ElectricityElectrical Machines II (year 6) · Power Plants and Raceways (year 7) · 144 h each · Topic 3 of 6 and topics 1 and 2 of 7

Power generation and generating sets

When the power goes out in a hospital, a factory or an antenna site, an engine starts by itself and, within a few seconds, brings the voltage back. Behind it there is an alternator that makes the electricity, an engine that turns it at constant speed, two regulators —one for voltage, one for speed— and a switchboard that decides when to connect the load. Understanding how they work together is understanding any power plant, from a 5 kVA set to a dam.

Synchronous alternator Turbines Diesel engine Generating set Automatic transfer Synchronizing

01The synchronous alternator in service

All serious generation, from a dam to a construction-site set, relies on the same principle: a rotating electromagnet (the rotor, fed with direct excitation current) induces an alternating voltage in the fixed windings of the stator (Magnetism and electromagnetism). It is synchronous because the frequency is tied to the speed: f = p · n / 60, with p pole pairs. The relation and the speed table for 50 Hz are in Electrical energy; here it is enough to remember that a common four-pole set turns at 1,500 rpm.

Rotor type	What it looks like	Where it is used
Salient pole	Many poles standing out from the shaft, large diameter and short length. Turns slowly.	Hydro generators and 1,500 rpm generating sets.
Round (cylindrical) rotor	Solid slotted cylinder, two or four poles, long and slender. Withstands the centrifugal force at 3,000 rpm.	Steam and gas turbo-generators.

In modern sets the excitation is provided without brushes or slip rings: an exciter (a small alternator whose armature rotates with the rotor) feeds alternating current to a rotating diode bridge, which supplies the main field. Regulating the exciter current regulates the whole generator. Many machines add a permanent-magnet generator to power the regulator, so the alternator keeps its excitation even when the output voltage collapses in a short circuit.

Two variables, two regulators

In a set there are two things to control, and they hardly affect each other. The voltage is set by the excitation and governed by the automatic voltage regulator (AVR). The frequency is set by the engine speed and governed by the speed regulator (governor). The two powers also separate: the active power (kW) is set by how hard the engine pushes; the reactive power (kvar), by the load and the excitation.

Variable	Set by	Regulated by	If the regulator fails
Voltage	Excitation current and the connected load (power factor).	AVR: measures the output voltage and corrects the exciter current.	Without an AVR the voltage drops a lot when the set is loaded (tens of percent, as the example in generator testing shows) or rises uncontrolled at no load.
Frequency	Engine speed.	Governor: acts on the fuel (rack or injection) according to the measured speed.	The frequency drifts, the load motors change speed, and an overspeed can destroy the set.

Set rating: kVA, kW and the engine limit

The nameplate of an alternator says kVA: this is what it can deliver without overheating its windings, whatever the power factor of the load. What is usefully consumed is kW, and that comes from the engine shaft (Power in alternating current, Three-phase circuits):

S = \sqrt{3} \cdot U \cdot I P = S \cdot \cos φ

S in kVA is the apparent power the alternator can deliver; P in kW is the active power, which comes from the engine shaft.

By convention, sets are rated at power factor 0.8: a 100 kVA set is an 80 kW set. The rated output current is:

I_{n} = \frac{S}{\sqrt{3} \cdot U}

The weaker of the two sets the limit

A 45 kVA, 36 kW set is still a 36 kW set if the load has a power factor of 1: the alternator could give 45 kW, but the engine does not have it. With a low power factor the opposite happens: the engine has power to spare and the alternator overheats before the engine reaches its rating. To choose a set, both must be checked: kW against the engine and kVA against the alternator. Strongly capacitive loads (uncharged capacitor banks, some electronic equipment) deserve a separate note: they can make the voltage rise without the AVR being able to control it.

Voltage and frequency regulation

With the AVR in service the voltage is held within a small margin —on the order of ±1 % in steady state— between no load and full load. The hard part is the transient: when a large load is connected suddenly, the voltage sags and takes a fraction of a second to recover, and the engine loses speed until the governor increases the fuel. The larger the load step relative to the set rating, the deeper the sag; that ratio drives the sizing (section 06).

