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Controllable Network Transformers Deepak Divan and Jyoti Sastry

School of Electrical and Computer EngineeringGeorgia Institute of Technology, Atlanta, GA 30318 USA

ddivan@ece.gatech.edu

Abstract - The electric power grid is under stress, caused byincreasing loads, decreasing investments, and increasing penetration of dynamic sources and loads. Grid control techniques have not changed much over the last 50 years, with generator excitation providing the only ‘analog’ control handle. The basic control problem on the grid is to maintain node voltages while ensuring that branch currents do not exceed defined limits. This paper proposes the possibility of converting existing load tap changing (LTC) transformers into Controllable Network Transformers (CNT) using fractionally rated direct ac converters – providing vernier control of amplitude and phase angle. This would allow dynamic control to be distributed on the grid, enhancing response and reliability. The concept can be scaled to realistic power levels.

I. INTRODUCTION

The electric power infrastructure is under stress. As the penetration level of renewable energy increases, demand for energy soars, and ability to build new infrastructure remains compromised, the need to better utilize existing assets becomes very important. At the same time, the level of dynamic grid control that is needed is also dramatically increasing. Ensuring very high reliability levels, while cost-effectively meeting these targets is a challenging task.

There are various approaches that have been proposed to alleviate this gap between the growing need for power versus the ability of the power system, as it currently stands, to meet this growing need. One such solution is expansion of the existing infrastructure through the construction of additional transmission lines. This is both an expensive and difficult solution.

An alternate approach that is receiving a lot of attention is the concept of a “Smart Grid” that can more effectively utilize the current power system. A “Smart Grid”, which is reliable, self-healing, and fully controllable [1] can be implemented using distributed “smart”, “controllable” assets that augment the existing system assets to provide vernier control capability. These “Smart Assets”, would be capable of controlling the network using local information [2]. One such example is that of the Distributed Series Impedance (DSI) [3], where massively distributed assets are deployed to convert the existing “dumb” power lines to “smart” assets. This has been shown to result in tremendous improvement in the transfer capacity of the system.

Reliability is of vital importance for the power grid. The existing reliability of the system, which approaches 99.999%, cannot in any way be degraded. The need for higher reliability is moving utilities towards meshed or networked systems versus radial structures. The high

reliability that is inherent in meshed systems comes at a price, namely the poor controllability of current in the network. Networked systems are difficult if not impossible to control; some possible methods of control include Phase Shifting Transformers and Distributed Series Impedance modules [1].

The conventional approach to network operation is through the use of off-line optimal power flow and state estimation techniques, which are then used to dispatch generator excitation, LTC taps and shunt VAR compensation, so as to meet dual constraints of voltage regulation and controlled branch currents. In a highly interconnected meshed network, this represents a very challenging control problem. The non-trivial nature of the control problem has thus far discouraged actual implementation of any real time controllers that can fully control a network.

Some newer approaches to network control provide control of both voltage magnitude at a node, as well as phase angle. The angle control is instrumental in being able to control branch currents. Devices that provide such functionality include FACTS devices such as UPFC [4], and SSSC [5]. Shunt devices such as SVC’s and STATCOM’s [6] provide the VAR regulation required to maintain acceptable bus voltage levels in the network. Although FACTS devices have reached a high level of maturity, significant market penetration of these devices is yet to be seen. Phase angle regulators provide power flow control but are sluggish in response and cannot control bus voltages. The Intelligent Universal Transformer [7] is essentially a cascaded power converter, provides only unidirectional control of power flow, and is an expensive solution. Other approaches include the Power Electronics Transformer [8] which has a high switch count and cost due to the high frequency transformer, and the Sen Transformer [9], which provides a solution for power flow control with interconnection between phases thereby creating complex fault modes, as well as a high switch count.

