DIFFERENCE BETWEEN SOLID STATE AND OFFLINE UPS SYSTEM

Solid-state Controls systems differ from off-line designs in the following ways:

1. Solid-state Controls offers an on-line, double conversion UPS system. Therefore, the client’s critical load is being powered continuously from the inverter. The battery is always being floated by the fully-rated rectifier/charger and is always connected to the inverter input. As result, all components used in the Solid-state Controls design are fully rated to carry 120% of the load on a continuous basis over a 20-year life.

2. Solid-state Controls’ Ferro resonant design has a built in capacity to supply nonlinear (crest factor) loads of up to 3:1 without oversizing.

3. Solid-state Controls’ on-line UPS design is provided with a make-before-break static switch to aid in downstream fault clearing and to protect against possible system failures.

4. Solid-state Controls’ on-line UPS system is a double conversion type, converts AC to DC and then reconverts DC back to AC. It is also insensitive to frequency deviations at the primary input.

5. Solid-state Controls’ on-line UPS system is designed to handle input voltage deviations of +10% to  15% without causing the battery system to take over the inverter’s load. This reduces the UPS systems reliance on its battery, thereby increasing the life of the battery system.

PROBLEMS WITH OFFLINE UPS SYSTEM

The following is an outline of some of the major problems associated with off-line (stand by) UPS designs.

INPUT FREQUENCY/ VOLTAGE PASS THROUGH

The off-line unit is designed to pass through the input line voltage and frequency to the load. (Note: the power passed through is non-conditioned utility power.) While this may be fine for office environments, it is not acceptable for industrial settings with periodic voltage and frequency deviations. Due to the design of off-line systems, some of the deviations will be passed directly to the loads, causing loads to drop and/or loss of data. Off-line UPS suppliers could tighten input parameters so these levels of voltage and frequency are not passed through. However, this would require the systems’ batteries to assume the load more frequently.

BATTERY PICKUP

If the input voltage and frequency deviate outside of acceptable limits, the systems’ batteries will automatically assume the supply of the charger/inverter. While this mode of operation rectifies the problem of voltage and frequency pass through, it can cause other serious problems.

First, if the voltage and frequency deviate (for example every time a motor or pump starts up) then the system will be operating on its batteries. The batteries supplied with off-line systems are a valve-regulated “maintenance free” type.

These batteries are very sensitive to cycling. (Cycling is defined as any time that the battery supplies current to the load.) Cycling is not time dependent, so a one minute discharge is just as bad as a ten minute discharge. A battery is designed to supply only a certain number of cycles over life. (Note: valve-regulated batteries have a limited number of cycles, even less than other battery types.) Therefore, it’s not hard to imagine what will happen when an off-line system is supplied for an industrial setting. The constant starting of motors, pumps and other electrical devices will result in voltage deviations outside the limits of the pass through logic. This will result in the systems batteries being cycled each time it occurs. Eventually, you will exceed the limited number of cycles available and the batteries will fail. This will undoubtedly happen when you least expect it, and you will probably not be aware of the condition. The result is that you will drop your critical load and also have to replace your batteries.

Another problem associated with the battery pickup feature is that even if the batteries are functioning normally failure may occur because of a lack of recharge current. Typically, off-line UPS systems are not supplied with fully rated chargers. Instead the systems are supplied with a “trickle chargers.” These “trickle chargers” are not designed to quickly recharge the system’s batteries after a discharge. If the batteries are being cycled often, the result is that the “trickle charger” may not be able to fully recharge the batteries in-between discharges. The batteries can be discharged to a state from which they can no longer supply the required current to your critical loads. Not only does this damage the system’s batteries, but it will also drop critical loads.

INABILITY TO HANDLE NONLINEAR LOADS

Off-line systems do not handle nonlinear (crest factor) loads well. Therefore, in order to supply these types of loads, off-line systems must often be oversized. (Note: Typical nonlinear loads are DCS systems and computer loads.) If the systems are not oversized to handle these types of loads, they will deprive the load of necessary current, resulting in the “flat topping” of the current wave form. The result would be loss of data and/or system failures.

