Overload test

I. Overview of ship generator overloads and short circuits

In marine power systems, smaller overcurrents are generally referred to as overloads; severe overcurrents (e.g., short circuits) are referred to as overcurrents only. Putting1.1 times the rated current of the generator is called a small overload.；1.5 times the rated current of the generator is called the larger overload；A short circuit is called a severe overload。

The main cause of generator damage is the heat and electromotive force generated by short-circuit current, and the heat generated by prolonged over-current operation will cause insulation aging. The permissible operating current range of the generator is mainly determined by the current flowing through the stator armature windings.

Generator heat is related to the size and duration of the current, that is, determined by the thermal capacity of the generator. Generator overcurrent characteristics are: in 1.1 times the rated current of the generator allowable operating time does not exceed 2h; in 1.5 times the rated current of the generator does not exceed 5min. generally from the point of view of the safety of the provisions for:At 1.1 times the rated current of the generator, the permissible running time shall not exceed 15min; at 1.5 times the rated current of the generator, the permissible running time shall not exceed 2min.

The protection of the generator in stand-alone operation is mainly overcurrent protection. The generator can withstand the current overload, the current in 1.1 times the rated current of the generator allows running time for 2h, in 1.25 times the rated current of the generator for 30 min, in 1.5 times the rated current of the generator for 5min. active power overload is mainly determined by the type of the prime mover, the diesel engine to withstand 1.1 times the rated power to allow the running time of 2h; in 1.2 times the rated power of 30min; in 1.35 times the rated power of 5min. so from the generator itself allows a certain amount of time overload. So from the generator itself, it is allowed to overload for a certain period of time without requiring the main switch to trip immediately.

Second, the ship generator external short-circuit reason judgment and exclusion

(i) Overview of external short-circuit protection for ship's generators

Generator external short-circuit fault on the generator and electrical equipment, so the occurrence of short-circuit fault, the protection device should be quickly acted. However, in order to realize the selectivity of protection, a certain delay is also given. For generator external short-circuit protection, China Classification Society, "steel seagoing ship classification specification" made the following provisions: for the ship generator external short-circuit protection should be equipped with short-circuit short delay and short-circuit instantaneous action protection. When the short-circuit current reaches 2~2.5 times of the rated current, the protection device will delay 0.2~0.6 s to make the generator trip automatically; when the short-circuit current reaches 5~10 times of the rated current, the protection device should act instantaneously to make the generator trip automatically.

The most important issue of short-circuit protection for ship power grids (the ability to automatically remove the fault when a short-circuit occurs in the power grid) is the selectivity of the protective device, that is, when a fault occurs, the protective device only removes the faulty portion and does not cause the previous level of the protective device to operate. This ensures that other equipment that is not faulted can continue to operate normally.

In order to realize grid selective protection, it is usually possible to press thethe time principle和current principlePerform the calibration:

• The principle of time:It means to realize selective protection by the difference of the setting value of the action time of the protection devices at all levels. The action time should be ensured to increase step by step in the direction from the power-using equipment to the power source. In order to shorten the duration of the fault as much as possible, the closest switching time of the power equipment should be as short as possible, in order to achieve both rapid removal of faults, but also to ensure that the front and back of the two levels of protective devices have the purpose of selective action, and the key lies in the correct choice of the front and back of the two levels of protection action time difference. The key lies in the correct selection of the time difference between the front and rear protective actions. The general time difference for ship power grids is 0.15~0.58 s.
• Current Principle:It means to realize selective protection by the difference of the setting value of the action current of the protection devices at all levels. The action current should be ensured to increase step by step in the direction from the power-using equipment to the power source. The closer to the power supply, the higher the action current. The advantage of using the principle of selective protection according to the current is that the action is rapid in the event of a short circuit. Its action time depends only on the inherent action time of the protective device, usually about 0.1 s. Its disadvantage is often subject to the limitations of the switch current-breaking capacity, and is susceptible to interference by external factors, the inter-level coordination is also more difficult, so it is often used in the capacity of the ship power system.

Short-circuit protection of the ship's power grid follows a comprehensive principle that includes the time principle and the current principle. In the main distribution board and emergency distribution board on the feeder switch, generally not set up with delayed short-circuit protection, but more use of the current principle of the device type automatic switch, its electromagnetic instantaneous release time up to 0.02 ~ 0.05 s. In the power grid of the last level (the closest to the level of the power equipment), can be used to fuse on the end of the grid and the generator for short-circuit protection.

(ii) Analysis of the causes of external short-circuit faults in ship generators

The main causes of external short circuits in marine generators usually include the following:

1. Cable insulation damage
Long-term use of aging: long-term operation of the cable, the insulation layer aging, resulting in phase-to-phase or phase-to-ground short circuit.
Moisture or salt spray erosion: the ship environment is humid, the cable insulation layer absorbs moisture or is corroded by salt spray, resulting in leakage and short circuit.
Mechanical damage: the cable is subjected to external forces of extrusion, vibration or scratching, resulting in insulation layer breakage, forming a short circuit.

