High Voltage Instrument Transformers Handbook v16 PDF

High Voltage Instrument Transformers Copyright © 2020 by saVRee (UG haftungsbeschränkt) All rights reserved. This publication or any portion thereof may not be reproduced or used in any manner whatsoever without the expre...

High Voltage Instrument Transformers Copyright © 2020 by saVRee (UG haftungsbeschränkt) All rights reserved. This publication or any portion thereof may not be reproduced or used in any manner whatsoever without the express written permission of the publisher. First Edition, 2020 | saVRee (UG haftungsbeschränkt) | Schluderstrasse 2, Munich, 80634 Germany | www.savree.com | [email protected]  High Voltage Instrument Transformers Contents INTRODUCTION...........................................................................................................................................1 Location of Instrument Transformers in Substations.............................................................................2 PART 1: CURRENT TRANSFORMER (CT).......................................................................................................2 Basic Concepts and Operating Principles................................................................................................2 Metering and Protection Application......................................................................................................5 Open Secondary Winding........................................................................................................................7 Construction and Components...............................................................................................................7 Oil-immersed Current Transformers.......................................................................................................7 Construction of a Hair-pin (Tank Type) Current Transformer.................................................................9 Construction of a Top Core Current Transformer.................................................................................10 DESIGN CONSIDERATIONS AND SELECTION..............................................................................................11 Electric Insulation Level.........................................................................................................................11 Transformer Ratio.................................................................................................................................12 Accuracy and Burden Rating.................................................................................................................12 Polarity Markings...................................................................................................................................12 PART 2: VOLTAGE TRANSFORMER (VT).....................................................................................................14 Basic Concepts and Operating Principles of a CVT................................................................................14 Construction and Components.............................................................................................................17 Design Considerations and Selection....................................................................................................18 Electric Insulation Level.........................................................................................................................18 Rated Voltages.......................................................................................................................................18 Accuracy and Burden Rating.................................................................................................................19 CVT as a Coupling Capacitor in PLC Communication............................................................................19 www.savree.com | [email protected]  High Voltage Instrument Transformers INTRODUCTION In the electrical industry, current and voltage transformers are collectively called instrument transformers. Electrical instrument transformers are used in power systems to transform high magnitude currents and voltages to lower magnitude, easy to handle values which are then used for system monitoring, control, protection and measurement purposes. The main function of instrument transformers is to provide protection and metering devices with representative current and voltage signals that are an accurate (lower magnitude) reproduction of the corresponding primary quantities according to the specified transformation ratio. If instrument transformers are not used in high voltage grid systems, then the protection, control and metering devices would require an infeasible amount of electrical insulation and thermal robustness leading to an astronomical increase in their cost and complexity. By virtue of their construction, instrument transformers also provide galvanic isolation to the protection relay or metering devices (connected to the secondary windings) from the primary high voltage network. Furthermore, since the current and voltage ratings of the secondary windings of instrument transformers are standardised, this allows inter-changeability between different manufacturer’s relays and meters to be possible. Current Transformers in a High Voltage Substation 1 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Location