SIZING OF SYSTEM’S COMPONENTS

There are also component sizing concerns. (Some off-line manufacturers do not size the components utilized in the chargers and inverters to handle the systems full load on a continuous basis.) The thought process is that the station line voltage, frequency and current are going to be normally passed directly through to the loads. However, the off-line design, when utilized in an industrial setting, will require the inverter to supply the critical loads on a fairly regular basis. It stands to reason that the undersized components utilized in an off-line design will fail more often because they were never intended for continuous duty. It must also be noted that when the off-line system utilizes its charger/inverter, it is functioning in a most precarious position. If any component fails during this operation mode, your critical load will be dropped. However, in the off-line system you may never know if a component has failed until needed because the only time the charger/inverter components are turned “on” is when it is needed to supply your critical load. This is very much like a light bulb, it only blows when power is applied and you can never predict when it will happen.

LACK OF OVERLOAD/ SYSTEM PROTECTION

The off-line system would drop the load because it is not supplied with a static switch, there is no capability to supply high levels of fault current to the load. This point alone shows that the off-line design was never designed for primary use in an industrial environment. Industrial environments require a capacity for high fault-clearing capabilities. Office environments do not require this type of current capability.

Hence in an industrial atmosphere off-line systems fails. Off-line designs are not reliable for long term operation in industrial environments. Off-line systems are typically manufactured for office type environments which do not place the demands on the system that an industrial environment does.

DISTRIBUTION STATCOM D-STATCOM

The D-STATCOM is basically one of the custom power devices. It is nothing but a STATCOM but used at the Distribution level. The D-STATCOM is a voltage or current source inverter based custom power device connected in shunt with the power system. It is connected near the load at the distribution systems. The key component of the D-STATCOM is a power VSC that is based on high power electronics technologies. Basically, the D-STATCOM system is comprised of three main parts: a VSC, a set of coupling reactors and a controller. The basic principle of a D-STATCOM installed in a power system is the generation of a controllable ac voltage source by a voltage source converter (VSC) connected to a dc capacitor (energy storage device). The ac voltage source, in general, appears behind a transformer leakage reactance. The active and reactive power transfer between the power system and the D-STATCOM is caused by the voltage difference across this reactance. The D-STATCOM is connected in shunt with the power networks at customer side, where the voltage-quality problem is a concern. All required voltages and currents are measured and are fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals to drive the main semiconductor switches (IGBTs, which are used at the distribution level) of the power converter accordingly. The ac voltage control is achieved by firing angle control. Ideally the output voltage of the VSC is in phase with the bus voltage. In steady state, the dc side capacitance is maintained at a fixed voltage and there is no real power exchange, except for losses.

Figure: Basic structure of DSTATCOM in distribution system

OPERATION OF DSTATCOM:

DSTATCOM consists of an inverter, dc link capacitance C that providing the dc voltage for inverter, coupling inductance L used for current filter and reactive power exchange between D-STATCOM and power system and a control unit to generate PWM signals for the switches of inverter. Rdc and R respectively represents switching losses in inverter and winding resistance of coupling inductance. Exchange of reactive power between distribution system and D-STATCOM is achieved by regulating amplitude of the inverter output voltage V_i. The D-STATCOM operation is illustrated by the phasor diagrams shown in Figure 2.

Figure 2: Phasor diagrams for operation modes of D-STATCOM

If output voltage of D-STATCOM V_i is equal to AC system voltage V_s, exchange reactive power between D-STATCOM and gird will be zero and D-STATCOM operates in standby mode (Figure 1(a)).

If output voltage of D-STATCOM Vi is greater than ac system voltage V_s, D-STATCOM generate a capacitive reactive power (Figure 1(b)) and finally if output voltage of D-STATCOM V_i is lower than ac system voltage V_s, DSTATCOM absorbed an inductive reactive power (Figure 1(c)).