This paper proposes augmentation of an existing load tap changing transformer to provide dynamic vernier control of voltage magnitude and phase angle simultaneously over a meaningful control range. The Controllable Network Transformer (CNT) proposed is a “smart asset” that provides control of bus voltages and line currents in a meshed system; this is not achievable using conventional techniques. The converter used can be described as a Thin AC Converter, or TACC, a concept that is detailed in a companion paper [11].

978-1-4244-1668-4/08/$25.00 ©2008 IEEE

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CONTROLLABLE NETWORK TRANSFORMER –THE BASICS

II.

The Controllable Network Transformer or CNTprovides simultaneous control of bus voltage magnitudesand phase angles by augmenting an existing tappedtransformer with a small rated converter, as shown in Figure 1. The vernier control requirements of the systemallow for the converter to be rated fractionally withrespect to the rating of the transformer. The converterincludes two ac switches, a small filter capacitor andinductor. It may be assumed initially that the switches are controlled with fixed duty cycles D and (1-D). For a transformer with a tap ratio N, the voltage magnitude ofthe output voltage can be varied between (1+N) pu forD=1 to (1-N) pu for D =0. This can be implementedusing well known pulse width modulation techniques.

Figure 1: Controllable Network Transformer

By applying conventional pulse-width modulationtechniques only half the desired functionality is achieved,i.e. voltage magnitude control. The ability to controlphase angle is not possible as there are no energy storageelements that can provide the required energy during thezero crossings of the input voltage. To obtain an outputvoltage of controllable phase angle, the concept of “DualVirtual Quadrature Sources” [10] is applied. Figure 2aillustrates a simple ac chopper that operates like a buckconverter, generating an output voltage in phase with the input and of magnitude lesser than or equal to that of theinput, Figure 2b. Using conventional PWM techniques anoutput voltage with variable phase and/or harmoniccontent cannot be synthesized, as seen in Figure 2c.

To synthesize an output voltage of variablephase/harmonic content, two virtual sources are invokedin quadrature with the input voltage (Vdo), one at thefundamental (Vqo) and the second at the third harmonicfrequency (V3). The sum of the three components shouldat all instants of time satisfy the physical constraints on the system.

Figure 3(a) illustrates the concept of virtualquadrature sources, where the two virtual sources aresummed with the direct component of the voltage toresult in a voltage that has controllable phase and/orharmonic content. The resultant voltage is seen to lie within the envelope of the input voltage, thereby meetingthe physical constraints on the system. The sum of the direct (Vdo) and quadrature (Vqo) components at thefundamental frequency results in a phase shifted voltageat the fundamental frequency, as shown in Figure 3(b).The voltage synthesis technique used to generate thevirtual quadrature sources can be implemented using a simple real time modulation strategy (Even Harmonic

Modulation), the equivalence of the voltage synthesistechnique and the concept of the real time modulationstrategy (EHM), has been detailed in [10].

(a)

(b) (c)Figure 2: AC Chopper (a) Circuit topology (b) Achievable

output voltage

(a)

(b)

Figure 3: (a) Voltage synthesis using dual virtual quadraturesources, (b) Input and phase shifted output voltage

In one possible implementation of the control strategy, sine triangle PWM is used, with the controlreference voltage consisting of a DC component tosynthesize the desired Vdo, and a second harmonic of amplitude K2 and phase angle 2, which when multipliedwith the input voltage results in two components ofvoltage, one at the fundamental and the second at thethird harmonic frequency.

Applying EHM, the output voltage (V0) of thetransformer can be expressed as a function of the inputvoltage (VS), modulation signal (D) and tap ratio (N).

tVV mS sin (1)

SS VNDVDNV )1()1()1(0(2)

With the modulation signal given by (3),220 2sinKKD (3)

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Applying this technicontrolla

que to the proposedble network transformer enables the synthesis of

an output voltage of variable magnitude and phase angle,and also generates a by-product in the form of a triplencomponent which can be trapped.

