Power quality and machine protection
A grid that “has voltage” is not necessarily a grid that works: the voltage may be low, distorted, unbalanced, or missing for half a second, and any of these stops a production line or burns out electronics. This page teaches you to name, measure and correct those disturbances, and to protect the two most expensive machines of an installation —the transformer and the generator— from them and from their own faults.
01What power quality is and who requires it
Electrical energy is a product delivered with certain characteristics: a voltage of known value and shape, a stable frequency and continuous availability. Power quality is how closely what reaches the socket resembles what was promised. It is split into three parts, with different rules and different parties responsible:
What the voltage is like: its level, its waveform, its balance between phases and its disturbances (sags, transients, flicker, harmonics). It is measured with an instrument connected to the grid.
How much energy is missing: how many times it is cut and how long each cut lasts. It is measured from the utility’s records and with frequency and duration indices (section 04).
How the customer is treated: connection times, handling of complaints, billing accuracy. It is outside the scope of this page.
In Argentina electricity distribution is regulated under the framework of Law 24.065, which created the ENRE for utilities under national jurisdiction; the provinces have their own regulators (in Córdoba, the ERSeP, which oversees EPEC and the electric cooperatives). The utility’s obligation is not written in an electrotechnics manual but in its concession contract and in the quality regulation applied by the regulator: that is where the limits of each variable are set, together with how to measure it, how often it is checked and what penalties apply when it is not met. Those values change with the contract, the voltage level and the passage of time; for that reason this page does not copy them: in a real case, consult the contract and the current resolution of the relevant regulator.
The utility answers for the quality of the voltage it delivers at the point of connection and for the continuity of service. The customer answers for what their installation sends back to the grid: harmonics from their drives, flicker from their arc furnace, unbalance from their single-phase loads, poor power factor. A disturbance measured at one point can come from either side, and establishing which is part of the job (section 04).
To get an idea of how a limit is written, the European standard EN 50160 is useful (it does not apply in Argentina, but it is used as a reference): in low voltage it requires the rms voltage, averaged over 10 minutes, to stay within ±10 % of the nominal for 95 % of a week. Notice its form: not “always” but “a percentage of the time, evaluated over a period, with an averaging window.” Every quality regulation is written this way.
02The disturbances, one by one
Each disturbance has its own name, a typical cause and a different way of doing damage. It pays to tell them apart by their duration and their magnitude, because that determines how they are measured and how they are corrected. The figure shows four of them over a six-cycle window, which is what an analyzer sees when it triggers on an event.
| Disturbance | What it is | Usual cause | Effect |
|---|---|---|---|
| Slow voltage variation | The rms voltage stays above or below the nominal for minutes or hours. | Voltage drop in long or heavily loaded lines; wrong transformer tap; load changes during the day. | Motors with less torque and more current, lamps with less light; constant-power equipment that draws more current. |
| Voltage sag (dip) | Sudden drop to between 90 % and 1 % of the nominal voltage, lasting from half a cycle to one minute. | Short circuits on the grid, starting of large motors, energizing of transformers. | Contactors that drop out, PLCs and drives that reset or trip, interrupted processes. |
| Interruption (short interruption if very brief) | The voltage falls below 1 % of the nominal. In the European standard it is “short” up to 3 minutes and “long” beyond that. | Automatic reclosing of a line, operation of a protection or a fuse, a fault, a switching operation. | Stopped processes, lost data, reset of equipment without backup, discharge lamps that take time to relight. |
| Transient overvoltage (impulse) | A spike lasting microseconds to a few milliseconds, from hundreds of volts up to several kilovolts. | Lightning strikes on or near the line, switching of capacitors and inductive loads. | Insulation breakdown and destruction of electronics; the damage can be cumulative and appear months later. |
| Temporary overvoltage (swell) | The voltage exceeds 110 % of the nominal for a few cycles or seconds. | Disconnection of a large load, a ground fault on one phase (the other two rise), neutral problems. | Heating and ageing of lamps, power supplies and windings; it can damage sensitive equipment. |
| Rapid fluctuation (flicker) | Repeated amplitude variations of a few percent, at a rate of a few hertz. | Fast-changing loads: arc furnaces, welders, compressors and sawmills with frequent starts. | Flickering lighting, annoying to the eye even though the rms value barely changes. |
| Unbalance | The three line voltages are not equal in magnitude or are not 120° apart. | Uneven distribution of single-phase loads; an open fuse; a faulty contact. | Motors that overheat and lose service life; neutral and negative-sequence currents. |
| Harmonics | The waveform is no longer sinusoidal: it contains components at multiples of 50 Hz. | Non-linear loads: rectifiers, drives, switching power supplies, LED lamps, UPS, welders. | Heating of cables, transformers and capacitors; overloaded neutrals; resonances; trips and measurement errors. |
A 40 ms sag lasts about as long as a blink, and a multimeter averages several cycles: the reading does not even move. These disturbances only show up on a recorder or power quality analyzer set to capture events (or on an oscilloscope with a well-chosen trigger). If “the machine stops by itself” and the multimeter says 220 V, the grid is not ruled out: it is measured with the right instrument.
