Q9Power System Instrumentation

(a) Capacitive Voltage Transformers (CVT)

A Capacitive Voltage Transformer (CVT), also called a Capacitor Coupled Voltage Transformer (CCVT), is a voltage-sensing device used at extra-high and ultra-high transmission voltage levels, combining a capacitive potential divider with a conventional electromagnetic voltage transformer to economically step the very high transmission line voltage down to a standard secondary voltage suitable for metering and protection relay use.

Construction and working: the CVT consists of a capacitor stack forming a capacitive potential divider, connected between the high-voltage transmission line and ground, with two (or more) series capacitors C1 (a smaller-value capacitor connected to the line, forming the bulk of the withstand voltage) and C2 (a larger-value capacitor connected to ground, at the intermediate tap point) — by the standard capacitive-divider action, the voltage appearing at the intermediate tap point between C1 and C2 is a fixed, much-reduced fraction of the full line voltage. This intermediate, reduced voltage is then fed to a conventional electromagnetic step-down voltage transformer (further reducing it to the standard secondary voltage, typically 110V line-to-line), together with a series compensating reactor connected between the capacitor tap point and the voltage transformer's primary, whose inductive reactance is specifically selected to resonate with (cancel out) the capacitive reactance of the divider network at the power system's operating frequency, minimizing the phase-angle and ratio errors that the capacitor divider's own reactance would otherwise introduce into the voltage transformation.

Economic advantage over conventional VTs: at extra-high transmission voltage levels (220kV and above), a conventional electromagnetic voltage transformer designed to directly step down from the full line voltage requires very extensive, costly high-voltage insulation for its primary winding; a CVT, by contrast, uses relatively inexpensive high-voltage-rated capacitors (which are simpler and more economical to insulate for very high voltage than an equivalent electromagnetic winding) to perform the bulk of the voltage step-down, requiring only a comparatively modest, much less costly conventional voltage transformer for the final stage of reduction from the already-much-lower intermediate tap voltage — this substantial cost advantage is the primary reason CVTs are the preferred voltage-sensing device at EHV and UHV transmission voltage levels, despite offering somewhat inferior transient response and accuracy compared to a conventional electromagnetic VT (particularly during fast transient/switching-surge conditions, an important consideration when CVTs are also used to provide the voltage signal for high-speed distance protection relays).

Additional practical feature - power line carrier coupling: since the CVT already provides a convenient, insulated tap-off point between the high-voltage line and ground via its capacitor stack, CVTs are frequently also used to couple power line carrier communication (PLCC) signals onto the transmission line for teleprotection signaling and voice/data communication between substations, using the same capacitor stack (via an additional carrier-frequency coupling connection) that also performs the CVT's primary voltage-sensing function, providing a dual-purpose, cost-effective combined solution widely used on EHV transmission lines.

Ferroresonance in CVTs and Its Mitigation

A significant design concern specific to the CVT, not shared by a conventional electromagnetic voltage transformer, is ferroresonance — an abnormal, self-sustaining, non-linear resonance phenomenon that can arise from the interaction between the CVT's own capacitor divider network, its compensating reactor and the non-linear (saturable) magnetizing inductance of the intermediate voltage transformer's iron core. Ferroresonance can be triggered by a system transient (such as a sudden change in the line voltage during fault clearance, or energization/de-energization of the CVT itself), causing the circuit to settle into a sustained oscillation at a sub-harmonic or other frequency different from the normal power frequency, during which the intermediate voltage transformer's core is driven into saturation, and which can produce severely distorted secondary voltage waveforms, sustained overvoltages, and, in an uncontrolled condition, excessive heating and possible damage to the CVT's components. To mitigate this risk, CVTs are fitted with a ferroresonance-suppression (damping) circuit, most commonly consisting of an additional damping resistor (or a more elaborate active/passive electronic damping circuit) connected across the tertiary or an auxiliary winding of the intermediate transformer, or across the secondary itself, designed to absorb energy specifically from any ferroresonant oscillation (whose frequency and waveform differ from the normal fundamental-frequency secondary voltage) while remaining essentially inactive and not affecting the CVT's accuracy during normal, non-resonant operation. Correct design and sizing of this damping circuit, verified through transient simulation studies during CVT design, is essential to ensure the device reliably self-extinguishes any incipient ferroresonant condition rather than sustaining it indefinitely.

