How to size a generator set

July 14th, 2014, Published in Articles: Vector

 

Many factors influence generator set sizing, making it time consuming to manually calculate required generator capacity.

Parameters that determine the required generator set size include minimum generator set load; maximum allowable step voltage dip and step frequency dip; altitude and temperature; duty cycle, fuel, phase, frequency and voltage.

Sizing software, such as Cummins Power Generation’s GenSize, greatly simplifies this process and allows analysis of alternative to optimise the generator size. Knowing manual calculation methods helps the engineer understand how each factor contributes to a correctly-sized genset.

While most sizing exercises are best done with sizing programmes or with the help of the manufacturer, it remains important to understand the factors affecting your generator set. The first step in sizing and selecting a generator set is to establish project parameters.

Project parameters

Minimum generator set load/capacity

Running a generator set under light load can lead to engine damage, reducing reliability. It is not recommended to run generator sets at less than 30% of rated load. Load banks should supplement the regular loads when loading falls below the recommended value.

Maximum allowable step voltage dip

As you reduce the maximum allowable step voltage dip during initial startup, when loads cycle under automatic controls or when high peak loads operate, you must increase the size of the generator set specified. Choosing lower allowable voltage dip requires a larger generator set.

Maximum allowable step frequency dip

As you reduce the maximum allowable frequency dip, you increase the size of the generator set needed.

Altitude and temperature

Based on the site location, the size of the generator set must increase for a given level of performance as altitude and ambient temperature rise.

Duty cycle

Generator set size is also influenced by whether the application is for standby power, prime power or utility paralleling. Standby power systems generally have no overload capability. Prime power systems generally have a minimum of 10% overload capacity. Generator sets which are intended to operate extended hours at steady constant load should not be operated in excess of the continuous rating.

Fuel

The preference for petrol, diesel, or LP will affect generator set choices. Often, generator sets running on petrol or LP must be oversized due to derating. Emergency systems must typically be supplied by fuel stored locally.

Phase, frequency, voltage

Select either single or 3-phase. The 3-phase selection permits single-phase loads but the assumption is that the single-phase loads will be balanced across the three phases. Frequency should be either 50 or 60 Hz. Voltage choices are usually a function of selected frequency.

Loads

The next and most important step in sizing a generator set is to identify every type and size of load the generator set will power. In general, when non-linear loads are present, it may be necessary to oversize the alternator.

Power factor (PF)

The inductances and capacitances in AC load circuits cause the point at which the sinusoidal current wave passes through zero to lag or lead the point at which the voltage wave passes through zero. Capacitive loads, overexcited synchronous motors, etc. cause leading power factor where current leads voltage (see Fig. 1).

Fig. 1: A typical alternator curve of reactive power (kvar) capability. A reasonable guideline is that a generator set can carry up to 10% of its rated kvar capability in leading power factor loads without being damaged or losing control of output voltage. The most common sources of leading power factor are lightly loaded UPS systems with input filters and power factor correction devices for motors. Loading the generator set with lagging power factor loads prior to the leading power factor loads can improve stability.

Fig. 1: A typical alternator curve of reactive power (kvar) capability. A reasonable guideline is that a generator set can carry up to 10% of its rated kvar capability in leading power factor loads without being damaged or losing control of output voltage. The most common sources of leading power factor are lightly loaded UPS systems with input filters and power factor correction devices for motors. Loading the generator set with lagging power factor loads prior to the leading power factor loads can improve stability.

Lagging power factor, where current lags voltage, is more generally the case and is a result of the inductance of the circuit. Power factor is the ratio of kW to kVA and is expressed as a decimal figure (0,8) or as a percentage (80%).

Three-phase generator sets are rated for 0,8 PF loads and single-phase generator sets for 1,0 PF loads. Lower PFs require larger alternators or generator sets to serve the load properly. Caution should be used whenever applying generator sets to leading power factor loads. Only slightly leading power factor can cause generator sets to lose voltage control.

Single-phase loads, load imbalance

Single phase loads should be distributed as evenly as possible between the three phases of a 3-phase generator set to use generator set capacity fully and to limit voltage imbalance.

Peak loads

Peak loads are caused by loads which cycle on and off – such as welding equipment, medical imaging equipment, or motors. Taking cyclic loads into account can increase the size of the recommended generator set significantly despite painstaking efforts to place loads in a step starting sequence.

