SCADA and Systems Monitoring

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2 SCADA and Systems Monitoring SCADA stands for supervisory control and data acquisition system. There are locations, in most of the power systems, where operations, such as opening and closing circuit breakers, must be done from time to time, but the cost of providing and maintaining operator attendance cannot be justified. Sending an operator to such locations could results in delay in some cases, as a result the outage may be lengthen and end up in total failure in some cases.

3 SCADA and Systems Monitoring Moreover, the cost of providing attendance at remote stations or providing personnel to perform switching continues to rise. This makes operator attendance of remote stations even more uneconomical. Devices to control equipment remotely have been used for many years. The need for remote indication as well as control led to the development of equipment that could perform operations, monitor them, and report back.

4 SCADA and Systems Monitoring At the same time it is often important to transmit such information as loads and bus voltages to an operating center, to make corrective action in case of a violation. Almost all modern dispatch and operating centers of power systems are now provided with at least some SCADA system equipment. These equipment have proven to be efficient and economical for power system operations. It is a very effective aid for station operators, making it possible for them to maintain relatively complete knowledge of conditions on the portions of the system

5 SCADA and Systems Monitoring The SCADA system allows a few operators to monitor the generation and high-voltage transmission systems and to take action to correct overloads or out-of limits.

6 Control and Supervision The term supervisory control is normally applied to remote operation of devices such as motors and circuit breakers. The term supervision means signaling back to indicate that the intended operation has been carried out. Simple supervisory control systems have been used since early days of utility operations. The supervision was provided by lights in early systems. Green light to indicate an open device and red to indicate the contrary

7 Control and Supervision In such systems, wires between the control point and the controlled device being were required for each device being controlled and supervised remotely. When there are many such devices involved, the cost and complexity of supervisory schemes increased directly with the number of devices to be controlled remotely. Moreover, when significant distances were involved, the associated costs were increasingly high and reliability suffered because of the induced electrical noise in the control circuit. The possibility of physical failure has also increased due to such long control circuit.

8 Control and Supervision Some of the shortcomings of direct-wire circuits between the controlling and controlled equipment on a one-for-one basis were minimized by use of the selective relays. By such means it was possible to select the device to be controlled, operate it, and transmit a supervision signal back to the control center over a single control circuit. This type of systems became complex, and sometimes difficult to maintain. They were also limited in speed of operation, and in maximum data transfer when the number of controlled and supervision devices became large.

9 Control and Supervision The arrival of electronic communication methods and digital data transmission provide a means of greatly increasing the capabilities of supervisory control. Sequential scanning and supplied with remote supervisory terminal units, made it possible for one master station to control several remote stations and many devices at each remote station. It was possible to telemeter the control actions taken by the remote units as well as live analog data on current, voltage and many other items needed for the complete supervision of remote station.

10 Control and Supervision Reduction in data transferred between the remote units and the master station was done by a procedure known as exception reporting. This means certain data are transferred only when they change or fall outside previously set limits. These developments made it possible for central location equipped with a supervisory masters station to have almost complete control, and information concerning the status of stations under the control of a single master station.

11 Control and Supervision In most of the systems, the master unit sequentially scans the remote terminal units (RTUs) by sending short message. If it does, the RTU will send a message back to the master, and the data received will be stored into the computer. If necessary, a control message will be sent to the RTU, and an alarm or message will be printed on the master typewriter and displayed on the cathode-ray-tube (CRT) screen. The scan cycle of RTUs is 2s

12 Control and Supervision However, in the event of trouble at a remote station, a message will be sent from remote station to master unit. In some cases, the master units can automatically perform predetermined control action.

13 Control and Supervision All modern supervisory control systems are computerbased; that is the master unit consists of a digital computer with input and output equipment. The input and output equipment are used to send and receive information from RTUs. The received information is displayed on CRTs and/or printed on electric typewriters for permanent records. CRTs can also display graphic information, such as oneline diagrams of the remote stations. In many control centers, overall system status is also shown on wall diagrams, which are kept current with existing conditions by data from the remote terminals.

14 Configuration of SCADA systems Basic elements in a SCADA system are a master unit and RTUs. Links with MU and RTUs were made through communication channels. A number of configurations can be used depending on the requirement, availability of communication channel, and cost.