Reactive and active power that generated or (absorbed) by D-STATCOM respectively is given,

Q=VsX(Vs−ViCosδ)

P=VsViXSinδ

Where X is reactance of coupling inductance and δ is phase angle between fundamental voltages of D-STATCOM and AC grid.

DC GENERATORS

Principle: An electrical generator is a machine which converts mechanical energy into electrical energy. The energy conversion is based on the principle of the production of dynamically induced emf, where a conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faraday’s Laws of electromagnetic Induction. This emf causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux. The following figure shows a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnets or electromagnets. The two ends of the coil are joined to two slip-rings ‘a’ and ‘b’ which are insulated from each other and from the central shaft. Two collecting brushes (of carbon or copper) press against the slip-rings. Their function is to collect the current induced in the coil and to convey it to the external load resistance R. The rotating coil may be called ‘armature’ and the magnets as ‘field magnets’.

As the coil rotates in clock-wise direction and assumes successive positions in the field the, flux linked with it changes. Hence, an emf is induced in it which is proportional to the rate of change of flux linkages (e = NdΦ /dt).

1) When the plane of the coil is at right angles to lines of flux i.e. when it is in position 1, then flux linked with the coil is maximum, but rate of change of flux linkages is minimum. Hence, there is no induced emf in the coil.

2) As the coil continues rotating further, the rate of change of flux linkages (and hence induced emf in it) increases, till position 3 is reached where θ= 90⁰, the coil plane is horizontal i.e. parallel to the lines of flux. The flux linked with the coil is minimum but rate of change of flux linkages is maximum. Hence, maximum emf is induced in the coil at this position.

3) From 90⁰ to 180⁰, the flux linked with the coil gradually increases but the rate of change of flux linkages decreases. Hence, the induced emf decreases gradually till in position 5 of the coil, it is reduced to zero value.

4) From 180⁰ to 360⁰, the variations in the magnitude of emf are similar to those in the first half revolution. Its value is maximum when coil is in position 7 and minimum when in position 1. But it will be found that the direction of the induced current is the reverse of the previous direction of flow.

For making the flow of current unidirectional in the external circuit, the slip-rings are replaced by split-rings. The split-rings are made out of a conducting cylinder which is cut into two halves or segments insulated from each other by a thin sheet of mica or some other insulating material. As before, the coil ends are joined to these segments on which rest the carbon or copper brushes. It is seen that in the first half revolution current flows along (ABMLCD) i.e. the brush No.1 in contact with segment ‘a’ acts as the positive end of the supply and ‘b’ as the negative end. In the next half revolution, the direction of the induced current in the coil has reversed. But at the same time, the positions of segments ‘a’ and ‘b’ have also reversed with the result that brush No.1 comes in touch with the segment which is positive i.e. segment ‘b’ in this case. Hence, current in the load resistance again flows from M to L. The waveform of the current through the external circuit is as shown in below. This current is unidirectional but not continuous like pure direct current.

1) The position of brushes is so arranged that the changeover of segments ‘a’ and ‘b’ from one brush to the other takes place when the plane of the rotating coil is at right angles to the plane of the lines of flux. It is so because in that position, the induced emf in the coil is zero.

2) The current induced in the coil sides is alternating as before. It is only due to the rectifying action of the split-rings (also called commutator) that it becomes unidirectional in the external circuit.

METHODS OF STARTING OF SYNCHRONOUS MOTOR

(1) By using a starting motor. This motor is directly coupled to the motor. It may be an induction motor which can run on a synchronous speed closer to the synchronous speed of the main motor.

