Improved Controls for LCC-VSC Hybrid HVDC System t Premila Manohar , Vijetha Kelamane, Darshan Kaushik, and Wajid Ahmed Department of Electrical and Electronics Engineering M. S. Ramaiah Institute of Technology Bangalore, India t
Email:
[email protected]
High Voltage Direct Current (HVDq transmission
control and the freewheeling anti-parallel diodes acts as bridge
systems continue to be an excellent asset in modern power
rectifier and feed the fault resulting in large dc line current. As
Ab s tract
-
systems. The classical HYDC system and YSC-HYDC system
a consequence of this, the capacitor on the faulted line starts
showed greater advantages in power transmission and industry.
discharging and the voltage of the healthy pole increases to a
Meanwhile
major
disadvantages
like
commutation
failure,
reactive power requirement in classical system and high cost, high switching losses and inability to handle DC line fault in VSC-HVDC system lead to the invention of LCC-VSC hybrid HYDC system. With
no
commutation
failure,
less
reactive
power requirement and reduced losses, the hybrid system is best suited for unidirectional power transmission scenarios, such as power transmission to islands and remote load centers, where the
very high value and hence their application is limited to only for bi-polar underground cables. The possible solutions to overcome the limitations ofVSC HVDC system are: •
construction of new transmission lines is prohibitively expensive.
•
The present work aims to study various aspects of hybrid HVDC
•
system. The emphasis will be on developing appropriate control schemes at both rectifier and inverter terminals for efficient operation of the hybrid system and achieve best performance of the system under steady state and
transient condition.
The
simulation study is carried in the PSCAD/EMTDC environment.
Keywords-YSC-HVDC
system,
LCC-HVDC
system,
hybrid
system, commutation failure, simulation, PSCADIEMTDC.
H ybrid HVDC system [7,8] Using multilevelVSC converters [9] Combination of chopper controlled resistor and superconducting FCL for DC line fault recovery [10]
The hybrid HVDC system can be realized by using LCC and VSC converters either in series at a terminal or in parallel one at each station. The hybrid system with LCC on the rectifier and VSC at the inverter side offers all the advantages ofVSC-HVDC system [11-12], especially at the inverter and also reduces the cost of the system. This paper aims to study
I.
the dynamic performance of LCC-VSC hybrid HVDC system
INTRODUCTION
with emphasis on the design of best controllers.
The recent developments in solid state technology have led
II. VSC-HVDC SYSTEM
to IGBTs and GTOs of higher ratings. These devices have controlled tum on and tum off characteristics and have
With the rapid development of power electronic devices
resulted in the development of new converters called Voltage
having turn-off capability, the VSC based systems are getting
Source Converters (VSC). Switching of these devices is
more and more attractive for HVDC transmission. The VSC
The VSC-HVDC system
converters uses insulated gate bipolar transistor (lGBT) valves
eliminates the problem of commutation failure, harmonics and
and pulse width modulation (PWM) for creating the desired
permits inversion into weak AC systems. The converter
voltage wave form.
controlled by PWM technique.
operations are self-sufficient and provide independent control
III. LCC-VSC HYBRID HVDC SYSTEM
of active and reactive power which reduces the lower order harmonic content, hence making the converter station more
The VSC-HVDC system is advantageous because of its
compact and modular [I]. They have black start capability and
ability to independently control active and reactive power.
also capable of supplying passive load [2]. The applications of
Also the VSC's ability for independent control of output
these
voltage magnitude and phase angle is an outstanding feature
converters
include
power supply to
isolated loads
( islands and offshore platforms), city center in-feed, wind
that enables a wide number of control objectives to be
farm connection and multi-terminal systems [3,4,5,6].
achieved in the connected AC network. H owever, serious
On the other hand, these systems are expensive, have high
drawbacks such as high power losses, high dielectric stress on
switching loss and larger dielectric stress on insulation [8].
equipment
Further, their recovery from DC line fault poses severe
widespread use ofVSC-HVDC transmission.
problems.
The
major
difficulty
in
using VSC-HVDC
transmission for dc overhead lines is the over voltages and over currents during dc line fault. H ere, the IGBTs lose
insulation,
and low
device
rating
restrict
the
A typical LCC-VSC hybrid HVDC system, with rectifier as LCC and inverter asVSC, is shown in Fig. 1.
