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1. Hybrid HVDC Converters and Their
Impact
on
Power System Dynamic Performance
Abstract: results is also compared with a
Hybrid converter conventional HVDC scheme.
HVDC transmission is a new
hybrid transmission system for I. NOMENCLATURE
connecting two ac systems. Voltage Sourced Converter:
Because it uses different (VSC); Line Commutated
converters, this new Converter:(LCC); Forced
configuration offers several Commutated Converter: (FCC);
advantages over conventional Series Hybrid Converter: (SHC);
HVDC Commutation Failure: (CF); Pulse
systems. This paper Width Modulation: (PWM).
demonstrates the superior
performance of hybrid II. INTRODUCTION
converter based HVDC THE conventional HVDC
transmission systems with transmission systems that utilize
respect to increased stability Line Commutated Converters
and terminal ac voltage control. (LCC) have advantages over
A control system is developed HVAC systems such as their
for the hybrid system and its ability to connect ac networks
dynamic performance is non-synchronously and their
investigated. The hybrid system ability to carry power
performance with emphasis on economically over large distances.
commutation failure during Unfortunately, these
severe disturbances and its

2. schemes do have certain LCC converter, at the same time
disadvantages such as a need for introducing the additional benefits
reactive power, commutation of the VSC converter. Several
different topologies suitable to
failures, higher over-voltages and such combination have been
poorer recovery especially when proposed in literature.
they are connected intoweak This paper introduces a
terminating ac networks. hybrid topology which includes a
Unlike the LCC which series dc--side connection of an
relies on the ac voltage for LCC and VSC as shown in
successful valve commutation, the Fig. 1. The paper studies dynamic
Voltage Sourced Converter (VSC) control performance, fault
uses special devices that can be recovery transient performance
turned off with and commutation failure
appropriate control signals. While susceptibility of the proposed
maintaining most of the scheme and shows it to be
advantages, VSC based HVDC superior to a purely conventional
schemes also overcome a LCC based scheme.
number of disadvantages inherent
to conventional systems. Rather
than consume reactive power,
their ability to generate III.PROPOSED HYBRID
lagging or leading reactive power CONVERTER
permits them to operate and
provide voltage support to very The hybrid topology may
weak ac networks. Thus they are employ the LCC and VSC
an ideal option for providing converters connected either in
reliable power to remote locations parallel or series on the dc side.
such as offshore plants. Their More complex schemes may not
disadvantages include higher be justified easily due to control
costs, sensitivity to dc-side faults, complexities, expenses, need for
higher power losses due to the larger space, etc.
high frequency of switching, and In a parallel hybrid
smaller ratings in comparison to configuration the converter
conventional converters. voltage rating is limited to the
Appropriately sized VSC and highest voltage level permissible
LCC converters can be for the VSC converter, which is
incorporated into a single much lower than that of a
composite “hybrid” converter comparable LCC and
which combines the lower costs consequently limits the power
and robustness of the conventional rating of the topology. The

3. proposed hybrid converter is of the CIGRE benchmark. At the
labeled a “Series Hybrid inverter side, the VSC used is a
Converter” (SHC), as it includes modified version of [6] which
one LCC and one VSC in series. considering its optimal rating, the
In contrast to some earlier LCC has been re-sized so as to
approaches in which the VSC has keep the overall ratings identical
only been used for reactive to that of the CIGRE benchmark
support or for active filtering the systems.
proposed topology uses both This paper describes the
converters for real power transfer. principles of the proposed SHC
system along with its main control
IV. BAISCS OF THE strategies including the terminal
PROPOSED SHC SYSTEM voltage control, real power control
MODEL at the receiving end and inverter
The schematic for the proposed dc capacitor voltage control.
SHC has been depicted in Fig. 1.
The sending end (rectifier side) A. Optimizing the SHC’s Power
has been assumed to be a LCC and Voltage Rating
converter station and the receiving Based on the nominal
end (inverter side) is a LCC-VSC power (Pnom) of the HVDC
series connection, along with its system the power / voltage rating
harmonic filters. for the hybrid-side converters may
be calculated. To find the
appropriate voltage level on the
converters an optimization
concept is employed to establish a
connection between inverter-side
voltage ratings, and major system
components’ prices.
Assume that the price for
each converter is proportional to
its MVA rating. Based on this
assumption minimizing the total
MVA of the inverter side
The “First CIGRE HVDC converters (SLCC + SVSC) which
Benchmark System” [4] has been also equals to sum of their
used as the test bed for the corresponding transformers’
performance of the proposed SHC ratings has
system to be compared to. In the the same meaning of minimizing
proposed SHC system the rectifier the total converter expenses.
side is structurally identical to that Using an engineering-based