(2) Starting as an induction motor. This is the most usual method in which the motor is provided with a special damper winding on rotor poles. The stator is switched on to supply either directly or by star delta/reduced voltage starting. When the rotor reaches more than 95% of the synchronous speed, the dc circuit breaker for field excitation is switched on and the field current is gradually increased. The rotor pulls into synchronism

(A) Pull-in torque. It is the maximum constant load torque under which the motor will pull into synchronism at the rated rotor supply voltage and rated frequency, when the rated field current is applied

(B) Nominal pull in torque. It is the value of pull in torque at 95 percent of, the synchronous speed with the rated voltage and frequency applied to the stator when the motor is running with the winding current.

(C) Pull out torque. It is the maximum sustained torque which the motor will develop at synchronous speed for I minute with rated frequency and with rated field current.

(D) Pull up torque. It is the minimum torque developed between standstill and .pull in point. This torque must exceed the load torque by sufficient margin to ensure satisfactory acceleration of the load during starting.

(E) Reluctance torque. It is fraction of the total torque with the motor operating synchronously. It results from saliency of the poles. It is approximately 30% of the pull-out torque.

(F) Locked rotor torque. It is the maximum torque which a synchronous motor will-develop at rest, for any angular positions of the rotor at the rated voltage and frequency.

USES OF DC GENERATORS

1. Shunt generators with field regulators are used for ordinary lighting and power supply purposes. They are also used for charging batteries because their terminal voltages are almost constant or can be kept constant.

2. Series generators are not used for power supply because of their rising characteristics. However, their rising characteristic makes them suitable for being used as boosters in certain types of distribution systems particularly in railway service.

3. Compound generators: The cumulatively-compound generator is the most widely used dc generator because its external characteristic can be adjusted for compensating the voltage drop in the line resistance. Hence, such generators are used for motor driving which require dc supply at constant voltage, for lamp loads and for heavy power service such as electric railways. The differential-compound generator has an external characteristic similar to that of a shunt generator but with large demagnetization armature reaction. Hence, it is widely used in arc welding where larger voltage drop is desirable with increase in current.

CHARACTERISTICS OF DC GENERATOR

Following are the three most important characteristics or curves of a dc generator:

1. No-load saturation Characteristic (E₀/I_f):

It is also known as Magnetic Characteristic or Open-circuit Characteristic (O.C.C.). It shows the relation between the no-load generated MMF in armature, E₀ and the field or exciting current I_f at a given fixed speed.

It is just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately-excited or self-excited.

2. Internal or Total Characteristic (E/I_a):

It gives the relation between the MMF E actually induces in the armature (after allowing for the demagnetizing effect of armature reaction) and the armature current I_a. This characteristic is of interest mainly to the designer.

3. External Characteristic (V/I):

It is also referred to as performance characteristic or sometimes voltage-regulating curve. It gives relation between that terminal voltage V and the load current I. This curve lies below the internal characteristic because it takes into account the voltage drop over the armature circuit resistance. The values of V are obtained by subtracting I_aR_a from corresponding values of E. This characteristic is of great importance in judging the suitability of a generator for a particular purpose. It may be obtained in two ways

(i) By making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator or

(ii) Graphically from the O.C.C.

Provided the armature and field resistances are known and also if the demagnetizing effect (under rated load conditions) or the armature reaction (from the short-circuit test) is known.

BENEFITS OF UTILIZING FACTS DEVICES

The advantages of using FACTS devices in electrical transmission systems are described below.

1. MORE UTILIZATION OF EXISTING TRANSMISSION SYSTEM

In all the countries, the power demand is increasing day by day to transfer the electrical power and controlling the load flow of the transmission system is very necessary this can be achieved by more load centers which can change frequently.

Addition of new transmission line is very costly to take the increased load on the system; in that case FACTS devices are much economical to meet the increased load on the same transmission lines.

2. MORE INCREASED TRANSIENT AND DYNAMIC STABILITY OF THE SYSTEM

The Long transmission lines are inter-connected with grids to absorb the changing the loading of the transmission line and it is also seen that there should be no line fault creates in the line / transmission system. By doing this the power flow is reduced and transmission line can be trip. By the use of FACTS devices high power transfer capacity is increased at the same time line tripling faults are also reduces.