of Instrument Transformers in Substations Instrument transformers are used to supply measured quantities of current and voltage in an appropriate form to controlling and protective apparatus, such as energy meters, indicating instruments, protective relays, fault locators, fault recorders and synchronisers. Instrument transformers are thus installed when it is necessary to obtain measuring quantities for the purposes mentioned above. Typical points of installation are switch bays for lines, feeders, transformers, bus couplers, and busbars. The figure below shows an example of a suitable location for a current transformer (CT) and voltage transformer (VT) in a typical substation arrangement. Instrument Transformers in a Typical Substation The next sections discuss the working principles, construction, components and design aspects of instrument transformers applied in high voltage air-insulated substations (AIS). Part-I deals with current transformers whereas voltage transformers are covered in Part-II. PART 1: CURRENT TRANSFORMER (CT) Basic Concepts and Operating Principles A current transformer (CT) is connected in series to carry the full rated load (and short circuit current) of the system. In its simplest form, a CT consists of a magnetic core and two windings, commonly designated as the primary and secondary windings which are insulated from each other. Alternating current (AC) flowing in the primary produces a magnetic flux which is coupled or linked to the secondary winding through the CT core, thereby inducing an MMF (magnetomotive force) and current in the secondary winding. In many ways, a CT is similar to a single-phase power transformer except that the current in its primary winding is stiff, i.e. it’s controlled by the primary network. Note: In a power transformer, the current in the primary winding is controlled by the load connected to the secondary winding. 2 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Schematic of a Current Transformer The figure above shows the simplified schematic of a CT. In the case of an ideal current transformer, the magnetomotive force (MMF) in the primary must be the same as the magnetomotive force in the secondary, in which case the following equation applies: I1 N1 = I2 N2 Or by rearranging, I1 N2 = I2 N1 Where, I1 is the current in the primary winding (the line current) I2 is the current induced in the secondary winding N1 is the number of turns of the primary winding N2 is the number of turns of the secondary winding The ratio N1/N2 is known as the transformer ratio. The specified ratio N2/N1 is referred to as the current transformation ratio. The standard values for rated secondary current are 1 A or 5 A. For high voltage grid applications, the current transformation ratio is usually high, for example, 2000:1. This means 2000 turns of secondary winding around the magnetic core to 1 primary single turn conductor. As a result, 2000 A flowing in the primary circuit would translate into 1 A flowing in the secondary output of the current transformer. Protection and control equipment can then use this output signal as required to monitor and execute the safe operation of the substation equipment and associated network. In reality, the current transformer is a transformer and must be modelled by the equivalent circuit of “The Real Transformer” which is displayed below. 3 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Equivalent Circuit for a Real Transformer In the case of a current transformer, the coil resistance of both the primary and secondary is extremely small. The primary is usually a busbar that goes straight through the CT once, and the secondary winding is constructed using a heavy gauge (diameter). The secondary coil is a toroid, resulting in very little leakage inductance. Therefore, R1 & R2 may be neglected. Thus, all the inaccuracies may be attributed to the excitation impedance. Since the currents are related by the inverse of the turns ratio of the transformer, the following equivalent circuit referred to the secondary side may be used. Equivalent Circuit of a Current Transformer Referred to the Secondary Side Here the load impedance is commonly known as the burden on the CT and is specified in units of ohms (Ω). The load impedance or burden includes the impedance of the relays and meters connected to the secondary winding of the CT as well as the leads (physical wires & cables) connecting the secondary winding terminals of the CT (installed in the outdoor switchyard) with the protection and metering equipment (installed in the indoor control house building). The diagram shows that not all primary current is transformed (transferred) to the secondary circuit because some part of it is consumed by the CT core in the form of excitation or magnetising current (Ie). The relationship between the primary and secondary currents of a CT is therefore modified as follows: 4 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers N1 I2 = I − Ie = I'1 − Ie N2 1 The equation above corresponds to the CT equivalent circuit referred to the secondary side. Therefore, the quantity I’1 is the primary current referred to the secondary side. This means that the primary current is not reproduced exactly, which introduces an error both in magnitude and phase. The error in amplitude called the current or ratio error (ε) is defined as: I'1 − I2 Ie ε= = I'1 I'1 The error in phase is called the phase