1- فکر می کنم بد نباشد سیستم تحریک و تجهیزات مرتبط بازبینی شود زیرا عدم تعادل جریانی تا این حد ممکن است ناشی از سوختن بعضی تریستورها یا اتصال کوتاه شدن حلقه به حلقه سیم پیچی تحریک باشد.
2- بطور کلی هر دو ژنراتور هم ولتاژ و هم فرکانس را می توان با هم موازی کرد اما برای کارکرد مطلوب و قابلیت اطمینان مناسب سیستمهای موازی شونده باید از حداکثر انطباق برخوردار باشند. این انطباق نه تنها شامل سیستمهای کنترل بلکه شامل مشخصات فنی گرداننده ژنراتور و خود ژنراتور نیز می شود. در زیر مشروح مواردی که جهت حفظ بیشترین انطباق بین واحدها باید رعایت شوند با استفاده از کتابچه فنی شرکت کومینز آمده است.
Simply speaking, generator sets in a paralleling system are compatible when
-Compatible load sharing control systems
-Compatible interfaces to other monitoring and control systems,
including local and remote monitoring, “first start” controls, manual controls, and load demand controls
To a limited degree, systems can operate successfully with less than completely compatible equipment, but these incompatibilities may result in the need for added equipment in the system or limitations in the flexibility or operation of the system.
Before any modifications to any existing system are made, it is critical that the equipment to be modified is fully tested to verify that it can operate at full rated load with proper voltage and frequency control.
If a machine can’t perform properly with a dedicated load from a load bank, there is no way that it will operate successfully in parallel with other machines.
The real power (kW) provided by a generator set operating in parallel with others is a direct function of engine real power output. Compatible engines can share load nearly equally, at all load levels, while operating at steady state load levels and during transient loading conditions. Conversely, if incompatible engines are paralleled, load sharing problems can occur, particularly on application or rejection of large load steps. As loads are added to a generator set, particularly in large increments, the generator set frequency will momentarily drop until the engine governor can drive more fuel into the engine to recover to its nominal speed (frequency). The amount of speed drop and the recovery time are a function of the inertia in the rotating components of the system and how fast the governing and air intake systems can increase the fuel rate in the engine. The generator set’s recovery rate is determined by the type of governing system, the engine’s fuel and air intake system designs, and the engine’s combustion cycle (two-stroke or four-stroke).
Load sharing during transient conditions is a concern because differently sized machines often accept and reject load with different levels of ability. For example consider a paralleled 250 kW and a 500 kW generator set. Application of a 250 kW load on a 250 kW generator set will result in a voltage dip of approximately 25%, and a recovery time of 3 seconds. For the 500 kW generator set, a 500 kW load results in a voltage dip of 30%, and a recovery time of 5 seconds. So if a 750 kW load (a full load step) is applied on the two machines, or drops 750 kW in one step from the two machines, they will not share loads equally during the transient period. It is possible to have the system exposed to potential overcurrent conditions on the faster machine or nuisance reverse power faults on load rejection.
At lower load levels, voltage and frequency transients are lower, and recovery times are shorter, so as load step size drops, it eventually gets to the point that transients of a specific level are very similar between machines. This means that dissimilar transient performance of the machines can be dealt with by adding and shedding loads in smaller steps than might be used in a system that has all the machines of the same size. A system designer can compare the single step load pickup and load rejection performance of the various machines in the system to determine if there is a potential problem with engine compatibility. When that is done, actual assembled generator set test data should be used in the evaluation, not just alternator voltage dip or engine (alone) transient performance. As a general rule, there will be no negative impacts due to difference in engine performance if the transient load steps are less than 25% of the rating of the smallest generator set in the system.
Nuisance reverse power trips on load rejection can often be addressed by increasing the time delay on reverse power to outside the recovery time for the slowest machine in the system. Do not address the issue by increasing the “Reverse Power” limit on the generator set. This could desensitize the system to the point that reverse power protection is lost.
Alternators are compatible if they can operate in parallel without damaging or disruptive neutral currents flowing between them. Depending on the generator set’s temperature rise characteristics, age and insulating ratings, neutral current flow between generator sets is not necessarily damaging, but neutral currents can cause disruption in protective relay operation, particularly for ground fault sensing.
Neutral current flow between generators occurs when the voltage between the two machines is different. The voltage difference can occur because of in accuracies in the kVAR load sharing adjustments, or because of differences in the voltage waveform shapes due to differences in alternator voltage harmonics as is illustrated in Figure below.
