B4-208
21, rue d'Artois, F-75008 Paris
http://www.cigre.org
Session 2004
© CIGRÉ
System Benefits derived from the 500MW Back to Back HVDC scheme at
Sasaram, India
R N Nayak1, D Kumar1, B N Kayibabu3*, R Gulati2, M H Baker3
1
PowerGrid Corporation of India Ltd (India)
2
AREVA T&D (India)
3
AREVA T&D Power Electronic Activities (UK)
Abstract - The paper describes the Sasaram HVDC back-to-back scheme which connects the
northern and eastern electrical regions in India. Reactive power and AC voltage support,
overload control, frequency control and power modulation are among the functions available
from this scheme to support the two AC systems.
Keywords- HVDC, Modulation, Interconnection, Frequency control.
SASARAM
EAST
SASARAM
N ORTH
400kV
400kV
507.4MW
205kV
112Mvar
Double Frequency
112Mvar
Double Frequency
112Mvar
Double Frequency
*
112Mvar
Double Frequency
Figure 1:
112Mvar
Double Frequency
BIHARSHARIFF BIHARSHARIFF
LINE 1
LINE 2
112Mvar
Double Frequency
112Mvar
Double Frequency
57.1Mvar
Reactor
112Mvar
Double Frequency
57.1Mvar
Reactor
57.1Mvar
Reactor
ALLAHABAD
LINE
57.1Mvar
Reactor
SARNATH
LINE
Simplified Single Line Diagram
bruno.kayibabu@areva-td.com
1
SCHEME DESCRIPTION
The Sasaram Back-to-Back HVDC transmission scheme links the northern and eastern 400kV
AC networks in India, each of which terminates in a breaker-and-a-half busbar on the Sasaram
site. One 400kV line to Allahabad and one 400kV line to Sarnath form the points of connection
to the northern network and two 400kV lines to Biharshariff form the connection point to the
eastern network, as shown in Figure 1. A bypass link is provided via the two filter busbars,
whereby the two AC networks can be connected together if required.
The continuous rating of the scheme at maximum ambient temperature is 500 MW. 550MW can
be carried for up to 2 hours, or 667 MW for up to 5 seconds. During low ambient temperature,
the continuous capability increases to a maximum of 650 MW. The thyristor valves are rated for
205kV, each valve containing 50 thyristors rated 5.2kV. The converter transformers are of single
phase, 3 winding type, and are rated for 234MVA each.
The thermal capacity of the equipment is monitored continuously and utilised to achieve these
power transfers, starting from any steady state operating condition. Currents greater than the
steady state maxima are employed during the time required to reposition the converter
transformer tap-changer.
There is no DC reactor1. Discontinuous current operation may occur for brief periods (during
disturbances), but not for long enough to dictate the design of the on-valve electronics, which are
well able to provide the necessary additional firing pulses to maintain conduction for short times.
Multiple Pulse Resistor protection (MPRP) is provided to prevent the damping resistors of BOD
fired levels in the thyristor valves from experiencing thermal overload under these rare events.
The settings of this protection depend on initial conditions and the number of pulses per cycle
experienced by the thyristor valves. These settings are within the withstand capabilities of the
thyristor valve components, which have thermal time-constants sufficiently long to accommodate
this operating condition.
Valve hall fault currents are limited, mainly by the converter transformer reactance, to values for
which records of valve proving tests are already available. To protect the converter equipment
from exposure to externally applied overvoltage, the switchyard is protected from direct lightning
strokes by shielding, and the voltage between the terminals of all major equipment such as
converter transformers is limited by the presence of gapless zinc oxide surge arresters.
To protect the environment from the action of the converter station, the screened valve hall is
treated as a "containment" for high frequency noise generated by converter switching. The
converter transformer 400kV bushings are connected to the main busbars via high frequency
filters, which limit the exposure of the 400kV network to the effects of converter-generated
interference in the power line carrier and higher frequency ranges.
CONTROL SYSTEM AND OPERATIONAL FACILITIES
The Sasaram HVDC back-to-back scheme uses ALSTOM T&D’s latest state-of-the-art fully
digital controller, which is fully redundant, the dual lanes being based on VME technology
using DSPs for fast data acquisition and processing and Pentium processors for general
control.
The HVDC control system is responsible for the operation of the main scheme equipment
(thyristor valves, transformer tapchangers and breakers for filters and shunt reactors). It
ensures that the equipment is operated within its limits to achieve the desired power transfer
within the specified control parameters. It is also responsible for the start up and shutdown
sequences of the scheme.
2
The operator interface is provided by an integrated and duplicated SCADA system and a
control mimic, which allow the station operator to apply station level commands to the
control system and to observe salient system conditions. This allows the operator to initiate
start or stop sequences, request power demands and amend operating conditions (e.g. voltage
or reactive power exchange target). The primary interface is a control mimic situated in the
control room. In addition SCADA workstations are installed which provide a back-up to the
mimic and data logging facilities. The SCADA system also allows some operational functions
to be carried out remotely via telecommunications links to either of the two Regions load
despatch centres.