Two disturbances that are calculated by hand
The slow variation is expressed as the percentage deviation of the measured voltage from the nominal:
And the voltage unbalance, in the simple form used by many motor manufacturers (the IEC standard uses symmetrical components, but this is enough for a first diagnosis):
1) A panel at the end of a long line measures 201 V phase to neutral, the nominal being 220 V:
A −8.6 % calls for checking the voltage drop of the line (see Substations and distribution lines) or the transformer tap.
2) At the same three-phase panel, the line voltages are 385 V, 378 V and 371 V. The average and the maximum deviation are:
1.85 % seems small, but a three-phase motor is very sensitive: a small voltage unbalance produces a current unbalance several times larger, and that is why motor manufacturers ask for derating when the unbalance exceeds a value of about 1 %. See also Three-phase circuits and Single-phase and three-phase AC motors.
03Harmonics and distortion: the THD
A linear load (a resistor, a motor without a drive) draws a sinusoidal current from a sinusoidal voltage. A non-linear load —a rectifier with a capacitor, like the one in a computer power supply— draws current only at the voltage peaks and the resulting shape is no longer a sine. Any periodic wave like that can be broken down, as Fourier analysis teaches (Signal generation and processing), into a 50 Hz fundamental plus harmonics: waves of 100, 150, 200 Hz … Single-phase loads mainly produce odd harmonics: 3rd (150 Hz), 5th (250 Hz), 7th (350 Hz).
The measure of how far the wave departs from a sine is the total harmonic distortion, THD: the square root of the sum of the squares of the harmonics, divided by the fundamental.
A load draws a fundamental of 100 A with harmonics of 33 A (3rd), 20 A (5th) and 14 A (7th). The current THD is:
The total rms current seen by the cables and the transformer is not 100 A but:
8 % more current than the useful working current, which only produces heat. Check by another route: the square root of the sum of the squares of all the components, = 108.1 A.
In a three-phase load with neutral, the fundamentals of the three phases are 120° apart and in a balanced system they cancel in the neutral. But harmonics whose order is a multiple of 3 (3rd, 9th …) are in phase in all three phases and they add up. With the 33 A of third harmonic per phase from the example:
The neutral conductor carries almost as much as each phase without the phase current giving any warning, and in old installations, with an undersized neutral or one shared between circuits, it overheats without the phase breaker tripping. In installations with a lot of electronics (offices, data centres, LED lighting) the neutral is sized equal to or larger than the phase conductor.
| Harmonic | Sequence | Main effect |
|---|---|---|
| 3rd, 9th … | Zero (homopolar) | They add up in the neutral and circulate in the delta winding of transformers, which overheats. |
| 5th, 11th … | Negative | In a motor they produce a field rotating the opposite way: they brake and heat. |
| 7th, 13th … | Positive | General heating; together with the 5th they are the characteristic harmonics of six-pulse three-phase rectifiers. |
What damage they cause
- Heating of cables, windings and cores: eddy-current losses grow with the square of the frequency, so a transformer under harmonics loses more than one of the same rating with a linear load (that is why “K” transformers exist).
- Resonance between power factor correction capacitors and the grid inductance, which can amplify a harmonic several times over and burn out the bank (see Power in alternating current).
- Nuisance tripping of protections and measurement errors in instruments that do not measure true rms (see AC measurements).
- Voltage distortion: a harmonic current, flowing through the grid impedance, deforms the voltage seen by every other user at that point. For that reason regulations limit both the voltage THD of the grid and the harmonic current each user may inject.