Id
0 1 [H] 1,5lohml
IV.
l.5lohml
---i
CONTROLLERS
A system shows better performance with good controls and hence controllers are very vital in a HVDC system. In this
Converter Transformer
�I
Inv AC
section, the controllers used in the work will be elaborated.
o
Having VSC on the inverter end not only avoids commutation failure but also offers independent control of active and reactive power. With LCC on one end, the cost of the system will be brought down. The different controllers used here are:
Vac
Vvsc
Current controller
A.
Inverter iVSC based)
Rectifier iLCC based)
The current controller that is widely used in LCC system is used here at the rectifier. It is basically used to obtain constant current characteristics. The operation of the controller is based on measurement of the dc system current and
Fig. 1. LCC-VSC hybrid HVDC system
comparing to a reference value. The resultant current error is then fed to the PI regulator. It will control the triggering angle
A.
of the thyristors, in turn maintaining rated voltage and dc
Converters The hybrid HVDC system uses LCC based rectifier and
VSC
based
inverter,
commutation failure.
thus
overcoming
In rectifier,
the
problem
of
B. DC voltage controller at inverter
converters are thyristors
based with 12 Pulse bridge and depend on AC circuit for its commutation. The output DC voltage of the converter is controlled by controlling the triggering angle of the thyristors. The inverter uses VSC which is based on IGBT. These devices are operated using PWM technique having high switching frequencies. This system has better controllability compared to LCC based systems. B.
power.
DC link voltage of VSC can be maintained by controlling charge on the large capacitors located on the dc side of the VSC. This controller is used to achieve active power balance during
it is a 12 pulse converter. They are connected in Y-Y and V-A configuration to eliminate 5th and
DC
voltage
is
controlled
by
receiving end converter.
DC voltage controller will maintain
the balanced DC power throughout the DC link. A simple PI
ih harmonics. The voltage
controller
changer, which will maximize the reactive power flow. At the receiving end, 6 pulse converters are used with a single transformer of A-Y configuration to have reduced lower order harmonics.
output
signal
is
used
in
reference
sine
wave
generation part of inverter. C. A C voltage controller at inverter
level on the inverter side can be controlled using a tap
AC Voltage controller is required in a system to limit the over voltages, flickers and commutation failure. The AC voltage measured at inverter end is measured and compared with a reference voltage. The error signal is used to get the modulation index of the converter after it is processed in a PI controller. Modulation index a ratio of reference wave to the
C. A C Filter
fundamental
The
controller can control power flow by adjusting the phase shift
Two identical transformers are used at rectifier side since
AC
transients.
angle between converter voltage and ac system voltage. The
Converter Transformer
The
the
adjusting the phase angle of the AC side voltage of the
voltage AC
output
component
of plus
the
VSC
contains
higher-order
the
harmonic,
derived from the switching of the IGBT's. These harmonics need to be eliminated.
carrier wave which determines the magnitude and frequency of the output AC wave in a PWM controlled converter. D. Reactive power controller
AC filters are thus used to obtain the
desired levels of individual harmonic distortion level total harmonic distortion (THO) and telephone influence factor (TIF). The filter configuration varies from application to application. They also serve as source of reactive power. D. DC link Capacitor The PWM switching of high power VSC introduces the
Reactive power controller is required to reduce dynamic over voltages. It improves the system performance and brings back the system into stable state, quickly. Reactive power of a system is dependent on magnitude of the AC voltage and hence it is used to get the modulation index of the PWM waveform. With this control, the HVDC system is made self sustaining in reactive power supply to the loads.
harmonics in the DC current flowing in the transmission line. This generates the harmonics in the DC voltage which is strongly related to the converter AC voltage. To eliminate these harmonics DC capacitors are used on the DC side of VSC system. The main objective of the DC capacitors is to provide an energy buffer to keep the power balance during transients and reduce the voltage ripple on the DC side.
V.
SYSTEM STUDY
The paper presents two types of control schemes for the LCC-VSC hybrid HVDC system. In order to understand the performance of the controllers, the system under consideration is studied dynamically for both the sets of controllers and the performance of the system is compared. The specification of the LCC-VSC hybrid system is shown below: Pdc
=
300 MW, Vdc
=
110 k V, Idc
=
2.7 kA, Cdc
=
2000 flF
Rectifier:
VI.