3. INCREASED MORE QUALITY OF SUPPLY FOR LARGE INDUSTRIES

New industries wants good quality of electric supply, constant voltage with less fluctuation and desired frequency as mentioned by electricity department . Reduce voltage, variation in frequency or loss of electric power can reduce the manufacturing of the industry and cause to high economical loss. FACTS devices can helps to provide the required quality of supply.

4. BENEFICIAL FOR ENVIRONMENT

FACTS devices are becoming environmentally friendly. FACTS devices does not produce any type of waste hazard material so they are pollution free. These devices help us to deliver the electrical power more economically with better use of existing transmission lines while reducing the cost of new transmission line and generating more power.

5. INCREASED TRANSMISSION SYSTEM RELIABILITY AND AVAILABILITY

Transmission system reliability and availability is affected by many different factors. Although FACTS devices had ability to reduce such factors and improves the system reliability and availability.

APPLICATIONS AND TECHNICAL BENEFITS OF FACTS DEVICES

The basic technical benefits of the FACTS devices includes

(1) Problems of voltage limit

(2) Addressing in steady state applications

(3) Problems of thermal limits,

(4) Problems short circuit levels and

(5) Problems of sub-synchronous resonance

WEAK BUS IDENTIFICATION

Pilot bus or weak bus is defined as the bus which, when supported, improves voltage profile at all the buses and also ensures additional security to the system, in terms of increased loading margin. Usually, placing adequate reactive power support at the weakest bus enhances static voltage stability margins. The bus which is close to experience voltage collapse is the weakest bus. Changes in voltage at each bus for a given change in system load is available from the tangent vector, which can be readily obtained from the voltage collapse proximity index prediction index (VCPI) is calculated at every bus. The value of the index determines the proximity to voltage collapse at a bus.

The technique is derived from the basic power flow equation, which is applicable for any number of buses in a system. The power flow equations are solved by Newton Raphson method, which creates a partial matrix. By setting the determinant of the matrix to zero, the index at bus k is written as follows:

V_m is the phasor voltage at bus m

V_k is the phasor voltage at bus k

Y_km is the admittance between bus k and m

Y_kj is the admittance between bus k and j

k is the monitoring bus

m is the other bus connected to bus

N is the bus set of the system.

By finding the VCPI index we can find the weak bus and weakest bus. Thus for these weak bus and weakest bus we are implementing the FACT device like STATCOM and TCSC.

THYRISTOR CONTROLLED SERIES COMPENSATOR TCSC

The basic Thyristor Controlled Series Capacitor scheme was proposed in 1986 by Vithayathil with others as a method of "rapid adjustment of network impedance". A TCSC can be defined as a capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor-controlled reactor in order to provide a smoothly variable series capacitive reactance. In a practical TCSC implementation, several such basic compensators may be connected in series to obtain the desired voltage rating and operating characteristics. However, the basic idea behind the TCSC scheme is to provide a continuously variable capacitor by means of partially canceling the effective compensating capacitance by the TCR. The basic conceptual TCSC module comprises a series capacitor, C, in parallel with a thyristor controlled reactor.

Figure: Structure of TCSC

PRINCIPLE OF OPERATION:

A TCSC is a series-controlled capacitive reactance that can provide continuous control of power on the ac line over a wide range. From the system viewpoint, the principle of variable-series compensation is simply to increase the fundamental-frequency voltage across a Fixed Capacitor (FC) in a series compensated line through appropriate variation of the firing angle, α. This enhanced voltage changes the effective value of the series capacitive reactance. A simple understanding of TCSC functioning can be obtained by analyzing the behavior of a variable inductor connected in parallel with an FC. The maximum voltage and current limits are design values for which the thyristor valve, the reactor and capacitor banks are rated to meet specific application requirements.

