Generator Maintenance& Engine Overhaul

A single generator sized for the whole site is simple, but it has three weaknesses: it is a single point of failure, it is forced to run even when the load is small (wasting fuel and wet-stacking), and it cannot grow with the business without being replaced. Paralleling — running two or more sets connected to a common bus — solves all three. You gain redundancy (lose one set, the others carry on), efficiency (bring sets on and off line so each running set stays in its economical band), and scalability (add a set and synchronize it rather than scrapping the lot).

The catch is that you cannot simply close a breaker and connect two live alternators. Each is an independent AC source with its own voltage, frequency and instantaneous phase. Tie them together when those do not match and the result is a violent equalising current — effectively a short circuit between two stiff sources — that can trip breakers, shear couplings, distort shafts and destroy windings. Synchronization is the disciplined process of making the two sources close enough to identical that they merge smoothly.

Synchronization also applies when connecting a set to the utility grid (closed-transition transfer, peak-shaving, grid support) — there the generator must match the grid, which is an effectively infinite bus. The principles are the same; only the protection and the utility's interconnection rules become stricter.

Before a paralleling breaker may close, four conditions must all be satisfied between the incoming set and the live bus. Miss any one and the closure is unsafe.

(1) Voltage magnitude must match — typically within a few percent. A voltage difference drives a circulating reactive (kVAr) current the instant the breaker closes. (2) Frequency must match very closely; in practice the incoming set is run a touch fast (a small positive slip, e.g. +0.1–0.2 Hz) so that on closing it immediately takes a little load rather than being motored. (3) Phase sequence (rotation)— A-B-C — must be identical; a reversed rotation is catastrophic and is checked once at installation, not on every start. (4) Phase angle — the two waveforms must cross zero at the same instant (angle difference near 0°) at the moment of closing.

The diagram below shows the incoming set (amber) with a small slip and phase offset against the running bus (cyan). Synchronization is the act of nudging the amber wave until its amplitude, wavelength and start point all line up with the cyan one — then closing in the brief window when they coincide.

The classic instrument that shows all of this at a glance is the synchroscope. Its needle rotates at the slip frequency — the difference between the incoming and bus frequencies. If the incoming set is fast the needle turns clockwise (toward "FAST"); if slow, anticlockwise. Twelve o'clock is the in-phase point. The operator trims the incoming governor until the needle is turning slowly clockwise (set slightly fast) and closes the breaker just before it reaches 12 o'clock, so the contacts make exactly at zero phase difference. A needle spinning fast means the frequencies are far apart — never close on a fast-spinning scope.

On modern installations a check-synchronising (25) relay — or the synchronising function inside the genset controller — supervises all four conditions automatically and only permits (or commands) the close when they are within set windows. It is the electronic guardian that prevents an out-of-sync closure even if a human gets it wrong. The animated synchroscope here shows the idea: only close in the green "SYNC" zone at the top.

There are three levels of automation. Manual synchronizing uses a synchroscope (or the lamp methods below) and a human closing the breaker — instructive, still used on small or legacy plant, but slow and error-prone. Semi-automatic uses a check-sync relay to supervise a manual close: the operator lines things up but the relay blocks an unsafe closure. Automatic synchronizing — standard on modern paralleling controllers — has the controller adjust the incoming set's governor and AVR to match frequency, phase and voltage, then issue the close command itself, all in a second or two.

The traditional teaching methods deserve a mention because colleges still use them. In the three-dark-lampmethod, lamps connected across the open breaker poles go dark when the sources are in phase (close at darkness). In the two-bright-one-dark method, the rotation of brightness shows whether the incoming set is fast or slow and indicates the in-phase instant. These lamp methods illustrate the physics beautifully but are not used on serious modern switchgear, where relays and controllers do the job precisely.

The following is the disciplined sequence we follow to bring an incoming set onto a live bus. Each step exists to protect the machines and the people near them.

Step 1 — Confirm the bus is live and healthy. Verify the running set(s) are stable, voltage and frequency nominal, and the bus is energised. Know which set is the "lead" holding the bus.

Step 2 — Start and warm the incoming set. Start it off-load, let oil pressure and temperature stabilise, and let it reach nominal voltage and frequency before attempting anything.

Step 3 — Confirm phase rotation. On a new or re-wired set, verify A-B-C rotation matches the bus (once, at commissioning). A reversed rotation must be corrected before any close is attempted.

Step 4 — Match the voltage. Trim the incoming set's AVR until its voltage equals the bus within the permitted window (a few percent). A voltage mismatch becomes circulating reactive current on close.

Step 5 — Match the frequency, slightly fast. Trim the governor so the incoming set runs a fraction above bus frequency (small positive slip). This ensures it picks up a little load on closing instead of being motored.

Step 6 — Watch the synchroscope / sync indication. The needle should turn slowly clockwise. If it spins quickly, the frequencies are too far apart — adjust the governor until the rotation is slow.

