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Most networks are coupled to a large interconnected power system. This interconnected power system mostly is represented by an external grid, possibly with a reduced order network model, representing the most important interconnections. In the case that a network is not connected to an external grid, it is considered an isolated network or an “island grid”.
The traditional loadflow technique is based on solving the nodal voltages from the system of network equations. Each system is modelled using as many network equations as there are nodes in that system. At each node a fixed complex power may be injected. The solution process for this set of equations requires the complex voltage on one node as a reference. This reference node is called the “Swingbus”. For this node the voltage is fixed, both in magnitude and angle.
In the standard loadflow calculations the swingbus is integrated in the external grid. Its voltage is the nominal voltage, multiplied by the per unit reference voltage (Uref) as specified by the user. In most cases the reference voltage angle is zero, but this can be altered by the user.
Since the swingbus voltage is fixed, the complex power (P+jQ) can be freely chosen. This means that the difference between the total generated power and load plus the network losses within the whole system will be absorbed/delivered by the swingbus. The external grid, containing this swingbus and is thus able to provide or absorb the active and reactive power shortage or surplus in the network. Without a swingbus a loadflow calculation cannot be performed.
Island networks exist aboard of ships and off-shore installations. Also an industrial network may sometimes become isolated from the external grid. In an isolated network, that is not connected to any interconnected system, there is no possibility of importing or exporting any power shortage and surplus, so this power has to be provided by the generators in the network. Any network calculation method, however, still needs a swingbus reference. The solution is to configure the generators in such a way that the swingbus does not provide or absorb any power. In that case the network generators should be in balance with the loads and the network losses.
Island mode calculation is done automatically. Vision controls the power of the existing synchronous generators in such a way that the generation is in balance with the load in the network. The power exchange with the swing bus is then reduced to zero.
Primary frequency control
The governor control of synchronous generators controls the generated power output, depending on any frequency changes in the system. The governor acts according to the power/frequency characteristic, determined by the governor droop. Each power disturbance involves a system frequency change. The governor control will respond with a change in prime mover input to the generator in order to restore the balance of generated power versus load and system losses.
Governor droop
A generating unit with governor control will produce more power after a frequency drop. The amount of extra power is determined by the power/frequency characteristic and the governor droop. The droop has been defined as the quotient of frequency change and power change.
The larger the governor droop, the smaller the contribution the generating unit makes to a change in power.
Governor control constant
The governor control constant is a derived quantity, indicating the amount of power change resulting from a system power change.
The generating unit power change will be derived from the frequency drop and the governor control constant.
Power system control constant
The sum of all generating units governor constants is called the power system control constant (KN). Using this system constant the frequency change after a power change can be calculated. The frequency change is equal to the quotient of the system power change and the power system control constant.
Even large interconnected systems, like the European UCTE system, are island networks. Every power change leads to a frequency change. All interconnected power generating units are equipped with a governor control and act according to this principle.
Parallel operation of interconnected generating units
The contribution (Delta Pm) of a single generating unit, after a frequency change (Δf), results from the corresponding governor control constant (Km) and the power system control constant.
Isochronous control
In case of generators with droop control the network frequency varies during load changes. There may be also generators present in the network that drive the frequency back to the nominal value. Active power of such generators is independent from the frequency (isochronous control). If there are multiple generators with isochronous control present in an island, an active power unbalance is split between these generators.
Load flow calculations
Every isolated part of the network can be solved in island mode. There are two conditions:
•Each island network must have one or more synchronous generators in service, equipped with voltage control and governor control (droop or isochronous).
•The generating power in the island network must be sufficient for the total island system load plus losses.
Synchronous generators with a zero governor droop and disabled isochronous control are considered as constant power generators and do not contribute to the frequency control. The principle used for sharing of active power unbalance between generators participating in primary frequency control is as follows. First, the unbalance is divided between the generators with isochronous control. If there is still active power unbalance present, it is split further between the generators with governor droop control. The frequency in this case will deviate from the nominal value.
As a consequence to the solution method, the proper choice of the system base power (Sbase) has become more important. A general equation for a proper value can not be given. If the solution process might not converge or if the maximum number of iterations would be reached, another value of Sbase should lead to a better calculation. In case links are present in the network, the value of the link impedance can have an influence on the convergence process. In situations with convergence problems the adjustment of the link impedance (Options, tab Calculation | General) can provide a solution.
Generated power not sufficient
In an isolated network, that is not connected to any interconnected system, there is no possibility of importing or exporting any power shortage or surplus, so this power has to be provided by the generators in the network. In all cases the following relation has to hold:
If the total load power is larger than the total available generator power, the extra power needed to solve the loadflow equations, will be delivered by the swingbus generators and a warning will be printed that the generated power is not sufficient.
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