DECS-150 Digital Excitation Control Systems offer precise excitation control and machine protection in a compact package. DECS-150 adaptability to many applications is assured through configurable contact inputs and outputs, flexible communication capabilities, and programmable logic implemented with the provided software. The high flexibility and powerful functionalities makes this device particularly useful in those projects where machines are paralleled to other generators and/or the utility system, since the requirements from the grid codes are very demanding.
The DECS-150 is suitable for the Mecc Alte medium and high voltage series ECO46 and ECO49. In fact, the DECS-150 can be fed indifferently by the PMG3, MAUX or even shunt-excited.
Excitation power is supplied from the DECS-150 by means of a filtered, switching power module that uses pulse-width modulation. It is capable of supplying 7 Amps at 70°C ambient temperature and has a forcing capability of 11 Amps for 10 seconds.
STATOR WINDING PT100
Stator winding temperature sensing is accomplished in a different way compared with the low voltage. The probes are fitted inside the stator slots, 100mm inward. 3 wires PT100s are used. If no.3 probes are required, they are installed at the NDE side, 120° shifted. In case of no.6 probes, 3 of them are installed at the NDE, the others at the DE. Having the temperature on both sides of the stator helps in recognizing serious thermal issues. As a rule of thumbs, the lower the difference between the two side, the better the cooling. Double PT100s, housed in the same case, are available as well.
Bearings PT100s are installed in the main bearing ring, close the outer raceway.
ECO46 series is optionally provided with one PT100 for each bearing.
ECO49 series is optionally available with one or two PT100s for each bearing, in two diametrically opposed position.
This is to immediately detect a different temperature and consequently a local hotspot.
Single axle accelerometers are used to sense the vibration in the alternators. They are installed close the bearings, or couplings, in the main 3 axles. They are mostly used for the detection of the following issues:
- Rotor unbalancing
- Bearings failures (in combination with the bearing temperature sensing)
- Engine-alternator improper alignment
ECO49 are standardly provided with the predisposition for no.3 vibration sensors at the DE and no.3 at the NDE. The housing is on the shields or the bearing caps.
ECO46 are optionally available with the predisposition.
The accelerometers themselves are available upon request.
They can be provided loose or fitted and cabled to the auxiliary terminal box.
The most popular configuration is: one sensor at the DE Y axle + one at the NDE Y axle.
The sensors are designed to be connected to an external controller, which is housed in the auxiliary terminal box or in the customer cubicle.
The controller is available upon request.
Mecc Alte Power Products has chosen the best available technology in the market, for Diesel Genset application.
We use class H, VOC free (volatile organic compound), low viscosity Epoxy resin in combination with glass and mica tapes. Main advantages:
- Outstanding behavior after ageing at high temperature (Thermal index 200°C - IEC 60216)
- Excellent flexibility and mechanical properties
- Good thermal conductivity
- Good electrical properties
- Approved from American Petroleum Institute (API) for harsh environmental application
Our insulation system passed the Functional-System Test according to IEC 60034-18-31 / IEEE 1776 which include thermal, vibration, humidity and electrical endurance tests. Certificates are available upon request.
MARINE GRADE EPOXY OVERCOATING
Windings are covered with a red treatment (marinization). Red varnish, is a high temperature insulating enamel that forms a tough and flexible film, with excellent moisture and chemical protection. It is water and oil proof, and also protects windings from abrasion. It is applied spraying an over coating layer over the impregnated winding.
Epoxy overcoating is standard for the stator windings for both the ECO46 and ECO49.
The overcoating is standard for ECO49 rotor.
The overcoating is optionally available for the ECO46 rotor.
ANTIDEFLAGRANT DIAPHRAGM ON TERMINAL BOX
An arc flash event can result in significant injury to workers.
To mitigate the risk for an explosion in the event of an arc flash inside the terminal box, we design a specific gas evacuation path and a brass diaphragm for all our medium and high voltage alternators.
This is standard for all the units.
AMAGNETIC SIDE PANELS
Medium voltage alternators, both series ECO46 and 49 are standardly provided with the an amagnetic panel for the cables outlet.
The panels are made of stainless steel AISI 304 which avoids the arising of Eddy currents and consequently eliminates the hotspots near the cable glands.
The anticondensation heaters for both the ECO46 and 49 are detachable and replaceable.
The heaters are available as an option.
In the ECO46 they are installed inside, both at the NDE and DE side, between the stator windings heads and the frame. In the event of a fault, the shilelds might be removed to replace the component. The total burden of the heaters is 300W (2 X 150W).
In the ECO49 the heaters are housed in four pockets in the bottom side of the alternator.
The total burden of the heaters is 600W (4 X 150W). The heaters can be easily replaced without removing any parts of the alternator
BEARING INSULATION & SHAFT GROUNDING
All rotating machines potentially have a bearing current problem whether it is DC or AC, and either large or small power in size. These rotating machines have three basic sources of shaft voltage:
- Electromagnetic induction
- Electrostatic coupled from internal and external sources
Electromagnetic induction from the stator winding to the rotor shaft is more prevalent in long axial machines. The shaft voltage is due either to small dissymmetry of the magnetic field in the air gap or dissymmetry in rotor geometry. Both are inherent in a practical machine design. Irregularities in the magnetic circuit may cause a small amount of flux to link the shaft, with the result that an electromotive force is generated between the shaft ends.
Electrostatic sources are usually represented by Variable Speed Driver (VFD) or Power supplier.