15 Configuration of SCADA systems (a) One-for-one SCADA system. M R

16 Configuration of SCADA systems (b) STAR or HUP SCADA system. R R M R R

17 Configuration of SCADA systems (c) Party-line SCADA system. M R R R R

18 Configuration of SCADA systems (c) Combination of STAR and Party-line SCADA system. R R M R R R R R

19 Configuration of SCADA systems Reliability of SCADA systems can also be increased by providing alternative communication channels. The overall reliability of a system can be no better than that of the communications communication channels are exposed to various hazards, they usually the least reliable part of SCADA systems.

20 Supervisory Master Units Master unit of SCADA is the heart of the system All operator-initiated operations of an RTU are made through the master unit and are reported back to the master from the RTUs. Modern supervisory master units consist of a digital computer and equipment to permit communications between the master unit and RTUs. Such equipment includes modems to convert the digital pulses used by computer to a form that can be transmitted and received from and RTUs.

21 Supervisory Master Units In addition to the computer, peripheral necessary for the proper operation of the system is provided. (a) (b) (c) (d) A control console Keyboards or other means for the purpose of entering the data CRTs or monitors Printers In some cases, a simplified one-line diagram of the power system is supplied in the form of a large wall map that shows when stations are normal and abnormal conditions exist.

22 Supervisory Master Units When an operation is performed from a SCADA master unit, every effort is made to ensure that the desired device is selected and that the correct operation is chosen. The operator at the master station follows a procedure called the select before operate method.

23 Supervisory Master Units Select before operate method o The operator selects the remote station. o The remote station acknowledges that it has been selected. o The operator selects the device to be operated. o The RTU acknowledges that the device has been selected. o The operator performs the operation. o The RTU carry out the operation and signal back to the master unit that it has been performed.

24 Supervisory Remote Units These are located at some selected stations. They are either wired to perform certain pre-selected functions, or in modern units, equipped with microcomputers which have memory and logic capabilities. RTUs with microcomputer called intelligent remotes Can perform some functions without the direction of the master unit.

25 Supervisory Remote Units However, any operation performed reported back to the master unit in the next scan. The RTUs can also drive a new element, the programmable controller (PC), a dedicated controller with memory and logic. The RTUs are also equipped with modems so that they accept messages from master and signal back to the master Relays located at the RTUs are used to open or close the selected control circuits to be controlled equipment on command from master unit

26 Supervisory Remote Units Transducers in the remote units are used to convert measurement such as voltage, current, watts, and vars to direct current or voltage signals proportional to the respective measured quantity. Then A/D converters are used to convert the measured quantity into digital form.

27 Communication for SCADA Systems Communication is an essential part of SCADA systems. The communication links can be wire circuits, microwave channel, or power line carrier channels. Any communication circuit that provides an adequate signal to noise ratio and has a bandwidth capable of passing the data signals at the rate at which they are transmitted can be used.

28 Communication for SCADA Systems Communication is an essential part of SCADA systems. The communication links can be wire circuits, microwave channel, or power line carrier channels. Any communication circuit that provides an adequate signal to noise ratio and has a bandwidth capable of passing the data signals at the rate at which they are transmitted can be used.

29 Communication for SCADA Systems In most application, a normal telephone voice channel of about 400 to 3400 Hz bandwidth is satisfactory. For low speed data transmission, a narrow bandwidth, usually located above the highest voice frequency, can be used. In such cases, the voice band is restricted to about 400 to 2200 Hz, and the data are transmitted from 2200 to 3400 Hz.

30 Communication for SCADA Systems This type of operation is called Speech-plus, and it provides for both voice and data communication on a single voice channel, with a somewhat degraded voice channel. It should be stressed that communication is of primary importance for SCADA system. Poor communication results in errors or lost messages. A system cannot function properly without reliable and adequate communication channels.

