2011 8th International Multi-Conference on Systems, Signals & Devices

IMPROVEMENT OF FAULT CRITICAL TIME BY HVDC

TRANSMISSION

Raouia AOUINI1 Khadija BEN KILANI1 Bogdan MARINESCU2 Mohamed ELLEUCH1

1
University of Tunis El Manar, ENIT-L.S.E.-BP 37-1002 Tunis le Belvédère, Tunisia,
2
ENS-Cachan SATIE 61 Avenue du Président Wilson 94235 Cachan Cedex, France

Email: aaouinii @yahoo.com, khadija.kilani@ept.rnu.tn, bogdanmarinescu@hotmail.fr,

melleuch2005@yahoo.fr

ABSTRACT HVDC systems. In a purely AC system these control

devices consist of voltage controls and turbine governor
This paper investigates the impact of High Voltage Direct controls. Transmission in Direct Current (DC) offers
Current (HVDC) transmission on the transient stability of attractive features such as fast controllability of power
a two-machine power system, considering three through converter control, bulk power transmission, ability
transmission line configurations: parallel HVAC-HVAC, to enhance transient stability problems associated with
parallel HVDC-HVDC, and a hybrid HVAC-HVDC HVAC lines, asynchronous interconnections,
operation. The faults are balanced three-phase short- environmental and economical advantages [4].
circuits in AC lines, and single phase faults on DC lines, Controllability in a DC link is assured by
applied in the mid-point of the interconnection. For each hierarchically organized controls: the master control, the
configuration, transient stability of the AC systems is pole control and the converter control [1]. The pole control
assessed in terms of the fault critical clearing time (CCT), system is responsible for the firing of the thyristor. On the
and for different DC power levels. The results indicate the rectifier side, a proportional integrator (PI) regulator
contribution of HVDC transmission in increasing the maintains the DC current, whereas on the inverter side, a
critical clearing time; and therefore enhancing the systems (PI) regulator controls DC voltage so that minimum losses
stability margin and operational security. are archived.
Several studies have emphasized the performance
Index Terms— HVDC transmission, transient stability, improvements of power systems brought on by HVDC
fault critical clearing time. controllability functions [3]. However, in earlier HVDC
works, voltage and angle rotor stability have not
1. INTRODUCTION thoroughly been addressed. Only in recent years, have
papers [5 - 9] addressed the issue of voltage stability in
Fault critical clearing time (CCT) is commonly used as the AC/DC interconnection. It has been recognized that weak
conventional transient stability measure of power system AC/DC interconnection points can limit the power transfer
robustness to withstand a given disturbance [1]. This capability of DC links during dynamic operation on
parameter corresponds to the maximum time duration that account of the transient voltage stability phenomenon [8].
a disturbance may last without losing the system capacity Transient stability has been investigated for a parallel
to recover to a steady-state stable operation. The CCT AC and hybrid AC-DC configurations in [10]. Both
depends on both the initial operating state of the system, configurations were subjected to a fault near inverter bus.
the location ant the severity of the disturbance. It has been shown that the presence of a DC link suitably
Transient instability in a power system is exhibited in controlled improves the transient stability of the system.
the form of swings in machine angle and grid power. In an HVDC schemes in parallel operation with AC
interconnected network, power swings in one area may transmission are prone to both angle and voltage
affect the connected areas and cause the instability of the instabilities even for relatively “strong” AC systems [9]. In
entire network. The instability of frequency or voltage [9] it has been shown that a HVDC with constant DC
may cause loss of load in an area, transmission line power control does not contribute to system synchronizing
tripping, equipment damage and degradation of power torque even with supplementary damping signal.
system performance [2]. Fundamentally, transient stability is assessed based
Several control schemes may be used to prevent this on synchronizing and damping torques. Lack of sufficient
event, for example using power electronic equipment synchronizing torque results in an aperiodic instability,
devices, in the form of Flexible Alternative Current and lack of damping torque results in an oscillatory
Transmission Systems (FACTS) devices, or employing instability [1].

