Download ENHANCEMENT OF TRANSIENT STABILITY FOR AN AC/DC

ENHANCEMENT OF TRANSIENT STABILITY FOR AN AC/DC
INTER-CONNECTED SYSTEM USING HVDC LIGHT
K. Lalaiah1, N. Gangaswami2
1Assistant
2Student
professor, Electrical Engineering, SIET, Telangana, India
M.Tech, Electrical Engineering, SIET, Telangana, India
ABSTRACT: The necessity to deliver cost
effective energy in the power market has
become a major concern in this emerging
technology era. Therefore, establishing a
desired power condition at the given points
are best achieved using power controllers
such as well-known High Voltage Direct
Current (HVDC) transmission systems.
High voltage direct current transmission is
an economic way for long distance power
delivery
and/or
interconnection
of
asynchronous grids. The system planner
must
consider
DC
alternative
in
transmission expansion. The factors to be
considered are cost, Technical performance,
and Reliability.
A HVDC Light is a new technology
utilizing forced-commutated voltage source
converters which can control both active
and reactive power independently without
commutation failures in the inverter side. It
doesn’t
require
reactive
power
compensators resulting much smaller
equipment size. HVDC Light can be applied
to the voltage support in the receiver
systems,
interconnection
between
asynchronous power systems, and grid
connection of large wind farms of offshore
wind farm and subsea power transmission.
HVDC light can bring a new approach for
the AC/DC interconnected transmission
systems and multi- in feed HVDC
transmission system.
Keywords - power controllers, High Voltage
Direct Current (HVDC), asynchronous grid,
forced-commutated voltage source converters,
I.
INTRODUCTION
High Voltage Direct Current (HVDC)
transmission systems are now getting more
popular due to their low power losses and
fast controllability of power in DC lines as
compared to AC systems. In order to
protect equipment from overvoltage, surge
arresters are installed in the HVDC system
to limit the voltage across the terminals of
the equipment. The nonlinear resistors
should have high resistance for normal
condition to avoid negative effects on the
system and low resistance during surge so
that overvoltage is limited. It is more
economical for the HVDC transmission
system to transfer more power as the
power factor is almost near to unity and
the energy loss is low. Surge arresters are
useful to overcome over voltage and
protect the transformers. HVDC Light is a
new
technology
utilizing
forced-
commutated voltage source converters. It
can enhance the voltage support and
improve the system stability. HVDC
systems are responding and can be
controlled within tens of milliseconds.
HVDC Light is composed of transformer,
filters, converters and DC capacitors.
Transformer is used to step down the AC
voltage to satisfy the demand of self
commutated
solid-state
devices.
An
AC/DC parallel transmission system and a
three in feed HVDC system are developed
by MATLAB/SIMULINK platform. The
proposed method is feasible and effective
control scheme.
II.
Normally, HVDC system operates in
constant power control mode. Power order
is given by the user. Current order (I order)
derived from the power controller, which
is send to the VDCOL (voltage dependent
current order limiter) and into the current
control amplifier (CCA). The alpha order
HVDC
TRANSMISSION
from the CCA is send to the converter
firing control which determines the firing
SYSTEM
A high-voltage,
electric
HVDC control
direct
power
current (HVDC)
transmission system
uses direct
current for
transmission
of
electrical
the
power,
bulk
in
contrast with the more common alternating
current systems.
For
instant of valves.
The primary function of HVDC controls
are:

long-distance
between the terminals under steady
transmission, HVDC systems may be less
expensive and suffer lower electrical
state and transient operation.

losses. HVDC allows power transmission
between unsynchronized AC distribution
Fast protection of ac and dc system
faults.

systems, and can increase system stability
by preventing cascading failures due to
Fast and flexible power control
It minimizes over voltage across
the valves.

It reduces the short circuit current
phase instability from propagating from
through the valves and lines/cables
one part of a wider power transmission
it reduces the reactive power
grid to another. HVDC also allows transfer
consumption.
of power between grid systems running at
different frequencies, such as 50 Hz vs.