Motor loads

Calculating specific motor loads is best handled by sizing software which will convert types of motor into load starting and running requirements. For this discussion, however, it is sufficient to characterise loads broadly as high inertia or as low-inertia loads for the purpose of determining engine power needed to start and accelerate motor loads.

Low-inertia loads

These include fans and centrifugal blowers, rotary compressors, rotary and centrifugal pumps.

High-inertia loads

These include elevators, single and multi-cylinder pumps, single and multi-cylinder compressors, rock crushers, and conveyors.

Motors over 50 HP

A large motor started across the line with a generator set represents a low-impedance load while at locked rotor or initial stalled condition. The result is a high inrush current, typically six times the motor rated (running) current.

The high inrush current causes generator voltage dip which can affect other systems. The manner in which generator voltage recovers from this dip is a function of the relative sizes of the generator, the motor, engine power (kW capacity) and generator excitation forcing capability.

Depending on the severity of the load, the generator should be sized to recover to rated voltage within a few seconds, if not cycles. Various types of reduced-voltage motor starter are available to reduce the starting kVA of a motor in applications where reduced motor torque is acceptable. Reducing motor starting kVA can reduce the voltage dip, the size of the generator set, and provide a softer mechanical start.

However, these starting methods should only be applied to low-inertia motor loads unless it can be determined that the motor will produce adequate accelerating torque during starting.

Variable frequency drive (VFD) motors

Variable frequency (or variable speed) drives are non-linear loads used to control the speed of induction motors induce distortion in generator output voltage. Larger alternators are required to prevent overheating due to the harmonic currents induced by the VFD and to lower system voltage distortion by lowering alternator reactance. For example, VFD loads on a generator must be less than approximately 50% of generator capacity to limit total harmonic distortion to less than 15%.

Other loads

Uninterruptible power supply (UPS) loads

A UPS system uses silicon controlled rectifiers or other static devices to convert AC voltage to DC voltage for charging storage batteries and are another type of non-linear load. Larger alternators are required to prevent overheating due to the harmonic currents induced by the rectifiers and to limit system voltage distortion by lowering alternator reactance. Past problems of incompatibility between generator sets and static UPS devices lead to many misconceptions about sizing generator sets for this type of load. Most UPS manufacturers have addressed these issues and it is now more cost effective to require UPS devices to be compatible with the generator set than to oversize the generator for the UPS significantly. Use the full nameplate rating of the UPS for determining load to allow sufficient capacity for generator set battery charging and accommodating full UPS load capacity.

Battery charger loads

A battery charger is a non-linear load requiring an oversized alternator based on the number of rectifiers (pulses) – up to 2,5 times the steady-state running load for three-pulse; to 1,15 times the steady-state running load for 12-pulse. These loads are typically found in telecommunications systems.

Medical imaging loads

These include CAT scan, MRI, and X-ray equipment. The generator set should be sized to limit the voltage dip to 10% when the medical imaging equipment is operated with all other loads running to protect image quality.

Lighting loads

In addition to lamp wattages, ballast wattages and starting and running power factors should be considered.

Regenerative loads

For loads such as elevators, cranes and hoists, the power source is often relied upon for absorbing power during braking. That is usually not a problem when the utility is supplying power because it can be considered as an infinite power source with many loads. A generator set, by comparison, is able to absorb far less power, especially with no other loads connected. Generally, the regeneration problem can be solved by making sure there are other connected loads which can absorb the regenerative power. Excessive regenerative load can cause a generator set to over-speed and shut down.

Load step sequencing

In many applications, the generator set is sized to pick up all loads in one step. In some applications it is advantageous to start up the loads which cause the largest starting surge first and then the rest in multiple steps – the “largest motor first” rule. Codes may require sequenced load starting to start emergency and life safety loads within as little as ten seconds, while allowing other loads longer periods of time. In general, sequenced startup allows the smallest generator set in relation to the steady state load. When cycling motor loads exist, it will still be necessary to size the generator set to start the largest cycling motor last, with all other loads connected.

Contact Robin Kuriakose, Cummins, Tel 011 589-8604, robin.k.kuriakose@cummins.com

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