Frequency can be regulated in two modes. In droop mode the speed falls slightly as the load increases, which is what allows the power to be shared among several sets in parallel (section 05). In isochronous mode the regulator is electronic and holds the exact speed at any load; it is used on a single isolated set, and in parallel it needs a communication line between regulators to share the load. The droop is defined as:

R = \frac{f_{no-load} - f_{full}}{f_{n}} \cdot 100 %

With f_n = 50 Hz, a 4 % droop means the frequency falls 2 Hz between no load and full load.

Alternator efficiency

The alternator loses energy in the copper (Joule heating in stator and rotor), in the iron (hysteresis and eddy currents) and in friction and windage. Its efficiency is the ratio between the power it delivers and the power it receives:

η_{alt} = \frac{P_{out}}{P_{out} + P_{Cu} + P_{Fe} + P_{mech}}

P_Cu: copper losses in the stator and rotor; P_Fe: iron losses; P_mech: friction and windage.

In the alternators of small and medium sets, efficiency is around 85 to 95 %, better the larger the machine and the closer to full load. The difference leaves as heat, which the alternator fan must carry away: hence the importance of letting the cooling air circulate freely (section 04).

02Turbines: types and characteristics

The turbine is the prime mover of large plants: it converts the energy of a moving fluid —steam, hot gas or water— into the rotation of a shaft. It is more compact and smoother than a reciprocating engine, but it needs a pressurized fluid to be available; at small ratings the reciprocating engine dominates. The plants of each kind and their efficiencies are in Electrical energy; here the point is what each turbine looks like and what characterizes it.

Turbine	Fluid and principle	Characteristics	Regulated by
Steam	Steam from a boiler or a reactor expands in nozzles and blades. It can be impulse (the steam is accelerated in fixed nozzles and the rotor only receives the impulse) or reaction (the expansion continues in the moving blades). Large ones combine both, in several casings.	Runs at 3,000 rpm (two poles) or 1,500 rpm (four). A condensing turbine uses the maximum pressure drop; a back-pressure turbine delivers steam at medium pressure for a process (cogeneration).	Valves that regulate the admitted steam.
Gas	Brayton cycle: a compressor raises the air pressure, a combustion chamber heats it by burning fuel, and the gases expand in the turbine.	More than half of the turbine work is spent driving the compressor. Starts fast (minutes), from tens of kW (microturbines) to hundreds of MW. The very hot exhaust feeds the combined cycle.	Fuel flow.
Pelton (hydraulic)	Impulse: one or more high-speed water jets hit buckets on the rim of the wheel, at atmospheric pressure.	For high heads (hundreds of metres) and small flows. Each jet pushes the double buckets of the wheel by impulse.	Needle that closes the nozzle and a deflector that diverts the jet.
Francis (hydraulic)	Reaction: water enters through a spiral casing (volute), crosses the runner radially and leaves through the centre, losing pressure in the runner.	For medium heads (tens to a few hundred metres) and medium flows. It is the most widespread.	Wicket gates (movable guide vanes).
Kaplan (hydraulic)	Reaction, axial flow, with propeller blades that can be tilted.	For low heads (up to a few tens of metres) and large flows: lowland dams and rivers.	Guide vanes and blade pitch.

The head ranges are indicative: the limits between one type and another overlap, and the choice is made by efficiency, cost and specific speed in the design of each plant. For a hydraulic plant, the power is computed from the net head and the flow:

P = η \cdot ρ \cdot g \cdot Q \cdot H

ρ = 1,000 kg/m³, g = 9.81 m/s², Q flow in m³/s, H net head in m; the result is in W.

Example · A micro hydro plant on a mountain stream

A drop uses a net head of 45 m with a flow of 2.5 m³/s. The overall efficiency of turbine and generator is 0.88. What power is obtained and which turbine suits? If the generator is coupled directly to a 300 rpm turbine, how many pole pairs does it need for 50 Hz?

P = 0.88 \cdot 1000 kg/m³ \cdot 9.81 m/s² \cdot 2.5 m³/s \cdot 45 m = 971 kW

With a 45 m head and a medium flow, the natural choice is the Francis (a Kaplan would be possible with a larger flow and a smaller head). For the poles:

p = \frac{60 \cdot f}{n} = \frac{60 \cdot 50}{300 rpm} = 10 pole pairs

This is a salient-pole machine with twenty poles, as befits a slow generator.