Each lane contains a set of control system equipment and two redundant sets of pole
protection equipment, thus providing quadruplicate protection of the key functions. Although
the protection functions are grouped in with the lane concept, they are physically and
electrically separated from the equipment associated with the control system. The primary
outputs from the control and protection functions within each lane are a Firing Word and
Thermal Word (temperature information), which are connected optically to the Valve-based
Electronics (VBE).
VBE consists of two decoders/drivers and up to 18 transceiver cards. The VBE
decoders/drivers operate as a duplicate system. Both VBE decoders/drivers receive both
Firing and Thermal Words. Only one set of lane firing and thermal words are used,
determined by the Lane Select signals from the Changeover Unit.
Both VBE
decoders/drivers issue firing commands to the appropriate thyristor valves. The firing
commands from the VBE decoders/drivers are put in parallel through an OR gate before
being issued to the valves via the transceiver cards. Therefore, in the event of a VBE
decoder/driver failure an alarm is issued but scheme operation can continue with the
remaining single channel decoder/driver, until the next normally scheduled shut down.
Since there are two converters, a rectifier and an inverter, and most control functions are
specific to an individual converter on one side of the scheme. Separate Converter Control and
Protection (CCP) functions are provided for each converter.
Main Control System Functions
Station Control is responsible for controlling the influence of the scheme on each AC system.
Such control features are station power transfer, system voltage, system frequency, and
reactive power exchange. Sub-synchronous damping is included as a secondary control
function. Detailed functions of pole and phase controls are as earlier described for similar
schemes 2,3,4 and robust methods are used5.
DC Protection independently oversees the system and ensures equipment safety. Within each
CCP lane there are two parallel DC protection systems with the same functionality and both
are always active. If either system detects a condition where equipment integrity may be at
risk, protective actions are performed. Ultimately, a protective action can result in a converter
block and a main breaker trip.
Valve Base Electronics (VBE) contains the functions of firing pulse encoding, optical
transmission of firing information to the valves, optical reception of valve monitoring
information, and decoding of the monitoring information (databack). VBE receives the firing
information (“firing word”) from Phase Control in both CCP lanes but only uses the
information from the active control lane. VBE also receives the thyristor junction
temperature information from Thermal Model in CCP. Databack information from both
decoders is sent to the active CCP lane for thyristor level fault monitoring.
3
BENEFITS TO THE AC NETWORKS
The Sasaram HVDC link provides an asynchronous link between the two systems to enable
power to be exchanged at will in a reliable and controlled manner. Although both AC
networks to which the Sasaram HVDC back-to-back link is connected operate at the same
frequency, they are not always in synchronism. However synchronised operation is possible
with limited power transfer capability and security between the two. Apart from this
traditional capability of HVDC links, the Sasaram HVDC back-to-back scheme incorporates a
number of features which enhance and reinforce the networks to which it is connected. These
are summarised below.
Reactive Power and AC Voltage Support
The link is designed to provide extra reactive power capability to support both networks when
the need arises. This reactive power capability allows for the minimum number of AC filters
required to meet the harmonic performance and is dependent on the transmitted power as
shown in Figure 2 below. This capability can also be used to control the network voltage.
500
Reactive Power Capability (MVAr)
(Power into AC System)
400
300
200
100
0
0
20
40
60
80
100
-100
-200
-300
-400
Transmitted Power (%)
Maximum
Figure 2:
Minimum
Station Reactive Power Capability
The minimum continuous power for the Sasaram scheme is 10% of the rated power. Below
this level the converter is not available for the control of reactive power and therefore the
capability of the station is purely determined by the sizes of the reactive power elements
(absorption capability of 114MVAr, generation capability up to 448MVAr).
The AC filters for Sasaram have been designed such that only two filters are required to meet
the reactive power requirements over the full range of the transmitted power. Therefore at
low power levels, the converter needs to absorb extra reactive power if the target of the
reactive power exchange with the system is low or negative. For power levels below
approximately 60% of rated power, the reactive power absorption capability of the converter
station is about 100MVAr. Above this, the absorption capability increases with increased
transmitted power. This capability is achieved by operating the converter with higher control
4
angle and low direct voltage, maintaining the ordered power by increasing the direct current.
The limiting factors being the valve surge arrester PCAV (peak continuous applied voltage),
harmonics injected into the system and the thermal capability of the thyristor valves.
The Reactive Power Controller (RPC) incorporated in the HVDC control system can operate
in one of the two main control modes: Reactive power exchange mode (RPEM) or AC
voltage control mode (ACVCM). Under RPEM, the operator selects a reactive power
exchange target and the RPC will switch the reactive power elements (ac filters and shunt
reactors) as necessary to ensure that the reactive power exchange with the network is within
set limits, which are determined by the target value. Generally this requires switching of
reactive power elements. However when the target is low, the minimum number of filters is
reached and all available shunt reactors are energised, the converter is used to absorb the
surplus, by operating at reduced direct voltage.