2. Short-circuiting of load equipment
Motor winding short circuit: motor stator or rotor winding insulation is damaged, resulting in a short circuit between phases or turns.
Heating equipment failure: electric heating equipment (such as boiler heaters, electric heaters) due to insulation damage or aging short circuit.
Short circuit of power distribution equipment: wrong wiring or insulation failure inside the distribution box and control cabinet, resulting in a short circuit.

3. Failure of electrical components
Circuit breaker, contactor burnout: excessive current or aging of equipment, resulting in contact ablation and short circuit.
Loose switches and connecting terminals: Vibration or long-term operation leads to loose terminals, triggering phase-to-phase or phase-to-ground short circuits.
Capacitor and transformer failure: short-circuiting of filter capacitors or transformer windings, resulting in generator tripping.

4. Dampness or contamination
Humid environment: Marine electrical equipment is in a high humidity environment for a long period of time, the insulation performance decreases and it is easy to short-circuit.
Accumulation of grease and dust: Conductive dirt accumulates inside distribution cabinets and junction boxes, leading to short circuits.

(iii) Determination of external short-circuit faults in ship's generators

1. Preliminary judgment
For power stations with automatic station management systems, when a generator main switch trip occurs and the main grid loses power, and there is no response from the nacelle other than the alarm and the alarm indicates a short-circuit fault, it means that an external short-circuit fault of the generator has occurred.
For conventional power stations, when the generator main switch tripped, and this trip does not occur at the same time to start several large loads, not in the use of the ship's cargo lifter for loading and unloading operations, not in the first occurrence of a drop in speed and then the main switch tripped, not in the first occurrence of a drop in voltage and then tripped (from the brightness of the lamps can be discerned), it can generally be judged to have occurred in the generator outside the short-circuit fault. Generator external short-circuit failure, but also does not exclude the operation of personnel errors, such as generator parallel operation improper generator current reaches the short-circuit protection setting value, may also be caused by the main switch itself fault trip.

2. Specific judgments
When an external short circuit occurs in a ship generator, an abnormal increase in current and tripping of the circuit breaker usually occur. In order to accurately determine the location and cause of the fault, the following methods can be used:
(1) Observe fault phenomena: generator tripping, alarm indication (short circuit alarm from PMS), smoke/burning of cables or equipment.
(2) Circuit breaker step-by-step investigation: after the generator no-load operation to observe the normal, one by one close the branch switch to observe which one is tripped after closing the circuit breaker.
(3) Insulation resistance test: Use an insulation resistance tester to measure the relative ground and phase-to-phase resistance, and there is a risk of falling below 1 MΩ.
(4) Equipment check: look for cable condition, load equipment, and circuit breaker burnouts or contacts.

(iv) Troubleshooting of external short circuits in ship's generators

After locating the short-circuit point by the above method, repair it by the following measures:
1. Cable repair: Replace damaged cables, damp cables can be dried by electric heating or hair dryer.
2. Maintenance of equipment: repair or replacement of faulty equipment and cleaning of contaminated distribution boxes.
3. Tighten loose wiring: Ensure that connections are secure and that a thermal imager can be used to detect abnormal warming.
4. Replacement of damaged parts: Replacement with new circuit breakers or fuses and readjustment of protection parameters.

(v) Prevention of external short-circuit faults in ship's generators

Regularly check the insulation, reasonably use the protection device, keep the equipment clean, and avoid overloaded operation of the load.

(vi) Practical training operation of external short-circuit faults of ship's generators

(1) Use the turbine simulator to set the point of failure, as shown in Figure 6-3, the main circuit of the main seawater pump is short-circuited, and the short-circuit point is at the main seawater pump circuit breaker inlet, and the main seawater pump distribution switch is not able to be de-energized.

Figure 6-3 Main switch protection device parameter setting interface

(2) Observed phenomena: generator main switch tripped, generator abnormal alarm.
(3) Analysis: After eliminating the factors of misoperation, overload and undervoltage, it can be assumed that a short circuit has occurred.
(4) Find the short-circuit point: after removing all the load, close the load air switch one by one, and the generator trips when closing the main seawater pump circuit to determine the external short-circuit point. At the same time to determine the main seawater pump distribution switch itself has a fault or time parameter setting error.
(5) Troubleshooting: Remove the equipment and restore power to the grid. Repair the main seawater pump distribution switch and short circuit point.

III. Judgment of causes and elimination of ship generator overload faults

(i) Overview of generator overload protection on ships

The principles of generator overload protection for ships are: on the one hand, to protect the generator from damage; on the other hand, to ensure that the power supply is not interrupted as far as possible. Therefore, generator overload protection is widely usedAutomatic graded unloading protectionThat is to say, after the generator is overloaded, the automatic graded unloading device will first unload part of the secondary load to eliminate the generator overload phenomenon and issue an alarm signal. If the overload cannot be removed within a certain period of time, in order to protect the generator from being damaged, the overload protection device should issue an automatic generator overload trip signal to remove the generator from the busbar.

