|The designing of AC generators, such as the AvK® DIG 142 and STAMFORD® P80, is increasingly being driven by the need for grid code compliance|
To minimise the impact of unstable grids, TSOs are devising grid codes that specify performance expectations on power plants. One area where this is having a profound effect is embedded generation – power plants consisting of engine-driven generating sets.
Sridhar Narayanan, Cummins Generator Technologies, UK
Global electricity demand is growing at a rate of 2.4 per cent every year, according to the International Energy Agency, and one of the quickest ways to address this demand is embedded generation – ad hoc power plants consisting of engine-driven generating sets.
Along with renewables, these embedded generation units make up a significant portion of modern grids. However, although there are environmental and economical benefits, distributed generation makes the grid less stable. In order to ensure continuity of power supply, electricity transmission and distribution system operators all over the world, with Europe in the lead, are enforcing regulations – grid codes that define performance expectations on power plants.
Table 1 summarises the key aspects of a grid code document for Germany – voltage, power factor, frequency under steady-state conditions and the required connection period during a fault ride through. The grid codes can broadly be classified as static (steady-state) and dynamic (transient) operating conditions. Designing power plants and associated components to comply with grid codes is challenging, especially for generating sets smaller than 5 MW. The grid conditions in Table 1 mean that the generator is exposed to thermal and electrical stresses. Combined with the need to comply with the codes, these requirements challenge the generating set components in numerous ways.
Steady state condition: Under steady-state conditions, the grid codes demand the generator to supply the ratings plate KVA for a wide range of voltage, power factor and frequency conditions. Although the requirements from the lagging power factor condition is not so demanding, the leading power factor operating conditions expose the generator stator to higher currents, higher mechanical stresses because of the stator currents, and also reduces the electromagnetic coupling between the stator and the rotor. The weak electromagnetic coupling means the generating set is mechanically stressed and is close to its operating margin under steady-state conditions.
Fault ride through: Figure 1 is a representation of the voltage profile of the ‘grid’ during a fault ride through event. The voltage at the point of common coupling drops to a very low value and the active load on the generating set also drops. The kinetic energy in the shaft accelerates the rotor while the generating sets work hard to not pole slip (rotor magnetic field trying to catch up with stator magnetic field, which stresses the rotor) for the duration of the fault as required by the grid codes.
|Figure 1: A voltage profile of a fault ride through event|
The acceleration of the shaft and the rotor stresses the generator mechanically, while the low voltage at the generator terminal induces short-circuit level currents in the stator windings, which expose them to thermal and mechanical stresses.
Post fault ride through: The grid starts to recover depending on the local condition and the generator that had previously accelerated is now attempting to connect to the grid with a different frequency and phase than the grid. This is similar to a game of ‘tug of war’ with the grid and generator attempting to regulate each other’s phase and frequency. This results in huge mechanical forces on the generator. Figure 2 is a simulation of the electromagnetic forces on the alternator windings during fault condition.
|Figure 2: Simulation of electromagnetic forces on the windings of the alternator under fault conditions|
The requirements already described would imply that a generator design to support grid code-compliant generating sets would involve more than just complying – it would need to be robust and durable. There is a need to understand the critical parameters influencing both compliance and robustness. Cummins Generator Technologies’ engineers determine these critical parameters using Cybergen1, a tool that was developed to study the effect of transients on generators. Figure 3 is a schematic of the tool.
|Figure 3: Schematic of Cybergen, specicially developed to study the effect of transients on generators|
Cybergen combines the detailed finite element models of generators with mathematical models of the engine, controls and the application, thereby running accurate simulations of the generator and allowing evaluation of both the mechanical and electromagnetic performance of the generator. Combined with test data, the tool was used to define the critical generator characteristics/parameters that impact design for grid code compliance.
Using Cybergen, simulations of the generating set connected to the grid were run at various operating points. The results of the simulation can be split into static grid codes, fault ride through, and robustness – the three key requirements to be satisfied.
The key issues related to designing generators for static grid code are:
Thermal: Due to the leading and lagging power factor conditions and the huge voltage range, the rotor and stator windings of the generator are exposed to high currents and therefore high temperatures. It is crucial to understand the temperature rise in various scenarios. The current and losses in the rotor and stator and the effectiveness of cooling in the generator influence the temperature rise.
Mechanical: The currents in the windings of a generator induce an electromagnetic force which causes the windings to vibrate at twice the fundamental frequency of the supply. It is therefore essential to consider these vibrations while designing the windings of a generator. Care should be taken to avoid resonance. The geometry of the windings – overall length, mass, and insulation material – have a significant impact on the mechanical performance of the generator.
