Synchronization of Generators is the process of matching the output of one generator with the electrical parameters of another power source (such as a power grid or another generator) before connecting them together. In industrial and commercial power systems, multiple generators often operate in parallel (also called generator paralleling) to increase capacity, enhance reliability, or facilitate maintenance. However, paralleling generators is only possible when all units are properly synchronized – meaning their voltages, frequencies, and phase relationships align within strict tolerances. This article provides an in-depth technical overview of generator synchronization, covering how it works, why it’s crucial for power systems, methods and equipment used, necessary conditions, consequences of faults, and answers to frequently asked questions. Engineers and designers of power generator systems will gain a comprehensive understanding of synchronization requirements and best practices.
Generator synchronization refers to the coordination of key electrical parameters of a generator with those of a live power system so that they can be safely connected together. In practice, this means ensuring the generator’s voltage, frequency, and phase angle (and for three-phase systems, phase sequence) match the system it will connect to. When two AC generators or a generator and the grid are in sync, their alternating voltage waveforms rise and fall in unison – effectively becoming one source. If they are out-of-sync and connected, large fault currents will flow due to the differences, potentially causing severe equipment damage
Illustration of two AC voltage waveforms (blue for reference bus, red for incoming generator) with a phase difference. To synchronize, the incoming generator’s frequency and phase must be adjusted so both waveforms align (zero phase difference). Even a 30° phase misalignment can cause high transient currents if the breaker is closed at that instant.
In essence, synchronization is about matching the incoming generator to the running system. The main parameters to match are:
Frequency (Speed): The generator’s frequency (determined by engine speed and alternator poles) must equal the system frequency (e.g., 60 Hz or 50 Hz).Any difference in frequency means the phase angle between the sources is constantly changing.
Voltage Magnitude: The RMS voltage of the generator must be the same as the bus voltage (within a small tolerance, typically a few percent difference at most). A voltage mismatch can cause high reactive currents or voltage disturbances when paralleled.
Phase Angle: The relative phase angle between the generator’s voltage waveform and the system’s voltage waveform should be as close to zero as possible at the moment of closing the connection. Zero phase angle means the waveforms are in phase (peaks and zero-crossings aligned).
Phase Sequence: For three-phase generators, the phase rotation (order of phases A, B, C) must be identical. A mismatched phase sequence (e.g. ABC vs ACB) is equivalent to two phases being 120° out of phase, which is unacceptable.
Only when these conditions are met is a generator considered “in sync” with the system, and a circuit breaker can be closed to connect them. Synchronizing effectively makes multiple generators operate as one larger source, sharing load in the power system. Without proper synchronization, a generator cannot deliver power to an electrical network without causing disturbance or damage.
How Generator Synchronization Works
Synchronizing generators involves controlling the incoming unit’s speed and voltage, and timing the breaker closure, so that all parameters match at the instant of connection. Consider a scenario of adding a standby generator to an energized bus (or a second generator to one already running):
Match Frequency (Speed Control): The generator’s prime mover (engine or turbine) is sped up or slowed down until the generator’s frequency equals the bus frequency. In practice, operators adjust the governor set-point or throttle. If the generator is running slightly slower or faster than the system, the phase angle between them will drift. The goal is to have zero slip (difference in speed) at the moment of closure, or a very small slip so that phase alignment occurs just as the breaker closes. Modern digital governors often have a fine frequency control to achieve this.
Match Voltage (Excitation Control): The generator’s output voltage is raised or lowered by adjusting the excitation of the alternator (via the Automatic Voltage Regulator, AVR). The generator’s voltage must closely match the bus voltage in magnitude. If the voltages differ, there will be a reactive power surge when connected – the higher voltage source will feed vars into the lower, possibly tripping protective relays or causing voltage flicker.
Phase Alignment: Even with equal frequency, the sinusoidal waveforms might not be in phase at a given instant. The operator or control system monitors the phase difference. If the generator is at exactly the same frequency, the phase difference stays constant. Often, the incoming generator is adjusted to run just slightly faster than the bus (a small positive slip frequency). This causes the generator’s phase to slowly advance relative to the bus. The breaker is then closed at the precise moment the two waveforms align (phase angle ~0°). This technique ensures that if there is a tiny frequency error, the generator will not remain out-of-phase for long; it “sweeps” through phase alignment and closes at the correct instant.