(b)(a)Figure 4: (a) Control region (b) Variation of the phase angle

The degr ble is determinedby the ra

with a control variable

ee of control achievating of the converter. For example, for a tap ratio

(N) ± 15% of the nominal voltage, the range ofachievable phase angle control is illustrated in Figure4(b). The maximum achievable phase shift for this ratingis ±5 degrees. This does not sound like much, but is sufficient to control current on short to medium lengthlines in a meshed network. The power flow in a line, representing the active current is given by

sinVVP 21o X

(4)

where V1 and V2 are voltages at the ends of a transmission line, X is the reactance of the line, and d is the angle between V1 and V2.

(a)

(b)

Figure 5: (a) Experimental re Even Harmonic modulation

chopper that phase angle

Totransformer, p hown.The

sults.signal, modulation signal with even harmonic and dc

component, and switching pulses for ac switches, (b) Input and output voltage for an ac chopper.

gures 5(a) and 5(b) show experimental results for an acis providing amplitude and

Fi

control using virtual quadrature sources, and validates thecontrol characteristics for the ac chopper, which is thebasic building block for the CNT.

III. SIMULATION RESULTS – CONTROLLABLENETWORK TRANSFORMER

illustrate the performance of the proposedreliminary simulation results are s

ac chopper is rated at ± 10% of the nominal voltage. The transformer is simulated at a voltage level of the138KV.

(a)

(b)

(c)

Figure 6 (a) Switch Voltage and Current, (b) Switch voltage andcurrent waveforms illustrating ching instances, (c) Input and

Output voltages swit

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Figure 6(a) and (b) show the switch voltage and current.The switch voltage has a peak value of 35KV and the

A.To test thepeak current is 2600 functionality of theControllable Network Transformer in a simplifiedsystem, the CNT in Fig 7a is used in a simple two bustwo line system as shown in Fig 7b.

(a)

15kV1380kV138

(b)

Figure 7: (a) Controllable Network Transformer, (b) 2 busystem

s

(a) (b)

Figure 8 (a) Line-voltage after 3rd harmonic trap, ( eacross the 3rd harmonic trap, (c) RMS of the line current in Line

1 and ine 2

B

two lines can be cont l. The third harmonic

r voltagesand

b) Voltag

L

y varying the amplitude and phase angle of the voltageacross the transformer, the current and power flow in the

rolled at wilvoltage generated due to the VQS control can beabsorbed by the harmonic trap if required. In this simplesystem, the CNT has ±15% control. This translates to ±5 degrees phase angle control, thereby allowing for thecontrol of the current in line 2 of the 2 bus system. Therms current can be seen to decrease from 500A down to400A and then increase to 600A. The rms of the line

current in Line 1 however, remains constant over theentire period as there is no control in line 1.

It is also important to see how the ControllableNetwork Transformer can be scaled to highe

power levels. A companion paper discusses theconcept of Thin AC Converters (TACC) and multi-leveldirect ac converters [11]. Figure 9 shows a four leveldirect ac converter wrapped around a LTC transformer to implement a TACC based CNT.

(a)

(b)ntrollable Network Transformer

Synthesis of output voltage

voltage and power levels. For instance, for a four levelTAC

The CNT equ valent circuit for use in meshednetwork cbeha

Figure 9: (a) Realization of Cousing a 3 level ac converter, (b)

The TACC based CNT allows scaling to high

C as shown using simulation in Figure 9(a) and 9(b),using readily available 1700 volt 1000 A IGBTs, it wouldbe possible to realize a CNT rated at 12 MVA at asecondary voltage of 13.8 kV, with a primary voltage atsay 69 kV or 115 kV, and a control range of +/-15% ofnominal voltage and +/-5o of phase angle control. Thisdemonstrates the ability of the CNT to provide value inan interconnected meshed network, and to do so atrealistic power and voltage levels.

