I. Introduction

HVDC (High Voltage Direct Current) is a technology that has been implemented since the 1950s as a complement of AC (Alternating Current) power systems. Because of its different characteristics as compared to AC systems, it offers great advantages over these in many aspects. Perhaps one of its main advantages, and initial purpose for implementation, is the capability to interconnect two unsynchronized systems through a DC link. Likewise, HVDC offers improved efficiency for the transmission of large amounts of electric power through long distances. These two main applications of HVDC systems have given rise to two common terms used in industry, namely Back-to-Back and Point-to-Point installations. Back-to-Back installations are those in which two systems are interconnected with two converter stations tied together via a DC bus in close proximity to each other. On the other hand, Point-to-Point installations are those in which the main objective of the system is to transmit power over long distances, via a HVDC transmission line.  

Throughout the years, improvements in the cost and efficiency of the components used in HVDC systems have made them more economically viable and are now competing with AC systems.

Protection and control schemes for AC systems have been developed for over one hundred years, and are now very well established. Most utilities have very well-written and thorough standards for the protection of their systems. These standards, however, apply only to AC systems and, although the concepts could be passed on to HVDC systems, the schemes ought to be modified and new ones have to be developed due to the differences in equipment used. Due to the increasing implementation of HVDC systems, it is of utmost importance for power systems engineers to understand how to protect and control these systems in a more efficient and reliable manner.

II. Existing Systems

In order to have a good understanding on the applications of HVDC systems, existing installations must be discussed. It is important to note that, although the use of HVDC has increased over the last five decades, most installations can be mentioned in the context of this paper, which shows that there is a lot of potential for future implementations.

A.  North America

• Chateauguay Station: The Chateauguay station is the largest Back-to-Back system in North America. With a capacity of 1000MW, this station interconnects Hydro-Quebec’s system with the New York Power Authority. The station was commissioned in 1984 and modernized in 2009.
• Pacific DC Intertie: The Pacific DC Intertie has a capacity of 3100MW and transmits power from the Northwest United States, where cheap hydroelectric power is available, to the Los Angeles area in California. Although the system often transfers power from north to south, it can transmit power both ways. Thus, is acts as a good complement since, during winter time, the north consumes a significant amount of power, and, during the summer, the south consumes significantly greater amounts of power.
• Celilo Converter Station: The Celilo Converter Station is the north terminal of the Pacific DC Intertie and it’s configured of Light-Triggered Thyristors (LTT) groups that eliminate environmental risks of mercury and reduce maintenance costs.
• Sylmar Converter Station: Located north of Los Angeles, the Sylmar Converter Station is configured of a single pair of 12-pulse converters built by ABB. 

B. Japan

The Japanese power system is unique since it is divided into two systems operating at different frequencies. Due to this, in order to interconnect the entire Japanese system, several converter stations have been installed over the last five decades.

• Sakuma: Built near the Sakuma dam in 1965, it was one of the first HVDC stations built. Similarly, after its upgrade in 1993, it was the first converter station in the world to use LTTs.
• Hokkaido-Honshu: The Hokkaido-Honshu is a Point-to-Point system connecting the two islands in Japan. In total, the transmission line is 193km long and consists of 149km of overhead transmission line and 44km of submarine cable.
• Kii Channel: The Kii Channel is presently the highest power submarine HVDC cable and connects Shikoku with the island of Honshu and operates at a bipolar voltage of ±500kV.

C. Europe

Since most of the research on HVDC has historically been done in Europe, it is not surprising that the European power system has a significant amount of HVDC systems and have implemented them in as early as 1954.

• Gotland: Going in service in 1954, Gotland was the first fully commercial HVDC system. It could transfer 20MW of power over a submarine cable 98km long between Sweden’s mainland and the island of Gotland at a voltage of 100kV.
• Cross-Channel: Connecting the British and French power systems, the Cross-Channel was built in 1961. It consisted of a bipolar submarine cable, and had a capacity of 160MW. It is important to note that a bipolar cable was used in order to minimize the impact of the magnetic fields on the compasses of passing ships. Due to increasing demand, the system was upgraded in 1986 to provide a maximum power transmission of 2000MW.
• Italy-Greece: The Italy-Greece HVDC link consists of a monopolar submarine cable, operated at 400kV, with a capacity of 500MW.

