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Glossary of Generators – Part One


Glossary of Generators – Part Two


First: Reasons for having onsite generators
Second: Applicable performance standards for generator sets
Third: Selection Factors Used For Generators Sizing Calculations
 Generator Power Ratings
 Application type


Third:
Selection Factors Used For Generators Sizing Calculations
3 Location
Considerations,
4
Fuel Selection Considerations,
5
Site Considerations,


Third:
Selection Factors Used For Generators Sizing Calculations
6
Environmental Considerations,
7
System Voltage and Phase,


Today, we will continue explaining other Selection factors used for Generators Sizing Calculations.
Third: Selection Factors Used For Generators
Sizing Calculations

Here
we will describe preliminary factors for selecting a generator for certain
project, which will be as follows:
Generator Power Ratings,
Application type,
Location Considerations,
Fuel Selection Considerations,
Site Considerations,
Environmental Considerations,
System Voltage and Phase,
Acceptable percent of voltage & frequency dip,
Acceptable duration of the voltage & frequency dip,
Percent and type of loads to be connected,
Load step sequencing,
Future needs.

8 Acceptable Percent Of
Voltage & Frequency Dip
9 Acceptable Duration Of The
Voltage & Frequency Dip

Introduction
Dips
are usually happened when there is a transient condition applied to a
generator.
Transient
Response definition
Whenever a load is applied to or removed
from a generator set, the engine speed rpm, voltage and frequency are
temporarily changed from its steadystate condition. This temporary change is
called transient response.
Figure.1:
Transient Response
So, the two cases cause transient
response are:
Case#1: Applying a Load
When a significant load is applied, the
engine speed temporarily reduces (generally referred to as frequency or
voltage dip) and then returns to its steady state condition. See Figure.1
The degree of this dip depends on the
following:
The amount of active power (kW) and reactive power (kVAR) changes based upon the voltage regulator settings,
The total capacity and dynamic characteristics of the generator set
The electrical inertia of the other loads in the system.
Case#2: Removing a Load
On removal of load, the engine speed
increases momentarily (generally referred to as overshoot), then returns to
its steadystate condition. See Figure.1

Recovery time definition:
The time required for the generator set
to return to its normal steadystate speed is called recovery time. See Figure.1.

Voltage
Dip
Voltage dip is the dip in voltage that results when a load is added,
occurring before the regulator can correct it, or resulting from the functioning
of the voltage regulator to unload an overloaded enginegenerator.
We must differentiate between the
following two expressions: see Figure.2
Figure.2:
Instantaneous & Sustained voltage dip
1 Maximum instantaneous voltage dip:
2 Sustained voltage dip:
The sustained voltage value is determined by applying stepped load increases until the alternator does not recover to 90% of sustained voltage. This is called by NEMA MG1 part 32, which states: “The generator set shall be capable of recovering to a minimum of 90% of rated no load voltage following the application of the specified kVA load at near zero power factor.”
Notes:
The amount of voltage dip is independent of the level of load already carried by the generator, particularly where that load is of a mixed nature (i.e. it consists of heating, lighting, and general power).

The
Maximum Allowable Voltage & Frequency Dip
Notes:

Typical Voltage Dip Limitations According
To The Facility Type
Typical voltage dip limitations are found
in Table1 for
various facilities.
Typical Voltage Dip
Limitations

Facility

Application

Permissible Voltage dip

Hospital,
hotel, motel,
Apartments,
libraries, schools, and stores.

Lighting
load,
large
Power
load,
large
Flickering
highly
Objectionable.

2%
Infrequent

Movie
Theaters (sound tone requires constant frequency. Neon flashers erratic)

Lighting
load, large Flickering objectionable.

3%
Infrequent

Bars
and resorts.

Power
load, large Some flicker acceptable

5%
 10% Infrequent

Shops,
factories, mills, laundries.

Power
load, large Some flicker acceptable

3%
 5% Frequent

Mines,
oil field, quarries, asphalt, plants.

Power
load, large Flicker acceptable.

25%
 30% Frequent

Greater voltage fluctuations permitted with emergency
power systems.

Table1: Typical
Voltage Dip Limitations

Transient Response Standards  ISO 8528
Four performance classes are designated
in ISO 852817 to describe a generator in terms of voltage and frequency. Table2
below lists the performance class and their criteria and application
examples.
Note:
Generator’s Performance Class

Performance
Class Criteria

Application
Examples

G1

Required
for applications where the connected loads are such that only basic parameters
of voltage & frequency need to be specified.