CHARACTERISTICS OF TCSC:

Below shows the characteristics of TCSC. α is the delay angle measured from the crest of the capacitor voltage or equivalently, the zero crossing of the line current. Therefore, with the usual TCSC arrangement in which the impedance of the TCR reactor X_L is smaller than that of the capacitor, X_C, the TCSC has two operating ranges around its internal circuit resonance.

Figure: Characteristics of TCSC

STATIC SYNCHRONOUS COMPENSATOR STATCOM

STATCOM is a Static synchronous generator operated as a shunt-connected static VAR compensator whose capacitive or inductive output current can be controlled independent of the ac system voltage. STATCOM is one of the key FACTS Controllers. A STATCOM is a controlled reactive power source. It provides voltage support by generating or absorbing capacitors banks. It regulates the voltage at its terminals by compensating the amount of reactive power in or out from the power system.

When the system voltage is low the STATCOM injects the reactive power to and when the voltage is high it absorbs the reactive power. The reactive power is fed from the Voltage Source Converter (VSC) which is connecting on the secondary side of a coupling transformer as shown in the Figure 1. By varying the magnitude of the output voltage the reactive power exchange can be regulated between the convertor and AC system. STATCOM is such a device in which the modern power electronic converters have been employed. These converters are capable of generating reactive power with no/very little need for large reactive energy storage elements.

Figure 1: Block diagram of STATCOM

OPERATING PRINCIPLE OF STATCOM:

The STATCOM generates a balanced 3-phase voltage whose magnitude and phase can be adjusted rapidly by using semiconductor switches. The STATCOM is composed of a voltage-source inverter with a dc capacitor, coupling transformer, and signal generation and control circuit.

Let V₁ be the voltage of power system and V₂ be the voltage produced by the voltage source (VSC). During steady state working condition, the voltage V₂ produced by VSC is in phase with V₁ (i.e. =0) in this case only reactive power is flowing. If the magnitude of the voltage V₂ produced by the VSC is less than the magnitude of V₁, the reactive power is flowing from power system to VSC (the STATCOM is absorbing the reactive power). If V₂ is greater than V₁ the reactive power is flowing from VSC to power system (the STATCOM is producing reactive power) and if the V₂ is equal to V₁ the reactive power exchange is zero. The amount of reactive power can be given as:

Q=V1(V1−V2)X

V-I CHARACTERISTICS OF STATCOM:

STATCOM exhibits constant current characteristics when the voltage is low/high under/over the limit. This allows STATCOM to delivers constant reactive power at the limits compared to SVC. Since SVC is based on nominal passive components, its maximum reactive current is proportional to the network voltage. While for STATCOM, its reactive current is determined by the voltage difference between the network and the converter voltages and therefore, its maximum reactive current is only limited by the converter capability and is independent of network variation.

Figure 2: V-I Characteristics of STATCOM

ADVANTAGES AND DISADVANTAGES OF PHOTOVOLTAICS

ADVANTAGES OF PHOTOVOLTAICS:

1. Fuel source is vast and essentially infinite
2. No emissions, no combustion or radioactive fuel for disposal (does not contribute perceptibly to global climate change or pollution)
3. Low operating costs (no fuel)
4. No moving parts (no wear)
5. Ambient temperature operation (no high temperature corrosion or safety issues)
6. High reliability in modules (>20 years)
7. Modular (small or large increments)
8. Quick installation
9. Can be integrated into new or existing building structures
10. Can be installed at nearly any point-of-use
11. Daily output peak may match local demand
12. High public acceptance
13. Excellent safety record

DISADVANTAGES OF PHOTOVOLTAICS:

1. diffuse (sunlight is a relatively low-density energy)
2. High installation costs
3. Poorer reliability of auxiliary (balance of system) elements including storage
4. Lack of widespread commercially available system integration and installation so far
5. Lack of economical efficient energy storage

