|
|
|
Update:
Our Latest Forum Schedule
|
FREE
Email
Newsletter
Monthly
Magazine
Member Of
Download
Our
Issues
|
EXCITATION SYSTEM REPORT
Designing an Excitation System With A Standby Generator (Part Two)
By Larry Perez
Continued from Issue 7, 2002 of Electricity Today
Marine generator applications follow the same basic engineering practices as the standby system with the exception of the ship's individual load profiles and the standard requirements of the Coast Guard and American Bureau of Shipping. Both USCG and ABS inspectors put requirements on the equipment that must be used onboard the vessel.
The generator set is no exception.
For the application, the generator to be used is rated at:
- 480/277 Vac, 4 Wire, Grounded Neutral
- 60 Hz
- 2,000 Kw
- 3600 RPM
- Brushless Excited
- Full Load Exc: 63 Vdc @ 8.5 Adc
- P.M.G.: 260 Vac No Load/240 Vac Full Load
- 240 Hz
The load profile of the ship is similar to the loads that would be seen in a normal building application, except for communications equipment (such as radar or radio apparatus), large pumps and navigational equipment. These loads may require special excitation equipment to provide the generator with high performance and long term reliability. Many ships are equipped with devices called bow thrusters -- electric motors that drive propellers to help steer the ship in tight quarters while docking and maneuvering. These motors are controlled by large SCR drives and create harmonics/distortion on the generator's terminal voltage. This distortion may cause misoperation of the regulator, which could generate EMI at levels high enough to cause misoperation of the ship's communication system. So, extreme care must be taken in selecting a regulator for this application.
Another very common concern in shipboard applications is the ability to control the excitation system from a remote location. The operator is generally some distance from the generator and the excitation system needs to control the generator's output. It is important for the regulator to directly take a contact input.
As previously mentioned, the USCG and ABS put requirements on the excitation system. Now they require a manual back-up to the automatic regulator, and a fault current support system (CBS or SBO) is required for generators over 100kW.
There are two basic types of manual voltage controllers (MVC). The first is the variable transformer (Variac) type, which offers manual control of the excitation for system trouble shooting and back-up to the automatic regulator. This type of MVC, model MVC 104, offers operation while completely isolating the regulator for maintenance purposes (as shown in Figure 14). A lower cost alternative to MVC 104 is an MVC 300, a device that does not include total isolation of the automatic regulator but, because it is solid state, offers a more constant output voltage than the variable transformer type.
On this shipboard application, there is also a requirement for fault current support. We could fulfill this requirement with a CBS or SBO as with the simple standby generator in Example #1 (as printed in Issue 7, 2002 of Electricity Today), but since this generator is equipped with a permanent magnet generator (PMG), we will not need to add another current boost system. The PMG will provide input power to the regulator during fault conditions or motor starting events.
The PMG is a separately mounted generator connected to the shaft of the main generator for the sole purpose of providing a power input to the voltage regulator. The only considerations to be made when using a PMG are the output ratings and the regulator's compatibility. PMGs typically have more poles than the main generator and, therefore, have a higher frequency output. The regulator must be capable of accepting this higher frequency for proper regulator operation. Care must also be taken in the event the PMG is a three-phase device when the regulator is a single-phase device. If the regulator only loads two of the three lines of the PMG, the kVA rating of the PMG has to be de-rated by as much as one third.
For this example, a DECS 63-15 was selected for several reasons. The DECS unit is microprocessor based and will take contacts directly for remote control. This helps to meet the economic goals of this project, because separate motor operated potentiometers for remote control do not need to be bought. Secondly, the DECS can also accept a PMG input, single or three phase, at any frequency from 50 to 400 Hz. This means that compatibility with the PMG is not a concern. Thirdly, DECS has a pulse width modulated (PWM) power stage that is excellent for use with non-linear loads like bow thrusters. Finally, DECS offers high field forcing for improved motor starting characteristics.
Also, an MVC must be added to meet the requirements of the USCG and ABS. A solid state device for low cost and output regulation was selected. Care must be taken with a solid state MVC so that it is compatible with the PMG. Alternatively, power for the MVC may be taken from generator output voltage, since manual control is an emergency mode of operation.
