Power Flow Control Solutions in the Romanian Power System Under High Wind Generation Conditions

Costel CONSTANTIN, Mircea EREMIA, Lucian TOMA Department of Electrical Power Systems

University Politehnica of Bucharest Bucharest, Romania

constantin_costel_ro@yahoo.com

AbstractThis paper presents three options to control the power flows in a transmission network by using three FACTS devices, that is the Thyristor Controlled Series Capacitor (TCSC), Static Synchronous Series Compensator (SSSC) and Phase Shifting Transformer (PST). Simulations are performed in the Eurostag software on a database of the Romanian transmission system. The control is focused on the south-east area of the country, where a large amount of power was installed in wind power plants.

Index Terms-- Power systems, Power flow control, Thyristor Controlled Series Capacitor, Static Synchronous Series Compensator, Phase Shifting Transformer.

I. INTRODUCTION In the ENTSO-E system the power exchanges between

countries are increasingly larger, i.e., in 2011, the biggest net exporting countries were France (57.1 TWh), the Czech Republic (17 TWh) and Bulgaria (10.5 TWh). The main net importers were Italy (45.8TWh) and Finland (13.9 TWh). Also in the ENTSO-E system, both exports and imports were higher in 2011 than in 2010 [1]. This phenomenon is stressing for the ENTSO-E system where the increase of cross border exchanges between national systems, which result from the uneven allocation of the generation, is suturing several interconnections [2]. Also, the integration of the renewable energy sources leaded to the power transfers grows and the power system can become increasingly more difficult to operate, and the system becomes more insecure, with unscheduled power flows and higher losses.

In Romania the most of the wind power plants are concentrated in the South-East area (Dobrogea area), which is characterized by a power surplus. The closest large important consumer is the Bucharest city, so that diverting the energy directly to it is of great importance. High risks are experienced when, during certain conditions, active power flows are naturally diverted to flow on the tie lines between Dobrogea and Bulgaria, which finally may trigger disconnection of some lines by specific automata. In order to avoid these situations, three solutions for power flow control

are analyzed, i.e. installing a Thyristor Controlled Series Capacitor, a Static Synchronous Series Compensator or a Phase Shifting Transformer.

II. THYRISTOR CONTROLLED SERIES CAPACITOR The Thyristor Controlled Series Capacitor is a FACTS

device which consists of a series capacitor bank connected in parallel with a thyristor-controlled reactor in order to provide a variable series capacitive reactance and thus continuous control of the power flow on the transmission line [3].

A. Steady-state model The TCSC power flow model presented in this section is

based on the simple concept of a series reactance, that can be automatically varied so that to allow more power or less power to flow across the line, as required by the operating conditions. The value of this variable reactance is efficiently determined using Newton-Raphson method. The reactance XTCSC, shown in Figure 1, models all elements that are part of the TCSC, when operating in either the inductive or the capacitive regions [4].

Vk Vm

Ik Im

XTCSC

k m

Figure 1. Thyristor controlled series compensator equivalent circuit.

Appling the Kirchhoff theorems, for the above circuit, the nodal equations result:

1( )

1( )

k k mTCSC

m m kTCSC

I V VjX

I V VjX

Equations (1) can be written in matrix form:

The work has been funded by the Sectoral Operational ProgrammeHuman Resources Development 2007-2013 of the Romanian Ministry ofLabour, Family and Social Protection through the Financial AgreementsPOSDRU/107/1.5/S/76903 and POSDRU/89/1.5/S/62557.

1 1

1 1TCSC TCSCk k

m m

TCSC TCSC

j jX X VI

VI j jX jX

From the matrix equation (2), the nodal admittance matrix is obtained:

TCSC TCSCkk km

nn TCSC TCSCmk mm

jB jBY

jB jB

For inductive operation we have:

1

1

TCSC TCSCkk mm

TCSC

TCSC TCSCkm mk

TCSC

B BX

B BX

whereas for capacitive operation the signs are reversed.

The active and reactive power equations at bus k are:

2sin( )

cos( )k k m km k m

k k kk k m km k m

P V V BQ V B V V B

B. Dynamic model The control of the value of the reactance XTCSC is

performed according to the diagram from Figure 2 [5].

Figure 2. Modeling of the XTCSC for dynamic simulations.

The PSS loop based on the signal for the amount of active power to be transferred by TCSC is represented in Figure 3.

Figure 3. Modeling of the POD Power Oscillation Damper signal.

The control loop of active power flow on the line is performed as shown in the diagram from Figure 4.

Figure 4. Modeling of the PLINE signal.

The TCSC parameters are presented in Table I.