978-1-4577-0411-6/11/$26.00 ©2011 IEEE

 An important measure for system security is the
DC Line
maximum time interval by which a fault must be cleared in
order for the system to preserve its stability this maximum Three-phase AC
Three-phase AC
duration is the fault critical clearing time (CCT) 12-pulse System 12-pulse
Rectifier System Inverter
In this paper, we investigate the transient stability of
a two-machine power system, considering three
transmission line configurations: parallel HVAC, parallel
HVDC, and a hybrid HVAC-HVDC. The faults are
applied in the mid-point of interconnections. For each AC System Filtrer AC System Filtrer

configuration, transient stability of the systems is assessed

in terms of the CCT. The results indicate the importance of Figure 1. Single pole 12-pulse HVDC transmission system
installing HVDC to grid power system not only on
increasing the stability margin and the tie-line power Rcr RL Rci
control but also increasing the power system critical
clearing time.
Id
Vdr Vdi
2. SYSTEM MODELING
Vdr 0 cosD Vdi0 cos J

2.1 HVDC system

Figure 2. Equivalent HVDC transmission system
HVDC transmission systems transport very large
amounts of electric power which can only be 3 X ci
accomplished under tightly controlled conditions. Direct Rci (5)
S
current and voltage are precisely controlled to affect the
Vdr Vdi  RL I d (6)
desired power transfer Pd. It is necessary therefore to
continuously and precisely measure system quantities Pac Pd Vd I d (7)
which include at each converter bridge, the DC current Id, Qac Pd tgM (8)
its DC side voltage, the delay angle D , and for an Vdr R I
inverter, its extinction angle J . The principles of HVDC cos Mr cos D - cr d (9)
2Vdr 0 Vdr 0
control can be well described by the two-terminal
where Vd0r and Vd0i represent the converter transformer no-
monopolar HVDC link schematized in Figure 1, where the
load DC voltage of the rectifier and inverter respectively.
rectifier and the inverter are 12pulse converters using two
The equivalent commutation resistances of the rectifier
6pulse thyristor bridges connected in series. The and inverter are respectively Rcr and Rci. The commutation
corresponding equivalent circuit is shown in Figure 2. The reactance of the converter transformer at the rectifier and
strength of the two AC systems connected by HVDC
inverter are respectively Xcr and Xci. The DC line
transmission has a significant impact on the AC/DC
resistance is denotes by RL. The firing angle for rectifier
system interactions. This impact could be measured by the
is D , the extinction angle for inverter is J ; the current of
short-circuit ratio (Kcc) when AC filter ratings are
excluded [1]: DC line is Id, the DC terminal voltage of the rectifier and
shortcircuit (MVA) of AC system the inverter ends are defined by Vdr and Vdi respectively.
K cc (1) Although several control strategies are suggested for
DC converter (MW) rating DC link operation, most DC transmission systems use the
control concept of constant extinction angle at the inverter
Based on this parameter, the AC system is considered with constant current control at the rectifier end. The shift
strong or weak. If the Kcc > 3.0 the AC system is logic of these controllers is implemented by the current in
considered strong and control systems stability and DC line Id which could be derived from the equivalent
robustness is not a problem. If Kcc < 3.0, the AC system is circuit:
considered weak. For such a system, converter controls are Vdr 0 cos D  Vdi 0 cos J
difficult to adjust and appropriate tuning is needed [11]. A Id (10)
minimum Kcc level of 2.5 has often been used as a lower Rcr  Rci  RL
limit for acceptable DC operation [12]. Figure 3 illustrates the basic control scheme of an
The basic converter equations, for both rectifier and HVDC link. An HVDC system can be divided into several
inverter operations, describing the relationship between levels. The master control layer determines the reference
the AC and DC variables can be written as follows [1]: current Id_ref, which indeed decides the active power to be
transmitted. The actual reference current Id_ref_lim used by
Vdr 2(Vdr 0 cos D  Rcr I d ) (2) the controllers is limited by the Voltage Dependent
Current Order Limiter (VDCOL), which is implemented to
3 X cr help the fault recovery. This control automatically reduces
Rcr (3)
S the reference current Id_ref set point when Vd_mes decreases
Vdi 2(Vdi 0 cos J  Rci I d ) (4) (as, for example, during a DC line fault or a severe AC
fault).
 Reducing the Id_ref currents also reduces the reactive
Id_ref 1. The Master Control Up to 10 s
power demand on the AC system, helping to recover from
the fault. The difference between both settings is the
current margin Imargin and its value is normally fixed in the 2. The Pole Control 10 ms to 500 ms

range of 10%15% of the system rated current. Id_ref *Id_min Id_ref

The current error Imargin provides a transition between Id_ref_lim
the current control and voltage control to facilitate control
stabilization. The Pole control is the core of HVDC control
and activates the appropriate controller of the rectifier and
Vd_meas
inverter station according to the state of AC/DC systems. VDCOL
Then it produces the firing angle D ord for both rectifier and ref

inverter stations. The constant current mode at the rectifier Id

meas meas inv Imargin Id meas rec
is achieved using a typical PI regulator, which acts
according to the error obtained from the measured and the
desired current to keep the direct current constant. Pole
control of the inverter station includes a constant current PI Gamma PI Current PI Current
(CC) controller and a PI a constant extinction angle (CEA) Regulator Regulator Regulator
controllers in order to maintain a constant voltage at the ord ord ord
inverter end. Finally, the bridge or converter unit control
determines the firing instants of the valves within a bridge.
This has the fastest response within the control hierarchy. Inverter System Rectifier System
Firing Firing