A steam turbine is not started all at once

The rotor and the casing expand with heat, and if they heat at different rates they rub. That is why the shaft is turned very slowly with a turning gear before steam is admitted, and speed and temperature are raised in stages, following a procedure. The same precaution applies when stopping: a shaft that stops hot would bow if it did not keep turning slowly.

03Internal combustion engines, fuels and lubricants

Below a few MW, the usual drive for a generator is a reciprocating internal combustion engine: pistons that turn the pressure of burned fuel into crankshaft rotation. In practice there are two families:

Diesel engine (compression ignition): it draws in and compresses only air, with a compression ratio of about 16:1 to 22:1; at the end of the compression the air is so hot that the injected diesel ignites on its own. It is the engine of medium-power and emergency sets: it consumes little, withstands a lot of work and its fuel is less flammable.
Otto engine (spark ignition): it compresses an air-fuel mixture (petrol, natural gas or LPG) with a compression ratio of 8:1 to 12:1 and a spark plug ignites it. It is the engine of small portable sets (1 to 10 kVA) and of gas-fuelled sets.

Both are usually four-stroke: intake, compression, expansion (the only one that delivers work) and exhaust, in two crankshaft turns. The engine must hold the alternator speed (1,500 rpm) even when the load changes: that is the governor’s job. The shaft power is the product of torque and angular speed:

P = M \cdot ω = M \cdot \frac{2 \cdot π \cdot n}{60}

P in W, M (torque) in N·m, n in rpm. At 1,500 rpm, 40 kW corresponds to a torque of 255 N·m.

Figure 1. Energy balance of a 45 kVA diesel set at full load, animated. Of every 100 units of fuel energy, only 33.2 reach the terminals as electricity; almost all the rest is lost as heat. The exhaust and coolant percentages are typical values.

The efficiency of the engine is obtained from the specific consumption b (grams of fuel burned per kWh delivered at the shaft) and the heating value of the fuel:

η_{m} = \frac{3.6 MJ/kWh}{b \cdot LHV}

b: specific consumption in kg/kWh; LHV: lower heating value of the fuel in MJ/kg (3.6 MJ = 1 kWh).

With b = 230 g/kWh and a diesel of 42.7 MJ/kg, the efficiency is 3.6 / (0.230 · 42.7) = 36.7 %. The specific consumption gets worse at low load, because the mechanical losses are almost fixed: an engine working at 25 % load burns considerably more per kWh than one at full load (a table with measured values is in section 06).

What a set engine includes

Besides the block, cylinders and crankshaft, a set engine has its auxiliary systems, which are the ones that fail most often:

Fuel: tank, filters (with water separator), pump and injection system.
Air: air filter and, on larger engines, a turbocharger with an intake air cooler, which raises power per displacement.
Cooling: water circuit with antifreeze, pump, thermostat and radiator with fan; small engines are air-cooled.
Lubrication: oil pump, filter and, sometimes, an oil cooler; oil pressure is the parameter that most protects the engine.
Starting: 12 or 24 V electric starter and batteries, with charger.
Exhaust: manifold, flexible joint, silencer and a duct to the outside.

Rating categories

The nameplate power is not a single value: it depends on the kind of duty for which the manufacturer guarantees the machine. The ISO 8528-1 standard defines continuous power (COP, at constant load with no limit on hours), prime power (PRP, at variable load with no limit on hours and a limited average load) and limited-time power (LTP), and manufacturers often add emergency standby power (ESP, for backing up the grid a few hours per year). The same engine has a different value in each category, and the emergency rating is the highest: it must not be used to size a set that works every day. In addition, the values apply at reference conditions (in ISO 8528-1, 100 kPa, 25 °C and 30 % relative humidity); at higher altitude or temperature the engine delivers less, and the manufacturer gives the derating curve.

Fuels

Fuel	Engine	Approx. energy	Points to keep in mind
Diesel fuel	Diesel	10 kWh/L (0.84 kg/L)	Sold by the litre. What matters: the cetane number (how easily it ignites), viscosity, cold filterability (wax clogs filters in frost) and the content of water and sediment.
Petrol	Otto	9 kWh/L (0.74 kg/L)	Highly flammable and degrades within months of storage; poorly suited to long-wait standby.
Natural gas	Gas Otto	about 10 kWh/m³	Taken from the network: no tank and no limited runtime, but a gas cut or a pressure drop leaves the set without fuel.
LPG (propane-butane)	Gas Otto	about 12.8 kWh/kg	Stored in pressure tanks; clean and without degradation, but with its own safety requirements.

