Course Project: Control of HVDC converters
School of Electrical and Information Engineering
Table of Contents
1.
Introduction .............................................................................................................................. 1
2.
HVDC Control Overview ............................................................................................................. 2
3.
Control Principles for HVDC Converters ..................................................................................... 3
4.
5
3.1
Alpha minimum characteristics at the rectifier ................................................................... 4
3.2
Constant current characteristics at the rectifier .................................................................. 4
3.3
Constant extinction angle (minimum γ) characteristics ....................................................... 5
3.4
Alpha minimum characteristics at the inverter ................................................................... 5
3.5
Current margin characteristics ........................................................................................... 5
HVDC Converter Control System Implementation ...................................................................... 7
4.1
Voltage Dependent Current Order Limiter (VDCOL) ............................................................ 7
4.2
Current Control Amplifier ................................................................................................... 8
4.3
Converter Firing Control ..................................................................................................... 8
4.4
Converter Firing Control Modes of Operation ..................................................................... 8
4.5
Rectifier alpha limiter (RAML) ............................................................................................ 9
4.6
Voltage controller .............................................................................................................. 9
4.7
Gamma0 controller ............................................................................................................ 9
Recovery from AC and DC Faults ................................................................................................ 9
5.1
Commutation failure ........................................................................................................ 10
5.2
DC system faults............................................................................................................... 10
5.3
AC system faults............................................................................................................... 10
5.3.1
Rectifier side AC system faults .................................................................................. 10
5.3.2
Inverter side AC system faults................................................................................... 11
5.
Conclusion ............................................................................................................................... 11
6.
References............................................................................................................................... 12
Appendix A ...................................................................................................................................... 13
1. Introduction
Early transmission systems were developed with direct current. However, the availability of
transformers in the 20th century together with the advances in induction motor technologies led to
the utilization of alternating current (AC) transmission systems over direct current (DC) transmission
systems [1][2]. The increase in size and complexity of AC transmission system also lead to the increase
in issues associated with AC bulk power transmission such as reactive power support, and system
stability.
Due to the fast development of converters, over the years High voltage direct current (HVDC)
transmission has become more practical and favourable when long distances are to be covered or
when power has be to deliver over long distances using cable networks. Additionally HVDC
transmission is distinguished by a number of advantages such as lower overall cost, easier integration
with renewable energy sources and most importantly higher transmission stability and power quality
[3]. Moreover, they can also solve a lot of problems that are associated with AC transmission such as
voltage stability of the AC power network, reduction of fault currents and optimal management of
electric power.
An HVDC transmission power system is characterized by the conversion of power that takes place at
its sending and receiving end. The conversion from AC occurs at the sending end or rectifier station
and the conversion of DC back to AC occurs at the receiving end or inverter station. Early HVDC
schemes utilized mercury arc valves and in the 1960s control electrodes were developed and
integrated into silicon diodes thus giving rise to silicon controlled rectifiers (SRCs) or thyristors. As
such HVDC converters that were implemented using SCRs were categorized as line-commutated
converters (LCC). In addition, due to advances in power electronics in 1997 the first voltage source
converter (VSC) was installed and it used transistor based technology usually insulated-gate-bipolar
(IGBT) instead of SCRs [3].
The main advantage of an HVDC transmission system is the rapid controllability of the power that is
transmitted between the rectifier converter terminal or sending end and the inverter converter
terminal or receiving end through the control of firing angles of the both converters [1] [4]. This built
in ability to control the transmitted power can be used to stabilize the AC network, to control the
frequency of the receiving network and the frequency of the generator connected to the HVDC
transmission rectifier. Also, the reactive power the HVDC converter uses is dependent on the values
of the control angles therefore the reactive power between the converter and AC network controlled
while stabilizing the AC voltage. Present-day converter controls are not only fast but also very
dependable and they can be employed in protection against line and converter faults. From time to
time HVDC converters have to respond in an appropriate manner to normal conditions as well as to
disturbances such as faults on the DC side, faults on the AC side and well as power system oscillations .
Consequently this report looks at different control techniques for HVDC converters.
2. HVDC Control Overview
The figure shows an illustration of an HVDC link.
Figure 1: Illustration of an HVDC link [5]
The DC current, Id shown in Figure 1 is represents the current that flows from the rectifier to the
inverter and can be expressed using the equation below [5].
Id =
Vd0r cos α−Vd0i cos γ
RL +Rcr+Rci
(1)
Where Vd0r and Vd0i represent the no load DC voltage in the rectifier and the inverter respectively.