2.2 AC and DC transmission line model 3. The Converter Control 1 ms

The DC transmission line is represented using the

distributed parameter line model with lumped losses. For Figure 3. Basic HVDC control scheme
an AC transmission line, the resistance, inductance, and
capacitance are uniformly distributed along the line. For each AC system, the synchronous generator is
Unlike the distributed parameters line block, which has an represented by a sixth order dynamic model; the excitation
system is represented by a third order dynamic model, the
infinite number of states, the S sections linear model has a
model parameters are given in the Appendix. The AC
finite number of states that permits to compute a linear
systems have identical nominal voltages, frequency, and
state-space model.
short-circuit powers (U = 500 kV, f = 50 Hz; Scc = 5000
MVA). They are considered as strong systems since the
3. TRANSIENT STABILITY CRITERIA
corresponding short-circuit ratio (Kcc) is equal to 5). The
Transient stability is the ability of the power system to model presented in Figure 4 includes two types of buses
maintain synchronism when subjected to a severe transient according to Table 1:
disturbance such as a short-circuit on a transmission line.
The resulting system response involves large excursions of Table 1: Model load flow data
generator rotor angles and is influenced by the nonlinear Generated Power (MW) P load (MW)
power-angle relationship [1]. The conventional transient Bus 1 2915 1000
stability measure of power system robustness to withstand Bus 2 1000 3000
a given disturbance (e.g., its stability degree) is named
Critical Clearing Time (CCT), which is the maximum time
duration that the disturbance may act without losing its The DC system is a bipolar, 12 pulse, rated 1000 MW
capacity to recover to a steady-state (stable) operation. The (1000 A, 500 kV). The rectifier (sending end) and the
CCT depends on both the initial operating state of the inverter (receiving end) are separated by a 100 km
system, the location and the severity of the disturbance [1]. transmission line. The HVDC transmission line has the
configuration with two ends (point to point). The
converters stations use thyristor valves. The simulations
4. TEST POWER SYSTEM are performed with the Toolbox SimPower Systems of
MATLAB.
The proposed test power systems consist of two finite The no-load DC voltage is computed by:
control areas interconnected via different HVDC and 3 2 3 2 0.96 * 200
HVAC parallel line configurations. Each control area is Vdr 0 Vc 288.1kV (11)
S S 0.9
represented by a turbine generator set equipped with The equivalent commutation resistance of the rectifier is
power frequency and voltage reactive power control computed from equation (3):
systems, an area load (Pload), and a system interconnection
3 X cr
bus. An overview of the test power systems is shown in Rcr 9.429: (12)
Figure 1. S
 Qv Qv
Pac Qv Qv
Control System Control System
Control System Pd Control System

Turbine G1 Pac G2 Turbine

DC Line
Turbine G1 G2 Turbine
Pd
Pt Pt Pt
Pt
Control System Control System
Control System DC Line Control System

Figure 4(c). Parallel HVDC transmission line

Figure 4(a). Parallel HVAC transmission line

- Configuration 1: Parallel HVAC-HVAC (Fig 4(a))

Qv
Pac Qv - Configuration 2: Hybrid HVAC-HVDC (Fig 4(b))
Control System Control System - Configuration 3: Parallel HVDC-HVDC (Fig 4(c))
Turbine G1 G2 Turbine
Pd 5.1 Comparative Analysis
Pt Pt The simulation tests were carried out to compare system
Control System
DC Line Control System
dynamic responses of the above three configurations for
four scenarios are tested:
Figure 4(b). Hybrid HVAC-HVDC transmission line
Case 1: Configuration 1- Parallel HVAC-HVAC
Where Vc is the line-to-line RMS commutating voltage
which depends on the AC system voltage and the A three phase short-circuit fault occurs at the mid-point of
transformer ratio. The terminal DC voltage Vdr at the the AC transmission line. A critical time noted CCTAC//AC
rectifier end is computed from its expression in (2): is determined (Table 2). The simulation results depicted in
Figure 5 shows the speed machine 2 for the stable and the
Vdr 2(Vdr 0 cos D  Rcr I d ) 514.7 kV (13) unstable cases.

