Grid Infrastructure for Mega Cities - AC & DC Transmission

Article 4:  Power Transmission Technologies

Electricity generation is, in most cases, located in remote regions where hydro power, wind or solar is abundant. At these locations, economy of scale allows power to be generated far more efficiently and transmitted to the end users via electrical cables.

The power transmission from generation sites to load centers is performed through overhead lines or underground/subsea cables using either Direct Current (DC, proposed by Edison) or Alternating Current (AC, proposed by Tesla). Direct Current works by applying a constant electric voltage, from which most devices will draw a constant electric current. Batteries are a common source of direct current, and most modern electronics require direct current in order to operate. In an alternating current scheme, the voltage oscillates as a function of time - usually at a rate of 50 or 60 Hz.

Both types of electricity are equally capable of powering light bulbs, electric motors, and most types of appliances. What makes alternating current popular from a power transmission point of view is the fact that due to the principle of magnetic induction, it is very easy and cheap to raise or lower the voltage by means of an iron-core transformer. Since the power dissipated in a transmission line scales as the square of the current, and since increasing the voltage using a transformer decreases the current, one can dramatically reduce losses by using high-voltage power transmission. The principle of magnetic induction only works for alternating currents, and this is the reason that, for the past century, almost all commercial electricity has been produced and transmitted via alternating current.

However, AC transmission lines are characterized by a reactive current that reduces the lines power transfer capability and limits its transmission length. This reactive current may also cause a voltage drop that would require some sort of compensation to maintain the voltage at acceptable levels.

With the advances of power electronic devices, conversion between AC and DC at high voltage levels allowed High Voltage Transmission Systems (HVDC) to become the de-facto solution for transmitting power over long distances and especially for offshore applications.

The final decision to choose HVDC instead of conventional AC transmissions depends on a number of factors and the main reason often changes between different projects. Some advantages with HVDC compared to HVAC are the following:

• Controllability of the power flow: Both the power direction and the power level can be controlled very accurately and rapidly
• Losses: HVDC transmission lines have lower losses than AC lines for the same power capacity. The losses in the converter stations have of course to be added, but above a certain break-even distance, the total HVDC transmission losses become lower than the AC losses. HVDC cables also have lower losses than AC cables 
• Offshore loads: For long submarine cable links HVDC is the only possible technical solution
• Asynchronous grids: When connecting two asynchronous AC networks together, a HVDC link has to be implemented

4.1. AC Transmission

AC transmission lines and cables are a vital element of any AC power network. They are used to transfer electrical power from the power generating stations to the distribution system, which then supplies the electrical power to the consumers.

As the power generating stations in an AC power network can be quite distanced from the centers of energy consumption, AC transmission lines often have to transfer electrical power over great distances. This particularity, coupled with the fact that AC transmission lines are primarily inductive, has many effects on the operation of AC transmission lines.

One of the main effects is that a significant voltage drop occurs at the receiver end of AC transmission lines. This voltage drop must be continually compensated in order to maintain the receiver voltage within acceptable ranges. This is commonly achieved using capacitors and inductor banks connected in parallel to the line. When an AC transmission line is particularly long, substations containing parallel connected compensation devices must be added at regular intervals along the line.

When AC transmission lines are used to transfer electrical power in interconnected power networks, the flow of active power and reactive power between any two regions must be carefully controlled to maintain grid stability.

4.1.1. Line model and power transfer

Consider the model in Figure 4.1 to analyze the transfer of AC power between two buses across a line:

Figure 4. 1: Power transfer on a line

4.1.2. Charging current and critical cable length

A power cable has the capability to store and release electrical energy with the voltage variation; it works as a shunt capacitance generating a capacitive current which is in quadrature with the resistive or load current. The capacitive or charging current has a limiting effect on cable rating capacity (MW).

This effect is quantified by the fact that when intended to supply energy to a resistive receptor (active load consumer) in a radial network, via a power cable circuit, it is needed to inject a higher current at the source to compensate for cable capacitance.

The charging current is calculated with the following equation:

As the capacitance increases with the length of the cable, the charging current increases. This charging current generates heat losses that can amount to the entire thermal rating capacity of the cable. For long and uncompensated cables, the entire rating capacity could be consumed by the circulation of charging current and no real power can be transmitted.

The length of a cable at which the thermal capacity is consumed by the charging current is called “critical length” and is calculated using the following formula:

Figure 4.4 shows the typical critical length and the length at which the current rating is reduced by 20% for major high voltage cables (138 to 500kV voltage steps) based on North American experience.