Retail diesel includes a biodiesel blend set by current regulations, and like any organic fuel it collects water and can develop fungus if it stands still. In an emergency set the tank must always be full (also so that moisture does not condense), the fuel is renewed or treated periodically, and water is drained from the bottom valve. Tanks and their containment walls are designed to the fire-protection requirements of Law 19,587 and Decree 351/79 (see Occupational health and safety).

Lubricants

Engine oil does five jobs at once: it lubricates, cools the zones the coolant does not reach, cleans (detergent-dispersant additives keep soot in suspension), seals between rings and cylinder and protects against corrosion. It is chosen by its viscosity, given as an SAE grade (a set diesel usually takes a multigrade such as SAE 15W-40), and by its API service level (the “C” series, for diesel). Oil and filters are changed by running hours or by time, whichever comes first, following the manufacturer’s manual; an emergency set, even if it runs few hours, still needs the time-based change because the oil ages. A periodic analysis of the used oil (viscosity, water, soot, wear metals) is a predictive-maintenance tool, like thermography (Maintenance of electrical components). The general criteria for choosing fuels and lubricants in the workshop are in Electrical workshop supplies.

A diesel that hardly works gets sick

A diesel running for a long time at low load (below roughly 30 % of its power) does not reach working temperature: the fuel burns poorly and leaves soot and unburned diesel in the exhaust and cylinders. That is why emergency sets are tested under load, not at no load, and why oversizing a set “to be safe” is rejected.

04The medium and small generating set

A generating set (motor-generator) is an engine and an alternator coupled on a common bedplate, with their auxiliaries and a control panel. “Small” is the portable or site set, a few kVA, almost always single-phase and petrol-fuelled; “medium” is the three-phase diesel set, fixed or trailer-mounted, from some tens to a few hundred kVA, which backs up a building, a plant or an antenna site.

Part	What it is for	What is checked in maintenance
Bedplate and mounts	Rigid frame joining engine and alternator; anti-vibration mounts that isolate the floor.	Coupling alignment, condition of the mounts, fasteners.
Engine	Delivers the torque to the shaft.	Levels, belts, leaks, filters, running hours.
Alternator and voltage regulator	Converts the rotation into three-phase voltage.	Insulation, terminals, cleanliness of ventilation, AVR condition.
Cooling	Carries off the heat of engine and alternator.	Coolant level and concentration, hoses, radiator free of dirt.
Fuel	Base tank, day tank, filters and drain.	Level, water at the bottom, filters, tank venting.
Exhaust	Takes the gases to a safe place.	Tightness, thermal insulation, silencer.
Starting	Batteries, charger and starter.	Battery voltage, electrolyte density, charger, terminals.
Control panel	Starts and stops; measures and protects (oil pressure, temperature, overspeed, failure to start).	Alarms, emergency stop, sensor calibration.
Coolant heater	Keeps the engine warm so it starts quickly and takes the load in a few seconds.	Element, thermostat.

The generator room

A well-chosen set can fail because of its room. It needs combustion and cooling air in quantity (an inlet and an outlet louvre of the size the manufacturer gives), an exhaust that discharges outdoors away from openings, fuel that is contained and away from heat sources, space for maintenance and a base that supports the weight and the vibration. General maintenance of the switchboard components is explained in Maintenance of electrical components, and planning in Maintenance management.

A set in automatic starts by itself

Before touching a set whose panel is in automatic mode, it must be prevented from starting: turn the selector to “out of service”, disconnect the batteries and hang the padlock and tag. Only then can the fan, the belt or the coupling not move with hands inside. Also: exhaust carbon monoxide is lethal, so a set is never tested in a closed room; the exhaust and the manifold burn; batteries give off hydrogen (explosive), and the noise calls for hearing protection.

The automatic transfer switchboard

A backup set is connected to the installation through an automatic transfer switchboard (ATS). It contains two contactors or motorized breakers —one for the grid, one for the set—, interlocked mechanically and electrically so they are never closed at the same time, and a controller that measures the voltage and frequency of the grid and runs the sequence. It has three modes: automatic (it serves the grid), manual (the operator switches) and test (it starts the set and transfers on purpose, to exercise it).