Case 2a: Configuration 2a - Hybrid HVAC-HVDC

Referring to equations (8) and (9), the power factor and
A three phase short-circuit fault occurs at the middle of
the reactive power consumption are about:
AC transmission line. A critical time noted CCTAC//DC-a is
determined (Table 2). Figure 6 shows the angular speed of
Vdr Rcr I d
cos Mr cos D  0.893 (14) machine 2 for the last stable simulation (FCT<CCTAC//DC-a)
2Vdr 0 Vdr 0 and for the unstable case (FCT > CCTAC//DC-a).
Qac Pd tgM 503MVAR (15)
Case 2b: Configuration 2b- Hybrid HVAC-HVDC
A single phase short-circuit fault occurs at the mid-point
of the DC transmission line. The critical clearing time
5. SIMULATION RESULTS
noted CCTAC//DC-b is determined (Table 2) and the results
The time domain simulation method is chosen to assess are shown in Figure 7.
the transient stability of a power system because it is the
most accurate method compared to direct methods. The Case 3: Configuration 3- Parallel HVDC- HVDC
differential equations to be solved are nonlinear ordinary A single phase short-circuit fault occurs at the mid-point
equations with known initial values. In this work, the of the DC transmission line. The critical clearing time
trapezoidal technique is used considering the fact that it is noted CCTDC//DC is determined (Table 2, Figure 8).
widely used for solving electro-mechanical differential
algebraic equations. In all the above cases, the CCT is obtained by
When a three-phase fault occurs at any line in the increasing the fault duration gradually until the system
system, a breaker will operate and the respective line will loses its stability.
be disconnected at the Fault Clearing Time (FCT) which is
set by the user. If the relative rotor angles with respect to Table 2: Critical clearing time for different cases
the slack generator remain stable after the fault is cleared, CCTAC//AC CCTAC//DC-a CCTAC//DC-b CCTDC//DC
it implies that FCT < CCT and the power system is stable. (ms) (ms) (ms) (ms)
However, if the relative angles go out of step after the fault 424 519 100 180
is cleared, FCT > CCT and the system is unstable.
Methodologically, the type of contingencies According to Table 2, the fault critical clearing
considered are three-phase balanced faults created at the time for case 1 estimated at CCTAC // AC = 424 ms is less
mid-point if the AC tie-lines and single phase faults than for the mixed system of case 2a where the CCT was
applied on the DC tie-lines. For comparison, the study estimated at CCTAC//DC-a = 519 ms. As a result, the addition
considers three different tie-line configurations: of parallel DC lines to AC transmission circuit improved
 Case 1 Case 2b
1.25 1.4
Angular speed (p.u)
1.2 1.3

Angular speed (p.u)

1.15 1.2
425 ms
1.1
1.1
CCT=424 ms 1
1.05
0.9
1
0.8
0.95
10 20 30 40 50 60 70 0.7
0 10 20 30 40 50 60 70 80
Time (s) Time (s)

Figure 5. Angular speed of machine 2 Figure 7. Angular speed of machine 2

Case 2a Case 3
1.15
1.016

Angular speed (p.u)

1.014
Angular speed (p.u)

1.1
1.012
1.01
1.05 1.008
520 ms 519 ms
1.006
1.004
1 1.002
1
0.998 0
0.95 5 10 15 20 25 30 35 40 45 50
30 35 40 45 50 55 60 Time (s)
Time (s)
Figure 8. Angular speed of machine 2
Figure 6. Angular speed of machine 2