Step 7 — Close at the in-phase instant. Issue the close just before the needle reaches 12 o'clock (or let the check-sync/auto-sync function close). The breaker contacts should make at near-zero phase difference.

Step 8 — Confirm the set is paralleled and stable. After closing, the set should immediately settle and begin sharing — watch for any oscillation (hunting) between sets.

Step 9 — Transfer / share load deliberately. Ramp the incoming set's governor up to take its share of real power (kW) and trim its AVR to take its share of reactive power (kVAr), so the load divides in proportion to set ratings.

Step 10 — Document and monitor. Log the parallel operation, watch reverse-power and load-sharing, and define the sequence for de-loading and opening a set cleanly when demand falls.

Closing the breaker is only half the job. Once paralleled, the sets must share the load fairly, and real power (kW) and reactive power (kVAr) are shared by two different controls. Real power (kW) is set by the engine governors — more fuel to an engine makes it try to speed up, and on a stiff parallel bus that extra torque becomes extra kW rather than extra rpm. Reactive power (kVAr) is set by the alternator excitation/AVRs — more excitation raises that machine's voltage tendency, which on the common bus becomes more kVAr from that set.

Governors share kW in one of two modes. Droop lets frequency sag slightly with load (e.g. 3–4% from no-load to full-load); sets in droop share load by their droop settings, but bus frequency falls as load rises. Isochronous holds frequency dead constant; with multiple isochronous sets, a load-sharing line (or the controllers' data link) tells each engine its fair share so they do not fight. Most modern multi-set plants run isochronous load sharing over the controller network, giving constant frequency and clean proportional sharing.

Mismatched sharing is a classic fault: if one set hogs the kW (governor set high) while another is light, or one set carries all the kVAr (excitation high) while others run at unity, the plant is unbalanced and a lightly-loaded set can even be driven into reverse power — motored by the bus — which the 32 relay must trip to protect it.

Sizing a multi-set plant starts, like any generator job, from a measured load study and the worst-case motor start — but adds the question of how many sets and of what size. The total installed capacity must cover the peak demand with the chosen redundancy: for N+1, install one more set than the peak needs, so any one can fail or be serviced with the rest still carrying the load. Equal-sized sets are easiest to share load and stock spares for; mixed sizes are possible but complicate sharing.

The art is choosing set size so that across the daily load curve, the running sets stay in their efficient, wet-stacking-safe band (above ~30% each). Several smaller sets that stage on and off as load varies beat one big set that idles — this is the efficiency win of paralleling. The controller's load-dependent start/stopbrings sets online as load rises past thresholds and sheds them as it falls, keeping the running fleet well-loaded.

The common bus, busbar and the incomer to the load must be rated for the sum of the paralleled sets, and — critically — the switchgear must withstand the combined prospective short-circuit current of all sets feeding a fault together (calculated per IEC 60909). Paralleling multiplies fault current, so breaker breaking capacity and busbar bracing are sized for the worst case of every set contributing at once.

Each set's cables to the paralleling breaker carry that set's full current, sized for current-carrying capacity, volt-drop and fault withstand to IEC 60364 — the same rules as any feeder, but with fault levels raised by paralleling. The common busbar is sized for the total paralleled current and braced for the combined fault current. Conductor runs between sets should be kept balanced in length where practical so impedances (and therefore current sharing at the physical level) are even.

The subtle, dangerous detail in paralleled systems is the neutral and earthing arrangement. If every set's neutral is solidly earthed and all neutrals are also bonded to a common point, triplen (3rd, 9th…) harmonic currents circulate between the machines through the neutral, overheating windings and nuisance-tripping earth-fault protection. The accepted solutions are to earth only one point of the system (a single bonded neutral for the paralleled bus) or to switch neutrals so only one set's neutral is connected at a time. Getting this wrong is a common cause of mysterious overheating and earth-fault trips on multi-set installations.

Earthing must also satisfy protection and safety: a low-impedance earth so earth-fault relays see and clear faults, and step/touch-potential safety at the plant. On a paralleled system the earthing scheme is designed as a whole, not set by set, precisely because of the circulating-current issue.

Paralleling switchgear is more than breakers. Each set needs a circuit breaker capable of synchronised closing (motorised, with the controller commanding the close at the in-phase instant), and a suite of protection relays identified by their IEEE C37.2 device numbers. The non-negotiables are the 25 sync-check (permits closing only in sync), 32 reverse-power (trips a set being motored by the bus), 46 negative-sequence (unbalance), 40 loss-of-excitation, 27/59 under/over-voltage, 81 under/over-frequency, and on larger machines 87 differential protection.

These functions live either as discrete relays or — on modern plant — inside the genset paralleling controller, which integrates synchronising, load sharing, protection and metering in one device. The switchgear also provides the load-sharing interconnection (hard-wired lines or a controller data bus), the bus metering, and the maintenance-isolation and bus-tie arrangements that let a set be worked on safely while the others run.