General
purpose
applications
Lighting & electrical loads

G2

Required
for applications where the demand on voltage is very much the same as for
the commercial power system.
When
load changes, temporary deviations in voltage and frequency are acceptable.

Lighting
systems, pumps, fans and hoists

G3

Required
for applications where the connected equipment may make severe demands on
voltage and frequency and waveforms.

Telecommunications
equipment

G4

Required
for applications where the demands on voltage, frequency, and waveform are
extremely severe.

Dataprocessing
&
Computer
equipment

Table2: Generator’s Performance Class
Table3
shows the acceptance (dip)
and rejection (overshoot) parameters identified by ISO 85285. Class G4 is
reserved for limits that are unique and must be agreed upon by the
manufacturer and customer. ISO 85285 also sets limits on recovery times for each
class and identifies how recovery time is measured.

Class
G1

Class
G2

Class
G3

Class
G4

Frequency %
Acceptance

–15

–10

–7

AMC

Frequency %
Rejection

18

12

10

AMC

Voltage %
Acceptance

–25

–20

–15

AMC

Voltage %
Rejection

35

25

20

AMC

Time
Seconds

5

5

3

AMC

Table3:
ISO
85285 acceptance (dip) and rejection (overshoot) Limits
Note:
AMC:
Agreed
between Manufacturer and Customer.

Voltage Regulators
The voltage regulator is a key component
in determining the amount of voltage/frequency dips and recovery time.
There are several different types of
regulators:
1 Constant Voltage Regulator:
It attempts to maintain rated voltage as
the load is applied. Since the generator is maintaining rated voltage, it is
maintaining applied load (ekW).
The relationship between ekW and bkW is:
ekW = pf x kVA
bkW
= Speed x Torque/lambda
bkW = ekW/ eff + Fan Demand
where:
kVA =
kVA output of generator
pf =
power factor of connected load
ekW =
electrical power (electrical kW)
bkW =
engine power (brake kW)
eff =
generator efficiency
Therefore, when a constant voltage regulator is used, it imposes increasing torque on the engine during frequency dips. Since most generator engines are not designed for increasing torque, significant amounts of frequency dip can occur. Engine speed rpm decreases as any load is imposed on a limited bus (generator). This causes frequency/voltage to dip accordingly.
2 The “Volts per Hertz” (1:1 Volts/Hz)
regulator
If the speed dips 15%, the Volts/Hz regulator will cause the voltage to dip by 15%. This will reduce current flow into the load by 15%. The kW absorbed by the load is then 0.85 (85% of volts) multiplied by 0.85 (85% of current), or 72.25% of rated power. The engine only has to produce 72.25% of rated hp. mathematically, the formula for this concept is:
0.85
volts x 0.85 current = 0.7225 power = 0.85 speed x 0.85 torque
3 The 2 Volts/Hz regulator
A reduced motor starter may not be required with the 2 Volts/Hz regulator when starting large motors depending on system design and applied load steps. However, the entire connected system would have reduced voltage, not just the motor.
4 Digital Voltage Regulator

Generator
Voltage Dip Calculation
We have
two methods to calculate/determine the Generator
Voltage Dip as follows:
By using equations,
By using generator manufacturers’ performance data.
1 By using equations
The magnitude of the voltage dip at a generator's terminals, following load switching, is a direct function of the subtransient and transient reactances of the machine. It can be calculated from the following equation:
Notes:
The equation above can be used to show that for a given machine rating (same kVA, same voltage, same frequency), the lower the X'd, the lower the amount of voltage dip for an applied load. This is based on physics and holds true for all generator set manufacturers.

2 By using generator manufacturers’ performance data
The performance curves are for a specific frame size. The terms 'minimum' and 'maximum' voltage refer to the bottom and top ends of the standard range of voltages available from the particular winding applied to the machine. Typically, these may be 346 volts and 480 volts, respectively. The 'maximum rated kVA of the frame' is its standard (industrial) continuous maximum rating.
Fig3: Voltage dips related to impact load at low, lagging power
factors

In the next article, we will
continue explaining other Selection factors used for Generators Sizing Calculations.
So, please keep following.
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