The AC transmission line voltage goes to as high as 765kV, but beyond this voltage level, the T/L circuit will incur so much power loss in the form of heat dissipation due to high dielectric loss.
DC transmission line incurs a bigger cost when the DC voltages are transformed to lower voltages, with or without advanced power electronic technology, but these are offset by lower transmission line loss and number of cable to be used with cable size that may be smaller than AC transmission system. This reduces the T/L pylon loads that which will greatly decrease its cost.
AC transmission line operating above 765kV to transmit power over long distances is deemed not suitable, because of high dielectric loss that reduces the transmission line’s efficiency to considerable degree. Dielectric lose is the inherent dissipation of electromagnetic energy into heat. It can be expressed as “loss angle d (pronounced “delta”) or loss tangents d” - phasors with reactive components and resistive component. The resistive component is the electromagnetic field’s lossy part, while the imaginary or reactive component its lossless counterpart.
The T/L length, efficiency and capacity dictates the sending voltage level that must be used in order to design an efficient T/L system. However, the T/L’s length is inversely proportional to its capacity to transmit power to the receiving end, i.e., its energy capacity transfer efficiency decreases as the T/L length increases. This can be counteracted by increasing the diameter of the T/L cables and constructing bigger and stronger transmission towers to bear the extra mechanical stress of larger and heavier power cables; and by increasing the sending voltage level. These factors, thus, will considerably increase the investment cost to build the T/L because of higher cost of power transformers and dielectric materials and supports to be used.
When we consider the advantages of DC system, we tend to conclude that the DC system is indeed superior than AC system; but why almost all of transmission line system AC?: because transformers make it easier and cheaper to step-down or step-up the voltage, AC circuit breakers are cheaper than DC circuit breakers and also the ease of using voltage and current transformer for protection, monitoring and control of the grid.
"Large single phase three winding transformer with its side valve bushing mounted for entering the valve hall" - Courtesy of Asea Brown Boveri
The first HVDC project in the Philippines was the interconnection of Luzon Grid to the Visayas Grid. The submarine cable link portion is about 21 kms and traversed the San Bernardino Strait from Matnog, Sorsogon to Allen, Samar. The HVDC link has a power rating of 440MW with DC voltage of 350kVDC from 230kVAC end. It was commissioned and went into operation in 1988. Below is the project detail of the HVDC Leyte – Luzon high voltage direct current transmission link courtesy of Asea Brown Boveri:
"The HVDC Leyte–Luzon is a high voltage direct current transmission link in the Philippines between geothermal power plants on the island of Leyte and the southern part of island of Luzon. The HVDC Leyte – Luzon went in service on 10 August 1998.
The HVDC Leyte – Luzon begins at Ormoc converter station (Leyte Province) and ends at Naga converter station (Province of Camarines Sur). It consists three sections:
The length of submarine cable is 21 kilometers (13 mi) and the total length of overhead lines is 430 kilometers (270 mi). The HVDC Leyte – Luzon can transfer an output of 440 MW. It is implemented as monopolar line (see the Monopolar Line in the banner above) for a voltage of 350kV.
The crossing of San Juanico Strait is realized as overhead crossing with a tower on an island in the strait.
Aim of the HVDC link is to feed the AC grid in the Manila region. Beside of overall connection of grids, the HVDC Leyte – Luzon stabilize the AC network. The interconnector is manufactured by the ABB Group in cooperation with Marubeni Corporation and it's operated by state owned National Transmission Corporation (Now National Grid Corporation of the Philippines or NGCP).
The grounding electrodes are situated at Albuera at 10°54'01?N; 124°42'24?E and near Calabanga at 13°43'59?N ; 123°14'29?E . They are connected with the converter stations by 25 respectively 15 kilometers long overhead lines."
HVDC converter transformer supplies AC voltages into two separate windings that feed the rectifier with a phase shift of 30deg in order to reduce 5th and 7th harmonics and provides a galvanic barrier between the AC and DC systems to prevent the DC voltage from flowing to the AC system. Likewise the converter transformer’s AC supply reactive impedance controls the rate of rise in the valve current during commutation and reduces short circuit current during fault conditions.
The converter/inverter station converts AC to DC and DC to AC. This means power can be fed or received from any side. The HVDC transformer connects and isolate the AC system to the rectifier system (thyristors). A rectifier is an electrical device that allows the flow of current only in one direction. The output can only be +DC or -DC depending on how the thyristor is configured, since only a half of a periodically reversing alternating current wave can pass in the valve.
HVDC converter transformer act as an interface equipment between AC and DC inverter/converter at both ends of a long DC high voltage transmission line. The converter/inverter station converts AC to DC and DC to AC. This means power can be fed or received from any side. The HVDC transformer connects and isolate the AC system to the rectifier system (thyristors). A rectifier is an electrical device that allows the flow of current only in one direction. The output can only be +DC or -DC depending on how the thyristor is configured, since only a half of a periodically reversing alternating current wave can pass in the valve.
HVDC converter transformers can either be 1-ph or 3-ph and can have one or two thyristor valve winding per phase. These valve windings are under AC and DC stress, which require them to have suitably designed insulation. Further, the load has high harmonics content that causes higher loses and noise.
For the special DC and polarity reversal test that have to be performed on valve winding see: IEEE Xplore: Calculation and Analysis of Polarity Reversal Electric Field in Valve Winding End of Converter Transformer.
A big difference between the properties of AC and DC line is that AC lines are synchronized with the grid frequency. When two AC grids not synchronized with each other are bridged with HVDC line the DC voltage inverted back to AC must be synchronized at the converter/inverter station AC take-off end. For this reason, DC power lines are often referred to by power systems engineers as “asynchronous links”. A DC asynchronous link is the frequency synchronization of the AC interconnected grids via the HVDC submarine cable or overhead HVDC transmission system.
Old HVDC converters used mercury arc valve, mercury-vapor rectifier, or mercury-arc rectifier to convert high voltage AC to DC. Invented in 1902 by Peter Cooper Hewitt, it was the method of high voltage rectification prior to the advent of solid state devices that made possible the invention of thyristors . ”A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act exclusively as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward-biased (that is, while the voltage across the device is not reversed).” Thyristors are the main component of HVDC Converters for converting high voltage AC to DC and vice-versa. HVDC converters are capable of converting up to 2GW of power with voltage of up to 900kV.
HVDC Converters are actually a converter/inverter combination and most HVDC Converter are such, making them inherently bidirectional i.e. they can convert from either AC to DC (rectification) and DC to AC (inversion).