Electromagnetic: It is important that the generating set remains stable for all operating points within the boundary specified by the grid codes. The generator must be designed with the correct value of synchronous reactances – Xd and Xq. These parameters affect the ‘load angle’ of the generating set. The higher the load angle, the closer the set is to stability limits.
Fault ride through: Even though the dynamic grid codes involve much more than fault ride through, the design of the generator itself is focused on the fault ride through. Achieving dynamic grid code compliance would involve the generating set packagers, engine manufacturers, generator manufacturers and control systems specialists working together to integrate a ‘grid code compliant genset’.
The fault ride through requirements in grid codes state that the generating set stays connected to the grids for a specified duration of the fault on the grid (150–250 ms). The acceleration of the rotor described in the previous section during a fault ride through would mean it would be difficult to maintain the stability of the generator under dynamic conditions.
Some of the key factors influencing the behaviour of the generator during a fault ride through are:
Initial conditions: The steady state load angle of the generating set is one of the most significant contributing factors to determining dynamic stability. As the rotor accelerates, care must be taken so that the ‘dynamic load angle’ does not reach a critical value. Smaller the steady state load angle, longer it takes for the rotor to accelerate to this angle and therefore the generating set can stay connected stably for longer.
Inertia of the generator: The laws of physics state that heavier a body, smaller its acceleration for a given force. So the inertia (mass and diameter of the rotor) is fundamental to solving the problem of dynamic stability. However, there are practical limits on the amount of mass that can be added to the rotor and how much the diameter of the rotor can be increased by.
Back-swing: Back-swing is the phenomena where the generator slows down for a few tens of milliseconds after the fault before it starts to accelerate again. This buys the generating set additional time. The factors affecting back-swing are the subtransient reactances – X”d, X”q and their ratio (subtransient saliency).
Robustness: To design a robust generator for grid code compliance, the stresses in the various components of the generator need to be studied in order.
Stator windings: The mechanical and thermal stresses caused by the increased current levels in the stator windings need to be minimised. The vibration signature of the generator must also be studied so ‘resonant modes’ can be avoided. Figure 4 shows simulation results from a study conducted to determine the normal modes of vibration of the stator windings. Studies on whether additional bracing would be required to protect the windings are also required.
|Figure 4: The results of a simulation study carried out to dtermine the normalmodes of vibration of the stator windings|
Damper cage: The long-fault ride through scenarios means the damper cage becomes hot and is also exposed to serious mechanical stresses. Thus, numerous tests and simulations are required before designing the damper cage or evaluating the suitability of the existing damper cage.
Shaft: Figure 5 showed the extent of damage a failed synchronization event could do to the shaft of a generator. If the masses of the generator and the engine are not matched correctly, then some of this torque could be transmitted to the engine as well. So it is essential to understand the nature of torque transients that occur in the generator. Designing the shaft to accommodate these transients would ensure a robust shaft design.
|Figure 5: Highlighting the damange that torque transients can have on a generator shaft|
Rectifiers: Low leading power factors and huge current in-rush can lead to a high negative voltage on the rectifiers commonly used in the rotor electrical circuit thereby damaging them. Understanding the magnitudes of these voltages is key to designing measures to protect them.
While distributed generation is a quick solution to the ever-growing electricity demand, the conditions imposed by grid codes pose challenges to the design of generating sets for distributed power generation.
It might appear that using permanent magnet machines with full power converters may be a straightforward solution, but high initial costs and the effect on grid stability of having too many permanent magnet machines need to be understood. Modern tools can be used to make design recommendations, but practical considerations and time to implement vs time to enforce codes make the implementation of these changes even harder.
With that in mind, the grid codes have transformed what seemed to be a stable concept – ‘synchronous generators’ – into an exciting area where numerous innovations are possible. With the uncertainty in the rare earth metal prices, wound field synchronous generators could well become the preferred electrical machine type for use in grids.
1. N.L. Brown & A. Michaelides. Cybergen: Modelling the design challenges for small embedded synchronous alternators connected to increasingly unstable networks, Proceedings of Power Electronics Machines and Drives 2010, UK.
About the author
Sridhar Narayanan works as the Global Product Engineering team leader at Cummins Generator Technologies in UK. He received his Master of Science degree in Physics from the University of Florida, US. For more information on Cummins Generator Technologies, please visit https://cumminsgeneratortechnologies.com
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