Prime Power Applications
Prime power applications are unique in several ways. Because the generators are generally larger in kW rating, they typically have medium- or high-voltage outputs. This higher voltage makes step-down transformers for the sensing and power inputs necessary. Prime power applications also have a concern about system integrity and redundancy for critical generation sites. These applications also require direct communications because many of these sites are remote and unmanned.
Potential Transformers/Fuses
For generator voltages above 600 volts, fused potential transformers with adequate accuracy for metering are required. Single phase or three phase PTs with secondary voltage ratings of 120 or 208/240 volts may be supplied. The PTs may be used only for voltage regulator sensing or may be combined with meters and relays within the burden capacity of the PT. PTs are also used for "Dead Front" Switchboard design for 480 or 600 volt generators, in the same manner as for voltages above 600 volts.
Power Isolation Transformers and Fuses
Excitation power for generator voltages above 600 volts, and sometimes below 600 volts, is provided by a stepdown transformer, rated to match the regulator's input power requirements. The secondary voltage is based on the regulator specs. The maximum transformer rating is also taken from voltage regulator specs, but the required transformer rating depends on the exciter field resistance.
For example, the AEC63-7 specifications are listed below along with some generator specifications. By sizing the power transformer based on field resistance, the transformer size and cost is kept low.
AEC63-7 Generator
-------- ------------
63Vdc 63 Vdc, exciter
7 amps 15 ohms resistance
1100 volt-amperes burden
240 volts ac input
9 ohms minimum field resistance
9 = .6 .6 x 1100 VA = 660 VA
---
15
A 660 VA transformer may be used instead of the 1100 VA based on the regulator's maximum burden rating.
Prime power application specifications almost always specify the need for "black start" capability. This is the ability of the generator to build up voltage when there is no other power source available, except for the station batteries. To ensure positive voltage build-up, a field flashing circuit can be added. Most modern generators do not require flashing except in those rare occasions when residual is lost. An automatic flashing circuit can be added without much difficulty if a battery or other DC source is available. To determine how much flashing current is required, you can use a quantity equal to half of the no-load field current. This information is typically available from the generator manufacturer, but if no data is available, use a value equal to 10 per cent of the regulator's rating.
With the battery voltage rating, flashing current and field resistance, the value of a current limiting resistor can be calculated to limit maximum flashing current.
Battery: 24 Vdc R + 18 = 24 = 60
--
0.4
Regulator rating: 4 amps R = 42 ohms
Flashing current: 0.4 amps (10 per cent)
Use 50 ohms, 10 watts
Field resistance: 18 ohms
In addition to a battery and a current limiting resistor, a diode is required to prevent regulator output from charging the battery. Add a switch contact to start and stop the flashing to complete the system. The switch contact should close when the generator is rotating and should open when generator voltage builds up. These functions are often provided by a speed switch and an AC relay.
If a grounded battery is to be used to flash the field, a power isolation transformer is recommended to prevent DC-circuit grounds from destroying the regulator.
Most prime power applications are vital to the utility company to ensure there is adequate generation to provide their customers with all the power required to meet their needs. To ensure that the generation system is extremely reliable, the levels of complexity in excitation systems are increased. This expansion in excitation system complexity is not limited to the prime power application; it may also be true of a standby system used on an extremely critical load, such as a hospital.
In the case of a standby system, its whole purpose is to minimize the power outage to as short a time as possible, in most cases between five and ten seconds. For some applications, even this may be catastrophic. Many schemes exist to reduce or eliminate the power outage. They are commonly called uninterruptable power supply (UPS) systems. Although a number of UPS system designs involving static and rotating equipment have been devised, most of them use conventional excitation system equipment.
There are many ways to improve the overall generator system reliability, whether the system is a standby system or a prime power system. One method is to provide for a redundant or back-up voltage regulation system. These systems vary greatly in complexity and function and are based primarily on the application. These redundant systems can be as simple as an automatic regulator with a manual back-up device, or the system can be as complex as a completely redundant, automatic voltage regulator system with automatic transfer to the back-up channel. The systems can have auto-follower features that allow the back-up device to follow the operation level of the primary device.