OPERATION OF THREE PHASE THREE WINDINGS UNIFIED POWER QUALITY CONDITIONER

The shunt component is responsible for mitigating the power quality (PQ) problems caused by the consumer: poor power factor, load harmonic currents, load unbalance, DC offset, etc. The shunt active filter is responsible for power factor correction, compensation of load current harmonics and unbalances. It maintains constant average voltage across the dc storage capacitor C_dc. The shunt part of UPQC consists of a VSI connected to the common dc storage capacitor C_dc on the dc side and on the ac side it is connected in parallel with the load through the shunt interface inductors L_SH and a star-connected three-phase shunt coupling auto-transformer T_SH. The shunt interface inductors L_SH together with the shunt filter capacitors C_SH are used to filter out the switching frequency harmonics produced by the shunt VSI. T_SH is used for matching the network and VSI voltages.

The series component of UPQC is responsible for mitigation of supply side disturbances: voltage sags/swells, flicker, voltage unbalance and harmonics. It inserts voltages so as to maintain the load voltages at a desired level balanced and distortion free. And also series active filter is responsible for voltage compensation during supply side disturbances. The series part of the UPQC also consists of a VSI connected on the dc side to the same energy storage capacitor Cdc and on the ac side, it is connected in series with the feeder through the series low-pass filter (LPF) and three individual single-phase series coupling transformers T_SE.

CLASSIFICATION OF HVDC LINKS

HVDC links may be broadly classified into the following categories:

1. Monopolar Links
2. Bipolar Links
3. Homopolar Links

The basic configuration of a monopolar link is shown in figure. It uses one conductor, usually of negative polarity. The return path is provided by ground or water. Cost considerations often lead to the use of such systems, particularly for cable transmission. This type of configuration may also be the first stage in the development of a bipolar system.

Instead of ground return, a metallic return may be used in situation where the earth resistivity is too high or possible interference with underground/ under water metallic structures is objectionable. The conductor forming the metallic return is at low voltage.

COMPONENTS OF HIGH VOLTAGE DC TRANSMISSION SYSTEM

Figure: A schematic of a bipolar HVDC system identifying main components

APPLICATIONS OF HIGH VOLTAGE DC TRANSMISSION

1) CONNECTING REMOTE GENERATION

Some energy sources, such as hydro and solar power, are often located hundreds or thousands kilometers away from the load centers. HVDC will reliably deliver electricity generated from mountain tops, deserts and seas across vast distances with low losses.

2) INTERCONNECTING GRIDS

Connecting AC grids is done for stabilization purposes and to allow energy trading. During some specific circumstances, the connection has to be done using HVDC, for example when the grids have different frequencies or when the connection has to go long distances over water and AC cables cannot be used because of the high losses.

3) CONNECTING OFFSHORE WIND

Wind parks are often placed far out at sea, because the wind conditions are more advantageous there. If the distance to the grid on land exceeds a certain stretch, the only possible solution is HVDC - due to the technology’s low losses.

4) POWER FROM SHORE

Traditionally, oil and gas platforms use local generation to supply the electricity needed to run the drilling equipment and for the daily need of often hundreds of persons working on the platform. If the power is instead supplied from shore, via an HVDC link, costs go down, emissions are lower and the working conditions on the platform are improved.

5) DC LINKS IN AC GRIDS

HVDC links within an AC grid can be successfully utilized to strengthen the entire transmission grid, especially under demanding load conditions and during system disturbances. Transmission capacity will improve and bottlenecks be dissolved.

6) CITY-CENTER INFEED

HVDC systems are ideal for feeding electricity into densely populated urban centers. Because it is possible to use land cables, the transmission is invisible, thus avoiding the opposition and uncertain approval of overhead lines.

7) CONNECTING REMOTE LOADS

Islands and remotely located mines often have the disadvantage of a weak surrounding AC grid. Feeding power into the grid with an HVDC link, improves the stability and even prevents black-outs.

