This Is Why HVDC Transmission System Beats HVAC In These Essential Applications

What Is The HVDC?

For the most part, electric power is transmitted worldwide by means of ac. However, there are certain applications where dc transmission offers distinct economic and/or performance advantages.

These applications include long-distance overhead transmission, underwater or underground transmission, and asynchronous ties between power systems. The first practical application of dc transmission was in Sweden in 1954. But the wider applications of HVDC started after 1960.

In Canada, Vancouver Island is supplied through an HVDC cable. Another famous HVDC system is the interconnection between England and France, which uses underwater cables.

Typically, in an HVDC system, the ac voltage is rectified and a dc transmission line transmits the energy. An inverter that is located at the end of the dc transmission line converts the dc voltage to ac, for example, the Pacific HVDC Intertie that operates with ±500 kV voltage and interconnects Southern California with the hydro station in Oregon.

The bundled conductors are also used in HVDC transmission lines.

Contents:

• Overhead HVDC Transmission
• Comparison Of Power Transmission Capacity Of HVDC and HVAC
• HVDC Advantages
• HVDC Disadvantages
• Interesting HVDC projects (VIDEO)

Overhead HVDC Transmission

Figure 1 shows some of the typical circuit arrangements (links) for HVDC transmissions. In the monopolar arrangement, shown in Figure 1a, there is only one insulated transmission conductor (pole) installed and ground return is used.

It is the least expensive arrangement, but has certain disadvantages.

In order to eliminate the aforementioned electrolysis phenomenon, a metallic return (conductor) can be used, as shown in Figure 1b.

The bipolar circuit arrangement has two insulated conductors used as plus and minus poles. The two poles can be used independently if both neutrals are grounded. Under normal operation, the currents owing in each pole are equal, and therefore, there is no ground current.

As shown in Figure 1c, the rated voltage of a bipolar arrangement is given as ±V_d (e.g., ±500 kV, which is read as plus and minus 500 kV).

Figure 2 shows a dc transmission system operating in the bipolar mode.

It is possible to have two or more poles all having the same polarity and always having a ground return. This arrangement is known as the homopolar arrangement and is used to transmit power in dc systems that have very large ratings.

The dc tower normally carries only two insulated conductors, and the ground return can be used as the additional conductor.

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Comparison Of Power Transmission Capacity Of HVDC and HVAC

Assume that there are two comparable transmission lines: one is the ac and the other the dc line. Assume that both lines have the same length and are made of the same conductor sizes and that the loading of both lines is thermally limited so that current I_d equals the rms ac I_L.

Also assume that the ac line has three phase and three wires and has a power factor of 0.945 and the dc line is a bipolar circuit arrangement with two conductors.

Therefore, it can be shown that the dc power per conductor is:

P_(dc) = V_dI_d

P_(ac) = V_(L-N)I_Lcos θ W/conductor

where:

• V_d is the line-to-ground dc voltage in volts
• V_(L-N) is the line-to-neutral ac voltage in volts
• I_d is the dc line current in amperes
• I_L is the ac line current in amperes

Therefore, the ratio of the dc power per conductor to the ac power per conductor (phase) can be expressed as:

P_(dc) / P_(ac) = V_dI_d / V_(L-N)I_Lcos θ

or

P_(dc) / P_(ac) = √2 / cos θ

but since

cos θ = 0.945 then: P_(dc) / P_(ac) = 1.5

or

P_(dc) = 1.5 P_(ac) W/conductor

Furthermore, the total power transmission capabilities for the dc and ac lines can be expressed as:

P_(dc) = 2p_(dc) W and P_(ac) = 3p_(ac) W

Therefore, their ratio can be expressed as:

P_(dc) / P_(ac) = (2 / 3) × p_(dc) / p_(ac)

Substituting Equation P_(dc) / P_(ac) = 1.5 into this equation, we get:

P_(dc) / P_(ac) = (2 / 3) × 3 / 2 = 1

or

P_(dc) = P_(ac) W

Thus, studies indicate that a dc line generally costs about 33% less than an ac line of the same capacity. Furthermore, if a two-pole (homopolar) dc line is compared with a double-circuit three-phase ac line, the dc line costs would be about 45% less than the ac line.

In general, the cost advantage of the dc line increases at higher voltages. The power losses due to the corona phenomena are smaller for dc than for ac lines.

The reactive powers generated and absorbed by an HVAC transmission line can be expressed as:

Q_c = X_cV² vars/unit length

or

Q_c = ωCV² vars/unit length

and

Q_L = X_LI² vars/unit length

or

Q_L = ωLI² vars/unit length

where:

• X_c is the capacitive reactance of line in ohms per-unit length
• X_L is the inductive reactance of line in ohms per-unit length
• C is the shunt capacitance of line in farads per-unit length
• L is the series inductance of line in farads per-unit length
• V is the line-to-line operating voltage in volts
• I is the line current in amperes

If the reactive powers generated and absorbed by the line are equal to each other,

Q_c = Q_L

or

ω_cV² = ω_LI²

from which the surge impedance of the line can be found as:

Z_c = V / I = √(L / C)

Therefore, the power transmitted by the line at the surge impedance can be expressed as:

SIL = V²_L-L / Z_c W

Note that this surge impedance loading (or natural load) is a function of the voltage and line inductance and capacitance.