The equivalent resistances at the rectifier and inverter are represented by R cr and Rci respectively
while RL is the line resistance. The control of the DC current or active power is achieved through the
control of the voltages Vd0rcosα and Vd0icos γ. This is done by either adjusting the rectifier valve firing
angle α or inverter extinction angle γ [5].
The general configuration used in HVDC converters is a 12 pulse bridge converter. This configuration
employs two six pulse converter bridges that are connected in series with a single valve bridge being
provided [4] [5]. In HVDC transmission systems the control of power can be achieved in two ways
which are through the control of current or control of the voltage. Also, it is important to maintain
the voltage in the DC link constant and only adjust the current to in order limit the power losses.
Under normal conditions the rectifier station is responsible for current control while the inverter is
used to control the DC voltage [1][3]. Figure 2 shows the HVDC control overview.
Figure 2: HVDC control schematic [7]
The power order (Porder) in Figure 2 is given by the operator. From the power controller, the current
order (Iorder) is derived and sent to the voltage dependent current limiter (VDCOL), thereafter the
output is sent to the current control amplifier (CCA). The output of the CCA, the alpha order is sent
to the converter firing control (CFC) which determines the firing instant of the valves [7].
Under steady state and transient operation, rapid and flexible power control between the converter
terminals is required in order to stabilize the AC system. Also, fast protection against AC and DC
system faults is required to help with following (i) Minimize overvoltages across the valves (ii) Reduce
short circuit current through the valve and lines (iii) Minimize reactive power usage and (iv) Prevent
repetitive commutation failure.
3. Control Principles for HVDC Converters
The most important control techniques for HVDC converters are the constant extinction angle (CEA)
control and constant current (CC) control [6] [8]. Under normal conditions the rectifier operates
under constant current control while the inverter operates at constant extinction angle control.
When the AC voltage at the rectifier is reduced, the current control must be shifted to the inverter in
order to prevent the rundown of the DC link. As a result the current controller must also be provided
at the inverter in addition to the CEA so that smooth transition from CEA to CC or vice versa can occur
effortlessly when the DC link current is reduced. Additionally, to prevent a clash between the two
current controllers, the reference current at the inverter is kept below that of the rectifier by a value
called the current margin.
The control characteristics of both rectifier and inverter stations are shown in the figure below.
Figure 3: Combined rectifier and inverter Vd-Id control characteristics [10]
The intersection point of the two characteristics curves at point A represents the mode of operation
of the sending end station functioning as a rectifier at constant current control and receiving end
station operating as an inverter with constant extinction angle control (minimum γ). In a DC link
there are three modes of operation for same direction of power flow. This is dependent on the
voltage limit at the rectifier which determines the point of intersection of the two characteristics
curves. The aforementioned operational modes are discussed in the following page.
Mode 1 - Point A is the normal mode of operation with constant current control at the rectifier and
CEA control at the inverter.
Mode 2 - When there is a slight decrease in the AC voltage, the point of intersection moves to point
B which implies minimum alpha (α) at the rectifier and minimum γ at the inverter.
Mode 3- When there is a lower AC voltage at the rectifier the mode of operation shifts to point C
which implies constant current control at the inverter and minimum α at the rectifier.
Table 1 shows a summary of the operating characteristics of the Vd/Id characteristics curve in Figure
3.
Rectifier
ab
bc
cd
Inverter
fg
gh
hj
A,B and C
Type
minimum α
constant current
minimum γ
Operating point
Table 1: Operating characteristics
3.1 Alpha minimum characteristics at the rectifier
This characteristics is determined by the shown below
Vdc = VdiO ∙ cos α − (dxN + drN ) ∙
VdioN
IdcN
∙ Idc
(2)
The above equation is used to determine the DC voltage across the rectifier. Practically a minimum
alpha limit of 5 degrees is assumed in order to ensure that there is sufficient positive voltage across
the valve before firing. Also the transformer reactance (d xN+diN)۰VdiO/IdcN is always kept constant.
Thus increasing the DC current reduces the DC voltage and results in the negative slope, line ab in
Figure 3 is determined by the transformer reactance and DC current. Additionally, once αMIN is
reached it is not possible to further increase the rectifier voltage as such the rectifier will operate in
constant ignition (CIA) angle control [7].
