Lesson 9 Overvoltage Protection - OU and SPD PDF

OVERVOLTAGE PROTECTION 2. Overvoltage (Excess voltage +/-10% for long periods) The term overvoltage here refers to the situation...

OVERVOLTAGE PROTECTION 2. Overvoltage (Excess voltage +/-10% for long periods) The term overvoltage here refers to the situation situat where the There are mainly two types of over voltage: nominal voltage (230v) exceeds the accepted maximum 1. Surge voltage voltage by 10%.. However such overvoltage continues for a A surge voltage is a short burst of high voltage that occurs longer period of time. There are different causes for this in a very short time like micro-seconds. A surge protection situation, namely three: device is used to protect against this type of voltage.  Overvoltage injected on the supply system due to a Typical causes of surge voltage are: generated higher voltage. A typical cause for this is the  Lightning installation of photovoltaic cells whose voltage must be  Heavy current switching slightly higher that at the supply voltage.  Heavy current faults  Overvoltage caused by a combination of variation in load For this type of overvoltage, a Surge Protection Device and voltage regulation settings on the substation (SPD) is used to protect againstt such surges surges. transformer.  A faulty situation where the main neutral conductor is broken. This will generate wild voltages across each phase. For this type of overvoltage, an over and under voltage relay is used to protect against this situation. 1 Legal requirements Over and under voltage relay According to S.L.545.24 (LN225 of 2010, the local Electrical Installations Regulations), any new, modified or extended An over and under voltage relay is a unit that is used to protect installations after April 2010, must have an overvoltage against variations in voltage that are beyond the +/-10% protection. fluctuation level. The O/U relay is primarily a voltage sensor that operates an internal relay when the voltage rises or falls It is recommended by the regulating authority (REWS) that the below a pre-set value. protection required is against any fluctuating voltage and therefore the use of over and under voltage relay protection The current that the internal relay can handle is often very small is recommended. and definitely much smaller than the current that the whole installation absorbs. Thus the O/U relay operates a larger relay According to clause 20 paragraph 4, of the same S.L., “An that can handle the larger currents handled by the entire over voltage protective device should be installed installation. immediately after the main switch fuse or circuit breaker”. Recently on the market there were introduced units of O/U relays complete with a power relay to handle high currents. This new technology can be bought as one unit and can be thus installed in the consumer unit between the main switch and the RCD. 2  Single unit type, over/under voltage relay, including power relay. (2 module type). Separate over and under voltage relay and a modular type relay 3 Surge protection The Surge Protection Device (SPD) is a component of the electrical installation protection system. This device is connected in parallel on the power supply circuit of the loads that it has to protect. It can also be used at all levels of the power supply network. An SPD connected in parallel to the supply system has a high impedance. Once the transient overvoltage appears in the system, the impedance of the device decreases so surge current is driven through the SPD, bypassing the installation or sensitive equipment. 4 The three types of SPD: Connection of SPD Type 1 SPD An SPD is always connected across the phase and neutral (or 3 phases and neutral in case of a 3 phase system). It should The Type 1 SPD is recommended in the specific case of be connected between the main switch or circuit breaker and service-sector and industrial buildings, protected by a lightning RCD. protection system. It protects electrical installations against direct lightning strikes. Type 2 SPD The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard, it prevents the spread of over-voltages in the electrical installations and protects the loads. Type 3 SPD These SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads. Fig 7 5 Critical length of connecting conductors for SPD`s. To gain maximum protection the connecting conductors to SPDs must be kept as short as possible, to minimize additive inductive voltage drops across the conductors. The total lead length (a+b) should preferably not exceed 0.5m but in no case exceed 1.0m; see figure. Refer to the SPD manufacturer’s instructions for optimal installation. Where more than one type of SPD is used, the IET recommends that the coordinated SPDs are to be of the same make or manufacture unless the designer is satisfied that the devices of different makes will coordinate as required. OCPD = Overcurrent Protection Device SPD = Surge Protective Device E/L = Equipment or installation to be protected against overvoltage 6 OVR`s are made of electronic components and therefore sensitive to surge voltages. Where the supply is by overhead lines and therefore exposed to lightning voltage surges, OVR`s become useless as they are easily damaged by these surges. To protect OVR`s against these voltages, SPD`s are required to be wired as per drawings below. Typical single phase SPD 7 8