HYBRID LOAD-ADAPTIVE VARIABLE-SPEED GENERATING SET:
NEW SYSTEM TOPOLOGY AND CONTROL STRATEGY
W. Koczara *, L. Grzesiak**, Warsaw University of Technology, Poland
00-661 Warszawa
Tel +48 22 6294991, Fax. +48 22 6256633
*Visiting Professor to the University of Pretoria, South Africa
Tel +27 12 420 3190 Fax. +27 12 362 5000
*E- mail: koczara@isep.pw.edu.pl **E-mail: grzesiak@isep.pw.edu.pl
M. da Ponte, Volt-Ampere, Pretoria, South Africa, P.O. Box 4245, Pretoria 0001
Fax +27 12 324 4203, Tel. +27 12 328 6551
E-mail: vanetcom@cis.co.za
ABSTRACT
The paper notes the changing circumstances of the electricity industry, in which generators will
have an increasing role to play at consumer level, for base load, peak shaving and UPS purposes.
The characteristics of various constant and variable-speed engine-driven generating systems are
reviewed. A novel, load-adaptive generation technology with automatically adjusted speed –
namely the Hygen system – is introduced, a working model (already in service) described, and
some test results provided.
The novel application of a controllable converter to the alternator output and the unique topology
and control strategy of a power electronic converter system have made possible the successful
integration of engine, alternator, energy storage devices and microcomputer control to produce
an intelligent energy delivery system with manifold variations in application, and power ratings
into the megawatt range.
The greatest benefits of the system are that it delivers reliable, no-break, high quality power at
constant voltage and constant frequency, suitable for all loads including modern electronic
appliances and other sensitive equipment; and that it maintains its performance and high quality
of supply under even the most abusive load conditions with which conventional generators and
UPS units or combinations thereof cannot cope without sustaining damage or compromising
their performance or output quality. Moreover, it does so with considerably greater fuel
economy than constant speed gensets; alternatively it can be optimised for a number of other
priorities e.g. maximum power.
INTRODUCTION
It was James Watt’s steam engine that made possible the large scale commercial generation and
distribution of electricity, and in turn enabled the quantum leaps we have seen in electronic
engineering, for example in the fields of computing and communications. Sadly, however,
electricity generation and distribution is still by and large based on the technology and
philosophy of a century ago. Though some refinements have been made in materials, design
and manufacturing processes, so much capital and technological investment has been made in
centralised generation feeding large grids, that even the most adventurous lateral thinking stood
little chance of finding alternatives to this tried and trusted system, now at the end of its design
life.
As we tread the threshold of the new Millennium, it is the “offspring” of electricity – namely
electronics and power electronics – that have come to the aid of the ageing parent industry by
making possible the economic generation of electricity on smaller (and even small!) scale. In
brief, these new technologies are enabling fresh approaches to power generation theory and
practice that were inconceivable only a few years ago, in respect of centralised power stations
feeding national and even international grids, and also in respect of distributed and island power
generation. The purpose of this paper is to look at developments in this essential industry, and
introduce an attractive new technology – namely Hygen, the novel hybrid load-adaptive
adjustable-speed system with programmable engine torque control.
A NEW SCENARIO - DISTRIBUTED AND ISLAND GENERATION
In effect, the move towards deregulating the electricity utilities is stimulating distributed
generation, which will crystallise in much smaller local power stations – on and off-grid - and a
proliferation of distributed generating units at consumer level [1]. A new electricity industry will
emerge [2] in which small power plants (up to several Megawatts) comprising new technology
reciprocating and/or turbine engines driving alternators at variable and/or high speed [2], [3]
[4], [5] and [6], will feature predominantly. Waste heat exploitation is also developing rapidly
and will drive overall efficiencies above the eighties.
The fledgling fuel cell technologies [7] and [8] are attractive alternatives which show great
potential, but they have yet to progress up the path of experimental development to maturity. A
technical barrier which poses a major obstacle in the application of fuel cell technology is the
limitation in output power ramp, e.g., 10% of rated power per minute [7]. The varying nature of
load profiles imposes load changes which almost without exception require a complementary
system such as high capacity energy storage or other means of supplementing the fuel cell
system in a load levelling function.
THE UNFORTUNATE LEGACY OF GENERATOR SET SIZING
Engine-driven generating plants are also sensitive to varying load power demand and the nature
of the load current, which affect the allowable voltage variation – both in the steady state and
under transient conditions. It is common practice to oversize generator sets to compensate for
voltage-distorting non-linear loads and cope with large step loads as well as intermittent
overloads and impact loads such as those caused by the starting of induction motors. Naturally,
in larger generating plants where the rated power of the induction motor is relatively small in
comparison with the rated power of the generating plant, this problem is insignificant.
Conventional gensets are heavily oversized in relation to prevalent load conditions.
The established practice of sizing generator capacity for worst case load scenarios according to
volt ampere power (kVA) and a power factor of 0.8, has resulted in generator sets operating with
light loads for long periods. Installations in which generator sets operate at an average load of
20% to 30% of installed capacity are common in smaller power systems, e.g. up to 300kVA. For
light loads, the specific fuel consumption is very poor, in that incomplete combustion does not
utilise all the fuel. Worse still - the unburnt fuel dilutes the oil in the cylinders and thereby
causes excessive wear in the cylinder walls, cylinder glazing and carbon build-up. These harmful
and destructive conditions result in severe deterioration in engine performance and inflict
premature engine failure – typically at 20% of design life. To avert these harmful effects,
manufacturers insist that engines operate with loads above 50%.
