1. ATS, the automatic transfer switch, is to select one of two powers to load device. Each power supply is interlocked by electricity and machinery, ensuring only one power supply at any time.

2. ATS Configuration

ATS has the following conditions:

1) AC contactor type

The AC contactor with mechanical interlocking adopted. This type has small breaking capacity, fit for small capacity with protection for power supply inlet terminal.

2) Breaker type

Two air breakers and mechanical interlocking adopted. As the air breaker has short circuit and over-current protection as well a large breaking capacity, it can be directly transferred load from power supply.

3) Transfer switch type

Transfer switch adopted, means that integrates two automatic control separating switches, with large breaking capacity but without protection, fit for protection with power supply inlet terminal.

3. ATS Control

The control of ATS has the following conditions:

1) ATS controller type

The special ATS controller controls the ATS and provides the special communication agreement.

2) Genset controller type

The genset controller with ATS control function adopted.

3) Voltage relay type

Voltage relay and medium relay is adopted to monitor and logically control the main power supply and standby power supply.

1. Paralleling Theory

The set in parallel means that several generator sets are in operation, the power provided by each generator set is connected to a single power to load. In modern generation plant, multi synchronizing generator sets are always paralleling, each plant is connected to a strong power supply system. Many diesel generator sets (maybe different brands and types) are paralleled to charge with load, which find favors in eyes of more and more companies home and abroad with standby power supply. On the basis of the existing diesel generator sets, only increase parallel cabinets can make up the standby power grid in full use of the primary diesel generator sets.

2. Advantages of grid operation

1) The disadvantages of single genset is obvious, which doesn’t not only ensure the quality (stability of voltage and frequency), but also achieve the flexibility and economy of power supply. However, these disadvantages can be improved by parallel operation.

2) The power grid is comprised by several generators or plants in parallel. In modern generation plant, several synchronous generators are paralleled to be connected to the common Bus. Several plants are paralleled to compose a strong power grid in a district. In order to ensure the power supply when mains supply fails, some large companies or factories often adopt many diesel generator sets to be paralleled, forming a small emergency power grid, which enhances the electrical reliability. As the gensets are increased, the electrical quality is high, that is to say, the voltage and frequency is more stable.

3) Power grid supply has many advantages compared to single power supply:

�?Enhance the reliability of power supply. That, a generator breaks down or carries out regular check doesn’t result in power failure.

�?Achieve the flexibility and economy of power supply. For instance, when the hydropower plant and thermal power plant are paralleled, they could generate power with adjustment under dry period and wet period, making full use of water resource. In peaks and troughs, the number of generators into power grid is flexibly decided, with the increase of generation efficiency and supply flexibility.

�?Enhance the quality of power supply. That, the capacity of power grid is huge (compared with single generator or some load), single generator is start or shutdown as well as some load changes shows little influence on power grid. The voltage and frequency that judges the quality of power supply may be considered to be a constant. The power grid can be considered to be infinite power grid or BUS to single generator. After the synchronous generator is paralleled to power grid, its operation condition is limited by power grid, that is to say, the voltage and frequency are in keeping with power grid, without alone changes.

3. The disadvantages of parallel operation

1) The synchronizing operation technology is more complicated;

2) The number of maintenance equipment is large, with lots of works;

3) The consequence of operation is serious. The lesser will cause impact current and impact moment, resulting in failure shutdown and power supply interruption. However, the more serious will result in generator winding deformation or engine weary for overlarge impact movement.

As all kinds of semi-automatic and full-automatic synchronizing parallel are adopted, the technological problems of parallel operation have got resolved better.

4. Paralleling conditions

1) same sequence 2) same frequency 3) same voltage 4) same voltage phase 5) same waveform

    Only the above five conditions are satisfied, can the parallel of generator sets be free of impact. Meanwhile, the sequence and waveform are satisfied in design and installation, the frequency, voltage as well as phase needs to be mainly checked in parallel operation.

5. Paralleling methods

I. Asynchronous paralleling method

      1) Self synchronous paralleling method

      2) General synchronizing paralleling method

II. Quasi-synchronizing paralleling method

      1) Manual quasi-synchronizing paralleling method

      a、Lights quenching method

      b、Lights rotation method

      c、S synchronoscope method

      d、MZ-10 combined synchronoscope method

      2) Semi-automatic quasi-synchronizing paralleling method

      3) Full-automatic quasi-synchronizing paralleling method

Asynchronous paralleling method

1) Self synchronous paralleling method

   The generator speed is adjusted to or higher than the synchronizing speed of power grid in parallel. Without excitation, that is, close is put into parallel operation, then excited voltage is added, making generator like synchronizing electro-motor be put into synchronization. This is called self synchronous paralleling method.

The disadvantages are that the impact current is large, 3.5~5 times of rated current, the voltage drops to 30%~40%, affecting the operation of power grid seriously. It is rarely adopted now.

2) General synchronizing paralleling method

   This parallel operation is called general synchronizing paralleling method, also known as resistance synchronizing paralleling method. The generator speed and voltage is adjusted to technical data of power grid in parallel. Then, the generator connected resistor in series is switched on power grid. After the delay of 4~8s, when the generator is put into synchronization and the main breaker is closed as the impact current is small, the resistor is removed, then the parallel operation is completed.

The disadvantages are that there is asynchronous process in parallel, and the impact torque on the revolving shaft, increasing resistors, with some impact current, impact torque, large equipment as well as complicated wiring.

Quasi-synchronizing paralleling method

1) Manual quasi-synchronizing paralleling method

   That, the voltage regulation, speed regulating, the instance of least phase angle, the control of generator main switch as well as all manual operation is called manual quasi-synchronizing paralleling method. There are several methods of monitoring voltage difference, frequency difference and phase difference, as follows:

    a、Lights quenching method

    b、Lights quenching method

    c、S synchronoscope method

    d、MZ-10 combined synchronoscope method

The disadvantages are the highly technological level of operators and low safety.

2) Semi-automatic quasi-synchronizing paralleling method

   The voltage regulation and speed regulation of generator are manually adjusted, while the capture of the least phase angle instance and the control of generator main switch close are adjusted automatically, which is named as semi-automatic quasi-synchronizing paralleling method.

  The adopted semi-automatic program is mainly adopted synchronizing relay to finish monitoring phase and closing main switch automatically at present. The voltage regulation and speed regulation is adjusted by voltage meter, frequency meter and synchronoscope.

3) Full-automatic quasi-synchronizing paralleling method

   The generator voltage regulation, speed regulation, the capture of the least phase angle instance and the control of generator main switch close are all adopted automatic equipment, meanwhile, the active power and reactive power of parallel operation are both automatically controlled, which is called full-automatic quasi-synchronizing paralleling method. This program adopts module parallel operation, that is, the automatic synchronizing module makes the generator be paralleled to the Busbar, meanwhile, the automatic load share power meanly distributes load according to the ration of genset capacity and percentage, providing auto peak regulation.

6. Type of parallel cabinet:

   1) Semi-automatic parallel cabinet

   2)  Full- automatic parallel cabinet