Initial Start-Up
Preparing a generator for parallel operation requires special attention. Before you attempt to parallel units for the first time, check all the units for the following three conditions.
- Same phase rotation
- Same alternating current frequency
- Same voltage adjustment
- Check the phase rotation.
Units that operate in parallel must have the same phase rotation. There are two methods that may be used in order to determine if the incoming unit and the unit that is on-line have the same phase rotation. These methods are listed below:
- Using a phase rotation meter
- Using a set of three light bulbs
Use the procedure below to determine the proper phase rotation by using three light bulbs.
- Connect the light bulbs with rated voltage between the generator leads and the corresponding line phase. For example, connect terminal 1 to line 1 across the open circuit breaker.
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Illustration 1 g00695380
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When servicing or repairing electric power generation equipment:
Make sure the unit is off-line (disconnected from utility and/or other generators power service), and either locked out or tagged DO NOT OPERATE. Remove all fuses.
- Start the units that will be in operating in parallel. Bring the units up to speed. As the units approach the same speed, the lights will start to blink.
- If the lights blink in sequence, one of the units is connected backward. In order to correct the problem, stop the units. Remove generator leads 1 and 3 at the circuit breaker. Exchange these generator leads. This reverses the direction of phase rotation. Terminal 2 should always be connected to line 2. Go to 5.
- Both generators have the same phase rotation when the lights blink in unison. The first condition of "Initial Start-Up" has been met.
- If the lights blink in sequence, one of the units is connected backward. In order to correct the problem, stop the units. Remove generator leads 1 and 3 at the circuit breaker. Exchange these generator leads. This reverses the direction of phase rotation. Terminal 2 should always be connected to line 2. Go to 5.
- Using a phase rotation meter
- Adjust the frequency.
The units that will be operating in parallel must operate at the same speed. Speed is proportional to the alternating current frequency.
- Allow each electric set to run under load for about 30 minutes.
- Adjust the governor control in order to give the rated frequency at full load.
- Remove the load and check the high idle speed. The high idle speed should be approximately 2 to 5 percent above full load speed for governors that are equipped with droop. If these speeds cannot be obtained, contact your Cat dealer.
- For the most consistent results, repeat 2.b and 2.c until the second condition of "Initial Start-Up" has been met.
- Adjust the voltage.The potentiometers should be at the initial settings. Refer to Operation and Maintenance Manual, "Voltage Regulators".
- Preset the unit for parallel operation by connecting the current transformer to 1 and 2. Set potentiometer P1 for quadrature droop in the center position. Apply the rated load. The voltage should drop for 2% to 3%. Switch the positions of the two incoming secondary wires of the current transformer if the voltage increases.
Note: Make sure that the speed droop is identical for all of the engines before adjustments are made to the generator.
- The no-load voltages should be identical for all the generators that are operating in parallel. Connect the generators in parallel. Try to obtain a 0 kW power exchange by adjusting the speed of the generator. Try to minimize the circulating currents between generators by altering the voltage setting with potentiometer P1, or remote potentiometer on one of the generators.
- Apply the available load. The setting is correct only if a reactive load is available. Equalize the Kilowatts or divide the rated power of the units proportionally by altering the speed. Alter the quadrature droop potentiometer (P1) in order to equalize the currents or divide the currents.
Note: Do not change the voltage settings after this step.
Starting Multiple Units
Use the procedure for starting single units in order to start multiple units. Refer to Operation Section, "Single Unit Operation".
Paralleling Multiple Units
Units may be paralleled at no load. Units may also be paralleled with units under load. After the initial conditions for start-up are satisfied, verify for the following requirements:
- One of the governors can be an isochronous governor. Electronic load sharing governors are an exception.
- Generators must have voltage droop compensation or cross current compensation.
- Start the unit which will be paralleled.
- Turn on the synchronizer lights.
- After the engine has run a few minutes, bring the engine up to synchronous speed. This means that the frequency of the incoming unit will be the same frequency as the unit that is on-line. The synchronizing lights will begin to blink.
Note: The frequency of the incoming unit should be slightly greater than the line frequency. The greater frequency will allow the incoming unit to assume some of the load instead of adding to the system load.
- By using the governor control, adjust the engine speed until the lights blink slowly.
- The lights are off when the voltages of the two units are in-phase. When the lights are off, quickly close the breaker while the lights are out.
- Use governor controls in order to share kW load between engines.
