Download HVDC Technology 1.Introduction The development of HVDC(High

HVDC Technology
1.Introduction
The development of HVDC(High Voltage Direct Current) transmission system dates to the 1930’s when
mercury arc rectifiers were invented. In 1941 , the first HVDC transmission system contract for a commercial
HVDC system was placed: 60 MW were to be supplied to the city of Berlin through an underground cable of 115
km in length. In 1945, this system was ready for operation. However, due to the end of World War II, the system was
dismantled and never became operational. It was only in 1954 that the first HVDC (10 MW) transmission system was
commissioned in Gotland. Since the 1960s, HVDC transmission system is now a mature technology and has
played a vital part in both long distance transmission and in the interconnection of systems.
HVDC transmission systems, when installed, often form the backbone of an electric power system. They
combine high reliability with a long useful life. Their core component is the power converter, which serves as
the interface to the AC transmission system. The conversion from AC to DC, and vice versa, is achieved by
controllable electronic switches (valves) in a 3-phase bridge configuration.
An HVDC link avoids some of the disadvantages and limitations of AC transmission and has the following
advantages:
. No technical limit to the length of a submarine cable connection.
. No requirement that the linked systems run in synchronism.
. No increase to the short circuit capacity imposed on AC switchgear.
. Immunity from impedance, phase angle, frequency or voltage fluctuations.
. Preserves independent management of frequency and generator control.
. Improves both the AC system’s stability and, therefore, improves the internal power- carrying capacity, by
modulation of power in response to frequency, power swing or line rating.
Figure 1.1 shows example applications of HVDC transmission systems in which the labeling is as follows:
1.
2.
3.
4.
5.
6.
7.
8.
Power transmission of bulk energy through long distance overhead line.
Power transmission of bulk energy through sea cable.
Fast and precise control of the flow of energy over an HVDC link to create a positive damping of
electromechanical oscillations and enhance the stability of the network by modulation of the transmission
power by using a Back-to-Back.
Since an HVDC link has no constraints with respect to frequency or to phase angle between the two AC
systems, it can be used to link systems with different frequencies using an Asynchronous Back-to-Back.
When power is to be transmitted from a remote generation location across different countries or
different areas within one country, it may be strategically and politically necessary to offer a connection to
potential partners in the areas traversed by using a multi- terminal DC link.
An HVDC transmission system can also be used to link renewable energy sources, such as wind power, when
it is located far away from the consumer.
VSC (Voltage-Source Converter) based HVDC technology is gaining more and more attention. This new
technology has become possible as a result of important advances in the development of Insulated Gate Bipolar
Transistors (IGBT). In this system, Pulse-Width Modulation (PWM) can be used for the VSC as opposed to the
thyristor based conventional HVDC. This technology is well suited for wind power connection to the grid.
Since reactive power does not get transmitted over a DC link, two AC systems can be connected
through an HVDC link without increasing the short circuit power; this technique can be useful in generator
connections.
2.Advantages and Disadvantages of HVDC over AC
2.1.Advanteges of HVDC over AC
The classical application of HVDC systems is the transmission of bulk power over long distances
because the overall cost for the transmission system is less and the losses are lower than AC transmission. A
significant advantage of the DC interconnection is that there is no stability limit related to the amount of power
or the transmission distance.
Long Distance Bulk Power Transmission. When large amounts of power are to be delivered
over long distances, DC transmission is always an alternative to be considered. AC transmission becomes limited by:
.
.
.
Acceptable variation of voltage over the transmission distance and expected loading levels.
Need to maintain stability, that is, synchronous operation across the transmission, after a disturbance, both
transiently and dynamically.
Economic effects of additions necessary to correct the above limitations.
The DC line, requiring as few as two conductors (one only for submarine with earth return) compared to
the AC line’s use of three, requires a smaller right of way and a less obtrusive tower. Figure 1.2 shows
schematically the tower configurations for 1200 MW (two circuits AC,
Figure 1.2 Tower configurations for AC and DC transmission.
Despite alternating-current being the dominant mode for electric power transmission, in a number of
applications HVDC is often the preferred option.
•
Undersea cables (Ex: 250 km Baltic Cable between Germany and Sweden)
•
Increasing the capacity of an existing power-grid in situation where additional wire are difficult or
expensive to install
•
Allowing power transmission between unsynchronized AC distribution systems.