Figure 2. Sequence of an automatic transfer switchboard, animated: normal grid, failure, set start, transfer and return of the grid. The times of each step are adjustable in the controller.

Timer	What it does	Usual value
Failure delay	Waits before giving the start order, to ignore micro-outages.	1 to 5 s
Cranking	Maximum time given to the engine to start; if it does not, it retries and then raises an alarm.	10 s per attempt (3 attempts)
Transfer delay	Waits for voltage and frequency to be stable before closing the set.	a few seconds
Return delay	Waits for the grid to be stable before going back to it.	1 to 10 min
Cool-down	Leaves the set running unloaded before stopping it.	3 to 5 min

These values are indicative; each installation’s design sets them. Where the interruption must be minimal (an operating theatre, a data room), an uninterruptible power supply (UPS) is added to cover the seconds until the set takes the load.

Backfeed: the risk you cannot see

If the set were connected to an installation still tied to the utility grid, it would energize the street cables through the distribution transformer, at high voltage, with a worker believing them dead. That is why the connection is made only through the interlocked ATS and never with a plug or a jumper. The case is also discussed in generator testing. The AEA 90364 regulation asks that a standby source cannot be paralleled with the grid by mistake; and a permanent parallel connection with the utility grid needs authorization and special protections.

The ATS design also defines what happens with the neutral: a four-pole switchboard also switches the neutral and prevents the return current from taking unplanned paths; the set’s neutral is earthed according to the earthing system of the installation, and the set’s earthing and its continuity are measured (Earthing, insulation and generator testing).

05Paralleling and synchronizing

Two or more sets can feed the same busbar. It is done for redundancy (if one fails, the other keeps running), to be able to maintain a set without cutting the service, to grow by adding a machine, and so that only the necessary sets run, close to their best efficiency. The price is complexity: the breaker of each set must be closed at a precise instant and then the load must be shared.

The four conditions to close

A generator can be connected to a live busbar only when the voltage (RMS), the frequency, the phase sequence and the phase angle all match those of the busbar. The sequence is checked once, at commissioning, with a phase-sequence meter; the other three are adjusted every time, and watched with a voltmeter, a frequency meter and a synchroscope, whose pointer turns at the speed of the frequency difference (toward “fast” if the incoming set is faster) and points to 12 o’clock when the waves are in phase.

Figure 3. Conditions for synchronizing, animated. Each state leaves one condition unmet: different voltage, different frequency, different phase angle; only when all four match is the breaker closed. The differences are exaggerated.

There are two ways to do it. Manual synchronizing uses the synchroscope, or three lamps: in the “dark lamp” method the set is closed when all three go out together; if they are cross-connected the light “rotates” and its direction of rotation says whether the set is faster or slower. Automatic synchronizing is done by a controller that adjusts the speed and the voltage and closes the breaker when the conditions are met and hold for a few moments (a synchronism-check relay).

Closing out of synchronism breaks machines

If the breaker is closed with the waves out of phase, the instantaneous voltage difference produces a short-circuit current and a sudden torque on the coupling and the crankshaft: shafts, couplings and windings have been broken. A breaker is never closed while the synchroscope is spinning fast, and the control system must include synchronism checking (synchronism-check relay, function 25).

Sharing the active power: droop

Once in parallel, the machines turn together at the same frequency. How they share the active power is decided by the speed characteristic of each engine: with droop, the frequency falls as the load rises. If two sets have the same droop in percent, they take the load in proportion to their rating; if one’s setting is raised, that one takes more. With the resulting frequency common to both, the sharing comes from drawing the two lines:

Figure 4. Load sharing between G1 (60 kW) and G2 (40 kW) with 4 % droop, animated. On the left the power of G1 grows and on the right that of G2; the common operating point is the horizontal line. Raising the G2 setting gives it more load; raising the total load makes the frequency fall.

The sharing of reactive power follows another path: it is governed by the AVRs, which share the reactive power through a small voltage droop (droop compensation) or a link between regulators. If the internal voltages of the alternators differ, a reactive current circulates between them that gives no useful power but heats them. In practice, the sets on one busbar are chosen with compatible regulators, and their fine adjustment is left to a technician with instruments (a network analyzer), not to trial and error.

