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Detecting Power Grid Synchronization Failure

Tarun Agarwal
Synchronization means the minimization of difference in voltage, frequency and phase angle between the corresponding phases of the generator output and grid supply. An alternating current generator must be synchronized with the grid prior to connection. It can’t deliver the power unless it is running at same frequency as the network. Synchronization must occur before connecting the generator to a grid. Synchronization can be achieved manually or automatically. The purpose of synchronization is to monitor, access, enable, and automatically take the control action to prevent the abnormalities of voltage and frequency.

Rules must be followed for Synchronization:

Voltage Fluctuation:

When a generator is synchronized with a power grid, normally there is a voltage fluctuation on the distribution line. During synchronization the voltage fluctuation should not exceed 3% at the at the point of common coupling.

Synchronization Limits:

The limits allowing for synchronization are

Phase angle- +/-20 degrees
Maximum voltage difference – 7%
Maximum slip frequency – 0.44%
Relays:

For checking the synchronization “synch check relay” must be used. The usage of relays can’t apply for induction generators. The use of synch check relay is to accept as a backup during synchronization and to ensure a generator will not connect to a dead distribution line.

Synchronization of Induction Generators:

For the synchronization of induction generators it just needs to be run up to synchronization speed and connected. For this purpose standard motor controllers will be used. To mechanically drive the generators up to synchronization speed turbine shaft power will be used. The speed of the motors depends on supplied frequency and number of poles on the generators.

Synchronization of Synchronous Machines:

For synchronous generators output waveform should be in phase with the grid voltage waveform or with in specified limits. The rate of change of phase angle between the grid and the machine (generator) must be with in the specified limits.

Some other rules are variable speed drive arrangement to maintain a constant output frequency, inter connection protection between the generator and distribution system.

Failure of Synchronization:

Synchronization circuit may fail for responding to a received input pulse when the received input pulse is shorted than the sampling period of the synchronizer. Then no synchronized representation will take place. When the pulse rate of the input signal is higher than the synchronization rate of the synchronizer then also it may fail to respond. Some times the synchronizer it self may fail by ignoring input events. These all are the circumstances which could create problems if not detected. There are various reasons for failure of power grid synchronization.

Synchronization Failures and their Detections:

There are some situations where the generators and some local loads have become disconnected from main distribution lines. Due to this reduction in quality of supply, and it may prevent automatic reconnection of devices. This is called as islanding. For this reason islanding must be detected immediately and producing power must be stopped immediately.

Due to Islanding following hazards may take place

Generally distributed lines are earthed only at the sub station. When the distributing lines and generators are disconnected the line is not earthed. Due to this reason line voltages may excessive.
The fault level contribution from the grid to sub-station may be lost. This will effect the operation of protection on distributed lines. Due to this sufficient current may not be generated.
Due to Islanding synchronization can’t be maintained. When the grip attempts to reconnect with distribution line it may be out of synchronization at the reconnection point. Due to this, suddenly large power may flows which cause damage for generators, distribution units and consumer products.
Some other disadvantages due to islanding are the voltage levels may go outside of normal operating limits, and the quality of supply may be reduced.

Detection methods of Islanding:

Detection of islanding can be done through active and passive methods. Passive methods look for transient events on the grid and active methods will probe the grid by sending signals from the distribution point of grid. Loss of Mains Protection (LoM) will be designed to sense the disconnection of generators and loads when an island has been created. The most utilized LoM detection methods may fail to detect islanding when the production matches closely with the consumption in the island zone. This blind area is called Non Detection Zone (NDZ). The size of NDZ can be reduced by tightening the LoM setting relays.

Active Methods:

Impedance measurement, Detection of impedance at specific frequency, slip mode frequency shift, Frequency bias, and frequency jump detection methods are some passive methods for islanding detection. The advantage of impedance measurement method is an extremely small NDZ for a single inverter. Slip mode frequency shift method is relatively easy to implement. It is highly effective in islanding prevention when compared to other detection methods.

Passive Methods:

All grid-connected PV inverters are required to have Over/Under frequency protections methods and Under/Over voltage protection methods that cause the inverter to stop supplying power to the utility grid if the frequency or voltage of grid at the point of coupling.

Power
Under/Over Protection for Voltage/Frequency
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These protection methods protect consumer’s equipment and also servicing as anti-islanding methods. Voltage Phase Jump detection, and Detection of voltage harmonics are some more passive methods for detection islanding. The Under/Over voltage protection methods and the Under/over frequency methods are required other than preventing islanding. Several islanding prevention methods produce abnormal voltage, and frequency. Under/Over voltage protection methods and Under/Over frequency protection methods are low cost methods for the detection islanding.

Applications of Power grid failure Detection:

Lighting is one of the main causes for power system faults. The entire power system is consisting electrically of power plants, sub-stations and transmission lines, distribution feeders and power consumers. Detecting the synchronization failure between generators and power grid is the major advantage such as saving the energy. Then we can avoid the loss of power consumption by disconnecting from the power consumption devices.

Power Grid
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When there is an Under/over voltage or under/over frequency, then the comparator will detect the difference the actual power and reactive. If there is no failure of power grid synchronization then the detectors will give the zero values. Based on the Under/over voltage and under/over frequency values the power suppliers will be disconnected if any out of limitation values are observed.