Tests with Combined and Composite Voltages

Abstract

In power systems, the over-voltage stresses of insulations are often combinations of the operational voltage with over voltages. This can be neglected as long as the over-voltage value includes the contribution of the operational voltage. It cannot be neglected when the insulation between phases or of switching devices is considered. In that case, the resulting voltage is the combination of two voltage stresses on three-terminal test objects. In other cases, the stressing voltage is composed of two different voltage components, e.g. in certain HVDC insulations, as a composite voltage of AC and DC components. This chapter is related to the definition, generation and measurement of combined and composite test voltages on the basis of IEC 60060-1:2010 and IEEE Std. 4. Also some examples for tests with combined and composite voltages are given. It should be mentioned that these voltages are sometimes called “hybrid” or “superimposed” voltages, also the summarizing term “mixed” voltages is in use. This book follows the terminology of the Standard IEC 60060-1 (2010).

8.1 Combined Test Voltage

The definitions of combined and composite test voltages are related to the position of the test object to the test voltage sources. When the test object is arranged between the two test systems, a combined voltage stresses the test object via two different HV terminals and to ground (three-electrode test arrangement). When the two test systems are directly connected, a composite voltage is generated stressing the test object from one HV terminal to ground (two-electrode test arrangement). In both cases, each test system must be protected against the voltage generated by the other system by means of an element that lets pass its own voltage and blocks the voltage of the other one (coupling/protecting element).

8.1.1 Generation of Combined Test Voltages

The combined voltage appears between the two HV terminals of a three-terminal test object with the third terminal grounded (Fig. 8.1a). Typical three-terminal test objects are disconnectors and circuit breakers. Also the insulation between phases in three-phase systems, e.g. metal-enclosed busbars of GIS and three conductor cables, form a three-terminal test object. For a HV test of a GIS, two phases are connected each to one HV source, the third phase and the enclosure are grounded.

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig1_HTML.gif — Fig. 8.1
Generation and measurement of combined voltages. a Schematic circuit diagram. b Combined test voltage as a difference of two voltages

The test object is stressed by the difference of the two single voltages (Fig. 8.1a: V_c = V_AC − V_SI). Figure 8.1b shows as an example the combined test voltage of an AC and a SI voltage. As long as the test object withstands the combined voltage stress V_c, it separates the two HV sources from each other. But when it breaks down, each test system is also stressed by the voltage of the opposite HV source. Then, it must be protected by a suited protection element that blocks the other voltage, at least down to an acceptable voltage stress. But this element must also couple—and not block—the voltage of the protected HV source. This means that elements must be applied that have different impedances for different voltages. For instance, an inductor has no impedance for DC voltage, but an high impedance when stressed with LI voltages. Therefore, it may be used for protecting a DC voltage generator. It must be considered that the coupling/blocking elements influence the two components of the combined voltage. Therefore, the measurement of the two voltages has to be made after the coupling/protecting elements in parallel to the ground insulation of the test object (Fig. 8.1a). Compared with the impedance of the test object, the impedances of the coupling/protecting elements should be low.

Table 8.1 summarizes the characteristics of coupling/ protecting elements . The preferred application of an element is shown in the first row, a second application with different parameters of the element is given in brackets. The arrows in brackets indicate a low (↓) or high (↑) value of the parameter (L, R, C). For instance, a capacitor of large capacitance (low impedance for power frequency) couples AC voltage of power frequency but one of low capacitance (high impedance) may block it. Triggered switches can be switched to positions “closed = coupling” or “open = protecting” and can consequently widely be applied. It has to be considered that a—possibly triggered—spark gap requires a certain voltage for ignition (see Fig. 7.6b). When it is reached, the resulting voltage jumps to that value (Fig. 8.8). After an impulse stress, it does not easily extinguish at an AC or DC voltage which is short circuited. Non-triggered gaps have the disadvantage of the dispersion of their breakdown voltage and that it is difficult to reach standard LI and SI voltage shapes. The alternative coupling element to the sphere gap is a capacitor, in the superposition it maintains the two voltages as they are (e.g. Fig. 8.5). Capacitors are expensive because their capacitances should be remarkably higher than that of the test objects (Felk et al. 2017).

Table 8.1

Coupling/protecting elements for combined/composite test voltage circuits

Test voltage elements	DC voltage	AC voltage	SI voltage	LI voltage
Inductors (L)	Coupling	Coupling (L↓) (Protecting, L↑)	Protecting (L↑) (Coupling, L↓)	Protecting
Resistors (R)	Coupling (R↓) (Protecting, R↑)	Coupling (R↓) (Protecting, R↑)	Protecting (R↑) (Coupling, R↓)	Coupling (only low R↓)
Capacitors	Protecting	Coupling (C↑) (Protecting, C↓)	Coupling (C↑) (Protecting, C↓)	Coupling
Switches as triggered gaps, semiconductors	Coupling or protecting	Coupling or protecting	Coupling or protecting	Coupling or protecting

Example When a DC/LI combined voltage shall be applied, the right element between DC voltage test system and test object is an inductor, because it couples the DC voltage and protects against LI voltage. Between the LI test system and the test object, a capacitor is the right element, it couples the LI voltage and protects the impulse generator against a DC voltage stress. Also the application of a (-possibly triggered-) sphere gap can be taken into consideration.

