Abstract
This paper analyzes the importance of generator synchronization to the power grid, the required conditions to perform this procedure and the consequences of imprecise synchronization. For a generator to be synchronized to a grid, the voltage magnitude, frequency, phase sequence and phase angle must match. Manual synchronization depends on an operator to modify generator voltage and frequency, confirm phase sequences of generator and grid, and monitor phase angle difference to timely initiate breaker closure. Automatic synchronizers perform all these function through a computerized system that consists of complex algorithms and control systems, and are categorized in phase lock type and anticipatory type. Comparisons between methods and description of new developments are also covered in this paper.
Introduction
Generators are rarely used to independently supply power to a specific load, as they are most commonly connected to power grids composed of numerous generators. This allows for a combined power output that is able to cover a larger demand, increases system reliability and improves flexibility of maintenance and repair operations. Generators connected to the same grid are considered to operate in parallel; thus, in order to add a new machine to the network, careful consideration must be given to the synchronization of its operational parameters to the grid.
According to Chapman [1], the conditions that must be met to achieve the proper synchronization of the generator to the grid are equal voltage magnitudes, phase sequences (in three-phase systems), phase angle and frequency. Failure to assure that these parameters are in sync before connecting the generator may result in severe damage to the machine due to excessive mechanical stress as well as disturbances to the power system.
This paper will analyze the theory behind generator synchronization, the different methods used to perform this procedure, compare the approaches and present possible improvements.
Voltage Magnitude
The sinusoidal wave that describes the generator’s voltage must have a magnitude equal to the grid’s to avoid the establishment of a large undesired flow of reactive power. Thompson [2] states that if the generator voltage is lower than system voltage, it will fall into a condition of under-excitement and absorb VARs to increase its output, which may cause voltage dips to the system if it is unable to supply this demand. Conversely, if the generator’s voltage magnitude is higher than the system’s, it will supply VARs to the grid.
Phase Sequences
For the two voltage groups to be identical, besides being of equal voltage magnitude, they must also have the same phase sequences (abc or acb). If they differ, one pair of voltages will be equal but the other two will be dephased by 120°, which will subsequently generate large currents through these phases, potentially causing damage to the machine.
Frequency
The frequency of the voltage induced in the generator’s stator must closely match the grid frequency. If this condition is not met before connecting the machine, the generator will be out-of-step with the power system and the rotor will change speed to match the system. If the difference is too large, the rotor and stator would be slipping poles, and significant transient torque would be generated to accelerate or decelerate the machine, which may cause long-term or instantaneous damage to it due to excessive stress.
Phase Angle
According to Csanyi [3], “the phase angle between the voltage produced by the generator and the voltage produced by the grid must be zero”. The generator’s and grid’s voltage groups may have the same magnitude and phase sequence, but they must be in phase at the moment of connection, otherwise the voltage magnitude differences will generate currents that may be larger than the winding’s rating, thus potentially damaging them.
Synchronization Standard Requirements
IEEE Standard for Salient-Pole 50 Hz and 60 Hz Synchronous Generators and Generator/Motors for Hydraulic Turbine Applications Rated at 5 MVA and Above [4] and the IEEE Standard for Cylindrical-Rotor 50 Hz and 60 Hz Synchronous Generators Rated 10 MVA and above [5] specify the maximum tolerable voltage, frequency and phase angle differences for synchronization, which are:
Voltage: 0 – 5% magnitude difference.
Phase Angle: ±10 degrees.
Frequency Slip: ±0.067 Hz
It is should be noted that the standards recommend to limit the frequency slip to the positive range, 0 to +0.067 Hz, to reduce transient torque when the generator is connected to a grid with higher frequency.
Methods to Synchronize a Generator to the Grid
Manual Synchronization
Synchronization of a generator to a power grid can be performed by a trained operator who has access to a synchronizing panel that allows him to visualize the different system variables involved in the procedure. Through this method, the operator manually adjusts the field current of the exciter circuit until the voltage at the generator’s terminals matches the system voltage magnitude. Additionally, the operator modifies the prime mover speed, therefore altering the output voltage system, to make it match system frequency.
Before proceeding to close the corresponding breaker, the operator must make sure the phase sequences of both generator and system are the same. To do this, he can connect a small induction motor to the generator terminals, observe the direction of the rotor’s rotation, and repeat the procedure connecting the motor to the grid using the same phase order; if the direction of rotation is the same, than the generator and grid phase sequences are equal. Similarly, three lamps can be connected at the terminals of the breaker that links the generator to the system, and sequence can be determined by observing the lamp’s behavior: if they appear bright and turn off at the same time, the phase sequences are equal.
After it has been confirmed that voltage magnitude, frequency and phase sequences match, the operator can proceed to close the breaker when the generator and system voltages are in phase. A synchroscope is used to determine the correct instant for switch closure. According to Patil, More, Magar and Kamble [6], when the synchroscope’s pointer is stationary and pointing upwards, the alternator and the grid are synchronized, thus can be connected. The synchroscope has replaced previous method of verifying phase angle, such as the bright lamp and dark lamp methods. The dark lamp methods consists of connecting a lamp in one phase, and cross-connecting another two lamps. The lamps bright up in rotation indicating phase angle or frequency differences between the grid and the generator. When the key lamp is off and the other two are turned on and show equal brightness, synchronization has been achieved and the circuit breaker may be closed. This method, though rudimentary, has proven to be effective.
