Why Do Two Sets 1500kVA Diesel Gensets Equip with ACB Breaker for Parallel？

Do you have a project that requires the parallel operation of two 1500kVA diesel generator sets? If so, today's article may be helpful to you. Let's take a look together!

The four key elements for achieving parallel operation are: same voltage, same frequency, same phase, and same phase sequence. So why is it necessary to equip with a ACB circuit breaker?

Main reason: Parallel operation demands extremely high requirements for breaking capacity, protection logic, synchronous control, and interlocking reliability, which cannot be met by molded case circuit breakers (MCCBs). Therefore, only air circuit breakers (ACBs) can be used. Let's analyze the role of ACBs together below.

1. L: Long time-delay overload protection (inverse time limit, overload heating trip)

Function: Prevents cables and generator windings from being burned due to long-term overload and heating.

Setting content: Current Ir and delay time Tr are adjustable.

Ir: adjustable from 0.4 to 1.0 times the rated current. For example, if the frame is rated at 1000A and Ir is set to 800A, any sustained load exceeding 800A is considered overload;

Tr: After overload, there is a delay of several seconds before tripping (5s/10s/15s/30s settings).

Action logic (inverse time limit): Slight overload: Gradually delay the trip (leaving a margin for short-term overload, and not tripping easily due to the impact load of generator startup); the more severe the overload, the faster the trip.

Generator purpose: To prevent the unit from operating at overload for a long time and prevent the stator from overheating and burning the coil.

2. S: Short-time delay short-circuit protection (definite time, selective protection, unique to ACB)

This is the most important section for generator sets.

Function: In case of a branch short circuit, only the branch molded case will trip, and the main switch ACB will not bypass the lower level and trip.

Setting: Short time delay current Is and short time delay Ts are adjustable.

Is: generally adjustable from Ir×2 to Ir×10 times;

Ts: Time delay options of 0.1s, 0.2s, 0.3s, and 0.4s are available (critical time difference).

Operating principle: In the event of a short circuit in the downstream load, the branch MCCB (Mini Circuit Breaker) trips instantly within 0.02 seconds. The ACB (Air Circuit Breaker) is set with a short time delay of 0.2 seconds. If the fault is cleared by the downstream switch within 0.2 seconds, the main switch does not trip. If the downstream switch fails to trip and the short circuit persists beyond the set time (ts), the ACB trips.

Essential for parallel operation power stations: avoid short-circuiting of one load circuit, which may lead to the complete shutdown of the entire unit/entire parallel operation system.

3. I: Instantaneous short circuit protection (no delay, instantaneous tripping for extremely large short circuits)

Function: In the event of an extremely large short circuit near the busbar or generator outlet, it instantly cuts off the power supply.

Setting: The instantaneous current Ii is adjustable, typically Is×(1~10) times, with no delay (≈0.01s).

Action logic: In case of extremely high short-circuit current (such as busbar sparking or direct contact between generator outgoing lines), if the current exceeds Ii, the circuit breaker will immediately trip without any delay to protect the circuit breaker itself and the generator.

Distinguish between: small short circuits with short time delay S, and devastating large short circuits with instantaneous I.

4. G: Ground fault protection (zero-sequence ground protection G)

Function: Phase line touching the shell, cable insulation damage causing leakage to ground and fire, generator stator grounding.

Setting: The grounding current IG and grounding delay TG are independently adjustable. IG typically ranges from tens of amperes to hundreds of amperes, while TG can be set between 0.1 and 0.5 seconds.

Applicable scenarios (high-frequency faults in generator sets) include cable insulation damage leading to grounding, motor insulation aging resulting in grounding, and marine unit hull leakage. When the leakage current is not large, L/S/I will not actuate, and the G protection will trip to prevent electric shock and fire.

Some clients say, we purchase and install the parallel cabinet locally ourselves, and doesn't the parallel cabinet have an ACB circuit breaker? Can't we use a molded case circuit breaker (MCCB) on the diesel generator set?

Next, we'll give some suggestions: why can't we casually use a molded case MCCB as the generator outlet switch under parallel operating conditions?

Core pain point: The MCCB breaker lacks S short-time delay protection (crucial). The customer's outdoor parallel operation cabinet (inside which is an ACB frame main switch + parallel operation protection) has the following structure: generator's own circuit breaker → cable → customer's parallel operation cabinet's incoming main ACB. The normal hierarchical logic is: a lower-level fault trips the generator's own switch, while the upper-level parallel operation cabinet's main switch does not bypass and trip.

The parallel cabinet's main ACB features a short time delay of 0.2s with S setting; if the generator's outgoing circuit breaker is an MCCB (only with L overload + I instantaneous settings, without short time delay), when the generator's outgoing terminal is short-circuited, the short-circuit current simultaneously exceeds both the instantaneous setting of the MCCB and the short time delay setting of the parallel cabinet's ACB. Specifically, the MCCB's instantaneous setting is 0.01s, and the ACB's short time delay setting is 0.2s, seemingly allowing for hierarchical protection. However, in cases of internal stator short-circuit within the generator or a major short-circuit near the busbar, bypassing of the hierarchical protection is highly likely to occur: when one unit experiences a faulty short-circuit and the molded case fails to trip in time, the upstream parallel cabinet's ACB will directly trip, resulting in all three units shutting down and the parallel operation collapsing.

Specification for parallel operation of multiple machines: The circuit breaker of the generator body must have a short time delay S to achieve selective protection, and only ACB has four-section protection.

2. Parallel operation circulation and grounding hazards: Three-phase imbalance and single-point grounding leakage are prone to occur during parallel operation of three machines. The MCCB lacks independent adjustable G grounding protection; once a single stator insulation is grounded, the grounding current is small, and the molded case L/I does not operate, causing the fault to continue heating and burning the generator, or even leading to a short circuit in the parallel operation. The ACB comes with an independent grounding G setting, and a single grounding will trip the machine independently, without affecting the grid-connected operation of the other two machines.

3. Parallel operation and maintenance requirements: ACB can be electrically stored and remotely opened and closed. The parallel cabinet requires a grid-connected controller to remotely open and close the switch of a single unit. The cost of adding an electric mechanism to a common molded case is much higher than that of a small current frame (63~250A small ACB). MCCB manual switches cannot achieve automatic grid-connected opening logic.

After review above information, we think you have known the ACB breaker is most important for parallel operation of two 1500kVA diesel generators.

Dingbo Power has done diesel generator business for more than 20 years, power range from 20kVA to 3750kVA, options: Cummins, Perkins, Volvo, Yuchai, Shangchai, Weichai, MTU etc. If you have purchasing plan, welcome to contact us by email [email protected] or WhatsApp +8613481024441, we will guide you to choose suitable product.