HVDC Vs HVAC |Comparison Of HVAC And HVDC

HVDC vs HVAC: In this post, a Comparison of HVAC and HVDC transmission is given.HVDC stands for High Voltage Direct Current and HVAC stands for High Voltage Alternating Current. Lets compare HVAC and HVDC transmission.

HVDC vs HVAC

There are some characteristic problems associated with HVDC in comparison to AC transmission and these include:

HVDC requires expensive converters at each end of a DC transmission link; whereas, AC transmissionrequires only transformer stations
Converters require high reactive power (up to 50%of the active power rating) both for rectification andinversion. The reactive power requirement is met with installation of synchronous or static capacitors.
Convertors generate a lot of harmonics both on the DC and the AC sides. Filters can be used to reduce the amount of harmonics transferred to the AC system but DC system requires additional reactor components; thus adding to the cost.
Circuit breaking is difficult with DC due to absence of natural current zero. The lack of this protection equipment imposes restrictions on increasing the transmission voltage beyond pre-set values.
Voltage transformation is not easy. Converters lack the overload capacity available in transformers.

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HVDC vs HVAC Transmission

Comparison of AC and DC Transmission

The merits of two modes of transmission (AC & DC) should be compared based on the
following factors.

1) Economics of transmission
2) Technical Performance
3) Reliability

Economics of Power Transmission

In DC transmission, inductance and capacitance of the line has no effect on the power
transfer capability of the line and the line drop. Also, there is no leakage or charging current of
the line under steady conditions.

A DC line requires only 2 conductors whereas AC line requires 3 conductors in 3-phase
AC systems.

The cost of the terminal equipment is more in DC lines than in AC line. Break-even distance is one at which the cost of the two systems is the same.

It is understood from the below figure that a DC line is economical for long distances which are greater than the break-even distance.

Technical Performance

Due to its fast controllability, a DC transmission has full control over transmitted power,
an ability to enhance transient and dynamic stability in associated AC networks and can limit
fault currents in the DC lines.

Furthermore, DC transmission overcomes some of the following problems associated with AC transmission.

Stability Limits

The power transfer in an AC line is dependent on the angle difference between the
voltage phasors at the two line ends. For a given power transfer level, this angle increases with
distance. The maximum power transfer is limited by the considerations of steady state and
transient stability. The power carrying capability of an AC line is inversely proportional to
transmission distance whereas the power carrying ability of DC lines is unaffected by the
distance of transmission

Voltage Control

Voltage control in ac lines is complicated by line charging and voltage drops. The voltage
profile in an AC line is relatively flat only for a fixed level of power transfer corresponding to its
Surge Impedance Loading (SIL).

The voltage profile varies with the line loading. For constant voltage at the line ends, the midpoint voltage is reduced for line loadings higher than SIL and increased for loadings less than SIL.

The maintenance of constant voltage at the two ends requires reactive power control as
the line loading is increased.

The reactive power requirements increase with line length.

Although DC converter stations require reactive power related to the power transmitted, the DC
line itself does not require any reactive power.

The steady-state charging currents in AC cables pose serious problems and make the break-even distance for cable transmission around 50kms.

Line Compensation

Line compensation is necessary for long distance AC transmission to overcome the problems of line charging and stability limitations.

The increase in power transfer and voltage control is possible through the use of shunt inductors, series capacitors, Static Var Compensators (SVCs) and, lately, the new generation Static Compensators (STATCOMs).

In the case of DC lines, such compensation is not needed.

Problems of AC Interconnection

The interconnection of two power systems through ac ties requires the automatic generation controllers of both systems to be coordinated using tie line power and frequency signals.

Even with coordinated control of interconnected systems, the operation of AC ties can be
problematic due to:

The presence of large power oscillations which can lead to frequent tripping,
Increase in fault level, and
Transmission of disturbances from one system to the other

The fast controllability of power flow in DC lines eliminates all of the above problems.

Furthermore, the asynchronous interconnection of two power systems can only be
achieved with the use of DC links.

Ground Impedance

In AC transmission, the existence of ground (zero sequence) current cannot be permitted
in steady-state due to the high magnitude of ground impedance which will not only affect
efficient power transfer, but also result in telephonic interference.

The ground impedance is negligible for DC currents and a DC link can operate using one conductor with ground return (monopolar operation).

The ground return is objectionable only when buried metallic structures (such as pipes)
are present and are subject to corrosion with DC current flow.

While operating in the monopolar mode, the AC network feeding the DC converter station operates with balanced voltages and currents.

Hence, single pole operation of dc transmission systems is possible for extended period,

while in AC transmission, single phase operation (or any unbalanced operation) is not feasible
for more than a second.

Disadvantages of DC Transmission

The scope of application of DC transmission is limited by

High cost of conversion equipment.
Inability to use transformers to alter voltage levels.
Generation of harmonics.
Requirement of reactive power and
Complexity of controls.

Over the years, there have been significant advances in DC technology, which have
tried to overcome the disadvantages listed above except for (2). These are

Increase in the ratings of a thyristor cell that makes up a valve.
Modular construction of thyristor valves.
Twelve-pulse (and higher) operation of converters.
Use of forced commutation.
Application of digital electronics and fiber optics in the control of converters.

Reliability

The reliability of DC transmission systems is good and comparable to that of AC
systems. The reliability of DC links has also been very good.

There are two measures of overall system reliability-energy availability and transient
reliability.

Energy availability

Energy availability = 100 (1 – equivalent outage time) %
Actual time

Where equivalent outage time is the product of the actual outage time and the fraction of
system capacity lost due to outage.

Transient reliability

This is a factor specifying the performance of HVDC systems during recordable faults on
the associated AC systems.

Transient reliability = 100 X No. of times HVDC systems performed as designed
No. of recordable AC faults

Recordable AC system faults are those faults which cause one or more AC bus phase
voltages to drop below 90% of the voltage prior to the fault.

Both energy availability and transient reliability of existing DC systems with thyristor
valves is 95% or more.

Comparison of AC and DC Transmission

The comparative characteristics of AC and DC transmission are listed as follows.

Power transmitted

Power transmitted: The power transmission capabilities of an AC link and a DC link are different.

Tower size

Tower size: The DC insulation level required is lower than that for AC, for the same power transmission.

Also the DC line will only need two conductors instead of three conductors (or six) as required
for AC to obtain the same reliability. Thus, based on these electrical and mechanical considerations; smaller tower are suitable for transmitting DC.

Ground return

Ground return: For the same length of transmission, the impedance of the ground path is lesser for DC than for the corresponding AC transmission, s the DC spreads over much larger width and depth.

Hence use of ground return is possible.

The ground path resistance in case of DC is almost entirely dependent on the earth’s electrode resistance at the two ends of the line and not on the line length.

However, on the flip side ground currents may cause electrolytic corrosion of the buried
conductors and also interfere in the operation of signalling.

Skin effect

Skin effect: In AC systems, the current is not uniformly distributed over the cross-section of the
conductor.

The current density is higher in the outer region (attributed to skin effect) and results
in under-utilisation of the conductor’s cross- section.

Skin effect is completely absent in DC and hence there is a uniform current in the conductor and the conductor metal is better utilised.

Read Also: Skin Effect and Proximity Effect

Stability and synchronous interconnections

Stability and synchronous interconnections: The DC link is an asynchronous link and
hence any AC supplied through converters or DC generation does not require synchronisation with the link.

For AC links, interconnections between power systems must be synchronised and thus, systems of different frequencies cannot be interconnected.

Such systems with different frequencies can be easily interconnected through high voltage
DC links.