Synchronization Check & Breaker Closure: Finally, when frequency, voltage, and phase are within acceptable synchronization tolerances, the connection is made. This can be done manually by an operator observing instruments, or automatically by a synchronizer device. Ideally, the breaker closes when the phase angle is as close to zero as possible to avoid transients. Any remaining minor differences at closing will result in a small exchange of power (e.g., the incoming generator might momentarily take some motoring power or immediately pick up a bit of load) to settle into equilibrium with the system.
After the generator is connected, it will naturally fall into step and share load with the other source. At that point, the generator’s governor and AVR typically switch to droop settings to maintain stable load sharing (this is discussed more later). For all this to work, specialized equipment is used to measure differences and assist the operator or control system in closing the breaker at the right time.
In modern power systems, synchronization is often performed automatically. Digital generator control modules (from manufacturers like Cummins PowerCommand, Woodward, DEIF, etc.) can control engine speed and excitation and issue the close command once sync conditions are met. These controls continuously monitor the difference in frequency (slip rate), voltage, and phase. Some systems even calculate an advanced angle to trigger the breaker slightly before perfect alignment, accounting for the breaker closing time, so that by the time the contacts touch, the phase angle is nearly zero. This level of precision is difficult to achieve manually, especially for large generators, which is why automatic synchronizers are prevalent in modern installations.
Why Synchronization of Generators Is Crucial in Power Systems
Proper synchronization is absolutely critical whenever multiple AC power sources operate in parallel. The main reasons include:
Avoiding Electrical Damage: If a generator is connected while out-of-sync (frequency or phase not matched), the difference in phase is effectively a short-term fault. The two sources will fight each other, driving a high current surge between them.This can blow fuses, trip breakers, or even damage windings and transformers. Only by synchronizing first can we avoid these destructive circulating currents.
Mechanical Protection: A generator synchronized out-of-phase will experience a sudden torque shock. The machine’s rotor will jerk into synchronism with the grid in a fraction of a second, imposing extreme mechanical stress on the shaft, couplings, and the prime mover. Large angle or speed mismatches at closing have been known to bend shafts or shear keys. Even if nothing breaks immediately, such transients can cause cumulative fatigue damage to turbine blades, crankshafts, and bearings. Correct synchronization prevents these sudden mechanical transients by ensuring the machine is already nearly in step before connection.
Stable Load Sharing: In a multi-generator power station or an industrial facility with several generators, synchronization is what allows them to share the electrical load smoothly. If each generator is not in sync, they cannot maintain a constant share of the load – one would overspeed while the other is dragged – leading to instability or outages. Synchronized generators, however, can seamlessly divide the kW and kVAR demand among themselves according to their governors and AVR settings, keeping the overall system stable and the frequency/voltage within limits.
Preventing Blackouts and Equipment Trips: Many protective relays are installed to prevent out-of-sync conditions. For example, a synchronism-check relay (ANSI 25) will block breaker closure if parameters are outside limits. If somehow a breaker were closed out-of-sync (due to operator error or device failure), protection like reverse power relays (ANSI 32) or differential protection may trip the generator offline almost instantly. While this protects the equipment, it also means the generator cannot connect to support the system, potentially leading to power shortages or blackouts. Thus, correct synchronization is crucial to ensure continuity of power without tripping any safety devices.
Efficient and Reliable Power Management: In commercial and industrial applications, using multiple synchronized generators (instead of one large unit) provides redundancy and flexibility – one generator can be taken offline for maintenance while others carry the load, or extra units can start during peak demand. This parallel operation strategy only works with proper synchronization; without it, the generators could not operate together reliably. In facilities like hospitals, data centers, or manufacturing plants, this reliability is often mandated. For example, closed-transition transfers (where a backup generator is momentarily paralleled with the utility to avoid any power interruption) require tight synchronization to meet regulatory standards and prevent any disturbance during the transfer.
In summary, synchronization of generators is fundamental to the safe and efficient operation of power systems with multiple sources. It ensures that power can be added or removed from the system without causing harm, thereby enabling redundancy, scalability, and stability in power generation.