For generator short-time overload (overload caused by large motor startup, multiple motor startup at the same time and short-circuit at the remote end of the power grid), the protection device should avoid such short-time overload, i.e., the overload protection should have a certain time-delay characteristic. For generator overload protection, the "steel seagoing ship classification specification" stipulates that: for the generator without automatic grading unloading device, when the overload reaches 125%~135% rated current, the protective device delays 15~30 s action, so that the generator automatically tripped; for the generator with automatic grading unloading device, when the overload reaches 150% rated current, the protective device delays 10~20s action, so that the generator automatically tripped. ~For generators with automatic grading unloading device, when the overload reaches 150% rated current, the protective device will delay for 10-20s to cause the generator to trip automatically.

Ship generator overload protection is generally realized by the overcurrent striker in the automatic air circuit breaker. Priority overcurrent relay action current setting is based on the generator overload protection of long-delay setting current. For example, the rated current of a ship generator is 770 A, and its priority overcurrent relay is set to 90% of the long-delay overcurrent detector setting current, then: long-delay setting current = 770 × 1.1 = 847 A, priority overcurrent detachment setting current = 847 × 0.9 = 762.3 A, therefore, the priority overcurrent detachment setting current is 99% of the generator's rated current. priority overcurrent detachment The setting of the delay time not only requires that the setting of the action current of the overcurrent relay and the generator overload protection of the long delay current are coordinated with each other, but also requires that the setting of the delay time is well coordinated. In the actual design, the delay time of the long-delay tripper is usually set to 15-30s, so the delay time of the overcurrent relay for priority tripping should be set to less than 15s.

Priority cut off the non-important load, usually according to the nature of the load, the size of the power to adjust. For example, a container ship is divided into 2 levels of disconnection: the first level cuts off mechanic tools, kitchen equipment, water machine, etc.; the second level cuts off the power supply of refrigerated containers. How much load is prioritized depends on the number of generators running in parallel and the load rate.

(ii) Analysis of the causes of ship generator overload failure

Ship generator overload fault refers to the generator's output power exceeds its rated power, resulting in excessive current, temperature rise, and even trigger the protective device tripped. The following are the common causes analyzed:

1. Increased loads leading to overloads
(1) The total load power exceeds the rated power of the generator: new high-power equipment or multiple equipment started at the same time. For example, a bulk carrier loading and unloading operations, sudden load caused a voltage drop tripped.
(2) Abnormal operation of load equipment: motor blockage, short circuit or aging of electrical equipment. For example, the ballast pump bearing of a chemical ship was damaged, resulting in the current exceeding 35%.
(3) Inadequate reactive power compensation: low power factor operation leads to an increase in total current.

2. Reduced generator output capacity
Failure of the excitation system, deteriorated or damaged windings, bearing or cooling system failure, and AVR failure (resulting in low voltage and increased current).

3. Operational environmental factors
High ambient temperatures, high humidity or salt spray erosion and mechanical vibration cause wiring to loosen.

4. Control system issues
(1) Uneven load distribution: when several units are connected in parallel, one unit takes too much load. For example, an oil tanker is overloaded by another unit due to an abnormal governor.
(2) Malfunctioning of the protective device: Setting value too low.
(3) Wrong parameter setting: e.g. PMS fails to automatically start the standby machine causing the running machine to overload and trip.

(iii) Determination of ship generator overload faults

Generator overload caused by the main switch tripped, generally occurs in a single machine running high load start large loads, multiple large loads started at the same time or parallel operation when the graded unloading failure and other occasions. Judgment methods are as follows:

• (1) Intuitive judgment: Observe the abnormal heat, burning smell and control panel alarm.
• (2) Instrument measurement: Check for abnormal voltage drop and active power excess.
• (3) Load analysis: whether there is any new high-power or simultaneous start-up equipment.
• (4) Test method: gradually reduce the load to observe the current change, or replace the parallel generator test.
• (5) Diagnostic Log: View the PMS history.

(iv) Troubleshooting of generator overloads on ships

Adjustment of load distribution, optimization of operation mode (grid connection), checking of thermal environment and calibration of power system control settings.

(v) Preventive measures against overloading of ship generators

Regular measurement of parameters, rationalization of loads, installation of PMS systems and regular maintenance of equipment.

(vi) Ship generator overload protection practical training operation

Overload protection (long delay time release) is usually simulated by using a main switch calibrator and placing the switch in the “test” position. The procedure for using the turbine simulator is as follows:
(1) Setting the point of failure: When a single machine is running at a high power, turn on another large load.
(2) Observed phenomenon: main switch tripped, abnormal alarm.
(3) ANALYSIS: Identify the priority release indication and determine that it is an overload.
(4) Troubleshooting: Directly close the gate to restore the critical load. Turn on the parallel into the standby to distribute the load before turning on the large load.