Figure 24 shows one of the most common and basic types of redundant regulation systems. This type offers both an automatic voltage regulator and a manual voltage control system. With this system, either the automatic voltage regulator or the manual voltage controller is in operation at any given time. Therefore, if a failure of one device occurs, a transfer from the failed device to the standby device is performed. The generator may remain on-line during the transfer. This system offers many desirable features. One such feature is the ability of some manual voltage controllers to provide total isolation of the automatic regulator for maintenance purposes, while continuing to generate power in the manual mode. Another feature of the scheme is the ability of the Manual Voltage Control to operate without the need for a sensing input. This is an essential feature, should the sensing transformer fail or should a fuse on the sensing input to the regulator open. When one or both of these happen, the automatic regulator will turn full on because it has lost its reference. Since the manual regulator does not need a reference to operate, its output is controlled manually, dependent only upon the operator. The generator can stay on-line producing power, even with a loss of sensing input.
The redundant scheme in Figure 24 consists primarily of an automatic voltage regulator, manual voltage controller and transfer switch S1. There are also components shown that are common to most redundant schemes. These components are the fuses in the power input circuits of each regulator and the diodes across the exciter field. If a power semiconductor short circuits in the automatic regulator, the fuses associated with that regulator will open, removing it from control of the generator and prohibiting that failed component from affecting the operation of the back-up manual voltage controller. The series diode pair provides a discharge path for the highly inductive field circuit during the transfer from one regulator to the other. This discharge path allows for transfer from auto to manual and back to auto while the generator is operating. The diodes are typically rated at 800 to 1000 PIV and 1.5 to two times the rated field current. There is also an internal single diode across the regulator's output that does the conducting during normal SCR commutation.
This type of transfer switching arrangement may create a severe system disturbance, or bump, during the transfer from auto to manual control. If the excitation system is operating in automatic voltage regulation and a transfer is performed, a bump will occur unless the manual voltage controller happens to be at exactly the same DC output level. To prevent this bump, some method must be used to allow the output of the two devices to be matched prior to transfer. The most basic method of matching the outputs of the two control devices is with a nullmeter (see Figure 25). This device compares the outputs of the two devices and indicates zero volts, or a null, when the two sources are matched. The nullmeter indicates zero at the center scale position.
The addition of a nullmeter necessitates both auto and manual devices to be turned on simultaneously. This simultaneous power-up creates problems for certain types of manual voltage controllers. If the manual voltage controller has an SCR power stage, the output of that device will charge up to an extremely high level of output voltage. A loading resistor is applied across the output of the manual voltage controller to diminish the high DC voltage. With the addition of the loading resistor, the output voltage of the manual controller can be accurately compared to that of the automatic regulator. Care must be taken in the selection of the loading resistor value so that it does not force the total parallel resistance (the resistance of the loading resistor and the field circuit) to be below the minimum required by the regulator. If the total resistance is less, the regulator may see a decrease in its life because of the excessive field currents drawn due to the lower total field resistance, as shown in Figure 26. The value of the load resistor may be obtained from the voltage regulator and manual control supplier. Basler recommends sizing the resistor to draw 250mA DC at the avr continuous voltage rating. Make sure this added current does not increase field current from the avr above its rating. R1 and R2 are normally the same rating.
Because both control channels are energized at the same time, and their F- outputs are tied together, it is recommended that an isolation transformer be added between avr and manual control to prevent a failure in one from causing a failure in the other. With the addition of the nullmeter, transformer and load resistor, the manual voltage controller output can now be accurately measured. The operator can precisely match the manual voltage controller's output, prior to transferring from automatic regulation to manual voltage control. With this method of nulling or matching the outputs of the two controlling devices, the transfer will be effectively bumpless and no generator disturbances will be noted.
The above systems require some sort of manual intervention prior to transfer. If the machine is operating at various load levels, varying amounts of excitation will be provided by the automatic regulator. In order to maintain a null between the auto and manual modes, the operator must periodically re-adjust the manual voltage controller's set point. If there is a requirement for automatic transfer to manual when a failed automatic regulator is detected, the manual mode must be adjusted as often as the load level/excitation level changes. For these systems, an auto-follower or auto-tracking device is used (Figure 27). This type of device automatically forces the manual mode to be continuously nulling the two systems. This means a transfer can occur at any time, and the result is that the generator will be at effectively the same load level as it was prior to the failure/transfer.
