across the critical load back to the nominal value

as shown in Figs. 24(b) and 24(c). The ES achieve this by

injecting about 190 V in series with the non-critical load

voltage across which drops to about 220 V as shown in Fig.

25 and Fig. 26 respectively. In order to support the voltage,

both ES and STATCOM inject reactive power (as indicated

by negative sign of Q) from the system.

(a) (b) (c)


with STATCOM





Comparing Fig. 27(a) and Fig. 27(b), 𝑄𝑉𝐴𝑟 of Electric Spring

(ES) is about 8 MVAr and 𝑄𝑉𝐴𝑟 of STATCOM is about 32

MVAr. Thus it is observed that the reactive power capacity

required for the ES is about 3 to 4 times less than that

required by the STATCOM.

Fig. 25: Electric Spring Voltage Fig. 26: Non-critical load voltage

(a) (b)


For both voltage suppression and voltage support modes of

operation, an ES connected to the critical load at bus 12 is

shown to achieve better voltage control than a STATCOM

connected to critical load at bus 13 because both loads at bus

12 and 13 are sensitive loads.

The study on the modified IEEE 14-node test feeder

network confirms the following.

Reactive power capacity required by ES is less than

that of required by STATCOM. This helps in better

total voltage control using Electric Spring (ES)

compared to STATCOM.

Modes

Critical Voltage

Electric

spring

Voltage

Non-

Critical

Voltage

Reactive Power

Without

controller

With ESs

& STAT-

COM

With

ES

With

STAT-

COM

Voltage

suppression

mode

Increases

from

400 V up

to 415 V

Suppress

back to

400 V

𝑉𝐸𝑆 =

170V

𝑉𝑁𝐶 =

225V

𝑄𝑉𝐴𝑟=

530 KVAr

𝑄𝑉𝐴𝑟=

2100

KVArr

Voltage

support mode

Decreases

from

400 V to

380V

Brings

back to

400 V

𝑉𝐸𝑆 =

190V

𝑉𝑁𝐶 =

210V

𝑄𝑉𝐴𝑟=

8 MVAr

𝑄𝑉𝐴𝑟=

32 MVAr

Table 1: Overall Results for both voltage supress mode and support

mode and its Reactive power exchange

VII. CONCLUSION

In this paper, a comparison is made between Electric Spring

and STATCOM with respect to reactive power control. For

a given range of supply voltage variation, reactive power

capacity required by both Electric spring and STATCOM is

compared by connecting to sensitive loads. 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.

Further, an IEEE 14-bus test feeder system is modified by

connecting transformer between bus 6 and buses 12, 13.

Also, Electric Spring and STACOM connected to the

critical loads at bus 12 and 13 respectively. Two modes of

operation Viz., voltage suppression mode and voltage

support mode are studied. In both modes, it turns out that

Electric Spring requires less reactive power capacity than

STATCOM. The ESs have the potential to solve the

stability problems arising from intermittent nature of

renewable energy sources and ensure that load demand will

follow power generation which is a new control method for

smart grid.

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Documents

Comparative Study of Performance of Electric Spring and ... · Fig.4: Power inverter used as a reactive power controller ENERGY STORAGE The vector equation for the electric spring