TABLE I. PARAMETERS OF THE TCSC MODEL

Parameters Value Ubase base voltage of the connection node [kV] 400 T2 time constant [s] 0.02 XMIN [p.u. (100 MVA, Ubase)] -0.0145 XMAX [p.u. (100 MVA, Ubase)] 0.0145 XSC [p.u. (100 MVA, Ubase)] -0.0045 TPSS 1.0 T1PSS 1.0 T2PSS

time constants characterizing the PSS loop [s]

1.0 KPSS POD signal gain 10 KPLINE 0.01 KILINE

gains characterizing the control loop of active power transit on the line 0

III. STATIC SYNCHRONOUS SERIES COMPENSATOR The Static Synchronous Series Compensator uses a shunt

capacitor which is connected to the transmission line via a voltage-source converter and a series transformer in order to produce a controllable voltage, in quadrature with the line current.

A. Steady-state model The steady-state model of the SSSC device in series with

a transmission line i-j, can be represented by a voltage source which has the voltage magnitude VSSSC and the voltage phase (Fig. 5) [6], [7].

Vi Vj

VSSSCZSSSCP +jQij ij P +jQji ji

Figure 5. Representation of the SSSC device in series with a transmission

line.

The active and reactive power equations which flows between the nodes i and j are:

2 cos sin

cos sin

SSSCij i SSSC i j SSSC i j SSSC i j

i SSSC SSSC i SSSC i

P V G VV G B

VV G B

2 sin cos

sin cos

SSSCij i SSSC i j SSSC i j SSSC i j

i SSSC SSSC i SSSC i

Q V B VV G B

VV G B

where: 1/SSSC SSSC SSSCG jB Z

Because the SSSC device can modify the power flow on the transmission line which connect the i node with j node, in the Newton-Raphson method the power values forced by the device will be added to the nodal powers of this two nodes:

new SSSCi i ijnew SSSCi j ij

P P PQ Q P

B. Dynamic model The basic control strategy of the SSSC device is adopted to

regulate the active power flow. Figure 6 presents the simplified dynamic model of the SSSC regulator for controlling the voltage angle of the voltage source converter [8]. A POD input signal is used for control the damping of the power oscillations. The maximum phase shift angle between the two ends of the SSSC is 9.2 degrees.

KPLINE

s

Activepowertransit

Setpoint valuefor active power

transit

++

+

++ 11+sT2

min

max

KPLINE

Figure 6. Modeling of the SSSC phase shift.

IV. STEADY STATE MODEL OF THE PHASE SHIFTING TRANSFORMER

A phase shifting transformer creates a phase shift between the primary and the secondary side. Except for very specific applications, the purpose of this phase shift is usually the control of power flow in a complex network.

The natural current distribution is dependent on the impedance of the lines. This natural distribution may be rather inefficient, if X1 and X2 are extremely different.

X1

X2

i1

i2itotal

a)

X1

X2

i i1+itotal

VPST

i i2+

b)

Figure 7. Current distribution over parallel lines without and with a PSTs boost voltage: (a) without PST. (b) with PST.

With the introduction of an additional voltage source, a circulating current can be generated, which equalizes the currents. Figure 7 gives a simple example for a current distribution over two lines with and without such an

additional boost voltage. To increase the power flow, the boost voltage must create an advance phase shift of the driving line voltage [9].

In steady state a PST can be modeled similar to a transformer but with complex voltage ratio in series with a transmission line (Fig. 8). It is assumed that the reactance of the transformer is included in the transmission line reactance [10].

yik

yik0

IiIi Ik

Vi Vk

Nik

yki0

Ik

Vk

Figure 8. Equivalent circuit of a PST.

For the scheme represented in Figure 8 we can write the following matrix equation:

0* 20ikik ik iki i

k kik ikik ik ki

Ynn

y y y N VIVI y N y y N

hence a complex voltage ratio ikN is used for the phase shifter in the nodal admittance matrix nnY .

Considering that the role of the PST is to modify the voltage angle in order to control the power flow through the line on which it is installed, then we can write:

1 jikN e and thus:

jii iknn jki kk

Y Y eY

Y e Y

The expression of apparent power for i bus in the presence of the phase shifting transformer is given by the next equation:

*jii iki i i kS V Y V Y e V

resulting the active and reactive power equations:

2

2

cos sin

cos sini i ii i k ik i k ik i k

i i ii i k ik i k ik i k

P V G V V G B

Q V B V V B G

V. CASE STUDIES Currently, the total capacity in the Romanian Power

System installed in wind power plants is 1940 MW, of which 1870 MW are located in the Dobrogea region, in the south-

east part of Romania (Fig. 9). This region encounters today a power surplus that is, sometimes, very difficult to control by the system operator. As more power will be installed in this region in the next two year, an efficient solution should be identified in order to manage the power transfer. One solution for this problem is to build new electric transmission lines, but due to the complex legislation, he high costs of the land, and the duration of construction, new solutions should be quickly identified.