In larger installations, the common problem of under-loading engines is overcome by employing
costly multiple generator sets running synchronously and controlled automatically so as to
operate with acceptable capacity factors at all times. Although the partial backup thus provided
is an important benefit, the complexity of such a system is obvious and loading is guaranteed
only up to 50% of capacity.
For higher capacity factors, some form of Demand Side
Management is required, which in effect, only manages the problem and does not solve it.
In small plants – particularly installations comprised of a single genset - dummy loads are
recommended, which adversely affect the fuel economy and drive operating costs through the
ceiling.
Generator set/UPS combinations to provide extended autonomy are increasing rapidly in number
and in size (up to 1 MVA). UPS systems are themselves often voltage-polluting non-linear loads
– there is the rare exception – and therefore exacerbate the problem of oversizing the generator
set with a multiplier effect. According to some application practices, the generator set is sized to
be two to three times the non-linear load power. If the generator set is sized according to volt
ampere power using the industry standard power factor of 0.8 (common practice), a 50kVA UPS
installation will require a 100kVA/80kW generator set. The typical active load power (kW) for a
50kVA UPS system is of the order of 35kW at full load. If for example the average load is 40%
of installed capacity, it will result in an average load of 14kW. This corresponds to 17,5% of the
active power (kW) rating of the machine. All other implications aside, this solution is without
question bad engineering practice and will culminate in disastrous results.
A better approach is to size the engine according to the active load power demand (kW) and the
generator according to the oversizing practice, namely, two to three times the volt ampere power
(kVA). Although this is a more sound engineering practice, such a design is still not optimal as
it addresses the symptoms rather than the cause. This approach requires specially designed
alternators or UPS systems with power factor correction, which draw sinusoidal current from the
supply.
Providing extended no-break power with a genset and UPS system combination – without
taking special measures - compounds the problem of oversizing and under-loading.
HYGEN IN BRIEF
The novel Hygen* system overcomes the abovementioned shortcomings effectively and paves
the way for a new approach to an old problem.
Hygen is an engine and environment friendly, load-adaptive adjustable-speed electricity
generating and Uninterruptible Power Supply (UPS) system with energy storage - in a hybrid
configuration.
Using systems engineering principles, it integrates engine, alternator, energy storage devices,
power electronic converters and microcomputer control to produce a fully integrated intelligent
energy delivery system with manifold and almost limitless variations in applications – base load,
UPS and peak shaving. It delivers reliable, no-break, high quality power at constant voltage
and constant frequency suitable for all loads including modern electronic appliances and other
sensitive equipment. It maintains its performance and high quality of supply even under the
most adverse load conditions with which conventional generators and Uninterruptible Power
Supply (UPS) units – or combinations thereof – cannot cope without sustaining damage or
compromise in performance or output quality. Unquestionably, it does so far more
economically than conventional gensets which operate at constant speed – with up to 70% fuel
saving and 500% longer engine life in some applications.
The Hygen system presented in this paper (designed for low power application) comprises an aircooled permanent magnet alternator (specially designed for rectifier loads) mechanically
integrated into a modified diesel engine (the outer rotor of the alternator replacing the engine
flywheel), a bank of lead acid batteries connected in series with boost charger and individual
equalisation chargers, a multiple power electronic converter system and a microprocessor control
system with interactive operator interface. A highly compact unit was achieved by “optimal”
integration of the modules. The system is equipped with comprehensive protection in the form
of surge protection, fuses and circuit breakers for all modules. The protection devices include
signal contacts which provide the microprocessors with information on their status or condition –
crucial to the self-diagnostic function of the system. Its operator interface displays information
on all the monitored and measured parameters of the system and empowers the operator to
control the machine and access its status, condition and performance information through a user
friendly and interactive panel. Facilities for remote monitoring and control by PC include a
RS232 serial port for direct connection to PC or via modem and a coupler for linking up with the
machine by means of fibre optic cable. Windows compatible software provides virtual
instrumentation and control features which can be easily customised. Routine checks, startup
tests and maintenance support are possible anywhere by hooking up to a notebook PC and a
telephone line, and clicking the mouse, even when in flight between continents. It is packaged in
a super-silent canopy with a 60 litre onboard fuel tank giving up to four days’ operation
depending on the load profile, and incorporates all necessary measures for compliance with
------------------------------------------------------------------------*
The Hygen system forms the subject matter of South African patent No. 97/11503 and
international patent application No. PCT/EP 97/07273 designating inter alia Europe, USA,
Australia, Canada, Japan and China.
stringent EMI standards. Details of the Hygen system** are given in Table 1, and a visual
presentation in the adjoining photograph.
