Why parallel diesel generators or alternators?

Why parallel diesel generators or alternators?

One of the many advantages of diesel generators is that they are versatile and can be used in a wide range of applications, resulting in a strong power supply and longevity to the equipment.

For greater reliability and scalability, with proper configuration and setup, multiple generators can be connected in parallel operation, further extending the versatility of power options. Among other features, it offers a wider range of power outputs that can be adjusted up or down to suit the exact requirements of a variety of electrical power. Running diesel generators in parallel creates a higher level of operating economy and efficiency than traditional single generator setups, especially when power output requirements vary and fluctuate frequently.

Today, I will tell you about 5 important reasons why generators should be considered for parallel operation.

  1. Higher reliability

The redundancy inherent in the parallel operation of multiple diesel generators provides higher reliability for critical loads than a single generator set can provide. If one unit fails, the critical load is redistributed among other units in the system on a priority basis. In many environments, critical loads that require the highest degree of reliable backup power typically represent only a small fraction of the total power produced by the system. In a parallel system, this means that even if one of the units fails, the most critical elements will have the redundancy needed to maintain power.

  1. Greater scalability

When sizing generators to match load requirements, it is often difficult to accurately predict load increases and adequately plan for expected additional demand. If the load forecast is too aggressive, the initial cost of the generator may be more than necessary. On the other hand, if the load forecast is insufficient, reliable backup power may be lost, or you may have to resort to expensive generator upgrades or even buy another unit entirely.

With parallel operating systems, it is easier to allow for load changes without going over budget or piling up expensive units that are rarely used. If necessary, a generator can be added to provide additional power when needed. Likewise, redundant generators can be detached from the unit and used independently at other sites.

  1. Flexibility

Using multiple units in parallel provides more flexibility than using a single large-capacity large generator. Multiple small generators running in parallel do not need to be grouped together and can be located in a distributed fashion, reducing the need for a single large generator to have a very large footprint.

Basement installations or setting up small generators in limited areas can be creative in several ways to find ways to fit them. Since these units do not require a collective large space side-by-side, these units can often be installed in small facilities or when space is a limiting factor.

  1. Ease of maintenance and serviceability

If a generator in the system fails or requires maintenance, individual units can be dismantled and serviced without disrupting the functionality of the other units. The inherent redundancy of paralleled systems provides multiple layers of protection and ensures uninterrupted power to critical circuits.

  1. Cost-effectiveness and quality performance

Parallel operation of generators allows higher power performance to be obtained using small capacity single units that typically cost less than single high capacity generator models. Combined with monitoring solutions such as remote monitoring, usage and performance data can be monitored for further analysis and informed decisions on optimal settings for generators in parallel operations, further reducing overall operating costs throughout the run.

So, what are the factors to consider when setting up parallel operations?

Speed control. Each individual generator operates at its own specified engine speed and frequency. When individual generators are coupled together, their engine speed is locked into the overall speed of the entire system.

Load balancing. The load shared by each generator determines the speed of its engine. In a parallel system, the entire load is shared by all generators.

Voltage regulation. Like the engine speed of a single generator, the voltage of each unit is locked to the voltage of the other units in the system. When any unit’s voltage set point is slightly higher than the other generators in the system, it will end up taking the entire voltage load of the system. The voltage regulators of the individual generators are interconnected in a reactive cross-current system that regulates the voltage set points of the individual generators by collecting inputs from all transformers that control the individual voltages.

Generator set controller. Controllers can be installed for automatic parallel operation, increasing the ease and speed of checking engine and alternator parameters for each genset in the system. Further strengthen the supervision and control of the system. Consideration should be given to adding remote monitoring capabilities to effectively monitor the performance of each generator in the setup and facility, as well as respond quickly and troubleshoot effectively should any problems arise.

Parallel operation is worth considering

As you can see, running generators in parallel has clear advantages and should be a serious and viable option for any company looking to achieve a high level of redundant, backup power for their projects. Diesel generators are built with parallel operation in mind and, combined with our in-house remote monitoring system, can be easily assembled to meet your specific needs.

What are the advantages of running diesel generators in parallel?

In a parallel circuit, the voltage across each component remains constant and the current is divided.

Everywhere in an electrical system, we need constant voltage to operate the electrical system in an efficient and efficient manner.

Take your office electrical connections, for example. Your wiring in the office is similar to a small electrical system, where there is only one power source and many loads to start. In order for the protective device to achieve the desired operation, it needs to be provided with a rated voltage, which can only be achieved by parallel connection.

Likewise, in large power systems, we interconnect various diesel generators at the same voltage level, the same frequency, and the same phase sequence.

Why should synchronous diesel generators be connected in parallel?

When a large number of alternators or synchronous generators are connected in parallel to an infinite busbar system with constant terminal voltage, constant busbar frequency and very small synchronous impedance, the connection is called alternator parallel operation. Parallel operation of alternators is often referred to as synchronization. Synchronization can be achieved using various methods, such as synchronizing lights, relays, or even synchronizers.