31 Energy Management Systems (EMS) For a power system to be able to supply to all its customers within acceptable voltage and frequency limits, it must be able to over come some unavoidable disturbances. Some of these disturbances could be quite abnormal, but nevertheless credible; such disturbances are: Line outages following shunt faults Equipment failure and subsequent isolation Switch surges and lightning strikes Mechanical damages (due to wind, snow storm)

32 Energy Management Systems (EMS) Some of these disturbances can be dealt with the help of protective devices and the system restored back to normal stage within few cycles. Other disturbances may cause transient oscillations, which could last for several seconds, producing large oscillations in power flow. This results in abnormal voltage and frequency and subsequent tripping of plant items. For this purpose and other reasons an Energy Management System is vital for any power systems.

33 Energy Management Systems (EMS) An EMS enables engineers to operate and control the network in real time and includes facilities to capture the current state of the system and to instruct generating plants and other controllable system components such that all the customers are supplied, at least cost, with security. Considerable back-up facilities are necessary, including special software programs, display facilities, and support staff.

34 Energy Management Systems (EMS) Hierarchy of power system with EMS Central Coordinating Control Level 3 Area Control Level 2 Local Control Level 1 Network and Substations Level 0

35 Energy Management Systems (EMS) At level 0 is the power system with isolators, switchgears, interconnections, transmission lines, cables, transformers, etc are located; At level 1 substation (local) controls are located. The local controls at level 1 may include protection relays, tap-change controllers, and compensator controls, with operating channels to the level 0 units.

36 Energy Management Systems (EMS) Level 1 controls often comprise digital/electronic devices for voltage and current measurement, inter locking and facilities for receiving and sending data to the next level up (area concentrators). In many cases, level 1 consists of racks of electronics within RTUs. At level 2, man-machine interfacing and data concentrators enable control and maintenance to be exercised so that the whole system can be kept in reliable and efficient condition.

37 Energy Management Systems (EMS) At the top level (3), the SCADA system resides, usually in a single control center (variously called Pool, National, or System control).

38 Energy Management Systems (EMS) The SCADA system accepts data from the various level 1 collectors and displays it in a meaningful way to the control engineers or operators, usually by one-line mimic diagram on a Colour video screen as seen in the previous section. The EMS processes SCADA data in various ways, including topology identification by using the dynamic data from switch-gear, isolators and other system connectors.

39 Energy Management Systems (EMS) Some EMS functions are: State estimation Security analysis Voltage reduction Load shedding Load forecasting

40 Energy Management Systems (EMS) State estimation (Power System State Estimation - PSSE) PSSE is a process in which data telemetered from network measuring points to a central computer, can be formed into a set of reliable data (the data base ) for control and recording purpose.

41 Flexible AC Transmission System Alternating current transmission systems incorporating power electronics-based and other static controllers to enhance controllability and increase power transfer capability

42 Large Electric Power Systems Simple illustration of the power transmission system Power system structure S = P + jq Pi Qi = PGenerator + PLoad + PCompensation = QGenerator + QLoad + QCompensation

43 Apparent Complex Power: S = P + jq Real Power: P V X 2 sin Reactive Power: 2V Q V * I * sin( / 2) * (1 cos ) X 2 V voltage X reactance phase angle I current

44 FACTS devices = Flexible Alternating Current Transmission System devices Direct control of power flow over designated transmission routes Fast Control Technology to overcome limitations to Power Transfer Capability through rapid response P V X 2 * sin

45 What is FACTS? The FACTS technology is a collection of controllers, which can be applied individually or in coordination with others to control one or more of the interrelated system parameters, such as series impedance, shunt impedance, current, voltage, and damping of oscillations.

46 What limits the Loading Capability? Thermal For overhead line, thermal capability is a function of ambient temperature, wind conditions, conditions of conductor, and ground clearance. The FACTS technology can help in making an effective used of newfound line capability. Dielectric Being designed very conservatively, most lines can increase operation voltage by 10% or even higher. FACTS technology could be used to ensure acceptable over-voltage and power flow conditions. Stability The stability issues that limit the transmission capability include: transient stability, dynamic stability, steady-state stability, frequency collapse. Voltage collapse, and sub-synchronous resonance. The FACTS technology can certainly be used to overcome any of the stability limits.