D. China

Due to its size, Point-to-Point HVDC is particularly viable in China. Moreover, natural resources are located far away from heavily populated areas and thus high-power high-efficiency transmission of power is needed.

• Gezhouba-Shanghai: The Gezhouba-Shanghai transmission system consists of a bipolar line operating at ±500kV with a maximum power transfer of 1200MW over a distance of 1046km.
• Three Gorges-Guangdong: Operating at a bipolar voltage of ±500kV, the Three-Gorges-Guangdong transmission line is 940km long and has a maximum power transmission rating of 3000MW.
• Three Gorges-Changzhou: Similar to the Three Gorges-Guangdong transmission line, the Three-Gorges-Changzhou transmission line is operated at ±500kV and has a maximum power transmission rating of 3000MW.

E. India

• Sileru-Barsoor: The Sileru-Barsoor transmission system went into service in 1989 and consists of a bipolar transmission line operating at ±200kV with a rating of 400MW.

F. Brazil

Similar to China, due to its size, Brazil offers good opportunities of the use of Point-to-Point systems.

• Itaipu: Connecting the second largest hydroelectric power plant in the world to the large city of Sao Paulo, the Itaipu transmission system consists of two bipolar transmission lines operating at a voltage of ±600kV over distances of 785km and 805km.
• Uruguaiana: Supplying power to the country of Uruguay from the Itaipu hydroelectric power plant, the Back-to-Back Uruguaiana station has a rating of 50MW.

G. Africa

• Cahora Bassa: The Cahora Bassa transmission system connects the Cahora Bassa hydroelectric power plant in Mozambique and the Apollo converter station in South Africa. The HVDC line is bipolar, operating at a voltage of ±533kV and has a maximum rating of 1920MW.

III. System Components

A. Converter Circuit

Figure 1 shows a typical 12-pulse bridge configuration used in HVDC converter stations. This arrangement uses two transformers connected in Y-Y and Y-Δ, in order to provide the necessary 30° shift. As shown in this Figure, 12 thyristor valves are used as the switches for conversion. The use of thyristors for HVDC converters is due to their power capabilities, as compared to other semiconductor devices. Modern thyristors have a blocking voltage of 5-8kV. Therefore, to achieve a typical blocking voltage of an AC bridge (~250kV), several thyrisitors must be connected in series, forming one valve. A control system is implemented to provide the firing signals to each valve, called the Valve Base Electronics. This system provides the signals to the gate electronic units adjacent to each one of the valves, which in turn provide the signals to each one of the individual thyristors. The gate electronic units also provide forward overvoltage protection for the thyristors.

Figure 1 12-Pulse Bridge for HVDC Applications

B. Converter Transformer

As mentioned previously, these transformers are essential for the 12-pulse bridge operation of the converter. Similarly, these transformers offer voltage regulation with on-load tap changers (OLTC) which regulate the voltage in order to compensate for deviations in the AC bus voltage from nominal values and internal voltage drops of the HVDC converter. While their main objective is to allow the phase shift for the bridge operation, they are also implemented to limit the short-circuit current.

C. Cooling System

Figure 2 shows a general cooling system for the thyristor valves. This system can provide either air or liquid cooling for the valves. While, as far as operation of the system, there is no difference between these two types, the selection of either air or liquid cooled systems depends on the specifications as far as cost, size, and maintenance required. Within the valves, the thyristors are the main source of heat, due to their current carrying requirements. The heat from the thyristors is transferred to the coolant via high efficiency heat sinks. In the case of air-cooled systems, the cooling air is forced through a central duct which passes through the valve and exists to the valve hall. On the other hand, for liquid-cooled systems, the coolant is piped directly to the heat sinks.  

