*
M. Noroozian A. Wilk-Wilczynski
ABB Power Systems
Reactive Power Compensation Division
S-721 64 Västerås, Sweden
C. Gama
Electronorte
SCN Q. 6 Conj. A Bl. B, Sala 1006
Brasilia, DF. 70718-900, Brazil
. Introduction
The North-South interconnection was planned to couple the two south-southeast and north-northeast systems in the beginning of 1999. The north-south interconnection, a single 500 kV compact transmission line with a length of 1020 km will connect Imperatriz substation (In the
North) to Serra da Mesa (in the south). This transmission line has a capacity of 1300 MW and will be reinforced within some years by a new line. Technical and economical feasibility studies related to the interconnection of these two systems have been performed since 1992.
Two transmission alternatives were considered and analyzed to establish the North-South Interconnection: a
DC bipole (

400 kV) and a single 500 kV AC compact transmission line (4x954 MCM bundle), 1,020 km long.
In both cases, this Interconnection should link the 500 kV substation of Inperatriz (North system) to the Serra da Mesa power plant (South system) and should be dimensioned to transmit up to 1,300 MW, with suitable
* Presented at the Visepope Conference in Recife, Brazil operation being required from no load up to maximum flow, in both directions. The generation installed capacity in South/Southeast and North/Northeast systems will be 48 GW and 14 GW, respectively, by the time of North/South Interconnection implementation.
Finally after extensive evaluation study the AC solution was selected due to a lower cost.
The Controlled Series Capacitors Banks (TCSCs) are being applied in the interconnection in order to eliminate the restriction related to the poorly damped low frequency oscillations [1][2][3]. The Fixed Series
Capacitors (FSC) for the North/South Interconnection aims at increasing the power transmission line capability. Fixed Series Capacitors (FSC) are also being applied in the existing Eletronorte 500 kV system in substations Marabá, Imperatriz and Presidente Dutra in order to increase the power transfer capacity of this transmission corridor, which is a need imposed by the implementation of the North/South interconnection.
The two systems will be connected via a 500 kV compact line with 54% fixed series compensation.

Furthermore two TCSCs banks are planned in order to damp the low frequency inter-area oscillations between the north and south systems. TCSC banks will be located near substations Imperatriz and Serra da Mesa.
These two TCSCs are planned to operate at -15.9 ohm
(6% of the line reactance) in the steady state and are continuously controllable from -13.3 ohm to -39.8 ohm
(corresponds to 5 to 15% of the line reactance). The possibility of bypassing the TCSCs, i.e. full conducting of thyristors are provided.
The main objective of installing the two TCSCs is damping of inter-area mode. This mode which is around
0.2 Hz, might be exited by small and large disturbances.
Each TCSC bank is proposed to operate in constant reactance control mode with an additional supplementary signal to provide damping for the electromechanical oscillations in the range of 0.1 to 0.3
Hz. The organization of this paper is as follows:

Section 2 examines the electromechanical oscillations behaviour of the inter-connected system.

Section 3 describes the procedure for POD control design. This covers a discussion about TCSC modelling for power swing studies, determination of the regulator structure, parameters and choice of input signals.

Sections 4 shows simulation results when using
TCSC for damping of electromechanical oscillations for a wide range of scenarios.
The content of this paper is based on the studies before the field tests.
2
Fig. 1: Brazilian North-South Interconnection
2. System Models
In this study two system models have been investigated:

A sixteen machine representation

A complete system representation
The sixteen-machine representation has been constructed based on the selection of some major machines in the north and south. The model is a very good representation of the total system. The relevant generators exciters and power system stabilizers have been modelled. The constructed 16-machine representation covers the original modes and has a very close response compared to the original system. The reasonable size of the 16-machine representation has made different types of study very effective . This has enabled the design of the regulator conveniently with some features like sensitivity of eigenvalues, etc.
. 1. Characteristics of the System Electromechanical
Oscillations
To examine the electromechanical stability two types of analysis are generally performed:

Small signal disturbance analysis

Large signal disturbance Analysis
Small signal disturbance analysis aims at study of the system response to small disturbances when a linearized system model can be used. In these cases, the eigenvalue analysis is a powerful tool, which indicates the modes of oscillations. Table 1 shows the result of an eigenvalue analysis for three different loading conditions. It is observed that in all loading conditions a mode with frequency around 0.2 Hz has a low damping.
Loading Condition Eigenvalues
Full load, flow from North to south -0.0230
 j 0.2091 Hz
Low load, flow from North to south
-0.0308
 j 0.2214 Hz
Full load, flow from South to North
-0.0160
 j 0.1918 Hz
Table 1: Eigenvalue analysis result (Only eigenavalues with real value larger than -0.2 are given )
Large signal disturbance analysis aims to study the performance of the system after the occurrence of a severe fault or a major outage. In this situation the linear analysis is just valid for a specific operating point and the full system response can be examined best through non-linear time domain simulations. Simulations confirm that the studied system is stable in terms of first swing stability but however, poorly damped oscillations follow and last for some time. This leads to this conclusion that a control strategy based on damping of small signal disturbances could be effective for damping of 0.2 Hz. oscillation.

2. 2. TCSC Model
Fig. 2.1 shows the main circuit of a TCSC in the steady state:
X
R
U
C i
R i
L
U
C i
R i
L

X
C
Fig. 2.1: TCSC circuit with voltage and currents waveforms
The capacitor voltage varies by controlling the current pulses through the thyristor branch. Steady state relation of the apparent reactance of TCSC can be deduced by determining the fundamental frequency component of
U
C and dividing by I . Fig. 2.2 shows the
L characteristic of the TCSC apparent reactance at fundamental frequency.
Inductive
T C S C
A p p ar en t re ac tan ce
X
X
C
C
-
X
X
L
L p
/ 2
X
C
Capacitive l =
X
C
X
L
2 p l p

Fig. 2.2: TCSC steady-state reactance characteristic
For power swing damping studies, a TCSC can be modelled as a variable reactance. Fig. 2.3
shows the general block diagram of the TCSC model used for power swing studies: n t g l C t l
T C
X t a y
X
C
1
X
C
I t r a
Fig. 2.3: TCSC model as damping controller
Based on a control strategy a signal X TCSC ref
is determined. This signal is passed through a delay block.
The time constant T approximates the delay due to the main circuit characteristics and control systems. The output of the model is restricted by two limits.
1.
Fixed limit: The maximum apparent reactance
2.
Variable limit: The voltage across the TCSC
. POD Controller Model
Fig. 3-1 shows the suggested structure for the POD regulator.
Input
Signal
Measuring and filters
K g
Phase
Provider
X max
X order
X min
X
0
Fig. 3-1: Structure of POD regulator
.. Determination of POD regulator phase
Suppose that the uncontrolled power system has a critical eigenvalue l =   j
 . The sensitivity of this eigenvalue with respect to the gain of the regulator shows how the eigenvalue will change after the introduction of the regulator [4]. Fig. 4-2 shows the displacement of the eigenvalue after the POD control action. j
 j m l j

Fig. 3-2: Representation of sensitivity of eigenvalue
The phase angle j m shows the compensation angle which is needed to drive the eigenvalue to the -180 line.
.. Sensitivity of Eigenvalues
A model of the system based on the full loading and power flow from North to South was used for the sensitivity analysis. Table 1 shows that the eigenvalue l =-0.0230
 j 0.2091 has the least damping. Using the method above, the following compensation angle is determined: j m
= -
89 .
12

With this compensation, the sensitivity of the eigenvalue will be:
D l
D
K
= -
2 .
6995
 j 0 .
0479
This shows that the POD with the calculated parameters can contribute to damping.
3.3.
Influence of loading condition
The described method was repeated with a different tieline loading. We selected a situation which 300 MW power flows through the interconnection. It was observed that the change in the compensation angle is small compared to full load condition.
3

3.4. Determination of gain
The eigenvalue analysis shows that a gain equal to 0.05 provides sufficient damping. On the other hand this value can be increased by two times without any risk for instability. Therefore this value provides good damping with sufficient stability margin.
3.5. Influence of the POD Regulator on other eigenvalues
Table 2 compares the eigenvalues with and without the
POD regulator. It is seen that the damping of the interarea mode has increased considerably while the other modes have been affected slightly. without TCSC POD with TCSC POD
-0.0230 1/s
 j 0.2091 Hz -0.3357 1/s
 j
0.2208 Hz
Table 2: Comparison of eigenvalues with and without
POD regulator
3.6. Bypass strategy for POD
For a situation where inductive modulation is required, it might be beneficial to bypass the capacitor for a certain period. A control strategy for bypassing was developed and tested.
3.7. Variable gain strategy
The damping effect of a controllable series capacitor increases when the power through the transmission line increases. For low powers it is beneficial to increase the gain. This provides a higher voltage across TCSC and thereafter a higher effectiveness for power swing damping. If KGF is the optimum gain for high power flow (INF) and KGL is the optimum gain for low level of power flow ( INL), then for any other power flow level the gain can be determined from a linear interpolation of the above data.
4. Simulation Results
The tables below show the simulated fault scenarios.
The scenarios cover some of the most crucial faults which might occur in the studied power system. In each case the response of the system to the fault without the
TCSC POD regulators and with the TCSC POD regulators are shown.
 Loading Scenarios
No.
3
4
1
2
Description
1000 MW flow from north to south through one line
300 MW flow from north to south through one line
1000 MW flow from south to north through one line
1000 MW flow from north to south through two lines