3.2 Constant current characteristics at the rectifier
This part of the curve can also be explained using Equation 1. Operating with constant current and
variable α results in the vertical line bc in Vd-Id diagram. This is the normal mode of operation for the
rectifier where the DC current is controlled by varying α to meet the voltage on the DC side. Also the
current order is subjected minimum and maximum limits. With the former used to avoid issues at
low DC current and the latter determined by the overload capability [5] [7].
3.3 Constant extinction angle (minimum γ) characteristics
The inverter is normally operated at alpha max or CEA control in order to have a certain extinction
angle to commutate the valves without fail. Under normal operation, the inverter operates at γ = 17
degrees at 50 Hz. The assumption of gamma (γ) as constant and the DC current as variable gives the
negative slope characteristics. This slope is even steeper when the HVDC system is connected to
weaker AC system. The voltage at the inverter is expressed using the equation below.
Vdc = VdiO ∙ cos γ − (dxN − drN ) ∙
VdioN
IdcN
∙ Idc
(3)
3.4 Alpha minimum characteristics at the inverter
This segment of the (line fg) curve exists when there is power reversal in the system. Due to the
unidirectional nature of the converter valve, power reversal in an HVDC system cannot be obtained
by simply changing the direction of the current flow. It can only be obtained by reversing the polarity
of the DC voltage. This is done by increasing the current order of the inverter to a value higher than
that of the rectifier. This means that the inverter is now operating as a rectifier and the rectifier as
an inverter. Thus, it is very important to provide both the CEA and CC controllers at both converter
terminals [1]. In an effort to ensure that the inverter does not operate in the rectifier region in the
event of a fault, an alpha minimum limit of approximately 110 degrees is imposed in the inverter.
Furthermore, it is necessary for the inverter to have a minimum counter voltage to commence
current flow after fault clearance.
3.5 Current margin characteristics
The two converter stations are provided with equal current orders but a current margin order ΔI is
subtracted in the inverter. Normally the current margin order ΔI is a value that is approximately 1015% of the rated current [9] [5]. The current margin order is chosen such that it is large enough that
the rectifier and inverter constant current modes do not cross as a result of errors in measurements.
In HVDC systems this control technique is called the current margin control method. The main
advantage of this control strategy becomes more distinct when there is sudden voltage drop at the
rectifier AC bus. The operating shifts from point B to point Y as shown in Figure 4 below.
Figure 4: Vd-Id characteristics at a reduced voltage
As a result the transmitted current will be decreased to 0.9pu of it previous value and the rectifier
will take over the voltage control responsibility. Nevertheless the power transmitted is largely
maintained at approximately 90% of its original value [4]. To solve this problem and also improve
the system behaviour during disturbances the control strategy employs the following modifications,
voltage dependent current limit (VDCOL) and constant beta control.
At the rectifier converter terminal the control strategy employs voltage dependent current limiter.
This improvement is done to limit the DC current as a function of either DC voltage or AC voltage.
Also this improvement helps the DC link to recover quickly from faults. Figure 5 shows the VDCOL
characteristics.
(a)
(b)
Figure 5: (a) Steady-state V-I characteristic with VDCOL, minimum current limiter and firing angle limits [10] (b) Modifications
As part of the modifications the rectifier is also provided with DC current (ID) minimum limit which is
typically in the range 02-0.3 pu. This modification is done to ensure a minimum DC current to avoid
the likelihood of DC current extinction caused by the valve current dropping below the hold-on
current of the silicon controlled rectifiers (SCRs). This phenomenon described above can occur
transiently because of the presence of harmonics on the low value of the DC current [4]. Moreover,
the resultant current chopping would cause overvoltages to appear on the valves. Furthermore the
size of the smoothing reactor has a significant impact of the magnitude of I D-min.
On the other hand when the inverter operates into a weak AC system, the slope of the CEA control
mode will be even steeper thus causing many crossover points with the rectifier characteristics. To
avoid the likelihood of this from happening, the inverter CEA characteristic is improved into a
constant beta characteristics or constant voltage characteristic within the current error region.
Instead of using CEA to regulate γ to a constant value, a closed-loop control voltage control may be
used to maintain a fixed or constant voltage at a preferred point in the DC line, usually at the sending
end. Compared to the CEA control, the constant voltage control has a flat inverter Vd-Id characteristic
(Figure 5b) and is less prone to commutation failures [11]. On the other hand constant beta method
is similar to the alpha max control which is discussed in section 3.3.
4. HVDC Converter Control System Implementation
Both the rectifier and the inverter are provided with equal control functions with a slight difference
in individually parameter settings [9].