Reverse-power protection (the set that, instead of delivering, “motors” dragged by the others) and the other generator protections are explained in Supply quality and machine protection.

06Sizing, testing and protecting a set

Choosing a set is solving a problem with several possible answers; a small one sags when motors start, an enormous one always runs at low load and gets sick. The method has six steps:

Survey the loads the set must feed (the essential ones, or the whole installation): power, power factor, efficiency and type of starting.
Add kW and kvar; the total apparent power is not the sum of the apparent powers, but the composition of the active and reactive ones.
Add a reserve margin (for growth and to avoid running at full load), of about 20 %.
Check the starting of the largest motor: the transient voltage dip must be acceptable.
Correct for altitude and temperature and choose the duty category (standby or prime) and the next commercial size.
Check the fuel and the runtime the service requires.

Example · A backup set for a workshop

A workshop, supplied at 400/230 V, needs backup for: lighting and sockets, 9 kW at power factor 0.95; a compressor with a 7.5 kW motor (shaft output, efficiency 0.88, power factor 0.84); and a pump with an 11 kW motor (efficiency 0.89, power factor 0.85). The set is a diesel at 1,500 rpm, with x′_d = 0.20 and an alternator efficiency of 0.905. Choose its rating.

Load	Electrical P (kW)	Power factor	S (kVA)	Q (kvar)
Lighting and sockets	9.00	0.95	9.47	2.96
Compressor: 7.5 / 0.88	8.52	0.84	10.14	5.50
Pump: 11 / 0.89	12.36	0.85	14.54	7.66

The active and reactive powers are added; the apparent power is composed:

P_{tot} = 9 + 8.52 + 12.36 = 29.88 kW Q_{tot} = 2.96 + 5.50 + 7.66 = 16.12 kvar

S_{tot} = \sqrt{{29.88}^{2} + {16.12}^{2}} = 33.95 kVA \cos φ = \frac{29.88}{33.95} = 0.88

A 20 % reserve is added and the next commercial size is taken:

S_{set} \geq 1.2 \cdot 33.95 kVA = 40.7 kVA \to 45 kVA P_{set} = 45 \cdot 0.8 = 36 kW

The engine gives 36 kW against a 29.88 kW load:

\frac{29.88 kW}{36 kW} = 0.83 (83 %) I_{n} = \frac{45000 VA}{\sqrt{3} \cdot 400 V} = 65 A

Starting the pump (the largest motor). Its rated apparent power is 14.54 kVA (table). With direct-on-line starting it draws about six times the rated current, hence six times that apparent power:

S_{st} = 6 \cdot S_{n} = 6 \cdot 14.54 kVA = 87.2 kVA \frac{U_{st}}{U_{n}} = \frac{1}{1 + 0.20 \cdot \frac{87.2}{45}} = 0.72

The voltage falls to 72 %, a dip of 28 %: unacceptable, because the usual criterion is not to exceed 15 % (at that dip, in addition, the motor starting torque is halved and contactors may drop out). With star-delta starting (one third of the current) or a soft starter:

S_{st} = \frac{87.2 kVA}{3} = 29.1 kVA \frac{U_{st}}{U_{n}} = \frac{1}{1 + 0.20 \cdot \frac{29.1}{45}} = 0.886

The dip is 11 %: acceptable. Conclusion: a 45 kVA set works if the pump starts star-delta or with a soft starter (AC motors, Variable-speed drives); with direct starting, a considerably larger set would be needed. The model is simple (it ignores the load already connected) and serves to decide; the manufacturer’s catalogue gives the finer curves.

Fuel. The shaft power is 29.88 / 0.905 = 33.0 kW; with 230 g/kWh and diesel of 0.84 kg/L:

P_{shaft} = \frac{29.88 kW}{0.905} = 33.0 kW C = \frac{33.0 kW \cdot 0.230 kg/kWh}{0.84 kg/L} = 9.0 L/h

With a 120 L tank:

runtime = \frac{120 L}{9.04 L/h} = 13.3 h

If the service requires 24 hours, the tank must be larger or refuelling must be planned.

Equipment tests

Before accepting a set, an acceptance test is made. The electrical tests of the alternator (winding resistance, insulation, no-load and short-circuit characteristics, synchronous impedance, load test with a bank of resistors and inductors) and the earthing measurement are developed in Earthing, insulation and generator testing; those of motors and transformers are in Motor and transformer tests. Here come those that look at the whole: the engine, the transfer and the real service.