Fault Scenarios
2
3
No.
1
Description
1phase to ground at Tucuri with loss of one line between
Tucuri-Maraba
Loss of 700 MW generation at TUCURI (North West)
1-phase to ground fault at MIRACEMA (intertie), disconnection of fault after 100ms, opening of the line, closing the line after 600ms
Different fault scenarios have been investigated under different loading conditions in order to evaluate the effectiveness of the POD control. For each case the variation of active power and the apparent reactance of the TCSC for controlled (solid line) and uncontrolled mode (dashed line).
4.1. Scenario 1
This scenario is defined for loading scenario no. 1 and fault scenario no.1:
I(P)
1
1+sT av
2
IN
KGL
KG
KGF
KG
INL IN INF
Fig. 3.3: Varying gain strategy
3.8. Interconnection with two parallel line
The proposed control strategy is valid for the situation when two parallel tielines become under operation. In that case the input signal of the regulator will be the sum of power or current through the two lines.
Fig. 4.1: Variation of line power and TCSC apparent reactance for scenario 1
4

4.2. Scenario 2
This scenario is defined for loading scenario no. 2 and fault scenario no.1:
Fig. 4.2: Variation of line power and TCSC apparent
4.3. Scenario 3 reactance for scenario 2
This scenario is defined for loading scenario no. 1 and fault scenario no.2:
Fig. 4.3: Variation of line power and TCSC apparent reactance for scenario 3
4.4. Scenario 4
This scenario is defined for loading scenario no. 3 and fault scenario no.3:
Fig. 4.4: Variation of line power and TCSC apparent reactance for scenario 4
5
. Conclusions
This study has examined the low frequency electromechanical oscillations behaviour of the
Brazilian system. The impact of TCSC POD regulators for improving damping of the inter-area mode was examined. The assumption in this study was that the two
TCSCs on the intertie have the same POD regulator structure. Based on the analytical studies and results from time simulations, the following conclusions can be drawn:

Each TCSC increases the damping of the inter-area oscillation mode. The damping effect of the two
TCSCs is larger than the damping effect of one
TCSC. This means that the TCSCs do not detract the damping effect of each other.

The damping effect of TCSCs is effective both for small and large disturbances.

The TCSCs damping is effective under disturbances which result in the change of network topology.

The damping effect of the TCSCs is higher for the conditions when sufficient power is flowing through the intertie. In the cases where the power flow through the intertie is low, the damping effect of the
TCSCs is lower. The proposed varying gain strategy has improved this performance.

The natural damping of the network with the two lines in the interconnection will become higher. The
TCSCs damping contribution is similarly effective when the second line is under the operation.

For the case when the required reactance of TCSCs should be inductive for power swing damping, a bypass strategy could drive the TCSC to full inductive.
5. References
[1] C. Gama “Prospective Application of Controlled
Series Compensation to Improve Transmission System
Performance”, Cigre´ 35 session, Paris, 1994.
[2] M. Noroozian, P. Halvarssonp and H. Othman.
“Application of Controllable Series Capacitors for
Damping of Power Swing”. In Proceedings of V
Symposium of Specialists in Electric Operational and
Expansion Planning, vol. 1, pp. 221-225. May. 1996,
Recife, Brazil
[3] M. Noroozian and G. Andersson. “Damping of
Power System Oscillations by Controllable
Components”. IEEE Transactions on Power Delivery, vol. 9, No. 4, Oct. 1994, pp. 2046-2054.
[4] L. Rouco and F.L. Pagola, “An Eigenvalue sensitivity approach to Location and Controller Design of Controllable Series Capacitors for Damping Power system Oscillations”. IEEE Transactions on Power
Systems, Vol. 12, No. 4, Nov. 1997, pp. 1660-1666.