There are unique devices to perform the auto-tracking function. They compare the output of the automatic voltage regulator to the output of the manual voltage controller, and either increase or decrease the output of the manual voltage controller to create the null.
This device eliminates the need for human intervention where an operator adjusts the manual voltage controller as the excitation varies. Because the RA-70M continuously adjusts the output of the manual voltage controller, a transfer can be initiated at any point by some type of protective relay scheme. Care must be taken in selecting the auto-tracking device to ensure that the device has some intentional delay in the tracking response as a standard feature. This delay will ensure that the manual controller does not track the automatic regulator into a fault. An example of this could be in the case of loss of sensing. When a loss of sensing occurs, the automatic controller will immediately turn full on, forcing maximum output into the field. If the RA-70M were to respond instantaneously, it would follow the automatic regulator's output to a maximum level and the manual mode would be at the same higher excitation point. Some delay, typically in the range of two to five seconds, is incorporated to allow the protective relays time to transfer to manual mode while the manual mode is still at the excitation level prior to the loss of sensing.
Obviously, during a fault the transfer to manual control is anything but bumpless. In the example of loss of sensing, there is first a generator overvoltage/over-excitation condition and then, after the transfer, a return to a normal level. The term "bumpless" applies only to a normal, non-faulted transfer to the back-up mode of operation where the generator's output is effectively the same before and after the transfer between auto and manual modes.
In Figure 27, transfer relay K1 is incorporated to perform the function of the transfer switch, because the transfer needs to be automatic and initiated by the protective relays. This transfer relay also could have been an 86 device or lockout-type relay that required a manual reset to the automatic regulator. This manual reset mechanism assures that someone has to make the decision to reset after first investigating the trip to manual mode.
The types of protective relays utilized are dependent upon the generator and its use. If the system is stand alone (not connected to any other generator or the utility grid), the relays will probably be as simple as an overvoltage/undervoltage device.
If the generator is paralleled to a utility grid, the relay types could be loss of sensing, over/underexcitation relays and reverse vars or loss of excitation relays. Any number of different scenarios can be used to initiate a transfer.
An additional facet of the auto-manual regulation system is the ability to transfer from the manual mode back to the automatic regulator in a bumpless fashion (Figure 28). This type of transfer is possible, but it tends to be more of an operator-assisted type of transfer and is very difficult to automate. The actual transfer from manual controller to automatic regulator is very similar to the process used when manually synchronizing a generator with a synchroscope. The nullmeter replaces the synchroscope and the operator looks at voltage difference instead of phase angle. While in the manual mode, the operator slowly adjusts the automatic regulator's set point. As the regulator nears the regulation (null) point, the nullmeter will start to move off the extremes (full clockwise or counter-clockwise) of the meter back to the center/null position. Once the meter is at the null point, the operator can initiate the transfer and obtain a virtually bumpless transfer. The operator's skill level and the rate of adjustment of the regulator's set point have a direct impact on the magnitude of the bump seen on the system.
As with the manual voltage controller, the output of the regulator cannot be left open in order to get proper voltage measurements. To obtain accurate output comparison measurements, a loading resistor is applied to the output of the regulator. The sizing of this loading resistor is the same as the one used on the manual device and the same criteria is used. It is likely that the resistor will be identical to that chosen for the manual mode.
Automatic/Automatic Regulation Systems
Redundant regulator systems that incorporate two automatic voltage regulators are common in unmanned, or remote, generation facilities. Incorporating two automatic regulators has the advantage of always being in the auto mode. In other words, no manual controller and, therefore, no human intervention is required. If a transfer occurs, system performance, such as load on/off transients, will be handled automatically by the back-up automatic regulator, and system performance is identical to that of the prime regulator. No manual adjustment of excitation is required. After the initial regulator failure and system disturbance have occurred, the only noticeable indication of the back-up regulator being in control should be the status of the transfer relay.
Figure 29 illustrates the basic Automatic/Automatic (auto/auto) system. It is similar to the basic auto/manual system in many aspects. There are two devices capable of generator control. There will be some form of transfer switching or relaying and there will be an isolation transformer between the two regulators. One of the automatic voltage regulators will be considered the primary regulator and the other, the back-up. The primary regulator will be in control at all times until it fails to perform its primary function, to regulate voltage or VARS.
