Parallel Operation of DC Generators

Large power systems usually use several synchronous generators on an AC network. This page instead covers DC generators connected to a common DC bus. DC machines do not need phase or frequency synchronisation before connection. The incoming generator needs correct polarity and a terminal voltage close to the bus voltage. A voltage mismatch can drive a large circulating current, so the connection also needs suitable switching and protection.

Connection of Parallel DC Generators

1. Generators in a power plant can feed heavy conductors called bus-bars. Connect each positive generator terminal to the positive DC bus and each negative terminal to the negative bus. Confirm the polarity before closing a generator breaker.
2. Keep the incoming generator disconnected from the bus while its prime mover reaches the required operating speed. Close switch S₄ only in the sequence defined for the machine’s field circuit and starting equipment.
3. Use the circuit breaker as the generator’s controlled bus connection. With the incoming generator disconnected, connect instrument V₂, the voltmeter, across the open switch S₂ as shown. Adjust generator 2 excitation with the field rheostat until its voltage matches the bus magnitude with the correct polarity.
4. Close the main switch S₂ after the polarity and voltage checks pass. If generator 2’s generated EMF exactly balances the bus voltage, it carries nearly zero current and is said to be floating on the bus.
5. Increase generator 2 excitation gradually so its generated EMF rises enough to supply the required share of bus current. Adjust the other generator excitation as needed to keep the bus voltage within its permitted range. Keep both machines within their current, commutation and thermal ratings.

Ignoring brush drop in the simplified relation, generator output current I is given by Where, R_a is the armature-circuit resistance, including the relevant armature winding resistance.

Load Sharing of Parallel Connected DC Generators

In a DC power plant, load sharing follows each generator’s generated EMF and external voltage-current characteristic. A synchroscope is an AC synchronising instrument and does not control DC generator sharing. For two generators connected to the same bus voltage, their simplified current relation isThe current supplied by each machine depends on E₁ and E₂, its armature-circuit resistance and any brush drop. Adjust the field rheostats or automatic voltage regulators gradually while monitoring bus voltage and each machine current.

Advantages of Parallel Operation of DC Generators

Cost Effectiveness:- Running only the units needed for the load can keep each machine nearer an efficient operating range. The operating plan must still account for start-up costs, reserve capacity and the Electric power demand profile.

Smooth Supply of Power:- Several units can improve continuity because one generator can be isolated while healthy units remain on the bus. Continuous supply still depends on sufficient spare capacity, selective protection and a fault that does not collapse the common bus.

Easy to Maintain:- Operators can remove one generator for planned maintenance while the remaining units carry the permitted load. This requires enough available capacity and safe isolation of the stopped unit.

Easy to Increase Plant Capacity:- A compatible generator and its protection can be added to the bus when more power generation capacity is needed. The bus-bars, switchgear and controls must also have adequate ratings.

Precautions During Parallel Connection

1. Bring each prime mover to its specified operating speed before connecting the generator. DC generator shafts do not lock to a common electrical speed after paralleling, so each speed controller must remain stable.
2. Share the load without exceeding any generator’s current, power, speed, commutation or thermal rating.
3. Use controls and protection that monitor bus voltage, generator current, field condition, speed, temperature and reverse current as required by the installation.
4. Coordinate voltage regulation so one unit does not motor or take excessive current after a voltage drop. Check the individual generator currents as well as the common bus voltage.
5. Prove correct bus polarity before closing the incoming breaker. Reversed polarity creates a short-circuit path limited mainly by circuit resistance and protective-device operation.