The brain that synchronises and load-shares is the genset controller, and Kenya's installed base spans many families. DeepSea Electronics (DSE) is the regional workhorse: the DSE73xx/74xx (single-set AMF), and the paralleling DSE8610 MkII, DSE8660 (bus-tie/mains), DSE7560 and DSE8620 handle synchronising and isochronous load sharing over the DSENet/MSC link. Woodward easYgen (3100/3200/3400/3500) and PowerWizard (1.0/2.0, on many engine-OEM sets) are equally common, as is ComAp(InteliGen, InteliSys, IG/IS-NT, InteliMains for bus-tie).

Beyond those, the same job is done by Caterpillar EMCP, Cummins PowerCommand (PCC), SmartGen (HGM/HMC paralleling), Datakom, Lovato (RGK series), Basler (DGC-2020), Mecc Alte (DST4602 Evo), Sices (GC600/DST), Kutai, Emko, Crompton/Selco sync relays, Be1/Beckwith, Thomson, ASCO paralleling switchgear, Deif (AGC-4/AGC-150) and Maru/Mebay controllers. They differ in configuration, fault codes and communications, which is exactly why controller-specific expertise matters when a plant will not synchronise.

Whatever the brand, the controller does the same fundamental things: it measures bus and set voltage/frequency/phase, adjusts the governor (speed bias) and AVR (voltage bias), commands the synchronised breaker close, then runs isochronous kW and kVAr sharing — while applying the protection functions above. We configure, commission and troubleshoot across all of these families.

When a plant will not parallel or will not share, the symptom points to the cause. The most common is the set won't come into sync: the synchroscope spins fast and never settles. That is a frequency mismatch the governor cannot close — check the speed-bias output from the controller to the governor/actuator, the governor mode (it must be in the controllable mode, not fixed droop), and that the controller is actually commanding speed bias.

The breaker trips immediately on closing (or there is a heavy thump) almost always means it closed out of phase — verify the 25 sync-check windows are not set too wide, that the close relay timing/breaker closing time is compensated in the controller (advance the close command by the breaker's closing time), and that the PT/VT sensing for bus and set is correct and in phase (a swapped or mis-wired sensing transformer is a classic culprit).

A set trips on reverse power (32) shortly after paralleling means it is being motored — its governor is set too low so the bus drives it. Raise its speed bias / kW set-point so it takes load. The opposite, one set hogging all the kW, means its governor/load-share is set high relative to the others — check the load-share line continuity (a broken MSC/load-share link makes sets fight) and that all controllers are in isochronous load-share, not a mix of droop and isochronous.

kVAr not sharing (one set carries all the reactive load, runs hot, or trips on over-excitation) points to the AVR cross-current/quadrature-droop compensation: the AVRs' reactive-droop or cross-current compensation must be set and wired so excitation is shared. Hunting / oscillation between sets — power swinging back and forth — is a governor or AVR stability problem: detune the gains, check for a sticking actuator, and confirm the load-share signal is clean (noise on the line makes sets chase each other).

Sync-check relay blocks every close even when the scope looks fine: check the relay's voltage, slip and angle windows and its sensing inputs — often the bus and incoming sensing are swapped or one phase is missing. Voltage won't match: AVR fault, wrong AVR mode, or sensing error. Earth-fault or overheating with no obvious load fault on a multi-set plant is the circulating-neutral problem from section 8 — revisit the neutral earthing (single-point or switched neutral). Methodically, every paralleling fault traces to one of: sensing (PT/VT wiring), bias outputs (governor/AVR), settings (windows, droop mode, share gains), or the neutral/earth scheme.

Paralleling is live, high-energy work: a mis-close releases enormous fault energy, so it is done by competent persons, behind proper protection, with the switchgear and relays commissioned and tested before the plant carries real load. Commissioning includes verifying phase rotation, proving the 25 sync-check windows and the breaker close-time compensation, testing the 32 reverse-power and other protections by injection, and a load-bank exercise that proves the sets synchronise, share and shed load correctly across the range.

The work is governed by ISO 8528 (set ratings and parallel operation), IEEE practice for device functions and interconnection, and IEC 60364/60909 for the installation and fault levels. Documentation — settings, test certificates, the one-line diagram and the operating sequence — is handed over so the plant can be operated and maintained safely for its life. A paralleled plant that is undocumented is a plant nobody can safely modify later.

Reduced to its essence: synchronizing means matching voltage, frequency, phase angle and rotation, closing in the brief in-phase window (slightly fast), then sharing kW with the governors and kVAr with the AVRs, all supervised by protection (25, 32, 27/59, 81, 46, 40, 87) and orchestrated by a controller (DSE, Woodward/PowerWizard, ComAp, Deif and 20+ others). Size the plant for peak-plus-redundancy, rate the bus and breakers for the combined fault current, and earth the neutral at a single point to avoid circulating currents.

Master those ideas and the diagrams above and you can read any paralleling switchboard in the country. This material is free to use for teaching in engineering and technical colleges — and when the theory meets a real plant that will not synchronise, our engineers commission and troubleshoot every controller family on this page.