AC source: 230 kV, 50 Hz, SCR
=
3.7,
RESULTS AND ANALYSIS
The dynamic system analysis is carried out for the LCC
Transformers: 300 MVA, 230/55 kV
VSC hybrid HVDC system for both the proposed sets of controllers. The steady state performance curves are shown in
Inverter: AC source: 230 kV, 60 Hz, 32.52/80° n, SCR
=
Fig. 2. A few indicative transient conditions are simulated to
5.42
check the performance of the controllers being used. The
Transformer: 300 MVA, 115 /230 kV
different transient studies for hybrid system with first set of controllers are explained below.
Controllers:
Controller 1
Converter Rectifier
Inverter
(LCC)
The single line to ground or unbalanced fault is created at (0.05 sec). The performance curves are shown in the Fig 3.
Current control
During fault condition, current dips to 2.2 kA and reactive
AC voltage control
Reactive power control
and
and
DC voltage control
Single line to groundfault
2.5 sec at inverter and the duration of fault is 2 and half cycles
Controller 2
Current control
(VSC)
A.
CONVERTER CONTROLLERS
TABLE I.
power dips to -1000 MVAR. Voltage peak is 165 kV. The system recovery is quick and recovers within 0.15 second.
DC voltage control
B.
Three phase to groundfault The balanced or three phase to ground fault is created at
2.5 sec at inverter and duration of fault is 2 and half cycles
I- dcVotta!!e Inv
- DC Votts Rect
GO()
•
50()
reactive power goes up to -3000 MVAR. Current dips to zero
40()
� '" '" 2
�
and recovery is slow. It takes 1.1 sec to come back to steady
30()
state. Similar transient studies are carried out for the hybrid
20() 10() ()
r-
system with second set of controllers and the results are
II
presented.
-10()
VII.
-20() _30()
I- dclinelnv
- DC Cu rrent Rect
4.()
•
1: �
::;
U U 0
1.() ().()
is modelled separately for the purpose of comparison. It can
11\ i V
be observed that,
control. It is seen that three phase to ground fault at the ,
1 ()
,
,
2.()
3 ()
,
4.()
5.()
I•I G ra p h s
I- Piny
- Prect
inverter end is critical. During this condition, in VSC system,
,
voltage peak is up to 330 kV and it takes around 1.6 seconds
-I •
c;; '"
10()
0..
()
20()
'" " U '"
OJ
voltage rise is only around 225 kV, and the recovery is very
I I III 1/ ,
fast within 0.3 sec. The above comparison will say that the reactive power control and DC voltage control at the inverter
-10()
end would be a better choice for the better performance of
-20 1}
LCC-VSC hybrid HVDC system.
-30()
().5k
•
Qinv
•
1.0k
0..
voltage goes up to 550 kV and also it takes 0.8 seconds to reactive power controller is used at the inverter end, the
40() 30()
to recover. Considering hybrid system with controller 1, the recover, but in the hybrid system with controller 2, where
50()
0
in
reactive power goes to -1550 MVAR, but in LCC-VSC hybrid
GO()
c;; '"
ground fault
HVDC system with controller 2, reactive power is well under
Rectifier Inverter
�
to
-2.()
-I
:li!
single line
-1.()
, ().()
'" '"
during
hybrid system with controller 1 and in VSC system, the
-4 . ()
0
The comparison of transient performance of two proposed HVDC system with dc values as 300 MW, 110 kV, and 2.7 kA,
-3.()
f
COMPARISON OF CONTROLLER PERFORMANCE
controllers with VSC-HVDC system is briefly done. A VSC
3.() 2.()
�
(0.05 sec). The performance curves are shown in the Fig 4. During fault condition, DC voltage peak is 550 kV and
VIII.
-
().()
CONCLUSIONS
-().5k
The LCC-VSC hybrid HVDC system is observed to have a
-1.0k
lower cost and would give less switching losses compared to
-1.5k
VSC-HVDC system since VSC system is used only on
-2.0k
inverter side of the hybrid system. The work is concentrated
-2.5k -3.0k
,
,
().()
1 ()
,
2.()
,
3 ()
,
4.()
,
5.()
1·1
-I Fig.
2. Steady state perfonnance curves
on control part of the hybrid system and a brief comparison is made. By the evaluation of transient state results of the above studied hybrid systems,
it is observed that system with
reactive power control and DC voltage control at the inverter end shows a better performance and fast response.
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