Figure 9. South-east area of Romanian transmission system.

During winter, the maximum load in the Romanian power system is about 9500 MW, and the share of wind generation may reach up to 20% of the total generation. The transfer of large amount of energy from the Dobrogea region, where an 1400 MW nuclear power plant is in operation, become a challenging activity for the power system operator.

The active power produced by the wind power plants shows significant variations during short time intervals, sometimes from zero to the maximum capacity. During summer, after dry period, the hydro power plant may not be appropriately available to balance the additional variation created by the wind farms.

Furthermore, large variations of the power flows are in the transmission system may be observed during short time intervals.

Figure 10 illustrates a SCADA record showing the variation in the power generated by the wind power plants located in the analyzed area, during one week [11].

Figure 10. Active power generated in wind power plants

These large variations from one hour to another of the power generated by the wind power plants, lead to large changes of the power flows on the nearby transformers and the interconnections lines of the Dobrogea region with the rest of the Romanian power system and with the Bulgarian power systems (Table II). Also, the actions taken by the system operator in case of disturbances in the transmission network are strongly influenced by power generated in the wind power plants (WPP).

TABLE II. ACTIVE AND REACTIVE POWER FLOWS

S [MVA] Network element 30 % WPP 70 % WPP 100 % WPP 400/110 kV T1 Medgidia 68.3 + j23.8 -36.2 + 31.2 -115.1 - j37.2 400/110 kV T1, T2 Tulcea -2.2 + j13.0 -69.4 + j26.4 -119.2 + j35.5 400/110 kV T1, T2 Constana 55.5 + j32.5 5.5 + j36.3 -32.9 + j37.2 400 kV OHL Smrdan-Gutina 392.8 - j5.9 523.2 - j48.3 615.1 - j63.1 400 kV OHL G. Ial.- Buc. S. 101.8 - j43.5 282.3 - j63.1 416.1 - j94.8 400 kV OHL Pelicanu-Buc. S. 125.3 - j42.6 277.9 - j63.4 390.6 - j100.8 400 kV OHL Rahman-Dobrudja 83.9 - j28.9 189.6 - j44.4 266.8 - j64.2 400 kV OHL Stupina-Varna 88.3 -j 24.4 207.7 - j41.9 295.2 - j65.5 220 kV OHL Barboi-Focani 134.3 - j6.6 161.1 - j9.1 180.4 - j9.3 110 kV OHL Slob. S.-Drag. V. 9.5 + j0.5 17.5 - j2.2 22.9 - j5.9

Table II reveals that for large values of the generated

power by the WPPs the direction of the active power flowing through the power transformers is from the distribution network to the transmission network. This occurs because a large number of WPPs are connected in the distribution network.

The impact of the TCSC, SSSC and PST devices on the power flows control have been analyzed in this paper. The simulations have been performed on Dobrogea region of the Romanian Power System, which has a generated power of 2970 MW, of which 1310 MW are produced in wind power plants and 1650 MW are exported to other regions.

A. TCSC device in series with the 400 kV OHL Bucureti S.-Gura Ialomiei The purpose is to increase the power flowing on the 400

kV OHL Bucureti S. Gura Ialomiei, and the TCSC device is set to operate in the capacitive domain. For a capacitive compensation of 15% of the inductive reactance of the line the active power will increase by 25 MW, whereas for a compensation of 30 % the power will increase by 50 MW.

In the case of a disturbance that leads to a change in the power flow on the studied line, the TCSC device will modify the compensation reactance in order to maintain the power at the desired value. Figure 11 shows that for a decrease in the power flow of 15%, TCSC will command the modification of the compensation reactance from -0,0045 p.u. to -0,011 p.u. so that to force the power flow to reach the desired value.

0 2 4 6 8 10 12 14 16 18 20

285

290

295

300

305

s

MW

[var_-15%] ACTIVE POWER : LINE GIAL41-R-BUCU4 -1 Unit : MW

(a)

0 2 4 6 8 10 12 14 16 18 20

-0.014

-0.012

-0.010

-0.008

-0.006

s

[var_-15%] MACHINE : TCSC-IN VARIABLE : XC Unit : p.u.

(b)

Figure 11. Active power flow on the OHL (a) and TCSC reactance (b).