Starting and load pick-up: time from the order to stable voltage and frequency, and to taking 100 % of the load.
Load by steps (25, 50, 75 and 100 % of the rating) with a load bank, recording at each step voltage, frequency, current, kW, temperatures, oil pressure and fuel consumption.
Step response: sag and recovery of voltage and frequency when a large load is connected suddenly.
Transfer sequence: times of each ATS step, interlock and return.
Protections: trip on low oil pressure, high temperature, overspeed and emergency stop, simulated.

A load test of the 45 kVA set from the example, with consumption measured by tank level difference, gives the following table. The consumption per electrical kWh is computed as b_el = (L/h · 840 g/L) / kW, and the overall efficiency from the fuel energy (9.96 kWh/L):

Load	Power (kW)	Consumption (L/h)	Consumption (g/kWh)	Overall efficiency
25 %	9	3.6	336	25.1 %
50 %	18	5.9	275	30.6 %
75 %	27	8.4	261	32.3 %
100 %	36	10.9	254	33.2 %

The specific consumption at full load (254 g/kWh electrical) is that of the engine (230 g/kWh at the shaft) divided by the alternator efficiency (0.905), and it shoots up at low load: at one quarter load, each kWh costs 32 % more fuel. It is one more reason to size without exaggerating.

Generator protection

Besides the protections of the output circuit (a thermal-magnetic or circuit breaker with overload and short-circuit settings), a generator needs its own protections: overload, short circuit, reverse power, over- and undervoltage, over- and underfrequency, loss of excitation and stator earth fault; on large machines, differential protection. They are developed in Supply quality and machine protection; the fundamentals of low-voltage protection are in Electrical protection.

What to take away from this page

A set is a chain: fuel, engine, alternator, switchboard. Voltage is set by the excitation and frequency by the speed. It is chosen by kW (the engine) and kVA (the alternator), with a margin and checking motor starting. And it is connected to the installation only through an interlocked switchboard, and to another set only in synchronism.

07In the workshop

Practice 1 · Survey a generating set

Materials: a workshop set (stopped, with the batteries disconnected), form, flashlight, multimeter, insulation tester. Steps: copy the nameplates of the engine and the alternator (power, voltage, current, power factor, rpm, duty category); identify each system in the table of section 04; check oil, coolant and fuel levels; measure battery voltage and density; measure the alternator insulation between phases and to frame (method in generator testing). Acceptance criterion: complete form, no fluid leaks, insulation above the minimum of the IEEE 43 rule (U_n in kV plus 1 MΩ: 1.4 MΩ for 400 V) and charged batteries.

Practice 2 · Load test with a resistor bank

Materials: set, three-phase resistive load bank, network analyzer or clamp with voltmeter and frequency meter, fuel measuring cylinder or flow meter, infrared thermometer. Steps: start the set and let it run unloaded for a couple of minutes; load it in steps of 25, 50, 75 and 100 %; at each step wait for stabilization and record voltage, frequency, currents, kW, oil pressure, coolant temperature and consumption; compute the g/kWh of each step; reduce the load and let it cool. Acceptance criterion: voltage within ±5 % of 400 V and frequency between 49 and 51 Hz at all steps (the contractual value is that of the manufacturer’s specification); no alarms; consumption close to the catalogue value.

Practice 3 · Transfer switchboard sequence

Materials: training ATS with a low-power set, lamp load, stopwatch, multimeter, phase-sequence meter. Steps: with the grid present, check the phase sequence of both sources; open the supply breaker to simulate the failure; time with the stopwatch how long until the set starts and until the lamps light again; restore the grid and time the return delay and the cool-down; with the switchboard de-energized and the teacher present, try to close both contactors by hand and check that the interlock prevents it. Acceptance criterion: transfer with no simultaneous closing in any test, times within the settings and a complete record.

