Protective chokes for compensation systems

Protective chokes of ED3F type protect the capacitive batteries against overloading with harmonic currents.

Fig.1 Diagram of one stage of compensating battery with suppression chokes.
Description: Dławik – Choke, Bateria - Battery

As a result of installation of many non-linear receivers the level of higher harmonics of voltage and current is dangerous for transformers, motors and, in particular, batteries of capacitors. Correct determination of the parameters of ED3F protective chokes decides on the compensating battery efficiency. For that purpose it is almost necessary to measure the contents of higher harmonics at the planned place of compensating battery operation. On the basis of measured spectrum the appropriate resonance frequency for battery-choke system is selected. Usually it is the frequency intermediate between the frequencies of these voltage and current harmonics which are not present in the measuring spectrum or have the smallest share in it. The battery-choke resonant system will show an insignificant impedance for harmonic frequencies located in the vicinity of resonance frequency. Simultaneously it will strongly suppress the currents with frequencies distant from fresonance frequency.
Selection of optimal resonance frequency of the system (e.g. fR=189Hz) and knowledge of network frequency  (fN=50Hz) at which the battery will operate allow to determine the p% suppression factor(1), which will be then used in simplified calculations.

 (1)

Table 1. Examples of resonance frequencies in battery-choke system:

 Suppression factor - p% 5% 5,67% 7% 12,5% 14% Resonance frequency - fR ~224 Hz ~210 Hz ~189 Hz ~141 Hz ~134 Hz

Each stage of a compensating battery is protected with a protective choke of inductance determined for the battery stage capacitance and assumed resonance frequency of the system. It is necessary to determine the power (QCN) and rated voltage (UCN) of capacitors which form the individual stages of the compensating battery. It allows to conduct correct calculations in order to determine the IS currents in the circuits and the inductance (LN) of protective chokes.
On the basis of equation describing the capacitor power (e.g. 25kVar/440V 50Hz), the capacity of compensating battery stage can be determined as follows (2):

 (2)

Then the capacitive reactancy (XC), inductive reactancy (XL) and the resultant reactancy of battery with chokes (XBAT) (3,4,5) are determined, with assumption that the resonance frequency of the system should  ~189Hz (p=7%).

 (3) (4) (5)

On this basis the LN phase inductance of choke and IS current forced by the battery can be determined (6,7):

 (6) (7)

The actual voltage (UCR) on the battery terminals will achieve the following value (8):

 (8)

Hence the actual reactive power (QCR) of 25kvar/440V 50Hz capacitive battery, reactive power of protective choke (QL) and the resultant reactive power of battery stage (QBAT) can be determined (9, 10, 11):

 (9) (10) (11)

ED3F protective chokes are required to operate properly in the conditions of changeable load. The share of harmonic currents loading the battery and chokes is varying depending on the configuration of factory network and the number of converters or other non-linear receivers operating at a given time. Therefore the parameter called the magnetic linearity was determined. This parameter represent the maximal choke current (ILIN), for which the inductance tolerance should be within the range L ≥ 0.95LN. It is the measure of choke parameters stability during overloading.
The intensity of network voltage sinusoid deformation at the place of capacitive battery operation may depend, to a significant extent, on non-linear receivers operating in neighbouring companies supplied from the same switching stations. Attention should be paid that the number of deformations (i.e. THDU, THDI) may change radically within only several months (as the successive devices, e.g. converters, are installed in the neighbouring companies). Therefore the installed compensating systems should be equipped with protective chokes, in order to avoid the operational problems.

ELHAND choke intended for protection of ED3F-22,2/400/7-25/440 battery