04How it is measured: analyzer, recording and continuity indices
The power quality analyzer
The instrument for this task is the power quality analyzer, which unlike a multimeter or a clamp meter samples the waveform at high speed, calculates rms values, harmonics and events, and stores them in memory for days. So that two devices from different makers give the same result there is a standardized method, the IEC 61000-4-30 standard, which defines “class A” (highest accuracy, the one used to verify a contract) and averages the quantities over 10-minute intervals.
| What you want to know | How it is recorded | Precautions |
|---|---|---|
| Voltage level (slow variations) | Rms value of the three voltages, 10-min average, minimum and maximum of each interval. At least a full week. | Cover working days and the weekend: the load changes. |
| Sags, overvoltages, interruptions | Capture thresholds (for example, 90 % and 110 % of the nominal) and recording of the event waveform with its duration. | A threshold that is too tight fills the memory; one that is too loose lets important events through. |
| Harmonics and THD | Spectrum up to order 40 or 50, 10-min averages, in voltage and in current. | Flexible (Rogowski) or core clamps of the right range; measure before and after the suspect equipment. |
| Flicker | Pst (10 min) and Plt (2 h) indices calculated with the IEC 61000-4-15 method. Pst = 1 is the threshold of irritation. | Only useful if the disturbing load ran during the recording. |
| Unbalance | Ratio of negative- to positive-sequence components, or the calculation in section 02. | Check the phase order and the direction of the clamps. |
Measured at the point of connection (the utility’s meter) you see what the grid delivers to the customer and what the customer sends back. Measured downstream, at a machine’s panel, you see how that machine suffers or produces the disturbance. To know whether the origin of a sag is “outside” or “inside,” you measure at both points and compare: if the sag appears at both at the same time, it comes from the grid; if it only appears downstream, an own load caused it (a start, an internal short circuit). It is the same reasoning as in section 01 on who answers for what.
Continuity: how many times and for how long
For technical service we do not look at a waveform but at a list of outages. The most widely used indices, defined in the IEEE 1366 guide, summarize that list in three numbers: the average interruption frequency per customer (SAIFI), the average total duration per customer (SAIDI) and the average duration of each outage for those who suffer it (CAIDI).
Argentine concession contracts use equivalent indicators, normally referred to installed capacity (kVA) instead of the number of customers, with names such as FMIK (average interruption frequency per kVA) and TTIK (total interruption time per kVA); they are also checked per individual customer. The idea is the same; which interruptions are counted (for example, those longer than a minimum time), which values must not be exceeded and how much customers are compensated when they are exceeded are set by the contract and the regulator’s regulation.
A medium-voltage feeder serves 2,000 customers. During the year it had four interruptions: the first affected all 2,000 customers and lasted 45 min; the second, 500 customers, 120 min; the third, 1,200 customers, 40 min; the fourth, 300 customers, 180 min.
Each customer suffered, on average, two interruptions in the year. The average total duration is:
That is, 126 min (2 h 6 min) without power per customer per year, and each outage lasted on average:
Check by another route: dividing the total customer-minutes without service (252,000) by the total of customers affected by each outage (4,000) also gives 63 min. The same SAIDI can come from many short outages (which hurt industry with sensitive equipment) or from a few long ones (which hurt those without backup): the three numbers together tell the story.
05How it is improved: UPS, filters and compensation
Mitigating means choosing the remedy according to the disturbance. And the first question is always the same: is it better to correct it at the source, on the grid, or to protect the sensitive equipment? Sometimes the cheapest solution is not to generate the disturbance (starting a motor with a soft starter instead of direct on line) and at other times, to shield the equipment that matters.
| Problem | Remedy | Comment |
|---|---|---|
| Slow variations | Transformer tap changer, voltage regulator, larger line cross-section, capacitors. | First the voltage drop is calculated; the tap is changed with the transformer de-energized and unloaded. |
| Sags and short interruptions | UPS for critical loads; contactors with drop-out delay or with a backed-up coil supply; soft starting. | Backup is sized only for what must survive the sag: PLC, control, servers, not the whole building. |
| Long outages | Generating set with automatic transfer, sometimes in cascade with a UPS (see Power generation and generating sets). | The set takes seconds to pick up the load: the UPS covers that interval. |
| Transient overvoltages | Surge protective devices (SPD) in cascade, equipotential earthing, surge arresters in MV. | See Electrical protection, section 04. |
| Harmonics | Line reactors on drives, tuned passive filters, active filters, “K” transformers, reinforced neutral, 12-pulse or active-front-end drives. | A passive filter is designed for one harmonic and can resonate with others: measure before and verify after. |
| Flicker | Supply the disturbing load from a point with higher short-circuit power; static compensators for furnaces and welders. | The disturbance depends on the ratio between the load variation and the short-circuit power of the grid. |
| Unbalance | Distribute single-phase loads among the phases; check contacts and fuses; phase-loss protection. | The correction is about organizing the installation, not about equipment. |
| Low power factor | Automatic capacitor bank, with detuning reactors if there are harmonics. | See Power in alternating current, section 04. |
The UPS: three types and when each one is useful
The load is fed from the grid; if it fails, the unit switches to the inverter. It has a transfer time of a few milliseconds and does not correct the voltage. The cheapest type: suitable for personal computers.