If an operating system exhibits symptoms of incompatibility between alternators (such as current flow that can’t be adjusted out by kVAR load sharing), a harmonic analysis test of the neutral current flow between the machines when operating with balanced linear load (or even no load) can be conducted. If the fundamental frequency of the current is the same as the system operating frequency, the current flow is a result of inaccurate kVAR load sharing. If the fundamental frequency is 150 hertz or greater, the current flow is almost certainly due to alternator incompatibility, and a decision is needed as to how to deal with it. Harmonic neutral current flow is caused by differences in the voltage waveform shape between two (or more) paralleled machines.
Alternator designers can control the harmonics produced in an alternator by manipulating several design factors, the most important of which is alternator pitch. The pitch of a generator set is a design parameter that can be used to optimize the generator set waveform shape and minimize costs. The pitch refers to the mechanical design characteristic of a generator set. It is the ratio of the number of slots enclosed by each coil to the number of winding slots per generator set pole. Alternators that are identical are obviously compatible. Similarly, alternators that have 2/3 pitch will have waveforms that are compatible for paralleling with each other and/or with the utility.
Compensating for pitch differences
When faced with a requirement to parallel dissimilar generator sets, a system designer has several options to avoid harmonic problems associated with generator set incompatibility:
- If possible, require that new equipment be identical to existing equipment. This may require replacement of one or more alternators in the system. Considering that the effective life of an alternator is approximately 25 years in a standby application, and that engines often be fully functional at that age, this is not an uncommon choice. Requiring a 2/3 pitch on all alternators, even if they are not paralleled, makes them compatible for future paralleling. Replacing alternators is more practical at line voltage level than at medium and high voltages, due to significantly higher cost for higher voltage machines.
- Use a 3-wire primary distribution system. By avoiding a solid neutral connection, there is no path for the neutral current flow, so the most disruptive problem of the incompatibility is removed. Note that the neutral of the dissimilar machine must not be bonded to ground when other machines are connected. Harmonic currents will still flow in the phases between the generators, but will be less apparent, and the system will not be hampered with problems associated with high neutral current flow. Transformers may be used to develop necessary neutral connections for single phase loads.
- Connect neutrals of like machines only, and prevent the dissimilar machines from being the first to close to the bus.
- Add neutral contactors to the switchgear to connect the neutral only on the first unit to close to the bus. This, however, introduces several other potential problems and is not normally recommended.
- Install reactors in the neutral leg of each generator set to limit current flow at third and higher order frequencies. Reactors can be tuned to specific frequencies that are the biggest problems.
- Compensate for the incompatibility by over-sizing the neutral conductor, and derating the alternators.
- The designer may allow system operation with the neutral current and compensate by derating the generator set.
Compatible load sharing control systems
In a paralleled arrangement, the voltage and frequency outputs of the generator sets are forced to exactly the same values when they are connected to the same bus. Consequently, generator set control systems cannot simply monitor bus voltage and speed as a reference for maintaining equal output levels. If, for example, one set operates at a higher excitation level than the other sets, the reactive load will not be shared equally.
Similarly, if a generator set is regulated to a different speed than the others, it will not share kW load properly with other generator sets in the system. Each generator set in the system has two active control systems always in operation: the excitation control system regulating voltage and the fuel control system regulating engine speed. Generators can be sharing kW load and have problems sharing kVAR load, and vice versa.
So real power sharing (expressed as kW) depends on speed matching between the generator sets and fuel rate control; reactive power (expressed as kVAR) is primarily dependent upon voltage matching and excitation system control between the generator sets.
Although it is sometimes possible to integrate systems of different manufacturers, generator set governors and load sharing controls should be of the same manufacturer to avoid conflicts in responsibility for proper system operation.
Several types of load sharing control are available:
- Droop governing and voltage regulation (reactive droop compensation)
- Isochronous kW load sharing
- Cross current compensation for kVAR load sharing
- Isochronous Voltage kVAR load sharing
Compatibility with other control systems
Individual generator sets in a paralleling system may interface to each other and the balance of the facility in a number of ways, including:
- Generator sets must have a means to determine which generator set will close to the bus first in a “black start” (first start) situation.
- Generator sets often provide status information to a system master control, for the purpose of displaying data, allowing the master control to control system power capacity, and for central system load management.
- Generator set may be monitored by a facility monitoring system, or by an external monitoring system, such as for service contract facilitation. In general, these communication tasks are handled by commonly available communication practices such as discrete signals or by digital communication such as RS485/Modbus1 register maps. So the main concern in
dealing with them is to simply plan for them carefully.