Automatic Synchronization
This method replaces the operator for a computerized system that can perform all the required adjustments (voltage magnitude, frequency, etc.) to synchronize the generator to the grid through the use of control systems that can alter the parameters of the AVR (Automatic Voltage Regulator) and governing unit as well as prime mover speed. Automatic synchronizing devices have incorporated synchroscopes that, along with phase angle prediction algorithms, calculate the exact moment for appropriate circuit breaker closure. These panels are generally composed of four output contacts through which pulse width signals are sent to the exciter circuit, in order to modify voltage and frequency. Control variables are used to reduce error to acceptable levels.
Synchronism-check relays monitor the synchronization in automatic mode (and manual mode, also available in these devices) by calculating the time frame during which the phase angle difference is tolerable and the breakers can be closed without compromising the machine nor the system. Advances in technology are evident in the many different relay options currently available in the market from several manufacturers. For instance, General Electric’s Synchronism Check Relay [7] allows to set desired frequency slip, phase angle and voltage magnitude differences through a screen interface. Moreover, automatic synchronizing panels typically have voltage relays as well (used to further monitor the closing command), phase sequence indicators, frequency meters, voltmeters, ammeters, indicator lamps and push buttons.
Automatic synchronizers can be divided in phase lock type and anticipatory type. The former calculates the time window for voltage and phase angle acceptance and automatically commands a relay to close the contacts that energize the breaker coil, thus initiating the closure operation; according to the Basler Electric Company [8], these synchronizers operate on the principle of providing correction signals to the governor and AVR until the waveforms are matched in phase and magnitude. The latter functions in a similar way to the phase locked type, but also has the capability to initiate breaker closure in advance of phase coincidence to further approximate the phase angle difference to 0° by taking into account the breaker closing and relay operation time, through slip measurement and calculation of the advanced angle required to compensate for these delays. The advanced angle is calculated using the following equation, where TCLS is the delay for the circuit breaker closure operation:
An equivalent and simplified formula follows:
Where TB represents the circuit breaker closing time, TR is the delay caused by relay response time and FS is the measured frequency slip.
Comparison of Manual and Automatic Methods
Automatic synchronizing technologies were developed to reduce human error in the connection of offline generators to power grids, given that effectiveness of the procedure in manual synchronizing is highly dependent on the operator’s experience, which can greatly vary, thus introducing an undesired factor of uncertainty to the process. The operator may wrongly adjust voltage magnitude and frequency, or may not promptly initiate breaker closure which may cause a generator-grid connection with a phase angle different than zero.
Anticipatory type automatic synchronizers calculate the advance angle required to compensate for frequency slip and equipment delays when closing the breaker, through a series of algorithms and control variables. In contrast, highly experienced operators may how fast the phase angle gap closes (by observing the synchroscope) and energize the breaker coil accordingly in order to approximate the 0° angle difference.
There is also a hybrid manual-automatic method in which the operator performs all the necessary adjustments, but the breaker closure operation is monitored by synch-relays, which do not allow the connection if the synchronizing conditions are not met. This is typically used in application where the need for synchronization is occasional and it is not practical to install a completely automated synchronization panel.
According to Upadhyay and Yadav [9], “automatic synchronization is much preferred because successful manual synchronization requires a highly skilled operator and unsuccessful synchronization can easily damage equipment on the grid”. An approach used by many industries that rely on manual synchronization is the use of simulators to improve the operator’s performance.
Improvements in Synchronizing Technology
Thompson and Ravikumar [10], describe new developments in synchronizing technology that can improve operator performance in manual synchronization as well as advances that can reduce cost and simplify design for automatic synchronizing devices. Among these, the synchrophasor stands out: a device which measure phase angle differences and synchronize them with universal time references, allowing to better understand the system’s nature by displaying real-time data throughout the power system, with an average accuracy of ±100 nanoseconds. Thompson and Ravikumar [10] also affirm that this information helps the operator to better control and monitor the system.
A study conducted by Seeley, Craig and Rainey [11] showed that the implementation of synchrophasor communications allowed for a group of generators to be in permanent synchronization with a reference source, significantly improving generator control flexibility.
Other important developments in the field are the establishment of communication links through fiber-optic, which allow the operator to remotely monitor the system and also reduces overall panel cost, and the inclusion of event recording functions in modern synchronizing panels that store valuable information, which can be used to analyze event if faults occur do to improper synchronization.
Conclusions
The connection of a generator to a power grid is a procedure that requires the cautious revision of system parameters. Synchronization requires matching of voltage magnitude, frequency, phase sequence and phase angle of the generator and network. If these conditions are not met, the connection could result in high flow of reactive power and potentially-damaging mechanical stress for the generator.
The synchronization procedure can be realized either manually or automatically, the latter being preferred due to the reduction of human error in the process. However, manual synchronization is still used in systems that were created before the automatic technology was developed and in applications where the need to synchronize generators to the grid is infrequent. In manual synchronization, it is the responsibility of the operator to alter the voltage and frequency variables required for the synchronization, and determine the appropriate time for breaker closure. For this purpose, the operator can use archaic methods to calculate the moment of 0° phase angle moment such as the dark lamp method or, he can rely on the use of a synchroscope. On the contrary, automatic synchronization devices perform all these functions through computerized systems.
Advances to improve the safety of synchronization operations are in constant development, as engineers design more sophisticated panels that are cost-efficient and allow for simpler monitoring of system variables. More studied need to be conducted to further develop control systems.
References