47 A Simple Example of FACTS

48 Basic types of FACTS Controllers Series controllers: The series controller could be a variable impedance or a variable source both are power electronics based. In principle, all series controllers inject voltage in series with the line. Shunt controllers: The shunt controllers may be variable impedance connected to the line voltage causes a variable current flow hence represents injection of current into the line. Combined series-series controllers: The combination could be separate series controllers or unified series-series controller--- Interline Power Flow Controller. Combined series-shunt controllers: The combination could be separated series and shunt controllers or a unified power flow controller

49 Relative Importance of Different Types of Controllers For a given MVA size, the series controller is several times more powerful than the shunt controller in application of controlling the power/current flow. Drawing from or injecting current into the line, the shunt controller is a good way to control voltage at and around the point of connection. The shunt controller serves the bus node independently of the individual lines connected to the bus. Series connected controllers have to be designed to ride through contingency and dynamic overloads, and ride through or bypass short circuit currents. A combination of series and shunt controllers can provide the best of effective power/current flow and line voltage. FACTS controllers may be based on thyristor devices with no gate turn-off or with power devices with gate turn-off capability. The principle controllers are based on the dc to ac converters with bidirectional power flow capability.

50 Relative Importance of Different Types of Controllers Energy storage systems are needed when active power is involved in the power flow. Battery, capacitor, superconducting magnet, or any other source of energy can be added in parallel through an electronic interface to replenish the converter s dc storage. A controller with storage is more effective for controlling the system dynamics. A converter-based controller can be designed with high pulse order or pulse width modulation to reduce the low order harmonic generation to a very low level. A converter can be designed to generate the correct waveform in order to act as an active filter. A converter can also be controlled and operated in a way that it balances the unbalanced voltages, involving transfer of energy between phases. A converter can do all of these beneficial things simultaneously I the converter is so designed.

51 Brief Description and Definitions of FACTS controllers Shunt connected controllers Series connected controllers Combined shunt and series connected controllers

52 Shunt connected controllers

53 Series connected controllers

54 Combined shunt and series connected controllers

55 Other controllers

56 SVC, Static VAR Compensator (shunt connected controller) Note: The control strategy usually aims to maintain the transmission line voltage at a fixed level.

57 STATCOM, Static Compensator---Advanced Static VAR Compensator (shunt connected controller) The main features: 1. Wide operating range 2. Lower rating than SVC 3. Increased transient rating and superior capability to handle dynamic system disturbances

58 TCSC, thyristor-controlled series capacitor (series-connected controller) Note: The TCSC behaves as a tunable parallel LC-circuit to the line current. As the impedance of XL is varied from its maximum (infinity) toward its minimum wl, the TCSC increases its capacitive impedance.

59 UPFC, unified power flow controller (combined shunt and series connected controllers) 1. The UPFC consists of an a series STATCOM and a shunt SATACOM with a common DC link. 2. Power control is achieved by adding series voltage Vinj to Vs, thus giving the line voltage VL. 3. With two converters, the UPFC can supply active power in addition to reactive power.

60 UPFC Capabilities Increase transmission line capacity Direct power flow along selected lines Powerful system oscillation damping Voltage support and regulation Control of active and reactive power flow at both sending- and receiving-end

61 Operation Reactive power is generated or absorbed by the shunt inverter to control bus voltage Reactive power is generated or absorbed by the series inverter to control the real and/or reactive power flow on the transmission line. A portion of the real power flow on the transmission line is drawn from the bus by the shunt inverter to charge the DC capacitor. Real power is inserted into the line through the series inverter.

62 Power flow in a transmission line V S jx V R P SR VV S X R sin and V V jxi R S P SR V R jxi V S To increase P SR, increase

63 V A - V inj + V B V B V inj V A V S jx V R - V + inj VV S R V P sin R SR X V R P SR V inj jxi V S

64 How is V inj created? a 1 b 1 c 1 + V a 2 b 2 c 2

65 a 1 on, b 1 on, c 1 off V ab =0, V bc =V, V ca = -V a 1 a 2 b 1 b 2 c 1 c 2 + V a 1 on, b 1 off, c 1 off V ab =V, V bc =0, V ca = -V a 1 b 1 c 1 a 2 b 2 c 2 + V a 1 on, b 1 off, c 1 on V ab =V, V bc =-V, V ca = 0 a 1 b 1 c 1 a 2 b 2 c 2 + V

66 Introduction Electric power transmission was originally developed with direct current. The first commercial HVDC line built in 1954, a 98 km submarine cable with ground return between the island of Gotland and the Swedish mainland. Thyristors were applied to D C transmission in the late 1960 s and solid state valves became a reality. D.C. transmission is now an integral part of the delivery of electricity in many countries throughout the world.

