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. Power transmission of bulk energy through long distance overhead line.2. Power transmission of bulk energy through sea cable.3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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 HVDCsystem 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 ConverterLine 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 multi-terminal links – a parallel or serial type.

Figure 1.12 Operational characteristics of a voltage-source HVDC system.