TOPOLOGY OF THE HYGEN SYSTEM
The production of high quality output power with constant voltage and constant frequency, using
a variable-speed electricity generating unit, is made possible by the application of new
technology power electronic AC/DC/AC converters and micro-computer control [5]. A block
diagram of the load-adaptive adjustable-speed system is shown in Figure 1. The speed of the
engine- generator unit IEG, comprising an engine ENG and a permanent magnet AC generator
PMG, is automatically adjusted according to the active load power demand and thus generates a
three phase output voltage and frequency which is proportional to speed. The voltage and
variable frequency (three phase) output of the generator PMG is fed to an active boost rectifier
ABR wherein it is rectified by rectifier R to form the primary DC voltage, DC link1. The voltage
of DC link1 is boosted by booster B to provide a regulated secondary DC voltage, DC link2,
referred to as the DC Bus. An energy storage system ESS is connected to the DC Bus through a
bi-directional converter BDC to form the power conditioning energy storage PCES module. The
DC Bus supplies a DC/AC inverter IF which in conjunction with an output filter produces a
sinusoidal, constant voltage and constant frequency output. The protection module P consists of
surge suppression devices and circuit breakers. The load LD represents typical real-world loads
which produce diverse load profiles with dramatic variations in load power demand.
A multiple micro-processor control system MMC communicates with all the modules via a
number of sensors, controls the operation of the modules individually and manages their
interaction at system level to produce a high quality electrical output from an “optimised” energy
efficient fuel input. The control system incorporates an operator-machine interface which
displays information on all the mechanical, electrical and electronic parameters of the system
including the status of the protection modules. In addition, it provides a RS232 serial port and a
fibre optic coupler for a remote control and monitoring computer RCM.
POWER AND ENERGY FLOW MANAGEMENT
The Hygen system presented herein uses a diesel engine-generator unit as primary energy source
and a lead-acid battery bank as secondary energy source. Figure 2 depicts the power flow
between the engine-generator unit, the energy storage battery and the load. There are four basic
states of operation namely, S1, S2, S3 and S4, as illustrated in figures 2a, 2b, 2c and 2d
respectively.
ƒ
In figure 2a (S1) power (Pe) from the engine-alternator and active boost rectifier ABR, flows
directly to the load Pl (Pe =Pl).
------------------------------------------------------------------**
The Hygen system was designed, developed and hand manufactured by the Volt Ampere
Group of Pretoria, South Africa in co-operation with the Warsaw University of
Technology, Poland and the University of Pretoria, South Africa.
ƒ
In figure 2b (S2) power flows from the battery Pb to the load Pl (Pb = Pl) when the battery is
charged and engine operation is uneconomical or undesirable. In this operating mode, the
Hygen system is similar to the well known and widely used UPS systems. The engine is in
standby mode and automatically cuts in when the battery is discharged to a pre-determined
level.
ƒ In Figure 2c (S3), the power flows from the engine Pe to the load Pl and the battery Pb
(Pe = Pb + Pl). The engine supplies power to the load and replenishes the battery energy.
ƒ In figure 2d power flows in parallel, from the engine Pe and from the battery Pb to the load Pl
(Pl = Pe + Pb). The battery supports the engine and augments the output power to produce
high peak power for relatively short periods. The system peak power capability is thus the
sum of maximum engine power and maximum battery power.
The battery can be sized to meet the load power demand during periods when engine operation is
undesirable or uneconomical, and to support the engine and augment the output power to meet
peak load power demands of short duration. Therefore, in applications where high peak load
power demands are of relatively short duration, the engine-generator unit does not need to be
sized according to the peak load power demand. A smaller and cheaper engine which is more
economical to run will suffice, as it is supported by the battery. This contrasts with the need to
over-size a conventional generator set to cope with peak loads.
This intermittent high peak load capability also enables the system to maintain its high
quality output voltage in transient conditions e.g. switching high step loads or impact loads.
At the lower end, when the load power demand is too small, under-loading is averted by utilising
some of the engine power to charge the battery as depicted in Figure 2c. When the battery is
fully charged and if the light-load power demand persists, the engine is automatically stopped
and the battery supplies power to the load as depicted in Figure 2b. Small plants are particularly
exposed to under-loading while in larger units it is not so common. There is thus no need for
fuel sapping dummy loads to prevent under-loading, which makes the Hygen system a very
economical one to operate in applications with extended low power demands e.g. low power
demand at night.
STRATEGY OF CONTROL
The AC generator PMG output which varies in voltage and frequency as a function of speed is
converted to DC by rectifier R, forming the primary DC link, DC link1. The voltage of DC link1
is boosted by the booster B in the active boost rectifier ABR to form the secondary DC link, DC
link2, referred to as the DC Bus. The active boost rectifier ABR is a variable ratio converter
which boosts the primary DC link voltage and regulates it according to a predetermined voltage
reference (the first DC Bus threshold). A current control loop regulates the current according to
a predetermined current reference signal which is representative of the engine torque. This
current reference signal may be altered and shaped in a signal conditioner so that it represents
any desired engine torque/speed characteristic. Figure 3 illustrates the engine continuous
torque/speed characteristic Tr and the desired torque/speed characteristic Th which will result in
the lowest fuel consumption for load-adaptive adjustable-speed operation between a constant
minimum speed operating point (ωmin) and a constant maximum speed operating point (ωmax).
The specific fuel consumption contours, F1 to F5 are also shown. F1 represents the region of
lowest fuel consumption, known as the “sweet spot”. The curve Tp represents the part-load
torque/speed characteristic and resembles the engine’s rated torque curve Tr. Engine designers
are compelled to compromise the engine’s fuel efficiency at rated torque Tr1 to accommodate a
reasonable fuel efficiency at part-loads Tp e.g. Tp < load torque < Tr1.
