Q4Protection of Power System

The Merz-Price current balance principle, discussed in relation to another question in this examination for generator stator protection, is applied equally to power transformers, comparing the current entering the transformer's primary winding against the current leaving its secondary winding, appropriately scaled to account for the transformer's own turns ratio, since under any healthy, through-load condition, the primary and secondary ampere-turns must remain very closely balanced (differing only by the small magnetizing current the transformer itself draws to establish its working flux).

In practice, CTs are installed on both the primary and secondary sides of the protected transformer, with CT ratios specifically selected (in conjunction with any necessary auxiliary interposing CTs) so that the resulting CT secondary currents on both sides are equal in magnitude under normal, healthy through-load conditions of any magnitude, and are connected in a differential circuit configuration (analogous to the generator stator differential scheme) such that these two balanced secondary currents circulate around the pilot wire loop connecting the two CT sets and cancel out at the differential relay under healthy conditions, while an internal transformer fault (a winding-to-winding fault, a winding-to-earth fault, or an inter-turn fault) causes the primary-to-secondary ampere-turn balance to be disrupted, producing a net differential current that operates the relay and trips both the primary and secondary circuit breakers of the transformer.

Shortcomings of the Basic Scheme

• CT ratio and phase mismatch across a delta-star transformer: since a delta-star (or star-delta) transformer introduces a 30-degree phase shift between its primary and secondary line currents, simply matching CT ratios alone is insufficient; the CTs on the star side must be connected in delta and the CTs on the delta side connected in star (an interconnection scheme deliberately opposite to the transformer's own winding connection) to correct for this 30-degree phase shift and restore the correct balance condition at the differential relay under healthy through-load conditions.
• Unwanted operation due to magnetizing inrush current: when a transformer is initially energized (switched in) from no-load, it draws a very large, transient magnetizing inrush current on the energized side only, which appears to the simple differential scheme exactly like an internal fault current (since this inrush current flows into the primary winding but is not matched by any corresponding secondary current, the secondary being initially unloaded), risking unwanted, nuisance tripping of the differential relay every time the transformer is energized, discussed in further detail for another question in this examination.
• CT saturation during heavy external (through) faults: during a severe external fault well outside the protected transformer's zone, the very high resulting through-fault current can drive one or both sets of CTs into magnetic saturation; if the two CTs (primary-side and secondary-side) do not saturate identically (due to slightly different burden, remanent flux, or core characteristics), their respective secondary currents will differ even though the actual through-fault condition is, in reality, perfectly balanced and external to the protected zone, again producing an unwanted differential (spill) current that risks incorrect tripping for what is actually a healthy through-fault condition outside the protected zone.
• Tap-changer-induced ratio mismatch: an on-load tap changer alters the transformer's actual effective turns ratio as it moves between tap positions, meaning the CT ratios (fixed and matched to only one specific tap position, typically the nominal or mid-tap position) will show some degree of mismatch whenever the transformer is operating away from that reference tap position, producing a small differential current proportional to the tap deviation even under perfectly healthy conditions.

How These Shortcomings Are Overcome

• Percentage (biased) differential relaying: rather than a simple, unrestrained differential relay that operates on any differential current above a fixed threshold, a percentage differential relay additionally passes the through-current (the larger of the two CT currents, or their average) through a restraining coil, and the relay is designed to operate only when the differential (operating) current exceeds a set percentage of this restraining (through) current, rather than a fixed absolute threshold; this percentage bias characteristic automatically accommodates the small, roughly proportional spurious differential currents caused by CT saturation during heavy through-faults and by tap-changer-induced ratio mismatch, since these unwanted differential currents scale roughly with the through-current itself, precisely the quantity the percentage bias characteristic is designed to compensate for.
• Harmonic restraint: to specifically distinguish magnetizing inrush current from genuine internal fault current, a harmonic restraint feature (discussed in detail for another question in this examination) is added to the differential relay, exploiting the fact that inrush current contains a very substantial second-harmonic (and other even-harmonic) component that genuine fault current does not contain, restraining the relay from operating whenever this characteristic harmonic content is detected in the differential current, even though the differential current magnitude alone might otherwise be sufficient to cause tripping.
• Appropriate CT interconnection (delta-star reversal) matched to the protected transformer's own winding connection, as described above, directly correcting the phase-shift mismatch problem for any delta-star or star-delta protected transformer.

Buchholz Relay

The Buchholz relay is a gas-and-oil-actuated protective device installed in the pipe connecting an oil-filled transformer's main tank to its conservator (expansion) tank, providing mechanical, non-electrical detection of certain internal transformer faults that develop gradually or that generate gas as a natural byproduct of internal arcing or overheating, complementing (rather than replacing) the electrical differential protection scheme discussed above.

The Buchholz relay housing contains two hinged flap or float elements. The upper element responds to the slow accumulation of gas bubbles generated by minor, incipient internal faults (such as a slowly developing inter-turn short circuit, poor core lamination insulation causing localized overheating, or a poor electrical joint) that decompose the transformer oil and surrounding insulation at a low but sustained rate; as gas slowly accumulates in the relay housing (having risen from the main tank through the connecting pipe), it displaces oil and causes the upper float to progressively tilt downward, eventually operating a mercury switch (or an equivalent electronic sensor in modern designs) connected to sound an alarm, alerting operators to the developing minor fault before it escalates into something more severe, allowing for a planned, controlled shutdown and inspection rather than an unplanned trip. The lower element responds instead to a sudden, severe internal fault (such as a major winding short circuit) that generates a violent surge of gas and oil displacement in a very short time, causing a rapid oil flow (a shock wave) through the connecting pipe from the main tank toward the conservator; this rapid oil flow strikes and displaces a baffle plate or vane connected to the lower element, causing it to rapidly operate a second mercury switch (or equivalent) that is wired directly to trip the transformer's circuit breakers immediately, providing the fast tripping response appropriate for a severe, rapidly developing internal fault.

Merits and Drawbacks of the Buchholz Relay

• Merit: the Buchholz relay can detect certain incipient fault conditions (slow gas generation from minor insulation breakdown, core lamination faults, or poor joints) well before they develop into a severe fault that would be detectable by the electrical differential protection scheme, providing valuable early warning that allows planned maintenance intervention rather than an unplanned, more disruptive shutdown.
• Merit: being a simple, purely mechanical (gas and oil flow actuated) device, the Buchholz relay is inherently robust, requires minimal maintenance, and operates independently of the electrical protection and CT circuits, providing a genuinely independent, complementary layer of protection.
• Drawback: the Buchholz relay can only be applied to oil-filled transformers equipped with a conservator tank and connecting pipe of the appropriate design, and cannot be used on dry-type transformers or on transformers without a conservator arrangement.
• Drawback: because gas must physically accumulate and rise through the connecting pipe (for the alarm function) or a rapid oil surge must physically propagate through the same pipe (for the trip function), the Buchholz relay inherently responds somewhat more slowly than a purely electrical differential relay for a given severe internal fault, meaning it is generally used as a complementary, backup protection rather than as the sole primary means of transformer fault protection.
• Drawback: the Buchholz relay can potentially give a false alarm or, in rare cases, false operation due to gas accumulation from causes unrelated to an actual electrical fault, such as air trapped in the oil system following maintenance or oil filling, or mechanical vibration and shock during transportation, requiring careful commissioning procedures (allowing trapped air to fully vent) before the relay is placed into service.