In an AC system, voltage conversion is simple. An AC transformer allows high power levels and high insulation levels within one unit, and has low losses. It is a relatively simple device, which requires little maintenance. Further, a three-phase synchronous generator is superior to a DC generator in every respect. For these reasons, AC technology was introduced at a very early stage in the development of electrical power systems. It was soon accepted as the only feasible technology for generation, transmission and distribution of electrical energy.
However, high-voltage AC transmission links have disadvantages, which may compel a change to DC technology:
- Inductive and capacitive elements of overhead lines and cables put limits to the transmission capacity and the transmission distance of AC transmission links.
- This limitation is of particular significance for cables. Depending on the required transmission capacity, the system frequency and the loss evaluation, the achievable transmission distance for an AC cable will be in the range of 40 to 100 km. It will mainly be limited by the charging current.
- Direct connection between two ACsystems with different frequencies is not possible.
- Direct connection between two AC systems with the same frequency or a new connection within a meshed grid may be impossible because of system instability, too high short-circuit levels or undesirable power flow scenarios.
Line-Commutated Current Sourced Converters
The invention of mercury arc rectifiers in the nineteen-thirties made the design of line-commutated current sourced converters possible.
In 1941, the first contract for a commercial HVDC system was signed in Germany: 60 MW were to be supplied to the city of Berlin via an underground cable of 115 km length. The system with ±200 kV and 150 A was ready for energizing in 1945. It was never put into operation.Since then, several large HVDC systems have been realized with mercury arc valves.
The replacement of mercury arc valves by thyristor valves was the next major development. The first thyristor valves were put into operation in the late nineteen-seventies.Further development went via air insulated air-cooled valves to the airinsulated water-cooled design, which is still state of the art in HVDC valve design. The development of thyristors with higher current and voltage ratings has eliminated the need for parallel connection and reduced the number of series-connected thyristors per valve. The development of light-triggered thyristors has further reduced the overall number of components and thus contributed to increased reliability. Innovations in almost every other area of HVDC have been constantly addingto the reliability of this technology with economic benefits for users throughout the world.
Self-Commutated Voltage Sourced Converters
Voltage sourced converters require semiconductor devices with turn-off capability. The development of Insulated
Gate Bipolar Transistors (IGBT) with high voltage ratings have accelerated the development of voltage sourced converters for HVDC applications in the lower power range. The main characteristics of the voltage sourced converters are a compact design, four-quadrant operation capability and high losses.
The advantages of a DC link over an AC link are:
- A DC link allows power transmission between AC networks with different frequencies or networks, which cannot be synchronized, for other reasons.
- Inductive and capacitive parameters do not limit the transmission capacity or the maximum length of a DC overhead line or cable. The conductor cross section is fully utilized because there is no skin effect. For a long cable connection, e.g. beyond 40 km, HVDC will in most cases offer the only technical solution because of the high charging current of an AC cable. This is of particular interest for transmission across open sea or into large cities where a DC cable may provide the only possible solution.
- A digital control system provides accurate and fast control of the active power flow.
- Fast modulation of DC transmission power can be used to damp power oscillations in an AC grid and thus improve the system stability.
For a given transmission task, feasibilitystudies are carried out before the final decision on implementation of an HVAC or HVDC system can be taken. Fig.1-1 shows a typical cost comparison curve between AC and DC transmission considering:
- AC vs. DC station terminal costs
- AC vs. DC line costs
- AC vs. DC capitalized value of losses
The DC curve is not as steep as the AC curve because of considerably lower line costs per kilometer. For long AC lines the cost of intermediate reactive power compensation has to be taken into account.
The break-even distance is in the range of 500 to 800 km depending on a number of other factors, like country-specific cost elements, interest rates for project financing, loss evaluation, cost of right of way etc.
An HVDC transmission system is basically environment-friendly because improved energy transmission possibilities contribute to a more efficient utilization of existing power plants.
The land coverage and the associated right-of-way cost for an HVDC overhead transmission line is not as high as thatof an AC line. This reduces the visual impact and saves land compensation for new projects. It is also possible to increase the power transmission capacity for existing rights of way. A comparison between a DC and an AC overhead line is shown in Fig. 1-2.