•
Reducing the profile of wiring and pylons for a given power transmission capacity
•
Stabilizing a predominantly AC power-grid
Long undersea cables have a high capacitance. This causes AC power to be lost extremely quickly in
reactive and dielectric losses, even on cables of a modest length.
HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC
line is lover than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor
spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power
consumption, which can lower the costs!
2.2. Disadvantages of HVDC over AC
The disadvantages of HVDC are in conversion, switching ,control ,availability of link capacity and
maintenance.
HVDC is less reliable and has lower availability than AC systems, mainly due to the extra conversion
equipment. Single pole systems have availability of about 98.5%, with about a third of the downtime unscheduled
due to faults. Fault redundant bipolar systems provide high availability for 50% of the link capacity, but availability
of the full capacity is about 97% to 98%
The required static inverters are expensive and have limited overload capacity. At smaller transmission
distances the losses in static inverters may be bigger than in an AC transmission line!
In contrast to AC systems ,realizing multi-terminal systems is complex, as is expanding existing schemes to
multi-terminal systems.
High voltage DC circuit breakers are difficult to build because some mechanism must be included in the
circuit breaker to force to zero!
Operating a HVDC scheme requires many spare parts to be kept, often exclusively for one system as HVDC
systems are less standardized than AC systems and technology changes faster!
3. HVDC System Costs
It is very much cost effective for a long distance DC power transmission compared to AC power transmission.
In case of undersea cables where the intersections of the bold lines are located at a relatively short distance as
shown in Figure 1.3, the DC system is much more economical.
In Figure 1.3,(1) illustrates the initial cost for HVAC power transmission and (2) illustrates the initial cost of
HVDC power transmission with a bigger initial cost due to a higher valve cost for HVDC transmission. In addition, (3)
and (5) represent the cost for transmission line construction in HVAC and HVDC power transmissions, respectively
and they demonstrate that HVDC power transmission has a lower cost for transmission line construction.
In the case of HVAC power transmission, a shunt capacitor must be installed typically at every 100 km or
200 km because of its electrostatic capacity. In other words, the increase in total cost for power transmission lines is
accompanied by additional costs due to shunt capacitors. In the same Figure 1.3, (6) and (7) illustrate losses of
HVDC and HVAC systems during power transmission. It is shown that an HVDC system has a smaller loss if the same
amount of electric power is delivered. Therefore, HVAC transmission is favorable for distances less than about 450 km
and HVDC transmission is favorable for distances exceeding 450 km.
Figure 1.3 Transmission distance and investment costs for AC and DC power transmission lines
4.Overview and Organization of HVDC Systems
HVDC transmission refers to that the AC power generated at a power plant is transformed into DC
power before its transmission. At the inverter (receiving side), it is then transformed back into its original AC
power and then supplied to each household. Such power transmission method makes it possible to
transmit electric power in an economic way through up-conversion of voltage, which is an advantage in
existing AC transmission technology and to overcome many disadvantages associated with AC power
transmission as well. The overall structure of an HVDC system is as shown in Figure 1.4 and its basic
components are described below.
AC Breaker. This is used to isolate the HVDC system from the AC system when the HVDC
system is malfunctioning. This breaker must be rated to carry full load current, interrupt fault current, and
energize the usually large converter transformers. The purposes of this breaker are for the interface between AC
switch yards or between AC bus-bar and HVDC system
(Figure 1.5).
AC Filters and Capacitor Bank. The converter generates voltage and current harmonics at both the AC and
DC sides. Such harmonics overheat the generator and disturb the communication system. On the AC side, a
double tuned AC filter is used to remove these two types of harmonics. In addition, the reactive power sources
such as a capacitor bank or synchronous compensator are installed to provide the reactive power necessary
for power conversion (Figure 1.6).
Figure 1.4 Basic structural diagram of a bipolar HVDC system.
Current (i)
Zero crossing 60/s (60 Hz)
Time (t)
Figure 1.5 Blocking at the zero crossing of AC current.