Adds an autotransformer that regulates high or low voltage without using the battery. Transfer is also a few milliseconds. The middle ground for small offices and network equipment.
Always converts the grid to DC and back to AC: zero transfer time, stable voltage and frequency, isolation from transients. The most expensive, and the one used for PLCs, process control, servers and medical equipment.
A UPS is sized by apparent power (VA) and by runtime: how long it sustains the load on batteries. If more runtime is needed than a battery justifies, a generating set takes over. And since batteries age, they are part of the maintenance plan.
Improving service from the grid
The utility improves continuity with measures of another kind: automatic reclosers on overhead lines (most MV faults are transient, and a fast reclose clears them without the customer losing more than a short interruption), ring networks with remote-controlled switches, which allow the faulted section to be isolated and the rest to be re-energized, preventive maintenance of the lines (tree trimming, insulators, thermography) and fast fault detection and location. All of that lowers SAIFI and SAIDI; the industrial customer, for its part, decides with the indices in hand whether it pays to invest in its own backup.
06Protection of transformers and generators
A cable is protected by a fuse or a thermal-magnetic breaker that watches the current; a transformer or a generator are expensive machines with distinct internal faults (insulation, gas, oil heating, loss of excitation) and cannot be repaired in a day. That is why they carry several relays, each watching one thing. They are named with the ANSI/IEEE C37.2 standard device number, which is used in drawings and manuals all over the world.
Transformer protection
| Protection | No. | What it detects and how it acts |
|---|---|---|
| Medium-voltage fuse | — | Clears short circuits in small and medium transformers. It must withstand the inrush current at energization. With a striker, when it blows it opens the three-phase switch-disconnector, so that motors are not left supplied with only two phases. |
| Overcurrent instantaneous and time-delayed | 50/51 | A relay fed by CTs. The 51 acts faster the higher the current; the 50 is instantaneous and is set above the inrush. The earth version (50N/51N) detects ground faults. |
| Differential of the transformer | 87T | Compares the input and output currents, already compensated for the ratio and the vector group. In an internal fault those currents stop matching and it trips without delay. It has a second-harmonic restraint so as not to mistake the inrush for a fault. It is justified from medium ratings upwards. |
| Buchholz relay | 63 | Located in the pipe between the tank and the conservator of oil transformers. An arc or internal overheating breaks down the oil and releases gas: gas accumulating slowly gives an alarm; a sudden surge of oil, a trip. |
| Thermal image and thermometer | 49/26 | Simulates the hot-spot temperature of the winding from the oil temperature plus the load current. It alarms and, if the temperature keeps rising, trips. It is the protection against sustained overload, which is not a short circuit. |
| Pressure relief valve and oil level | — | Releases the overpressure of an internal arc so that the tank does not burst; a low oil level gives an alarm. |
A 630 kVA, 13.2/0.4 kV transformer is protected on the MV side with a 50/51 relay fed by 50/5 A CTs. The rated current of the primary is calculated:
The 51 is set above the normal load current, with a margin for permitted overloads, which the coordination study defines (here 1.25 times the rated current is used, an exercise value), and converted to the CT secondary:
The 3.5 A tap is chosen, equivalent to 35 A in the primary. The 50 must stay above the inrush current, which for transformers of this kind is of the order of 8 to 12 times the rated current for a few tenths of a second:
If the 50 were set below 220 A, every time the transformer was energized it would trip with no fault at all. Final values are set by protection engineering with the manufacturer’s data, the grid and the coordination with the other protections (see Electrical protection, section 03).