As with the auto/manual system, the auto/auto system also has some standard circuit criteria that have to be met (refer to Figure 29). The auto/auto scheme utilizes the series, diode pair for field discharge during transfer from prime to back-up regulator. The power isolation transformer is again installed to isolate the inputs of the two regulators, since their outputs are commoned at F-. Loading resistors are again used to provide for accurate measurement of the regulator's output voltage.
To provide auto tracking of the two regulators' set points, an RA-70 solid state reference adjuster (with two separate outputs), or a motor operated/manual potentiometer with two pots on a single shaft, can be used. These types of potentiometer arrangements provide for very close tracking of the two regulators' outputs. As one potentiometer is adjusted, the other is automatically adjusted because of the mechanical tie between the two potentiometers. The accuracy of tracking is limited only to the linearity of each potentiometer.
The RA-70 has an advantage over the mechanical devices because it also offers two pre-positioned set points. These set points are preset and, when a contact is input to the appropriate terminal, the RA-70 instantly goes to a value of resistance that may be associated to either off-line rated voltage or a value of resistance that is equivalent to rated on-line VARS out of the generator.
There are auto/auto systems that use the dual voltage adjust potentiometer technique, but they still set the regulator's internal voltage adjust slightly differently. An example would be to set the prime regulator for the nominal voltage and the back-up regulator one per cent lower. This lower voltage can allow for both regulators to be powered up simultaneously. The regulator with the lower regulation point will be turned off because it is trying to bring the voltage down. By being simultaneously powered up, if a problem occurs in the prime regulator, the generator's voltage will fall off until it reaches the regulation point of the back-up regulator. At this point, since the back-up regulator has its power supplies energized, the back-up regulator will start to conduct and the transfer will be virtually undetectable. This scenario assumes the regulator has failed in the "full-off" mode. If the regulator fails in the "full-on" direction, the generator's voltage or Vars will increase drastically. There will inherently be a significant system disturbance until the transfer to the back-up regulator can occur. With this type of transfer arrangement, the bumpless transfer to back-up, for maintenance purposes, is difficult.
Because each regulator is a high gain circuit, very small variations in line voltage can cause drastic swings in the regulator's output. This makes automatic tracking of two automatic voltage regulators very difficult. Some systems simply do not transfer to the back-up regulator while the generator is on-line. The only time a transfer is performed is off-line and on a periodic basis just to verify the back-up regulator is operational. Because the transfer is to an automatic regulator and not a manually adjusted device, the system disturbance is short-lived and usually only as long as the response time of the regulator. This may be on the order of several cycles.
If a transfer to the back-up automatic regulator is performed under load during a failure, the system disturbance can not be forecasted or avoided. If a bumpless transfer to the back-up device is necessary, a manual transfer scheme is shown in Figure 31.
With the addition of the nullmeter and two separate voltage adjusters, a manual transfer scheme can be provided. This system works identically to the operation of the auto/manual system when transferring from manual to automatic mode. In this case, we are transferring from auto to the back-up auto mode.
It is necessary to put load resistors on outputs of each of the automatic devices. This will provide for accurate measurements of the regulator's output voltages. This nulling method is again similar to the synchroscope method except that the operator waits for the nullmeter to read zero or null before a transfer is initiated.
In order to maintain the automatic tracking of the two regulator's set points, two RA-70 devices are used (Figure 31). By providing simultaneous raise and lower signals (via K2), the two RA-70 will raise and lower at the same rate and, therefore, track in their adjustment of the regulator's set point.
Overall system reliability is of extreme importance to the prime power application. Communications requirements with the excitation systems may be made a requirement. This requirement may be as simple as providing a contact input for adjustment or a four to 20 ma signal input for control. The communications may be as sophisticated as master/slave communications through computer language over telephone lines or even fibre optics. These communications will help with the addition or removal of available generation to make the utility grid ultra reliable.
Generator systems vary greatly, and the excitation systems used on them are even more diverse. The end result of a correctly selected excitation system is to have a generator function properly over a long life for every system condition possibility. ET
|
|
|
|
|
LATEST ISSUE