CVT Versus Conventional VT: Accuracy Class Comparison

Conventional electromagnetic voltage transformers are generally capable of achieving tighter accuracy classes (such as 0.1 or 0.2 class, used for precision revenue metering) than CVTs, because a CVT's overall accuracy is influenced by additional factors beyond the intermediate voltage transformer alone, including the temperature-dependent capacitance stability of the divider capacitors, the tuning accuracy and stability of the series compensating reactor, and stray/leakage effects in the capacitor stack, all of which introduce additional sources of ratio and phase-angle error that a conventional VT does not have to contend with. Consequently, CVTs are typically supplied to somewhat less stringent accuracy classes (commonly 0.5 or 1.0 class for metering duty, or the corresponding protection accuracy classes for relaying duty) compared to the best conventional VTs, and CVTs also generally exhibit poorer transient response, taking a brief but finite time (a few milliseconds) to settle to the correct secondary voltage following a sudden change or a fault-induced collapse in the primary line voltage, due to the stored energy in the capacitor divider and compensating reactor, whereas a conventional electromagnetic VT responds essentially instantaneously to a primary voltage change. This slower transient response is an important consideration when a CVT's secondary voltage is used as an input to high-speed distance protection relays, where relay algorithms must be designed to tolerate or compensate for the CVT's characteristic transient voltage transients following a fault, and is a factor engineers weigh against the CVT's substantial cost advantage at EHV and UHV voltage levels when selecting between CVT and conventional VT technology for a given application.

(b) Protection Circuit of Current Transformers

A current transformer (CT) is designed to operate with its secondary circuit always connected to a burden (a relay, meter, or other secondary-connected instrument, presenting a relatively low impedance) — this is in fundamental contrast to a voltage transformer, whose secondary is designed to operate essentially open-circuited (very high burden impedance) under normal conditions.

Why the CT secondary must never be open-circuited while energized: a current transformer's primary winding is connected in series with the load/feeder circuit whose current is being measured, meaning the primary current is determined entirely by the external power system load current, completely independent of whatever secondary circuit condition exists (unlike a normal transformer, where primary current is determined by the secondary load). If the secondary circuit of an energized CT (with primary current flowing) is suddenly open-circuited (for example, by inadvertently disconnecting a meter or relay from the CT secondary terminals without first providing an alternative low-impedance path), the CT's magnetizing flux — no longer opposed by any secondary ampere-turns, since no secondary current can flow through the now-open circuit — rises dramatically, essentially following the full primary ampere-turns without any compensating secondary demagnetizing effect, driving the core deep into saturation and inducing an extremely high, potentially dangerous, peaked secondary voltage (which can reach several kilovolts even from a nominally low-voltage CT), posing a serious safety hazard to personnel and a risk of insulation breakdown in the secondary wiring and connected equipment.

Protection circuit / practice: to prevent this hazard, standard practice mandates that a CT secondary circuit must always be short-circuited (using a dedicated shorting link, shorting switch, or shorting-type terminal block specifically provided for this purpose) before any secondary-connected device (meter, relay) is disconnected or removed for maintenance, ensuring an alternative low-impedance path for the secondary current at all times while the primary circuit remains energized — many CT terminal blocks and test-switch assemblies are specifically designed with automatic (self-shorting) mechanisms that short the CT secondary automatically as soon as a test plug or connected device is withdrawn, precisely to guard against the possibility of an operator forgetting to manually apply a shorting link before disconnecting a CT-fed instrument, making this shorting protection circuit/practice an absolutely essential safety requirement in the design, wiring, and maintenance procedures of any CT-based metering or protection installation.