Diesel Generator Controller

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The functions of the diesel generator controller  are to provide: control, protection and indication.  They are designed to provide the state of the generator at all times and the ability to change that state. Equipment required in the control system include:

  • Measuring instruments (meters)
  • Status indicators
  • Protection relays
  • Warning/Alarm
  • Engine and alternator regulation
  • Command signalling

Today most systems contain microprocessors allowing to provide complex features such as:

  • True RMS output
  • Component monitoring
  • Configurable inputs
  • Engine ECU interface
  • Remote communication

Controllers range from basic start/stop operation to being able to monitor and control remotely. It is important that the application requirements and operating staff qualifications are considered in determining the correct system. If not this can lead to having an underutilised system or a system that cannot meet the requirements of the application.

Monitoring

Typical metering outputs are:

  • Hour run
  • Engine Speed
  • Engine oil pressure
  • Engine coolant temperature
  • Voltage
  • Current
  • Frequency
  • Battery Voltage
  • Battery Current
  • Power
  • Energy

Control Functions

Start – Enabled only when:

  • Engine is not running
  • Emergency stop button is not engaged
  • No alarms are present

 Auto Start

  • Engage starter for a time period (crank time)
  • Dis-engage starter upon engine start (crank disconnect)
  • Dis-engage starter after a time period of engine not starting
  • Repeat start attempt after a defined rest time and no of attempts (crank rest)
  • Upon engine no start signal fail to start (fail start)

Protection delay

Period between the generator reaching up to speed and voltage condition and protection circuits arming.  This is to allow oil pressure to build and voltage and frequency to stabilize.

  • Load breaker switching

Period between the generator reaching up to speed and voltage condition and closing the breaker to accept load. 

Status Indicators

Control systems provide the current mode of the generator. Common indicators that are useful for operators are:

  • Start Delay
  • Return delay
  • Cooling run
  • Fuel off
  • Fuel on
  • Cranking or Starting
  • Crank rest or Start rest
  • Failure to start
  • Crank disconnect
  • Running up to speed
  • Up to speed and volts
  • Running
  • On load
  • Breaker fail to close
  • Engine at rest 

Warning/Protection/Alarm

The control system monitors several conditions in order to limit generator faults and damage to connected load. Warning alarms are non-critical conditions however failure to address may lead to generator failure to start and or damage. Shutdown alarms are critical conditions that will trip the main circuit breaker and cut supply to the fuel solenoid.  Conditions may have an alarm and or warning parameter.

Typical conditions monitored are:

  • Low engine oil pressure
  • High coolant temp (Warning, Alarm)
  • High oil temp
  • High alternator winding temp (Warning, Alarm)
  • High alternator bearing temp (Warning, Alarm)
  • Low coolant level
  • Low fuel level (Warning)
  • Under Voltage
  • Over Voltage
  • Under speed
  • Over speed
  • Under frequency
  • Over frequency
  • Over Current
  • Earth Fault
  • Reverse power 

Telemetry

With the advancement of microprocessors incorporated in control systems it has become easier to communicate remotely.  SCADA suite software, web based and mobile apps are the various solutions used to link generator controller to remote device.

The use of remote communication provides numerous benefits:

  • Travel to site may be avoided
  • Sites can be left unmanned
  • System parameters can be monitored to develop trends and aid in early failure detection
  • All operation, maintenance and performance data can be stored on PC and retrieved when required.
  • Instant alerts
  • Fault finding

Conclusion

The diesel generator  controller is the brains of the generator. Consisting of electronics and microprocessor, it can provide various functions ranging from basic to complex. The application and end user requirements need to be assessed in order to select the appropriate controller.  Parameters, inputs and outputs can be configured to ensure customers specifications are met.  Highly sophisticated systems require specialist engineers to provide the desired outcome.

Sizing Diesel Generator

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Note: Diesel Generator applications can be quite complex, involving many different site specific elements to be considered.  This discussion presents a basic guide to sizing a diesel generator.  It assumes knowledge of power calculations.

Sizing diesel generator involves calculating the following load factors:

  1. The continuous load operating at any given time
  2. Derating factors
  3. Transient performance
  4. Non Linear loads

The size required will be determined by the maximum demand from one of the above load factors.

We will now expand on each one.

  1. TOTAL CONTINUOUS LOAD (run kVA)
  • Calculate the total kVA for all motors and machinery that will be operating at one time
  • Calculate the total kVA for all heating/air conditioning that will be operating at one time
  • Calculate the total kVA for all lighting that will be operating at one time

The run kVA (rkVA) equals the sum of the maximum possible load demand at one time.

 

  1. ALLOW FOR ANY DE-RATES

Common de-rate elements are:

  • Altitude
  • Ambient temperature
  • Dust
  • Humidity
  • Power Factor

Each diesel generator will have their own de-rate characteristics to each item, hence their technical data must be referred to. If more than one de-rate applies the multipliers become cumulative.

When de-rates apply the continuous rating of the generator is reduced by the multiplier.

 

  1. TRANSIENT LOAD PEFORMANCE

When a motor starts its initial kVA (start kVA) is larger than its run kVA. This occurs due to initial inrush current required to start the motor. The skVA can be several times the rkVA causing an initial larger load demand and resulting voltage dip at the terminals. In general, a voltage dip greater than 25% can cause the generator to stall/shutdown, hence becoming an important element to consider.  Generators have specific starting voltage characteristics that must be considered.