67 Introduction A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy currents required by the cable capacitance. HVDC allows power transmission between unsynchronized AC distribution systems, and can increase system stability by preventing cascading failures from propagating from one part of a wider power transmission grid to another.

68 Why HVDC systems?

69 Why HVDC An overhead d.c. transmission line with its towers can be designed to be less costly per unit of length than an equivalent a.c. line designed to transmit the same level of electric power. However the d.c. converter stations at each end are more costly than the terminating stations of an a.c. line and so there is a breakeven distance above which the total cost of d.c. transmission is less than its a.c. transmission alternative. The d.c. transmission line can have a lower visual profile than an equivalent a.c. line and so contributes to a lower environmental impact. There are other environmental advantages to a d.c. transmission line through the electric and magnetic fields being d.c. instead of ac. If transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km but d.c. cable transmission systems are in service whose length is in hundreds of kilometers and even distances of 600 km or greater have been considered feasible. Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them are quite small. It is physically impossible to connect the two together by direct A.C methods in order to exchange electric power between them. However, if a d.c. converter station is located in each system with an interconnecting d.c. link between them, it is possible to transfer the required power flow even though the a.c. systems so connected remain asynchronous.

70 HVDC Advantages 1. Interconnections between asynchronous systems. 2. Deliver energy from remote energy sources. Where generation has been developed at remote sites of available energy, HVDC transmission has been an economical means to bring the electricity to load centers. Gas fired thermal generation can Import electric energy into congested load areas. 3. Increasing the capacity of existing a.c. transmission by conversion to d.c. transmission. 4. New transmission rights-of-way may be impossible to obtain. Existing overhead a.c. transmission lines if upgraded to or overbuilt with d.c. transmission can substantially increase the power transfer capability on the existing right-of-way. 5. Power flow control. A.c. networks do not easily accommodate desired power flow control. Power marketers and system operators may require the power flow control capability provided by HVDC transmission. 6. Stabilization of electric power networks. Some wide spread a.c. power system networks operate at stability limits well below the thermal capacity of their transmission conductors. HVDC transmission is an option to consider to increase utilization of network conductors along with the various power electronic controllers which can be applied on a.c. transmission.

71 Advantages over HVAC Bulk Power transfer with lower capital costs and with lower losses than AC. In many applications HVDC is more effective than AC transmission. Undersea cables, where high capacitance causes additional AC losses. Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps'. Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install Power transmission and stabilization between unsynchronized AC distribution systems Connecting a remote generating plant to the distribution grid. Stabilizing a predominantly AC power-grid, without increasing prospective short circuit current Reducing line cost. HVDC needs fewer conductors as there is no need to support multiple phases. Thinner conductors can be used since HVDC does not suffer from the skin effect Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies Synchronize AC produced by renewable energy sources

72 BASIC HVDC Topology

73 HVDC transmission The HVDC transmission has the major advantage of flexibility in power exchange in comparison with HVAC. Moreover, in case of transmission of bulk electrical energy over long distances (more than 50 km) by submarine cable, the only applicable technology is the HVDC transmission. HVDC is an intelligent link. The power flow can be controlled and changed very quickly practically independently from frequency at the terminal busses, voltage or power angle at the terminal busses like in HVAC transmission. Due to the inherent properties of the electronic equipment which forms the converter bridges, the direction of the transmitted power may be also changed very quickly.