Generator protection
A synchronous generator (see Power generation and generating sets) fails in other ways: it is driven by a prime mover that can stop delivering, it has a field that can be lost, and a voltage and frequency that depend on regulators that also fail.
| Protection | No. | What it detects and why it matters |
|---|---|---|
| Overcurrent | 50/51 | Short circuits and overloads. A generator’s fault current decays after a few cycles and, without excitation forcing, can fall below the rated current; for that reason the 51 is used with voltage restraint (51V), which lowers the setting when the voltage drops. |
| Reverse power | 32 | The generator, in parallel, stops delivering and starts absorbing power from the grid: it is motored and drives the prime mover. A diesel engine can be damaged and a turbine can overheat. The setting is defined by the prime-mover manufacturer. |
| Loss of excitation | 40 | If the field is lost, the machine absorbs reactive power from the grid and behaves like an induction generator: the rotor overheats. It is detected by the reactive power absorbed or by the impedance seen by the relay. |
| Under- and over-frequency | 81 | Under-frequency indicates an overload or a prime-mover fault; over-frequency, a load rejection or a speed-governor failure. Both are a risk for the machine and for the loads. |
| Under- and over-voltage | 27/59 | Failures of the automatic voltage regulator. Overvoltage overheats the iron and stresses the insulation; undervoltage indicates an overload or a fault. |
| Generator differential | 87G | Compares, phase by phase, the current leaving through the terminals with that at the neutral side of the winding. It detects internal stator faults, which would burn the iron in a few seconds if not cleared immediately. |
In larger machines these are joined by protection against negative-sequence current (46, for unbalanced loads), against stator earth fault (64), against overfluxing (24) and against loss of synchronism (78). In small generating sets most of the functions live inside the set’s electronic controller; in large ones, in multifunction protection relays.
The differential (87) protects one zone bounded by its CTs: it only sees what happens between them. Overcurrent (50/51) is a backup protection that looks further and acts slower, to cover the case where the main protection or its breaker fails. If the times are not coordinated, a short circuit in one load can leave the whole plant without voltage. This is the principle of selectivity.
Working on these machines: safety plans and live working
Every adjustment, test or repair of a transformer, a generator or their relays is carried out within a safety plan: the work is described, it is decided who does it and who authorizes it, the supply is cut and locked out (from the grid and from the backup, which can feed back), the absence of voltage is verified and the equipment is earthed. When cutting is impossible —or when cutting causes greater damage— live working is used, which only qualified personnel perform, with a written procedure and under Law 19.587, Decree 351/79 and SRT Resolution 592/2004. That legal and methodological part is developed in Live working, and the basics of occupational health and safety in Occupational health and safety.
A CT behaves like a current source: if its secondary is left open while the primary carries current, the voltage across the terminals rises to values dangerous for people and destroys the insulation. Before disconnecting a relay or an instrument, short-circuit the CT secondary. And remember that a transformer or a capacitor bank can hold charge and voltage after being disconnected.
07In the laboratory
Materials: a power quality analyzer (or a voltage recorder) with its leads and its fuse, a workshop socket and a spreadsheet. Steps: 1) with the teacher, set the nominal voltage to 220 V, the averaging interval to 10 min (or 1 min) and event capture with thresholds of 90 % and 110 %; 2) connect, note the start time and leave the instrument recording for 24 hours, including the times when machines are started; 3) download the record and tabulate the minimum, maximum and average of each interval; 4) count how many events were captured and at what time. Measured: minimum, maximum and average voltage, and percentage of time within a ±10 % band (used only as an exercise: the real limits are those of the contract). Acceptance criterion: a report with a voltage versus time graph, the list of events and an explanation of the most serious event based on what was running in the workshop at that moment.
Materials: an analyzer or quality clamp with harmonic recording (or an oscilloscope with a current probe and FFT function); an incandescent lamp, a computer power supply, an LED lamp and a variable-speed drive with a small motor. Steps: 1) connect each load, one at a time, to a socket with a residual-current device; 2) record the current waveform and its spectrum up to the 15th harmonic; 3) note the rms current, the THD and the dominant harmonic; 4) calculate the THD with the formula from the harmonic amplitudes and compare it with what the instrument shows. Acceptance criterion: a table with the four loads ranked by THD, the difference between the calculated and the measured value explained (for example, by the higher-order harmonics the calculation left out), and a sentence explaining why the incandescent lamp has the lowest distortion.
Materials: a single-phase variable autotransformer fed through an isolation transformer, a contactor with a 220 V coil and a latching circuit, a start pushbutton, a true-rms voltmeter. Steps: 1) build the circuit with the contactor coil fed from the autotransformer and check it with the teacher before energizing; 2) start at 220 V and confirm that the contactor holds; 3) lower the voltage very slowly until the contactor drops out and note that value; 4) repeat three times. Measured: the drop-out voltage as a percentage of the nominal. Acceptance criterion: average and spread of the three measurements, compared with the manufacturer’s data, and a comment on which sag in Figure 1 would cause that drop-out.