Figure 2 HVDC Valve Cooling System

D. Overhead Conductors

The configuration of HVDC transmission lines can either be monopolar or bipolar. Although these two types are technically feasible, bipolar lines are the most common, only in special circumstances are monopolar lines implemented.

During the construction stage of a new HVDC transmission line, monopolar systems can be implemented while the second pole of the system is being commissioned. Thus, although for a period of time the system operates as monopolar, they are usually intended for a final use as bipolar. However, certain systems are designed to operate indefinitely as monopolar, such as the Italy-Greece submarine cable. This is due do the fact that power can be transmitted using a single conductor, and the sea as the return path. Another example of a monopolar system is the Cahora Bassa transmission line which, because of the inaccessible terrain through which it passes, operates as a monopolar system for a portion of its extent, providing power transmission at a reduced rating with return path through the earth.

Figure 3 shows a typical single-circuit bipolar transmission tower configuration. As seen in this Figure, the conductor insulators are arranged in a V-shape in order to reduce the swing width under maximum wind. This arrangement reduces the right-of-way needed. However, it increases the conductor surface gradient which unfavorable increases the exposure to lightning strikes. For this reason, HVDC overhead lines are usually equipped with an overhead ground wire over their length and, typically, two ground wires for its last spans close to the converter stations in order to guarantee an absolute reliable protection at these sections to avoid overvoltage surges that could enter the station.

Figure 3 Bipolar HVDC Transmission Line

Figure 4 and Figure 5 show two different configurations for double circuit bipolar transmission lines. When power transmission capabilities make it necessary for a transmission line to have two circuits, double circuit towers offer significant advantages since a narrower right of way is needed, and cost of construction is lower. Figure 4 shows the two circuits on different levels, while Figure 5 shows both circuits on the same level. Out of these two, the configuration shown in Figure 4 is less expensive and is less visually obstructive. However, the configuration shown in Figure 5 makes it easier to build one circuit and put it in service while the other one is constructed at a later stage.

Figure 4 Two-Level Double Circuit Transmission Line

Figure 5 Single-Level Double Circuit Transmission Line

E. Earth Electrodes

The earth electrodes in an HVDC system provide the necessary reference point for the definition of voltage to ground necessary for insulation coordination and overvoltage protection. The earth electrode also provides a means to increase the availability of the overall system since, for a failure in one of the poles, the system would still be able to transmit power at half its full rating.

F. Cables

As previously mentioned, cables can also be used to transmit power in special configurations such as underground transmission or for transmission over a sea. The construction of these cable is theoretically not very different from that of HVAC (High Voltage Alternating Current) cables. However, there are certain phenomena which are different to that of HVAC. One of the main differences is that the cable capacitance of HVAC cables result in a charging current which reaches the level of the nominal current at about 50km to 80km. On the other hand, charging current is significantly less for HVDC cables, thus increasing its advantages for long cables. Another advantage of HVDC cables over HVAC is that they do not experience the skin effect phenomena, which increases their power carrying capability for the same amount of material, as compared to HVAC.

G. Telecommunication

Due to the fact that both converters in a Point-to-Point HVDC system are involved in the overall control of the system, communication between the two converter stations is essential. Set points used for current and voltage regulation, dynamic control signals used for damping of electromechanical oscillations, control signals used for start-up and shot-down of an HVDC system, and disturbance signals used for protection schemes must be transmitted from both stations to each other in order to provide reliable operation of the overall system.

H. Current Sensors

There are two main types of current sensors used for HVDC, namely, Zero Flux Current Transducers (ZFCT), and Optical Current Transducers (OCT).

The ZFCT is a contact-less measuring system consisting of a core and coil assembly which wraps around the current-carrying conductor. The system has an electric module which consists of a burden resistor and an output amplifier. The core and coil assembly has three individual cores. The output current of the core and coil assembly passes through the burden resistor. The amplifier is connected in parallel to the burden resistor and therefore amplifies the voltage across it, providing the measurement signal.

The OCT, on the other hand, is based on electro-optical effects. It offers advantages over the ZFCT, such as electrical isolation from the high voltage system, prevention of electromagnetic interference of the current sensor, and reduction of the ground current effects. For this reason, the OCT is the most used type of current transducer for HVDC applications.