Hence, the costs of necessary series capacitor and shunt reactor compensation should be taken into account in the comparison of ac versus dc lines. The dc line itself does not require any reactive power. However, the converters at both ends of the line require reactive power from the ac systems.

Underground cables used for ac transmission can also be used for dc, and they can normally carry more dc power than ac due to the absence of capacitive charging current and better utilization of insulation and less dielectric wear. However, an HVDC transmission cable is designed somewhat differently than that of an ac transmission cable.

Since a power cable employed for dc power transmission does not have capacitive leakage currents, the power transmission is restricted by the I²R losses only.

Furthermore, submarine or underground ac cables are always operated at a load that is far less than the surge impedance load in order to prevent overheating.

Thus, compensating shunt reactors are to be provided at regular intervals (approximately 20 mi). Contrarily, dc cables do not have such restrictions. Thus, the power transmission using dc cable is much cheaper than ac cable.

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HVDC Advantages

The major advantages of the dc transmission can be summarized as follows:

1. If the high cost of converter stations is excluded, the dc overhead lines and cables are less expensive than ac overhead lines and cables. The break-even distance is about 500 mi for the overhead lines, somewhere between 15 and 30 mi for submarine cables and 30 and 60 mi for underground cables.
   
   Therefore, in the event that the transmission distance is less than the break-even distance, the ac transmission is less expensive than dc. Otherwise, the dc transmission is less expensive. The exact break-even distance depends on local conditions, line performance requirements, and connecting ac system characteristics.
2. A dc link is asynchronous. That is, it has no stability problem in itself.
   
   Therefore, the two ac systems connected at each end of the dc link do not have to be operating in synchronism with respect to each other or even necessarily at the same frequency.
3. The corona loss and RI conditions are better in the dc than the ac lines.
4. The power factor of the dc line is always unity, and therefore, no reactive compensation is
   needed.
5. Since the synchronous operation is not demanded, the line length is not restricted by stability.
6. The interconnection of two separate ac systems via a dc link does not increase the short-circuit capacity, and thus the circuit breaker ratings, of either system.
7. The le line loss is smaller than for the comparable ac line.

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HVDC Disadvantages

The major disadvantages of the dc transmission can be summarized as follows:

1. The converters generate harmonic voltages and currents on both ac and dc sides, and there- fore, filters are required.
2. The converters consume reactive power.
3. The dc converter stations are expensive.
4. The dc circuit breakers have disadvantages with respect to the ac circuit breakers because
   the dc does not decrease to zero twice a cycle, contrary to the ac.

The potential applications for HVDC transmission across the world and in the United States are as follows:

1. Long-distance overhead transmission
2. Power in-fed into urban areas by overhead transmission lines or underground cables
3. Asynchronous ties
4. Underground cable connections
5. East–west and north–south interconnecting overlays
6. DC networks with tapped lies
7. Stabilization of ac systems
8. Reduction of shorter-circuit currents in receiving ac systems

In the future, the application of HVDC transmission will increase due to the following two basic reasons:

Reason #1 – The availability and ever-increasing prices of important oil imports are making coal and hydro more attractive.

Reason #2 – The ever-increasing pressures by the environmental concerns to locate new power plants remotely from densely populated urban areas.

Hence, obtaining sites for new power plants are becoming extremely difficult. Because of this difficulty, utility companies will be forced to locate them several hundred miles away from their load centers.

Thus, the need for economical long-distance transmission of large blocks of electric energy will increasingly dictate the use of HVDC transmission lines.

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Interesting HVDC projects (VIDEO)

Brazil-Argentina HVDC Interconnection

This HVDC back-to-back station located between Brazil and Argentina involved considerable innovation in manufacturing and construction techniques for both transmission lines and converter station. The scheduled time to deliver was only 22 months.

The first phase went into commercial operation in 1999 and the second phase in 2002.

Gotland – the world’s first HVDC Light project

The push for renewable forms of energy has brought wind power plants to southern Gotland, a Swedish island in the Baltic Sea. The transmission link between the southern part of Gotland and the city of Visby is rated 50 MW and was put into operation in June 1999.

It is the world’s first HVDC link using VSC technology.

North-East Agra – UHVDC transmission link

The 800 kV North-East Agra UHVDC (ultrahigh-voltage direct current) link will have a record 8,000 MW converter capacity, transmitting clean hydroelectric power, equivalent to the generation of 8 large power plants, from India’s northeast region to the city of Agra, a distance of 1,728 km.

The North-East Agra project is ABB’s fifth HVDC transmission link in India.

800 kV HVDC convertor transformer

Alstom has created history by manufacturing the first “made in India” 800 kV HVDC convertor transformer from its world-class facility at Vadodara in Gujarat. The convertor transformer is part of Power Grid’s 3000 MW advanced UHVDC system.

It meet the bulk power transfer requirement from Chhattisgarh region – a hub of Independent Power

Producers of thermal power – to the load centre located in the northern region of the country, through a 1,365 km transmission line, creating an “energy highway” of clean, efficient power.

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Reference // Electrical Power Transmission System Engineering by Turan Gönen (Purchase hardcover from Amazon)