They cause a common voltage mode in the stator, which induce an AC shaft voltage trough the parasitic stator-rotor capacitance.
Once these voltages reach a level sufficient to overcome the dielectric properties of the bearing grease, they discharge along the path of least resistance — typically the generator bearings — to the generator housing. During virtually every VFD switching cycle, induced shaft voltage discharges from the motor shaft to the frame via the bearings, leaving a small fusion crater (fret) in the bearing race. Regardless of the type of bearing or race damage that occurs, the results is the early fault of the alternator.
To avoid shaft currents circulation through the bearings, two methods are usually implemented: bearing insulation and shaft grounding. They can be used together or separately.
INSULATED BEARING - CERAMIC COATING
Both the drive end and the opposite drive end bearing in the ECO46 are optionally available with an insulated ceramic coating.
The coating is usually made of aluminum oxide power, sprayed with a plasma jet onto the outer or inner ring. When applied in this manner, the oxide layer adheres extremely well to the base material. The insulated layer offers a DC resistance in the order of 500 MΩ – 10 GΩ and guarantees a dielectric strength of at least 500VDC.
The insulated bearings are interchangeable with the standard ones.
INSULATED BEARING - FIBERGLASS RING
For series ECO49, a special arrangement for the bearing box is optionally available.
Contrary to the ECO46 series, where the bearings are insulated, here the bearing box is completely insulated by means of insulating fiberglass rings. This solution is robust and offer a very high insulation between the shaft and the alternator housing.
Both the drive end and opposite drive end are optionally available with the insulation.
This solution is not retrofittable hence has to be required in the order.
SHAFT GROUNDING RING OR BRUSH
The grounding rings or brushes provide reliable shaft grounding to divert harmful shaft voltages to ground and extend bearing life. Depending on the application and the type of bearing, the ground ring can be installed at the drive end or the opposite drive end.
Generally, for the gensets, the shaft grounding is accomplished at the drive end side, in combination with an insulated bearing at the opposite drive end.
Both the ECO46 and ECO49 are optionally available with a shaft grounding ring.
CURRENT & VOLTAGE TRANSFORMERS
Current and voltage transformers are necessary in MV alternators, for the following reasons:
- To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments.
- To insulate the metering circuit from the primary high voltage system.
- To provide possibilities of standardizing the instruments and relays to a few rated currents and voltages.
The standard configuration provides:
- No.1 Ph-Ph VT, Un/100V 15VA cl.05
Dedicated to the voltage regulation with DER2 or Basler AVR
- No.1 CT, In/1 A 15VA cl.05 FS10
For customer integration or droop controller (not available with DER2)
ADDITIONAL CT & VT
Customers may require specific configurations according to their needing for protection and metering.
For series ECO46 and ECO49, the total number of VTs and CTs which can be housed in the standard terminal box is six. Usually customers require three VTs for three-phase voltage sensing and three CTs for measuring and protection.
The VTs and CTs configuration is fully customizable, but the most required configurations are the following:
- Single phase voltage sensing and three-phase current sensing (fig. 2 below)
No.1 VT Ph-Ph, Un/100V 15VA cl.05
No.3 CTs for metering and protection In/1A 15VA cl.05 FS10 /1A 15VA 5P10
- Three-phase voltage sensing and three-phase current sensing (fig. 3 below)
No.3 VTs Ph-Gr, Un:√3/100:√3 V 15VA cl.05
No.3 CTs for metering and protection In/1A 15VA cl.05 FS10 /1A 15VA 5P10
Other customizations are available such as:
Rated secondary currents 1-5 A or other
Different secondary voltages, 100-110-120 VAC
Different standard IEC, DIN, VDE, ANSI, BS
Different precision class (included class X), and burden
Matched CTs for differential protection
ADDITIONAL TERMINAL BOX
For sectors such as Oil & Gas and Energy production, the specification for VTs and CTs are more demanding. It is not uncommon to find specification where seven CTs and three VTs are required or even more.
In this case the room available in our standard terminal boxes is not enough so that an additional terminal box is necessary. This is available for series ECO49.
With the additional terminal box, the VTs-CTs configuration reaches no. 7 CTs and 3 VTs as you can see in the picture below. Normally the additional terminal box is dedicated to the center star and the neutral output
ROTOR EARTH FAULT (ANSI 64R) DETECTION
Generator rotor winding is typically fully insulated from the earth (i.e. ground).
Therefore, single connection of this circuit to earth will not cause flow of any substantial current. However, if second earth fault appears in this circuit circumstances can be quite serious. Depending on the location of these two faults such operating condition may cause:
- Partial or total generator loss of field
- Large dc current flow through rotor magnetic circuit
- Rotor vibration
- Rotor displacement sufficient to cause stator mechanical damage
Therefore, practically all bigger generators have some dedicated protection which is capable to detect the first earth fault in the rotor circuit and then, depending on the fault resistance, either just to give an alarm to the operating personnel or actually to give stop command to the machine.
Typical protection relay includes an AC injection module which shall be connected to one of the rotor poles (either plus or minus). The resulted injected current signal is then connected to an analog input, able to discriminate capacitive and resistive component, hence the insulation impedance. When the rotor is fully insulated that impedance is pure capacitive. Once one point goes to earth the relay detects a resistive component and triggers an alarm or give a stop command.
A specific brush to inject the monitoring signal is optionally provide for series ECO49.
This option is in bundle with the shaft grounding option.