In addition to the heating effect of charging current, the cable capacitance may have an impact on steady-state voltages across the power system to which the cable is connected.

The voltage may rise, especially at low loads, due to charging current. In this case the network voltage stabilization is carried out by adjusting the voltage magnitude at generator by reducing the field excitation or by lowering the voltage taps on transformers.

For long cables, compensation of charging current is ensured using shunt reactors at the receiving end of the cable. The size of the shunt inductor is determined by a load flow study.

4.1.3. Frequency Impact

If we examine the equations derived above we find that the frequency is a major parameter in the characterization of the impedance and conductance of the cables. higher frequency tends to increase voltage drop and shorten the transmission distance of the cable. In the other hand, a lower frequency reduces the voltage drop and increases the transmission distance. So the question is:

What happens if the grid frequency is reduced to zero?

When the frequency is set to zero, the electrical equations of impedance, voltage, current, and power are simplified as shown in Table 1. In this case, we are operating the grid with Direct Current (DC) and we get the following advantages:

• The voltage drop caused by the inductance is eliminated because the impedance of the line is reduced to the line resistance R
• The stray capacitance of the cable becomes infinite and does not conduct current leading to lesser leakage and more power transfer
• The charging current is eliminated, the transmission distance of the line becomes infinite, the power losses becomes substantially reduced, and reactive power compensation is no longer required

Table 1 : Transmission Cable – Frequency Effect

When the power is transmitted at a grid frequency equal to zero and at high voltage we call that High Voltage Direct Curent (HVDC) transmission system.

4.2. HVDC Transmission

As stated in the last paragraph, the term HVDC stands for High Voltage Direct Current and is today a well proven technology employed for power transmission all over the world. The method is very flexible and can be used to transfer electrical power over long distances using overhead lines or cables.

Two converter stations are to be used: one rectifier and one inverter. Electrical power is taken from one point in an AC network and converted to DC with the help of the rectifier station. The DC power is transmitted by overhead lines or by cables, converted back to AC power at the inverter station and finally injected into the receiving AC network. 

Two types of technologies are actually used for HVDC applications: one is using Current Source Converters (CSC)s and the other is using Voltage Source Converters (VSC)s. These two technologies are described in the following sections.

4.2.1. HVDC classic

The conventional HVDC transmission is called HVDC Classic and is based on a CSC technology, which is a line-commutating converter. The valves in the converter are made out of several thyristors connected in series and parallel configuration. The valves are then connected in so called Graetz bridge modules, as shown in Figure ‎4‑5. In the figure, phase to ground voltages are identified as Va, Vb, and Vc. The transformer is represented by its leakage reactor X and the thyristor switches are tagged T1 through T6. The thyristors conduct in pairs as (T1-T2), (T3-T4), and (T5 - T6) with overlap conduction between adjacent legs during current commutation. The output of the bridge is a DC current Id, and a DC voltage Ud.

Figure ‎4‑5: Simplified 6-pulse converter bridge

 A 12-pulse converter bridge can then be built by connecting two 6-pulse bridges in series. The bridges are then connected separately to the AC system by means of converter transformers, one of Y-Y winding structure and another of Y-∆ winding structure, as shown previously in Figure ‎4‑6. 

Figure ‎4‑6: Simplified 12-pulse converter bridge

Regarding operation and control of an HVDC link, two basic methods for generating gate pulses are discussed below:

1.  Individual phase control (IPC): used in early HVDC projects. The main feature of this scheme is that the firing pulse generation for each phase (or valve) is independent of each other and the firing pulses are rigidly synchronized with the commutation voltages. The major drawback of IPC scheme is the aggravation of harmonic stability problem, characterized by magnification of non-characteristic harmonics in steady state. 
2. Equidistant pulse control (EPC): In this scheme, the firing pulses are generated in steady state at equal intervals of 1/pF, where p is the number of pulses and F is the fundamental frequency. There are three variations of the EPC scheme; pulse frequency control (PFC), pulse period control, and pulse phase control (PPC).

Although EPC scheme has replaced IPC scheme in modern HVDC projects, it has certain limitations. The first drawback is that under unbalanced voltage conditions, EPC results in less DC voltage compared to IPC. EPC scheme also results in higher negative damping contribution to torsional oscillations, and it has more impact when HVDC is the major transmission link from a thermal station. 