When the engine is running at the minimum constant speed point (ωmin), and LD increases, the
current in the active boost rectifier ABR, and hence the alternator current, is allowed to increase.
The engine’s load torque is proportional to the alternator current and therefore increases
accordingly. When the engine’s load torque reaches point P1 on the desired torque curve Th, the
current control loop in the active boost rectifier ABR regulates the current according to the
reference signal which represents the desired engine load torque characteristics Th for the most
economical operation.
When the load LD exceeds the output power of the engine at a given moment – the power is the
product of its speed and torque – the regulated current in the active boost rectifier ABR limits the
alternator output. Consequently, the alternator, and hence the engine cannot meet the load
demand. The DC Bus is therefore “starved” of power and the voltage decreases as the DC Bus
capacitors deliver power to the load.
The voltage control loop detects the lower voltage and activates the engine speed governor so
that the engine speed increases. The alternator voltage, which is proportional to the speed,
increases accordingly. Therefore the alternator output power and the engine’s output power
increase accordingly. When the voltage is restored to the predetermined DC Bus voltage, the
first threshold, the engine speed stabilises at the higher speed and hence higher power level,
thereby restoring the balance of power between the engine-alternator output and load. The
alternator current, and hence the engine load torque, will follow the desired curve Th as the speed
increases with increased load power demand. When the load power demand corresponds to the
torque at maximum speed P2, the engine runs at constant speed. Beyond this load, the alternator
current will be permitted to increase up to its maximum rated current, which corresponds to the
maximum continuous engine torque and hence maximum power at the maximum speed, point P3
on the rated torque curve Tr
Conversely, when the load power decreases to P2, the power balances will be disturbed and the
voltage in the DC Bus will rise above the reference threshold. The voltage control loop will
activate the engine speed governor to reduce the speed until the voltage is restored to the
reference threshold. The engine speed will again stabilise at the lower operating point. When
the load torque decreases to P1, the engine will run again at minimum constant speed and the
current will decrease as the load decreases.
The torque curve Ti represents the maximum intermittent torque of the engine. The difference in
torque between the Ti and Th represents the excess torque Ta which enables engine acceleration
under load. Note that at minimum speed the desired regulated torque curve Th is made to deviate
from the most economical curve so that acceleration torque Ta is available at all times.
Whilst in this example the engine torque versus speed characteristic is programmed for fuel
efficiency, it should be noted that the system can be programmed to optimise other
performance parameters e.g. maximum load.
Referring back to Figure 1, the power conditioning and energy storage system PCES supports the
DC Bus when the power flow from the generator is insufficient to meet the power demand of
load LD. In this instance, the DC Bus voltage falls below its first threshold. Energy from the
battery is pumped to the DC Bus, thereby regulating the DC Bus voltage at a second threshold,
marginally lower than the first threshold, that will meet the minimum requirements of the
DC/AC inverter. The energy delivered to the load from the battery is replenished when the
power balance is restored and there is excess engine capacity.
Figure 4 shows a simplified diagram of the relationship between the total output power Pl when
the generator power Pe is supported by the battery power Pb, and the speed. Note that the battery
charging power Pb1 is considerably lower than the battery output power Pb2 in order to extend
battery life. (Pb1<< Pb2). Constant speed operating points are indicated by ω min for low speed
and ω max for high speed operation respectively. The generator power is represented by the curve
Pe (Pe = ωT). The total available power Pl therefore also follows a similar curve Pe and is
augmented by the battery power Pb (Pl = Pe + Pb, with Pb being constant as depicted).
The inverter is controlled by an additional control loop and high quality power is produced at the
inverter and output filer IF, even under the most adverse load conditions.
A REVIEW OF SOME ALTERNATIVE GENERATING SYSTEMS
Constant speed generators?
Constant speed electricity generators need no introduction. Suffice it to say that it is the most
widely used system in applications ranging from watts to the largest of power stations.
Substantial development efforts costing millions of dollars, over many years, have been put into
improving the speed governors of engines so that the speed deviation from synchronous speed is
minimal e.g. 1 –2 % speed regulation. In Figure 5, the torque versus constant speed is depicted
by the vertical line Tc1. The engine load torque is directly related to the load; the engine speed
governor merely responds to load charges by increasing or decreasing the fuel rate in an attempt
to maintain the speed constant. The engine torque follows the load torque directly up or down
the constant speed line as depicted by the vertical torque line Tc1 for a constant speed engine.
More often than not, the rated torque at P4 does not correspond to the engine’s best point of
operation, the “sweet spot”. But assuming that the rated torque does correspond to the engine’s
“sweet spot” and that an imaginary engine map with fuel consumption contours is appropriately
placed on the graph, with P4 in the “sweet spot”, it will be evident that economical operation at
constant speed is restricted to a very small region in the map. Furthermore, in most engine maps,
the fuel consumption contours do not cross the torque line Tc1 for torque values below, say, 50%
of rated torque. Engine manufacturers explain that the engine should in any event not operate in
this region for long periods.
An equivalent Hygen system will use a much smaller engine with a lower rated torque to
produce the same power (Pe = ωT) corresponding to the equal powers at P4 and P3 for constant
and variable speed respectively. It is evident in Figure 3 that with the Hygen technology, the
engine torque is regulated so that the most economical operation is maintained over a wide range
of speed and hence power. Furthermore, operating in the under-loaded region is averted because
the part of the torque range which falls outside the fuel consumption contours of the engine map
is very small - and load torque can be sufficiently increased by charging the battery.