4. estimation for filter reactive power The LCC generates voltage
(Qfilt) hybrid converter’s complex harmonics. In its 12-pulse
power may be written as: configuration, the harmonics at
11, 13, 23 and 25 times the
fundamental frequency (60 Hz)
are present. As the VSC switches
at rather high frequencies it will
In (1), the (Pnom – PLCC ) term only add high frequency
is equal to PVSC. Also under harmonics to the system. Selecting
normal working conditions the a switching frequency of 27th of
term (0.6 * PLCC ) approximates the fundamental frequency
the QLCC ; the last expression generates harmonics at 25th and
simply equals the reactive power 29th order harmonics. To cancel
that has to be generated by VSC out the detrimental effects of these
(QVSC). Differentiating (1) with harmonics and help to meet the
respect to PLCC and setting that system’s harmonic requirements
equal to zero, the PVSC will be the 11th, 13th, 23rd, 25th and 29th
determined. Based on this value order harmonic filters were
the appropriate voltage rating for installed, with total static reactive
hybrid-side converters, and as the power support of around 80
last step the LCC and VSC’s MVAR. These filters will provide
transformer ratings, will be voltage-dependent reactive power
determined. The optimized supply to the inverter side and
magnitudes are given in help to meet the standards for
Appendix. It has to be reminded system harmonic levels. During
that even at the design level there the steady state conditions the
are other expenses that could be VSC has to provide the un-
considered but as the converters supplied reactive power needed
and transformers are the most for the conventional converter part
“expensive” parts of each of inverter side. During the
converter, only these two major transient conditions it has also to
devices have been included in supply the extra reactive power to
optimization. Other design provide voltage support at inverter
philosophies may bring equally terminal. The ability of VSC to
valid results for the purpose of supply voltage support depends on
converter rating design. its electrical rating and the
coordination between LCC and
VSC controls. The dynamic
response is also a function of the
B. SHC’s Filter and dc capacitor dc capacitor(s) size. A suitable dc
considerations capacitor size has been selected to

5. give an acceptable dynamic
response; however, in this paper,
no attempt has been made to
optimize this
perfectly.
V. CONTROL OF THE SHC
SYSTEM
The system design outputted from terminal power
philosophy has been based on two error signal.
control objectives:
1) Terminal voltage of the hybrid
converter must be maintained at 1
P.U.
2) Power delivered to terminal
during normal working conditions The VSC inverter control
must be 1 P.U. system shown in Fig. 3 has two
The SHC control block diagram degrees of freedom. The first is
has been shown in Fig. 2. In this used by the VSC’s dc voltage
figure the upper and middle parts capacitor controller which
depict the rectifier and inverter generates the reference real
angle controls, respectively. The current (Idref) signal. The second
bottom part is used to control the terminal
illustrates the VSC controls. voltage via the reference reactive
The basis for SHC current (Iqref) signal. The d and q
controllers is a coordinated current errors are used to generate
version of LCC-HVDC [4] and the corresponding voltage orders
VSC-HVDC system [5] controls. (Vd and Vq) through a decoupled
In the SHC presented controller block. These are
here, the rectifier’s LCC works in converted into a modulation index
current control mode while the magnitude (m) and phase (φ)
inverter’s LCC works in signal. A phase locked loop (PLL)
extinction angle (γ) control mode, is used to synchronize with the ac
using the current control as its network voltage and generates the
backup [2]. The current order synchronizing angle signal (θ)
signal that would end up to LCC’s which is used to generate the
inverter angle order (αinv) is firing pulses for the IGBT devices
of the VSC.

6. commutation margin resulting
from a sudden change in the ac
VI. OPERATION ISSUES IN voltage phase. Having larger
HVDC SYSTEMS commutation margin in normal
A. Commutation Failure operation improves the system’s
Phenomena CF
Commutation failure (CF) susceptibility, but this also results
is one of the most onerous in a poorer power factor and
transient events experienced by potential over-voltage problems
HVDC systems. Its causes include on load rejection.
sudden transient reductions and/or In a conventional HVDC
phase shifts in the ac voltage and converter, the fault induced CF
sudden transient reductions in the leads to power disruptions. In
direct current. The sensitivity of a some cases there are repeated CF
HVDC inverter to CF depends on occurrences from which recovery
the main circuit design and its is not possible without a full re-
control system. In conventional start. Additionally, CF also causes
converters, commutation failure over-current in the valves.
likelihood is significant when The VSC in the hybrid
there is a 10% or larger voltage HVDC converter cannot suffer
reduction caused by an ac system CF. Thus HVDC transmission
disturbance. systems with hybrid converters are
The main reason for CF is less susceptible to CF related
that the excessive reduction in the power disruptions. Also, the same
extinction angle during its fault which would have resulted in
initiating system disturbance. This serious system failure in the
decrease could be caused by an conventional converter has a much
increase in the converter’s overlap smaller impact on the hybrid
angle due to ac voltage reduction converter.
or due to a change in the