error or phase displacement. It can be visualised by examining the phasor diagram below. In an ideal transformer, the vector angle between the primary and secondary is zero. Phasor Diagram of Currents in a CT The error of the CT directly depends on the size (magnitude) of the connected burden. For smaller burdens, the excitation current Ie is small. Therefore, current transformers work at their best when they are connected to low-impedance burdens. When applying current transformers in substations it is important to take into account the amplitude and phase displacement errors. Metering and Protection Application The requirements expected from a current transformer can vary depending on whether it is intended for a metering or protection application. To meet the varied requirements (as briefly explained below), typically a single high voltage current transformer unit houses multiple metering and protection cores (up to five in total) made from different materials to provide the requisite characteristics. Normally, one or two cores are specified for metering purposes, and two to four cores for protection purposes. Metering Cores When used for metering/revenue billing purposes, current transformers should provide current signals that are very accurate representations of the primary line values for steady-state and overload conditions. Furthermore, the secondary output from the current transformer should saturate in the event of a short circuit fault, or system transient, to protect the metering devices connected to it from being damaged by high currents during the fault conditions. 5 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers An important factor for the metering core of a current transformer is the Instrument Security Factor. The rated Instrument Security Factor (FS) indicates the overcurrent as a multiple of the rated primary current at which the metering core will saturate (the primary current limit). It is thus limiting the secondary current to FS times the rated current. The safety of the metering equipment is greatest when the value of FS is small. Typical FS factors are 5 or 10. Protection Cores When used for protection purposes, the current transformer outputs must be capable of transforming primary line values during both steady-state and transient conditions. The secondary output from the current transformer should not saturate even during very high primary currents. As a result, lower accuracy is permitted for protection cores than for metering cores. A key factor for the protection core of a current transformer is the Accuracy Limit Factor. The Accuracy Limit Factor (ALF) indicates the overcurrent as a multiple of the rated primary current up to which the rated accuracy of CT is fulfilled with the rated burden connected. The excitation curves below show the different saturating behaviour and response of a metering and protection CT core. Typical Excitation Curve for Protective and Metering Cores (Source: ABB) To summarise: Current Transformers for Metering Designed to operate at rated steady-state current Saturation typically commences at 1.2 – 1.5 x primary rated current 6 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Current Transformers for Protection Designed to operate at system faults and disturbances Saturation typically commences at 20 – 30 x primary rated current Open Secondary Winding A secondary winding should never be open-circuited when the CT is in operation. Current flow in the primary winding of the current transformer produces an induced current flow in the secondary winding. The ampere-turns of the primary and secondary windings are equal and opposite and therefore cancel out the effect of each other. If the secondary winding is open-circuited while current is flowing through the primary, there will not be a counter MMF (magnetomotive force). The absence of the opposing ampere-turns of the secondary winding results in abnormally high flux in the transformer core. As a consequence of this magnetic flux build-up, the core experiences higher losses with heating and a high voltage develops across the secondary terminals. The induced high voltage can cause damage to the insulation and poses a safety hazard. Moreover, the excessive MMF and residual magnetism in the core can be detrimental to the accuracy of the current transformer. Therefore, the secondary windings of a current transformer should always be connected to a burden or shorted out and earthed (in case of spare cores). Construction and Components High voltage current transformers are commercially available in three insulation designs: Oil-immersed Current Transformers High dielectric strength oil-filled designs utilise paper and foil shields impregnated with refined dehydrated and degassed transformer oil. SF6 Gas Insulated Current Transformers For these designs, the combination of oil and paper insulation is replaced by SF6 gas, which has excellent dielectric properties and is not flammable. Epoxy Moulded Current Transformers In epoxy insulated current transformers, the primary winding and the secondary cores are embedded in epoxy resin. The application of epoxy-insulated current transformers is limited up to 72.5 kV. Oil-immersed Current Transformers The majority of high voltage current transformers manufactured and installed today are of the oil- immersed type, which we will focus on in this section. Depending on where the core and secondary windings of the CT are located, they can be classified into two main types: 7 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers 1. Tank type - with the cores situated in a tank close to the ground. The primary conductor is U-shaped (hair-pin) or coil-shaped (eye-bolt). This type of design is also referred to as dead tank design because the tank is at ground (earth) potential. 