Influence of TCSC control systems on
oscillations damping
Cite as: AIP Conference Proceedings 2552, 040009 (2023); https://doi.org/10.1063/5.0112238
Published Online: 05 January 2023
Tokhir Makhmudov
AIP Conference Proceedings 2552, 040009 (2023); https://doi.org/10.1063/5.0112238
© 2023 Author(s).
2552, 040009
Influence of TCSC Control Systems on Oscillations
Damping
Tokhir Makhmudov1, a)
1
Tashkent State Technical University, Power Plants, Networks and Systems Department,
100095 Tashkent, Uzbekistan
a)
Corresponding author: tox-05@yandex.com
Abstract. In the world, special attention is paid to determining the stability margins of electrical systems in connection
with the high growth rates of demand for electricity. In this regard, special attention is paid to research aimed at
improving the stability and dynamic properties of electric power systems, including through the use of modern control
methods based on intelligent systems. The application of HVDC and FACTS transmission technologies have added new
control methods to the power system, increased the throughput of electrical networks, and improved monitoring
capabilities. Power system stabilizers (PSS) are traditionally used to damp electromechanical low-frequency oscillations
in the frequency range 0.2–1.5 Hz. This paper proposes the use of thyristor-controlled series capacitors (TCSC) control
systems as a oscillations damper. For the compiled model of the POD-controller, a procedure was performed to optimize
its parameters in order to minimize the deviation of the mutual load angle of the two power systems, and an algorithm for
parametric optimization was presented. The simulation results on a digital model confirm the possibility of using TCSC
regulators to suppress low-frequency oscillations in power systems.
INTRODUCTION
Serial compensation has been used successfully for many years in electrical networks. With the use of sequential
compensation, it is possible to increase the transmission capacity of the existing transmission line with lower
investment costs and in a shorter period of time compared to the construction of a new line.
TCSC series compensation control circuits include controlled reactors connected in parallel with the capacitor
bank sections. This combination allows for smooth regulation of capacitance in a fairly wide range [1].
TCSC controllable longitudinal compensation devices have some major advantages over their shunt
counterparts. With series capacitors, the reactive power increases as the square of the line current, while with shunt
capacitors, the reactive power is generated in proportion to the square of the bus voltage. To achieve the same
systemic advantages as in series capacitors, it is necessary to use shunt capacitors whose rated reactive power is
three to six times that of series capacitors [2]. In addition, bypass capacitors usually need to be connected at the
midpoint of the line, while series capacitors do not.
The use of thyristor control in series compensation devices potentially provides the following advantages [3-6]:
1. Continuous control of the level of serial compensation of the transmission line.
2. Dynamic power flow control in selected transmission lines in the network to ensure optimal conditions for
power flow.
3. Damping of power swings from local and intersystem oscillations.
4. Increased level of protection of series capacitors. Fast shunting of series capacitors can be achieved by
thyristor control when large overvoltages occur on the capacitors after emergency situations. Likewise, capacitors
can be quickly re-enabled with thyristors after the fault has been rectified.
5. Maintaining the voltage level. TCSC devices in combination with series capacitors can generate reactive
power, thereby helping to regulate the voltage level in the local network.
Rudenko International Conference “Methodological Problems in Reliability Study of Large Energy Systems” (RSES 2021)
AIP Conf. Proc. 2552, 040009-1–040009-6; https://doi.org/10.1063/5.0112238
Published by AIP Publishing. 978-0-7354-4307-5/$30.00
040009-1
6. Reduction of short-circuit current. During the passage of short-circuit currents, TCSCs can switch from
variable capacitance mode to variable inductance mode, thereby limiting short-circuit currents.
THE PRINCIPLE OF OPERATION OF TCSC
Consider the operation of the TCSC by analyzing a circuit with variable inductance L, controlled by thyristors T1
and T2, connected in parallel with a fixed capacitor C, as shown in Fig. 1. Traditionally, thyristor converters are
described using the firing/actuation anJOHĮDVWKHFRQWUROYDULDEOH>-9].
FIGURE 1. Functional diagram of the TCSC device.
The equivalent impedance Zeq of this LC circuit is expressed as
X C ˜ X L (D )
Z eq (D )
j
X L (D ) X C
1
.
(1)
1
ZL
Here, WKHUHVLVWDQFHRIWKHIL[HGFDSDFLWRULVGHILQHGDVíM Ȧ& ,IȦ&– Ȧ/ !WKHQWKHFLUFXLWZLOOKDYHD
capacitive character.
,IȦ&– Ȧ/ WKHSKHQRPHQRQRIYROWDJHUHVRQDQFHRFFXUVZKLFKOHDGVWRDQLQILQLWHHTXLYDOHQWFDSDFLWLYH
reactance, and therefore to significant overvoltages.
,I Ȧ&– Ȧ/ WKHQ WKH UHDFWDQFH RI YDULDEOH LQGXFWDQFH EHJLQV WR SUHYDLO LQ WKH FLUFXLW 7KLV VLWXDWLRQ
corresponds to the inductive operating mode of the TCSC.
In Fig. 2 shows the waveforms associated with TCSC during steady state operation [10].
ZC FIGURE 2. Operation of TCSC in steady state.
In this case, the thyristor switch-on intervals are significantly shorter than the half-period of the mains frequency,
and they arise near the amplitude values of the power line current curve.
In TCSC, all or part of the capacitor bank is equipped with parallel thyristors that transmit current pulses, then
they are summed in phase with the line current to increase the voltage. Each thyristor is triggered once per cycle and
has a conductance interval that is shorter than half the cycle of the rated mains frequency [11].
