Merz Price Protection or Differential Protection of An Alternator
Merz price protection is also called differential protection. The Merz Price Protection method is the most common method for protection against faults in the stator winding of an alternator. The Merz Price Protection system is active to keep the alternator fault-free by always checking for faults occurring inside the alternator. The Merz Price method is basically just a version of the differential circulating current protection system.
For alternators, the most serious consequence of a fault is damage to the stator. For this, Alternator Differential Protection or Merz Price Circulating Current Protection System is used which protects the alternator stator from at least 80% burnout.
Even though the alternator is disconnected from the system, the current continues to flow through the fault. So the generated voltage should be suppressed as soon as possible. So this can be done by interlinking the field current with the circuit breaker.
The neutral of the alternator is usually earthed through a resistance. If there is an earth fault in the alternator, excessive current flows through its windings. A resistor is used to limit the value of this current to a safe value. In this condition the entire stator winding connected to the star cannot be protected in the manner described earlier. A zone near the star point will be unprotected.
To overcome this difficulty, two relays are set at the two centers of the alternator, and equalization is maintained by connecting an impedance relay in the third phase. Then the three phases are connected by a star and the earth fault relay is connected from the star point. When there is an earth fault with the winding the earth fault relay is activated and disconnects the alternator from the system through the circuit breaker.
In this system, two rated CT sets are placed at both ends of each phase winding. CTs at both ends are star connected (usually stator windings are star connected) If delta connection, CTs at winding ends are delta and CTs at circuit breaker are star connected.
The phase and neutral of CTs are connected through pilot wires in such a way that they block each other’s current flow. The qualities or characteristics of the CTs should be identical. Otherwise, an unwanted current (spill current) will operate the relay unnecessarily. The inverse time overcurrent relay is connected to the pilot wire. Here the relay is usually connected to the ground of the pilot wire. But many times for the convenience of operation the circuit breaker is provided close to the CT connection.
In this case, a high-quality resistance is connected in series between the relays from the nearby CT to get equal voltage to the relay from both ends, it is called balancing resistance. It is shown in the figure above.
Under normal conditions, the current flowing through the relay is zero because the CTs at both ends are equal. But in abnormal conditions that is, if there is a fault between two phases or between phase and earth, the balance of current is lost in CT at both ends, and circulating current flows in the relay due to unequal current. The relay then operates and trips the circuit breaker.
Merz price protection scheme or differential circulating current protection scheme works on the concept of comparing two currents in and out of the stator coil.
Disadvantages of merz price protection system of Alternator
(1) This arrangement is ineffective if a short circuit occurs between two five turns of the same phase of the stator winding.
(2) This system is not effective for faults. A fault outside the protective zone is called a throw fault.
Differential circulating current protection scheme or Merz Price circulating current scheme is the most popular protection scheme for alternator stator protection. It works on the idea of comparing two currents in and out of the stator coil. Under normal conditions, the two currents will be the same, if a fault occurs there will be some difference, and the Merz Price Circulating Current scheme works by detecting this difference or differential current.