the transient stability of the network. For case 3, the A three short-circuit fault at the mid-point of the AC line
critical fault duration is estimated at CCTDC//DC = 180 ms. is applied and the corresponding CCT is determined.
When compared to the DC line fault in the hybrid
configuration case 2b (CCTAC//DC-b = 100 ms), the former Two different cases were considered: the case when the
case resulted in a higher allowable fault duration time. capacity of the parallel AC transmission Pac is much less
This result further confirms the improvement of than the DC power Pd with R=0.63; and the case when the
system transient stability by adding the DC transmission. capacity of the AC transmitted power Pac is higher than the
DC power Pd with R=1.66. The results presented in Table
5.2 Influence of AC-DC interconnection ratio 3 show the positive impact of the increased ratio on the
CCT.
In the following, the influence of the relative capacity
of the HVDC and the parallel AC transmission is The increase of the DC current significantly reduces
investigated. The ratio between the active power the momentary AC power transfer capability; improving
transmitted and the continuous power to HVDC line is therefore the CCT, and hence increasing the stability
defined by a ratio R: margin of the network when the AC line becomes weaker.
Pac In fact, in a DC transmission, DC transmitted power is
R (16)
Pd independent from the AC transmission angle . This
where Pac is the active power transmitted by the AC line possibility is often used to improve the performance and
(MW) and Pd is the continuous power line DC (MW). efficiency of the connected AC networks. The
According to [9], the strength of an AC line is classified as controllability of the HVDC power is often used to
high or low depending on the value of R: improve the operating conditions of the AC networks
x R : the AC line is strong where the converter stations are located.
x R  : the AC line is weak Direct Power AC power Pac Pac CCT
Pd (MW) (MW) R (ms)
The reference power of the DC line noted Pd_ref DC is varied Pd
(Pd_ref_DC = 800 MW, 1000 MW, and 1300 MW) keeping 800 1330 1.66 450
constant the total transmitted power: 1000 1130 1.13 519
1300 830 0.63 526
PT = Pd + Pac = constant (16)
Table 3. Critical clearing time with DC power levels
 5. CONCLUSION [10] N.A.Vovos, G.D. Galanos, Transient stability of ac–dc
system, IEEE Trans. Power Apparatus Syst. PAS-98 (4) (1979)
In this paper, the impact of HVDC interconnections on 1375–1383.
transient stability is investigated. Three transmission line [11] S. Lefebvre, M. Saad, R. Hurteau, Adaptive control for
configurations are considered: parallel HVAC, parallel HVDC power transmission systems, IEEE Transactions on
HVDC, and a hybrid HVAC-HVDC transmission line. For Power Apparatus and Systems, vol. PAS-104, No. 9, September
each configuration, transient stability of the system is 1985, pp. 2329–2335.
assessed in terms of the fault critical clearing time. Due to [12] Kala Meah, A.H.M. Sadrul Ula , A self-coordinating
the rapid and controllable features, HVDC systems can be adaptive control scheme for HVDC transmission systems,
Electric Power Systems Research 79 (2009).
used to improve the transient stability of interconnected
AC systems in terms of increasing the fault critical Appendix
clearing time.
The parameters of the system are:
Generators: Rated 5000 MVA, 13.8 kV
For the hybrid HVAC/HVDC transmission system, the Rotor type: Salient-pole
rapid and controllable features of HVDC system can also xl (p.u) : leakage Reactance = 0.18
be used to control the power flow in AC systems, so as to xd (p.u.) : d-axis synchronous reactance = 1.305 ,T’d0 (s): d-axis open
increase AC lines transmission capacity and transient circuit transient time constant =0.296
T’d0 (s): d-axis open circuit transient time constant = 1.01
stability margin. The protection systems in HVDC Xq (p.u ) : q-axis synchronous reactance = 0.053 ,Xq (p.u ) : q-axis
transmission are ignored in this work. synchronous reactance = 0.474 ,X’’q (p.u) : q-axis sub transient reactance
=0.243 T’’q0 (s): q-axis open circuit sub transient time constant=0.1 M =
2H (kWs/kVA): Mechanical starting time = 7.4
Governor control system:
Acknowledgment Permanent droop (statisme) R= 4 %
This work has been supported by the Tunisian-French Servo-moteur ka = 10/3, ta = 0.07 s
Regulation PID kp = 1.163, ki= 0.105, kd= 0
project CNRS/DGRDRT project Code: 09/R11-09 entitled Gate opning limit gmax= 0.01, gmax=0.97518
“Stability of interconnected power systems integrating Gate speed limit Vgmin =-0.1 Vgmax = 0.1(p.u/s)
decentralized production. Case of the south Excitation control system:
Mediterranean countries”. Amplifier gain Ka = 200, Amplifier time constant Ta = 0.001
Exciter gain ke = 1, Exciter time constant te = 0 s
"This work was supported by the Tunisian Ministry of Damping filter gain kf = 0.001, time constant te = 0.1 s
High Education, Research and Technology" Regulator output limits Efmin = 0, Efmax = 7
Initial value of terminal voltage Vt0 =1,
Initial value field voltage Vf0 = 1.35725
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