DAMPING OF POWER OSCILLATIONS
The transmission of power P along the line with serial compensation is determined by the expression [12;13]:
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U1 ˜ U 2 sin G
(2)
X L X C (t )
Active power oscillations in transmission systems can occur in communication lines between generating
facilities as a result of poor interconnection damping, especially during high power transmission. Such oscillations
can be caused by a number of reasons, for example, line disconnection, line switching or sudden change in generator
output power [14].
When the TCSC is properly controlled, the total transfer reactance changes over time so that the power
oscillations are attenuated.
In the POD (Power-Oscillation Damping) regulator, the input signals are local signals in the form of active
power and voltage on the line at the connection point, as shown in Fig. 3. The output signal of the POD-regulator is
the signal of the degree of compensation [15-17].
P(t )
FIGURE 3. Block diagram of the POD controller.
The POD controller consists of an amplifier Kp, a transfer function with a time constant T1, designed to extract
the variable component from the active power signal, which determines local oscillations, and filters with time
constants T2-T4. Thus, the POD controller generates a signal proportional to the oscillatory component of the power
flow, which is appropriately out of phase [18].
The filters are equipped with limiters for the Cmin and Cmax compensation signals to ensure that the POD dynamic
control does not exceed the adjustable TCSC range. The Cmax limit corresponds to the maximum allowable voltage
across the TCSC, and Cmin to the maximum power line impedance (inductive mode) [19].
RESULTS
Let us consider the effect of a thyristor-controlled TCSC device, which acts as a damper for power oscillations
using the example of an electric power system (EPS) shown in Fig. 4.
FIGURE 4. EPS circuit with a sequential compensation device.
Here, the first power system (EPS1) is adopted as the transmission system, and the second power system (EPS2)
is received. A TCSC device is connected in series to the power line.
In order to obtain transient characteristics, we will simulate an external disturbance, expressed in an increase in
power flow towards the second power system, and then determine the set of coefficients of the TCSC controller that
PLQLPL]HVWKHGHYLDWLRQRIWKHSKDVHDQJOHį12 į1–į2 between two EPSs.
The initial and optimized values of the parameters of the POD controller are given in Table 1.
Table 1. POD-controller parameter values.
Parameter
Value
KP T1=T3 T2=T4 Cmax Cmin
Initial
0,7
0.05
0.06
0.1
-0.1
Optimized
2
0.015 0.038
0.1
-0.1
The parameters of the POD controller were determined using parametric optimization, the block diagram of
which is shown in Fig. 5.
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FIGURE 5. Block diagram of parametric optimization.
The optimization procedure starts at preselected initial values of the POD controller parameters x0 = x0. Then the
coefficients are adjusted using a nonlinear programming algorithm until the objective function f(x) reaches its global
minimum. The parameters corresponding to this global minimum are optimal for the POD controller in the sense of
the minimum objective function
(3)
min ^ f ( x : h( x) 0, g ( x) t 0` ,
where f(x) is the objective function; x is a vector containing the parameters of the POD controller; h(x) - constraints
imposed by equalities; g(x) are inequality constraints.
In this case, the objective function was set in the form:
t1
f ( x)
¦ ³ G12 (t , x) G12 (0, x) dt
0
t1
³ 'G (t , x)dt ,
(4)
0
ZKHUHį12 W[ LVWKHGLIIHUHQFHLQSKDVHDQJOHVDWWLPHWRIWZR(36VZLWKLQWHUV\VWHPFRPPXQLFDWLRQį12(0, x) is
WKHLQLWLDOSKDVHDQJOHGLIIHUHQFHǻį W[ - change in the phase angle difference over time t; t1 is the observation
time.
The process of optimizing the parameters of the POD controller was carried out in the Response Optimization
package of the MATLAB environment.
In Fig. 6. shows the results of nonlinear mathematical modeling, showing a significant improvement in the
damping of oscillations in the EPS.
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FIGURE 6. Results of nonlinear modelling:
1 - POD-controller without optimization; 2 - optimized POD-controller.
Based on the obtained characteristics of transient processes 1 and 2, will evaluate the efficiency of the PODcontroller according to the following indicators [12, 15]:
- Delay Time (td): The time required to reach the half of the final value. Note that delay time is the time till first
reach is observed.
- Peak Time (tp): The time required for the response to reach the first peak of the overshoot.
- Settling Time (ts): The time required for the response to remain within a desired percentage (5%) of the final
value.
- Maximum (percent) Overshoot (Mp): The maximum peak value measured from the steady-state value.
Table 2. Transient Response Specifications.
Indicators of the quality of
the transients
Delay Time (td), sec.
1st characteristic
2nd characteristic
2,3
2,55
Peak Time (tp), sec.
3,2
3,8
Settling Time (ts), sec.
Overshoot (Mp), %
10,2
50
9,2
61,5
From the above results, it is obvious that despite the increase in overshoot when using a POD-controller with
optimized parameters, the damping of oscillations is improved, as evidenced by a decrease in the oscillation of the
transient characteristic 2 and a decrease in its decay time.
CONCLUSION
Active power oscillations limit the capacity of intersystem connections between parts of power systems. In some
cases, it is possible to install PSS system stabilizers on generators, especially with intersystem power oscillations
that tend to be low frequency (typically 0.2 Hz to 0.7 Hz). In this case, series capacitors with thyristor control TCSC
can be used to improve the characteristics of the power system, namely, to increase the stability margin, damping
power oscillations, and reduce subsynchronous resonance.
040009-5
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