Under ACVCM, the RPC controls the AC voltage to within a range around a target value
selected by the operator, subject to the inherent capability of the converter equipment, by also
using the reactive power elements and the converters as necessary. ACVCM is only available
on one side at any time with the other side normally operating in RPEM so that tight voltage
control using the converter firing angle control can be targeted at the priority Region,
normally the weaker region. Manual control of reactive power elements is possible but the
controller will automatically ensure that the minimum number of filters that are required for
harmonic performance remain energised.
Both these control features are available even when the converters are not transmitting any
real power. Therefore the converter station can be used as a pure reactive power or AC
voltage controller even when the converters are shut down.
If the AC voltage passes the steady state limits, the RPC will act automatically, regardless of
which control mode it is in, to minimise the voltage excursion by switching reactive power
elements or using the converters as necessary. The RPC overvoltage control is co-ordinated
with the busbar overvoltage protection and ensures that control actions are taken first.
Power Order
(out of ac system)
Power Order
(into ac system)
.
Po
Po
fo-'f
fo
fo 'f
Rectifier
Figure 3:
Frequency
f o -'f
fo
f o
'f
Frequency
Inverter
Frequency Control Characteristics
5
Frequency Control
The HVDC control system for Sasaram incorporates a frequency control function which is
always active in the background. The operator can set a target frequency (fo), the control
deadband (±∆f) and frequency slopes as shown in Figure 3. The converter station is normally
in power control where the power order (Po) is set by the operator. If the frequency on one
side of the scheme passes the limits determined by the frequency target and the control
deadband, then the scheme will automatically go into frequency control
The frequency slope allows the operator to apply a droop characteristic to the HVDC link
similar to that exhibited by the generators within the AC system. The frequency slope is
defined as the change in frequency corresponding to 1pu change in power. Thus, the
variations in frequency of the system will give rise to corresponding changes in the power
transfer through the link, thereby contributing to the power generation in the AC system.
Power Modulation
Generator Rotor Angle (degrees)
100
80
60
40
20
0
0
1
2
3
4
5
6
Time (s)
7
8
9
10
4
5
6
Time (s)
7
8
9
10
Generator Real Power (MW)
500
400
300
200
100
0
0
1
2
3
Figure 4:
System Recovery without DC Power Modulation
Rotor angles of generators in the North system (upper two traces) and the East system (lower
traces) are shown to have little or no effective damping when there is no modulation applied.
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The power modulation control is available within the HVDC control system and can be
selected by the operator to damp low frequency power swings within one or both networks.
Parameters are determined by system studies. Power Modulation operates through the
measurement of the changes in absolute phase angle of the connected network busbar
voltages. It produces an incremental change to the station power order corresponding to the
changes in the voltage phase angle in a sense to correct the changes. The contributions arising
from any power swings within both AC networks are summed to achieve optimum damping.
Figures 4 and 5 give the post-fault oscillations after two similar fault events near the eastern
side of the link without and with modulation applied.
Generator Rotor Angle (degrees)
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
Time (s)
9
10
Generator Real Power (MW)
500
400
300
200
100
0
0
1
2
3
4
5
6
7
8
Time (s)
9
10
Figure 5:
System Recovery with DC Power Modulation
Damping is introduced and the oscillations have faded within 4 to 6 seconds.
Overload Control
The control system includes an overload controller, which works out continuously the steady
state and short term overload capabilities of the converter station and makes this information
available to the operator and automatic controls through the mimic panel and the SCADA.
The overload capability of the scheme depends on many factors including the ambient
temperature, the availability of the redundant cooling, the loading history etc. This overload
capability can be utilised to support any demand by either AC system.
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CONCLUSIONS
The connection of the northern and eastern electrical networks in India using the Sasaram
HVDC back-to-back link allows secure and controlled transfer of high levels of power.
Furthermore, the Sasaram link includes features designed to support the connected AC
systems during steady state and transient conditions. These features include reactive
power/AC voltage control, overload control, frequency control and power modulation.
Power Modulation minimises power swings following clearance of major disturbance in the
AC systems.
ACKNOWLEDGEMENT
The authors wish to thank both PowerGrid Corporation of India Ltd and AREVA T&D for
permission to publish this paper and they wish to record that the views expressed herein are
their own and do not necessarily reflect those of the management of either company.
REFERENCES
[1]
[2]
[3]
[4]
[5]
R P Burgess, R Kothari, Design features of the Back-to-back HVDC converter
connecting the Western and Eastern Canadian systems (IEEE paper 89 SM 793-1
PWRD, July 1989).
B A Rowe, N M Kirby, H K Yu, Control system design at Chandrapur Back-to-back
HVDC station (CIGRE Colloquium on HVDC and FACTS, Montreal September
1995).
B R Andersen, D R Monkhouse, R S Whitehouse, J D G Williams, V K Prasher, D
Kumar, Commissioning the 1000 MW back-to-back HVDC link at Chandrapur,
India, (CIGRE Paper 14-114, Paris 1998).
K M Abbott, M Aten, Two HVDC schemes in close proximity; a coordination study
(CIGRE paper 14-109, Paris, 2000).
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schemes (Proc. IEE Generation Transmission and Distribution, Vol 150, No 6,
November 2003).
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