2. Inverted type (top core) - with the cores situated at the top of the transformer. The primary conductor is usually in the shape of a bar. The primary winding can also be coil-shaped. This type of design is also referred to as live tank design because the tank is at live line potential. Schematic of Hair-pin (left) and Top Core (right) CT Design A brief comparative summary of the advantages and disadvantage of hair-pin and top core current transformer types is presented in the table below. Hair-pin (Tank Type) Top Core Advantages Advantages - Lower centre of gravity. - Lower thermal losses due to short primary conductor. - Higher seismic resistance. - Higher ratings for rated and short circuit - Porcelain/composite not stressed by the current achievable. weight of heavy cores at the top. - The transformer is compact. - Oil circulation in the primary conductor (tube) alleviates thermal stresses and Disadvantages prevents hot spots. - Higher centre of gravity and lower - Core volume easily adaptable to seismic withstand. different requirements. 8 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers - Core located at the top stresses the Disadvantages support insulator. - Higher thermal losses due to longer - Limited core volume. primary conductor. - Short circuit withstand capability limited to less than or equal to 63 kA. Construction of a Hair-pin (Tank Type) Current Transformer In a hair-pin or tank type current transformer, the primary winding consists of one or more parallel conductors of aluminium or copper bent into a U-shape or hair-pin design. The winding is insulated with special paper which has high mechanical and dielectric strength. The insulation is graded with conducting/foil inserts placed at strategic locations to ensure uniform voltage distribution (capacitive layers perform a similar function in condenser type bushings). External terminals shaped as flats or studs are bolted to the two ends of the primary conductor (winding) for connection to the system. Construction and Components of a Hair-pin Current Transformer 9 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Cores for metering application are usually made of nickel alloy, which delivers low losses, high accuracy and low saturation levels. Protection cores are made of high-grade oriented steel strip providing higher saturation thresholds. Often cores may be included initially (for future substation extensions) that may not be connected until sometime later in the installation. Until they are used, the secondary windings of these spare cores can be simply shorted out and earthed. High grade enamelled copper wires are used for winding secondary turns on the cores. They are tightly wound and evenly distributed across the periphery of the core. The ends of the secondary windings are made accessible for external connection in the secondary terminal box at the bottom of the current transformer structure. The lower section of the transformer consists of an aluminium tank in which the secondary windings and cores are mounted. The insulator, fixed above the transformer tank, is made of high-grade glazed porcelain or composite polymer. The assembled transformer is vacuum treated and impregnated with degassed mineral oil. In addition to mineral oil, some manufacturers also use quartz filling to minimise the volume of required oil and to provide mechanical sturdiness to the cores and primary winding. To compensate for volume expansion of oil with varying ambient and load conditions, the current transformer is fitted with an expansion chamber at the top. The expansion system can be in the form of a gas (usually Nitrogen) cushion or stainless steel expansion bellows. Finally, the complete current transformer assembly is hermetically sealed. The sealing system consists of oil-proof O-ring gaskets. Construction of a Top Core Current Transformer In a top core current transformer, the housing at the top of the transformer (often called the primary head) contains the assembled cores and the secondary windings insulated with paper. The primary conductor passes through the centre of the head. Wire tails from the ends of the secondary windings are brought down through the porcelain or composite polymer insulator housing and terminated in the secondary terminal box fixed at the base of the CT. Insulating oil fills the void between the core, windings, head and support insulation. Expansion bellows made from stainless steel are located above the head housing of the current transformer; the bellows allow for expansion and contraction of the insulating oil due to temperature variation. Finally, the hermetic sealing of the housing protects the oil- paper insulation against atmospheric influences. 