Unlike a transformer, the variable autotransformer does not galvanically separate its output from the grid: between any terminal and earth there can be 220 V. That is why it is fed through an isolation transformer, with a residual-current device upstream. The circuit is built and modified always de-energized.
Materials: a multifunction protection relay or an overcurrent relay with a time curve, an adjustable current source (or a relay test set), a stopwatch or the unit’s own counter and the manufacturer’s settings sheet. Steps: 1) load the setting from the example in section 06 (pickup 3.5 A at the secondary); 2) inject currents of 1.5, 2, 5 and 10 times the setting, one at a time, and measure the trip time; 3) verify that below the pickup the relay does not act; 4) plot the times against the manufacturer’s curve. Acceptance criterion: all times within the tolerance declared by the manufacturer and no trip below the pickup. Work is done at the CT secondary current, with the secondary always closed.
08Common mistakes
| Mistake | Consequence |
|---|---|
| Measuring voltage quality with a multimeter | Sags, short interruptions and transients are not seen, and harmonics fool average-responding instruments. An analyzer is needed. |
| Recording for less than a week, or only at the convenient time | The load cycles are missed: the disturbance appears on the night shift or on Saturday and the report says “everything is fine.” |
| Compensating with capacitors an installation full of harmonics | Resonance with the grid: the bank overheats, burns out or amplifies the distortion. Detuning reactors or filters are needed. |
| Sizing the neutral smaller than the phase conductor in electronic loads | The third harmonic adds up in the neutral, which overheats without any phase protection tripping. |
| Buying a UPS without calculating runtime or power in VA | The unit shuts down after a few minutes or is overloaded at the first start of a high-current load. |
| Setting a transformer’s 50 below the inrush current | It trips every time it is energized, with no fault at all. |
| Resetting a Buchholz that tripped and re-energizing without looking | There is gas from an internal arc. Re-energizing can turn a small fault into the destruction of the transformer. A gas sample is taken and tests are done first. |
| Not setting the reverse power (32) of a set running in parallel | If the prime mover fails, the generator is motored and drives the engine, with risk of damage and fire. |
| Opening a CT secondary under load to “change the instrument” | High voltage at the terminals, risk to the person and destruction of the CT insulation. |
09Self-assessment
How do a voltage sag and a short interruption differ?
In how much voltage is left: in a sag it is between 1 % and 90 % of the nominal; in an interruption (or short interruption, if very brief) it is less than 1 %. Both can last the same, for example 40 ms.
A load draws 100 A of fundamental, 30 A of third harmonic and 10 A of fifth. What is its current THD?
The result is 31.6 %. Check: , and the square root of 0.10 is 0.316.
Why can a neutral end up more loaded than the phases even if the load is not unbalanced?
Because harmonics whose order is a multiple of 3 are in phase in all three phases and add up in the neutral, while the fundamentals, 120° apart, cancel. With 33 A of third harmonic per phase, the neutral carries 3 · 33 A = 99 A.
A feeder with 1,000 customers has a 2 h outage affecting 250 customers and another of 30 min affecting all 1,000. Calculate SAIFI and SAIDI.
Each customer suffered, on average, 1.25 interruptions with 1.0 hour accumulated without power. It is checked by adding the customer-hours (250 · 2 + 1,000 · 0.5 = 1,000) and dividing by 1,000.
Who sets the quality limits a utility must meet and what happens if it does not meet them?
They are set by the concession contract and the quality regulation of the regulator (the ENRE or the provincial regulator, such as the ERSeP in Córdoba). If they are exceeded, the contract provides for penalties; the concrete values must be looked up in the current contract and resolution, not in a textbook.
What does the Buchholz relay detect and how do its alarm and its trip differ?
It detects the gas and the sudden movement of oil produced by an arc or internal overheating of the transformer. Gas that accumulates slowly gives an alarm; a violent surge of oil gives a trip. After a trip, the transformer is not re-energized without studying the gas and testing it.
What is the reverse power protection (32) of a generating set for, and why does a generator’s fault current complicate overcurrent protection?
The 32 disconnects the generator when, in parallel, it stops delivering and absorbs power (it is motored), which could damage the prime mover. The generator’s short-circuit current decays after a few cycles and can drop below the rated value, so an ordinary 51 would not see it: that is why it is used with voltage restraint (51V) and complemented with the differential (87G).