4.2.2. VSC-based-HVDC

VSC-based-HVDC is the latest technology to transfer power by means of direct current. The first project using Light technology with insulated gate bipolar transistors (IGBT) was a 10 km long test transmission link between Hellsjon and Grengesberg, located in the central part of Sweden. The project transmitted 3 MW and was commissioned in March 1997. in a VSC-based-HVDC the valves are made out of devices using modules of IGBT switches with anti-parallel connected diodes. The method uses pulse width modulation (PWM) with a very high switching frequency.

In the development of HVDC, the hardware and software have gone hand in hand; the hardware has gone from vacuum tubes to transistors to integrated analogue circuits to integrated digital circuits to single board multiprocessor computers, and always at higher and higher clock speeds. This development has permitted the implementation of complexe functions into the control, and performing them digitally. At present, all functions in a HVDC converter station are performed digitally, including the most demanding ones, such as current control and the active filter control. An additional feature that the development has made possible and practical to achieve is full redundancy in control as well as in protection systems. To further increase the reliability and availability of the control systems as such, the self-diagnosing capabilities have increased dramatically, and even extended to the supervision of incoming signals, comparing them continuously against acceptability windows that are in turn updated by the prevailing conditions. The development has also touched the intra-station communication between different pieces of equipment; it is no longer the obvious answer to use hardwire signals (current, voltage, status) all the way to control rooms or through control islands; instead, the signals are converted to digital as close as possible to the source, and transmitted digitally, through optical fibers to the control room. 

The principles given above not only eliminate interference from the switchyard into the control building; they also permit an extremely high degree of integration of the different protection, control and monitoring functions. At present, full integration makes it possible to comprise not only the converter control, but the DC and AC sides as well, with, for example, transformer protection, tap changer control, reactive power control, transient fault recording, sequence of events recording, etc. The integration allows a much higher degree of utilization of the main circuit equipment capabilities. 

With the appearance of high switching frequency components, such as IGBTs it becomes advantageous to build VSC using PWM technology. The AC voltage is created by very fast switching between two fixed voltages. The desired fundamental frequency voltage is created through low pass filtering of the high frequency pulse modulated voltage.

In VSC-based-HVDC, PWM is used for generation of the fundamental voltage. Using PWM, the magnitude and phase of the voltage can be controlled freely and almost instantaneously within certain limits. This allows independent and very fast control of active and reactive power flows. From a system point of view, VSC acts as a zero-inertia motor or generator that can control active and reactive power almost instantaneously. Furthermore, it does not contribute to the short circuit power, as the AC current can be controlled. 

As a consequence, no reactive power compensation equipment is needed at the station, and only an AC filter is installed. The VSC-based-HVDC reactive power controller can automatically control the voltage in the AC network, without influencing the control of active power. In power supply applications, the active power is often controlled to match a set-point. Reactive power generation and consumption of a VSC-based-HVDC converter can be used for compensating the needs of the connected network within the rating of a converter. As the rating of the converters is based on maximum currents and voltages, the reactive power capabilities of a converter can be traded against the active power capability. 

The voltage control of a station constitutes an outer feedback loop and generates a reactive current demand signal in such a way as to maintain the set voltage on the network bus. Because of the short response time of the system for AC voltage control, the AC bus voltage can be kept constant during transients and other disturbances. Flicker is also mitigated to a higher degree. It is thus possible to use the controllability of VSC-based-HVDC to stabilize the voltage in fault situations.  

An interesting feature of VSC is its ability to supply a passive network having no generation components. When supplying a passive network, the static converter controls the amplitude and frequency of the AC voltage. Such a function can be of importance even if the VSC-based-HVDC system is connected to an active AC network, as it may become islanded and passive as a result of system faults. An island situation in an AC system can occur if a nearby AC breaker trips due to a fault. The VSC-based-HVDC system can be programmed to switch from active power regulation to frequency control following a breaker opening or in presence of frequency deviation. This makes VSC-based-HVDC very useful for transmission of electrical power to offshore platforms. VSC-based-HVDC can also be used for multiterminal operation, thus connecting together various platforms with one transmission link. 

4.2.3. HVDC applications

HVDC and VSC-based-HVDC systems are applied to transfer power over long distances, integrate wind farms to the grid, connect asynchronous systems, solve loop flow problems, damp power oscillations, and help with voltage stability concerns. Some applications are better done with VSC-based-HVDC due to its use of VSCs and PWM techniques. Such applications include flicker control, offshore power connection, and multiterminal systems using a common DC bus.

4.2.3.1. Asynchronous systems connection

The interconnected AC networks that tie the power generation plants to the consumers are in most cases large. Even when these networks operate at the same nominal frequency, there are always some variations that may complicate or make an AC connection between these networks impossible.