IV. Protection and Control

Having discussed all of the main components of an HVDC system, the protection and control schemes can now be discussed. The protection schemes need to be separated between AC and DC protections. As mentioned earlier, the AC protection schemes are more commonly known and follow common practices used over the last century in AC systems. Due to the fact that DC systems are not as common as AC systems, DC protection schemes are not so standardized. The following DC protections show schemes currently being implemented in HVDC installations.

A. AC Protections

1) Transformer Differential Protection

The objective of this protection is to detect faults in the converter’s transformers. It operates on a per-phase basis by comparing the currents on the AC bus side and the currents in the valve side of each transformer. When the transformer loses its ampere-turn balance, a transformer differential fault is declared. Although the general idea of the differential scheme is the previously mentioned, more complex schemes implement restraint functions based on the harmonic contents of the currents to ensure reliable operation under inrush conditions, and to detect overexciation conditions. Moreover, the AC bus may be included in the zone of protection of the transformer differential protection, in order to detect faults on the bus. Due to the inherent characteristic of this type of protection, coordination with other protective devices is not required.

2) AC Overcurrent Protection

The AC overcurrent protection serves as a backup to the transformer differential protection. The protective device measures the current in the bus side of the transformer and operates on an inverse time and/or definite time characteristic.

3) AC Filter Overload Protection

This protection measures the fundamental and harmonic currents in the filters to detect an overload condition. The protection has an inverse time characteristic which is set based on the damage curves of the filters.

4) AC Filter and Capacitor Bank Unbalance Protection

Each capacitor can in the capacitor bank has a number of capacitor elements that can be arranged either in series or parallel. The scheme detects a short circuit in one or more capacitor elements by comparing the currents in two capacitor chains, either parallel in the same branch or of different branches.

B. DC Protections

1) Converter/Valve Differential Protection

This protection scheme detects faults on the valves or on the DC side of the converter. It does so by comparing the AC side (converter transformer secondary winding) current to the DC side pole line and neutral bus currents.  The scheme only operates when the AC current exceeds the DC current significantly, indicating a ground fault or short circuit within the pole of the valve. This protection is the most critical of the HVDC systems since, if it does not operate currently, then the AC overcurrent protection may operate instead, resulting in a total loss of the system.

2) DC Overcurrent Protection

The DC overcurrent protection operates on the highest of the transformer valve side currents or the DC currents. It has an inverse time characteristic that is set based on the damage curve of the valves (overheating due to excessive current).

3) Commutation Failure Protection

This scheme detects failures in the 12-pulse bridge due to abnormal commutation conditions. It does so by measuring the AC currents and the DC currents and operating if the DC current is higher than the AC current, which indicates a commutation failure.

4) DC Harmonic Protection

This protection functions as a back-up to the commutation failure protection by measuring the harmonic content of the DC side current. If the fundamental frequency component of the DC current exceeds a certain value, a commutation failure is declared.

5) Voltage Stress Protection

The voltage stress protection protects the converter equipment from dielectric stresses caused by the AC side voltage. The AC voltage on the bus side of the converter is measured and compensated for by the tap-changer position and compared to a reference signal. If the measured voltage is greater than the reference, the system trips.

6) Valve Misfire Protection

The control system in the Valve Base Electronics provides the firing pulses to the thyristor valves. This information is compared with actual information about the firing of the valves. If the valves fired outside of the interval it was signaled to fire at (or did not fire at all) the protection operates due to a valve malfunction.

7) DC Ground Fault Protection

This scheme detects ground faults on the DC side of the converter by measuring the DC currents in both the low voltage and high voltage sides of the converter. A fault is detected if a difference exists between these two, by a set amount.

8) DC Overvoltage Protection

In order to detect an overvoltage on the DC line, this protection measures both the voltage and current through the line. A fault is detected by a combination of both an overvoltage and low current conditions.