7. The disruption of the HVDC alternatives. The
normal switching sequence parameters for the controllers
following a CF will lead to were selected for overall
considerable waveform distortion performance and were not
of the optimized for any particular
commutating voltage waveform disturbance event.
making the problem unsuitable for
analytical formulation. Therefore A. System step response
numerical simulation on an To investigate both
electromagnetic transients solver systems’ responses to set point
is required to assess the behavior changes, the conventional (CIGRE
of the system. Here the PSCAD / benchmark) converter
EMTDC software has been option was subjected to a 10%
selected for simulating the system change in power order. Also as the
and studying its behavior. hybrid system operates directly in
power control, its controller was
B. Dynamic Response and Fault subjected to a 10% change in
Performance power order. The results are
A well designed HVDC shown in Fig. 4.
system should show react rapidly
to set-point changes and also show
rapid recovery from In the system with the
system faults. In order to assess conventional converter only, the
these issues, the dynamic behavior steady state terminal voltage
of CIGRE benchmark and the settles to a different magnitude
proposed series hybrid converter after the change is applied, due to
HVDC systems will be compared. the resulting mismatch in reactive
The hybrid’s robustness under power. As the VSC in the hybrid
small and large dynamic control option is capable of
disturbances will be demonstrated. reactive power generation and is
Next, the two HVDC systems will tasked with
be compared by simulating their maintaining the ac voltage at rated
performance following single and magnitude, the ac voltage
3-phase to ground faults of eventually returns to its post-fault
varying severity. magnitude.
As can be seen from Fig.
VII. CASE STUDIES 4., both options show a quick and
The following section well damped response to the set-
contains simulated results for the point changes. However, the VSC
dynamic and fault performance of option shows a smaller settling
the conventional and hybrid

8. time with a slightly oscillatory
response.
capacitor limits the over voltages
B. System fault response of dc capacitor to 25%
To investigate both
systems’ responses to large Fig. 5 shows the pre and post
dynamic disturbances a fault terminal voltage and power
symmetrical three phase short curves obtained based on applying
circuit to ground at the inverter such a fault. In comparison to the
terminal was applied for 0.1 conventional option, the hybrid
second (6 cycles) to each systems’ option shows significantly faster
terminals. Various fault power recovery with 90% power
impedances were used, but the restored within 200 ms after fault
case reported below only shows clearance. The
the response to the most severe corresponding conventional option
fault which is a solid short circuit requires approximately 400 ms.
that reduces the terminal voltage However, the conventional
to essentially zero. One difference option shows a more gradual
between the two systems is that in voltage recovery without any over
the hybrid case the arrester voltage stress on the equipment.
connected across VSC’s dc The hybrid option, on account of

9. its voltage control function, causes phase faults under varying
the voltage to be rapidly regulated, inductances (not shown here)
and in doing so experiences a suggests less overall hybrid
modest 10% over voltage system’s sensitivity to
during recovery.
C. Commutation Failure
performance
Several other tests were
conducted with various different
fault impedance values (inductive)
to investigate the impact of fault
severity on the performance. The
VSC showed generally superior
fault recovery times in all cases.
Also for certain less severe (high
impedance) faults, the
conventional converter based
system experienced total power
loss whereas the hybrid system
managed to continue operation commutation failure comparing to
during the faulted period. a only-conventional converter
Fig. 6 shows the lowest ac based HVDC system, which
voltage and power magnitudes means that the hybrid system is
reached during the fault for immune to more sever faults
varying fault inductance values. comparing to a conventional
Both three phase-to-ground as HVDC system. Noticeably in case
well as single phase-to-ground of commutation failure in a hybrid
faults were applied. As can be converter system it is probable
seen the hybrid converter was able that it only experiences a
to maintain current and power to commutation failure (CF) in its
above 90% even with fault conventional converter part which
impedances of 1 H or higher for does not lead to total terminal
both types of fault, whereas the power disruption while even less-
conventional converter starts sever faults would cause CF and
experiencing similar reductions at power disruption in a conventional
a much less severe fault system.
inductance of 2.5 H. Examining The ranges for single phase
the power variation vs. extinction faults show similar trends with
angle curves for the two options fault and converter types, but the
subjected to three phase and single fault severity required to cause
failure is marginally smaller for

10. each case. The above tests were conditions without resorting to
conducted with the short circuit complicated control strategies
ratio of the ac system set to 2.5, with the added benefit of
which is considered fairly low and superior system performance (less
hence expected to cause power / voltage drop, less chance
challenges for the transmission of commutation failure and shorter
options. However, because of the recovery time using equal fault
VSC’s fast dynamic response to inductances).
reactive power demands the
proposed hybrid converter also IX. APPENDIX : MODEL
has the unique capability of DATA
working under even lower short HVDC system rating: 1000
circuit ratios (SCR) where the MW; 500KV DC Rectifier
conventional converter would not specifications:
be able to operate at all As per First CIGRE benchmark
model [4].
VIII. CONCLUSIONS Inverter specifications:
Using a coordinated Terminal voltage: 230 KV, L
controller for a SHC (Series SCR: 2.5
Hybrid Converter) HVDC LCC converter voltage (DC side):
transmission system results in 390 KV
superior inverter terminal VSC DC voltage: 110 KV
performance in response to small Line parameters: As per First
and large dynamic changes in CIGRE benchmark model [4].
comparison to a conventional
converter case (First CIGRE X. REFERENCES
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