74 HVDC transmission principles and characteristics In a HVDC system, the electricity is: - Taken from a 3-phase AC network - Converted to DC in a converter station - Transmitted by DC OHL or cable (underground or sub sea) - Converted back to AC in another converter station - Injected into AC network Controllable power injected where needed,supplemental control, frequency control - Bypass congested circuits no inadvertent flow - Higher power, fewer lines,no intermediate S/S needed - Lower losses - Facilitates integration of remote diverse resources with less impact on existing grid - Low expensive transmission line - Reactive power demand limited to terminals independent of distances - Narrower ROW, no EMF constraints - No limit to underground or submarine cable length

75 HVDC CONFIGURATION

76 MONOPOLAR AND EARTH RETURN 1. One of the terminals of the rectifier is connected to earth ground. The other terminal, at a potential high above or below ground, is connected to a transmission line. 2. The earthed terminal may be connected to the corresponding connection at the inverting station by means of a second conductor. 3. If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations. 4. Therefore it is a type of single wire earth return. The issues surrounding earth-return current include: Electrochemical corrosion of long buried metal objects such as pipelines Underwater earth-return electrodes in seawater may produce chlorine or otherwise affect water chemistry. An unbalanced current path may result in a net magnetic field, which can affect magnetic navigational compasses for ships passing over an underwater cable. 5. These effects can be eliminated with installation of a metallic return conductor between the two ends of the monopolar transmission line. 6. Since one terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmission voltage which makes it less costly than the high-voltage conductor. 7. Use of a metallic return conductor is decided based on economic, technical and environmental factors. 8. Modern monopolar systems for pure overhead lines carry typically 1,500 MW. If underground or underwater cables are used, the typical value is 600 MW. 9. Most monopolar systems are designed for future bipolar expansion. 10. Transmission line towers may be designed to carry two conductors, even if only one is used initially for the monopole transmission system.

77 BIPOLAR 1. In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. 2. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor. 3. Advantages to bipolar transmission. Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return. This reduces earth return loss and environmental effects. When a fault develops in a line, with earth return electrodes installed at each end of the line, approximately half the rated power can continue to flow using the earth as a return path, operating in monopolar mode. Since for a given total power rating each conductor of a bipolar line carries only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating. In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged. 4. A bipolar scheme can be implemented so that the polarity of one or both poles can be changed. 5. This allows the operation as two parallel monopoles. 6. If one conductor fails, transmission can still continue at reduced capacity. 7. Losses may increase if ground electrodes and lines are not designed for the extra current in this mode. To reduce losses in this case, intermediate switching stations may be installed, at which line segments can be switched off or parallelized.

78 BACK to BACK A back-to-back station (or B2B for short) is a plant in which both static inverters and rectifiers are in the same area, usually in the same building. The length of the direct current line is kept as short as possible. HVDC back-to-back stations are used for Coupling of electricity mains of different frequency (as in Japan; and the GCC interconnection between UAE [50 Hz] and Saudi Arabia [60 Hz] under construction in ± ) Coupling two networks of the same nominal frequency but no fixed phase relationship Different frequency and phase number. The DC voltage in the intermediate circuit can be selected freely at HVDC back-to-back stations because of the short conductor length.

79 OTHER HVDC CONFIGURATIONS

80 Components of HVDC System

81 Rectification and inversion use essentially the same machinery. Many substations (Converter Stations) are set up in such a way that they can act as both rectifiers and inverters. At the AC end a set of transformers, often three physically separated single-phase transformers, isolate the station from the AC supply, to provide a local earth, and to ensure the correct eventual DC voltage. The output of these transformers is then connected to a bridge rectifier formed by a number of valves. The basic configuration uses six valves, connecting each of the three phases to each of the two DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails. In addition to the conversion transformers and valve-sets, various passive resistive and reactive components help filter harmonics out of the DC rails.

82 HVDC Disadvantages The disadvantages of HVDC are in conversion, switching, control, availability and maintenance. HVDC is less reliable and has lower availability than alternating current (AC) systems, mainly due to the extra conversion equipment. The required static inverters are expensive and have limited overload capacity In contrast to AC systems, realizing multiterminal systems is complex, as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals HVDC circuit breakers are difficult to build because some mechanism must be included in the circuit breaker to force current to zero, otherwise arcing and contact wear would be too great to allow reliable switching. Operating a HVDC scheme requires many spare parts to be kept, often exclusively for one system, as HVDC systems are less standardized than AC systems and technology changes faster.