Variable speed generators?
Variable speed generators/alternators have been in use for decades. In aircraft, variable-speed
constant frequency generators have been used for many years. However, in these and most other
applications, e.g. wind generators, variable speed is a given condition and power
converters/conditioners merely convert the variable voltage and variable frequency to a constant
voltage and constant frequency. Wind generators with ratings into the Megawatt power range
are operating on wind farms and delivering electric power to the grid through double conversion
power electronic converters.
Manually adjusted speed generators?
A spin-off of the matured variable-speed generator technology is the “dial-a-speed” generator set
with manually adjusted speed, at the lower end of the power range. Figure 6 shows a block
diagram of the system in which an engine is equipped with a mechanism for manual speed
adjustment and drives a permanent magnet alternator. The speed is adjusted to operate at a
constant speed in accordance with the required maximum load. The torque characteristic of this
system is depicted by the constant speed, vertical line Tc2 in Figure 5. The principle of operation
is similar to that of constant speed operation depicted by the line Tc1. However, when the speed
is adjusted, the line Tc2 will move laterally depending on the adjusted speed and the rated torque
(and hence rated power) will correspond to P5 which follows the torque curve Tv. (Between
points P6 and P7, it is significantly lower than the engine’s rated torque Tr, so that unpredictable
load fluctuations are accommodated.) Its output power (P = ωT), voltage and frequency vary
with speed. The alternator output is rectified to form a DC Bus. A PWM inverter converts the
variable DC voltage to a specified constant AC voltage (e.g.110V) and constant frequency
(e.g.60Hz). The inverter can tolerate limited variations in voltage through its PWM operation as
long as the DC Bus voltage is higher than the required output voltage. However, restrictions in
voltage variation limit the speed range, and applications using this system are thus restricted to
small power installations e.g. mobile homes, boats or yachts, where there is good control over
the load and the operator is able to apply strict Demand Side Management. The step loads and
the speed range are also limited so as to avoid nuisance tripping of the protection devices and
prevent engines from stalling. Whilst this is an improvement on constant speed operation,
practical application of the system is restricted. Naturally, by adjusting the engine speed, and
hence its output power, the engine can be made to operate at a power level which marginally
exceeds the required maximum load. The benefits of this system are thus slightly lower fuel
consumption, lower noise levels and constant voltage/frequency, even when the speed is
manually adjusted (within limits) to a lower level during light load periods such as at night.
Under-loading also is thus mitigated and sometimes averted.
Load-following variable-speed generators?
An extension of the “dial-a-speed” system is the load-following variable-speed generator with
additional engine speed governor, a means of sensing the load, and a control system which
responds to load changes by effecting an appropriate change in engine speed. The topology of
this system is similar to that shown in Figure 6. There may or may not be an energy storage
system to support the engine/alternator and augment the output power. The torque versus speed
characteristic of this system is depicted by the curve Tv in Figure 5. Note the smaller speed
range and - again - the lower torque between minimum and maximum speed operating points P6
and P7!
The means of sensing the load may take the form of measuring the output current/power or the
voltage in the DC Bus, which falls when the load increases and rises when the load decreases. In
one of the control schemes, a voltage sensor senses the DC Bus voltage fall/rise and the control
system increases/decreases the engine speed, and hence the alternator and DC Bus Voltage,
respectively. In the load torque ranges below P6, and between P7 and P3, the system operates
similarly to the constant speed generators. However, for speed adjustment, the torque versus
speed characteristic between points P6 and P7 depends on the generator design, the DC Bus
characteristics and the control scheme. (The system is designed for a particular engine/alternator
match, or operating point or specific operating condition). As the load increases/decreases, the
speed is increased/decreased in order to maintain the DC Bus voltage within the limits required
by the inverter. This is a paradox to some extent because on the one hand the DC Bus voltage
must not exceed the limits of the inverter, whilst on the other hand the voltage will vary in
relation to speed (and load current, depending on the voltage versus load characteristic of the
alternator). However, without going into too much detail, these problems can be overcome by
imposing some restrictions on the system e.g. limiting the speed range, limiting the step load
capability and generally oversizing the system to allow for derating according to site conditions
e.g. ambient temperature and altitude above sea level, and thereby achieving some measure of
autonomy. This is clearly shown in Figure 5 by the torque versus speed curve Tv.
If the exercise of placing the imaginary engine map with fuel consumption contours is repeated,
it will be evident that this system, although an improvement on constant-speed operation, does
not operate in the most economical region.
The load-following variable-speed systems are limited in application to small sizes due to their
inherent design and to problems caused by real-world loads and load profiles. They require
special measures such as:
ƒ
ƒ
ƒ
Oversizing to cope with large step loads or, alternatively, imposing restrictions on the
nature and size of loads which can be switched on at any given time (Demand Side
Management).
Oversizing to guarantee performance under varying site conditions e.g. ambient temperature
and altitude above sea level.
Limitation of the operating speed range to avoid large variations in permanent magnet
alternator voltage, and to guarantee engine acceleration when responding to unpredictable
step loads.
Oversizing and a narrow speed range thus negate the benefits of variable-speed operation.
