Tuesday, 29 January 2019

Why to Compensate Reactive Power ? & Control & Regulations of Reactive Power

There are many simultaneously active loads in a conventional electricity network. Many are resistive, some have a capacitive component, i.e. the current curve hurries a bit ahead of the voltage curve (leading), and others have an inductive component, i.e. the current lags behind the applied voltage (for a more detailed explanation, see Annex on inductances, capacitances and reactive power).Resistive-inductive loads prevail in most networks, resulting in a resistive-inductive overall current. This incessant, undesired, oscillation of energy means an additional flow of current in cables and transformers. It causes additional resistive-losses and uses a potentially large part of their capacity.
Therefore the basic reasons for compensating are to avoid: 
  • The undesired demand on transmission capacity (additional current)
  • The energy losses caused by such
  • The additional voltage drops caused by such 

These extra voltage drops in the system are significant; a reactive current flowing in a resistance causes a real power loss. Even where the impedance is largely reactive, rapid changes in the reactive current  may cause flicker. A good example of this is a construction crane connected to a relatively small distribution transformer when a new home is erected in a residential area. The cranes are usually driven by relay-controlled three-phase induction motors which are quite frequently switched from stop to start, from slow to fast, and from downwards to upwards. 

The start-up currents of these motors are very high, several times the rated current, and have a very high inductive component, the power factor being around cos ϕ ≈ 0.3 (or even smaller with bigger machines). The voltage drop in the transformer is also largely inductive, so it has more or less the same phase angle as the start-up current of the motor. It will  add much more to the flicker than the same current drawn by a resistive load (Figure 1). Fortunately, this also means that this flicker can easily be mitigated by adding a capacitor to compensate the inductive component of the motor’s start-up current.  
It is normally desirable to compensate reactive power. This is quite easy to achieve by adding an appropriate capacitive load parallel with the resistive-inductive loads so that the inductive component is offset. While the capacitive element is feeding its stored energy back into the mains,the inductive component is drawing it, and vice versa, because the leading and the lagging currents flow in opposite directions at any point in time. In this way, the overall current is reduced by adding  a load. This is called parallel compensation. 

To do this properly requires knowledge of how much inductive load there is in the installation, otherwise over-compensation may occur. In that case, the installation would become a resistive capacitive load which in extreme cases could be worse than having no compensation at all. If the load –more precisely its inductive component – varies, then a variable compensator is required. Normally this is achieved by grouping the capacitors and switching them on and off group-wise via relays. 

This of course causes current peaks with the consequent wear of the contacts, risk of contact welding and induced voltages in paralleled data lines. Care must be taken in timing the switch on. When voltage is applied to a fully discharged capacitor at the instance of line voltage peak, the inrush current peak is equal to that of a short circuit. Even worse, switching on a short time after switching off, the capacitor may be nearly fully charged with the inverse polarity, causing an inrush current peak nearly twice as high as the plant’s short circuit current peak! If there are many switch-mode power supply loads (SMPS) being operated on the same system, then a charged compensation capacitor, reconnected to the supply, may feed directly into a large number of discharged smoothing capacitors, more or less directly from capacitance to capacitance with hardly any impedance in between. The resulting current peak is extremely short but extremely high, much higher than in a short circuit! 

There are frequent reports about the failure of devices, especially the contacts of the relays controlling the capacitor groups, due to short interruptions in the grid which are carried out automatically, e.g. by auto-reclosers, to extinguish a light arc on a high or medium voltage overhead line. It is often suggested that this doubling of peak value cannot occur with capacitors that are equipped with discharge resistors in accordance with IEC 831. However, the standard requires that the voltage decays to less than 75 V after 3 minutes, so they have little effect during an interruption as short as a few tens of milliseconds up to a few seconds. If, at the instant of re-connecting the capacitor to the line voltage, the residual capacitor voltage happens to equal the supply voltage, no current peak occurs. At least this is true if the compensator is viewed as a pure capacitance and the incoming voltage as an ideal voltage source, i.e with zero source impedance. But if the self-inductance of the system is taken into account, certain resonances between that and the capacitance may occur. 

Assume the following case: the residual voltage of the  capacitor is half the peak value and equal to the instantaneous line voltage, which is at 45° after the last zero voltage crossing, i.e. 

At this point in time the current in the capacitor is expected to be: 

However, this is not the case, because the capacitor has been disconnected from the supply up to this point in time. At the instant of connection, neglecting the system’s inductance, the current would rise up to this value immediately, and nothing would happen that would not have happened anyway in the steady state. But a real system is not free of inductance, so the current will assume
this value only hesitantly at first, then speed up and – again due to the inductance, its ‘inertia’ – shoot beyond the target way up to nearly double the expected value. Then it will come down again and so on, and thus perform a short period of oscillation that may be attenuated down to zero well within the first mains cycle after connection. The frequency of such oscillation may be rather high, since the mains inductance is low, and may cause interference to equipment in the installation. Only if the instantaneous line and residual capacitor voltages are both at their positive or negative peaks at the instant of re-connecting the capacitor, at which point in time the instantaneous current
would be zero anyway, will the resistive-inductive current start without oscillation. 

More precisely, there are two conditions to be fulfilled. Firstly, the sum of voltages across the capacitance and its serial reactance (be it parasitic or intentional detuning) must be equal to the line voltage. Secondly, the supposed instantaneous current, assuming connection had already taken place long before, has to equal the actual current, which of course is zero until the instant of switching. This second condition is fulfilled only at line voltage peak, which therefore has to equal the capacitor voltage. To achieve this, the capacitor is pre-charged from a supplementary power source. This practice has a  secondary minor advantage in that it makes sure that there is always the maximum possible amount of energy stored in the capacitor while not in use, so that at the instant of turn-on it may help to mitigate some fast voltage dip and prevent the subsequent flicker. 

Relays, however, are too slow and do not operate precisely enough for targeted switching at a certain point of the wave. When relays are used, measures have to be taken to attenuate the inrush current peak, such as inrush limiting resistors or detuning reactors. The latter are frequently used anyway for other reasons, and are sometimes required by utilities. Although this series reactor
replaces the inrush current peak at switch-on with a voltage peak (surge) at switch-off, it is still the lesser evil, since the reactive power rating of the reactor is just a fraction of the capacitor rating and so the energy available is less. 

Electronic switches, such as thyristors, can be easily controlled to achieve accurate point-on-wave switching. It is also possible to control switching so as to mitigate a fast flicker caused by a large unstable inductive load, such as the crane motor mentioned previously, an arc furnace or a spot welder. 

Now the latest solution is Emerich Energy's Premium product An advanced IGBT based one "Active Var Generator" (AVG) Which is Maintenance Free and Precise Compensation.

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