B. SSSC device in series with the 400 kV OHL Gutina-Smrdan

Assumes that the reference value of the active power flow on the OHL Gutina - Smrdan is 620 MW, with 100 MW bigger that in the case without the SSSC device. In order to maintain the power at the reference value, the phase shift between the voltages of the two sides of the SSSC is 4,48 degree.

In the case of a disturbance which leads to a decrease in the power flow on the analyzed line by 30 MW, SSSC will command the modification of the voltage angle at 5.68 degrees (Fig. 12).

0 5 10 15 20 25 30 35 40590

595

600

605

610

615

620

625

s

MW

[var_-15%] ACTIVE POWER : LINE SMIR4S -GUTI4 -1 Unit : MW

(a)

0 5 10 15 20 25 30 35 40

-5

-4

-3

-2

-1

-0

1

s

deg

[var_-15%] VOLTAGE ANGLE AT NODE : SMIR41 Unit : deg[var_-15%] VOLTAGE ANGLE AT NODE : SMIR4S Unit : deg

(b)

Figure 12. Active power flow on the OHL (a) and SSSC phase shift (b).

C. PST in series with the 400 kV OHL Rahman-Dobrudja The solution for which Transelectrica has shown interest

in solving the problem of parasite power flows through Bulgaria is the phase shift transformer (PST). Assumes that a PST is installed in series with the 400 kV OHL Rahman Dobrudja. The characteristics of the considered PST are the same as the ones of a PST implemented in 2007 in Germany: 1200MVA/400 kV/24/32 taps.

If a three phase short circuit will occur on the 400 kV OHL Pelicanu Bucureti S., the protection system will trigger the disconnection of the affected line. After the fault clearance, the OLTC will command a tap change by the PST, in order to restore the value of the power exported to Bulgaria to the reference value (Fig. 13). One can see that in this scenario, the PST successfully fulfills its objective, so that the active power is forced to flow on the 400 kV OHL Rahman - Dobrudja, with a mismatch of (2-3 MW). Obtaining a value as close as possible to the desired value depends on the number of taps of the PST and on the phase shift introduced by a single tap.

0 2 4 6 8 10 12 14 16 18 20

150

200

250

300

s

MW

[3f_fault] ACTIVE POWER : LINE RAHM41 DOBR-1 Unit : MW

Figure 13. Active power flow on the OHL in presence of the PST.

VI. CONCLUSIONS The simulations have shown that all the TCSC, SSSC and

PST devices represents effective solutions for active power flow control and may cope the problems about the power transfer from the Dobrogea region under the increasing installed power in wind power plants. However, a cost analysis should be performed in order to determine the most effective solution from economical point of view.

REFERENCES [1] European Transmission System Operators for Electricity (ENTSO-E),

System adequacy retrospect 2011, Technical Report, 2012. [2] P. Bresesti, M. Sforna, V. Allegranza, D. Canaver, R. Vailati,

Application of Phase shifting Transformers for a secure and efficient operation of the interconnection corridors, IEEE Power Engineering Society General Meeting, Denver, 2004.

[3] N. G. Hingorani, L. Gyugyi, Understanding FACTS: Concepts and technology of flexible AC transmission systems, IEEE Press, New York, 2000.

[4] E. Acha, H. Ambriz-Perez, C. Angles-Camacho, C.R. Fuerte-Esquivel, FACTS: Modelling and simulation in power networks, John Wiley & Sons, 2004.

[5] Tractebel Engineering - GDF SUEZ, Eurostag Users Manual/ Standard Models Library, 2010.

[6] L. Yao, P. Cartwright, L. Schmitt, X.P. Zhang, Congestion Management of Transmission Systems Using FACTS, IEEE/PES Transmission and Distribution Conference and Exhibition: Asia and Pacific, August 2005.

[7] S. Kamel, M. Abdel-Akher, M.K. El-Nemr, Implementation of SSSC model in the Newton-Raphson power flow formulation using current injections, 2010 Universities Power Engineering Conference, Cardiff, 2010.

[8] J. M. Gonzlez, C. A. Caizares, J. M. Ramrez, Stability Modeling and Comparative Study of Series Vectorial Compensators, IEEE Transactions on Power Delivery, Vol. 25, No. 2, pp. 1093-1103, April 2010

[9] W. Seitlinger,Phase Shifting Transformers Discussion of Specific Characteristics, CIGRE General Meeting, Paris, 1998.

[10] M. Eremia Study regarding the necessary tools for voltage regulation/ reactive power in the Romanian power system. The modeling of FACTS devices for applications at national level, University Politehnica of Bucharest, Technical Report for Transelectrica, 2010.

[11] www.transelectrica.ro

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