As mentioned before, the coupling/protecting elements influence the voltage generation of both voltage sources, and also the two sources show interactions. As a result, the combined voltage has not the shape as expected. The test of a disconnector shall be considered: The AC/SI combined test voltage shall be generated by a test transformer and an impulse generator (Fig. 8.2a). The AC voltage is dropped down if the AC source is not stiff enough (Fig. 8.2b: 20%). If no powerful AC test system is available, a supporting capacitor C_a ≫ C_t, larger than the test object capacitance C_t in parallel to AC source and test object, reduces the voltage drop remarkably (Fig. 8.2c: <5%) (Cui et al. 2009). When a combined (or composite) voltage test is planned, it is strongly recommended to analyse the test circuit by a suitable equivalent circuit.

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig2_HTML.gif — Fig. 8.2
Interaction between the two voltage sources. a Schematic circuit diagram. b Drop of the AC voltage without supporting capacitor. c Drop of the AC voltage with supporting capacitor C_a

8.1.2 Requirements to Combined Test Voltages

The test voltage value of the combined voltage is the maximum potential difference between the two HV terminals of the test object. Its tolerance, this means the difference between the specified value and the recorded value shall be within ±5% of the specified value. This includes that also a voltage drop does not exceed 5%. For each voltage component, the requirements mentioned above in the relevant Chaps. 3, 6 and 7 have to be applied. Furthermore, the time delay , this is the time difference between the two maxima of the voltage components, must be considered (Fig. 8.3). The tolerance of the time delay is 0.05 T_p, with T_p as the longer front parameter of the two voltages involved (where T_p is the LI front time or the SI time to peak or a quarter of an AC period).

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig3_HTML.gif — Fig. 8.3
Time delay of combined and composite voltages. a For two impulse voltages. b For an impulse voltage and an AC voltage

8.1.3 Measurement of Combined Test Voltages

The two HV test systems require a HV measuring system each for the adjustment of their output voltages that contribute to the combined or composite voltage. These measuring systems must be able to record also the interactions between the two HV test systems.

The stressing combined voltage acts between the HV terminals of the test object (Fig. 8.1). A usual measurement of this voltage is difficult because there is no earth potential involved. IEC 60060-1:2010 allows therefore the calculation of the combined test voltage from the measurement of its two voltage components: Each of the two voltage dividers shall be arranged as near as possible to its relevant HV terminal of the test object. The two voltages are recorded, and the combined voltage is calculated as its difference. The uncertainty estimation for the measurement of the calculated combined voltage must consider the influences of the test object and of voltage drops over the connection/blocking elements the calculated one (Fig. 8.1b), should be displayed using an identical time scale.

8.1.4 Examples for Combined Voltage Tests

Disconnector testing : The testing of EHVAC disconnectors and phase-to-phase air insulation with a combined voltage of AC and SI components is the classical example of a combined voltage test. The interaction between the test voltage sources may cause a voltage drop as considered above in Sect. 8.1.1 (Fig. 8.2). Garbagnati et al. (1991) found that the atmospheric corrections of IEC 60060-1 (see Sect. 2.1.2) deliver sufficient results when they refer to that component of the test voltage value between the HV terminals that causes the maximum of the combined voltage.

Combined voltage tests with DC voltage component: The broader application of HVDC transmission systems will require test voltages that can be understood as combined voltages (Gockenbach 2010). There have been early investigations about the combination of DC voltage and oscillating SI voltage (Fig. 8.4) in preparation of test systems for the Russian HVDC transmission (Lämmel 1973). Meanwhile LI/DC voltage investigations are extended to compressed gas insulations of N₂ and SF₆ (Wada et al. 2011). They show that a DC voltage component below 50% of the test voltage value has little influence on the breakdown voltage.

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig4_HTML.gif — Fig. 8.4
DC/SI combined voltage. a Circuit with coupling/connecting elements. b OLI voltage superimposed on DC voltage

8.2 Composite Voltages

8.2.1 Generation and Requirements

The connection of two different test voltages to one terminal generates a composite test voltage because of their superposition at that point (Fig. 8.5a). The connection is realized with suitable coupling/protecting elements (Table 8.1). In opposite to combined voltages, the composite voltage is the sum of the two components (Fig. 8.5b: V_co = V₁ + V₂). If the two voltage sources are connected together, the interaction between the two HV sources including their coupling/protecting elements may play a role and should be analysed.

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig5_HTML.gif — Fig. 8.5
Generation and measurement of composite voltages. a Schematic circuit diagram. b Composite voltage as the sum of the two voltages

The test voltage value of the composite voltage is the maximum absolute value at the test object and shall meet the specified value within ±5%. Also any voltage drop shall not exceed 5%. The time delay is defined as for combined voltages (Fig. 8.3) and should be again within ±0.05 T_p (T_p as defined in Sect. 8.1.1). For the single-voltage components, the requirements in the relevant chapters of this textbook shall be applied.