In HVDC systems, the current order is sent to the converter firing control system. Thereafter the
converter firing control system sends out firing pulses that will enable the system to maintain the
ordered current. Furthermore, the settings of voltage dependent current order limiter and current
control amplifier are used to determine the dynamics of the HVDC transmission system.
The control system block diagrams for each control function are shown in Appendix A.
4.1
Voltage Dependent Current Order Limiter (VDCOL)
The main purpose of VDCOL control is to decrease the current when the DC voltage is reduced due
to faults in the AC or DC system on the rectifier or the inverter [8]. This is done to prevent the high
consumption of reactive power and valve voltage stress. The figure below shows the VDCOL
characteristics with voltage limits.
Figure 6: Static characteristics of VDCOL [9]
The breakpoints UDLOW, UDHIGH and UdN vary depending on the application and strength of the AC
system. Furthermore it is advisable to place UDHIGH at close proximity to U dN when the AC system on
the receiving end is very weak. However under normal operating conditions the value of UDHIGH range
from 50-70% of the rated voltage [9].
An asymmetrical low pass filter is used to filter the DC voltage response before it is used to control
the maximum limitation of the current order. The filter has different time constants that are
dependent on whether the U D input increases or decreases at one of the converter terminals. The
time constant of the inverter should be higher than that of the rectifier in order to maintain the
current margin. Moreover, the difference between the time constants of two converters has a
significant impact on the restart time after a disturbance. Also the time constant for reducing the
value of UD is low, typically in the region of 10ms or less. This helps to rapidly force the current order
to a low value in the event of a disturbance.
4.2 Current Control Amplifier
Both the rectifier and inverter are equipped with a current control amplifier (CCA). However, there
may be a variation in the parameter settings for each converter terminal.
The main purpose of the CCA is to provide the current control loop with proper dynamics. The current
control loop is required to ensure the following (i) Fast response (ii) Zero current error at steady state
and (iii) stable current control. A comparison between the measured DC voltage and the current
order from VDCOL is made, thereafter the output signal which is the firing angle order is sent to the
firing control. Furthermore, CCA consists of a proportional and integral part. The integral part
provides a high gain for low frequencies. This means that the current error in steady state is zero [5].
4.3 Converter Firing Control
Firing control is used to convert the ordered firing angle α from the CCA into firing pulses. The firing
pulses are then transferred to the converter valves of corresponding phase and within a correct
interval. In addition, the converter firing angles vary between α=αMIN and α=αMAX with the minimum
extinction angle γ limit used to determine the latter. Moreover, changes to firing angle are dependent
on the mode of operation. The most important function of the firing control is to ensure that the
firing instant occurs within the designed time limitations for SRCs. There are two basic techniques
used for generating and synchronising converter firing pulses. These are Individual Phase Control
(IPC) and Equidistant Firing Control (EFC). In the IPC method, the firing instants for each valve are
produced individually depending on the zero crossing of the commutation voltage so that a constant
delay angle is sustained across all valves [11]. On the other hand the EFC overcomes some of the
shortfalls present in the IPC by using the phase lock oscillator technique. Equivalent time intervals
are maintained between consecutive firing pulses during fault conditions and under steady state. The
phase locked and phase limited oscillator is indirectly synchronised to the AC system to offer a stable
operation even if the system experiences a disturbance.
4.4
Converter Firing Control Modes of Operation
As discussed in section 3.1.5, the current order in the inverter is lower than the one received by the
rectifier by a factor called the current margin. Hence the current in the rectifier is higher than the
current delivered in the inverter. As a consequence the inverter attempts to counter this response
by increasing the firing angle. If the rectifier manages to drive the ordered current through the
inverter, α will reach its maximum value. For this reason the converter firing control (CFC) will operate
in the extinction angle control mode. Likewise, if the voltage at the rectifier is insufficient due to a
low AC voltage, the inverter will take over the current control because the current in the inverter will
be higher than that of the rectifier. So to deliver the ordered current, the value of alpha α has to be
reduced thus leading to the CFC to operate in the minimum alpha α mode of operation which is
obtained from the CCA output. The CFC is thus able to operate in any of the following modes that
are determined by the CCA (i) Minimum alpha mode or CIA (ii) Constant DC current (iii) minimum
extinction angle control and (iv) Constant DC voltage.