IV. SYSTEM IMPLEMENTATION AND IMPACT

Line 1 Line 2

9.2kV8.4kV

VV

7.6k6.8k

i aan be simplified as shown in Figure 10. CNT

vior can be analyzed in a two dimensional d-q plane.The dc component of the duty cycle D in Equation 3controls the ‘d’ component Vd, while the even harmoniccomponent of D controls the quadrature or ‘q’ componentVq. The CNT is also lossless and power is conserved atthe fundamental frequency (dynamic exchange of energyoccurs between the third harmonic and the quadraturecomponent). This leads to an equivalent circuit model forthe CNT that can allow investigation of more complexsystems with multiple CNTs.

qdo jVVV (5)

N)-(1KN)(1KN) 00(1-VV m(6)

d

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)2N)-(1(K-)

2N)(1(KVV 22mq

(7)

do

d IVV

Figure 10: CNT Equivalent Circuit

Control of a meshed system is a non-trivial problem due to interactions between the various interconnectedbranches. Knowledge of the state of the system and offline power flow simulations (optimal power flow)provides one approach to determine the set points for thecontrollable assets, such as CNTs, in the system.However, dynamic response to load, source or systemchanges is very poor, as it is based on latencies due tooff-line computation, as well as delays in the control andcommunication network.

An alternate approach would be to control the CNT using locally measured parameters. This is also an extremely difficult problem, as it is difficult to ensure thatmultiple controllers will not interact with each other. The ability to control massively distributed assets autonomously, based on locally measured information is possible and has been shown in [2]. A similar controlstrategy would be desirable for the controllable networktransformer, where local parameters such as node voltageand line current can be used to determine the set point foroperation of the transformer in order to have a systemlevel impact.

The levels of control required in any power networkcan be broken up into tiers, Tier 1 being control of thebus voltage magnitudes, say between 0.95 p.u to 1.05 p.u.Tier 2 would be control of the power flow in the networkto ensure maximum utilization of the transfer capacity while satisfying constraints enforced by the thermal limitsof the lines.

To study the dynamics of the transformer, a 4 busmeshed system has been used as shown in Figure 11a.The network has 2 generator buses and 2 load buses, and has anominal voltage of 79 kV. For the test system, thethermal limits of lines 2 and 5 limit the maximum powerthat can be transferred through the network. The control of the transformer is implemented assuming knowledge

of only local parameters, i.e. node voltage and branchcurrent.

(a)

(b)

Figure 11: Schematic of 4-bus system (b) Variation of busvoltages with (V2c, V3c) and without (V2, V3) CNT’s

The approach adopted in controlling thetransformers in a network follows the hierarchicalrequirements for any power system network. The first tier of control addresses bus voltage regulation for varyingloading conditions on the network. The second tier ofcontrol targets control of the line currents in the network, keeping the bus voltages within acceptable limits. Figure11b illustrates the drop in the bus voltages as the load onthe system is varied, with and without the inclusion of theCNT (with ± 10% control) at the 2 load buses, Bus 2 andBus 3.

The control of power flow in the network usinglocal parameters has been achieved by using the ability of the transformer to generate a voltage of arbitrary phaseangle. The voltage that is generated to control the powerflow is synthesized so that it is in quadrature with the current in the line. This then emulates an injection of inductance or capacitance in the line. The controlled line currents (Line 2 and Line 5) are shown in Figures 12a and12b, where the current in line 2 is controlled to 750 A,and line 5 700 A while regulating the bus voltagesbetween 0.98 p.u and 1.02 p.u. Table 1 details the linecurrents with and without control.

TABLE 1: LINE CURRENTS (L1 = 100 MW L2 = 80 MW)

Line LineCurrent (A)

ThermalLimit

RegulatedCurrent

1 498 750 5342 853 750 7503 95 750 994 260 700 3465 650 700 700

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(a) (b)Figure 12: Line Current (rms): (a) Line 2, (b) Line 5

(a) (b)Figure 13: Bus Voltage Magnitudes (rms): (a) Bus 2, (b) Bus 3

V. CONCLUSIONS

This paper has presented the concept of aControllable Network Transformer (CNT) that utilizes an existing tapped transformer and augments it with a fractionally rated direct ac converter. The existing “dumb” asset now has dynamic control capability. The controllable network transformer can providesimultaneous control of voltage and phase angle with noadditional energy storage elements, using the principle of dual virtual quadrature sources. This control capability is demonstrated experimentally. The two degrees offreedom can allow simultaneous control of node voltageand branch currents, although the control problem is seento be very complex.