8.2.2 Measurement of Composite Test Voltages

The stressing composite voltage acts between the HV terminal of the test object and the earth (Fig. 8.5). Therefore, it can be measured directly. The used measuring system shall fulfil the requirements of IEC 60060-2:2010 for both components. In case of a DC/LI composite voltage, e.g. an universal divider has to be applied (Fig. 2.10). The separate measurement of the two voltage components is necessary for the precise control of the two test voltage generators and for the verification of the correct relation of the two test voltages. All three voltages shall be recorded synchronously and displayed with an identical time scale (Fig. 8.5).The voltage measuring systems shall be calibrated for the measurement for the related voltage components as well as for the composite voltages to be measured. Based on that the uncertainty of the measurement of the composite voltages shall be estimated.

8.2.3 Examples for Composite Voltage Tests

A composite DC/AC test voltage can be generated when the smoothing capacitor of the DC generator is grounded via the HV winding of a test transformer (Fig. 8.6). The DC generator must be fed via an insulating transformer (Fig. 6.8). If the test transformer is designed to withstand the DC stress in case of a breakdown of the test object, no additional coupling/protecting elements are required. The photograph in Fig. 8.6 shows such a DC/AC test system at NIIPT, St. Petersburg, Russia, used for the operation of a HVDC test line, corona investigations and other basic research work. Both voltage components can be adjusted separately. For the application of composite DC/LI respectively DC/SI test voltages, the basic circuit (Fig. 8.5) with coupling/protecting elements according to Table 8.1 shall be used.

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig6_HTML.gif — Fig. 8.6
Test system for DC/AC composite voltage up to ±1300 kV DC and 400 kV AC (The principle is shown for one polarity only)

With respect to the characteristics of HVDC insulation (see Chap. 6, especially Sects. 6.2.2.2, 6.2.3, 6.3), composite voltages are mandatory for the testing of HVDC insulations. This shall be considered for HVDC gas-insulated systems (Cigre JWG D1/B3.57 (2017)) with the sensitive field strength distribution in the surrounding of the spacers. Figure 8.7 (Hering et al. 2017) shows the situation of the electrostatic field at AC voltage and of the streaming field at DC voltage just before a transient LI or SI stress occurs. For an HVAC test object—with lowest field strength at the outer electrode—the transient stress will change the intensity of the field distribution only. In case of the HVDC insulation—with the space charge-dominated high steady-state field strength at the outer part of the spacer—the transition of the surface-charge dominated streaming field to a capacitively-dominated quasi-electrostatic field must be considered. The latter is related to field strength enlargements, especially if the polarity of the transient stress is opposite to the steady-state conditions before (Fig. 8.8b, d).

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig7_HTML.gif — Fig. 8.7
Field conditions in an HVAC and an HVDC gas-insulated system before a transient stress (Hering et al. 2017). Consider the higher field strength on the outer side of the spacer at DC stress

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig8_HTML.gif — Fig. 8.8
Composite DC/LI voltage tests as definition for the LI voltage tests of a gas-insulated system (schematically) as proposed in CIGRE JWG D1/B3.57 2017. The blocking/connecting element is a triggered spark gap, causing a voltage jump when triggered (dotted blue lines). a Composite voltage of positive DC and positive LI component. b Composite voltage of positive DC and negative LI component. c Composite voltage of negative DC and negative LI component. d Composite voltage of negative DC and positive LI component

NOTE Fig. 8.8 is a proposal of the CIGRE JWG D1/B3.57 for the present draft of a CIGRE Brochure. It uses Fig. 8.8 for definitions of LI test voltages and defines separately a DC voltage pre-stress. It underlines the high LI stress when the two voltage components are of opposite polarity. But it seems to be better to consider the composite voltage because the two stresses act commonly.

Consequently composite DC/AC, DC/LI and DC/SI voltage tests (DC field in steady-state conditions) become necessary for gas-insulated HVDC insulations. Whereas test voltages and procedures have been considered within the CIGRE JWG, first values of the test voltages and test durations for a prototype installation test (see Sect. 6.2.3.1) are proposed by Neumann et al. (2017). It should be mentioned that this prototype insulation test is also a composite HVDC and load current test, using a suited current source for operating on HV potential. The testing of an open switching device of a gas-insulated HVDC system includes both, composite voltage test (Fig. 8.9a, b) and for the gap itself a combined voltage (Fig. 8.9c) test.

../images/214133_2_En_8_Chapter/214133_2_En_8_Fig9_HTML.gif — Fig. 8.9
Testing of a the switching device of a SF₆-insulated system (schematically). a DC/LI or SI composite voltage test, right side grounded. b DC/LI or SI composite voltage test, left side grounded. c DC/LI or SI combined voltage test

8. Tests with Combined and Composite Voltages