Methods of Generator Synchronization
Over the years, engineers have developed several methods and tools for synchronizing generators. These range from fully manual techniques to sophisticated automatic control systems. The goal of each method is the same: adjust the incoming generator’s conditions and close the breaker at the right moment. Below are the common methods and equipment used for generator synchronization:
Manual Synchronization (By Eye and Hand)
In traditional manual synchronization, a skilled operator performs the synchronization by observing instruments and manually controlling the generator. Key steps in manual syncing include adjusting the throttle and voltage and deciding when to close the breaker. Historically, operators used indicator lamps and analog meters to judge synchronization:
Three-Lamp Method: One of the oldest techniques uses incandescent light bulbs connected between the generator and bus terminals in a specific configuration. For example, a common setup uses two lamps each connected between the generator and bus on two phases (say Phase A to Phase A, and Phase C to Phase C), and a third lamp cross-connected (Phase B of generator to Phase A of bus). As the generator’s voltage comes into phase with the bus, the lamps get dimmer. When all three lamps go dark simultaneously, it indicates the voltages are in phase (no difference across each lamp). If the lamps are fully bright, the generator is 180° out-of-phase with the bus. The operator would adjust speed as needed to slow down or speed up the blinking rate of the lamps (which indicates slip frequency), and close the breaker at the moment the lamps go dark. This method is simple and requires no special instruments, but it can be hard to judge the exact moment of synchrony, especially under bright ambient lighting or for small phase angle errors.
Synchroscope Instrument: A synchroscope is a specialized analog instrument that directly displays the phase difference and relative frequency between the incoming generator and the system. It typically has a dial with a pointer that rotates. The direction of rotation shows whether the generator is running faster or slower than the system (e.g., pointer rotating clockwise might indicate the generator is fast with respect to the bus). When the pointer is at the 12 o’clock position, it means the phase angles are aligned (zero difference). An operator using a synchroscope will adjust the generator speed until the pointer rotates very slowly (indicating a tiny slip frequency), and then initiate breaker closure just as the pointer reaches the top (0° phase difference). The synchroscope often has markings like “Slow” and “Fast” to guide adjustments. Unlike the simple lamp method, a synchroscope provides a clearer indication of phase lead/lag and the rate of change, which improves the accuracy of manual synchronization.
Manual synchronization using these tools requires training and good judgment. The operator must anticipate the breaker closing time (often a few cycles of delay) and close slightly before exact phase alignment so that by the time the breaker contacts touch, the generator and bus are in phase. This is sometimes called the “synchronizing by ear and eye” skill in older power plants. While effective, manual methods are labor-intensive and prone to human error if not done carefully.
A classic synchroscope gauge by General Electric. The pointer rotates to indicate the phase difference between the generator and the bus. The operator adjusts the generator speed so that the pointer moves slowly to the “12 o’clock” (zero phase difference) position, then closes the breaker when aligned. Synchroscopes greatly aid manual synchronization by indicating if the incoming generator is “fast” or “slow” relative to the system.
Automatic Synchronization (Automatic Synchronizers)
Most modern generator systems use automatic synchronizing equipment to perform the process with precision and repeatability. An automatic synchronizer is essentially a controller or relay that takes over the tasks of adjusting speed, adjusting voltage, and closing the breaker at the right instant.
Automatic Synchronizer Controllers: Devices from companies like Woodward (e.g., MSLC controllers), DEIF, Basler, ComAp, and in-house generator controls (Cummins PowerCommand, CAT EMCP, Kohler Decision-Maker, etc.) can all perform auto-sync. These devices continuously measure the incoming generator’s voltage and the bus voltage. They output signals to the generator’s governor to slightly raise or lower speed and to the AVR to tweak the voltage, bringing the generator to the required setpoints. Once the frequency and voltage are within a preset “window” and the phase angle is approaching zero, the controller issues a close command to the breaker. Many synchronizer units calculate an advance angle – for example, if the breaker closing mechanism takes, say, 100 milliseconds, the unit will initiate the close when the phase angle is a few degrees before zero, so that during those 100 ms the generator “catches up” and is exactly in phase when the breaker closes. This technique ensures a near-perfect closure every time, something very difficult to do manually. The synchronization accuracy of automatic systems is typically within a few electrical degrees and a few tenths of a Hz or percent voltage, well within safe limits.