An AC tie between two asynchronous systems needs to be very strong to not get overloaded. If a stable AC tie would be too large for economical power exchange needs, or if the networks wish to retain their independence, than a HVDC link is the solution. In some parts of the world (Middle-East, South America, and Japan) 50 and 60 Hz networks are bordering each other and it would be impossible to exchange power between them using AC lines or cables. A HVDC link is then the only alternative.

4.2.3.2. Bottlenecks

The term Bottleneck is often interchangeable to congested transmission paths or interfaces. A transmission path or interface refers to a specific set of transmission elements between two neighboring control areas or utility systems in an interconnected electrical system. A transmission path or interface becomes congested when the allowed power transfer capability is reached under normal operating conditions or as a result of equipment failures and system disturbance conditions. The key impact of Bottlenecks is the reduction of system reliability; the inefficient utilization of transmission capacity and generation resources, and the restriction of healthy market competition. The ability of transmission systems to deliver the energy is dependent on several factors that are limiting the system, including thermal constraints, voltage constraints, and stability constraints. These transmission limitations are usually determined by performing detailed power flow and stability studies for a range of anticipated system operating conditions. Thermal limitations are the most common constraints, as warming and consequently sagging of lines is caused by the current flowing in the wires of the lines and other equipment. In some situations, the effective transfer capability of transmission path or interface may have to be reduced from the calculated thermal limit to a level imposed by voltage constraints or stability constraints. 

Bottlenecks may be relieved by the use of a HVDC link in parallel with the limiting section of the grid. By using the inherent controllability of the HVDC system, the power system operator can decide the amount of power to be transmitted by the HVDC system and consequently that to be transmitted by the AC link.

Longer AC lines tend to have stability constrained capacity limitations as opposed to the higher thermal constraints of shorter lines. By using the inherent controllability of a HVDC system in parallel with the long AC lines, the power system can be stabilized and the transmission limitations on the AC line can be relaxed.

4.2.3.3  Flicker control

Voltage Flicker can either be a periodic or aperiodic fluctuation in voltage magnitude i.e. the fluctuation may occur continuously at regular intervals or only on occasions. Voltage Flicker is normally a problem with human perception of lamp strobing effect but can also affect power processing equipment such as UPS systems and power electronic devices. Slowly fluctuating periodic flickers, in the 0.5 to 30 Hz range, are considered to be noticeable by humans. A voltage magnitude variation of as little as 1.0% may also be noticeable.

The main sources of flicker are industrial loads exhibiting continuous and rapid variations in the load current magnitude. This type of loads includes Electric Arc Furnace (EAF) in the steel industry, welding machines, large induction motors, and wind power generators. High impedance in a power delivery system will contribute further to the voltage drop created by the line current variation. 

A VSC working as a STATCOM is an effective method to address the problem of flicker. The unbalanced, erratic nature of an EAF causes significant fluctuating reactive power demand, which ultimately results in irritating electric lamp flicker to neighboring utility customers. In order to stabilize voltage and reduce disturbing flicker successfully, it is necessary to continuously measure and compensate rapid changes by means of extremely fast reactive power compensation. STATCOM uses voltage source converters to improve furnace productivity similar to a traditional SVC while offering superior voltage flicker mitigation due to fast response time.

4.2.3.4   Loop flow

The terms Loop Flow and Parallel Path Flow are sometimes used interchangeable to refer to the unscheduled power flows, that is, the difference between the scheduled and actual power flows, on a given transmission path in an interconnected electrical system.

Unscheduled power flows on transmission lines or facilities may result in a violation of reliability criteria and decrease available transfer capability between neighboring control areas or utility systems.

The reliability of an interconnected electrical system can be characterized by its capability to move electric power from one area to another through all transmission circuits or paths between those areas under specified system conditions. The transfer capability may be affected by the contracted path designated to wholesale power transactions, which assumes that the transacted power would be confined to flow along an artificially specified path through the involved transmission systems. In reality, the actual path taken by a transaction may be quite different from the designated routes, determined by physical laws not by commercial agreements, thus involving the use of transmission facilities outside the contracted systems. These unexpected flow patterns may cause so-called Loop Flow and Parallel Path Flow problems, which may limit the amount of power these other systems can transfer for their own purposes. 

4.2.3.5   Unbalanced load

An unbalanced load is a load which does not draw balanced current from a balanced three-phase supply. Typical unbalanced loads are loads which are connected phase to neutral and also loads which are connected phase to phase. Such loads are not capable of drawing balanced three-phase currents. They are usually termed single-phase loads.