10 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Construction and Components of a Top Core Current Transformer DESIGN CONSIDERATIONS AND SELECTION Design aspects, performance and other requirements for current transformers are usually stipulated in various national and international standards. Two of the most widely applied international standards in the industry for current transformers are: C57.13 – IEEE Standard Requirements for Instrument Transformers 61869-1 and 61869-2 – IEC Standards for General Requirements for Instrument Transformers and Additional Requirements for Current Transformers Some important factors when selecting and specifying current transformers are briefly described below. Electric Insulation Level During operation, the external and internal insulation of the current transformer is required to withstand system rated power frequency voltage and temporary overvoltages, switching surges and fast-front lightning impulses. The external supporting insulator should also provide adequate creepage (leakage) 11 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers distance to prevent excessive flow of leakage current and mitigate associated pollution-related flashovers. Note: The CT should also fulfil the specified limits for partial discharges and radio interference voltage (RIV). Rated Currents It is important to recognise that the current transformer is connected in series with the network. Therefore, in operation, the current transformer must withstand the continuous rated primary current and high magnitude short-time thermal/dynamic current during system faults, without exceeding the prescribed temperature rise limits. Note: The secondary rated current is standardised at 1 or 5 A. Transformer Ratio The primary rated current of a transformer is usually selected to be approximately 10% to 40% higher than the estimated maximum operating current. With selection of the rated primary current and adoption of a standardised secondary current, the required current transformation ratio can be specified. Commercially, current transformers are available in a wide range of ratios. Accuracy and Burden Rating The rated burden of the current transformer is the value of the impedance of the secondary circuit expressed in ohms or volt-amperes and on which the accuracy of the transformer is satisfied (i.e. the ratio and phase displacement errors remain within specified limits). To minimise errors, the burden rating of the transformer is selected to be as close as possible to, but higher than the calculated connected burden (measurement/protective devices and leads). The accuracy class of the current transformer is specified according to its application. For example, typical accuracy classes (as per IEC) are 0.2 for precision metering and 5P for protective relaying. The integer value in these class designations denotes the % error in the CT secondary output at the specified rated burden. Polarity Markings Polarity (or terminal) markings (⬤) on a current transformer designate the relative directions of instantaneous currents. The figure below shows terminal marking (in accordance with IEC convention) on a current transformer: 12 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers IEC Current Transformer Terminal Markings At the same instant that the primary current is entering the marked primary terminal, the corresponding secondary current is leaving the similarly marked secondary terminal, having undergone a magnitude change within the transformer. The importance of the correct application of polarity marks is illustrated in the figure below, which shows CT connections for differential protection. Connection of CT Secondary Windings Assume that the two CTs have the same ratio of 2000:1 and that the primary current is 2000 A. The left- hand diagram shows primary and secondary instantaneous current flows for normal loads or a fault outside the zone of protection (the zone of protection is between the two CTs). The burden current becomes zero. 13 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers The diagram on the right shows primary and secondary instantaneous current flows for a fault inside the zone of protection. In a healthy state, the secondary currents of current transformers (installed at the two ends of the busbar) are balanced, thus cancelling each other out. In the case of a fault on the busbar, the secondary currents are not equal but opposite, and thus a resultant current flows through the burden (relay) actuating the operation of the corresponding circuit breaker to isolate and clear the fault. PART 2: VOLTAGE TRANSFORMER (VT) In high voltage systems, voltage transformers are usually connected in parallel between phase and ground. From a design perspective, there are two types of voltage transformers: Inductive Voltage Transformer Capacitive Voltage Transformer Inductive voltage transformers (in a similar way to traditional transformers) comprise of a primary winding (connected to the network), a secondary winding and a core which provides magnetic coupling between the two windings. For high transmission voltages, the required transformation ratio is quite high. To achieve this with inductive voltage transformers, several voltage transformers need to be connected in series (often called cascade connection). Since using such an application is quite expensive for transmission voltages, capacitive voltage transformers are most commonly used. A capacitive voltage transformer (CVT) is sometimes referred to as a coupling capacitor voltage transformer (CCVT). This is because a capacitive voltage transformer can also be used as a coupling capacitor in combination with power line carrier (PLC) equipment for data communication between substations. The dual