There are, however, some environmental issues which must be considered for theconverter stations. The most important ones are:
- Audible noise
- Visual impact
- Electromagnetic compatibility
- Use of ground or sea return path in monopolar operation
In general, it can be said that an HVDC system is highly compatible with any environment and can be integrated into it without the need to compromise on any environmentally important issues of today.
The main types of HVDC converters are distinguished by their DC circuit arrangements as shown in Fig.2.1.
The following equivalent circuit is a simplified representation of the DC circuit of an HVDC pole. The current, and thus the power flow, is controlled by means of the difference between the controlled voltages. The current direction is fixed and the power direction is controlled by means of the voltage polarity. The converter is described in the next section.
The expression Back-to-back indicates that the rectifier and inverter are locatedin the same station.Back-to-back converters are mainly used for power transmission between adjacent AC grids which cannot be synchronized. They can also be used within a meshed grid in order to achieve a defined power flow as shown in fig.2.2.
Monopolar Long-Distance Transmissions
For very long distances and in particular for very long sea cable transmissions, a return path with ground/sea electrodes will be the most feasible solution.In many cases, existing infrastructure or environmental constraints prevent the use of electrodes. In such cases, a metallic return path is used in spite of increased cost and losses. For reference have a look at above fig.2.4.
Bipolar Long-Distance Transmissions
A bipole is a combination of two poles in such a way that a common low voltage return path, if available, will only carry a small unbalance current during normaloperation.
This configuration is used if the required transmission capacity exceeds that of a single pole. It is also used if requirement to higher energy availability or lower load rejection power makes it necessary to split the capacity on two poles.
During maintenance or outages of one pole, it is still possible to transmit part of the power. More than 50% of the transmission capacity can be utilized, limited by the actual overload capacity of the remaining pole.
The advantages of a bipolar solution overa solution with two monopoles are reduced cost due to one common or no return path and lower losses. The main disadvantage is that unavailability of the return path with adjacent componentswill affect both poles.
Bipole with Ground Return Path
This is a commonly used configurationfor a bipolar transmission system. The solution provides a high degree of flexibility with respect to operation with reduced capacity during contingencies or maintenance.
Upon a single-pole fault, the current of the sound pole will be taken over by the ground return path and the faulty pole will be isolated.
Bipole with Dedicated MetallicReturn Path for Monopolar Operation
If there are restrictions even to temporary use of electrodes, or if the transmission distance is relatively short, a dedicated LVDC metallic return conductor can be considered as an alternative to a ground return path with electrodes.
Bipole without Dedicated Return Path for Monopolar Operation
A scheme without electrodes or a dedicated metallic return path for monopolar operation will give the lowest initial cost.
Monopolar operation is possible by means of bypass switches during a converter pole outage, but not during an HVDC conductor outage. A short bipolar outage will follow a converter pole outage before the bypass operation can be established.
Functions of the HVDC Converter Transformer
The converter transformers transform the voltage of the AC busbar to the required entry voltage of the converter.
The 12-pulse converter requires two 3-phase systems which are spaced apart from each other by 30 or 150 electrical degrees. This is achieved by installing a transformer on each network side in the vector groups Yy0 and Yd5.At the same time, they ensure thevoltage insulation necessary in order to make it possible to connect converter bridges in series on the DC side, as isnecessary for HVDC technology. Thetransformer main insulation, therefore, is stressed by both the AC voltage and the direct voltage potential between valve-side winding and ground. The converter transformers are equipped with on-load tap-changers in order to provide the correct valve voltage.
The fig represents the HVDC transmission system.
- Eachconverter is of 12 pulses and its centre is earthed.
- One of the lines is kept at +200KV and other is kept at -200 KV.
- TheHVDC transmission which is linked by two ac system must be
- Thisis done so as the frequency remains constant. But the frequencies can be different also.
- To reduce the harmonicswhich are generated by converters, filters are used.
- The flow of power can be made in both the directions since the converter can operate both inrectifying mode and the inverting mode.
- Each thyristor is made of many thyristors to withstand the high voltages.
HVDC system is mostly operated in:
- Bipolar operation
- Monopolar operation with ground return Or metallic return
The HVDC breaker contains 3 branches in parallel,
- With breaking unit
- With non linear
- With a spark gapin series with capacitor
HVDC has following advantages: –
- The stability increases
- Limits the short circuit conditions
- Exchange is precise
- Energy can beconserved by reduction in transmission
- ELECTRICAL POWER SYSTEMS BY Sunil S RAO, UPPAL