 A single-phase load, since it does not draw a balanced three-phase current, will create unequal voltage drops across the series impedances of the delivery system. This unequal voltage drop leads to unbalanced voltages at delivery points in the system. Blown fuses on balanced loads such as three-phase motors or capacitor banks will also create unbalanced voltage in the same fashion as the single-phase and phase to phase connected loads. Unbalanced voltage may also arise from impedance imbalances in the circuit that deliver electricity such as untransposed overhead transmission lines. Such imbalances give the appearance of an unbalanced load to generation units.

An unbalanced supply may have a disturbing or even damaging effect on motors, generators, poly-phase converters, and other equipment. The foremost concern with unbalanced voltage is overheating in three-phase induction motors. The percent current imbalance drawn by a motor may be 6 to 10 times the voltage imbalance, creating an increase in losses and in turn an increase in motor temperature. This condition may lead to motor failure.

Modern electric rail system is a major source of unbalanced loads. Electrification of railways, as an economically attractive and environmentally friendly investment in infrastructure, has introduced an unbalanced and heavy distorted load on the three-phase transmission grid. Without compensation, this would result in significant unbalanced voltage affecting most neighboring utility customers. The VSC-based-HVDC inverter station can generate voltages to elegantly be used to restore voltage and current balance in the grid, and to mitigate voltage fluctuations generated by the traction loads.

4.2.3.6   Voltage instability

Voltage instability is basically caused by an unavailability of reactive power support in an area of the network, where the voltage drops uncontrollably. Lack of reactive power may essentially have two origins: firstly, a gradual increase of power demands without the reactive part being met in some buses or secondly, a sudden change in the network topology redirecting the power flows in such a way that the required reactive power cannot be delivered to some buses. 

The relation between the active power consumed in the considered area and the corresponding voltage is expressed in a static way by the P-V curve (also called “nose” curve) as shown in Figure 4‑3. The increased values of loading are accompanied by a decrease in voltage (except in case of a capacitive load). When the loading is further increased, the maximum loadability point is reached, beyond which no additional power can be transmitted to the load under those conditions. In case of constant power loads, the voltage in the node becomes uncontrollable and decreases rapidly. This may lead to the partial or complete collapse of a power system.

VSC-based-HVDC, when connected to the grid, can provide dynamic voltage support in response to system disturbances and balance the reactive power demand of large and fluctuating industrial loads. A VSC-based-HVDC is capable of both generating and absorbing variable reactive power continuously as opposed to discrete values of fixed and switched shunt capacitors or reactors. With continuously variable reactive power supply, the voltage at the bus may be maintained smoothly over a wide range of system operating conditions. This entails the reduction of network losses and provision of sufficient power quality to the electric energy end-users.

4.2.3.7   Power oscillations 

Oscillations of generator angle or line angle are generally associated with transmission system disturbances and can occur due to step changes in load, sudden change of generator output, transmission line switching, and short circuits. Depending on the characteristics of the power system, the oscillations may last from 3 to 20 seconds after a severe fault. During such angular oscillation period significant variations in voltages, currents, and power flows will take place. It is important to damp these oscillations as quickly as possible because they cause many power quality problems as well as mechanical wear in power plants. The system is also more vulnerable if further disturbances occur.  

The active power oscillations on a transmission line tend to limit the amount of power that may be transferred; this may result in stability concerns or utilization restrictions on the corridors between control areas or utility systems. This is due to the fact that higher power transfer can lead to less damping and thus more severe and possibly unstable oscillations.  

The HVDC damping controller is a standard feature in many HVDC projects in operation. It normally takes its input from the phase angle difference in the two converter stations. Figure ‎4‑7 from ABB website shows a comparison of power oscillation damping using different control strategies with VSC-based-HVDC.

 Figure 4‑7: Power oscillation damping using VSC-based-HVDC

4.3    Conclusion

An overview of AC and DC transmission technologies was conducted to assess the main difference and appropriate applications. General description and comparison of current source converter and voltage source converter based HVDC was provided. A survey of possible applications of VSC-based-HVDC was also discussed. These applications included transfer of power over long distances, connection of asynchronous systems, solving loop flow problems, damping of power oscillations, and helping with voltage stability issues. I also noted that some applications are better implemented with VSC-based-HVDC due to its fast action and PWM techniques. Such applications include flicker control, offshore power connection, and multiterminal systems using a common DC buses.