7. GENERATORS PARALLELING AND SYNCHRONISING
7.3. PARALLEL CONTROL OPERATION
7.3.1. Taking the load
Considering one generator on line on a network, in parallel with others generators, coupling (synchronising) has been successful, we want it to take some load.
Only 2 controls are available on the couple turbine/generator or engine/generator, the Voltage adjustment of the generator (AVR) and the speed control of the prime mover.
To take load for the “new arrived” generator, the speed control will increase/decrease the active power (“real” power on the shaft). When the voltage increases, the active power increases. In fact, the voltage on the network does not move. Instead the current output increases and the (reactive) power taken by the generator increases.
In manual control, the operator increase up to the desired value.
In automatic control, the “share” of the power will go (gradually) up to the pre-set value which can be in equal percentage for all generators or depending a ratio function of power capacity of the concerned generator.
7.3.2. Load sharing
Or the balancing of power between generators and this depending the configuration of the power plant
Identical generators
G 3Ph G 3Ph G 3Ph G 3Ph G 3Ph
On line Generators Stand-by Generators
Bus bars
Load distribution
G 3Ph
3 MW 3 MW 3 MW 3 MW 3 MW 3 MW
Figure 93: Identical generators load distribution
In this power plant, all generators identical, generally, power is shared in equal percentage
Training manual EXP-MN-SE060-EN
Last Revision: 29/09/2008 Page 80 of 3
Unbalanced power capacities
G 3Ph G 3Ph G 3Ph G 3Ph G 3Ph
On line Generators Stand-by Generators
Bus bars
Load distribution
G 3Ph
10 MW 10 MW 3 MW 3 MW 1 MW 0.6 MW
Figure 94: Non-identical generators
It is impossible to ask the last generator (0.6KW) to share same power with the 10MW ones. There will be a load sharing system which can be:
By ratio of individual power capacity, or by permanent manual adjustment
By centralised power sharing system, given independent orders to each generator control system, it is the “Load sharing” in which an additional control box is added to each unit in complement of its AVR and speed control
And if you are familiar with site electrical installation, Woodward control material is installed on numerous plants, here after some pictures of Load control, Load & speed control devices.
The 2301A Load Sharing and Speed Control is available in forward- or reverse-acting systems and in several speed ranges for applications requiring either droop or isochronous speed control. Models are
available with either accelerating or decelerating ramps.
Figure 95: Load sharing and speed control
The Automatic Generator Loading Control (AGLC) can be used with any Woodward load sharing and speed control system with either built-in or external load sensors. It is designed to provide soft loading or unloading of a generator set to an isochronous load sharing system or to base load setting at controlled rates. The electronic ramps are easily adjusted from five seconds to five minutes for 100% load change.
The load and unload ramp rates adjust separately Figure 96: Automatic Generator Loading Control
Training manual EXP-MN-SE060-EN
Last Revision: 29/09/2008 Page 81 of 3
And so on, we cannot edit here the complete Woodward operating and instruction manuals and anyway the load sharing + control of alternators is a complete course by itself.
Hereafter a schematic configuration with load sharing example. We shall see nevertheless in next paragraph the meanings of “isochronous” and “droop”.
G1 G2 G 3 G4 G5
Prime movers Gas Turbines
Load distribution bus-bars
G6
Communication bus or wires between LS’s
Total load and V volt
Figure 97: Load sharing principle example schematic
SCx is for Speed Controller receiving the rpm indication and sending back signal to prime mover governor
AVRx is the Automatic Voltage Regulator receiving voltage and current of its generator and sending back field current
LSx is the Load Sharing “box” dialoguing with each generator / prime mover SC & AVR and interconnected with the master LS0 which checks the total load
7.3.3. Droop and Isochronous mode of control
Isochronous operation provides constant turbine speed for single unit operation and for parallel units provides proportional division of load between units while maintaining fixed frequency on an isolated bus.