Figure 1.6 Double tuned AC filter for the 11th and 13th
Figure 1.7 Three-winding converter transformer
4.1. Converter Transformer.
This transforms the voltage from the AC system to be supplied to the DC system. It also provides a separation
between the AC and DC system. Specifically, when the two units of 6 pulse converters are serially connected to
generate a 12 pulse output, a 3-winding converter transformer is used.(Figure 1.7).
4.2.Thyristor Converter.
A converter, which is an essential component of HVDC power transmission, is developed using power
electronics. It is one of many research areas dealing with the transformation and control of power by switching
devices in the power converter. It performs the conversion from AC to DC or from DC to AC. It is mainly comprised
of a valve bridge and a transformer with a tap converter. Figure 1.8 shows the thyristor converter installed and
operating in Cheju Island. Its thyristor stack is configured with 6-pulses or 12-pulses and it is connected to the
voltage valve (Figure 1.9).
4.3.Smoothing reactors and DC Filters.
The smoothing reactor reduces the DC ripple current to prevent it from becoming discontinuous at low power
levels. Also, the smoothing reactor forms an integral component, together with the DC filter, to protect the
converter valve during a commutation failure by limiting the rapid rise of current flowing into the converter.
4.4.HVDC Controller Structure.
Figure 1.10 shows the basic control diagram of an HVDC system. An HVDC system can be divided into
several levels. Master control determines the power order or frequency order and calculates the current order for both
poles. Then, the current order that was received from master control is modified by control functions and limits in pole
control. Valve group control consists of a converter control and a valve firing control. The converter control
includes the current controller. The valve firing control distributes the firing signal to all thyristors.
4.5.Line Commuted Current Source Converter and Voltage Source Converter
Line Commutated Current Source Converter (LCC), as shown in Figure 1.11, consists of a 12-pulse
converter, AC filter and synchronous compensator. LCC depends on the AC system voltage for its proper operation.
LCC operates at a lagging power factor, because the firing of the converter has to be delayed relative to the voltage
crossing to control the DC voltage.
Figure 1.12 shows the concepts of Voltage Source Converter (VSC). VSC is based on forced commutated
devices that is, IGBTs or GTOs, which allows converter operation in all four quadrants of the P–Q plane. Since
commutation can be achieved quickly and independently of the AC system voltage, an entirely different type of
operation compared to the LCC converter is possible.
Figure 1.8 Thyristor converter
Figure 1.9 Thyristor stack
Figure 1.10 Basic control diagram of an HVDC system.
4.6.Point to Point System.
Most HVDC systems fall under this category. It consists of either cable or overhead lines or a combination of
these two. This type of system has one of the forms shown in Figure 1.13, depending on the number of overhead
lines and the polarity. Mono-polar HVDC. This type of HVDC link consists of a single conductor and a return path
either through the ground or sea.
This method is mostly used for power transmission using cables. Use of this type of system is dictated by
the costs of installing the cable. A metallic return path is preferred instead of through the ground when the
ground resistance is too high or the underground/undersea metallic components may cause some interference.
Bipolar HVDC. It consists of two poles, one positive polarity and the other negative polarity, and with their neutral
points grounded.
In steady state operation, the current flowing in each pole is the same and hence no current flows in the
grounded return. The two poles may be operated separately. If either pole malfunctions, then the other pole can
transmit power by itself with ground return. In a bi-pole the amount of power transmission is increased by a factor
of two compared to the mono-polar case. This creates fewer harmonics in normal operation as compared
to the mono-polar case. Reverse power flow can be controlled by converting the polarities of the two poles.
4.7. Back-to-Back system.
In this type of system, the rectifier and the inverter are located in the same station. In general, it is used for providing
an asynchronous interconnection for two AC systems. The amplitude of DC voltage is generally small, around 150 kV to
optimize the valve costs .
(A)
(B)
Figure 1.11 Operational characteristics of a line-commutated current-source HVDC system.
4.8.HVDC Multi-Terminal.
This refers to an HVDC system that consists of three or more transforming stations. Its architecture is
more complex compared to that of a two terminal point-to-point system. It requires a significant complexity to
facilitate communication and control between each transforming station. However, it is considered to be a
relatively new technology and has potential for a wide range of applications in the future. There are two types of multiterminal links – a parallel or serial type.
Figure 1.12 Operational characteristics of a voltage-source HVDC system.