Practice 4 · Synchronize two small sets

Materials: two single-phase or three-phase sets of a few kVA with an AVR, synchroscope or three lamps, voltmeters, frequency meters, clamp ammeter, resistive load, with the teacher in charge and each set’s own protections. Steps: start G1 and load it; check the sequence of G2; start G2 and adjust voltage and frequency to those of G1, with G2 slightly faster; watch the synchroscope and close the G2 breaker when the pointer passes slowly through 12 o’clock; share the load by adjusting the speed of each engine, and measure each set’s current. Acceptance criterion: closing without a noticeable current jerk on the clamp, and load sharing between equal sets with a difference of less than 10 % of the rated current.

In all four practices

The set is tested outdoors or with the exhaust discharged outside the workshop; nobody works on rotating parts or near the hot exhaust; and before touching wiring or connections the set is stopped, the batteries are disconnected and the absence of voltage is verified with an instrument. Nothing is loaded or connected to the workshop supply without the teacher.

08Common mistakes

Mistake	Consequence
Sizing by kW only (or by kVA only)	A set with an alternator to spare and an engine that falls short, or the reverse: the voltage sags or the machine overheats.
Forgetting motor starting	The voltage drops too much, contactors drop out and the motor does not start: the load is left without service just when it is needed.
Using the emergency rating for daily service	The engine works above what the manufacturer guarantees and ages quickly.
Testing the emergency set at no load	Soot and unburned diesel build up; when needed, it will not take the load. It is tested under load.
Connecting the set to the installation with a plug or a jumper	Backfeed to the utility grid: risk of death for whoever works on the line.
Closing the breaker of a paralleled set without watching the synchroscope	Short-circuit current and a mechanical blow to the coupling: broken shafts and windings.
Leaving the tank nearly empty and undrained	No fuel at the moment of the outage, and water and sediment that clog the filters.
Changing the oil only by running hours	On an emergency set the hours are few, but the oil ages anyway: it is also changed by time.
Installing the set in an unventilated room	Overheating, loss of power and, in the worst case, carbon monoxide in the building.

09Self-assessment

What does the AVR regulate and what does the governor of a generating set regulate?

The AVR regulates the voltage, acting on the alternator’s excitation current. The governor regulates the frequency, acting on the fuel to keep the engine speed constant.

A three-phase set of 100 kVA, 400 V and power factor 0.8. How many kW does it deliver and what is its rated current?

The active power is:

P = 100 kVA \cdot 0.8 = 80 kW

and the rated current:

I = \frac{100000 VA}{\sqrt{3} \cdot 400 V} \approx 144 A

The engine must also be able to deliver those 80 kW at the shaft (plus the alternator losses).

Why does a four-pole set turn at 1,500 rpm?

Because in a synchronous alternator the frequency is tied to the speed. For 50 Hz and p = 2 pole pairs:

n = \frac{60 \cdot 50}{2} = 1500 rpm

How do a Pelton, a Francis and a Kaplan turbine differ?

In the kind of drop they use. The Pelton is an impulse turbine, for high heads and small flows; the Francis, a reaction turbine, for medium heads and flows; the Kaplan, a reaction turbine with axial flow and adjustable blades, for low heads and large flows.

List the steps of an automatic transfer switchboard sequence upon a grid failure.

The controller detects the loss of voltage and waits for a delay; it gives the start order and opens the grid contactor; when the set is stable it closes the set contactor (never both at once). When the grid returns, it waits for a delay, retransfers and lets the set cool unloaded before stopping it.

Which four conditions must be met to synchronize a set with a busbar?

Equal voltage, equal frequency, equal phase sequence and equal phase angle. The synchroscope shows the last two: the pointer spins if the frequencies differ and points to 12 o’clock when the waves are in phase.

Two sets in parallel with 4 % droop (51 Hz at no load): one of 60 kW and one of 40 kW. The load rises to 90 kW. At what frequency do they run and how much does each take?

With the same droop, each takes the same fraction of its full load: 90 / 100 = 0.9. The common frequency is:

\frac{90 kW}{100 kW} = 0.9 \to f_{90 %} = 51 - 0.9 \cdot 2 = 49.2 Hz

G1 takes 0.9 · 60 = 54 kW and G2 takes 0.9 · 40 = 36 kW.

A set has an engine with 36.7 % efficiency and an alternator with 90.5 %. How much fuel is needed per electrical kWh?

The overall efficiency is:

η_{overall} = 0.367 \cdot 0.905 = 0.332

For every kWh at the terminals, 1 / 0.332 = 3.0 kWh of fuel is burned, that is, about 0.30 L of diesel.

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