COMPARATIVE FUEL CONSUMPTION FOR VARIOUS LOADS
In the Hygen system, the unique innovation and major step forward in adjustable-speed
generators is the application of a controllable boost rectifier which enables precise regulation of
the engine load torque according to a programmable characteristic. In this example, the engine
operation follows the “optimum” operating region in which the lowest specific fuel consumption
is achieved. See Figure 3 and 5. Referring to Figure 5, and if the “imaginary engine map” is
appropriately placed as previously described, Hygen’s superior performance and economic
operation are obvious. The gap is huge – a saving of up to 70% in fuel consumption in some
applications.
Figure 7 depicts the typical power versus speed curve and the corresponding fuel consumption
contours for a diesel engine. The fuel consumption for constant speed operation (4 pole and 2
pole), manually-adjustable speed operation, the load-following adjustable-speed operation and
Hygen are shown. The various lines relating to the power levels in the graph show the
corresponding fuel consumption for the four systems. Again, Hygen’s superior performance and
economic operation “stand out like an oasis in the desert”.
Fuel consumption for various loads are compared in the table below
Specific fuel consumption is designated by F1–F10 where:
F1 < F2 < F3 < F4 < F5 < F6 < F7 < F8 < F9 < F10
LOAD LEVEL
1-2-3-4-5
6-7-8-9
10-11-12-13
14-15-16
17-18-19
20-21
• (4)
(1)
(2)
(3)
(4)
NB:
HYGEN
F6 – F7
F1 – F2
F1
F1 – F2
F2 –F3
F5 – F6
F5 – F6
CS-4P
F8 – F9
F1 – F2
-(2)
-(2)
-(2)
-(2)
-(2)
VS
-(1)
F4 – F5
F3
F3 – F4
F6 – F7
F5 – F6
F5 – F6
MAS (3)
-(1)
F4 – F5
F3
F3 – F4
F6 – F7
F5 – F6
F5 – F6
CS-2P
-(1)
-(1)
F9 – F10
F8
F7 – F8
F6 – F7
-(2)
Operation not recommended (under-loading).
Operation not possible (over-loading). Clearly, at 4 pole speed a larger engine is
required!
In the manually-adjusted speed system, the speed can be adjusted between minimum and
maximum, according to the power versus speed characteristic of the load-following
variable-speed system. (This adjustment can be represented by moving the vertical line
in Figure 7 laterally.) The same limitations with reference to oversizing and speed apply.
At maximum load, Hygen’s fuel consumption is similar to the other systems. As the
part-load decreases, the benefits accruing from the Hygen increase dramatically.
With due consideration to real-world load profiles and their low load factors, it is clearly
evident from Figure 7 and the above table that Hygen’s performance in terms of fuel
consumption is superior by a wide margin. However, although the above refers to
optimisation for fuel efficiency, Hygen can, with programmable torque control,
optimise its operation to suit various criteria and applications.
HYGEN’S OUTPUT TESTS WITH REAL-WORLD LOADS
Although the voltage controller is programmed to limit the maximum switching frequency, the
quality of Hygen’s output voltage shown in Figure 8A is exceptionally high. Figure 8a shows
the voltage waveform and Figure 8b the harmonic spectrum with the highest harmonic at 0.32%
of the fundamental frequency. Because the quality of voltage is exceptionally high for all partload and rated load conditions, test results of only extreme load conditions are presented. Figure
9a shows the voltage waveform in a severe overload condition, i.e. 20 kW resistive load, which
represents an overload in excess of 100%. Even under this condition the highest harmonic in
Figure 9b is 1,4% of the fundamental frequency.
Figure 10a shows a most difficult and troublesome load for inverters, namely, a highly inductive
load, 12kVA with a very low power factor ( cos φ = 0,1564). In spite of an almost pure inductive
current with a wide lagging angle (φ= 810) the waveform remains virtually unaffected. Figure
10b shows the harmonic spectrum with the highest harmonic being 0,31% of the fundamental.
Figure 11 proves the reliability of the Hygen system with a step load of 14kW switched on while
the engine is running at minimum speed. The battery energy is pumped to boost the DC Bus and
augment the output power while the engine starts to accelerate. Figure 11 shows only a few
cycles of the acceleration period, sufficient to show that the quality of the voltage is unaffected.
Naturally, the frequency being synthesised in the software remains constant – compare Hygen’s
superior performance under this load condition with that of conventional or other more modern
variable speed generator sets.
Figure 12 shows yet another severe impact load condition, namely, the starting of a 2.5 kW
compressor motor. Again the quality of the output voltage is unaffected and the frequency
remains constant.
TO SUM UP
Hygen’s unique feature and technical achievement is the novel application of a controllable
converter (the active boost rectifier ABR) which regulates the alternator current, and
hence the engine torque, according to any specified torque characteristic.
As a result thereof, it is a highly flexible system, particularly in:
ƒ
ƒ
Automatically adjusting its operating speed across the engine’s full speed range so that it is
“optimally” loaded across a wide range of load power demands, with fuel consumption
directly related to active output power (kW).
Automatically derating the engine when operating conditions deviate from the standard
reference conditions e.g. altitude above sea level and ambient temperature.
ƒ
ƒ
ƒ
Automatically compensating for large variations in the alternator voltage caused by wide
speed variations - without compromising system efficiency, performance or safety.
Optimally and efficiently controlling the power flow between its energy sources, and the
load.
Adapting its torque characteristic to optimise operation for various diverse applications.