Avoids
repetitive
commutation
failures.
60 Hz. Such interconnections improve the
The limits of the P-I controllers as shown
stability of each grid, since they increase
in figure 3.3, are shown in the P-I blocks
the opportunity for any grid experiencing
only for the emphasis. However these
unusual loads to stay in service by drawing
limits are treated within the P-I algorithm
extra power from otherwise completely
used for simulation studies and the P-I
incompatible grids.
controllers used are non-wind-up. The top
one is the current controller and the bottom
one is the constant angle controller. The
outputs of these two are fed into a
minimum selector. The smaller of the two
firing angles generated, is then selected as
the inverter firing angle.
Figure 2.2 Power flow control loop
For controlling the DC power, the current
reference in the current controller is
derived from the ratio of the power order
and the inverter side DC voltage, as shown
in figure 2.2
III.
Fig.2.1 HVDC Control Diagram
HVDC LIGHT
Thus the minimum selector ensures that
Unlike conventional HVDC scheme that
only CC or CEA is active. This minimum
employs line commutated current source
selector does the controller switch over
converters with thyristors, HVDC Light is
from CEA to CC and vice versa. Besides
a
when one of the controllers is selected, the
commutated voltage source converters. By
other becomes saturated at its limit.
pulse wide modulation (PWM) control,
Inverter side DC voltage can be estimated
from
the
rectifier
side
voltage
by
subtracting the DC line voltage drop. The
rectifier substation as shown in figure 2.2
is provided with a power controller. The
current reference is calculated by dividing
measured dc power by measured dc
voltage. The upper limit of the dc
reference current is further adjusted by the
measured dc voltage.
new
technology
utilizing
forced-
HVDC Light realizes independent control
of active and reactive power control, and
does not need reactive compensation in
both rectifier and inverter side. It can also
be used with static synchronous series
compensation (SSSC) characteristics to
damp the power system oscillations.
HVDC Light applies self commutated
solid-state
device,
so
there
are
no
commutation failure issue. For AC/DC
interconnected transmission systems, the
introduction of HVDC Light can enhance
---- (1)
the voltage support and improve the
system stability. Two simulation models
Where Sb is the apparent power of HVDC
have been set up with HVDC Light built
Light
in; one is an AC/DC parallel transmission
P is the active power
lines system, and the other is a multi in
feed HVDC systems.
Q is reactive power.
HVDC Light is composed of transformer,
UF is the voltage of AC system
filters, and converters and DC capacitors.
UC is the output voltage of converter;
Transformer is used to step down the AC
voltage to satisfy the demand of self
ZR is the equivalent impendence of the
commutated solid-state devices, such as
converter system including the transformer
series and parallel of GTOs, IGBTs or
and reactors.
IGCTs.
High
frequency
components
caused by the switches of valves are
Active power and reactive power are given
respectively as:
isolated from power system by filters. The
key parts of HVDC Light are converters,
---- (2)
which can realize the conversion from AC
to DC bi-directly. DC capacitors are used
---- (3)
as the DC voltage source in HVDC Light,
which need being charged and recharged.
The rectifier controller is shown in fig3.2.
The controller Consists of four parts:
power flow control loop, reactive power
control loop, phases locked loop (PLL),
and PWM pulse firing. After a time delay,
the Pdc is compared with the desired DC
power Pdcref, and the comparator outputs
the error signals to the power flow
controller. The measured reactive power
Fig 3.1 HVDC Light converter
and the desired reactive power Qref are the
input of the controller. The outputs of
active and reactive power control loop are
The power transmitted by HVDC Light
is given as:
the inputs of PWM pulse generator. PLL
provides the synchronous signal of pulse
and they are interconnected according to
generator. PWM pulse generator sends the
the circuit flow and output is simulated
pulse signals to drive the valves in the
using the software and the results are
converter. The measured reactive power
shown in the oscilloscope.
and the desired reactive power Qref are the
input of the controller. The outputs of
active and reactive power control loop are
the inputs of PWM pulse generator. PLL
provides the synchronous signal of pulse
generator. PWM pulse generator sends the
pulse signals to drive the valves in the
converter.
Fig 3.2 the rectifier controller
Fig.4.1 Power response of HVDC light
It indicates the response of a power for
IV.
SIMULATION AND RESULTS
The HVDC LIGHT system has been
simulated
using
MATLAB-Simulink
software.
The control technique of HVDC Light
system i.e. PWM control, PI controller is
modeled
using
MATLAB
Simulink
software. This is done by using a set of
equations and several blocks are modeled
HVDC system for 10% change in power.
It shows that at 0.5 sec a fault is created
which is cleared at 1 sec.
Fig 4.3 Voltage and power responses for 10%
change in power
From the fig 4.3, compare the response of
voltage and power for traditional and HVDC
Fig 4.2 Fault analysis of HVDC Light
During the severe three-phase fault at station
2 at t = 0.5 s, the power is decreased to 0.5 Pu
during the fault and recovers fast and
successfully to 1.0 Pu after clearing the fault.
The transmitted power flow is reduced to
very low value during the fault and recovers
to 1.0 Pu after the fault.
light systems the HVDC light is giving better
performance than traditional system.
Fig 4.4 Voltage and power responses for 10%
CONCLUSION
change in voltage
HVDC Light is a novel power electronic
In the above result at t=0.5 s when the fault is
created the response of voltage and power are
shown. In which Traditional system is getting
more oscillations and recovering slowly when
compared to HVDC light system. For HVDC
light the responses are coming into steady
state at t=0.6 s itself, where as traditional
system is still in oscillating in nature.
device, which utilizes the technology of
VSC converter. By PWM control model, it
can control the output of active power and
reactive power independently. It is the
major difference from line commutated
HVDC transmission system. In AC/DC
interconnected transmission system, the
rotor angle stability of power systems can
be improved by the control of active
power. The AC voltage in inverter side can
be supported by the control of reactive
power.
In
multi-indeed
HVDC
transmission system, system faults can
cause commutation failure in several
HVDC systems. With the capability of
voltage support from HVDC Light, the AC
voltage can be supported to prevent the
commutation failure in some HVDC
systems. As a conclusion, HVDC Light is
a
novel
approach
to
improve
the
performance and stability of power system.
Fig 4.5 voltage & power responses for three
It has bright future to be applied in power
phase fault
systems in many fields.
The above fig 4.5 shows three phase fault.
The
experience
from
both
The fault clearing time is less for HVDC light
commissioning and operation so far shows
(at t=0.6s) but Traditional model is taking
that HVDC Light transmission based on
long time to come to steady state (at t= 1 s).
VSC techniques is a promising concept for
connecting large wind farms to ac systems.
However, at the design stage of the
windmills the dynamic speed increase at ac
Power Converter Introducing Zero-Current
system faults must be considered
Soft Switching Technique, " lET Gener.
Transm. Distrib. vol. 3, no. 4, pp. 315-324,
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