9) DC Line Protection

This scheme detects ground faults in the DC line by measuring the voltage on it. It operates in two different ways. First, if the voltage on the line drops below a minimum value, a fault is declared. Second, if the voltage drops below a certain value (higher than the minimum value) and with a derivative exceeding a certain value, a fault is also declared. This two-level algorithm provides improved reliability over a single level scheme. This protection is only active at the terminal operating in rectifier mode. When a fault is detected, the converter is forced into full inversion in order to extinguish the fault current. After a certain interval of zero current and zero voltage, the rectifier is restarted and, if the fault was not permanent, power transmission will be restored. This overall scheme is analogous to the reclosing sequence implemented after a line fault is cleared in an AC system, the rectifier end in this case being the live-or-dead-line (LODL) terminal in an AC system.

10) Electrode Line Open Circuit Protection

This scheme protects the neutral bus of apparatus from overvoltages. It operates on the current flowing through the lightning arrester, which is used to keep overvoltages down.

C. Protection by Control

In AC systems, since there is no control action that can interrupt a short circuit, faults are cleared by isolating the failed device via circuit breakers. This is feasible due to the fact that AC currents can be interrupted, as their instantaneous value is zero amperes every half cycle. In DC systems, however, the fault needs to be interrupted through other means if only the DC faulted equipment is to be taken out of service (i.e., the fault could also be cleared by tripping the AC side breakers, but it would take the entire system out of service). This is due to the fact that, if the fault were to be interrupted in the DC side, fatal damage to the semiconductor devices may occur. Because HVDC systems are based on fast semiconductor devices such as thyristors, control actions can be taken quickly to extinguish a fault.

1) Retarding the Converter

When the system detects a fault, the control pulses to the valves at the terminal operating as a rectifier may be retarded. That is, every control pulse is delayed with respect to the previous one which increases the firing angle until a full inverter operation is obtained. This operation is done to reverse the voltage of the rectifier converter in order to extinguish the fault current.

2) Blocking the Converter

The blocking action removes the control pulses to the valves in the converter. By removing the control pulses, the thyristors stop conducting once the current reaches zero. As mentioned previously, interruption of DC fault current can cause severe damage to the valves. For this reason, a bypass is first implemented by firing two opposite valves within the same AC phase.

V. Trends and Future Research

A. Modular Multilevel Converters

Introduced in 2003, the Modular Multilevel Converter (MCC) is a type of converter composed of six valves, each one with a number of IGBTs connected in series, therefore operating as its own controllable voltage source, as shown in Figure 6. As seen in this Figure, each submodule contains two IGBTs and a storage capacitor. The amount of submodules connected in series determines the converter’s ability to closely resemble a sinusoidal waveform. For an infinite amount of submodules, the converter can output a sinusoidal waveform with no harmonics. Although this is clearly not feasible, the MCC offers less harmonic contents than other types of converters.

Figure 6 Modular Multilevel Converter (MCC)

B. Embedded HVDC

As previously mentioned, the most common uses of HVDC systems are in Back-to-Back interconnection of unsynchronized AC systems and in Point-to-Point power transmission over long distances. The latter makes an important observation in that, in these systems, power is transferred between two converter stations with the sole purpose of bringing power from a remote source such as a hydroelectric plant, for example, to places where demand is higher. However, research has been done in the last decade to implemented HVDC transmission as part of a meshed AC system, thus providing, not only another path for transmission, but better reliability of the system.

C. 800kV HVDC Systems

Due to the increasing demand of power transmission over long distances, higher operating voltages have been considered. For example, in China, hydroelectric plants west of the Three Gorges are under development. These plants can be up to 2000km away from the load centers in eastern China. For these projects, 800kV HVDC Point-to-Point systems are planned due to their higher efficiency and cost benefits.

VI.  Conclusions

HVDC systems offer significant advantages over AC systems in certain areas. However, due to the limited use of these systems as compared to AC systems, protection and control schemes are not as well standardized. In this paper, an introduction to protection and control schemes currently used in industry was introduced by mentioning various projects, the system components used, and their applications. Through this study, the differences between AC and DC system protection can be observed. Future areas of improvements can be identified by analyzing these schemes.