The only protection against overcurrents does not ensure sufficient protection against internal unit breakdown; additional protection is therefore ne- cessary, especially when many units are involved. The optimal and efficient solution to guarantee the effectiveness and correct monitoring of the banks of capacitors is unbalance protection. There are several types of unbalance protection, the one most widely used involves measuring the current unbalance between the two star centers; the operation of this protection is based on checking the symmetry of the two star centres of the bank.
In a balanced three-phase system the current between the two star centers is practically zero, whereas if elements or units break down they shift with circulation of the residual current; installing a current transformer between two star centers it is possible to measure this residual current and, with a special relay, it is possible to promptly perform operations such as releasing the main switch, opening circuits or signaling a problem without causing damage to the good units. This very sensitive system is also able to detect failure of a single element and this prevents the remaining installed components from getting damaged; in addition, the protection and operating devices work on breaks in loads at nominal levels and not at short circuit levels.
Unbalance protection therefore comprises a current transformer and an homopolar residual current relay.
The function of the current transformer is to measure the currents coming from the unbalance of the two stars of the capacitor bank and to isolate neutral at the same level as the mains voltage. Enerlux Power Srl uses current transformers for indoor and outdoor installations; on request other types of transformers can be installed with different ratios, insulating voltage, performances, etc…
The unbalance protection relay is a homopolar residual current relay, insensitive to harmonics with independent time. This is a relay made with a conventional system that due to its characteristics of high reliability, easy settings and inexpensiveness is the one most widely used. The relay is equipped with a trip threshold that is associated with a relay with double output contacts in the standard version; for the protection of banks with high powers or where required, it is recommended to use two relays to create two thresholds (alarm and trip).
The use of a protection against overcurrents is usual- ly required on capacitor banks, additionally to unbalance protection; this protection is normally realized with No 3 line current transformers, No 1 unbalance current transformer and proper relay.
Protection functions can be realized with:
Due to the different types of available relays, we advise to contact Enerlux Power Srl technical department that will provide related technical information.
These are devices suitable for fast discharge of capacitor banks and batteries after disconnection from the network, reducing the residual voltage at the terminals of the bank extremely quickly (approximately 10÷15 seconds), differently from standard systems where the discharge time is of several minutes
This solution offers the following advantages:
We should point out that the operation of the fast discharge device must anyhow contemplate the thermal effects of the discharge currents crossing it; it is therefore wise to wait at least 5 minutes after two fast discharges before reconnecting the bank to the network.