Proposed Autonomous Power Network Design Process

The following graph (Figure 22) demonstrates a design methodology of a conventional autonomous power network. The first stage in the design process of any power network is the analysis of the requirements. The power requirements of each network are typically known at the very beginning even when someone starts the design process from a blank page. Power and operational requirements need to be specified and allocated to relevant

protection devices availability, required power converters, and overall system losses are some of the criteria that typically determine the basic parameters selection. More specifically, normal currents are the sizing factor for the main bus-bar of each network. The higher these currents are the heavier the bas bar will be. Furthermore, normal currents level also determine the fault currents level and hence the required rating of the switchgear. Switchgear devices have typically an interruption performance limit of around 40 kA (kilo Ampere) and ratings above this limit should be avoided (Malkin and Pagonis, 2013).

Figure 22 Design Process diagram of a conventional power network (Malkin and Pagonis, 2013)

Once these parameters are selected, an initial topology of the network will be defined. This topology will include the type and number of the main power sources (i.e. engines, generator sets, energy storage etc.), the number of switchboard sections, the converters and transformers of the system, and generally all the necessary equipment for the network. Depending on the network nature and requirements different architectures could be implemented such as bus, star, tree, ring, mesh, and/or a combination of them. The selection between AC and DC distribution is also a challenging task which requires

careful consideration in the initial stages of the design process of a power network since it dictates many of the equipment later being used.

The next stage of the design process will be an initial load analysis (i.e. steady-state low-abstraction model) of the power network which will determine the overall power system design. The load-flow study is basically a numerical analysis of the electric power flow in an interconnected system. Various power parameters of the network such as voltages, voltage angles, real and reactive power of each bus and line of the network are determined during this load flow analysis. These studies are important both for optimising the performance of existing networks but also for planning future expansion of already existed networks (Andersson, 2006). Generally the load flow problem is formulated by a set of non-linear equations.

f (x, u, p)=0 ^(3-1)

Where f is a n-dimension non-linear function, x is a n-dimension vector of the unknown component parameters (i.e. voltage magnitudes and angles in each node), u is a vector with known parameters (i.e. machines’ voltages), and p a vector including the network parameters (i.e. lines’ resistance and reactances).

Due to the non-linearity of the power flow analysis, this cannot be solved analytically and hence iterative solutions are commonly implemented. Newton-Raphson, Gauss-Seidel, and fast-decoupled-load-flow-method are just a few of the solution methods being used to deal with the non-linear set of equations of a network’s load flow. This analysis is of utmost importance to design the different power system components (such as alternators, transformers, transmission lines etc.) in order to be able to withstand any stresses they are exposed to during their steady state operation. These stresses could be a result

in high power applications and depending on the location could cause stability problems, mechanical and thermal stresses and interference with conductors.

The system needs to be protected from such faulty conditions by isolating the faulty parts of the network as quickly as possible typically with the use of protection devices such as circuit breakers. These devices ought to be capable of withstanding the maximum anticipated short circuits. As an example the calculation of a transient short circuit in a transmission line will be described (Andersson, 2006). It is known from circuit theory that the current of a circuit is composed of a steady state alternating current (𝑖_𝑠) and a transient direct current (𝑖_𝑡): wave when the short circuit occurs. However, the selection of circuit breakers is based on another short circuit value the so-called maximum momentary short circuit current (𝑖_𝑚𝑚) which corresponds on the first peak of the short circuit

waveform and can be as high as double the value of the symmetrical short circuit current:

𝑖_𝑚𝑚 ≤ 2√2𝑈

|𝑍|

(3-6)

Power quality is one of the main priorities while designing an electric power network. This can be jeopardised by the distortion in the voltage and current waveforms caused by the harmonics. The most common type of distortion is a periodic steady-state where the distorted waveform has a Fourier series with a fundamental frequency similar to the power system’s frequency (Ranade and Xu, 1998). Generally, the Fourier series for a regular periodic function is given by the following equation:

𝑓(𝑡) = 𝐶₀+ ∑ 𝐶_𝑛cos(𝑛𝜔𝑡 + 𝜃_𝑛)

∞

𝑛=1

(3-7)

Where 𝐶₀ is the dc value of the function, 𝐶_𝑛the peak value of the 𝑛_𝑡ℎ harmonic component, 𝜃_𝑛 is the phase angle, whilst 𝜔 is the fundamental frequency and is equal to 𝜔 = 2𝜋𝑓 𝑟𝑎𝑑/𝑠𝑒𝑐. An example of the synthesis of a waveform from harmonics can be seen in the next figure:

Figure 23 Synthesis of a waveform from harmonics

Generally, the propagation of each harmonic is studied separately since as it was described earlier the transmission system is typically simplified to a linear system. A small number of harmonics is typically considered in the early design stages. Harmonic studies are aiming on identifying the distortion levels in voltage and current waveforms of several points of the power network and evaluate the measures that need to be taken in order the harmonic caused problems not to affect the power quality of the whole network. The need for a harmonic study is typically indicated by the excessive measured distortion in systems which include several harmonic-producing equipment (i.e.

transformers, switching devices, rotating machines etc.).

Finally, one of the main objectives of the steady-state model is the optimisation of the protection system coordination. Protective device coordination could be defined in simple words as the process of determining the optimal solution in terms of current interruption in abnormal electrical condition circumstances.

Different protective zones are commonly set up in order to isolate potential faults in small regions of the network (Glover, Sarma and Overbye, 2010).

However, the main objective of the coordination study is to minimise the outage of any zone as much as possible.

Once the steady-state modelling is complete the key elements of the network will then might be resized and redefined in order to optimise the network performance during normal operation. A more comprehensive dynamic model is

the next step of the design process of an autonomous power network. Figure 24 summarises some of the dynamic phenomena being investigated during this stage. C and D dynamics have already been described as parts of the steady-state analysis.

Figure 24 Dynamic phenomena with their corresponding timescales in a power network: A. Electro-magnetic transients, B. Synchronous machine transients, C.

Quasi steady state, and D. Steady-state phenomena (Andersson, 2006) Transient motor starting analysis is another example of dynamic phenomenon in an electric power network. When motors start they typically require a high inrush of current (5-7 times their normal current) for a short period of time which could lead to excessive voltage drop. This drop needs to be monitored and analysed so that its effect both on the motor itself (i.e. possibility of stall) and on

Power transformers are typically one of the most expensive components in an electric power network and the excessive high current forces caused by these transients could affect their life expectancy (Ebner, 2007).

Stability issues are the main concern during this dynamic stage of the design process. The power system stability can be defined as the ability of an autonomous power network to regain a state of operating equilibrium after being subjected to any type of disturbance. Regaining an operating equilibrium does not necessarily mean returning to the initial steady-state condition but to a steady-state acceptable condition which will not result to protection actions causing further disturbance to the system (Andersson, 2006).

The latter stage (i.e. dynamic model topology) will again redefine the system’s requirements possibly leading to different network architectures and component ratings. The design process is basically a constant feedback procedure on how well a design satisfies the system requirements. Several modifications of the initial design concept will lead to a final design that will meet the total mission effectiveness requirements.

All the aforementioned steps can be better demonstrated with a current working example of such a network. Hence, the design process of a hybrid/electric ship example will be described in the following section. The selection of this network was based on the many similarities this network share with the TeDP concept the transmission of the electric power efficiently and reliably.