Its new topology and control strategy provide outstanding flexibility and application and a highquality grid-like power supply irrespective of adverse load conditions. It therefore takes variable
speed generators to an unprecedented level of technical sophistication and performance. It
extends the application of automatically-adjusted variable-speed generators to sizes in the
megawatt range and challenges conventional generating systems with fixed speed operation,
e.g. 1800/1500 and 3600/1500 rpm (60/50Hz).
In base load applications the system’s low fuel consumption, low maintenance costs and
extended engine life due to more efficient combustion, make it highly competitive in terms of the
ultimate bottom line – life cycle costs.
It can also be optimised (amongst its many possible configurations) to provide a compact, cost
effective and highly competitive Uninterruptible Power Supply (UPS) system for power
breaks, with engine driven back-up limited only by the availability of fuel. There is therefore no
need for large battery rooms and heavy duty, large and expensive diesel engines; a smaller and
less expensive automotive engine can suffice. The fact that this engine does not have the service
life of a rugged industrial diesel is of no practical consequence, since average annual usage
below 1000 hours can be expected. The same unit can provide a load levelling/ peak shaving
function, requiring additional running hours. In this instance, however, the engine will be
operating at reduced speed – thus also of no practical consequence. For larger applications, truck
engines, gas turbines or other “high” speed industrial engines will provide similar benefits.
CONCLUSION
Increasing and changing consumer requirements will make heavier peak period demands on
electricity utilities. The utilities will have to increase capacity, i.e. “oversize themselves”, with
resultant costs to themselves and consumers. Alternatively, they will have to encourage peak
shaving or load levelling through an appropriate price structure. The bottom line for the
consumers will be the cost factor – do they pay the price of high peak loads or do they install
their own generating capacity?
Furthermore, with computerised industrial automation and the proliferation of electronic
appliances such as computers and TVs etc, electricity consumers at all levels will demand high
quality, reliable (no-break) power. Generator capacity at plant level is more expensive than
normal utility rates. However, highly efficient plant level generation (in respect of fuel
consumption, maintenance and engine longevity) could make the difference, and prove
advantageous to consumer and utility alike. If an in-house generating facility can also provide
extended UPS as well as load levelling, it will be doubly attractive.
Conventional gensets are not efficient enough and do not provide these three functions, whilst
load-following variable-speed operation without “torque control” addresses only the symptoms
and is fraught with many problems in its practical application.
By contrast, the Hygen system overcomes these shortcomings through multi-disciplinary systems
engineering that successfully integrates mechanical, electrical, electronic and computer
engineering into a high performance system in respect of fuel economy, engine/environment
friendliness, output quality of voltage and frequency at all times, and immunity to abusive load
demands – conditions that frustrate and even damage conventional generator systems. As a
result, it has the capability to provide ultra-efficient generation for the electricity industry, and
thereby enable the industry as a whole (including both its outdated mega power stations as well
as the newly emerging distributed-generation middle-range power stations and lower-end
electricity generator and UPS systems) to reshape itself more appropriately to the circumstances
of the new Millennium. Naturally Hygen is equally well suited to base load applications.
The novel Hygen technology creates variable-speed with programmable torque control that
can be optimized for various applications. With this unique feature, it will thus replace the
century old technology and prove an indispensable element in the electricity generation
scenario of the 21st century.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
J Whitham: Utility Deregulation and The Role of Stand-by Generators. Conference
papers 9th International Conference & Exhibition of Power Generating Industries. PowerGen’96 December 4-6 1996, Orange Country Centre – Orlando, Florida. USA Book IV,
pp. 327-331.
J Loghead: A New Electrical Industry. Editorial Power Engineering Journal, February
1998, pp. 2-3.
Time for the Tine Turbines. Electrical Review. Vol. 231 No. 12 pp. 28-30.
L Grzesiak, W Koczara, M Da Ponte: Novel Hybrid Load-Adaptive Variable-speed
Generating System. Proceedings IEEE International Symposium on Industrial
Electronics ISIE’98, Pretoria, South Africa 7-10 July 1998, pp. 271.276.
Hygen Variable-speed Generating System for Quality Electric Power Supply, Technical
overview, Volt Ampere, Pretoria, South Africa 1997.
M Da Ponte, L Grzesiak, W Koczara, A Niedzialkowski, P Pospiech: Hybrid Generator
Apparatus, South Africa Patent No. 97/11503.
D R Glenn: Direct Fuel Cells: Putting Power Where You Need It. Conference papers
9th International Conference & Exhibition for Power Generating Industries. PowerGen’96 December 4-6 1996, Orange County Convention Centre – Orlando, Florida,
USA. Book IV, pp. 246-254.
D Dunnison, D Smith: PEM Fuel Cell Development for a Deregulated Marketplace.
Conference papers 9th International Conference & Exhibition for Power Generating
Industries. Power-Gen’96 December 4-6 1996, Orange County Convention Centre –
Orlando, Florida, USA. Book IV, pp. 231-244.