INTERLINE POWER FLOW CONTROLLER IPFC

Recent developments of FACTS research have led to a new device: the Interline Power Flow Controller (IPFC). This element consists of two (or more) series voltage source converter-based devices (SSSCs) installed in two (or more) lines and connected at their DC terminals. Thus, in addition to serially compensate the reactive power, each SSSC can provide real power to the common DC link from its own line. The IPFC gives them the possibility to solve the problem of controlling different transmission lines at a determined substation. In fact, the under-utilized lines make available a surplus power which can be used by other lines for real power control. This capability makes it possible to equalize both real and reactive power flow between the lines, to transfer power demand from overloaded to underloaded lines, to compensate against resistive line voltage drops and the corresponding reactive line power, and to increase the effectiveness of a compensating system for dynamic disturbances (transient stability and power oscillation damping). Therefore, the IPFC provides a highly effective scheme for power transmission at a multi-line substation. The IPFC is a multi-line FACTS device.

Figure 1: Schematic diagram of IPFC

An Interline Power Flow Controller (IPFC) consists of a set of converters that are connected in series with different transmission lines. The schematic diagram of IPFC is illustrated in Figure.1. In addition to these series converters, it may also include a shunt converter which is connected between a transmission line and the ground. The converters are connected through a common DC link to exchange active power. Each series converter can provide independent reactive compensation of own transmission line. If a shunt converter is involved in the system, the series converters can also provide independent active compensation; otherwise not all the series converters can provide independent active compensation for their own line. Compared to the Unified Power Flow Controller (UPFC), the IPFC provides a relatively economical solution for multiple transmission line power flow control, since only one shunt converter is involved. The IPFC also gains more control capability than the Static Synchronous Series Compensator (SSSC), which is like the IPFC but without a common DC link, because of the active compensation. From probabilistic point of view, the performance of the IPFC will be better when more series converter involves in to the IPFC system. However, because the converters are connected through the common DC link, the converters should be physically close to each other. The common DC link will become a location constrain for the IPFC and limits its commercial application in the future network. Therefore, a method which can eradicate the IPFC common DC link and provide the active power exchange between converters will be interesting.

FACTS TECHNOLOGY

The FACTS technology is not represented by a single high-power controlling device, but it is a collection of all the controllers, these individually or in coordination with the others give the possibility to fast control one or more of the interdependent parameters that influence the operation of transmission networks. These parameters include e.g. the line series impedance, the nodal voltage amplitude, the nodal voltage angular difference, then the shunt impedance and the line current. The design of the different schemes and configurations of FACTS devices is based on the combination of traditional power system components (such as transformers, reactors, switches, and capacitors) with power electronics elements (such as various types of transistors and Thyristors). The development of FACTS controllers is strictly related to the progress made by the power electronics. Over the last years, the current rating of thyristors has evolved into higher nominal values making power electronics capable of high power applications for the limit of tens, hundreds and thousands MW.

In general, FACTS devices can be traditionally classified according to their connection, as,

1) SHUNT CONTROLLERS:

The main devices of shunt controllers are the Static VAR Compensator (SVC) and the Static Synchronous Compensator (STATCOM).

2) SERIES CONTROLLERS:

It includes the devices like the Thyristor Controlled Series Capacitor (TCSC) and the Static Synchronous Series Compensator (SSSC).

3) COMBINED CONTROLLERS:

Elements such as the Thyristor Controlled Phase Shifting Transformer (TCPST), the Interline Power Flow Controller (IPFC), the Unified Power Flow Controller (UPFC) and the Dynamic Flow Controller (DFC) belong to this third category of FACTS.

FACTS devices are also classified according to the power electronics technology used for the converters as,

1) THYRISTOR-BASED CONTROLLERS:

This includes the FACTS devices based on thyristors, namely the SVC, the TCSC, the TCPST and the DFC.

2) VOLTAGE SOURCE-BASED CONTROLLERS:

These devices are based on more advanced technology like Gate Turn-Off (GTO) Thyristors, Insulated Gate Commutated Thyristors (IGCT) and Insulated Gate Bipolar Transistors (IGBT). This group includes the STATCOM, the SSSC, the IPFC and the UPFC.