4.5 Rectifier alpha limiter (RAML)
When a short circuit fault occurs in the AC network connected to the rectifier, the firing angle α gets
reduced to the smallest acceptable value αmin. When the fault is cleared and the AC voltage is still
being restored, the DC current will be high since the firing angle is at α min. To prevent this from
happening, a rectifier alpha minimum limiter controller is employed (RAML) [12]. The RAML
controller is activated when the AC voltage drops below a predefined value and increases the firing
angle to a predefined value. Moreover this control function forces the inverter into current control
mode during a short circuit fault at the rectifier end.
4.6 Voltage controller
This controller is used during reduced voltage operating conditions. However, it can also be employed
during normal operation. This controller uses a proportional integral (PI) regulator that responds
based on the minimum and maximum limits of the current controller. In rectifier operation it
increases the minimum alpha limit and in inverter operation it decreases the maximum alpha limit of
the CCA. At reduced voltage operations, the reference voltage is set lower than the operating voltage
thus leading to the controller to reduce the DC voltage. Also the reference DC voltage is set to slightly
higher in the rectifier in order to maintain the voltage control in the inverter.
4.7 Gamma0 controller
If the inverter is operational while the rectifier is blocked, the current controller will force the inverter
extinction angle γ 110 degrees. Consequently the direct voltage will increase rapidly to a reversed
polarity. To prevent this phenomenon, a gamma0 controller is used. This controller gets activated
when the direct voltage is lower than 0.6Vdref after a certain time delay and the firing angle is set to
the maximum value αMAX 110 degrees. This function resets after the voltage is restored to it its
nominal value.
5 Recovery from AC and DC Faults
Faults on the DC line, converter and AC system have a significant impact on the operation of HVDC
transmission system. The effect of the faults is reflected through the action of the converter controls.
Converter controls play an important role in the suitable response of HVDC systems to faults on the
DC as well as the AC systems [11].
5.1 Commutation failure
Commutation failure occurs when a converter valve that is meant to turn off continues to conduct
thus leading the current not to be transferred to the next valve in the firing sequence [12]. It mostly
caused by a sudden increase in the DC current and decrease in commutation voltage. Additionally,
its occurrence results in the temporary interruption of the transmitted power. While it is unavoidable
at first, it is however possible to prevent subsequent commutation failures during a fault at the
inverter AC network by advancing the firing angle in order to increase the commutation margin.
There are two control methods that are used to prevent subsequent commutation failures caused by
balanced and unbalanced faults. In unbalanced faults the fault is detected by a zero sequence voltage
detection and then it is compare with a predefined fault level. If the measured zero sequence voltage
is lower than the predefined value, the control strategy advances the firing angle and maintains it for
the duration of the fault. Since balanced faults don’t give zero sequence, the fault voltage is compare
directly with the predefined value. If the difference between the two is higher than a predefined
value, the firing angle will be decreased.
5.2 DC system faults
Mostly DC line faults are pole to ground faults. The HVDC converter control system plays an important
role in clearing DC fault. As soon as a DC fault is detected in the system, the forced retard function is
activated. This basically means that the rectifier is operated at a high alpha i.e. the rectifier is driven
to inversion while the inverter is also kept at an inversion state and this will send the energy back to
the ac system thus making the current zero current at DC system. During this process the current is
reduced to zero rapidly in approximately 10ms. To setup terminal voltages of correct polarity for fault
clearing, angle β is set to a maximum limit of about 90 degrees and the rectifier firing angle α is given
140 degrees limit.
5.3 AC system faults
When there is transient disturbances in the AC system, the DC system usually responds very much
faster compared to the AC system. Generally the DC system responds by either riding through the
disturbance with temporary reduction in power or shutting down completely until the AC system
recovers adequately to allow for resumption and restoration of power. Further, recovery from AC
system faults and commutation failure are regarded as important features for DC system operation.
5.3.1 Rectifier side AC system faults
Distant three phase faults cause the rectifier commutation voltage to decrease a little. This results in
the decrease of the rectifier DC voltage and hence the current. The employed current regulator
responds by decreasing α thus restoring the current by increasing the voltage. If the value of α
reaches the αmin limit, the rectifier shifts to constant ignition angle (CIA) mode of operation and this
transfers current control to the inverter [11]. If the low voltage continues, the tap changers will
operate to restore the DC voltage and current to normal. Subject to how low the voltage decreases,
the VDCOL may control the current and power transfer. Furthermore as opposed to distant three
phase faults, local three phase faults cause the rectifier commutation voltage to drop significantly.