Paralleling (Synchro) Panels: In multi-generator installations, there is often a dedicated paralleling switchgear or control panel. These panels incorporate synchronizer modules, sync-check relays, and breaker control circuits for each generator. The panel might allow for both auto and manual sync modes. For instance, a typical configuration is to have each generator’s controller handle syncing automatically, but the switchgear also has a synchronism-check relay (ANSI 25) that acts as a permissive device: it will only allow the breaker to actually close if the voltage, frequency, and phase are within prescribed limits (adding a layer of safety). Paralleling panels also include load sharing controllers that take over once the generators are connected, adjusting fuel to each engine to share kW load and adjusting excitation to share kVAR (reactive load) among the units. All these systems work hand-in-hand with synchronization. In the past, complex third-party switchgear with extensive relay logic was needed, but today many generator manufacturers offer integrated paralleling solutions where the sync and load share functions are built into the generator’s controller. This greatly simplifies the setup: the user can just enable the generators to parallel and the controllers handle the rest, coordinating via a network or analog load share lines.
Synch-Check Relays: Even with automatic sync, a protective relay known as a sync-check (device 25) is usually present as a backup. Its job is to verify that conditions are acceptable just before the breaker closes (or even immediately after, in case of very fast close). If the automatic synchronizer malfunctions or something drifts, the sync-check relay will prevent closure or immediately trip the breaker open if the angle or other parameters are out of range. Typical settings for a synchronism-check relay might be, for example: phase angle difference less than 10°–20°, frequency difference less than 0.1–0.2 Hz, and voltage difference within 2–5%. These ensure even a worst-case closure won’t be too harsh. Modern digital relays can also supervise the slip frequency to ensure the breaker doesn’t close if the phase is drifting too quickly (meaning frequency mismatch).
In automatic systems, the role of the human operator is largely supervisory – to enable the sequence and watch that it proceeds correctly. The automation handles the timing with far better precision. This reduces human error and allows synchronization of multiple generators to happen quickly (important in emergency start scenarios where generators must come online in seconds). Automatic synchronization is standard in utility power plants and large standby generator systems alike.
Other Considerations and Advanced Methods
Dead-Bus Paralleling: If the power bus is de-energized (dead), the first generator to start can close to the bus without matching another source (since there’s no voltage on the bus). This is called “closing on a dead bus” and does not technically require synchronization – instead the generator simply is started and its breaker closed, energizing the bus. However, when a second generator comes to join, then normal synchronization is required with the bus now alive from the first unit. Some control schemes have an automatic “dead bus arbitration” where the first generator to reach voltage takes the bus, and others wait and sync to it.
Reverse Synchronization: In some cases, a generator may be used to synchronize a bus to an existing running system (for example, when re-energizing a section of grid, a generator might be used to “synchronize the dead grid” before connecting to the main grid). This is a more complex scenario typically handled by utility transmission operations and beyond the scope of most generator paralleling in facilities, but it follows the same principles – control the voltage and frequency of one side to match the other before closing tie breakers.
Phase Sequence Checks: Modern synchronizers and relays usually include checks for phase rotation. When installing multiple generators or a new generator, verifying that the phase sequences match (often by phase rotation meters or simply by phasing out with a voltmeter) is an important step. If phase sequence is wrong, no amount of frequency/voltage matching will synchronize the systems – the connections of two leads must be swapped first to correct the rotation. In manual syncing with lamps, a tell-tale sign of wrong phase sequence is that the lamps will not go dark together but instead go out one at a time in succession (rolling dark), indicating the generator’s phases are cycling in a different order than the bus.
Requirements and Conditions for Synchronization
For successful synchronization of generators, certain conditions must be satisfied. These are often referred to as the synchronizing conditions or acceptance criteria that must be met before closing the paralleling breaker. Below is a summary of key requirements and typical acceptable limits:
| Parameter | Condition for Synchronization |
|---|---|
| Frequency | Must match the system frequency. A small difference (slip) is allowed to position the phase angle for closing, but typically within ±0.1 to 0.2 Hz of the target. In practice, many systems aim for <0.05 Hz difference at closing. |
| Voltage Magnitude | Generator voltage must equal bus voltage within a few percent. Often within ±5% or better (some standards use ±3% as a guideline). Too high a voltage difference causes reactive surges. |
| Phase Angle | Must be near 0° at the instant of breaker closure. Common relay settings allow closing below 10° or 15° phase difference, but the ideal target is essentially 0°. The closer to zero, the smaller the transient. Closed-transition transfer switches, for example, require <5° phase error. |
| Phase Sequence | Must be identical (e.g., ABC phase rotation matching ABC on bus). No tolerance – if not matching, the connections must be corrected prior to sync. |
| Waveform | For AC generators, the waveform shape should be similar (sine wave, low harmonic distortion). This is usually inherent in design (most large alternators use a 2/3 pitch winding to minimize triplen harmonics). Mismatched waveforms can cause circulating harmonic currents, though this is rarely a problem with modern generators of similar design. |
In addition to the above, voltage phase balance between phases should also be normal on both sides (e.g., not connecting a severely unbalanced source). But in practice, synchronization focuses on the main frequency, voltage, and phase timing factors.
