Smaller-scale power generation systems are creating new opportunities for OEMs and panel builders. But they need to understand the unique feature that generator breakers must include.
OEMs and panel builders who work in the low and medium voltage world are increasingly involved with renewable, microgrid, and critical power opportunities. In the past, the medium-voltage devices located upstream from panels or products were specified and supplied by the utilities on the power plant level. Today, however, power-generation components are more distributed and require increased expertise on the local level.
To understand distributed power generation, it’s helpful to understand what’s happening around generators of various sizes. This knowledge will enable you to offer better, more informed guidance to your customers and to develop products that will better meet the needs of current and future customers.
There are also some growing opportunities in applications that were formerly found only in the medium- to high-voltage range which are now moving closer to the applications where typical panel builders and OEMs usually work. For example, not that long ago power generation was almost exclusively the domain of large utilities operating hydroelectric or coal-fired facilities. Protection and control devices were massive, with breakers so large they would fill a room.
Today, there is an incredibly diverse array of power sources and microgrids, largely solar- or wind-powered but also many sources relying on gas turbines and smaller hydroelectric power plants and backup generators. There is a rapidly increasing number of much smaller power sources that are scaled to power a facility, campus, or data center.
These smaller systems incorporate breakers that can readily be housed in a typical metal-clad switchgear. Systems like these are on a scale that is well within the range of OEM’s and panel builder’s typical application, creating new business opportunities. But these applications require fresh knowledge about the technical requirements regarding the safety and protection of these systems.
The key feature of generator breakers
Most smaller scale power sources include backup generators connected to the grid “on demand”. This is especially true of renewable energy sources that are subject to “fuel shortages” i.e., no wind or sunlight.
Safely operating these systems requires a generator breaker or other device to clear system faults. When there’s a fault between the generator and breaker, the system is still being fed from the upstream source or utility and the fault can be cleared with any conventional breaker. But when the system is being fed by the generator and there’s a fault upstream of the generator breaker, you need the special characteristics of a generator breaker to clear such a fault. The most important of those characteristics is the breaker needs a current “zero crossing” to safely interrupt the current.
Medium voltage breakers all operate on the same basic principle. When a fault is detected, the breaker will trip and the contacts separate. But in most cases, the current will continue to flow until there’s a zero crossing of the current. Because zero crossings occur after each quarter of a cycle, the precise timing of the interrupt is critical. While system-fed faults are always symmetrical, regardless of where in the cycle the breaker trips, it will safely open the circuit.
It’s a different situation when the system is being fed by the generator and there’s an upstream fault. Generators have a very high DC current that does not naturally cross zero for many milliseconds. At the instant of fault and subsequent breaker opening, there continues to be a DC component of the current until the next zero crossing, which may not occur for 100ms or more.
This is a critical distinction between generator and other breakers. All distribution breakers are sized to handle arcs under 15ms at the worst case. In a generator-fed system, because of the delay in fault extinction, the system can experience 10 times that stress, which could create catastrophic damage to equipment and possible injury to employees in the area.
When selecting a generator circuit breaker it is important to consider the latest IEC/IEEE 62271-37-013 Standard. This is even more critical for generators above 5 MVA that have to be compact and fit in a medium voltage switchgear.
Generator protection and control systems have varying complexity based on the size and voltage level they are operating at. These systems have more failure modes than any other component in the power system. Such systems are also mandated to have adequate backup to adjacent power system components like transformer or bus bar that are fed by the generator. Hence the protection and control system can be massive.
Different faults and abnormalities that are to be considered in these systems include (1) stator faults (2) rotor faults (3) abnormal operating conditions and (4) faults in the connected power grid.
Ageing, overvoltage, or loss of cooling may cause stator short circuits. Stator earth faults are commonly observed in end portions of the stator coils. External short-circuits, improper synchronization and loss of synchronism may cause large currents and can displace the stator winding and cause an internal short circuit. This causes the electromechanical forces to cause strain on the windings, in some cases these may be more than 100 N/cm at sudden short circuits. Stator short circuit faults in generators can be reliably and selectively detected using generator differential protection. Such protection shall not operate in case of external short circuits and due considerations should be taken into account for current transformer saturation in case of external short circuits. The time constant of the DC component of the short circuit may be very long and in the order of 100 to 150ms. External short circuits with fully developed DC components puts severe demands on current transformer and differential protection. Stator ground fault protection varies based on the layout and the system grounding.
Line end ground fault protection
Neutral-point overvoltage protections, neutral-point overcurrent protections, zero-sequence overvoltage protection and residual differential protections are line end earth-fault protections.
Neutral end ground fault protection
An overvoltage (or overcurrent) generator earth-fault protection is straightforward but doesn’t detect faults near generator neutral and is not self-monitoring. It is possible to use third harmonic voltages, having zero sequence characteristics.
Field circuits of synchronous generators consist of rotor windings and associated circuits. These include field circuit breaker, rotating exciter etc. Modern field systems of generators are often operated ungrounded. Ground faults, interturn faults and open circuits occur in field circuits which require protection systems operating on different principles than that or stator. Over-excitation or unbalanced loading may overheat the rotor, and these require different detection mechanisms. Three different fault types exist in the rotor circuits of the generator (1) open circuits (2) ground faults (3) short circuits.
Abnormal operating conditions
Several operation conditions can result in damages to generators. Some generator protection functions shall detect such abnormal conditions and initiate tripping of the generator circuit breaker before internal faults occur. These operating conditions are (1) abnormal voltage (2) abnormal frequency (3) loss of synchronism (4) unbalanced loading.
Every OEM and panel builder understands basic current interruption and breaker functions. When it comes to generator breakers, though, there are additional requirements that must be addressed to adequately protect the generator and connected system. Protection systems can be designed using universal protection and control relays. Relays that are modular in hardware and multi-application oriented can provide a ‘one size fits all’ solution. Auxiliary functions such as auto-synchronization of the generator circuit breaker with the power grid can be accomplished in multi-application relays.
The special requirements for generator breakers, long recognized and documented, are now addressed in the new IEEE standard. Without the correct type and size of breaker, you risk using an undersized device to protect the generator at the most critical moment — during a generator-fed forward fault.
When dealing with a generator-fed forward application, you can rely on your distributor or relay/breaker manufacturer to help ensure the proper protection systems and breaker sizing.
With more distributed generation, and increased use of backup generators, relay and breaker manufacturers are working to create smaller, simpler solutions that put these applications within reach of panel builders and smaller-scale OEMs.
See related blog post “Apps build both safety and a path towards digitalization”.
Indoor Apparatus Marketing Manager
Product Specialist – Digital Substation Products