As a response, the DC system shuts down under VDCOL control whilst awaiting for the fault to be
cleared.
5.3.2 Inverter side AC system faults
Slight voltage dips at the inverter due to distant three phase faults result in the increase of the DC
current. This causes the rectifier CC and the inverter CEA controls to respond the changes. If the low
AC voltage continues tap changes will come about to restore the converter firing and DC voltage.
Additionally, if the voltage drop is substantial a reduction in the commutation voltages may occur
and lead to temporary commutation failure at the inverter. With the inverter operating at γ of 17
degrees, a voltage reduction by 10-15% will likely cause a commutation failure [11] [12]. Normally it
takes about 1 or 2 cycles to clear the fault. Subsequent to this some power may be transmitted with
the rectifier DC voltage reduced to equal the decrease in the inverter DC voltage thus resulting in the
increase of the reactive power consumption. To curb the reactive power consumption problem, the
DC current is reduced by VDCOL controller provided at the DC system. At extremely low voltage
conditions, recurring commutation failures are unavoidable as such it may be required to block and
bypass the valves until the AC voltage recovers.
5. Conclusion
The control of the DC current or active power in an HVDC system is done by either controlling the
rectifier valve firing angle or the inverter extinction angle. This report reviews and discusses the
traditional control system functions for HVDC converters. These include current control amplifiers,
voltage dependent current order limiter and the firing control. From the literature review conducted
for this study it was discovered that the most important control methods for HVDC converter are the
constant current control at the rectifier and constant extinction angle control at the inverter.
However, it is possible that there is a power reversal in the system, as such both the rectifier and
inverter are provided with current control. The DC voltage and DC current characteristics of the HVDC
system revealed that there are three different modes of operation in an HVDC system and they are
mainly influenced by the magnitude of the voltage. Furthermore, the current margin is essential in
preventing the misfiring of SCRs and ensuring that the inverter and rectifier current controllers do
not become active at the same time.
Even though commutation failure is unavoidable, it is possible to prevent subsequent commutation
failures by increasing the commutation margin. Moreover, VDCOL plays a significant role in system
recovery from disturbances or faults and commutations failures.
6. References
[1] P. Sridhar, Lecture notes on HVDC transmission. Hyderabad: Institute of Aeronautical
Engineering, 2019, pp. 1-20.
[2] M. Bahrman and B. Johnson, "The ABCs of HVDC transmission technologies", IEEE Power and
Energy Magazine, vol. 5, no. 2, pp. 32-44, 2007.
[3] L.W. Sheng, A. Razani and N.Prabhakaran, “Control of High Voltage Direct Current (HVDC) bridges
for power transmission systems” In 2010 IEEE Student Conference on Research and Development
(SCOReD) ,IEEE, 2010.
[4] R. Roy and M. Amin, "A Paper of Determination of Controlling Characteristics of the Monopolar
HVDC System", International Journal of Hybrid Information Technology, vol. 7, no. 3, pp. 105-120,
2014.
[5] J.Karlsson, “Simplified control model for HVDC Classic (Master’s thesis)”, 2006.
[6] V. Lackovic, Principles of HVDC transmission. Continuing Education and Development, 2016, pp.
23-25.
[7] A.Muthusamy, “Selection of dynamic performance control parameters for classic HVDC in PSS/E
(Master's thesis)”, 2010.
[8] M'Builu-Ives, S., “Stability enhancement of HVAC grids using HVDC links (Doctoral dissertation)”.
2016.
[9] Sari-energy.org, 2003. [Online]. Available: https://sarienergy.org/oldsite/PageFiles/What_We_Do/activities/HVDC_Training/Materials/1JNL100020842%20-%20PDF%20-%20Rev.%2000.pdf. [Accessed: 22- Jun- 2020].
[10] K. R. Padiyar, "HVDC Power Transmission Systems – Technology and System Interaction," New
Age International (P) Limited Publisher, New Delhi, Ch. I, 3 - 5, 1990
[11] P. Kundur, Power system stability and control. New York: McGraw-Hill, pp. 523-544.
[12] I. Oketch, "Commutation Failure Prevention for HVDC- Improvement in algorithm for
commutation failure prevention in LCC HVDC," Chalmers University of Technology, Gothenburg,
2016.
Appendix A
Figure 1A: Control block diagram for VDCOL
Figure 2A: Control block diagram for Current control amplifier
Figure 3A: Voltage controller control system
Figure 4A: RAML control system