Before attempting synchronization, operators typically verify these conditions using their control system or instruments:
Check List: Generator at rated speed and voltage, correct phase rotation verified, governor in the correct mode (usually droop mode or a special sync mode to allow fine speed adjustment), AVR in automatic and adjusted, no alarms, and sync-check relay enabled.
Synchronizing Window: The synchronism-check relay or the automatic synchronizer will have an allowable window for closing. For example, a relay might be set to only allow closing if the frequency difference is less than 0.1 Hz, voltage difference less than 3%, and phase angle within 10°. These settings are chosen based on the size of the system and how robust the equipment is. A smaller generator on a large stiff grid might need tighter settings (to avoid huge currents), whereas two generators of similar size sharing load might tolerate a slightly larger angle because both will adjust to each other upon connection.
Meeting all conditions does not guarantee zero disturbance, but it ensures any transient is minimal and within equipment design limits. As a reference, IEEE Standard 67 and other industry guidelines recommend keeping the synchronizing voltage (resultant of the difference) to a level that limits mechanical torque and electrical stress. One IEEE report noted that a phase angle less than 10° at closing is often required to keep torsional and electrical stresses low during synchronization events. Voltage difference should likewise be small, because a higher voltage on one side will push a rush of reactive power to the other upon closing, which can affect system voltage stability.
Consequences of Faulty Synchronization
Failing to properly synchronize generators before connecting can have serious consequences, both immediate and long-term. Some of the potential results of faulty synchronization include:
Large Current Surges: If the generator is out-of-phase with the system when connected, it’s akin to creating a short circuit between two voltage sources. A massive current surge will flow as the generator’s output tries to instantly align with the system. This surge can be several times the generator’s full load current. Such high currents can blow fuses or trip overcurrent relays instantaneously. Even fast-acting breakers may experience stress or contact damage from the fault-level current. Equipment not protected can be “severely damaged” by the electrodynamic forces and overheating in this scenario.
Mechanical Shock and Damage: The rotor of the generator will experience a sudden acceleration or deceleration torque if synchronization is off. For example, if the generator was a bit slow (lagging in phase) and got connected, the grid will pull the generator’s rotor into step, accelerating it abruptly. This can physically twist the generator shaft and coupling. Repeated occurrences can fatigue metal parts. In extreme cases (like very large phase angle differences, e.g., near 180°), the torque is comparable to a short-circuit condition on the generator – enough to crack shaft journals or damage the prime mover’s gearbox or turbine blades. A documented case in an IEEE report described how a generator step-up transformer failed after a 180° out-of-phase closing – the first out-of-sync event weakened the insulation, and a second one destroyed the transformer completely. This illustrates how even if the system survives one bad synchronization, the latent damage can cause a failure later.
Reverse Power and Loss of Field Trips: Modern generators have protective relays that sense abnormal conditions like power flowing into the generator (reverse power) or loss of excitation. When synchronization is bad, the generator can momentarily act as a motor (drawing power from the system instead of supplying) because it was behind in phase. This triggers reverse power protection to trip the generator offline to protect the prime mover (e.g., to avoid driving a diesel engine as a motor). Similarly, if the generator’s voltage was low compared to the system, a large reactive inflow can occur and the generator’s automatic voltage regulator may saturate trying to catch up. The sudden var inflow can mimic a “loss of field” condition leading to a loss-of-field relay trip. These protective trips will shut down the generator unit, meaning the synchronization attempt fails and the generator is disconnected for safety. While the equipment is saved, the facility loses the generator’s contribution until it can be reset and started again – which could mean a partial power loss in a critical moment.
