12510 Permissive Relaying

It is assumed here that the decision to operate and the starting of the prime mover are part of the control strategy in place. This discussion will center on the relay scheme. For successful parallel operation to result, the DG unit must be brought into synchronism with the grid-derived source. If the incoming source is an inverter or induction generator, synchronizing is quite readily achieved. Inverters' output circuits are connected to the grid and their solidstate switching circuitry is controlled to begin taking on load. For induction generators, a speed-sensing control initiates connection of the generator when it approaches synchronous speed. Upon connection, a loading control increases fuel flow to the prime mover. As the fuel flow increases, speed exceeds synchronous speed and the generator begins to produce output power. When this power flow reaches the desired level, fuel flow is held constant. For these two scenarios, the permissive relaying is built into the controls and usually consists of Dev. 27. The objective is to preempt the operation unless the grid-supplied voltage is acceptable. Since neither of these sources is capable of overpowering the grid, Dev. 81 is not required.

Where the power source is a synchronous generator, the permissive relay scheme will include a synchronizer or sync check relay, Dev. 25. The synchronizer will adjust the speed of the DG unit to closely match that of the grid, typically within 0.2 Hz, and will also adjust the voltage of the DG unit to match grid voltage, typically within 5%. When the frequency and voltage differences are acceptable, the synchronizer then reduces the phase angle difference between the two sources to an acceptable value, typically less than 5° (electrical). At AV = 5%, the voltage of the grid would be at 1.0 p.u. and that of the DG unit would be at 0.95 p.u. At AF = 0.2 Hz, the speed of the angular difference would be 72° per second. Switching equipment used in this application would likely have a closing time of 5 cycles, 84 milliseconds, or less. Given these conditions and a closing initiated when crossing 5° going toward zero, the contacts of the closing device would meet when the angle was 1.04° past zero. Using the law of cosines to find the value of the resultant vector and given a subtransient reactance of 0.1 pu, the maximum transient current exchange could not exceed 0.53 pu. However, because the generator is excited at the no-load value, the synchronizing current would be considerably less than 0.53 pu. Thus, system disturbance would be minimal, if distinguishable at all. Typically, a sync check relay would monitor relative phase angle only and not provide any matching. Consequently, when a sync check relay is used, it is recommended that Devs. 27 and 81 be provided on both the grid-and DG-derived buses.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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