- Generator temperature will be stabilized in approximately 1 hour. After the generator temperature has been stabilized, adjust the voltage droop rheostat of each generator. Adjusting the droop will share the reactive load, which will limit the circulating currents. Less droop increases the reactive current that is carried by the generator. Adjusting the voltage droop rheostat in a counterclockwise direction will decrease the droop. Adjusting the voltage droop rheostat in a clockwise direction will increase droop.
Load Division and Speed Droop (If Equipped)
Once two units have been paralleled, the share of the kW load is determined by the governor control setting. If two units of the same capacity and the same governor characteristics have the same governor control settings, the units will share the load equally. The total load must not exceed the capacity of the one engine.
In order to transfer the load from one engine to another engine, use the following procedure:
- Increase the governor speed control of one unit in order to increase the load.
- Reduce the governor speed control of the other unit in order to decrease the load on that unit.
- Raise the governor speed control or lower the governor speed control of both units in order to change system frequency.
Parallel Operation Of Governors
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| Illustration 2 | g00630841 |
Load Sharing Module (Typical Example) | |
The generator set load sharing module provides either the droop load sharing or the isochronous load sharing for parallel applications. The load sharing module has a synchronizing parallel module SPM-A input. The module provides the proportional load sharing. More information is available in the System Operation, Testing and Adjusting, SENR6565, "Generator Set Load Sensor and Generator Load Sharing Module".
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| Illustration 3 | g00637397 |
(1) Droop potentiometer | |
The Speed brick converter changes the analog signal of the speed potentiometer into a pulse width modulated signal. The engine electronic control can recognize this signal. The converter is mounted on the subpanel which is located within the control panel.
Function of The Engine Governor
This section describes the function of the engine governor in relation to load division between parallel electric sets.
The operator must understand two basic facts about load division between generator sets which are operating in parallel.
- The power which is supplied to the generator and to the load is a function of the engine. The setting of the engine governor and the position of the engine governor control determine the amount of power that is delivered by the engine. Therefore, the governor setting and the position of the governor control determine the kW load which is carried by the generator. A change in the engine power of any one of the units in a parallel operation will result in the same change in engine power for each of the other units in that parallel operation. In other words, the units that are in parallel operation will stay in parallel operation.
- The division of power is not determined by generator excitation or terminal voltage. The excitation will determine the power factor for a generator when the generator is operating in parallel with other generators.
Governors that are used with Caterpillar powered electric sets can be of two types:
- Governors with fixed speed droop
- Governors with adjustable speed droop
The values of speed droop which are commonly used are 3 percent and 0 percent. Governors with adjustable speed droop can be adjusted so the characteristics match closely with the characteristics of governors with fixed speed droop. If the governor is adjusted for 0 percent speed droop or isochronous operation, the same speed from no load to full load can be obtained.
Summary on Governor Operation
The preceding discussion of governor operation can be summarized below:
- Each governor should have a 3 percent speed droop in order to provide the simplest combination of governors for electric sets that are connected in parallel. If a constant frequency from no load to full load is required, one governor can be adjusted for isochronous operation. This isochronous unit will be called a "lead unit".
- In order for all paralleled units to accept the full share of the load, the following governor adjustments are required. The governors should have the same full load speed. The governors should have the same high idle speed in the case of governors which are adjusted for speed droop operation. Governor controls should be set to the high idle position so that the full range of the governor is available.
- Operating an isochronous governor in parallel with a speed droop governor requires special techniques.
- Any number of electric sets can be operated in parallel. However, only one governor of the group can be adjusted for isochronous operation. The exception will be some special cases of electronic governors with automatic load sharing.
Stopping
In order to remove a generator from the line, perform the following procedure.
- Check the load. The load must be less than the rated capacity of the remaining units.
- Be sure that the neutral of one of the remaining units is grounded.
- Remove the load from the outgoing unit. See the Parallel Operation, "Load Division - Speed Droop". The amperage may never go to zero due to circulating currents.
- Open the circuit breaker.
- Allow the engine to cool for 5 minutes.
- Stop the engine.
Circulating Currents
Understanding the circulating currents becomes very important when you parallel the units. These circulating currents are flowing between generators in parallel operation. The circulating currents are caused by voltage differences between the generators. The circulating currents are not doing useful work. The amount of the circulating current can be determined by subtracting the amperage which is going to the load from the total generator amperage.
The circulating current may be as high as 25 percent of rated amperes with cold generator sets. Such current may not even be considered harmful. The total generator current should not exceed the amperage rating.
As the generators warm, the circulating currents will decrease. The ammeter readings should decrease slightly, but the voltage meter readings should remain constant.