The VDEW publication ZLU (Zusätzliche Liefervereinbarungen für Umrichterantriebe in Kraftwerken (Supplementary Supply Agreements for Converter Drives in Power Plants), 1st Edition 1997) describes the design of frequency converters for power plant applications. It is noted in the preface of this document that the ZLU does not constitute a standard in the sense of a VDE regulation, but describes requirements and/or technical data and equipment details which apply additionally to converter drives used for power plant applications.
Several chapters are dedicated to describing the extensive range of equipment documentation. The KKS (Kraftwerkskennzeichnungssystem), a standardized identification system for power plants, must generally be applied throughout the documentation. As a minimum requirement, the correct KKS No. for the drive must always be stated in the documentation. The documentation must describe the converter interfaces in such a way that the destination and the type and number of used cables are clear even if these are not included in the scope of supply of the converter.
Furthermore, supplementary rules regarding documentation apply in every power plant and these must also be taken into account. These rules must generally be stated as special requirements, e.g. under "Engineer To Order – ETO" (X80), see Option descriptions.
Noise emission values which might require a special design are specified in Section 1.10. Where applicable, these must be stated or selected as special requirements in options.

The ZLU also describes the converter transformer and the motor. This information must be taken into account in the design of these components.

The electrical design specifications (Chapter 4) state that the operating device must also include a key-operated switch for local control of the converter. It is further stated that the incoming supply must be connected via a fuse switch disconnector and main contactor or via a circuit breaker. Generally speaking, requirements often deviate from this specification. The key-operated switch and the line-side power switchgear must be selected in a further option. Unless the order contains any additional information, option S12 must be selected. This includes the local key-operated switch and the circuit breaker. Option S02 is often selected as well.

Chapter 1.5 "Rated output" states that the drive may continuously output at least 105 %, but maximum 120 %, of the shaft power required in maximum-load operation of the driven machine. This means that the converters must always be designed with a 5 % power reserve. This must be taken into account in the drive design. Option X51 "Extended overvoltage range" does not include any power reserves, see "Engineer To Order – ETO" under Option descriptions.

The device testing procedures are described in Chapter 6 "Quality assurance, Testing, Acceptance". It includes a description of routine and type tests and prescribes type tests as mandatory for first deliveries. It is not acceptable to test a selected component from a series. The operator or the ordering party or his/her representative must be given the opportunity to carry out test inspections during the manufacturing process.
Furthermore, the total efficiency (transformer, converter, motor) must be verified by computation or in a test bay or at the site of installation.
Many tests are also specified as optional subject to agreement. These must be selected in the form of additional options. The test requirements specified in the ZLU are fulfilled with one of the options F82, F83, F84, F85 (warm-up with original motor).

Chapter 1.9 "Operational requirements" specifies special power system data in the event of load shedding of the turbine generator and offloading to auxiliary supply. The converters must remain in operation in the event of a brief frequency increase to 1.1 f_r for 5 s with simultaneous voltage increase to 1.25 Г— V_r for 2 s. Depending on operational requirements, it might be necessary to agree voltage values of up to 1.4 Г— V_r. It is not possible to implement this brief overvoltage load capacity on every converter (see option description X51 Extended voltage range).