function of CVTs as a voltage transformer and coupling capacitor makes them even more attractive and an economical alternative to inductive voltage transformers. In this section, we will focus on the design, working principles, and construction of oil-filled capacitive voltage transformers employed in high voltage air-insulated substations. Basic Concepts and Operating Principles of a CVT A CVT consists of a capacitor voltage divider (CVD) which contains two series-connected capacitors C1 and C2. Capacitor C1 is connected between the high voltage terminal and intermediate voltage terminal of the capacitor divider and is often called the high voltage capacitor. Capacitor C2 is connected between the intermediate voltage and low voltage terminal of the capacitor divider and is called the intermediate voltage capacitor. The CVD is connected to an electromagnetic unit (EMU). The EMU contains a small inductive voltage transformer and a tuning reactor. 14 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Voltage transformation in a CVT is carried out in two steps: first the capacitor voltage divider (CVD) reduces the primary line voltage to an intermediate value (say 12 kV), and then the inductive voltage transformer housed in the EMU reduces (transforms) the voltage further to a standardised low magnitude signal which can then be used for metering, protection, and control, of high voltage systems. The figure below shows a simplified schematic of a CVT. Schematic Diagram of a CVT The size of the capacitances C1 and C2 determine the voltage ratio (NC) of the CVD. The ratio of the capacitor voltage divider is given by: VT C1 + C2 = = nc V2 C1 The ratio of the intermediate inductive voltage transformer (in) is given by: V2 N2 = = nI V3 N3 N2 and N3 are the number of turns of the primary and secondary windings of the inductive transformer respectively. The total effective transformation ratio of the CVT (not) is: VT = nc ∙ n I = nt V3 15 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Due to the action of capacitors in the CVD, the voltage inside a CVT undergoes a leading phase-shift which needs to be corrected. Together, the inductance from the intermediate voltage transformer windings and the specially designed tuning reactor in the EMU compensate for the phase shift caused by the capacitive voltage divider. The value of the tuning reactor (L) is chosen so that the capacitive and inductive reactances of the CVT are tuned (resonate) at rated frequency and the following condition occurs: 1 ωL= ω(C1 + C2 ) Where ω is the angular velocity given in radians per second, which is related to the system frequency (in Hertz) by the following expression: ω = 2 π f. As a consequence of the inherent dependence of a CVT on the resonance between the capacitive and inductive reactance at the rated frequency, it cannot be expected that the CVT will have the same accuracy for frequencies deviating from the rated value (such as 50 or 60 Hz). Note: Due to the above frequency dependence, international standards specify a range of frequency for which the specified accuracy of a CVT is maintained. For instance, IEC standards stipulate that the CVT accuracy class will be satisfied for a variation of rated frequency between 99 – 101% for metering application and 96 – 102% for protection applications. Additionally, the EMU of a CVT contains a ferro-resonance protection (damping) circuit to protect the CVT against overvoltages or heat due to core saturation. All CVTs need to incorporate some kind of ferro- resonance damping, since the capacitance in the voltage divider, in series with the inductance of the intermediate transformer and the compensating series reactor, constitutes a tuned resonance circuit. The ferro-resonance damping circuit is connected in parallel with one of the secondary windings of the intermediate voltage transformer. Note: Ferro-resonance is a type of series-resonance which causes oscillating phenomenon in electrical circuits due to the complex interaction between a ferromagnetic saturable non-linear magnetic inductance and a capacitance (a CVT with its capacitor divider and its intermediate voltage transformer with non-linear excitation characteristics is such a circuit). Ferro-resonance is usually initiated through a transient disturbance such as the opening of a switch. The resulting overvoltages and/or high current spikes subject the electrical equipment to undesirable dielectric and thermal stresses. 16 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers Construction and Components The image below shows the basic construction of an oil-immersed capacitive voltage transformer. Construction and Components of a CVT Depending on the required voltage rating, the capacitor voltage divider (CVD) consists of one or more capacitor stacks (mounted on top of each other) and fixed onto the base tank. The capacitor stack contains the required number of capacitor elements which are connected in series. In general, these capacitor elements are comprised of aluminium foils with polypropylene film and oil-impregnated paper dielectric. The capacitor elements are hermetically sealed. The capacitor stacks are enveloped inside a porcelain or composite polymer insulator. The electromagnetic unit (EMU) which comprises of a medium voltage inductive transformer, compensating reactor and auxiliary elements (such as a ferro-resonance protection circuit) is housed inside a hermetically sealed aluminium base tank which is filled with mineral oil for insulation. The oil-impregnated medium voltage inductive voltage transformer has primary and secondary windings made of enamelled copper and a magnetic steel core. The compensating (tuning) reactor is placed in series between the capacitive voltage divider and the primary winding of the inductive voltage transformer. The CVD and EMU are connected internally through an epoxy bushing. 