Electrical Equipment Stress: Transformers connecting generators to switchgear, breaker contacts, bus bars, and other connected devices see significant stress during out-of-sync events. The dielectric stress on transformer windings from sudden phase jumps can degrade insulation. As noted, a generator’s step-up transformer (GSU) can fail if subjected to out-of-phase closings. Switchgear can experience rapid heating and mechanical force. Even the generator’s windings themselves will see huge transient currents that produce mechanical forces between the coils (trying to repel or attract due to the sudden current surge). All these effects reduce the lifespan of the equipment.
System-Wide Disturbance: A badly synchronized connection can cause a voltage dip or frequency wobble in the larger power system. For example, if a large generator connects out-of-phase on a regional grid (in a power plant scenario), it can cause a momentary fluctuation in grid frequency or knock other machines slightly off balance. Typically, the grid is robust and will absorb it, but in a weaker system (like an islanded microgrid), this could even cause other generators to trip or a portion of the system to go down. Thus, one mistake in synchronization could risk a cascading failure in a small power system.
In summary, the cost of synchronizing incorrectly ranges from nuisance trips (best case) to catastrophic damage (worst case). That’s why modern systems put so much emphasis on interlocks, relay checks, and automated control to ensure synchronization is done correctly every time. It is far better to have a synchronism-check relay prevent a closure (forcing a retry) than to have an out-of-sync closure. Additionally, many systems log synchronization attempts and any discrepancies, so any issues in the process can be corrected before they lead to equipment damage.
Frequently Asked Questions about Generator Synchronization
Q1. Can generators of different sizes or brands be synchronized in parallel?
Yes, it is possible to synchronize and parallel generators of different sizes or from different manufacturers, provided that their electrical parameters can be matched and they have compatible governing and voltage control systems. In practice, the generators need to have governors that can share load (often using a droop setting) and voltage regulators that can share reactive load. The phase sequence must be the same and the voltage/frequency must be brought to match before closing, regardless of size differences. One key consideration is load sharing: after synchronization, a smaller generator will take on proportionally less load than a larger one if both are set to the same droop characteristics. For example, if a 500 kW unit and a 250 kW unit are paralleled, the smaller unit might carry roughly one-third of the total load (depending on settings) because if it tries to exceed its capacity, its frequency droop will make it slow down slightly and the larger unit will inherently pick up more of the load. As long as protection settings and controllers are configured appropriately, mixed capacity gensets can operate together. Many vendors offer proprietary solutions to parallel dissimilar generators. It’s important to note that extremely mismatched sets (like a 2 MW turbine and a 50 kW generator) might not be practical to run in parallel due to stability issues, but in medium ranges it’s done in many microgrid and standby system designs.
Q2. How do paralleled generators share load once synchronized (droop vs isochronous)?
Once generators are synchronized, load sharing is governed by their control modes. Droop control is the most common method: each generator’s governor is set so that its speed set-point droops slightly as load increases, which means if one generator starts to carry more load, its frequency tends to dip, causing it to back off, while the other unit sees a slight frequency rise and increases output – resulting in an equilibrium where power output is divided. In droop mode, no single generator rigidly controls system frequency; instead, the frequency will be a little lower at full load than at no load (for example, a 4% droop means at full load the frequency might be 4% below nominal if it were alone). When multiple units run in droop, they inherently split the load based on their capacity and droop setting. Isochronous control, on the other hand, means the governor will try to hold the engine at a fixed speed regardless of load – which in a parallel system would normally cause conflict (only one unit can truly control the frequency). Therefore, usually only one generator (or an external device) is in isochronous mode at a time to set frequency for the group, while all others run in droop. For instance, in an isolated microgrid, you might designate one generator as the “swing” or master in isochronous mode (maintaining 60 Hz exactly), and the rest in droop mode so they follow and share load. In grid-parallel operation, the grid itself dictates the frequency, so all generators run in droop mode to share load according to their droop characteristics. Similar logic applies on the reactive power side with AVR droop (also called voltage droop or power factor control) to share kVARs. In summary, synchronizing puts machines in parallel, and droop settings ensure stable proportional load sharing, whereas isochronous mode is used sparingly (usually one unit at a time) when an isolated system’s frequency needs to be tightly maintained.
Q3. What if the phase sequence (rotation) is wrong when trying to synchronize generators?
