VOLTAGE TRANSFORMER (VT) EXPLAINED
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.
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:
The ratio of the intermediate inductive voltage transformer (in) is given by:
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:
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:
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.
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.
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.
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
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.