Various starting methods are used in applications to reduce the skVA. Below is a guide of starting method and skVA impact:

Starting Method skVA
Direct On Line 6 x rkVA
Star Delta 3 x rkVA
Auto Transformer 4 x rkVA
Soft Starter 2 x rkVA

 

For equipment used above each will have their own performance characteristics and will provide their starting impact. Technical data can be obtained from the manufacturers.

Motors that come on line at various stages, calculations are based on the highest possible demand scenario. When there is more than one motor starting simultaneously the total skVA is the sum of each of the motors skVA.

  1. NON LINEAR LOADS

Non-linear loads (NLL) are defined as electrical equipment that draw non sinusoidal current from the supply source (diesel generator) causing harmonics in the current waveform, which in turn causes distortion in the voltage waveform. Current distortion (ID) is a function of the number pulses. The degree of distortion can cause instability in the loads control system and in the generators excitation system.

Typical current distortion(ID) figures are:

Pulse ID%
12 14
6 30

 

Typical non-linear load applications and their acceptable levels of voltage distortion (VD) are:

NLL VD%
Soft Starters (SS) 25
Un Interruptible Power Supplies (UPS) 10
Variable Frequency Drives (VFD) 15

 

For a diesel generator to be able to limit the voltage distortion (VD) the alternator must have a sub-transient reactance (X”d) at a level (lower the better) that allows for this.

There are several guide/charts that take input of ID% and acceptable VD% and provide the X”d  that must not be exceeded.  A rough guide is below:

Pulse ID% VD(limit) % X”d  max pu
6 30 10 3.4
6 30 15 5.4
6 30 20 7
12 14 10 13.2
12 14 15 20

 

The diesel generator selected must have an alternator that operates within the X”d limit. For applications where the NLL is a high percentage of the total load an oversized alternator may be required.

DIESEL GENERATOR SIZE

Once the maximum kVA for each of the load factors are determined the size of the diesel generator will be that of the highest demand and hence meeting demands of remaining load factors.

As it can be seen sizing  diesel generator can be quite complex. This guide is basic and was written to provide awareness to the considerations required when sizing a generator.  This discussion can be expanded into great detail, in which consideration may be required to provide the correct sizing of the diesel generator.

Selecting a Diesel Generator

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A diesel generator is an electro mechanical machine that produces electricity. It comprises of a diesel engine coupled to an alternator. Its primary purpose is to provide power when the electrical grid is not available due to several factors (remote areas, grid stability, fault). Developing nations have grids that are not stable and as a result have a large requirement of diesel generators. Developed nations grid supply is generally stable and hence the applications are confined to areas such as critical power (hospitals, data centers, airports), commercial basis where loss of power will incur significant costs and remote areas (mines, communities) where grid supply is non-existent. Other applications include military, marine and telecom.

The following factors need to be considered when selecting a diesel generator:

• Application
• Prime or Standby Power
• Operating Conditions
• Commercial impact

The ideal diesel generator would have the following features:

• Reliability
• High performance
• Support
• Efficiency
• Durability
• Competitive pricing
• User friendly
• Functionality

Today there is a large range of generator manufacturers in the market place producing high quality product to simpler low end product. For the purpose of this discussion we will define the range by the following categories:

Tier 1 – high spec custom build generators for severe environments and for prime power applications. Eg. mine site no grid, dust, high temperatures, continuous operation where loss of power will cause large financial costs.
Tier 2 – quality mass produced generators that can be used for prime power applications and critical standby. Eg. emergency power for high traffic airport.
Tier 3 – low end mass produced product for minimal operation in standby application. Price sensitive. Eg. Standby power to a local restaurant that operates under a reliable grid.

Tier 1 generators are manufactured by companies who are based locally. They will either modify tier 2 product or custom build a generator. Components used will be specific to the application reliable and have high levels of support. The end product will be able to meet the demands of the application. Pricing can be up to 5 times tier 2 product however provides the best return on investment.

Tier 2 generators are manufactured by large engine companies based in the US and companies operating in large markets such as China and Europe. The large engine companies are conglomerates with several divisions. Their generators come under their power generation division. Due to their structure they are limited on their ability to produce tier 1 product. Typical constraints are:

• Company manufactured components must be used
• Approved suppliers only
• Mass produced
• Design variations limited

Large manufacturers based in Europe and China are not tied to any particular brand of component given them scope to assemble diesel generators that meet the requirements of the majority of the market. Due to their large plants they have scalability and can acquire componentry at competitive pricing. Labor costs are lower than that of developed nations. The limitations of their resources restricts them from producing tier 1 product. Any variation from their typical designs increases faults being reported. Their pricing is generally 30% lower than US engine conglomerates.

Tier 3 generators are produced by companies based in China. They use replicated components of well-known brands. They are not build to any particular standard. Reliability is low and support out of China is non-existent. Procurers can be attracted by the low pricing these generators are on offer for. Pricing in general can be up to 40% lower than top tier 2 counterparts.

In conclusion when selecting a diesel generator it is important to understand the application and environment that it will be operating in. This will provide guidance on which manufactures should be consulted providing the best return on investment.