Advantages of parallel operation of alternators:

Continuity of supply and maintenance:

Parallel connection is more conducive to repair and regular maintenance work. Maintenance is easier when using smaller single diesel generator sets as we can schedule maintenance for each unit, one after the other, without affecting the continuity of power generation.

Conversely, if it was a unit, it would require an entire shutdown for maintenance. This will greatly affect the electricity demand during the period.


According to the efficiency and load curve, the generator efficiency is the highest when the load is full, so the diesel generator set must run at full load. A single large unit is not economical for lower loads, but for smaller individual units, we can add or shut down gensets based on load requirements to meet maximum efficiency.

Expansion plan:

The current capacity of the power plant is assumed to be 500 MW. To expand the capacity to 700 MW to meet growing demand in the future, it would be more expensive to replace the entire unit and purchase a larger unit.

It is more economical to purchase a small stand-alone alternator unit that can be paralleled to the busbar system to reduce the initial capital investment.

Alternator power:

The higher the tier of the generator, the larger the scale of the setup. It is very difficult to manage a single large alternator that may be in the range of 1000MVA or more.

If we use a small single unit in parallel, it will be easier to maintain considering its size.

Maximize power system reliability:

If any one generator running in parallel trips due to a fault, the other parallel generators in the system will share the load. Therefore, the electrical system will not be interrupted by the trip of one alternator only if the shared load will not overload the other alternators.

Notes when connecting in parallel:

The voltage (RMS line-to-line terminal voltage) of all diesel generators must be equal

The voltage phase angles of all diesel generators must be equal

The phase sequence of all diesel generators must be equal

The frequency entered (no-load frequency) must be slightly greater than the frequency of the diesel generator connected to the system (no-load frequency)

Economically, a well-designed system provides backup power and variable output. Paralleling diesel generators becomes a cost-effective solution when the output power capacity of the standard diesel generator sets available in the market greatly exceeds the minimum requirements or falls below the maximum requirements. Synchronization of parallel diesel generators brings huge advantages. Parallel diesel generators provide maximum output when power demand peaks and minimum output when load demand is low. Therefore, paralleling diesel generators increases the reliability and efficiency of the power system.

The parallel connection of diesel generator sets should first meet the requirements of even power distribution. The uniform distribution of power includes two aspects: active power and reactive power.

The so-called even distribution means that the active power and reactive power undertaken by each generator should be proportional to their rated power. Our country has clear requirements for the parallel power distribution of diesel generator sets. When the generator generators are running in parallel, when the total rated power of the load changes within the range of 20%-100%, it should be able to run stably, and the power distribution error should be Meet a few requirements:

  1. The difference between the actual active power borne by each generator and the calculated value allocated in proportion to the rated power, when the rated power of the generator is the same, should not exceed ±10% of the rated active power of the generator. When the rated power of the generator is At different times, it should not exceed 110% of the rated active power of the maximum generator, and ±20% of the rated active power of the minimum generator.
  2. The difference between the reactive power actually undertaken by each generator and the calculated value allocated in proportion to the rated power of the generator should not exceed ±10% of the maximum rated reactive power of the generator.

If there is a large imbalance in power distribution, whether it is active power or reactive power, it will not only affect the efficiency and economy of diesel generator set operation, but also cause the failure of the entire power station.

If the active power distribution of parallel generator sets is seriously unbalanced, when the total load power is large, one generator set is often fully loaded or overloaded, while the other generator is still in a light load state, so that the power generation cannot be fully utilized. The capacity of the unit plays the role of the entire power station. The overload of diesel generator sets will not only bring serious harm to the diesel engine or generator, but also cause the protection equipment to work and affect the operation of the entire power station. On the contrary, when the total power of the load is very small, the imbalance of active power will often cause active circulation between the generator sets, so that some generator sets will switch to the motor state, which is also not allowed for diesel engines. When the diesel generator set has reverse power protection, the reverse power phenomenon will cause the reverse power to trip and cause a fault. For diesel power plants with frequent and large load changes, the above situation requires special attention.

The uneven distribution of reactive power will not seriously affect the diesel engine, but the rated value of the generator power is determined according to the apparent power. The unbalanced reactive power will inevitably cause the unbalance of the apparent power of the generator, which will also limit the power generation. The performance of the unit capacity, especially for the power station with low power factor, the load capacity of the unit does not depend on the rated power of the diesel engine, but on the rated current value of the generator.

The unbalanced reactive load can also be regarded as a circulating current between the two generators. Sometimes this circulating current will exceed the load current of the generator, resulting in additional losses inside the generator, even causing Generator overload. The reactive overload current will also trip the main switch of the generator, causing a failure. In addition, the magnitude of the generator’s reactive current is consistent with its excitation current, and the unbalance of the reactive current often occurs at the same time as the unbalance of the excitation current, which overloads the excitation system or even causes a fault. The rectifier diode of the compound excitation device is damaged, etc. The consequences of this failure are often quite serious. It can be seen that the uniform distribution of reactive loads and the uniform distribution of active loads have the same meaning.

Therefore, a reasonable choice of parallel connection is also very important.