DC surge protection of remote radio units RRU or remote radio head RRH intelec® 2014 conference proceedings

Traditionally, cellular radio antennae were connected to base station radio equipment using coaxial feeders. Microwave radio antennae were ether connected with waveguides or coaxial cables, which will collectively be called feeders herein. The feeders would carry the baseband frequency and the RF signal. RF feeders have served the industry extremely well. However as the frequency and the bandwidth transmitted increases, the losses in the feeder and connectors became more significant. There is a limitation on the length of the RF feeder before losses become intolerable and the error rate significant. The next generation of (microwave) radio equipment utilized remote radio units close to the antennae which would convert the frequencies to an intermediate frequency and this could be transmitted more efficiently on smaller coax feeders with losses being less of a problem. Modern cellular and microwave equipment utilize remote radio unit RRU or remote radio head RRH which is fed from the base station via optical fiber. This eliminates the loss issues on feeders and allows transmission to occur at much higher frequencies with larger bandwidth. Power to the RRU cannot be transferred from the base station to RRU or RRH via the optical fiber. Hence, power is fed separately as DC on copper cables. The copper cables are either separate from the fiber or are a composite fiber-copper cable. The DC feed acts as a source of lightning surges back into the equipment room. More precaution needs to be taken on how to control these surges, than ever before. In traditional radio, damage to equipment would normally be limited to the radio equipment. In the modern scenario damage can occur to the rectifiers or the whole DC power system, which would jeopardize other equipment installed at the site. 1) Location of SPD The simple solution to this may seem like installing Transient Voltage Surge Suppressors (TVSS) or Surge Protective Devices (SPD) on the DC feeds. However, there are intricac- es that involve ground loops & voltage drops associated with cable lengths that need to be understood before choosing the correct location of SPD. This paper will discuss the possible location of the SPD and the benefit and disadvantages with each location presented. 2) Sizing of SPD There is guidance on the sizing of AC SPD in various standards, including IEEE C61.42, IEC61643 & ITU K56. There is some guidance on the sizing of SPD´s for coaxial feeders and tower lights in ITU K56 guidelines. However the application of DC SPD on RRU is a relatively new concept and standards for sizing of these may not exist. The paper will look at methods of sizing AC SPD´s and SPD´s for traditional feeders and tower lights as a benchmark and propose suitable values for DC SPD for RRU. 3) Testing of SPD Finally, the application of SPD´s in DC applications poses some unique challenges that are not present in AC applications. One such challenge is ensuring that the DC voltage present does not cause the SPD to get into continuous conduction at any time. In AC systems there are many voltage crossing and hence there is opportunity for SPD to get out of conduction. This paper will look at a test setup that simulates the DC application and demonstrate examples of results obtained.