The CNT design can be extended to realisticpower levels through the use of Thin AC Converters. Forinstance, a four level direct ac converter design is seen to be suitable for 13.8 kV systems using commerciallyavailable IGBTs. Preliminary simulation results illustrate the performance of the network transformer in a stand-alonemode and in a meshed network. The ability to control the transformer in a network has been shown using a 4-busmeshed system, where each transformer has beencontrolled autonomously using only local parameters.Preliminary experimental results are shown that validate the ability of the ac chopper to provide amplitude and phase angle control capability.

ACKNOWLEDGMENT

The Intelligent Power Infrastructure Consortium (IPIC) at Georgia Tech is acknowledged for financial support.

REFERENCES

[1] D. Divan, H. Johal, “ A Smarter Grid for Improving SystemReliability and Asset Utilization”, IEEE Power Electronics andMotion Control Conference 2006, Vol. 1, pp. 1-7..

[2] D. Divan, H. Johal, “ Distributed FACTS – A New Concept forRealizing Grid Power Flow Control”, IEEE Power Electronics Specialists Conference (PESC) 2005, pp. 8-14.

[3] D. Divan, H. Johal, “Current Limiting Conductors: A DistributedApproach for Increasing T&D System Capacity and EnhancingReliability”, IEEE PES Transmission and Distribution Conferenceand Exposition 2005/2006, pp. 1127-1133

[4] A. Edris, A. S. Mehraban, M. Rahman, L. Gyugyi, S. Arabi, T.Reitman, “ Controlling the flow of real and reactive power”, IEEEComputer Applications in Power, Vol. 11, No. 1, Jan 1998, pp.20-25.

[5] L. Gyugyi., C. D. Schauder, K. K. Sen, “Static synchronous seriescompensator: a solid-state approach to the series compensation of transmission lines”, IEEE Transactions on Power Delivery, Vol.12, No. 1, Jan 1997, pp. 406-417.

[6] Noroozian. M, Pertersson. A. N, Thorvaldson. B, Nilsson. B. A,Taylor. C. W, “Benefits of SVC and STATCOM for electric utilityapplication”, IEEE PES Transmission and DistributionConference and Exposition 2003, Vol. 3, pp. 1192-1199.

[7] Jih-Sheng Lai, A. Maitra, A. Mansoor, F. Goodman, “Multilevelintelligent universal transformer for medium voltage applications”,IEEE Industry Applications Conference 2005, Vol. 2, pp. 1893-1899.

[8] E. C. Aeloiza, P. N. Enjeti, L.A. Moran, O. C. Montero-Hernandez, Sangsun Kim, “Analysis and Design of ElectronicTransformers for Electric Power Distribution System”, IEEETrans on Power Electronics , Vol. 14, No. 6, November 1999, pp.1133-1141.

[9] Kalyan . K. Sen, Mey Ling Sen, “Introducing the Family of “Sen” Transformers: A Set of Power Flow Controlling Transformers”,IEEE Trans on Power Delivery, Vol. 18, No. 1, January 2003, pp.149-157.

[10] Deepak. Divan, Jyoti Sastry, “Voltage Synthesis Using DualVirtual Quadrature Sources- A New Concept in AC PowerConversion”, IEEE Power Electronics Specialists Conference(PESC), June 2007. pp. 2678-2684

[11] Deepak Divan, Jyoti Sastry, Anish Prasai, Harjeet Johal, “ ThinAC Converters – A New Approach to Making Grid Assets Smartand Controllable”, PESC 08.