TECHNICAL INFORMATION
HYGEN H03011A
Modules:
ƒ Diesel engine - Perkins 103-10
ƒ Permanent magnet alternator – VATECH03011A1
ƒ Power electronic converter – VATRON H03022A1
ƒ Genesis battery G12V39A10EP [x8]
Basic Specifications:
ƒ Rated power:
9kW/18 kVA
ƒ Maximum power:
18kW, 25kVA (5 min)
ƒ Power factor:
0,5
ƒ Speed range:
1400 – 3400 rpm
ƒ Output voltage:
230V + 1%
ƒ Output frequency:
50Hz + 0,01%
ƒ Battery voltage:
96V
ƒ Battery capacity:
39Ah
ƒ Ambient temperature:
550C at 1700m asl
ƒ Dimension LxWxH:
1500 x 750x 980
ƒ Mass:
793 kg (ready to run)
Table 1
INTEGRATED ENGINE
GENERATOR UNIT
MAIN MULTI
PROCESSOR
CONTROLLER
MMC
ENGINE
ENG
ENGINE FEEDBACK &
CONTROL
GENERATOR
PMG
GENERATOR FEEDBACK
& CONTROL
RECTIFIER R
DC link1
VOLTAGE AND CURRENT
FEEDBACK & CONTROL
BOOSTER B
ACTIVE BOOST
RECTIFIER
ABR
BI-DIRECTIONAL CONVERTER
BDC
ENERGY STORAGE SYSTEM
ESS
DC VOLTAGE FEEDBACK
& CONTROL
BATTERY CHARGING &
EQUALISATION
POWER CONDITIONING
& ENERGY STORAGE
PCES
OUTPUT AC VOLTAGE
CONTROL
DC/AC INVERTER
& OUTPUT FILTER
IF
DC link 2
INTERNAL
DISPLAY & CONTROL
STATE OF SWITCHES &
PROTECTION UNITS
PROTECTION
P
AC OUTPUT
LOAD
LD
RCM
REMOTE CONTROL
& MONITORING
Fig. 1. Block diagram of the Hygen load-adaptive adjustable-speed electricity
generating and power supply system.
a)
ENGINE
GENERATOR
RECTIFIER
Pe
b)
DC/AC
INVERTER
Pl=Pe
DC/AC
INVERTER
Pl=Pb
LOAD
LOAD
Pb
BATTERY
STORAGE
c)
ENGINE
GENERATOR
RECTIFIER
Pe
DC/AC
INVERTER
Pl=Pe-Pb
LOAD
Pb
BATTERY
STORAGE
d)
ENGINE
GENERATOR
RECTIFIER
Pe
DC/AC
INVERTER
Pl=Pe+Pb
Pb
BATTERY
STORAGE
Fig. 2. Power flow of the Hygen system
a)
b)
c)
d)
S1 - Load powered from the engine,
S2 - Load powered form the battery,
S3 – Load and battery powered from the engine,
S4 – Load powered from the engine and from the battery
LOAD
Ti1
TORQUE
Tr1
Th
P3
P2
F1
F2>F1
Tp
F3>F2
P1
F4>F3
F5>F4
SPEED
ωmax
ωmin
Fig. 3. Torque vs. speed for the Hygen system with specific fuel
consumption contours.
OUTPUT POWER
TOTAL POWER
Pl =Pb + Pe
Pe2 + Pb2
Pe2
Pe1 + Pb2
GENERATOR POWER Pe
BATTERY POWER Pb2
Pe1
Pb2
Pb1
SPEED
ωmin
ωmax
Fig. 4. Output power vs. speed for Hygen with battery support.
ENGINE TORQUE
Ti2
Tr2
P4
Ti1
Tr1
Th
TA
P3
P5
P2
P6
P1
P7
Tv
TC1
TC2
SPEED
ωmin
SPEED RANGE
ωmax
Fig. 5. Torque vs. speed for constant speed generators, variable
speed generators and the Hygen system.
ENGINE
PM
ALTERNATOR
S
RECTIFIER
INVERTER
110V
60Hz
N
SPEED
Fig. 6. Block diagram for manually-adjusted speed and load-following variable
speed systems.
Hygen power curve with programmable torque control ………
Net intermittent power………………………………………….
Variable-speed……………………………………………….....
Engine rated power …………………………………………….
Load levels……………………………………………………...
Specific fuel consumption contours…………………………….
Maximum continuous power…………………………………...
Fig. 7. Power vs. speed for constant-speed generators, variable-speed generators
and the Hygen system.
a)
0.35%
b)
0.30%
0.25%
0.20%
0.15%
0.10%
0.05%
0.00%
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
Fig. 8. Output voltage for no-load operation
a) Voltage waveform.
b) Harmonic spectrum of the output voltage.
a)
VOLTAGE
CURRENT
1.60%
b)
1.40%
1.20%
1.00%
0.80%
0.60%
0.40%
0.20%
0.00%
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
Fig. 9. Output voltage and current for 20 kW resistive load (overload)
a) Voltage and current waveform.
b) Harmonic spectrum of output voltage.
a)
VOLTAGE
CURRENT
0.35%
b)
0.30%
0.25%
0.20%
0.15%
0.10%
0.05%
0.00%
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
Fig.10. Output voltage and current for a highly inductive load of 12 kVA
(cos φ = 0.1564 )
a) Voltage and current waveform
b) Harmonic spectrum of the output voltage.
VOLTAGE
CURRENT
Fig. 11. Output voltage and load current – Resistive step load – 14 kW.
VOLTAGE
CURRENT
Fig. 12. Hygen H3011A – Output voltage and load current – Impact
load of 2,5 kW induction motor coupled to an air compressor unit.