17 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers A secondary terminal or junction box is affixed onto the side of the base tank; the wire tails from the ends of the transformer output are connected to the protection and control cabling. Sometimes, trimming windings are used for fine-tuning the output signal to correspond with the accuracy class requirements. Insulating oil fills the voids inside the CVT assembly, and an expansion system at the top of the CVT assembly compensates for oil volume fluctuations due to changes in temperature. Design Considerations and Selection Two of the most prevalent industry standards which describe design aspects, performance and other requirements for capacitive voltage transformers are: C57.13 – IEEE Standard Requirements for Instrument Transformers 61869-1 and 61869-5 – IEC Standards for General Requirements for Instrument Transformers and Additional Requirements for Capacitor Voltage Transformers Some important factors when selecting and specifying CVTs are briefly described below. Electric Insulation Level The entire CVT assembly, including the capacitive voltage divider (CVD) and the electromagnetic unit (EMU), should adequately withstand all types of dielectric stresses such as power frequency overvoltages, lightning strikes and switching surges. Additionally, the external insulator (housing the capacitor stacks) should also provide sufficient creepage (leakage) distance to prevent excessive flow of leakage current and mitigate associated pollution-related flashovers. Note: The CVT should also fulfil the specified limits for partial discharges and radio interference voltage (RIV). Rated Voltages Since CVTs in substations are normally connected between phase and ground, the rated primary voltage is 1/√3 times the rated phase-phase system voltage. The rated secondary voltage is standardised (such as 100/√3 or 110/√3 V). Voltage Factor In the event of a disturbance (fault) in a three-phase system, the voltage across the CVT (connected phase-ground in the network) can increase up to the rated primary voltage, times the voltage factor (Fv). It is important that the CVT can not only withstand, but also reproduce these fault-induced overvoltages on its secondary side. 18 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers The required voltage factor depends on the system earthing conditions. As per IEC, the voltage factor for CVT is 1.5 for an effectively earthed neutral system and 1.9 for a non-effectively earthed neutral system. Accuracy and Burden Rating The rated burden of the CVT is the value of the impedance of the secondary circuit expressed in ohms or volt-amperes and on which the accuracy of the transformer is satisfied. Similar to current transformers, accuracy classes for CVT are specified separately for metering and protection purposes. However, it is important to note, that unlike current transformers where protection and metering secondary windings have independent cores, on a CVT provided with more than one secondary winding, these windings are not independent. Therefore, the burden requirement for a CVT is equivalent to the total burden of all the protection and metering equipment connected to a transformer. For example, typical accuracy classes (as per IEC) are 0.2, 0.5 or 1.0 (depending on application) for metering and 3P or 6P for protection purposes. The integer value in these class designations denotes the % error (voltage ratio error) in the CVT secondary output at the specified burden and primary voltage. CVT as a Coupling Capacitor in PLC Communication In substations, CVTs not only provide stepped down and accurate reproduction of primary voltages, but the capacitor voltage divider of the CVT can also be employed as a coupling capacitor (CC) for power line carrier (PLC) communication. This mode of communication uses high-frequency carrier signals (usually in the range of 20 – 700 kHz) that are superimposed on the power line conductors for application of remote control, voice & data communication, remote metering, protection, and so forth between substations. The figure below shows a simplified block diagram of a PLC communication system. Power Line Carrier Communication 19 / 20 www.savree.com | [email protected]  High Voltage Instrument Transformers The capacitive voltage divider of the CVT couples high-frequency power line carrier signals to overhead transmission line conductors. The coupling capacitor acts as a physical and frequency-dependent link or connection between the PLC equipment and the high voltage transmission line. The coupling capacitor together with a line tuner (also known as a coupling device) forms a series resonant filter, which provides a high impedance to power frequency (50 or 60 Hz) and low impedance to carrier signal frequencies. In this way, it allows the carrier frequency signals to enter the PLC equipment/panel whilst the power system frequency signals are blocked. 20 / 20 www.savree.com | [email protected]