Speed control, in isochronous mode, for each prime mover is (nearly) independent, the turbine (or engine) governor acts as a single regulator, watching and adjusting “its” speed to the fixed synchronism value.
Training manual EXP-MN-SE060-EN
Last Revision: 29/09/2008 Page 82 of 3
50 Hz
Figure 98: Isochronous response form for frequency on a bus bar
In Speed Droop the prime mover governor operates to decrease speed with increasing load. This is the mode that is commonly used to operate generators in parallel, as it allows them to share load in proportion to rated load.
50 Hz
Figure 99: Droop response
But, what about the frequency control? I want the network to be at 50 Hz permanently!
Other generators have to be in ‘isochronous’ on the network to keep the frequency at desired value. The one in “droop”, generally a smaller one, running “under speed” is just (by this system) maintaining its load at the same value leaving the other generator the task of taking the increase. The AVR, not concerned by the droop is still controlling at the set voltage.
Droop mode is pres-set as project/commissioning values; they are in the range of 3% or 5%, for 50%, 100% of load,
Training manual EXP-MN-SE060-EN
Last Revision: 29/09/2008 Page 83 of 3
50 Hz
5% droop for rated Hz at 100%
5% droop for rated Hz at 50%
3% droop for rated Hz at 100%
3% droop for rated Hz at 50%
Figure 100: Example of droop mode settings
Only one setting is applied to one machine, if all generators in parallel have the same droop curve, the frequency varies accordingly, the user have to accept the change in frequencies function of the load
Problems begin to occur when machines in parallel have different droop settings. Leave it to the specialist….
7.3.4. Load Shedding
It is common to see people mixing the terms and functions of “Load Shedding” and “Load Sharing” and not only because they are written nearly identically.
Principle :
Objective of a “Load Shedding’ is to keep a network on line not going to a general shutdown, when one generator trips revealing suddenly the insufficiency of available power. The only alternative, at time of the trip, is to cut immediately part of the distributed power, the one non essential.
This «Load Shedding » function is dedicated to a specific PLC (Programmable Logic Computer).
When a generator trips, the PLC, watching permanently the ratio between available and used power, can decide the opening of a certain number of breakers, this depending the analysed power to be “sacrificed’. PLC having acknowledged the trip of a turbine /
generator, analyse at that time, the value of frequency (generally) and if that one goes under a threshold, shedding is decided. The more frequency drop, the moreshedding)
Training manual EXP-MN-SE060-EN
Last Revision: 29/09/2008 Page 84 of 3
Circuits to be opened are pre programmed in the PLC, numbers being function of the power still available. Of course, all this shedding operation should be completed in the shortest time to avoid the overloading of generators kept on line. A lasting overload would conduct inevitably towards a general shutdown.
Generator trip detection + Frequency drop detection + Conditions analysis by PLC + Opening orders towards circuits + Response time of breakers = 40 to 50 milliseconds maxi.
The figure / diagram is self-explanatory for the understanding of the Load Shedding principle.
G 3Ph G 3Ph G 3Ph G 3Ph G 3Ph
Generators on line Generators in stand-by
busbars
Load Distribution
G 3Ph
3 MW 3 MW 3 MW 3 MW 3 MW 3 MW
G 3Ph G 3Ph G 3Ph G 3Ph G 3Ph
Generators on line Generators in stand-by
busbars
Load Distribution
G 3Ph
3 MW 3 MW 3 MW 3 MW 3 MW 3 MW
Instant ‘T’: 4 generators on line 12 kW available 11 kW used
Instant ‘T+t1’: 3 generators on line 9 kW available 11 kW used
Trip
x x x
Instant ‘T+t2: shedding = open non essential circuits to have total used power by consumers under available power from generators
Trip of 1 generator
Load Shedding action by PLC Analyse
P.avail. / P.used
Figure 101: Load Shedding principle
Training manual EXP-MN-SE060-EN
Last Revision: 29/09/2008 Page 85 of 3