If the phase rotation of the incoming generator doesn’t match the system, it is impossible to synchronize until that is corrected. Phase sequence (e.g. ABC vs ACB rotation) determines the order in which the three phase voltages reach their peaks. If one generator is spinning in effectively the reverse phase order, connecting it without correction would be equivalent to cross-connecting phases – leading to a huge fault. The signs of wrong phase sequence during a sync attempt include the synchronizing lamps not all going dark together (they might extinguish one after another in a rotating pattern), or a synchroscope that spins rapidly off-scale (if connected improperly). The fix is purely a wiring change: swap any two leads of the generator’s output (for example, swap generator leads B and C) to get the rotation to match the bus. This is typically verified during commissioning with a phase rotation meter or by a lamp method test before any synchronization is attempted. Modern digital systems may also detect phase rotation mismatches and give an alarm. In practice, once phase sequence is corrected, the normal synchronization procedure can take place. It’s a one-time setup issue per machine. All generators intended to run in parallel should be consistently wired so that their phase sequences are the same relative to their breakers and bus bars.
Q4. Which protective relays are involved in generator synchronization and paralleling?
Several protective relays ensure that synchronization and parallel operation do not harm the generators or the system. The primary one is the Synchronism-Check Relay (Device 25), which monitors two voltages (generator and bus) and permits breaker closure only if they are within preset differences of phase angle, frequency, and voltage. Essentially, it’s a safeguard against out-of-sync closing; even if an operator mistakenly tries to close or an automatic system glitches, this relay should prevent a bad closure. Another important element is the Reverse Power Relay (Device 32). When generators are paralleled, a reverse power relay monitors if power is flowing into the machine (which could happen if a prime mover fails or if the unit is motoring). If a generator goes offline but is still connected, it can draw power from the others or grid, acting as a motor – the reverse power relay detects this and opens the breaker to disconnect the unit, protecting the engine or turbine from being driven (for example, to prevent a diesel generator from being motorized which can damage it). A related relay is Loss-of-Field (Device 40) or an under-excitation limiter; if a generator’s excitation fails, it may slip out of sync or absorb excessive reactive power from the system, so this relay will trip the unit off to avoid instability or damage to the rotor. Overcurrent and Differential Relays (50/51, 87) protect against short-circuits or mistakes like wrong phase connection. They would act if, say, a synchronization attempt was wildly off and essentially caused a short. Lastly, modern generator controllers often include an integrated auto-synchronizer and phase mismatch alarm – if after a certain time the unit cannot synchronize, it will alarm for operator intervention. All these protections work together to make paralleling operations safe and to isolate any generator that is not behaving correctly in the network.
Q5. What happens if a generator is mistakenly connected to the grid out-of-phase?
This is a dangerous situation that every operator tries to avoid. If a generator is connected out-of-phase to the grid (meaning the breaker was closed when the phase angle difference was large, say tens of degrees or even completely 180° out), the result is a very large transient. The generator will experience a violent kick as it forcibly synchronizes to the grid. Immediately, a huge current flows to correct the discrepancy – this current spike can be several times the generator’s rating. Protective relays should act in a fraction of a second to trip the generator offline (for instance, the sudden current might trip an instantaneous overcurrent, or the abrupt power flow reversal might trigger the reverse power relay). The grid, being a vastly larger source, will hardly notice the incident beyond maybe a flicker, but the generator will likely suffer damage. Out-of-phase closings can bend rotor shafts, damage coupling bolts, and shock the generator’s windings. In the worst-case scenario, major damage occurs (as noted earlier, transformers have exploded and generator rotors have been ruined in such events). If the generator somehow remains connected, it will oscillate with the grid until it either falls in line or trips – these oscillations manifest as mechanical vibrations and electrical power swings. Usually though, protections cut it off quickly. Any time a bad synchronization occurs, the unit should be thoroughly inspected for damage before being used again. Hence the emphasis on sync-check relays and proper training – it’s a mistake you want to design out of the system because the consequences are severe and costly.
By understanding the principles and methods of generator synchronization, engineers can design and operate power systems that safely incorporate multiple sources. Whether it’s two diesel generators backing each other up in a hospital, or a large turbine generator connecting to a national grid, the fundamentals remain the same: match the voltage, match the frequency, match the phase, check everything twice, and only then close the breaker. Following these rules and using the right equipment (synchroscopes, automatic synchronizers, and protective relays) ensures smooth and reliable power system operation for industrial and commercial generator applications. Proper synchronization of generators is not just a technical formality – it is a critical practice that enables the flexibility and resilience of modern power systems.
